snow depth, soil frost and nutrient loss in a northern hardwood forest

Snow depth, soil frost and nutrient loss in a northernhardwood forest

Peter M. Groffman,1* Janet P. Hardy,2 Scott Nolan,3 Ross D. Fitzhugh,4

Charles T. Driscoll,4 and Timothy J. Fahey51Institute of Ecosystem Studies, Box AB, Millbrook, NY 12545, USA

2US Army, Cold Regions Research and Engineering Laboratory, Hanover, NH 03755, USA3Institute of Ecosystem Studies, Hubbard Brook Experimental Forest, Campton, NH 03223, USA

4Syracuse University, Department of Civil and Environmental Engineering, Syracuse University, Syracuse, NY 13244, USA5Cornell University, Department of Natural Resources, Ithaca, NY 14853, USA

Abstract:We have initiated a long-term experiment to examine the consequences of decreases in snowpack accumulationat the Hubbard Brook Experimental Forest (HBEF), a northern hardwood dominated forest located in the

White Mountains of New Hampshire. We are quantifying the e�ects of decreases in snowpack accumulation onroot dynamics of two key tree species in this forest (sugar maple, yellow birch), microbial biomass and activity,NOÿ3 and cation loss, the acid-base chemistry of drainage water, and soil±atmosphere trace gas ¯uxes. We are

calibrating an existing model (SNTHERM) to depict snow depth and soil frost dynamics given past or futureclimate scenarios for our site. In this paper, we describe the methods we are using for the manipulation studiesthat began in the winter of 1997/1998 and present preliminary results from our ®rst full year of treatment.Results from our methods development e�orts show that it is possible to keep plots snow free by shovelling

without disturbing the forest ¯oor. Preliminary test plot work showed that the SNTHERMmodel is capable ofdepicting snow depth and soil temperatures in both control and manipulated plots at our site. Results from our®rst full year of treatment showed that a relatively mild freezing event induced signi®cant increases in nitrogen

(N) mineralization and nitri®cation rates, solute leaching and soil nitrous oxide production and causedsigni®cant decreases in soil methane uptake. These results suggest that soil freezing events may be majorregulators of soil biogeochemical processes and solute delivery to streams in forested watersheds. Copyright# 1999 John Wiley & Sons, Ltd.

KEY WORDS snow; soil freezing; forests; soil nitrogen

INTRODUCTION

While much global change research has focused on the direct e�ects of long term changes in climate on thestructure and function of ecosystems (Ojima et al., 1991), there is widespread recognition that the mostdramatic consequences of climate change may occur due to indirect e�ects. Examples of these indirect e�ectsinclude changes in ®re frequency, invasion by exotic species, changes in hydroperiod, extreme temperatureand precipitation events, and changes in snowpack accumulation (Fajer et al., 1989; Smith and Shugart,1993; Vitousek, 1994; Schimel, 1995; Hornung and Reynolds, 1995; Su�ing, 1995). Characterizing and

CCC 0885±6087/99/142275±12$17�50 Received 1 June 1998Copyright # 1999 John Wiley & Sons, Ltd. Revised 10 November 1998

Accepted 18 March 1999

HYDROLOGICAL PROCESSESHydrol. Process. 13, 2275±2286 (1999)

*Correspondence to: P. M. Gro�man, Institute of Ecosystem Studies, Box AB, Millbrook, NY 12545, USA. Email: gro�[email protected]

Contract Grant sponsor: NSF/DOE/USDA/NASA Joint Program on TECO.Contract Grant number: DEB-9652678.

quantifying indirect e�ects represents a great challenge to global change research because these responses areecosystem-speci®c and di�cult to predict.

