enhancing nutrient retention in tropical tree …€¦ · 28 ecological applications, 14(1), 2004,...

28

Ecological Applications, 14(1), 2004, pp. 28–46q 2004 by the Ecological Society of America

ENHANCING NUTRIENT RETENTION IN TROPICAL TREE PLANTATIONS:NO SHORT CUTS

SETH W. BIGELOW,1,2,4 JOHN J. EWEL,2,3 AND JEREMY P. HAGGAR2,5

1Institute of Ecosystem Studies, Box AB, Millbrook, New York 12545 USA2Department of Botany, University of Florida, Gainesville, Florida 32611-8526 USA

3USDA-FS Institute of Pacific Islands Forestry, Pacific Southwest Research Station, 1151 Punchbowl Street, Suite 323,Honolulu, Hawaii 96813 USA

Abstract. In the humid tropics large quantities of nutrients can be rapidly leached whenthe soil is unprotected by actively growing vegetation. We established experimental plan-tations of three indigenous tree species on a fertile Andisol in Costa Rica and managedthem under 1- or 4-year cutting cycles with uncut stands as controls. Our goals were totest whether nutrient leaching was greatest under a regime of frequent disturbances thatreturned modest amounts of biomass to the soil surface (cutting and replanting on a 1-yrcycle) or less frequent disturbances that returned significantly greater amounts of planttissues to the soil (cutting and replanting on a 4-yr cycle), and to compare those cuttingcycles with nutrient leaching from uncut stands. Leaching of NO3

2 (over 9 yr), Ca21, Mg21,and K1 (over 4 yr) from upper soil horizons was monitored. Water balance was determinedby linking Penman-Monteith evaporation with changes in soil water storage modeled fromsoil physical parameters. Drainage water for solute measurement was sampled from porousceramic cups at 1.1 m depth in the soil.

Disturbance frequency proved to be an important determinant of NO32 leaching. Average

long-term NO32 leaching losses from stands on a 1-yr cutting cycle were extraordinarily

large: 442 mmolc·m22·yr21 (62 kg·ha21·yr21 of N), compared to 187 mmolc·m22·yr21 undera 4-yr cutting cycle and 71 mmolc·m22·yr21 from uncut stands. Elevated NO3

2 leaching wasprimarily due to increased concentration in the soil solution (rather than increased waterdrainage), because cutting usually resulted in a reduction of ,10% in evapotranspiration.Resilience of stands decreased with continued disturbance; under a 4-yr cutting cycle, standstended to take longer to return to the low levels of NO3

2 leaching characteristic of undis-turbed stands with each episode of cutting and replanting, while NO3

2 losses from annuallycut stands became increasingly variable over time. Due to high concentrations of soil Caat the site, the stands proved resistant to treatment-induced losses of base cations: noincreases in Ca21 or Mg21 leaching accompanied elevated NO3

2 leaching, although K1

leaching did increase under the 1-yr cutting cycle. Because of the potential for massive,sustained NO3

2 losses, development of land use systems for these soils should focus onminimizing frequency of disturbance.

Key words: hydrology; leaf area; nitrate and base-cation leaching; root length density; short-rotation forestry; tropical tree plantation.

INTRODUCTION

Actively growing vegetation diminishes nutrientleaching by taking nutrients out of the soil solution,decreasing drainage via transpiration, and adding or-ganic matter to the soil in the form of dead leaves,branches, and roots (Vitousek and Melillo 1979). Whenvegetation enters dormancy, when forests are cleared,or when agricultural crops and trees are harvested,these processes are altered: Uptake and transpirationtemporarily cease, and a pulse of organic matter is

Manuscript received 3 December 2002; revised 7 May 2003;accepted 28 May 2003. Corresponding Editor: A. R. Townsend.

4 Present address: USDA-FS Sierra Nevada Research Cen-ter, Pacific Southwest Research Station, 2121 2nd Street,Suite A101, Davis, California 95616 USA.E-mail: [email protected]

5 Present address: CATIE, Apartado P-116, Managua, Nic-aragua.

added to the soil. An excess, even short-term, of rainfallover the sum of evaporation and runoff then leads tonutrient leaching. This typically occurs in the temper-ate-zone spring, at the onset of the rainy season in theseasonally dry tropics, or almost any time in the wettropics. The high temperatures and readily availablemoisture that characterize humid tropical lowlands fa-cilitate rapid decomposition and nutrient release, there-by exacerbating these losses.

Leaching can be acute in tropical shifting agriculture,where nutrient losses are one of several factors thatlead practitioners to anticipate yield declines and shiftto new sites (Nye and Greenland 1960, Sanchez et al.1982, Trenbath 1984, Dagar et al. 1995, Juo et al.1995). Site impoverishment through post-harvest nu-trient leaching is not restricted to short-lived agricul-tural crops, however. It is a potential issue in plantationforestry, which today often involves rotation intervals

February 2004 29LEACHING FROM TROPICAL TREE PLANTATIONS

of only a few years, especially in the humid tropics(Fownes and Harrington 1990, Brown et al. 1997, Nam-biar and Brown 1997).

As rotation lengths shorten, nutrient leaching shouldincrease because root systems are poorly developed andany nutrients released into the soil solution are vul-nerable to loss. Under long-rotation cropping, post-har-vest nutrient losses in drainage water are considered tohave negligible effect on stand nutrient budgets (Mannet al. 1988), in part because the period of rapid loss isbrief in comparison to the duration of the cutting cycle(McColl and Powers 1984, Raison and Crane 1986).But the higher losses anticipated as rotations shortenmay be counterbalanced by the larger amounts of or-ganic matter added to the soil when older vegetationis felled. More biomass added to the soil at the timewhen resource-capturing structures are poorly devel-oped puts greater amounts of nutrients at risk of lossthrough leaching.

Thus, the relationship between rotation length andnutrient loss rates through leaching is uncertain, in-volving factors that exacerbate leaching losses both asa function of the time needed to develop leaching-re-ducing structures and processes on the one hand, andthe amount of nutrient-rich biomass added to the soilat the end of a rotation on the other. Our study wasdesigned to explore these trade-offs. Measurements ofleaching losses of macronutrients (nitrate, calcium,magnesium, and potassium) were made in experimentalplantations of three tropical tree species that were cutand replanted at intervals of one year or four years, orleft to grow undisturbed.

METHODS

Soil, site preparation, species

The study took place in the Atlantic lowlands (ele-vation 40 m) of Costa Rica at La Selva BiologicalStation. Mean annual temperature is 25.88C, and meanannual rainfall is 3962 mm; there is a dry season fromFebruary to April, although mean rainfall exceeds 100mm in all months (Matlock and Hartshorn 1999). Forannual summaries of data, we used a water year whosebeginning coincided with the onset of the wet season;water year 1992, for example, begins on 1 May 1992and extends to 30 April 1993. The experimental site isa flat, recently formed alluvial terrace: Rapid drainagein combination with flat topography and absence ofmicro-relief ensured that overland flow is virtually ab-sent even during intense rainfall.

