Forest Restoration in Abandoned Agricultural Land: a Case Study from East Africa

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  • 1301

    Conservation Biology, Pages 13011311Volume 13, No. 6, December 1999

    Forest Restoration in Abandoned Agricultural Land: a Case Study from East Africa

    COLIN A. CHAPMAN* AND LAUREN J. CHAPMAN*

    *Department of Zoology, University of Florida, Gainesville, FL 32611, U.S.A.Wildlife Conservation Society, 2300 Southern Boulevard, Bronx, NY 10460, U.S.A.

    Abstract:

    Millions of hectares of tropical forests have been converted to agricultural land and abandoned, so itis important that we understand the process of forest recovery and comprehend how pathways are modified bydifferent types of disturbance in different geographic regions. In a 4-year case study, we quantified the patternof forest recovery following clearing and 3 years of cultivation of a moist-evergreen forest in Uganda. Long-termobservation (746 hours) of frugivore visitation to the regenerating area demonstrated that birds were frequentvisitors (5.8 birds/hour), whereas large mammalian frugivores rarely used the area. Frugivore visitation ratesfacilitated seedling recruitment that averaged 0.51 seedlings/m

    2

    from 22 tree species by the end of the study. Re-cruitment included species with large seeds, despite the fact that seed-eating rodents were almost twice as abun-dant in the regenerating area than in mature forest. By the end of the study, however, only 20 trees were

    $

    0.5 mtall, and no trees were

    $

    2 m tall. This slow recruitment reflected high seedling mortality and dominance of thearea by elephant grass (

    Pennisetum purpureum

    ) and the herb

    Acanthus pubescens

    . After 4 years, trees

    $

    0.5 mtall attained a biomass of only 8.92 kg/ha, whereas the biomass of

    P. purpureum

    and

    A. pubescens

    had reached35,500 kg/ha and 18,100 kg/ha respectively. We provide an initial assessment of two programs designed to en-hance restoration of abandoned agricultural lands: planting of cuttings to act as dispersal foci and sowing ofseeds. Our results showed that density of seedlings growing in the management plot where we sowed seeds (0.35seedlings/m

    2

    ) and in the plot where we established cuttings (0.30 seedling/m

    2

    ) was lower than in the controlplot (0.51 seedlings/m

    2

    ). This East African site was only lightly disturbed, yet tree recovery was occurring slowerthan in heavily degraded sites described from South America. The rate of recovery seemed to be strongly deter-mined by interactions between tree seedlings and

    P. purpureum

    and

    A. pubescens

    .

    Restauracin de Bosques en Tierras Agriculturales Abandonadas: Caso de Estudio en Africa Occidental

    Resumen:

    Millones de hectreas de bosque tropical han sido convertidas a tierras agriculturales y aban-donadas; por ello, es importante que entendamos el proceso de recuperacin del bosque y comprendamoscomo sus posibles vas son modificadas por diferentes tipos de perturbacin en diferentes regiones geogrfi-cas. En un caso de estudio de cuatro aos de duracin, cuatificamos los patrones de recuperacin del bosqueposterior a un clareo y tres aos de cultivo en un bosque tropical hmedo siempreverde de Uganda. Observa-ciones de largo plazo (746 horas) de visitas de frugvoros a las reas de regeneracin demonstraron que lasaves fueron visitadores frecuentes (5.8 aves/hora), mientras que mamferos frugvoros grandes utilizaron elrea en raras ocasiones. Las tasas de visita de frugvoros facilit el reclutamiento de pltulas y promedi0.51 plntulas/m

    2

    para 22 especies de rboles al final del estudio. El reclutamiento incluy especies con semi-llas grandes a pesar del hecho de que la abundancia de roedores que se alimentan de semillas fue doble en lazona en regeneracin que en el bosque maduro. Sin embrago, al final del estudio solo 20 rboles tuviron

    $

    0.5 m de altura y no hubiron rboles de

    $

    2 m de altura. Este reclutamiento lento refleja una alta mortal-idad de plntulas y la dominancia en el rea del pasto elefante (

    Pennisetum purpureum

    ) y la hierba

    Acan-thus pubescens

    . Despus de 4 aos, los rboles de

    .

    0.5 m de altura obtuvieron una biomasa de tan solo 8.42kg/ha, mientras que la biomasa de

    P. purpureum

    y

    A. pubescens

    alcanz 35,500 y 18,100 kg/ha respectiva-mente. Proveemos una evaluacin inicial de dos programas diseados para incrementar la restauracin detierras agriculturales abandonadas: siembra de esquejes que acten como dispersores y siembra de semillas.

    email cachapman@zoo.ufl.eduPaper submitted May 7, 1998; revised manuscript accepted March 3, 1999.