In this paper, we describe an experiment that we have initiated to investigate the consequences of decreasesin snow cover on biotic functions and biogeochemical processes in the northern hardwood forest. Previousresearch (Likens et al., 1977; Edwards and Cresser, 1992; Pilon et al., 1994; Boutin and Robitaille, 1994;Hardy and Albert, 1995) has shown that snow depth is a key regulator of soil temperature, root andmicrobial dynamics, soil nutrient loss, drainage water acidi®cation and soil-atmosphere trace gas ¯uxes innorthern forests. Several studies have documented ecosystem and regional-scale e�ects of snowpackvariation on forest productivity, nutrient loss and other processes (Lewis and Grant, 1980; Auclair et al.,1992; Melloh and Crill, 1995; Mitchell et al., 1996; Goulden et al., 1996; Hobbie and Chapin, 1996; Williamset al., 1996; Brooks et al., 1997; 1998). We suggest that one of the most signi®cant consequences of globalclimate change on northern forests will be a reduction in snow cover. This disturbance will lead to increasesin soil freezing, nutrient loss and drainage water acidi®cation, and changes in trace gas ¯uxes and forestcomposition.

A lack of snow cover results in colder soil temperatures, more extensive soil freezing, and an increase infreeze/thaw cycles (Edwards and Cresser, 1992). Previous studies have suggested that these stresses result inroot and microbial mortality, releasing labile organic carbon (C) and nitrogen (N) to soil (via root andmicrobial death) and increasing soil moisture and available N (via reduced uptake by trees and microbes)(Sakai and Larcher, 1987; Pilon et al., , 1994; Boutin and Robitaille, 1994). These changes lead to increasesin net mineralization and nitri®cation rates, nitrate (NOÿ3 ) and cation leaching losses and acidi®cation ofdrainage waters (Skogland et al., 1988; Christensen and Christensen, 1991; DeLuca et al., 1992; Brooks et al.,1995; 1996). Over the long-term, we believe that di�erential resistance to freezing stress will be a keyregulator of species composition in northern forests under a warmer climate condition.

We have initiated a long-term experiment to examine the consequences of decreases in snowpackaccumulation at the Hubbard Brook Experimental Forest (HBEF), a northern hardwood dominated forestlocated in the White Mountains of New Hampshire. We are quantifying the e�ects of decreases in snowpackaccumulation on root dynamics of two key tree species in this forest (sugar maple, yellow birch), microbialbiomass and activity, NOÿ3 and cation loss, the acid-base chemistry of drainage water, and soil±atmospheretrace gas ¯uxes. In addition to experimental studies, we have initiated a long-term monitoring program ofsoil freezing events that will be closely coupled to ongoing long-term measurements of vegetation, rootactivity, microbial biomass and activity and drainage water chemistry. Long-term monitoring will allow us toevaluate the importance of natural freezing events and to assess the environmental relevance of ourtreatments. The experimental and monitoring work are closely linked to modelling work. We are calibratingan existing model (SNTHERM) that depicts snow depth and soil frost dynamics given past or future climatescenarios for our site. To evaluate the long-term implications of our results, we will use the SNTHERMmodel and a long-term database on streamwater chemistry to examine the e�ects of past natural freezingevents on nutrient loss from hardwood forests at HBEF, and to forecast future events given climate changescenarios.

In this paper, we describe the methods we are using for the manipulation studies that began in the winterof 1997/1998 and present preliminary results from our ®rst full year of treatment. The objectives of the paperare to demonstrate (1) that we can manipulate snow depth and soil freezing at our site, (2) that we cancalibrate the SNTHERM model to simulate snow depth and soil temperature in both reference andmanipulated plots at our site and (3) to present preliminary results on the e�ects of soil freezing on soilbiogeochemical processes.

METHODS

The experiment is taking place at the HBEF in the White Mountain National Forest in New Hampshire,USA. Hubbard Brook has been the site of a wide range of ecological and biogeochemical studies, and is a

Copyright # 1999 John Wiley & Sons, Ltd. HYDROLOGICAL PROCESSES, VOL. 13, 2275±2286 (1999)

2276 P. M. GROFFMAN ET AL.

NSF Long Term Ecological Research (LTER) site (www.hbrook.sr.unh.edu). Vegetation at HBEF isdominated by American beech (Fagus grandi¯ora), sugar maple (Acer saccharum) and yellow birch(Betula lutea); soils are acidic (pH 3.9) Typic Haplorthods.