The soil belongs to the order Andisol (Eutric Ha-pludand; Weitz et al. 1997), which comprises volcanicash soils characterized by variable charge, high cationexchange capacity, and amorphous allophane clays thatbind organic matter strongly. Soil at our site has typicalAndic properties. The upper mineral (A) horizon,which extends to ;0.2 m, is a highly aggregated clayloam with many fine pores. The lower mineral (B) ho-

rizon, extending to ;0.6 m, is a sandy loam, also withmany fine pores. There is a probable buried A horizonat ;0.7 m, followed by a C1 horizon of coarse sandwith few fine pores (A. F. Bouwman, personal com-munication). Cation availability is high in the A ho-rizon (cation exchange capacity . 400 mmolc/kg, basesaturation . 50%) and the B horizon (cation exchangecapacity . 300 mmolc/kg, base saturation . 30%; Hag-gar and Ewel 1994). The soil’s pH in water ranges from5.2 to 7.2; pH in KCl ranges from 4.6 to 5.6; organiccarbon content ranges from ;3 to 4% in surface ho-rizons and declines markedly with depth; and total Ncontent is ;0.3–0.4%. Mineralization and nitrificationrates are ;0.22 mg·g21·d21, or .100 kg·ha21·yr21 (Hir-emath and Ewel 2001). Despite the propensity of vol-canic-derived soils to bind P, N may be more limitingthan P at our site: Foliar N:P tends to be less than thethreshold value of 14 (Hiremath and Ewel 2001; seeKoerselman and Meuleman 1996), and soil P is highrelative to most tropical soils (.20 mg/g sodium-bi-carbonate extracted P; Haggar and Ewel 1994).

The experiment was carried out with three tree spe-cies that are indigenous to Mesoamerica; none of thespecies are known to have N-fixing symbionts. Hy-eronima alchorneoides Allemao (Euphorbiaceae) is anevergreen canopy emergent that can reach 50 m inheight (Clark and Clark 2001). It has simple, alternateleaves that are large when trees are young, but becomesmaller as the tree ages. Cedrela odorata L. (Meli-aceae) is a deciduous canopy tree with pinnately com-pound leaves that are shed during the dry season. Itproduces wood that is prized both for furniture andpaneling. Cordia alliodora (R. & P.) Cham. (Boragi-naceae) is a deciduous canopy tree that sheds its leavesat the onset of the wet season as an adult. The speciesis used in agroforestry (Greaves and McCarter 1990)and produces wood that is used for furniture and pan-eling as well as general construction (Opler and Janzen1983). Seed sources, nursery treatments, and out-plant-ing are detailed in Haggar and Ewel (1995, 1997).

In early 1991, the vegetation on the 8-ha site (anabandoned cocoa plantation) was felled, and the slashwas burned. Immediately following manual clearing ofcharred logs, three 60 3 40 m stands of each specieswere established in each of three blocks. Trees wereplanted in rows that were offset to provide a 2.0-mspacing between trees and an initial stand density of2887 trees/ha. Each 60 3 40 m stand was divided intothree plots: a 10 3 40 m plot for the 1-yr cutting cycle,a 20 3 40 m plot for the 4-yr cycle, and a 30 3 40 mplot for uncut stands. A self-seeded groundcover ofinvasive herbs was weeded every 2–6 wk.

One-year cutting-cycle stands were cut and replantedannually from 1992 to 2000, and the 4-yr cutting-cyclestands were cut and replanted in 1995 and 1999. Treesin the 1-yr and 4-yr cutting-cycle stands had reachedaverage heights of 1.6 m and 10.6 m, respectively, atthe time of cutting; uncut trees had reached an average

30 SETH W. BIGELOW ET AL. Ecological ApplicationsVol. 14, No. 1

height of 17.9 m by 1999. Operations were done fromMay to August, coinciding with the onset of the wetseason. All biomass was left in place after cutting toeliminate export of nutrients in aboveground biomassas a cause of differential leaching rates among treat-ments.

Fine root sampling

Fine root density (,2 mm diameter) of trees wasmeasured annually to assess the potential for nutrientuptake; methods and results from 1992 through 1994are presented in Haggar and Ewel (1995, 1997). In 1995and subsequent years, soil cores were taken from eightrandomly selected locations per plot in uncut stands.Sampling was done in 0.1 m increments to a depth of1.1 m, and in 1995, 1996, 1999, and 2000 samplingwas done to a depth of 2 m to check for deeper roots.Roots were separated from soil using a hydro-pneu-matic elutriator (Gillison’s Variety Fabrication, Ben-zonia, Michigan, USA), dead roots were discarded, andlive-root length was measured using the line-intersectmethod (Tennant 1975).

Water balance: precipitation, evapotranspiration,and soil moisture

The rate of water drainage at 1.1 m below the soilsurface was estimated from a water balance. Precipi-tation was measured directly, evapotranspiration wasestimated with a Penman-Monteith approach (Bigelow2001), and changes in soil water storage in the upper1.1 m of soil were estimated with a model based onsoil physical properties. Meteorological data for cal-culating evapotranspiration (ET) were obtained fromthe weather station of the Organization for TropicalStudies, ;1 km from the study site (Sanford et al. 1994,Matlock and Hartshorn 1999). Instrumentation con-sisted of a tipping-bucket rain gauge (model 6011-B,Qualimetrics, Sacramento, California, USA), a plati-num resistance thermometer and capacitative polymerchip for humidity housed in a radiation shield(HMP35C, Campbell Scientific, Logan, Utah, USA),and a pyranometer (LI-200 SZ, LI-COR, Lincoln, Ne-braska, USA) for measuring solar radiation (St). Datawere logged at 5-s intervals and recorded as hourlysums or means.

The Penman-Monteith equation requires inputs ofnet radiation (Rn), and three equations were tested forconverting St to Rn: (1) a linear equation Rn 5 a 1 bSt,where a and b are dimensionless constants, (2) differentlinear equations for wet and dry days, and (3) a mech-anistic equation, Rn 5 (1 2 r)St 1 Ln, where r is re-flectivity of short-wave radiation, and Ln is net long-wave radiation. Ln was estimated as c 1 sT4(d 2 1),where s is the Stefan-Bolzman constant, and c and dare universal fitting constants (Monteith and Unsworth1990). Data for model evaluation were 2647 obser-vations of mean hourly Rn (Bigelow 2001) and St mea-sured during 1995. The fit of data to model was eval-

uated using the sums of squares principle (Hilborn andMangel 1997), which adjusts the error sums of squaresfor the number of parameters in a model. The systemof different linear equations on wet and dry days re-sulted in the lowest adjusted sum of squares; regressionequations were Rn 5 0.667St (r2 5 0.602) and Rn 50.706St (r2 5 0.817) for dry and wet days, respectively(intercepts were omitted because they were nonsignif-icant).

As is typical for long-term meteorological recordsthere were a number of days of missing data. Whenavailable, data from nearby (,1 km) weather stationswere substituted for missing data (Bigelow 2001; H.Loescher, unpublished data). When substitute precip-itation data recorded at hourly intervals were not avail-able, gaps (104 d) were filled with daily totals by as-suming that rain fell from 07:00 to 11:00 hours. Miss-ing total solar radiation (St ) data (170 d) were filledusing linear equations that gave St as a function ofphotosynthetically active radiation. Equation parame-ters were obtained by linear regressions done on the10 d of data prior to each gap. The r 2 for every set ofparameters was $0.985. A continuous, 439-d gap ofrelative humidity (h) data was filled by substitutingsaturated vapor pressure at nighttime temperatures,es(Tmin), for actual vapor pressure, e, in the expressionh 5 e/es(T ). A test of this formula showed that esti-mated h was within 11% of measured h for 90% ofobservations in a year-long record. Other missing h andT data (26 d) were filled by substituting average valuesfor day and hour from the rest of the data record.

