Ecology, Evolution & Behavior

A conceptual model of Douglas-fir bark beetle (Dendroctonus pseudotsugae) dynamics and associated host tree mortality across multiple spatial and temporal scales was developed, then used to guide a study of the association between the occurrence of beetle- killed trees and factors that might render trees more susceptible to attack. Long-term records of beetle kill showed that beetle epidemics were associated

Recent studies suggest that the direction and magnitude of changes in soil organic carbon (soil C) pools following forest-to-pasture conversion in the tropics are dependent upon initial soil conditions and local factors (e.g. pre-conversion soil C content, soil texture, vegetation productivity, and management practices). The goal of this study was to understand how landscape-scale variation in soil-forming factors influenced the response of soil C pools to forest clearing and pasture establishment in northeastern Costa Rica. We measured soil C and its stable isotopic composition in 24 paired pasture and reference forest sites distributed over large gradients of edaphic characteristics and slope throughout a 1400 km 2 region. We used the large difference in stable C isotopic signatures of C3 vegetation (rain forest) versus C4 vegetation (pasture grasses) as a tracer of soil C dynamics. Soil C pools to 30 cm depth ranged from 26% lower to 23% higher in pastures compared to paired forests. The presence of non-crystalline clays and percent slope explained between 27 and 37% of the variation in the direction and magnitude of the changes in soil C storage following pasture establishment. Stable carbon isotopes (d 13 C) in the top soil (0-10 cm) showed a rapid incorporation of pasture-derived C following pasture establishment, but the vegetation in these pastures never became pure C4 communities. The amount of forest-derived soil C in pasture topsoils (0-10 cm) was negatively correlated to both pasture age and the concentrations of non-crystalline iron oxides. Together these results imply that site factors such as soil mineralogy are an important control over soil C storage and turnover in this region.

The goal of this study was to examine spatial variation of soil organic C and its stable isotopic composition (δ 13 C) in 1-ha plots of mature rain forest and a cattle pasture dominated by C4 grasses in Costa Rica. Soil samples were taken from 80 mapped locations per plot and analyzed for organic C and δ 13 C. The range of values for soil C concentrations was similar between forest and pasture, although the mean values were higher in the forest. δ 13 C was narrowly constrained in the forest (from −27.96 to −26.09‰) but varied from −15.09 to −28.59‰ in the pasture. Variograms revealed spatial autocorrelation of soil C and δ 13 C in the pasture and organic C concentration in the forest soil. The large range and spatial variability of δ 13 C in the pasture site may be due to varying contributions of C3 and C4 vegetation to the soil C pool, which may limit the usefulness of the natural abundance technique as a precise tracer of soil C dynamics in this pasture.

Although there are generalized conceptual models that predict how above and belowground biomass increase during secondary succession after abandonment from agriculture, there are few data to test these models for fine roots (defined as 2 mm diameter) in tropical forests. We measured live and dead fine roots (0-10 cm depth) in 18 plots of regenerating tropical dry forest in Costa Rica that varied in age from 5 to 60 yrs, as well as in soil properties. We predicted that both stand age and soil fertility would affect fine roots, with greater values in older forests on low fertility soils. Across two sampling dates and locations, live fine roots varied from 0.35 to 3.53 Mg/ha and dead roots varied from 0.15 to 0.93 Mg/ha. Surprisingly, there was little evidence that surface fine roots varied between sampling dates or in relation to stand age. By contrast, total, live, and dead fine roots averaged across sampling dates within plots were negatively correlated with a multivariate index of soil fertility (Pearson correlations coefficients were À0.64, À0.58, and À0.68, respectively; P < 0.01) and other individual edaphic variables including pH, silt, calcium, magnesium, nitrogen, and phosphorus. These results suggest that soil fertility is a more important determinant of fine roots than forest age in tropical dry forests in Costa Rica, and that one-way these plant communities respond to low soil fertility is by increasing fine roots. Thus, simple conceptual models of forest responses to abandonment from agriculture may not be appropriate for surface fine roots.

Northeastern Costa Rica is a mosaic of primary and secondary forests, tree plantations, pastures, and cash crops. Many studies have quantified the effects of one type of land-use transition (for example, deforestation or reforestation) on soil properties such as organic carbon (C) storage, but few have compared different land-use transitions simultaneously. We can best understand the effects of land-use change on regional and global ecosystem processes by considering all of the land-use transitions that occur in a landscape. In this study, I examined the changes in total soil C and nitrogen (N) pools (to 0.3 m) that have accompanied different land-use transitions in a 140,000-ha region in northeastern Costa Rica. I paired sites that had similar topography and soils but differed in recent land-use history. The following land-use transitions were represented: 12 conversions of primary forests to banana plantations, 15 conversions of pastures to cash crops, and four conversions of pastures to Vochysia guatemalensis tree plantations. The conversion of forests to bananas decreased soil C concentrations and inventories (Mg C ha Ϫ1 ) in the surface soil by 37% and 16.5%, respectively. The conversion of pastures to cash crops reduced soil C concentrations and in-ventories to the same extent that forest-to-banana cropping did. Furthermore, young Vochysia plantations do not appear to increase soil C storage, at least over the 1st decade. When data from all land-use transitions were pooled, the difference in root biomass and leaf litter pools between land-use pairs explained 50% of the differences in soil C concentrations and 36% of the differences in soil C inventories. Thus, reduced productivity or C inputs to the soil is one mechanism that could explain the losses in soil C pools with land-use change. In this landscape, losses of soil C due to cultivation are rapid, whereas reaccumulation rates are slow. Total soil N pools (0 -10 cm) were also reduced after the conversion of forests to banana plantations or the conversion of pastures to crops, despite fertilization of the cropped soils. This suggests that the added N fertilizer is not retained but instead is exported via produce, N gas emissions, and hydrologic processes.