Pilot study for methods testing and model calibration

We established a `pilot' site to develop methods for snow removal and to calibrate the SNTHERMmodel,near the HBEF LTER laboratories in the fall of 1996. We identi®ed two 10� 10 m plots, one designated asthe control plot, the other as the freeze plot. The site consists of mature, mixed deciduous trees. Prior to snowfall, we installed a vertical array of Betatherm thermistors to measure soil temperatures at 0.1, 0.2 and 0.3 mdepths. A Campbell Scienti®c CR10 datalogger measured soil temperatures every minute and stored themas one-half hour averages. The data were downloaded weekly to a personal computer. We measuredaccumulated snow depths on a weekly basis.

From mid November 1996 until late February 1997 the designated `freeze' plot was kept snow free tosimulate the e�ect of a reduced snow fall on soil freezing. Snowpack did not develop until mid-January of1997. The control plot accumulated snow at natural rates. As soon as practical after each snowfall, shovelswere used to clear the freeze plot of the new snow. We allowed a few centimetres of snow to compact on theground to protect plot installations and the forest ¯oor from shovel damage and to increase the albedo of theforest ¯oor to aid in soil freezing.

Snow and soil freezing model

SNTHERM is an internationally recognized, one-dimensional mass and energy balance model developedprimarily for predicting snowpack properties and processes (Jordan, 1991). However, the model is capable ofpredicting soil properties, and as part of this study, the soil component will undergo further testing andvalidation. The model simulates the average energy ¯uxes, snow depth, temperature and density, andproduces estimates of soil temperatures and moisture. SNTHERM was developed and validated for opensnow ®elds and was ®rst applied to forested environments by Hardy et al. (1997, 1998). They used physically-based modi®cations to improve the ability of SNTHERM to predict energy exchanges and snowpackproperties and processes in conifer and deciduous forests.

We ran SNTHERM using meteorological data from the HBEF from mid-December 1996 until mid-April1997. When modelling the snow depth and soil freezing in the freeze plot, we removed all precipitation in theform of snow (determined when Tair5 0 8C) from the meterological input ®le, until 5 February 1997. Datafrom rain events were not removed. We initialized and validated the model runs with soil temperatures frommeasurements at depths of 0.1, 0.2 and 0.3 m. Soil properties (mineral density, bulk soil density, heatcapacity, thermal conductivity) were estimated based on Oke (1987) and considered identical between thereference and freeze plots. The plots were snow-free at the start of the model runs. Since the meterologicalparameters were measured in a large, open area we made adjustments to the incoming solar radiation, windspeed and precipitation to account for reduced energy ¯uxes on the forest ¯oor. We based the adjustments onmeasurements of these ¯uxes in a boreal aspen forest (Hardy et al., 1998). The incoming, sub canopy solarradiation was reduced to 33% of that measured at the open site, while sub-canopy wind speeds wereestimated based on the following equation:

wsfor � max ��wsabove0�27� ÿ 0�24; 0� �1�

where wsfor is the subcanopy wind speed (m sÿ1) and ws above is the wind speed (m sÿ1) above the canopy.Precipitation beneath the canopy was reduced 5% from that measured in the open. Additionally, weestimated the long-wave radiation component based on a general equation for integrated atmospheric


SNOW HYDROLOGY 37: SNOW DEPTH, SOIL FROST AND NUTRIENT LOSS 2277

emissivity in a forest and the Stefen-Boltzmann equation. The e�ect of forest canopy and cloud cover on theatmospheric emissivity is modelled, after Dingman (1994), as

eat � �1 ÿ F��0�53 � 0�65 ea0�5��1 � 0�4C� � F �2�

where, eat is the integrated atmospheric emissivity, F is the canopy closure and C is the fraction of sky coveredwith clouds.