The Penman-Monteith equation was linked with acanopy drainage routine for intercepted rain, and theseequations required a number of parameters. Derivationof parameters for canopy conductance (gc), aerody-namic conductance (ga), canopy cover, and canopy wa-ter storage capacity are given in Bigelow (2001). Es-timates of leaf area index (LAI) were obtained annuallythrough harvest and allometry (Haggar and Ewel 1995,1997). The pattern of seasonal fluctuations of leaf areain the deciduous species, Cordia and Cedrela, was es-timated from visual observations and optical plant-can-opy analyses (LAI-2000, LI-COR) performed monthlyin all plots from mid-1999 through 2002.

Soil moisture was estimated using the modelSWAP93, which incorporates Richards’ equation forunsaturated flow through a soil profile (van den Broeket al. 1994). The profile was treated as having uniformproperties in four horizons: 0–0.1, 0.1–0.3, 0.3–0.5,and 0.5–1.1 m). Parameters describing soil physicalproperties (saturated soil moisture us, residual soilmoisture ur, and fitting parameters n and a; van Gen-uchten 1980) were derived by A. Weitz (unpublisheddata) for soil at a secondary forest site ;100 m fromour study plots on the same alluvial terrace.

Time-domain reflectometry measurements of soilmoisture (Trase II, Soil Moisture Equipment Corpo-ration, Santa Barbara, California, USA) were used to


evaluate the water balance. Pairs of stainless-steel rods,each of which sampled a cylindrical soil volume 1 mdeep by 0.2 m across, were installed vertically in threerandom locations in each plot. The model of Roth etal. (1990) was used to convert apparent dielectric con-stants into volume soil moisture, incorporating param-eters obtained for our site (porosity 5 0.711 m3/m3 andsoil geometry factor a 5 0.47; Weitz et al. 1997). Read-ings were taken in 1995, and the average time betweenreadings was 1 wk. Comparisons of simulated to mea-sured volume soil moisture were made with linear re-gressions (SAS 1990); adequacy of simulations wasassessed with the adjusted r2 statistic.

Soil water sampling and chemical analysis

Daily fluxes of nutrients in drainage water were es-timated by multiplying nutrient concentrations by dailywater drainage at 1.1 m soil depth. Solute concentra-tions were assumed to change linearly between sam-pling dates. Soil water was sampled with 48 3 60 mm,high-flow, round-bottom ceramic cups (Soil MoistureEquipment Corporation, Santa Barbara, California,USA) glued to 1.3 m long PVC pipes sealed at the topwith rubber stoppers. Stoppers were perforated by asemirigid 7.7 mm diameter polyethylene tube whosetip was inserted inside a soft neoprene tube, and theassemblage was sealed by clamping the soft tube. Ten-sion was placed on the samplers after $20 mm of rainhad fallen within 3 d, then water was collected afteran additional ;24 h. A tension of 216 kPa was usedbecause tensiometer measurements indicated that sat-urated soil at 1 m drained to approximately this tensionwithin 24 h (J. P. Haggar and A. Nobre, unpublisheddata), and our intent was to extract freely drainingwater rather than water that was tightly held by thesoil. To collect water from a sampler, a hand-pump wasapplied to a flexible tube that was passed through thestopper assemblage to the bottom of the sampler. Toinhibit nitrogen transformations, 0.05 mL of chloro-form was added to samples, and then they were re-frigerated. Soil water was first collected in June 1992,11 mo after establishment of the plantations, and con-tinued to be collected every ;3 mo for the first 2 yr,and then with varying frequency thereafter.

There were three soil water samplers in each of thenine stands (3 species 3 3 blocks) under a 1-yr cuttingcycle. Until mid-1995, there were two samplers in eachof the uncut and 4-yr cutting-cycle stands; prior tocutting the latter for the first time, additional samplerswere installed to bring the number of samplers in eachplot to three. Soil water from samplers within a plotwas combined prior to analysis.

Analysis of solutions for NO32 was done on-site with

an automated system (Technicon II, Scientific Instru-ments, Tarrytown, New York, USA); detection limitwas estimated as 0.003 mmolc /L. Analyses for Ca21,Mg21, and K1 were done using inductively coupledplasma spectroscopy (ICP). Only four years of cation

data are presented in this paper because of concernsabout accuracy of cation analyses collected from 1992–1994, and discontinuation of routine cation analysesafter mid-1999. Additional analyses were made onsamples collected in December 1999 to obtain a com-plete charge balance. Assays were done of CO3

22 andHCO3

2 by titration; Al31 by ICP; PO422 by ion chro-

matography; Cl2, SO422, and NH4

1 by automated col-orimetry, and Na1 by flame atomic absorption spec-troscopy.

Statistical analysis

The experiment was treated as a split-plot design inwhich block was a random effect, species was a fixedwhole-plot effect, cutting cycle was a fixed sub-ploteffect, and year was a fixed sub-sub-plot effect or re-peated measure (Haggar and Ewel 1997). Analysis ofvariance was done using the GLM procedure in SYS-TAT 9.0 (1998), and then error mean squares weretransferred to a spreadsheet to complete the appropriateF tests. Following common practice for this design, wedid not test for three-way interactions among block,species, and cutting cycle, nor for two-way interactionsamong block and cutting cycle (Montgomery 1984).Dependent variables were annual leaching losses ofNO3

2, Ca21, Mg21, and K1. Nitrate values reported as0 mmolc/L were treated as being equal to the lowerdetection limit (0.003 mmolc/L) for statistical analyses(Newman et al. 1989). Model adequacy was tested byexamining plots of residual against predicted values.There was little overlap of residuals of annual NO3

2

losses among cutting-cycle treatments, so annual NO32

losses were log-transformed prior to analysis.Multiple comparisons were used to pose directed

questions about NO32 leaching. Because some involved

multiple contrasts (e.g., comparing leaching from onespecies against two other species) Scheffe’s methodwas selected. Although this test is conservative andmay predispose towards Type II errors, we allowed arelatively high probability of making a Type I error (a5 0.1), which increased the likelihood of finding sig-nificant differences. All comparisons were made witha given water year, so the following parameter valueswere used for Scheffe’s test: N 5 27 (3 species 3 3cutting cycles 3 3 blocks) for number of data points;k 5 9 (3 species 3 3 cutting cycles) for number oftreatment groups, and s2 5 0.298 (sub-sub-plot errorfrom the analysis of variance) for the variance com-ponent.

Questions addressed with multiple contrasts were asfollows: First, was short-term NO3

2 leaching after cut-ting greater in 4-yr-old than in 1-yr-old stands? MoreN was sequestered in the biomass of the 4-yr-old stands,so we expected higher short-term (though not long-term) NO3

2 leaching rates there. Nitrate leaching wascompared in years in which stands on 4-yr (and 1-yr)cycles were cut, i.e., water years 1995 and 1999, withthe three species treated as a single group. Second, how


soon after cutting did stands again become retentive ofNO3

2? Annual NO32 leaching from stands on a 4-yr-

cutting cycle was compared to leaching from uncutstands in the years after the former had been cut, i.e.,water years 1996–1998 and 2000. Species were ex-amined individually because there were clear differ-ences in rates of recovery of nutrient retention. Finally,did established stands of the three species differ in theirpropensity to retain NO3

2? Annual leaching from uncutstands of Cedrela, which appeared to develop increas-ing NO3

2 leaching losses over the course of the ex-periment, was compared with leaching from the twoother species in the water years 1997–2000.

Factor analysis of solute concentrations from the De-cember 1999 collection was done to identify relation-ships among ions and to assist in evaluating the de-pendence of cation leaching on NO3

2 concentrations.In factor analysis, the covariances among a set of var-iables (in our case, solute concentrations) are convertedinto a set of principal components. Typically, the twocomponents that account for most variance are plottedagainst one another, and the position of the resultingvectors (one vector for each variable) can be viewedas a summary of the regression relationships amongvariables.

