Correction: Integrating tropical research into biology education is urgently needed.

[This corrects the article DOI: 10.1371/journal.pbio.3001674.].

Integrating tropical research into biology education is urgently needed.

Understanding tropical biology is important for solving complex problems such as climate change, biodiversity loss, and zoonotic pandemics, but biology curricula view research mostly via a temperate-zone lens. Integrating tropical research into biology education is urgently needed to tackle these issues.

Native tree species regulate nitrous oxide fluxes in tropical plantations.

Secondary and managed plantation forests comprise a rapidly increasing portion of the humid tropical forest biome, a region that, in turn, is a major source of nitrous oxide (N2O) emissions to the atmosphere. Previous work has demonstrated reduced N2O emissions in regenerating secondary stands compared to mature forests, yet the importance of species composition in regulating N2O production in young forests remains unclear. We measured N2O fluxes beneath four native tree species planted in replicated, 21-yr-old monodominant stands in the Caribbean lowlands of Costa Rica in comparison with nearby mature forest and abandoned pasture sites at two time points (wetter and drier seasons). We found that species differed eight-fold in their production of N2O, with slower growing, late-successional species (including one legume) promoting high N2O fluxes similar to mature forest, and faster growing, early successional species maintaining low N2O fluxes similar to abandoned pasture. Across all species, N2O flux was positively correlated with soil nitrate concentration in the wetter season and with soil water-filled pore space (WFPS) in the drier season. However, the strongest predictor of N2O fluxes was fine-root growth rate, which was negatively correlated with N2O emissions at both time points. We suggest that tree-specific variation in growth habits creates differences in both N demand and soil water conditions that may exert significant control on N2O fluxes from tropical forests. With the advent of REDD+ and related strategies for fostering climate mitigation via tropical forest regrowth and plantations, we note that species-specific traits as they relate to N2O fluxes may be an important consideration in estimating overall climate benefits.

Rapidly growing tropical trees mobilize remarkable amounts of nitrogen, in ways that differ surprisingly among species.

Fast-growing forests such as tropical secondary forests can accumulate large amounts of carbon (C), and thereby play an important role in the atmospheric CO(2) balance. Because nitrogen (N) cycling is inextricably linked with C cycling, the question becomes: Where does the N come from to match high rates of C accumulation? In unique experimental 16-y-old plantations established in abandoned pasture in lowland Costa Rica, we used a mass-balance approach to quantify N accumulation in vegetation, identify sources of N, and evaluate differences among tree species in N cycling. The replicated design contained four broad-leaved evergreen tree species growing under similar environmental conditions. Nitrogen uptake was rapid, reaching 409 (± 30) kg · ha(-1) · y(-1), double the rate reported from a Puerto Rican forest and greater than four times that observed at Hubbard Brook Forest (New Hampshire, USA). Nitrogen amassed in vegetation was 874 (± 176) kg · ha(-1), whereas net losses of soil N (0-100 cm) varied from 217 (±146) to 3,354 (± 915) kg · ha(-1) (P = 0.018) over 16 y. Soil C:N, δ(13)C values, and N budgets indicated that soil was the main source of biomass N. In Vochysia guatemalensis, however, N fixation contributed >60 kg · ha(-1) · y(-1). All species apparently promoted soil N turnover, such that the soil N mean residence time was 32-54 y, an order of magnitude lower than the global mean. High rates of N uptake were associated with substantial N losses in three of the species, in which an average of 1.6 g N was lost for every gram of N accumulated in biomass.

Impacts of individual tree species on carbon dynamics in a moist tropical forest environment.

In the moist tropical forest biome, which cycles carbon (C) rapidly and stores huge amounts of C, the impacts of individual species on C balances are not well known. In one of the earliest replicated experimental sites for investigating growth of native tropical trees, we examined traits of tree species in relation to their effects on forest C balances, mechanisms of influence, and consequences for C sequestration. The monodominant stands, established in abandoned pasture in 1988 at La Selva Biological Station, Costa Rica, contained five species in a complete randomized block design. Native species were: Hieronyma alchorneoides, Pentaclethra macroloba, Virola koschnyi, and Vochysia guatemalensis. The exotic species was Pinus patula. By 16 years, the lack of differences among species in some attributes suggested strong abiotic control in this environment, where conditions are very favorable for growth, These attributes included aboveground net primary productivity (ANPP), averaging 11.7 Mg C x ha(-1) x yr(-1) across species, and soil organic C (0-100 cm, 167 Mg C/ha). Other traits differed significantly, however, indicating some degree of biological control. In Vochysia plots, both aboveground biomass of 99 Mg C/ha, and belowground biomass of 20 Mg C/ha were 1.8 times that of Virola (P = 0.02 and 0.03, respectively). Differences among species in overstory biomass were not compensated by understory vegetation. Belowground NPP of 4.6 Mg C x ha(-1) yr(-1) in Hieronyma was 2.4 times that of Pinus (P < 0.01). Partitioning of NPP to belowground components in Hieronyma was more than double that of Pinus (P = 0.03). The canopy turnover rate in Hieronyma was 42% faster than that of Virola (P < 0.01). Carbon sequestration, highest in Vochysia (7.4 Mg C x ha(-1) x yr(-1), P = 0.02), averaged 5.2 Mg C x ha(-1) x yr(-1), close to the annual per capita fossil fuel use in the United States of 5.3 Mg C. Our results indicated that differences in species effects on forest C balances were related primarily to differences in growth rates, partitioning of C among biomass components, tissue turnover rates, and tissue chemistry. Inclusion of those biological attributes may be critical for robust modeling of C cycling across the moist tropical forest biome.