  • 1302

    Forest Recovery in East Africa Chapman & Chapman

    Conservation BiologyVolume 13, No. 6, December 1999

    Nuestros resultados muestran que las densidades de plntulas creciendo en los lotes manejados donde sesembraron semillas (0.35 plntulas/m

    2

    ) y en los lotes donde se establecieron esquejes (0.30 plntulas/m

    2

    )fueron ms bajas que en los lotes control (0.51 plntulas/m

    2

    ). Este sitio Africano fu perturbado solo ligera-mente, sin embargo, la recuperacin de los rboles ocurre mas despacio que en sitios altamente degradadosdescritos para Sudamrica. la tasa de recuperacin parece estar altamente degradados descritos paraSudamrica. La tasa de recuperacin parece estar altamente determinada por interacciones entre las semi-

    llas de los rboles y

    P. purpureum

    y

    A. pubescens

    .

    Introduction

    Tropical forests and the animals they support are in-creasingly threatened by accelerating rates of forest con-version and degradation (Lanly 1982; Brown & Lugo1990). Converted land is generally agriculturally unpro-ductive, biologically impoverished, and more flammablethan the forests it replaces (Uhl & Buschbacher 1985;Uhl 1987). Comparative data are generally not availableto allow generalizations about the process of successionthat follows different types of disturbance or the effec-tiveness of different forms of human intervention de-signed to facilitate recovery.

    Much of our knowledge concerning regeneration ofdamaged habitats is based on investigations of temperatesystems (Hutnik & Davis 1973; Johnson & Bradshaw1979; Cairns 1980, 1988

    a

    , 1988

    b

    ; McDonald & Stiles1983), but using this information to predict regenerationin tropical areas may be misleading (Lugo 1992). For ex-ample, most temperate areas lack major groups of seeddispersers that are important in the tropics (e.g., frugivo-rous primates and bats). Not only is there a paucity ofstudies of regeneration in the tropics, but the availablestudies are biased toward the Neotropics. Unfortunately,applying what is known from Neotropical studies toother tropical areas of the world may be inadequate. Forexample, it may be inappropriate to apply results fromthe Amazon to mid-elevation African forests. Unlike anumber of South American forests, mid-elevation Africanforests have few small-seeded colonizer genera (Chap-man et al. 1999), no leaf cutter ants (

    Atta

    ), and few treesthat rely on wind dispersal. Wind-dispersed trees consti-tute 15% of forest tree species in Brazil (Uhl 1988) butonly 2% in Uganda (C.A.C. and L.J.C., unpublished data).

    Several factors can delay or stop regeneration in de-graded areas, and the importance of these factors willprobably vary geographically. Agricultural activities canlead to topsoil erosion or nutrient exhaustion (Uhl et al.1982; Aide & Cavelier 1994); stump sprouting may belimited and seed banks depleted from repeated burningor weeding (Uhl et al. 1985; Nepstad et al. 1996); sourceplants that contribute new seeds may be distant orscarce (Guevara et al. 1986); and frugivore dispersersmay not be attracted to degraded areas (Duncan & Chap-man 1999). Even if seeds or sprouts are available in a de-

    graded area, regeneration may not proceed because themicroenvironment may be unsuitable (Brown & Lugo1994); aggressive herbaceous growth may dominate(Walker 1994); the tree community of a region may lackspecies that can take advantage of the conditions foundin large gaps (Chapman et al. 1999); seed and seedlingpredation may be extremely high (Nepstad et al. 1996);or human-initiated fires may prohibit succession fromproceeding (Uhl & Kauffman 1990). In general, data onthe importance of these factors are scarce, so examininggeographical variation of their importance is not possi-ble. Data on how different tree communities respond tolarge gap formation, however, indicate that geographi-cal variation can be dramatic. For example, Augspurger(1984) and Brokaw (1985) examined recruitment of sev-eral tree species in different conditions in Panama andfound that all the species exhibited higher growth andsurvival in the sun than in the shade. Similarly, Pompaand Bongers (1988) documented that in Mexico thegrowth of all species examined was enhanced in largeand small gaps, and in large gaps the growth was morerapid than in small gaps. Ganzhorn (1995) reported thatin Madagascar almost all overstory tree species regener-ate best in gaps larger than gaps created by the collapseof a single tree. In contrast, research in Uganda demon-strated that for some species, growth in small gaps wasnot faster than growth in the understory, and that formost species there is higher mortality for seedlingsgrown in large gaps than for those in the understory(Chapman et al. 1999).

    In a 4-year case study, we quantified the pattern of re-covery of forest following clearing and cultivation inKibale National Park, Uganda. First, we considered thepattern of seedling establishment and mortality in lightof what plant forms dominate the regenerating lands.Second, we considered the role of seed dispersers andseed predators in forest recovery by quantifying visita-tion rates of birds and other frugivores, seed rain, andchanges in rodent abundance. Third, we estimated thebiomass of establishing trees, herbs, and grasses. To fa-cilitate regional comparisons, these descriptions weremade by methods similar to those used in many otherNeotropical studies. Finally, we made an initial assess-ment of two management programs designed to en-hance restoration of abandoned agricultural lands: (1)

  • Conservation BiologyVolume 13, No. 6, December 1999

    Chapman & Chapman Forest Recovery in East Africa

    1303

    planting cuttings to act as dispersal foci and (2) sowingseeds.