Brief description of the main study plots

Our experimental design consists of eight 10� 10 m plots. The plots are deployed within four stands, twoof each species (80% dominance by the target species), with one snow reduction ( freeze) and one referenceplot in each stand. We are using maple and birch because of di�erences in cold hardiness between thesespecies, i.e. the elevation range of yellow birch normally exceeds that of sugar maple, and birch is a commonassociate of red spruce and balsam ®r in the lower subalpine zone in the northeastern US.

In fall and winter of 1996, we cleared minor amounts of understory vegetation from all plots (to facilitateshovelling), and installed soil solution samplers (zero tension lysimeters, Driscoll et al., , 1988), thermistorsfor soil temperature monitoring, water content re¯ectometers ( for measuring soil moisture), soil atmospheresampling probes, minirhizotron access tubes (Hendrick and Pregitzer, 1992) and trace gas ¯ux measurementchambers (Holland et al., in press). All plots are equipped with dataloggers to allow for continuousmonitoring of soil moisture and temperature. Our objective was to have all instruments installed in fall andwinter of 1996 to allow for any installation-related disturbance e�ects to subside before the treatment wasintroduced in fall/winter of 1997.

During summer and fall of 1997, we made a series of measurements to evaluate if disturbance e�ects weresubsiding and to assemble `pre-treatment' data on the plots. Lysimeters were sampled, trace gas ¯uxes weremeasured and video images of roots were collected in minirhizotron tubes several times during this period.

From late November 1997 until early February 1998 the designated `freeze' plots were kept snow-free tosimulate a reduced snow fall while the `reference' plots accumulated snow at natural rates. As soon aspractical after each snowfall, shovels were used to clear the freeze plots of the new snow. We allowed a fewcentimetres of snow to compact on the ground to protect plot installations and the forest ¯oor from shoveldamage and to increase the albedo of the forest ¯oor to aid in soil freezing.

During winter of 1997/1998 and presently continuing, we are sampling lysimeters, measuring trace gas¯uxes and collecting video images of roots at weekly to monthly intervals. Betatherm thermistors measuredsoil temperatures every 0.1 m to a depth of 0.5 m and snow temperatures every 0.2 m to a height of 0.8 m. Wemeasured snow and soil temperatures every ®ve minutes and data were stored as hourly averages on aCampbell Scienti®c CR10� datalogger. Every two weeks, we made manual measurements of snow depthvariability (n � 100), snow density and snow water equivalence at two sites. Net N mineralization andnitri®cation is measured using an in situ intact core method (Robertson et al., in press) and denitri®cation ismeasured using an acetylene-based intact core method (Gro�man et al., in press) on a monthly basis.Microbial biomass C and N content are measured using the chloroform-fumigation incubation method(Jenkinson and Powlson, 1976).

RESULTS

Pilot study and model calibration

Results from the pilot study conducted during winter 1996/97 were used to calibrate the SNTHERMmodel. Air temperatures during December 1996 were relatively warm (average mean temperature �ÿ1�9 8C). Snow did not begin to accumulate until mid-January (Figure 1) and this was coincident with muchlower air temperatures (mean temperature for the period 17 January±21 January � ÿ14�2 8C). From thisperiod on, the snow-free `freeze' plot had lower and more variable soil temperatures than the reference plot



(Figure 2). Once snow began to accumulate on the freeze plot (late February±shovelling stopped in earlyFebruary), temperatures increased and became more stable, but the soil remained frozen.

Our results using SNTHERM provide information on snow depths and depth of soil freezing in both thereference and freeze plots. The model runs imply that snow began to accumulate on the reference plot on day354 and reached a maximum depth of 62.1 cm on day 85 (Figure 1). Maximum snow depth in the freeze plotwas only 35.3 cm due to the snow removal treatment applied to the plot. Snow ablation was well underwayby early April. Although snow depth validation data are not available for the pilot study, the general modelresults are consistent with qualitative observations from the site and data from a similar site in northernVermont.

