Ann E. Russell

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.

Primary productivity and ecosystem development along an elevational gradient on Mauna Loa, Hawai'i

We measured aboveground plant biomass, aboveground net primary productivity (ANPP), detritus accumulation, and nitrogen and phosphorus uptake by aboveground vegetation in six Metrosideros polymorpha stands on the windward slopes of Mauna Loa, Hawai‘i, USA. Our objective was to quantify the effects of elevation (primarily temperature) on ecosystem properties during primary succession, as a key to understanding ecosystem–climate interactions. Four study sites were on 111- to 136-yr-old lava flows at elevations of 290, 700, 1130, and 1660 m. Two additional sites on 3400-yr-old lava were at 700 and 1660 m elevations. All sites were on solid pahoehoe (smooth or ropy-textured) lava substrates with gentle relief, were free of significant human disturbance, received abundant precipitation, and had similar vegetation composition. Total aboveground biomass, soil organic matter mass, and aboveground net primary production (ANPP) were all greater in the old sites than in young sites. Differences between young and old sites in aboveground live biomass, detrital mass, and ANPP all supported the conclusion that ecosystem development proceeded relatively faster at 700 m elevation than at 1660 m. However, aboveground biomass in the old sites (81 Mg/ha at 1660 m elevation and 123 Mg/ha at 700 m) was low in comparison with other wet tropical forests. Accumulations of N and P in live biomass and detritus followed the same trends as were observed for organic matter. Rates of soil carbon accumulation over the first 3400 yr of succession averaged 2.1 g·m−2·yr−1, similar to other reported soil chronosequences. Observed rates of N accumulation ranged from 0.1 to 0.6 g·m−2·yr−1 over the first 136 yr of succession. There were no monotonic elevational trends among young sites with respect to live biomass, detritus mass, or total N or P accumulation. Foliar nitrogen concentrations in the young sites were among the lowest reported from any tropical forests and tended to decline with increasing elevation. The growth and biomass of individual plant species varied in distinctive ways along the elevational gradient. Nevertheless, among young sites there was a direct, linear relationship between total ANPP and mean annual temperature of the site, with a similar pattern in the two old sites. For each 1°C increase in mean annual temperature, total ANPP increased by 54 g·m−2·yr−1. Community-level ANPP also was directly correlated with rates of N and P uptake by the vegetation, regardless of site age or elevation.

Both nitrogen and phosphorus limit plant production on young Hawaiian lava flows

We applied fertilizers in a 23complete factorial design to determine the effects of nutrient amendments on plant growth in Hawaiian montane forests growing on two different volcanic substrates: ‘a‘ā and pāhoehoe lava. Both sites were about 140 years old and their overstories were nearly monospecific stands of Metrosideros polymorpha. Fertilizer applications included N, P, a mixture of essential macro- and micronutrients excepting P and N, and all combinations thereof in each of four blocks. Additions of nutrients other than N or P had no significant effects on measured plant-growth variables. In contrast, additions of either N or P significantly increased tree height growth, diameter increments, biomass growth, and height growth of the understory fern Dicranopteris linearis in both sites. The effect of N was greater than that of P. Greatest growth rates occurred in plots receiving both N and P, and signficant N*P interactions occurred in several cases, suggesting a synergistic effect between these two elements. Plant growth on these young, poorly weathered, basaltic lavas is colimited by N and P availability. Growth in a similar-aged stand growing on a mixture of volcanic ash and cinders is N but not P limited, indicating that the texture of the parent material influences nutrient-availability patterns during early primary succession.

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.

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 CO2 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, δ13C 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.

Analysis of factors regulating ecosystem development on Mauna Loa using the Century model

We used the Century model to evaluateenvironmental controls over ecosystem developmentduring the first 3500 y of primary succession onpahoehoe (i.e., relatively smooth, solid) lava flowsof wet, windward Mauna Loa, Hawaii. The Century modelis a generalized ecosystem model that simulatescarbon, nitrogen and phosphorus dynamics forplant-soil systems. Preliminary results indicated theneed to modify the model to include the effects ofsoil C accumulation on soil water storage anddrainage. The modified model was parameterized tosimulate observed values of aboveground productivity,biomass and soil element pools on a 3400-y-old site at700 m elevation. Testing the model parameters at 1660m elevation indicated that N inputs were lower andsoil water drainage rates were slower at the higherelevation. We applied the modified and fullyparameterized model to simulate ecosystem attributesduring primary succession at five elevations, andconducted single-factor experiments with the model toidentify the specific influences of variations intemperature, nutrient inputs, and rainfall on modeledecosystem characteristics.Simulated aboveground productivity (ANPP), net N andP mineralization, and biomass element pools allincreased through time at each elevation, and alldeclined with increasing elevation at each point intime. After 3500 y of succession none of theseattributes had reached a stable asymptote, butasymptotes were approached more quickly, andsuccession was therefore faster, at lower than athigher elevations. Simulated soil organic matter(SOM) pools increased with elevation, despite thatplant productivity declined. These results, andsimilar comparisons among rainfall regimes, suggestthat SOM pools were more sensitive to factorscontrolling decay than production rates.Within elevations and temperature regimes, nutrientavailability was the most important factor controllingsimulated rates of plant productivity, biomass, anddetritus accumulation during ecosystem development. Through time, SOM accumulations alleviated nutrientlimitations to plants, but simulated productivityremained highly dependent upon externally suppliednutrients even after 20,000 y. Rainfall had two maineffects on nutrient availability within the model: (1)it increased rates of leaching, and thus depletednutrient supplies; and (2) it exacerbated soil floodingand thereby decreased nutrient turnover rates. Highrainfall on windward Mauna Loa maintains oligotrophicconditions through time despite continuous N and Pinputs.

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.

Carbon Dynamics in the Tropics

Native tree species differed in their effects on aboveand belowground carbon stocks and fluxes in these 16-yrold experimental plantations at La Selva Biological Station, Costa Rica. Results were explained primarily by differences in growth rates, C allocation, turnover rates, and tissue chemistry. In this experiment established in an abandoned pasture, all five tree species had attained biomass amounts similar to that of nearby mature forest, whereas the abandoned pasture control remained in arrested succession. Carbon sequestration averaged 5.2 Mg∙ha-1∙yr-1 across species, close to the annual per capita fossil-fuel use in the United States of 5.3 Mg C.