Nitrogen fertilizer effects on soil carbon balances in midwestern U.S. agricultural systems.

A single ecosystem dominates the Midwestern United States, occupying 26 million hectares in five states alone: the corn-soybean agroecosystem [Zea mays L.-Glycine max (L.) Merr.]. Nitrogen (N) fertilization could influence the soil carbon (C) balance in this system because the corn phase is fertilized in 97-100% of farms, at an average rate of 135 kg N x ha(-1) x yr(-1). We evaluated the impacts on two major processes that determine the soil C balance, the rates of organic-carbon (OC) inputs and decay, at four levels of N fertilization, 0, 90, 180, and 270 kg/ha, in two long-term experimental sites in Mollisols in Iowa, USA. We compared the corn-soybean system with other experimental cropping systems fertilized with N in the corn phases only: continuous corn for grain; corn-corn-oats (Avena sativa L.)-alfalfa (Medicago sativa L.; corn-oats-alfalfa-alfalfa; and continuous soybean. In all systems, we estimated long-term OC inputs and decay rates over all phases of the rotations, based on long-term yield data, harvest indices (HI), and root:shoot data. For corn, we measured these two ratios in the four N treatments in a single year in each site; for other crops we used published ratios. Total OC inputs were calculated as aboveground plus belowground net primary production (NPP) minus harvested yield. For corn, measured total OC inputs increased with N fertilization (P < 0.05, both sites). Belowground NPP, comprising only 6-22% of total corn NPP, was not significantly influenced by N fertilization. When all phases of the crop rotations were evaluated over the long term, OC decay rates increased concomitantly with OC input rates in several systems. Increases in decay rates with N fertilization apparently offset gains in carbon inputs to the soil in such a way that soil C sequestration was virtually nil in 78% of the systems studied, despite up to 48 years of N additions. The quantity of belowground OC inputs was the best predictor of long-term soil C storage. This indicates that, in these systems, in comparison with increased N-fertilizer additions, selection of crops with high belowground NPP is a more effective management practice for increasing soil C sequestration.

Fine root decay rates vary widely among lowland tropical tree species.

Prolific fine root growth coupled with small accumulations of dead fine roots indicate rapid rates of fine root production, mortality and decay in young tree plantations in lowland Costa Rica. However, published studies indicate that fine roots decay relatively slowly in tropical forests. To resolve this discrepancy, we used the intact-core technique to quantify first-year decay rates of fine roots in four single-species plantations of native tree species. We tested three hypotheses: first, that fine roots from different tree species would decay at different rates; second, that species having rapid fine root growth rates would also have rapid rates of fine root decay; and third, that differences in fine root decay among species could be explained by fine root chemistry variables previously identified as influencing decay rates. Fine roots in Virola koschnyi plantations decayed very slowly (k = 0.29 +/- 0.15 year(-1)); those of Vochysia guatemalensis decayed seven times faster (k = 2.00 +/- 0.13 year(-1)). Decay rates of the remaining two species, Hieronyma alchorneoides and Pentaclethra macroloba, were 1.36 and 1.28 year(-1), respectively. We found a positive, marginally significant correlation between fine root decay rates and the relative growth rates of live fine roots (R = 0.93, n = 4, P = 0.072). There was a highly significant negative correlation between fine root decay and fine root lignin:N (R = 0.99, P = 0.01), which supports the use of lignin:N as a decay-controlling factor within terrestrial ecosystem models. The decay rates that we observed in this single study location encompassed the entire range of fine root decay rates previously observed in moist tropical forests, and thus suggest great potential for individual tree species to alter belowground organic matter and nutrient dynamics within a biotically rich rainforest environment.

Temperature influences carbon accumulation in moist tropical forests.

Evergreen broad-leaved tropical forests can have high rates of productivity and large accumulations of carbon in plant biomass and soils. They can therefore play an important role in the global carbon cycle, influencing atmospheric CO2 concentrations if climate warms. We applied meta-analyses to published data to evaluate the apparent effects of temperature on carbon fluxes and storages in mature, moist tropical evergreen forest ecosystems. Among forests, litter production, tree growth, and belowground carbon allocation all increased significantly with site mean annual temperature (MAT); total net primary productivity (NPP) increased by an estimated 0.2-0.7 Mg C x ha(-1) x yr(-1) x degrees C(-1). Temperature had no discernible effect on the turnover rate of aboveground forest biomass, which averaged 0.014 yr(-1) among sites. Consistent with these findings, forest biomass increased with site MAT at a rate of 5-13 Mg C x ha(-1) x degrees C(-1). Despite greater productivity in warmer forests, soil organic matter accumulations decreased with site MAT, with a slope of -8 Mg C x ha(-1) x degrees C(-1), indicating that decomposition rates of soil organic matter increased with MAT faster than did rates of NPP. Turnover rates of surface litter also increased with temperature among forests. We found no detectable effect of temperature on total carbon storage among moist-tropical evergreen forests, but rather a shift in ecosystem structure, from low-biomass forests with relatively large accumulations of detritus in cooler sites, to large-biomass forests with relatively smaller detrital stocks in warmer locations. These results imply that, in a warmer climate, conservation of forest biomass will be critical to the maintenance of carbon stocks in moist tropical forests.