Results of modelling soil temperatures using SNTHERM in the reference and freeze plot (Figure 2) showthat SNTHERM is capable of successfully predicting soil temperatures in forested environments.SNTHERM accurately predicted the depth of freezing in both plots and did reasonably well predictingthe timing of freeze and thaw in both plots. In the reference plot, the model accurately predicted a soilfreezing depth between ÿ0.1 and ÿ0.2 m. For much of the winter, SNTHERM predicted slightly higher(51.0 8C) than measured soil temperatures in the reference plot. In the freeze plot, the model accuratelypredicted freezing soil temperatures to a depth of ÿ0.3 m and extensive freezing at a depth of ÿ0.1 m. Fromapproximately day 26 to day 34, SNTHERM predicted soil temperatures up to 6 8C colder than themeasured temperatures.

Preliminary results from main treatment plots

The winter of 1997/98 had relatively mild temperatures. However, all of the shovelled plots froze to at least10 cm depth (Figure 3). Plots remained frozen even after shovelling stopped and snow began to accumulate.The yellow birch sites had lower temperatures than the sugar maple plots, especially yellow birch site #2,which is north facing, while all the other sites are south facing.

The freeze treatment produced a series of signi®cant (p5 0.05) e�ects on soil biogeochemical processes.Soil nitrate concentrations (Figure 4) were higher just after snowmelt (April) in the freeze plots than in thecontrol plots. Di�erences were most marked in the yellow birch stands that had the lowest soil temperatures.

Figure 1. Modelled snow depths in the reference and snow removal test plots from December 1996±April 1997. The snow removal plotwas kept snow-free from mid December 1996±5 February, 1997 (day 35)



Figure 2. Measured and modelled soil temperatures at depths of 10, 20 and 30 cm depths in reference and snow removal test plots frommid December±April, 1997. The snow removal plot was kept snow-free from mid December 1996±5 February, 1997 (day 35)



Figure 3. Soil temperatures at 10 cm depth in two yellow birch (YB) and two sugar maple (SM) sites with freeze and control plots inwinter 1997/98



In situNmineralization and nitri®cation rates (Figure 5) were higher just after snowmelt (April) in the freezeplots than in the control plots, but only in the yellow birch stands. Fluxes of nitrous oxide were higher anduptake of methane was lower in the freeze plots than the control plots (Figure 6). There were markedincreases in solute concentrations in drainage water sampled in zero tension lysimeters in the freeze plotsrelative to the control plots (Figure 7).

DISCUSSION

Methods development

The pilot study produced several results that have important implications for our more extensivemanipulation study. First, it is quite feasible to keep plots snow-free by shovelling, i.e. it takes about an hourfor two people to remove several centimetres of snow from a 10 m by 10 m plot.

Second, it is possible to shovel plots without disturbing the forest ¯oor. We were quite concerned thatshovelling would disturb the surface organic layer of the soil, hurting ground vegetation and altering rates ofsoil biological processes. In the pilot study, ground vegetation was abundant and grew vigorously in thesummer following shovelling. The forest ¯oor visually appeared to be unaltered. The key to avoidingdisturbance is to leave a small (e.g. 2 cm) layer of snow on the surface. This layer does not signi®cantly inhibitfreezing, preserves ground vegetation and leaves the forest ¯oor intact.

Third, snow removal induced soil freezing as anticipated. Even though air temperatures were notextremely low during either winter period, soil on the shovelled plots was frozen. Interestingly, after snowremoval ended, soil temperatures on the shovelled plot increased, but did not match temperatures on thereference plot.

Model calibration

Results of the snow depth and soil freezing model runs using SNTHERM for the pilot study suggest themodel is capable of accurately predicting soil temperatures in snow-covered and snow-free plots. Thedi�erent response of measured soil temperatures in the reference and freeze plots, during the ®rst week (day

Figure 4. Soil nitrate concentrations in the forest ¯oor of two yellow birch (YB) and two sugar maple (SM) sites with freeze and controlplots, April 1998. Values are mean with standard error



349 to 356) suggests that, despite their proximity, the two soil temperature measurement pro®les may belocated in soils with di�erent thermal characteristics. In the freeze plot, the colder temperatures predicted bySNTHERM, compared to the measured temperatures, may be related to the soil thermal properties beingestimated for the pilot study. More data on soil characteristics are available for the main experimental plotsin the sugar maple and yellow birch forests. These data will allow us to initiate SNTHERM with measuredsoil properties, rather than estimated values.