    Methods

    Study Site

    Kibale National Park (766 km

    2

    ) is located in westernUganda, just east of the Ruwenzori Mountains (lat 0

    8

    13

    9

    0

    8

    41

    9

    N, long 30

    8

    19

    9

    30

    8

    32

    9

    E). Kibale is composed ofmature moist-evergreen forest, swamp, grassland, plan-tation, abandoned agricultural land, and colonizing for-est (Struhsaker 1975; Chapman & Chapman 1997 ). Ourstudy was conducted near Makerere University Biologi-cal Field Station. This area is located at an elevation of1500 m and receives an annual rainfall of 1700 mm(19841996) that is bimodal in distribution. May throughAugust and December through February tend to be drierthan other months. On average, the first rains of the year(MarchApril) are less severe than the second rains (Sep-temberNovember). Despite these bimodal trends, year-to-year variation in the magnitude, onset, and durationof wet and dry seasons is high. Mean daily minimumtemperature is 15.5

    8

    C, and mean daily maximum tem-perature is 23.7

    8

    C (19901996).We quantified the pattern of forest recovery following

    clearing and 3 years of cultivation of a moist-evergreenforest. Foresters have classified the forest that was origi-nally on this site as a

    Parinari

    forest, distinguished onphoto aspect maps by large, spreading crowns of

    Pari-nari excelsa

    (Kingston 1967; Skorupa 1988). The pres-ence of

    P. excelsa

    and the subdominants (

    Aningeria al-tissima, Olea welwitschii, Newtonia buchananii

    , and

    Chrysophyllum gorungosanum

    ) are thought to indicatea climax forest between 1370 and 1525 m (Osmaston1959). Detailed enumeration of the tree community ofthis area is provided by Chapman et al. (1997).

    We selected this area in which to quantify regenera-tion because it was one of a few areas of abandoned agri-cultural land within the northern section of the nationalpark; so it was possible to exclude fire for the durationof the project. In addition, detailed information wasavailable on its history of land use. It was a mixed-cropplot (the last crops were corn [

    Zea mays

    ] and yams[

    Dioscorea

    ]) that had been used for 3 consecutive yearsand then abandoned. As is typical for this area ofUganda, most trees were removed by the farmer, thearea was burned, remaining logs were removed, thesoil was hoed by hand, and stumps were removed. Eachyear before planting, the area was burned and hoed toremove colonizing grasses. Two plantings were madeeach year. This area was directly adjacent to a 300-haportion of relatively undisturbed natural forest known asforestry compartment K-30 (Skorupa 1988). Prior to1970, a few large stems (0.030.04 stems/ha) were re-

    moved from this area by pitsawyers, but this extremelylow level of extraction had little effect on the structureof the forest (Skorupa 1988; Struhsaker 1997).

    At least 26 rodent species have been trapped or seenin Kibale (Struhsaker 1997), and many of these speciesare seed predators (Basuta 1979). The rodents that areseed predators could potentially shape the trajectory ofthe regeneration. Because the abundance of different ro-dent species varies between habitats within Kibale andamong years (Basuta & Kasenene 1987), the mannerwith which they influence forest regeneration couldchange over time.

    Seedling Establishment and Mortality

    A 30

    3

    50 m area was selected and subdivided into 15,10

    3

    10 m subplots with corners marked with tallwooden stakes. To facilitate comparison with previousstudies, the size and shape of this area were made thesame as in prior studies in South America ( Jordan & Uhl1978; Uhl 1987; Saldarriaga et al. 1988). The location,size, and identity of any remaining trees were deter-mined. Tree seedling establishment and mortality weremonitored in the plot approximately every 3 months for44 months (September 1993April 1997). Each 10

    3

    10 msubplot was searched for seedlings, and every tree seed-ling encountered was identified and marked with an alu-minum tree tag tied with a loose string to the base of theseedling. Every month an effort was made to find as manyseedlings as possible. As the grass/herbaceous vegeta-tion became progressively more dense, it became in-creasingly difficult to locate new seedlings, so seedlingswere discovered only when they reached a larger size.Because of this bias, the seedling establishment and mor-tality rates that we report should be considered mini-mum rates.

    Dominant Cover and Aboveground Living Biomass

    We quantified changes in plot vegetation every 4 months.Four quadrats (65

    3

    65 cm) were placed systematicallyin each subplot, with each quadrat 2 m from the centerof the subplot in each of four compass bearings (north,east, south, and west). We described the dominant plantform in the quadrat, estimated the percentage of bareground, and ranked light availability at ground level on ascale of 1 to 4 (4 being full sunlight).

    To estimate aboveground living biomass of trees foreach year of the study, regression equations were bu...

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