Robin L. Chazdon

Beyond Deforestation: Restoring Forests and Ecosystem Services on Degraded Lands

Despite continued forest conversion and degradation, forest cover is increasing in countries across the globe. New forests are regenerating on former agricultural land, and forest plantations are being established for commercial and restoration purposes. Plantations and restored forests can improve ecosystem services and enhance biodiversity conservation, but will not match the composition and structure of the original forest cover. Approaches to restoring forest ecosystems depend strongly on levels of forest and soil degradation, residual vegetation, and desired restoration outcomes. Opportunities abound to combine ambitious forest restoration and regeneration goals with sustainable rural livelihoods and community participation. New forests will require adaptive management as dynamic, resilient systems that can withstand stresses of climate change, habitat fragmentation, and other anthropogenic effects.

Tropical forest recovery: legacies of human impact and natural disturbances

Land-use history interacts with natural forces to influence the severity of disturbance events and the rate and nature of recovery processes in tropical forests. Although we are far from an integrated view of forest recovery processes, some generalizations can be made. Recovery of forest structure and composition is relatively rapid following disturbances that primarily impact forest canopies, such as hurricanes. Recovery is considerably slower following disturbances that heavily impact soils as well as aboveground vegetation, such as bulldozing, heavy or long-term grazing, and severe fires, often with long-lasting effects on species composition. The landscape matrix plays a critical role in local recovery processes. Proximity of disturbed areas to remnant forest patches promotes more rapid recovery, which depends heavily on seed dispersal. Recovery of aboveground biomass is constrained by soil fertility and texture across regions as well as across soil types within a region. Restoration of soil fertility may be a prerequisite for forest recovery on sites with severely degraded soils. Despite evidence of rapid forest recovery following large-scale deforestation, many degraded areas of today’s tropics will require human assistance to recover forest structure, species composition, and species interactions typical of mature tropical forests.

PHOTOSYNTHETIC LIGHT ENVIRONMENTS IN A LOWLAND TROPICAL RAIN FOREST IN COSTA RICA

SUMMARY (1) Measurements of photosynthetic photon flux density (PPFD) in four sites within a lowland tropical rain forest were compared. The habitats investigated were a 0.5 ha clearing, a 400 m2 gap, a 200 m2 gap, and heavily shaded understorey. (2) Measurements were made during both wet and dry seasons under a variety of weather conditions. Quantum sensors were used to monitor continuously 10-min average PPFD over a 3-10 day period at each location. Daily average PPFD, total daily PPFD, and daily frequency distributions were analysed for two adjacent sensors per site on each sampling day. (3) Daily total PPFD in the understorey, 200 m2 gap, and 400 m2 gap were 1-2%, 9%, and 20-35%, respectively, of PPFD in the clearing. Daily total PPFD in the 400 m2 gap during the dry season was, on average, 2.4 times greater than in the 200 m2 gap, and 20-25 times those in the understorey. In the 200 m2 gap, daily total PPFD was nine times greater than in the understorey during the dry season. (4) In the clearing, PPFD was significantly different between seasons with 24% higher PPFD during the dry season. In the 400 m2 gap and understorey, PPFD was not significantly different between seasons. (5) The percentage of available PPFD reaching the understorey was highest on cloudy, overcast days and lowest on sunny days. No correlation was found between daily total PPFD in the 400 m2 gap centre and in the adjacent understorey measured on the same days. (6) In the clearing, a high proportion of 10-min averages were greater than 500 ,umol m-2 s-I; in the 400 m2 gap, a high proportion of 10-min averages were between 100,umol m-2 s-1 and 500 umol m-2 s-1; in the understorey over 70% of the 10-min averages were below 10,urmol m-2 s-1. The clearing exhibited the greatest diurnal variation in PPFD and the least day-to-day variation, whereas the understorey exhibited the least diurnal variation and the greatest day-to-day variation in PPFD.