RESULTS

Root length and leaf area

Fine root length density (Lv) in the upper 0.1 m ofsoil averaged 0.22, 0.06, and 0.13 cm/cm3 in estab-lished stands of Hyeronima, Cedrela, and Cordia, re-spectively (Fig. 1). Year-to-year variation in Lv wasminor. All species had fine root development at soildepths below 1.1 m; in 1995 and 1996, an average of27% of fine root length from 0–2 m soil depth occurredfrom 1.1–2.0 m. By 2000 and 2001, though, the pro-portion of fine roots concentrated in the 1.1–2.0 mdepth in uncut stands was ,4%. The presence of fineroots below 1.1 m indicates that nutrient uptake oc-curred below the level at which soil water was col-lected. In stands of 1-yr-old trees, in contrast, therewere few roots at 1.1 m depth (Haggar and Ewel 1995),so there would have been little uptake below this level.Thus, water sampled at 1.1 m beneath the surface of4-yr cutting cycle and uncut stands overestimatesleaching from those stands, biasing the nutrient-reten-tion interpretations toward the 1-yr cutting-cyclestands.

Hyeronima LAI in uncut stands eventually stabilizedat ;8 m2/m2, far higher than in uncut stands of theother two species, which were closer to 3 m2/m2 (Fig.2, lower panel). Accordingly, uncut stands differed inthe age at which they reached maximum LAI: Cordiaat 2 yr, Cedrela at 3 yr, and Hyeronima at 6 yr. Standson a 4-yr cutting cycle therefore spent a significantproportion of each cycle at less than maximum poten-tial LAI, and stands on a 1-yr cutting cycle had only

achieved a small proportion of potential LAI by thetime of each cutting.

Water balance

The long-term meteorological record showed annualvariation best when data were summarized at weeklyintervals (Fig. 3). The lowest weekly values of Rn, min-imum daily temperature, and maximum daily vaporpressure deficit occurred in late December/early Jan-uary. Simulated soil moisture closely matched mea-surements by time-domain reflectometry (Fig. 4). Ad-justed r2 varied from 0.522 to 0.702, and soil moisturein Hyeronima and Cordia stands was simulated betterthan in Cedrela stands (Table 1). Because the annualsoil moisture cycle was well reproduced by the simu-lations, we were confident that our simulated soil mois-ture was satisfactory. Annual wetting and drying cycleswere synchronized with the major dry season, as couldbe readily detected in most years (Fig. 5).

Simulated ET was constrained by the capacity of thesoil to supply water, ceasing when soil moisturedropped to the level specified by the residual watercontent parameter (0.300–0.335 m3/m3). The effect washardly noticeable in short cutting-cycle stands, whereaverage annual ET as a fraction of potential ET was0.98. The effect was greater in uncut stands becauseof their higher transpiration. Median ET as a fractionof potential ET was 0.97 in uncut stands, but in dryyears, such as 1995–1996 (Table 2), the fractiondropped as low as 0.86. Soil moisture limitation maynot have occurred in the field, though, because large,deep-rooted trees would have had access to waterstored in lower soil horizons (Fig. 1).

Variation in drainage due to treatments and specieswas minor, in part because of the ability of the ground-cover to compensate for differences in canopy-tree leafarea (Bigelow 2001). Mean annual drainage as a pro-portion of precipitation was 72%, with a range of 55–77% (Table 2). Most drainage occurred during theMay–December wet season, and in years when the dryseason was pronounced almost no drainage occurredin the dry season (Fig. 5). No clear El Nino–SouthernOscillation signal was observed, which is consistentwith observations on the principal river of the water-shed, the Sarapiquı (George et al. 1998).

Leaching losses

Nitrate concentrations were elevated (;0.4 mmolc/L) in all treatments when first measured (Fig. 6) dueto residual effects of site preparation and stand planting11 mo earlier. Cutting and replanting on a 1-yr cyclemaintained most mean NO3

2 concentrations between0.05 and 0.6 mmolc/L. In contrast, in stands that wereuncut or on a 4-yr cutting cycle, NO3

2 concentrationshad dropped to ,0.01 mmolc/L by late 1992, 18 moafter planting. Cutting of 4-yr-old trees in 1995 and1999 produced sharp peaks in NO3

2 concentrations,while NO3

2 concentrations in uncut stands remained


FIG. 1. Length density of fine roots (,2 mm diameter) of three tropical trees at La Selva Biological Station, Costa Rica.All values are from uncut treatments. Each datum was derived from a composite of eight cores. Means with standard errorsare from three blocks; measurements below 1.1 m are from one block only.

low, with the exception of a trend for somewhat higherNO3

2 concentrations in Cedrela stands.Largely as a result of the responsiveness of NO3

2

concentrations, cutting-cycle treatment had highlysignificant effects on annual NO3

2 leaching losses

(ANOVA, P , 0.001; Table 3). Annual NO32 leaching

from stands on a 1-yr cutting cycle (means of the threespecies) was 442 mmolc /m2, compared to 187 mmolc /m2 from stands on the 4-yr cycle and 71 mmolc /m2

from uncut stands. Variation due to species and block


FIG. 2. Leaf area (one-sided) development of the three study species under three cutting cycles. Arrows indicate cuttingand replanting of stands on 1-yr and 4-yr cutting cycles.

was less pronounced (P , 0.05 and P , 0.01, re-spectively).

Annual NO32 leaching losses viewed over time (Fig.

7) show that NO32 retention in the 4-yr cutting-cycle

stands was again comparable to that in uncut stands bythe second year after cutting (i.e., water year 1993),but that NO3

2 losses from 1-yr cutting-cycle standsremained high. Annual leaching losses from the formerstands were ,71 mmolc/m2 (10 kg/ha of N) in wateryears 1993 and 1994. Losses remained low for uncut

stands, but increased to 323–360 mmolc/m2 in 4-yr cut-ting-cycle stands when they were cut and replanted inwater year 1995. Nitrate leaching in this year was sig-nificantly greater by an average of 43% in stands on4-yr compared to 1-yr cutting cycles.

Nitrate losses from stands on a 4-yr cutting cyclevaried according to species and year after these standswere first cut in 1995, which accounts for the statisticalinteraction among year, cutting cycle, and species (Ta-ble 3). Cordia stands were the most resilient after cut-


FIG. 3. Meteorological record from 1992–2001 at La Sel-va Biological Station, Costa Rica. Values are weekly sums(net radiation [Rn]) or weekly averages of maximum dailyvalues (temperature and vapor pressure deficit [VPD]).

FIG. 4. Volume soil moisture during 1995 in upper 1 mof soil in Hyeronima stands under 1-yr cutting cycle. The lineis soil moisture modeled from water balance, and circles aresoil moisture measured by time domain reflectometry. Barsrepresent 1 SE (n 5 9).

TABLE 1. Coefficients of determination (r2) for modeled soilmoisture with respect to measured soil moisture in upper1 m of soil.

Cycle Hyeronima Cedrela Cordia

1-yr4-yrUncut

0.6870.6280.614

0.5520.5580.581

0.6520.7020.522

ting; NO32 leaching losses were indistinguishable from

those in uncut stands (Scheffe’s, a 5 0.1) one yearafter the 1995 cut, remaining so until the second cutof the 4-yr cutting-cycle stands in 1999. Cordia standsquickly resumed NO3

2 retentiveness after this secondcut (i.e., in water year 2000) also. Hyeronima standsalso quickly regained NO3

2 retentiveness after the 1995cut (i.e., NO3

2 leaching did not differ from uncut standsin water years 1996–1997), but in 1998, NO3

2 leachinglosses were significantly greater than in uncut standsand remained elevated after the second cut (Scheffe’s,a 5 0.1).