Much of the dry tropical forest biome has been converted to agricultural land uses over the past several centuries. However, in conservation areas such as those in the Guanacaste and Tempisque regions of Costa Rica, tropical dry forests are regenerating due to management practices including fire suppression. To better understand the patterns of secondary succession occurring in Costa Rican tropical dry forest, we established 60 20 Â 50 m plots in mature and regenerating forests in the Sector Santa Rosa (formerly known as Parque Nacional Santa Rosa) and Palo Verde National Park. Plots were stratified into three plant communities: tropical dry oak forest (Quercus oleoides) (SROAK), Santa Rosa tropical dry forest (SRTDF), and Palo Verde tropical dry forest (PVTDF). In these plots we measured and identified and all individuals >10 cm DBH, measured but did not identify stems <10 cm but taller than 1.3 m, counted woody seedlings (<1.3 m height) and analyzed soil chemical and physical properties.

Seasonally dry tropical forests (SDTFs) are globally extensive but understudied, especially from a biogeochemical perspective. Historically, much of the land covered by SDTF has been cleared for agriculture or other land uses, but forests are now regenerating in some areas. Quantifying biomass stocks in mature and regenerating SDTF is important for constructing global carbon budgets and for designing local policy and management tools designed to sequester and store carbon. We reviewed 44 published and unpublished studies that estimate aboveground biomass in SDTF around the world and examined patterns of biomass across successional stages and the climatic range in which SDTF occur. Aboveground biomass in mature SDTF ranged from 39 to 334 Mg ha À1 . A single climatic variable, mean annual precipitation (MAP), explained over 50% of the variation in aboveground biomass. Regenerating SDTFs at the wetter end of the precipitation spectrum (1500-2000 mm MAP) attained a greater biomass but did not appear to reach maximum biomass faster than SDTFs at drier sites. We used spatially explicit biomass, climate, and biome range data along with the derived precipitation-biomass relationship to estimate current and potential global SDTF biomass C stocks of 8.7 and 22.1 Pg respectively.

1. One way to simplify the high taxonomic diversity of plant species in vegetation models is to place species into groups based on shared, dominant traits. Many studies have suggested that morphological and physiological traits of tropical dry forest tree species vary with leaf habit (i.e. leaves from evergreen, deciduous or semi-deciduous species) and thus this characteristic may serve as a useful way to distinguish ecologically meaningful functional types. 2. In this study we examine whether 10 plant traits vary with leaf habit in replicated leaves and individual trees of 87 species from a tropical dry forest in Costa Rica. We also looked for evidence of phylogenetic conservatism, i.e. closely related species sharing similar trait values compared to more distantly related taxa. 3. While some of the traits varied within and among individual trees of the same species, interspecific variation accounted for 57-83% of the variance among samples. Four traits in addition to leaf habit showed evidence of phylogenetic conservatism, but these results were strongly dependent on the inclusion of the 18 species of legumes (Fabaceae) in our dataset. Contrary to our predictions, none of the traits we measured differed among leaf habits. However, five traits (wood density, leaf C, leaf N, N ⁄ P and C ⁄ N) varied significantly between legumes and other functional types. Furthermore, when all high-nitrogen non-legume taxa were compared to the high-nitrogen legumes, six traits excluding leaf N differed significantly, indicating that legumes are functionally different from other tree species beyond high N concentrations. Similarly, the 18 legume taxa (which all have compound leaves) also differed from other compound-leaved species for six traits, thus leaf type does not explain these patterns. 4. Our main conclusions are that (i) a plant functional type classification based on leaf habit alone has little utility in the tropical dry forest we studied, and (ii) legumes have a different suite of traits including high leaf carbon and wood density in addition to high leaf nitrogen. Whether this result generalizes to other tropical forests is unknown, but merits future research due to the consequences of these traits for carbon storage and ecosystem processes.