Estimation of tropical forest structural characteristics using large-footprint lidar

Quantification of forest structure is important for developing a better understanding of how forest ecosystems function. Additionally, estimation of forest structural attributes, such as aboveground biomass (AGBM), is an important step in identifying the amount of carbon in terrestrial vegetation pools and is central to global carbon cycle studies. Although current remote sensing techniques recover such tropical forest structure poorly, new large-footprint lidar instruments show great promise. As part of a prelaunch validation plan for the Vegetation Canopy Lidar (VCL) mission, the Laser Vegetation Imaging Sensor (LVIS), a large-footprint airborne scanning lidar, was flown over the La Selva Biological Station, a tropical wet forest site in Costa Rica. The primary objective of this study was to test the ability of large-footprint lidar instruments to recover forest structural characteristics across a spectrum of land cover types from pasture to secondary and primary tropical forests. LVIS metrics were able to predict field-derived quadratic mean stem diameter (QMSD), basal area, and AGBM with R 2 values of up to .93, .72, and .93, respectively. These relationships were significant and nonasymptotic through the entire range of conditions sampled at the La Selva. Our results confirm the ability of large-footprint lidar instruments to estimate important structural attributes, including biomass in dense tropical forests, and when taken along with similar results from studies in temperate forests, strongly validate the VCL mission framework. D 2002 Elsevier Science Inc. All rights reserved.

SPATIAL HETEROGENEITY OF LIGHT AND WOODY SEEDLING REGENERATION IN TROPICAL WET FORESTS

Variation in forest canopy structure influences both understory light availability and its spatial distribution. Because light is a major environmental factor limiting growth and survival of many forest species, its distribution may affect stand-level regeneration patterns. We examined spatial patterning in light availability and seedling regeneration in old-growth, second-growth, and selectively logged stands of tropical moist forest in northeastern Costa Rica. Our objectives were to determine how the frequency distribution and spatial pattern of understory light “microsites” differ among tropical wet forests; whether patterns of seedling regeneration are linked to spatial patterning of light availability; and whether these relationships differ among old-growth, second-growth, and selectively logged forest stands. We used both sensor-based and hemispherical photograph-based methods to measure light availability along three 130–160 m long transects in each of eight stands (three old-growth, three second-g...

The Potential for Species Conservation in Tropical Secondary Forests

In the wake of widespread loss of old-growth forests throughout the tropics, secondary forests will likely play a growing role in the conservation of forest biodiversity. We considered a complex hierarchy of factors that interact in space and time to determine the conservation potential of tropical secondary forests. Beyond the characteristics of local forest patches, spatial and temporal landscape dynamics influence the establishment, species composition, and persistence of secondary forests. Prospects for conservation of old-growth species in secondary forests are maximized in regions where the ratio of secondary to old-growth forest area is relatively low, older secondary forests have persisted, anthropogenic disturbance after abandonment is relatively low, seed-dispersing fauna are present, and old-growth forests are close to abandoned sites. The conservation value of a secondary forest is expected to increase over time, as species arriving from remaining old-growth forest patches accumulate. Many studies are poorly replicated, which limits robust assessments of the number and abundance of old-growth species present in secondary forests. Older secondary forests are not often studied and few long-term studies are conducted in secondary forests. Available data indicate that both old-growth and second-growth forests are important to the persistence of forest species in tropical, human-modified landscapes.

Rates of change in tree communities of secondary Neotropical forests following major disturbances

Rates of change in tree communities following major disturbances are determined by a complex set of interactions between local site factors, landscape history and structure, regional species pools and species life histories. Our analysis focuses on vegetation change following abandonment of agricultural fields or pastures, as this is the most extensive form of major disturbance in Neotropical forests. We consider five tree community attributes: stem density, basal area, species density, species richness and species composition. We describe two case studies, in northeastern Costa Rica and Chiapas, Mexico, where both chronosequence and annual tree dynamics studies are being applied. These case studies show that the rates of change in tree communities often deviate from chronosequence trends. With respect to tree species composition, sites of different ages differ more than a single site followed over time through the same age range. Dynamic changes in basal area within stands, on the other hand, generally followed chronosequence trends. Basal area accumulation was more linked with tree growth rates than with net changes in tree density due to recruitment and mortality. Stem turnover rates were poor predictors of species turnover rates, particularly at longer time-intervals. Effects of the surrounding landscape on tree community dynamics within individual plots are poorly understood, but are likely to be important determinants of species accumulation rates and relative abundance patterns.