Cedrela stands showed the most idiosyncratic NO32

leaching patterns. Despite the absence of disturbancein uncut stands, NO3

2 leaching in water years 1998 and2000 was significantly greater than from uncut standsof Hyeronima and Cordia (Scheffe’s test, a 5 0.1). Instands on the 4-yr cutting cycle, effective NO3

2 reten-tion did not resume rapidly; NO3

2 leaching in 1996 wassignificantly greater than in the uncut stands. In wateryears 1997 and 1998, NO3

2 leaching losses did notdiffer between uncut and 4-yr cutting-cycle stands, butmore as a result of elevated leaching losses in the uncutstands than resumption of effective retention in the cutstands. In water year 2000, after the second cut of thestands on a 4-yr cycle, NO3

2 leaching remained high(.200 mmolc·m22·yr21) in uncut and cut stands.

Cation concentrations (Table 4) were less variablethan NO3

2 concentrations, and responses to experi-mental treatments were weaker. The main source ofvariation in cation losses was year (Table 3), probablybecause of interannual variation in drainage. For ex-ample, water year 1995 was much drier than subsequentyears (Table 2), and cation losses also were low thatyear regardless of species or cutting cycle (Fig. 8).Potassium was the only ion for which there were sig-nificant increases in leaching losses resulting from cut-ting and replanting. Mean annual K1 losses in standson a 1-yr cutting cycle were ;50 mmolc/m2 greaterthan in uncut stands, but the only increase in K1 leach-ing in the 4-yr cutting-cycle stands occurred in Cedrelastands. Annual losses of Ca21 and Mg21 followed cours-es similar to each other, though the concentration ofthe former ion was over twice as great as the latter.There was a significant interaction between species andyear (Table 3); Ca21 and Mg21 leaching were highestin Cedrela stands in the water year 1996, but by 1998Hyeronima had highest Ca21 leaching.

Ion interrelationships and soil solutioncharge balance

Cations followed a consistent pattern in their con-tribution to solution charge. Calcium made the largestcontribution to positive solution charge, followed byMg21, Na1, K1, and NH4

1 (Fig. 9). Charge from Al31

was orders of magnitude less than charge from other


TABLE 2. Precipitation (P), evapotranspiration (Et), and soil water drainage (D) from stands of three tropical tree speciesunder three cutting cycles at La Selva Biological Station, Costa Rica (values are mm/yr).

Wateryear P

Hyeronima

1-yr

Et D

4-yr

Et D

Uncut

Et D

Cedrela

1-yr

Et D

199219931994199519961997199819992000

374735884515327541974428454845324032

1069986

1063115610391008113811171018

272126213419214331623371339534232999

119012171334118710301006107111431037

260823923149210131763364346633982979

121212301341150713851324150914161354

258723793141180028143053300931382655

1026974

103011291023

985111111161043

272526363448217131783392342334252974

Note: Annual hydrological measurements begin on 1 May and end on 29 April.

FIG. 5. Monthly precipitation, drainage, and mean soil moisture (upper 1 m) from 1992 to 2001 in Hyeronima stands atLa Selva Biological Station, Costa Rica, under 1-yr cutting cycle. Drainage (hatched bars) is superimposed on precipitation(open bars).

cations and is not shown. All cations except Al31 weretightly clustered in the factor analysis, and were highlyloaded onto factor 1 (Fig. 10).

In contrast to cations, anion concentrations werehighly variable with respect to each other. Most neg-ative charge in soil solution from stands on 1- and 4-yr cutting cycles came from NO3

2, but in the uncutstands most negative charge came from HCO3

2 (Fig.9). The sum of negative charge from HCO3

2, SO422,

Cl2, and NO32 was consistently less than the sum of

positive charge, indicating that unmeasured anions,

likely organic, contributed to negative solution chargeThis anion gap has also been observed in temperateforest soils (Haines et al. 1982), where organic anionscan contribute up to half of the negative charge in soilsolution (Dijkstra et al. 2001). The HCO3

2 concentra-tions we observed are consistent with those in tropicalsoils, such as ours, with solution pH . 6.0 (Nye andGreenland 1960). Chloride and SO4

22 contributed ap-proximately equal amounts of negative charge, andcharge from PO4

22 is not shown in Fig. 9 because itwould be less than the thickness of a line.


TABLE 2. Extended.

Cedrela

4-yr

Et D

Uncut

Et D

Cordia

1-yr

Et D

4-yr

Et D

Uncut

Et D

10621011120411801005

995119711401012

272826043276211131983388333034003005

105510051202129911611119126111841090

273426113278200830383257326233602925

1087995

1065116610641022116011331064

270126153414213431393354337234072952

120311971251114710411102131811401033

259424133218215331663281320633992985

112311121155123811191072118211171054

267124973317206030833307335234232963

Factor analysis showed NO32, SO4

2, and PO42 in a

cluster, indicating that they were positively correlatedwith each other. The cluster was orthogonal to HCO3

2,suggesting that concentrations of HCO3

2 and the otheranions varied largely independently. The two groupsof anions straddled the cluster formed by the dominantcations, suggesting only weak relationships betweenconcentrations of any given cation and anion.

DISCUSSION

Nitrate retention: loss of resilience withrepeated cutting

Although elevated soil solution NO32 is a common

response to forest cutting (Vitousek and Melillo 1979),leaching losses after cutting in our study were extraor-dinarily high. Stands under 1- and 4-yr cutting cycleslost an average 374 mmolc/m2 of NO3

2 (52.3 kg/ha ofN) in the first year after cutting. Ecosystem N losswould have been even higher had these been productionsystems in which N is exported in biomass. In com-parison, NO3

2 leaching of 128 mmolc·m22·yr21 occurredafter creation of a large gap in secondary tropical forestnear our study site (Parker 1985), and a review of nu-trient dynamics after clear-cutting in temperate forestsshowed a maximum NO3

2 loss of 78 mmolc·m22·yr21,with most sites losing ,25 mmolc·m22·yr21 (Mann etal. 1988). Large, persistent NO3

2 losses in our standsare likely related to the unusual properties of Andicsoils, which accumulate high amounts of N in stablecomplexes that form between organic matter and alu-minum constituents (Shoji et al. 1993). Nitrogen insuch complexes is considered recalcitrant; for example,an average of only 3.5% of the organic N in a rangeof Japanese Andisols was readily mineralized, in con-trast to 8.2% in non-Andic soils (Shoji et al. 1993),but conceivably the disturbance of harvesting and re-planting allows some of the recalcitrant N to becomemineralized.

We expected that NO32 leaching losses in the year

after cutting would be much greater in stands on a 4-yr compared to a 1-yr cutting cycle, because there wasfar more N accumulated in the tissues of 4-yr-old trees

(Bigelow 1998, Hiremath and Ewel 2001). This ex-pectation was met in water year 1995 after the firstcutting and replanting of stands on the 4-yr cuttingcycle, yet the 43% increase in NO3

2 leaching relativeto stands on a 1-yr cutting cycle was small given themuch larger biomass of the 4-yr-old compared to 1-yr-old trees. Processes such as microbial immobilizationor anion adsorption in subsoil (Sollins et al. 1988) maybe playing a role in regulating NO3

2 retention. Cu-mulative NO3

2 losses, of course, were much greater instands on a 1-yr cutting cycle because annual cuttingand replanting kept the stands from building up theroot density and growth demand necessary for effectiveNO3

2 retention.Although loss rates from stands on a 1-yr cutting

cycle did not diminish over the nine years reportedherein, the large losses of NO3

2 (442 mmolc·m22·yr21

or 60.5 kg·ha21·yr21 of N) dictate that N availabilitymust eventually wane. Indeed, NO3

2 retention in standson both 1-yr and 4-yr cutting cycles became progres-sively less resilient to disturbance over time. Year-to-year and block-to-block variability increased notice-ably in 1-yr cutting-cycle stands in the latter years ofthe experiment (Fig. 7), and in the 4-yr cutting cycleNO3

2 loss rates did not drop as low as in previousrotations once replanted stands were re-established.This loss of resilience of NO3

2 retention after repeatedcutting and replanting cautions against simple predic-tion of N loss rates by multiplying NO3

2 loss measuredduring an early cutting cycle by the number of antic-ipated cycles.