Forested soils in the tropics contain a large carbon pool that may respond to global environmental changes such as climate warming and land-use change. A better understanding of the distribution of tropical soil carbon (C) pools is necessary in order to manage soil C as well as to predict its potential responses to global change. The goals of this study were to quantify the relationships among soil C and environmental variables for 35 forest plots in a 140,000-ha landscape in northeastern Costa Rica, and to identify variables that can predict soil C storage at unsampled sites. The biophysical variables included indices of net primary productivity (forest floor mass, root biomass, and an index of productivity derived from satellite imagery), soil particle-size distribution and mineralogy, elevation, and slope. Soil carbon storage in these volcanic soils was relatively high, ranging from 51.1 to 138.6 Mg C ha À 1 in the top 30 cm of mineral soil. The relationships among forest soil C and biotic and abiotic variables were different for low-elevation (< 120 m) and high-elevation (120 -800 m) sites, and elevation explained much of the variability in soil C concentrations. Soil particle-size distribution and mineralogical variables are correlated in this landscape and co-vary in predictable ways along the elevation gradient. Thus, elevation represents a weathering gradient with younger, allophanic soils at higher elevations and older soils with gibbsite, goethite, and kaolinite as dominant clay minerals in the lowlands. We propose two mechanisms of C stabilization: soil C concentrations and contents are positively correlated to the amount of noncrystalline clays (e.g. allophane, imogolite, and ferrihydrite) in the high elevation soils, and positively correlated to aluminum in organo-metal complexes in the low elevation sites. The strong correlations among soil C concentrations, contents, and elevation (mediated through effects on soil mineralogy) indicate that it 0016-7061/02/$ -see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 6 -7 0 6 1 ( 0 2 ) 0 0 1 4 7 -7 * Corresponding Geoderma 109 165 -190 is possible to predict soil C in this landscape using variables that are easily mapped in a geographic information system. D

Understanding the natural variability in the stable carbon isotope composition of vegetation and soils can aid studies of soil carbon dynamics, and is a prerequisite for using 13 C as a natural tracer when vegetation communities change from one photosynthetic pathway to another (e.g. C4 pasture grasses replacing C3 forests). To examine the geographic and vertical patterns of d 13 C under mature rain forest vegetation, we measured the isotopic composition of leaf litter, roots, and mineral soil profiles in 35 plots located in a 140,000-ha study region in northeastern Costa Rica. We hypothesized that soil carbon isotopic composition would be closely coupled to the signature of detrital inputs (the average d 13 C of forest floor and fine-root biomass) and that the increase in d 13 C with depth would be related to abiotic variables that influence soil carbon turnover rates. The correlations between surface soil isotopic composition (0 -10 cm) and biophysical factors varied across the landscape. In multiple regression analyses, combinations of soil particle-size distribution and soil C/N ratio explained between 60% and 82% of the variation in surface soil isotopic composition for different regions in the landscape, but the isotopic composition of litter and roots did not explain any additional variation. The isotopic composition of litter, roots, and surface soils (0 -20 cm) was fairly similar over large, regional gradients of elevation and edaphic properties (the range of variation was 2.6x), but diverged considerably at depths below 20 cm (up to 5.8x differences). These large regional differences in the isotopic composition of soil C at depth are attributed to variation in soil carbon turnover rates across the landscape. D

Changes in soil carbon storage that accompany land-cover change may have significant effects on the global carbon cycle. The objective of this work was to examine how assumptions about preconversion soil C storage and the effects of land-cover change influence estimates of regional soil C storage. We applied three models of land-cover change effects to two maps of preconversion soil C in a 140 000 ha area of northeastern Costa Rica. One preconversion soil C map was generated using values assigned to tropical wet forest from the literature, the second used values obtained from extensive field sampling. The first model of land-cover change effects used values that are typically applied in global assessments, the second and third models used field data but differed in how the data were aggregated (one was based on land-cover transitions and one was based on terrain attributes). Changes in regional soil C storage were estimated for each combination of model and preconversion soil C for three time periods defined by georeferenced land-cover maps. The estimated regional soil C under forest vegetation (to 0.3 m) was higher in the map based on field data (10.03 Tg C) than in the map based on literature data (8.90 Tg C), although the range of values derived from propagating estimation errors was large (7.67-12.40 Tg C). Regional soil C storage declined through time due to forest clearing for pasture and crops. Estimated CO 2 fluxes depended more on the model of land-cover change effects than on preconversion soil C. Cumulative soil C losses under the literature model of land-cover effects exceeded estimates based on field data by factors of 3.8-8.0. In order to better constrain regional and global-scale assessments of carbon fluxes from soils in the tropics, future research should focus on methods for extrapolating regional-scale constraints on soil C dynamics to larger spatial and temporal scales.

Litter decomposition recycles nutrients and causes large fluxes of carbon dioxide into the atmosphere. It is typically assumed that climate, litter quality and decomposer communities determine litter decay rates, yet few comparative studies have examined their relative contributions in tropical forests. 2. We used a short-term litterbag experiment to quantify the effects of litter quality, placement and mesofaunal exclusion on decomposition in 23 tropical forests in 14 countries. Annual precipitation varied among sites (760-5797 mm). At each site, two standard substrates ( Raphia farinifera and Laurus nobilis ) were decomposed in fine-and coarse-mesh litterbags both above and below ground for approximately 1 year.