Forest structure and carbon exchange in a tropical forest

Collaborators: Steve Oberbauer, Florida International University; David Clark and Deborah Clark, University of Missouri-Saint Louis; Molly Cavaleri, University of Hawaii; John Bradford, USDA FS Northern Research Station, Ed Rastetter, Marine Biological Lab

Tropical rain forests (TRFs) contribute substantially to the global carbon cycle, accounting for ~40% of terrestrial net primary production, ~60% of forest biomass, and ~27% of carbon stored in forest soils. Knowledge of TRF carbon cycling is poor, and large uncertainties exist. Sizes of pools and fluxes are uncertain, and environmental controls over fluxes are very poorly known. While the few eddy covariance studies in TRFs suggest that these forests act as significant carbon sinks, this method may underestimate ecosystem respiration and overestimate a carbon sink. Further, carbon cycling in a warmer, drier climate may yield decreased net primary productivity (NPP) and increased respiration. Resolving these issues requires unbiased characterization of the structure and function of TRF canopies at the landscape scale, assessment of carbon budgets and annual fluxes using several different methods, and an understanding of the causes of interannual variability. Field sampling has shown that both canopy structure and function are very conservative and predictable, even though the biodiversity of these ecosystems is very high. Ryan is responsible for the respiration measurements, extrapolating ground-based measurements, and modeling the effects of forest structure and climate on ecosystem fluxes of carbon and water. The effects of forest structure on fluxes will apply to all forests (including subalpine), and the experience in extrapolating and modeling carbon and water fluxes will be used for similar work on subalpine forests in Colorado and Wyoming. Major findings: Wood CO2 efflux showed no evidence of seasonality over 2 years. CO2 efflux per unit wood surface area at 251 (FA) was highest for the N-fixing dominant tree species Pentaclethra macroloba, followed by other tree species, lianas, then palms. Small diameter FA increased steeply with increasing height, and large diameter FA increased with diameter. Soil phosphorus and slope had slight, but complex effects on FA. Wood CO2 efflux per unit ground area was 1.34 ± 0.36 mmol m^-2 s^-1, or 508 ± 135 gC m^-2 yr^-1. Small diameter wood, only 15% of total woody biomass, accounted for 70% of total woody tissue CO2 efflux from the forest; while lianas, only 3% of total woody biomass, contributed one-fourth of the total wood CO2 efflux (Cavaleri et al. 2006). Landscape leaf area index (LAI) was 6.00 ± 0.32 SEM, with a coefficient of variation of 37%. Trees, palms and lianas accounted for 89% of the total, and trees and lianas were 95% of the upper canopy. All vertical transects were organized into quantitatively defined strata, partially resolving the long-standing controversy over canopy stratification in TRF. Total LAI was strongly correlated with forest height up to 21 m, while the number of canopy strata increased with forest height across the full height range (Clark et al. 2008). Foliar respiration temperature response was constant within plant functional group, and foliar morphology drove much of the within-canopy variability in respiration and foliar nutrients. Foliar respiration per unit ground area was 3.5 ± 0.2 µmol CO2 m^-2 s^-1, and ecosystem respiration was 9.4 ± 0.5 µmol CO2 m^-2 s^-1 (soil=41%, foliage=37%, woody=14%, coarse woody debris=7%). When modeled with El Niño Southern Oscillation year temperatures, foliar respiration was 9% greater than when modeled with temperatures from a normal year, which is in the range of carbon sink vs. source behavior for this forest. Our ecosystem respiration estimate from component fluxes was 33% greater than nighttime net ecosystem exchange for the same forest, suggesting that studies reporting a large carbon sink for tropical rain forests based solely on eddy flux measurements may be in error (Cavaleri et al 2008). Leaf thickness is a major control over rates of photosynthesis and respiration. We found that height, not light, was the major controller of leaf thickness in the tropical rain forest. Height may exert control over leaf thickness through changing species composition or by decreased turgor pressure for cellular expansion (Cavaleri to be submitted). Steve Oberbauer’s Towers Page Steve Oberbauer’s Carbono Project Page		Tower after Sampling Molly Cavaleri
Publications Cavaleri MA, SF Oberbauer, DB Clark, DA Clark and MG Ryan. The classic sun/shade leaf model may not apply to forest canopies. To be submitted to Ecology (Fall 2008). Cavaleri MA, SF Oberbauer and MG Ryan. 2008. Foliar and ecosystem respiration in an old-growth tropical rain forest. Plant Cell & Environment 31:473-483. Clark DB, PC Olivas, SF Oberbauer, DA Clark, MG Ryan. 2008. First direct landscape-scale measurement of tropical rain forest Leaf Area Index, a key driver of global primary productivity. Ecology Letters 11:163-172. Cavaleri MA, Oberbauer SF and MG Ryan. 2006. Wood CO2 efflux in a primary tropical rain forest. Global Change Biology 12:1-17.		Paulo Olivas
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