Atmospheric inputs of N and subsoil retention ofNO3

2 can mitigate NO32 leaching losses. A study of

deposition using a sampler located ,1 km from ourstudy site showed atmospheric N inputs ,36 mmol/yr(Eklund et al. 1997), a modest amount equivalent toroughly half the N leached as NO3

2 from our uncutstands. Therefore, atmospheric inputs cannot be con-sidered a major factor offsetting the much larger NO3

2

losses from the stands on 1-yr and 4-yr cutting cycles.The role of anion retention in deep (.1 m) soil layersis less clear; NO3

2 retention in the subsoil has been


FIG. 6. Nitrate concentrations in soil water draining from tropical tree stands under three cutting cycles. Means andstandard errors (shown only once per year for clarity) are presented from three blocks. Arrows indicate cutting and replantingof stands.

documented in other tropical sites (Matson et al. 1987,Cahn et al. 1993, Schroth et al. 1999), but has not beenobserved locally (Sollins et al. 1994). Given the ra-pidity with which drainage occurs at our site (Fig. 5),we believe it is unlikely that much subsoil retentionhas occurred, but cannot discount this possibility. Inany event, under the 1-yr cutting cycle, roots do notgrow deeper than 1 m in the soil (Haggar and Ewel1994), and NO3

2 flushed past this point, whether re-tained in the subsoil or not, is inaccessible and shouldbe considered a permanent loss of the site’s nutrientcapital.

Besides the overwhelming effects of cutting-cyclelength on NO3

2 leaching, there were secondary effectsof species. These effects showed up clearly in the uncutstands, where low soil water NO3

2 concentrations(;0.01 mmolc/L) after establishment were comparableto those in established plantations and undisturbed for-ests in temperate and tropical regions (Sollins andMcCorison 1981, Vitousek and Matson 1984, Van Mie-groet et al. 1992, Brouwer 1996, MacDowell et al.1996). Consistently higher NO3

2 concentrations inCedrela stands beginning in 1997 (Fig. 6) resulted insignificantly elevated NO3

2 leaching losses in 1998 and


FIG. 7. Annual leaching (means and standard errors of three blocks) of NO32 from tropical tree plantations under three

cutting cycles. Water year was measured from 1 May to 30 April.

2000, though earlier analyses did not detect differencesin leaching among established stands of the three spe-cies (Bigelow 1998). It is unusual to find differencesin NO3

2 leaching among established stands of plantsof the same life-form, none of them with known nitro-gen-fixing symbionts. Leaching from Cedrela stands isrelated to seasonal leafing phenology: Elevated NO3

2

in drainage water occurs most commonly in the earlymonths of the year when trees have shed their leaves(Fig. 6).

Controls on cation losses

Concentrations of K1 and Mg21 in soil solution werecomparable to those from other tropical sites (Russelland Ewel 1985, Brouwer 1996, Poels 1987), but Ca21

concentrations were extraordinarily high: Mean Ca21

concentration was 0.55 mmolc/L, in comparison to amaximum of 0.15 mmolc/L reported in the other stud-ies. The combined concentration of all cations was ;1mmolc/L (Fig. 9), more than twice the highest valuefrom 20 temperate forests (Van Miegroet et al. 1992)and three times higher than the maximum value re-ported on more acid soils at La Selva Biological Station(Johnson et al. 1977). Despite such high solute loadsin draining water, cation loss rates were less than max-imum rates from undisturbed tropical forests. A review

of hydrologic cation outputs of seven undisturbed trop-ical forests showed annual Ca12 losses of 49, 101, 124,430, 813, 2908, and 3247 mmolc/m2; Mg12 losses of41, 85, 140, 355, 362, 420, and 642 mmolc/m2; and K1

losses of 9, 24, 31, 38, 55, 125, and 194 mmolc/m2

(Bruijnzeel 1990). In our study, leaching loss rates av-eraged across all species, treatments, and years were1580 mmolc·m22·yr 21 of Ca21, 592 mmolc·m22·yr21 ofMg21, and 179 mmolc·m22·yr 21 of K1. High Ca21 leach-ing rates are clearly related to the high values of ex-changeable Ca21 at our study site (Haggar and Ewel1994), but the origin of the Ca remains unknown be-cause there is no obvious source of unweathered, Ca-rich rock at the site.

Hydrology exerted more control than any other fac-tor on cation leaching rates, as shown by the effect ofyear on cation leaching (P , 0.001; Table 3). Still, theeffects of experimental treatments on drainage rateswere not large enough to affect leaching of Ca21 andMg21. These weak effects were unexpected given thatdrainage can increase dramatically after conversion oflowland tropical rainforest to other uses (Parker 1985,Abdul Rahim 1988, Malmer 1996). For example, drain-age at 1-m depth increased by 49% in first year afterconversion of lowland Amazonian rainforest to plan-tation forest (Klinge et al. 2001). In our experiment,


TABLE 3. Analyses of variance (split-plot/repeated-measures design) on annual leaching losses of nitrate (nine years; log-transformed) and base cations (four years) in drainage water at La Selva Biological Station, Costa Rica.

Source

NO32

df MS ES F df†

Ca21

MS ES F

BlockSpeciesSpecies 3 blockCycleSpecies 3 cycleSub-plot error

2, 42, 44, 122, 124, 1212

7.1825.7410.394

102.51.7960.631

0.3940.3940.6310.6310.631

18.2**14.5*

0.65162***

2.84

2, 42, 44, 122, 124, 1212

117 978574 219794 170

66 047131 378155 754

794 170794 170155 754155 754155 754

1.280.725.09*0.420.84

YearYear 3 blockYear 3 speciesYear 3 species 3 blockYear 3 cycleYear 3 species 3 cycleSub-sub-plot error

8, 3216, 3216, 3232, 9616, 9632, 96

96

7.2920.3650.9830.4423.9370.7480.298

0.4420.4420.4420.2980.2980.298

16.4**0.822.22*1.48

13.2**2.51*

3, 126, 126, 12

12, 366, 36

12, 36

2 186 80497 316

319 49155 311

125 90046 06923 288

55 31155 31155 31123 28823 28823 288

39.5**1.755.77**2.37*5.40*1.97

Note: MS is between-treatment mean square, ES is error mean square, and F is the quotient of MS and ES; df are numeratorand denominator degrees of freedom, respectively. Boldfaced F values are significant at the 0.05 level, and asterisks denotelevel of significance: * P , 0.05; ** P , 0.01; *** P , 0.001.

† These degrees of freedom refer to Ca, Mg, and K.

TABLE 4. Mean (61 SE) annual concentrations (mmolc /L) of base cations in drainage water.

Cation andcycle Species 1995–1996 1996–1997 1997–1998 1998–1999

Ca21

1-yr H. a.C. o.C. a.