Preliminary results from main treatment plots

We were surprised at the marked response that we observed to the relatively minor freezing event that wewere able to induce with treatment of our main plots in the relatively mild winter of 1997/98. Even thoughsoil temperatures never dropped below ÿ4 8C, we observed signi®cant increases in N cycling and loss in thefreeze plots relative to the controls. These data strongly suggest that snow depth and soil freezing events areimportant regulators of N availability and solute delivery to streams in forested watersheds in thenortheastern US. These results support observations by Mitchell et al., (1996) who suggested that regionalincreases in streamwater NOÿ3 in the northeastern US during summer 1990 were caused by widespread soilfreezing in December 1989.

The signi®cant response to mild freezing that we observed was especially surprising given laboratorystudies that we conducted that suggested that we would not observe any signi®cant e�ects on N cyclingwithout very hard freezing (Nielsen et al., in preparation). In those studies, where soils were frozen atÿ3 andÿ13 8C, we did not observe any stimulation of N mineralization at ÿ3 8C, and we did not observe anystimulation of nitri®cation at either temperature. The marked response that we observed in the ®eld, wheretemperatures barely reached ÿ3 8C, suggests that plant-microbial interactions (which were eliminated in thelaboratory studies that had only soil material) are critical to the freeze response. We are currently conducting

Figure 5. In situ net N mineralization and nitri®cation rates in the forest ¯oor of two yellow birch (YB) and two sugar maple (SM) siteswith freeze and control plots, April 1998. Values are mean with standard error.



another round of laboratory studies, with sugar maple seedlings planted in reconstructed soil pro®les. Wewill expose these seedlings to the same temperatures we used in the `soil only' laboratory experiments. Weexpect to observe much more dramatic responses than we did with the soil only experiments.

In addition to our surprising biotic results, we were also surprised that soils in the freeze plots did not thawonce we stopped shovelling and snow began to accumulate. These observations suggest that one of the mostimportant e�ects of soil freezing may be to increase runo� and decrease in®ltration of snowmelt. We willconduct detailed analysis of leachate volume in lysimeters and continuous soil moisture data from watercontent re¯ectometers and will make observations of the nature and extent of soil frost in our plots (e.g.,granular versus concrete frost) to quantify this e�ect.

Ultimately we hope to be able to evaluate the e�ects of soil freezing on solute delivery to streams bycoupling the SNTHERM model to models capable of depicting the e�ects of freezing events on soil bio-geochemical processes and running the coupled models at the watershed scale using past climate data. Wewill `hindcast' past freezing events and compare model predictions of the e�ects of these events on solutedelivery to streams with the long term streamwater chemistry record at HBEF. We will also `forecast' thenature and extent of soil freezing events under alternate future climate scenarios to evaluate the importanceof soil freezing events as controllers of watershed processes in the context of `global change'.

ACKNOWLEDGMENTS

The authors are grateful for the hard work of A. Welman and J. Demers, for meteorological data providedby A. Bailey and technical assistance from R. Jordan. This research was supported by NSF Grant #DEB-9652678 from the NSF/DOE/USDA/NASA joint program on Terrestrial Ecosystems and Global Change(TECO). The support of C. Eagar and W. Martin is gratefully acknowledged. This is a contribution to theHubbard Brook Ecosystem Study.

Figure 6. Nitrous oxide and methane ¯uxes measured in in situ ®eld chambers. There were three chambers placed in freeze and controlplots in two yellow birch (YB) and two sugar maple (SM) sites. Values are mean with standard error of nine sampling dates

between October 1997 and April 1998



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snow depth, soil frost and nutrient loss in a northern hardwood forest

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