THE IMPORTANCE OF SUNFLECKS FOR FOREST UNDERSTORY PLANTS : PHOTOSYNTHETIC MACHINERY APPEARS ADAPTED TO BRIEF, UNPREDICTABLE PERIODS OF RADIATION

n many forests with closed canopies, only a small fraction (0.55%) of the solar radiation incident above the canopy reaches the understory. Understory plants of these forests experience a highly dynamic light environment, with brief, often unpredictable periods of direct solar irradiance (sunflecks) punctuating the dim, diffuse background irradiance. The contribution of sunflecks to the daily and seasonal light environment of forest understory plants has long been appreciated (Evans 1956, Lundegarth 1921), but only within the last ten years have detailed studies focused on the extent to which photosynthesis and growth of understory plants are influenced by sunfleck activity (Chazdon 1986, Chazdon and Pearcy 1986a,b, Pearcy 1983, 1987, Pearcy et al. 1985, Pfitsch and Pearcy 1989a,b). These studies have revealed that sunflecks are a vital resource for light-limited understory plants (Chazdon 1988, Pearcy 1988, 1990).

Integrating agricultural landscapes with biodiversity conservation in the Mesoamerican hotspot.

CELIA A. HARVEY,∗‡‡‡ OLIVER KOMAR,† ROBIN CHAZDON,‡ BRUCE G. FERGUSON,§ BRYAN FINEGAN,∗∗ DANIEL M. GRIFFITH,†† MIGUEL MARTINEZ-RAMOS,‡‡ HELDA MORALES,§ RONALD NIGH,§§ LORENA SOTO-PINTO,§ MICHIEL VAN BREUGEL,∗∗∗ AND MARK WISHNIE††† ∗Department of Agriculture and Agroforestry, CATIE, Apdo 7170, Turrialba, Costa Rica †Programa de Ciencias para la Conservacion, SalvaNATURA, Colonia Flor Blanca, 33 Avenida Sur #640, San Salvador, El Salvador ‡Department of Ecology and Evolutionary Biology, University of Connecticut, Storrs, CT 06268-3043, U.S.A. §Departamento de Agroecoloǵia, El Colegio de la Frontera Sur, Carretera Panamericana y Periferico Sur s-n, San Cristobal de Las Casas, Chiapas, Mexico ∗∗Department of Natural Resources and Environment, CATIE, Apdo 7170, Turrialba, Costa Rica ††Biodiversity of BOSAWAS Biosphere Reserve, Saint Louis Zoo, Managua, Nicaragua ‡‡Centro de Investigaciones en Ecosistemas, UNAM, AP 27-3 Santa Maŕia de Guido, CP 58089, Morelia, Michoacan, Mexico §§Centro de Investigaciones y Estudios Superiores en Antropoloǵia Social, San Cristobal de las Casas, Chiapas, Mexico ∗∗∗Centre for Ecosystem Studies, Wageningen University, P.O. Box 47, 6700 AA Wageningen, The Netherlands †††Equator Environmental, LLC, 250 Park Avenue South, New York, NY 10003, U.S.A.

Photosynthetic Responses of Tropical Forest Plants to Contrasting Light Environments

Across the complex matrix of microsites that compose tropical forests, light availability varies more dramatically than any other single plant resource. On a sunny day, instantaneous measurements of photosynthetically active radiation range over 3 orders of magnitude, from less than 10 µmol m-2 s-1 in closed-canopy understory of mature forests to well over 1000 µmol m-2 s-1 in exposed microsites of gaps and large clearings, or at the top of the forest canopy (Chazdon & Fetcher, 1984b; Figure 1.1). Among the environmental factors that influence plant growth and survival in tropical forests, light availability is likely to be the resource most frequently limiting growth, survival, and reproduction (Chazdon, 1988; Fetcher, Oberbauer & Chazdon, 1994). Photosynthetic utilization of light is therefore a major component of the regeneration responses of forest species within the larger context of forest dynamics and succession.