0.512 6 0.0700.660 6 0.0600.514 6 0.045

0.574 6 0.0900.683 6 0.0080.546 6 0.073

0.516 6 0.0470.423 6 0.0630.435 6 0.017

0.607 6 0.0970.478 6 0.0550.359 6 0.021


0.581 6 0.1160.668 6 0.1330.588 6 0.060

0.467 6 0.0310.666 6 0.0860.492 6 0.099

0.488 6 0.0940.573 6 0.0900.466 6 0.078

0.594 6 0.0820.529 6 0.0740.528 6 0.057

Uncut H. a.C. o.C. a.

0.585 6 0.1210.559 6 0.0330.453 6 0.037

0.675 6 0.0970.570 6 0.0390.495 6 0.036

0.526 6 0.0840.503 6 0.0320.413 6 0.070

0.749 6 0.1190.480 6 0.0400.561 6 0.011

K1


0.075 6 0.0180.084 6 0.0050.080 6 0.023

0.076 6 0.0160.083 6 0.0060.075 6 0.021

0.060 6 0.0060.054 6 0.0130.062 6 0.021

0.060 6 0.0130.066 6 0.0030.071 6 0.013


0.046 6 0.0170.085 6 0.0180.063 6 0.021

0.044 6 0.0090.085 6 0.0140.052 6 0.015

0.032 6 0.0060.072 6 0.0140.046 6 0.010

0.037 6 0.0080.073 6 0.0130.057 6 0.018


0.043 6 0.0170.067 6 0.0100.052 6 0.009

0.056 6 0.0170.061 6 0.0110.054 6 0.005

0.027 6 0.0050.052 6 0.0130.036 6 0.006

0.055 6 0.0190.061 6 0.0080.078 6 0.027

Mg21


0.198 6 0.0200.248 6 0.0120.187 6 0.018

0.195 6 0.0180.249 6 0.0220.180 6 0.017

0.203 6 0.0080.144 6 0.0180.145 6 0.012

0.218 6 0.0230.185 6 0.0110.124 6 0.005


0.233 6 0.0420.290 6 0.0990.244 6 0.055

0.181 6 0.0150.262 6 0.0630.188 6 0.052

0.189 6 0.0320.233 6 0.0720.177 6 0.048

0.248 6 0.0410.222 6 0.0560.221 6 0.063


0.205 6 0.0440.221 6 0.0310.173 6 0.009

0.229 6 0.0410.216 6 0.0320.182 6 0.013

0.176 6 0.0310.179 6 0.0430.138 6 0.028

0.275 6 0.0530.189 6 0.0300.226 6 0.008

Notes: The period 1995–1999 encompasses the second cycle of stands on a 4-yr cuttingcycle. Abbreviations are: H. a., Hyeronima alchorneoides; C. o., Cedrela odorata; and C. a.,Cordia alliodora.

drainage from frequently cut stands was usually ,10%,and at most 20%, greater than drainage from uncutstands. There are several explanations for the modestimpact of our experimental treatments on drainage.

First, even by tropical standards, the ;4 m of rain thatfalls annually at La Selva is large relative to potentialevapotranspiration, so fluctuations in ET have limitedeffect on water budgets. Second, favorable soil physical


TABLE 3. Extended.

K1

MS ES F

Mg21

MS ES F

44 08328 41224 88333 234

81407504

24 88324 883

750475047504

1.771.143.31*4.42*1.08

147 861111 962224 209

98 59519 72994 329

224 209224 209

94 32994 32994 329

0.650.492.371.040.20

29 25011732725

8852712

5901639

885885885

163916391639

33.0**1.323.07*0.541.650.36

284 45716 67049 314

932924 146

89335257

932693269326525752575257

30.5**1.785.28**1.774.59**1.69

FIG. 8. Annual leaching of Ca21, K1, and Mg21 (means and standard errors of three blocks) from tropical tree plantationsunder three cutting cycles. Water year was measured from 1 May to 30 April.

characteristics mean that water is usually available inthe upper soil layers and can be accessed even by shal-lowly rooted vegetation, tending to even out transpi-ration between tall and short vegetation. Additionally,underestimation of transpiration in uncut stands in drieryears due to a modeling constraint on extraction ofwater from deep in the soil profile would have slightlylimited our ability to detect true differences in drainageamong treatments. The cumulative effect of these fac-

tors was to limit the measurable impacts of our ex-perimental treatments on drainage and associated nu-trient leaching.

Hydrological effects aside, the modest impact of ourexperimental manipulations of vegetation on cationleaching was surprising on two counts. First, NO3

2 pro-duction is an important cause of cation leaching in arange of systems (Nye and Greenland 1960, Raney1960, Likens et al. 1969), because hydrogen ions pro-


FIG. 9. Charge balance of ionic species in drainage water collected December 1999. Tree species are indicated by firstinitials: H. a., Hyeronima alchorneoides; C. o., Cedrela odorata; C. a., Cordia alliodora). Groups of three adjacent barsrepresent three blocks. Phosphate and Al31 are not represented because bar thickness would be less than the thickness of aline.

FIG. 10. Factor analysis of ionic species in drainage watercollected in December 1999. Factors 1 and 2 explain 37%and 17% of total variance, respectively.

duced during nitrification can exchange with base cat-ions on soil colloid complexes, bringing the cationsinto solution where they are vulnerable to leaching.Our cutting treatments had a strong effect on NO3

2

leaching, yet there was no concomitant increase inleaching of base cations other than K1. Second, nutrient

uptake affects leaching because a molecule once takenup from soil solution and stored in biomass is moreresistant to being leached. In highly weathered, acidsoils of the southeastern United States, differential up-take and sequestration of base cations among vegeta-tion types can be a major determinant of Ca leaching(Johnson and Todd 1990). There were large differencesin cation sequestration among our experiment treat-ments, given that after each cut in stands on 1- and 4-yr cutting cycles sequestration in living biomass of allnutrients was reduced to nil. Several factors must beinvoked to account for the negligible impact of ourcutting-cycle treatments on Ca21 and Mg21 leaching.

High solution strength moderated increases in neg-ative charge from excess NO3

2 production due to cut-ting. For example, a 20-fold increase in NO3

2, from0.01 to 0.2 mmolc/L, would only engender a 20% in-crease in solution cation charge, e.g., from ;1 to 1.2mmolc/L. Further, such increases in solution cationcharge tended to be distributed evenly among cations(indicated by clustering of cation concentrations in Fig.10), diminishing the impact on leaching of any indi-vidual cation. Concentrations of K1, the only cationwith increased leaching under the cutting treatments,were poorly correlated with NO3

2 concentrations (Fig.10), implicating a mechanism other than NO3

2 pro-duction for elevated leaching. Lack of uptake effectson cation leaching also is likely related to high cationexchange capacity (300–400 mmolc/kg) and high base


saturation (30–50%) in our soils. In such fertile soils,a cation taken up from soil solution can be immediatelyreplaced by one from the exchange complex, counter-ing any potential effect on leaching (Nye and Green-land 1960). In contrast, on the acid Ultisols of thesoutheastern USA where Ca uptake had a strong effecton leaching, cation exchange capacity in the A horizonswas 41 mmolc/kg and base saturation was 39% (John-son et al. 1985). Finally, net rates of accumulation atour site were small compared to leaching; after eightyears of growth there was an average Ca21 content of1800 mmolc/m2 in biomass of uncut stands, little morethan the 1580 mmolc/m2 of Ca21 that was leached inan average year (Fig. 8). In every case, the reason forlack of cutting-cycle effects on Ca21 and Mg21 can betraced to the tremendous native fertility of the studysite soil.

A further concern in soils with pH-dependent, var-iable charge such as ours is that nitrification will de-crease soil pH (Van Miegroet and Cole 1984), resultingin loss of negative charge density on soil colloid com-plexes and subsequent wholesale flushing of cationsinto soil solution (Sollins et al. 1988, 1994). Again,the well-buffered nature of the soil solution made itresistant to changes in pH from experimental treat-ments, and there was no evidence of rapid leaching ofcations such as would be expected with progressiveloss of cation exchange capacity.

Short-cycle cropping and sustainability

A central concern with short-cycle cropping systemsis that stands spend a large portion of each cycle be-coming established, allowing leaching of nutrients pri-or to full-site occupancy (McColl and Powers 1984,Raison and Crane 1986, Heilman 1992). In the presentstudy, NO3

2 leaching patterns suggest that biotic con-trol over nutrient losses had become largely re-estab-lished within 1.5 years of cutting and replanting stands,which is consistent with trends in re-establishment ofroot length and leaf area. Root length density (Lv) inestablished stands was lower than in many other sys-tems; mean Lv in the upper 1.0 m of soil was 0.09,0.03, and 0.05 cm/cm3 respectively for Hyeronima,Cedrela, and Cordia. In comparison, tree plantationscan have Lv of 0.1–1.0 cm/cm3 (Van Rees and Com-erford 1986, Hughes and Gandar 1993, Jama et al.1998) or $1 cm/cm3 (Roberts 1976, Heilman et al.1994) in the upper 1.0 m, although fruit orchards onfertile soils can have Lv , 0.1 cm/cm3 (Hughes et al.1995).

In areas of the wet tropics such as our study site,where heavy storms can occur daily, it is likely that anLv of 0.1 cm/cm3 in the upper 0.1 m of soil (wherenitrification is most rapid) is necessary for effectiveNO3

2 retention. We obtain this result from depletionzone theory (Nye and Tinker 1977). In a stand with Lv

of 0.1 cm/cm3, the mean distance to the midpoint be-tween neighboring roots is 1.8 cm (5 (pLv)20.5), and

in one day a molecule of NO32 can diffuse up to 1.9

cm (5 2 3 (1025 cm2·s21 3 1 day)0.5) or far enough tomeet the depletion zones formed by neighboring roots.Stands of Cordia had almost reached an Lv of 0.1 cm/cm3 in the upper 0.1 m within 12 mo of planting, Ced-rela stands approached an Lv of 0.1 cm/cm3 within 24mo of planting, and although Hyeronima had Lv $ 0.4cm/cm3 within 6 mo of planting, its roots initially werehighly clumped around individual plants (Haggar andEwel 1995). Despite this variation among species inthe time to reach adequate Lv for NO3

2 retention, thevalues are reasonably close to the 1.5-year point atwhich nutrient retention resumed.

An infrastructure of roots must be accompanied bydemand in the form of growing plants for effectiveextraction of NO3

2 from the soil, and LAI is an indi-cator of the strength of such demand. Because LAI didnot peak until ;2.5 yr after cutting and planting (Fig.2), it is likely that full demand strength did not developuntil about this time; thus, the stands’ demand for NO3

2

lagged behind the ability of root systems to take it up.Nevertheless, the re-establishment of stand-level con-trol over leaching of NO3

2 by 1.5 yr indicated thatdemand was sufficient to halt most NO3

2 leaching bythis time. Thus, stands on a 4-yr cutting cycle spendone-third to one-half of the cycle in the vulnerable pre-establishment phase. Stands on a 1-yr cutting cycle, ofcourse, were continually maintained in the pre-estab-lishment phase resulting in massive long-term NO3

2

leaching losses.Are remedies available to counter the loss of soil

nutrients under short-cycle cropping? In the context ofindustrial short-rotation forestry, it has been suggestedthat the cost of replacement of nutrients removed inharvested parts would be negligible (Wise and Pitman1981, Mackensen and Folster 2000). This perspectiveavoids the question of hydrological losses of nutrients,for which few data are available, and the present studysuggests that such losses may be larger than antici-pated. In any event, there are often impediments tofertilization of tree crops, especially for small-holders,so that self-sustaining systems are preferable (Nambiarand Brown 1997). Further, it may not be feasible torestore native soil fertility by application of inorganicfertilizer (Hiremath and Ewel 1998): An experiment isunderway at our study site to test this proposition.

The seasonal pattern of rainfall and drainage in theAtlantic lowlands of Central America suggests that har-vesting and planting activities might be timed for thedrier season, thereby confining soil disturbance to pe-riods when drainage rates are low (Fig. 5). Yet the onsetof the wet season is the traditional time for planting ofperennial crops and for good reason. Even the com-paratively mild dry season at our study site is enoughto desiccate young plants, and the pulse of nitrificationthat occurs with the onset of the wet season (Greenland1958, Hiremath and Ewel 2001) is probably importantfor establishment of young plants. Consequently, there


is limited ability to moderate leaching losses by chang-ing planting to a different season.

CONCLUSION

Andisols constitute ,1% of soils in the developingworld, but have an importance that belies their smallarea because they are among the most productive anddensely populated soil types, in some regions sup-porting densities .400 people/km2 (van Wambeke1992, Shoji et al. 1993). Our study, in tropical treeplantations on an Andisol in Costa Rica, showed thatdisturbance frequency was more important than mas-siveness of the stand structure disrupted by cutting indetermining average long-term rates of NO3

2 leaching.Under the most frequent disturbance regime, hugeleaching losses of NO3

2 were sustained over almost adecade, though there was progressive loss of resiliencein NO3

2 retention with repeated disturbance. Effects oftree species on leaching, although less drastic than dis-turbance frequency, were large enough to warrant con-sideration from the standpoint of plantation manage-ment. In contrast to many other systems, leaching ofbase cations (other than K1) due to elevated NO3

2

leaching did not prove to be a concern. Given the im-pact of disturbance frequency on nutrient leaching, de-velopment of cropping systems that minimize distur-bance frequency, possibly through incorporation of pe-rennial plants, should be a priority for these soils.

ACKNOWLEDGMENTS

Antje Weitz generously shared her soil physics expertiseand unpublished data with us. We thank V. Alvarado, M.Cifuentes, F. Chaverri, A. Hiremath, O. Paniagua, G. Hurtado,R. Gomez, and S. Villegas for collecting data and maintainingthe field experiments; M. Costantino Curtis and T. Cole fordatabase management; T. Cole for data analysis; J. Harrisonfor statistical advice; the Organization for Tropical Studies,Incorporated, and M. Keller and coworkers for logistical as-sistance and meteorological data; J. Bartos, C. Chase, E. Mo-lina, J. Perez, M. A. Sanchez, M. J. Sanchez, and D. Schmidtfor chemical analyses; H. Loescher for sharing unpublisheddata; and G. Lovett and two anonymous reviewers for com-ments that improved the manuscript. The first author thankshis dissertation committee at the University of Florida (N.Comerford, J. Putz, K. Williams, and T. Sinclair) for guid-ance, and thanks C. Canham and N. van Breemen for supportduring manuscript preparation via NSF grant DEB-9815390and the Netherlands Geosciences Foundation/Netherlands Or-ganization for Scientific Research, respectively. This paperis a contribution to the programs of the Institute for EcosystemStudies, and the Institute of Pacific Islands Forestry and theSierra Nevada Research Center of the USDA Forest Service’sPacific Southwest Research Station. Research was funded byNSF grants BSR 900089, DEB 9318403, and DEB 9975235and by the Andrew W. Mellon Foundation.

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