The CARBONO Project

LaSelvaEach day, forests take up a tremendous quantity of carbon by way of photosynthesis. Over time, this carbon accumulates in plant tissues, where it resides until trees are cut down or die naturally and decompose, or are burned to clear land for agriculture. In addition to sequestering carbon through photosynthesis, forests continually release carbon to the atmosphere, as trees and soil microorganisms, like all living organisms, respire carbon. The worldwide uptake of carbon by forests exceeds that released to the atmosphere, sequestering about one quarter of anthropogenic CO2 emissions annually, slowing CO2 accumulation in the atmosphere.

Estimates of carbon fluxes from forest inventories and the atmosphere suggest that tropical forests contribute substantially to this global carbon sink. Although future levels of atmospheric CO2 depend on the long-term persistence of this sink, which is a function of its underlying causal mechanisms, these mechanisms remain uncertain.

The net carbon balance of tropical forests with respect to the atmosphere can be tipped by many factors, including temperature, precipitation, solar radiation, atmospheric CO2, topography, soil type and fertility, species composition, and disturbance history. The CARBONO project aims to elucidate these relationships using a well-studied old-growth tropical rain forest at La Selva Biological Station in the Atlantic lowlands of Costa Rica as a detailed case study.

The project is funded through 2019 by a grant from the National Science Foundation and consists of a suite of core measurements within 18 plots measuring 50 × 100 m. Plots are stratified by gradients of soil fertility and topography. In each plot, all woody plants > 10 cm stem diameter at breast height or above basal irregularities such as buttresses are identified, mapped and measured annually. Several important findings concerning the influence of climate on tropical carbon fluxes have emerged from the CARBONO project: (1) declines in tree growth in relatively hot and dry years are associated with a net carbon flux to the atmosphere from terrestrial ecosystems in the tropics (Clark et al. 2003), (2) old-growth forest at La Selva was in equilibrium with its disturbance regime over 8.4 years, contrary to the hypothesis that old-growth tropical forests are uniformly accumulating carbon due to rising atmospheric CO2 or other mechanisms (Kellner et al. 2009), (3) forest sensitivity to drought depends on topography, with alluvial stands experiencing less water stress (Silva et al. 2013), (4) minimum temperature and dry season water stress explain 73% of the annual variation in net annual carbon uptake associated with aboveground wood production (Clark et al. 2013), and (5) there is a small but statistically significant positive effect of the concentration of CO2 in the atmosphere on tree growth, but it is too small to compensate for the negative effects of warming and daytime water stress.

In 2015 we are working to integrate high-frequency and high-spatial resolution observations from airborne imaging spectroscopy made using the Brown Platform for Autonomous Remote Sensing into the suite of CARBONO measurements. B-PAR completed its first flights in Costa Rica in May of 2015. Data collected by B-PAR will provide physiologically informative measurements of solar-induced chlorophyll fluorescence, which is related to instantaneous carbon uptake by photosynthesis.

Collaborators: David B. Clark and Deborah A. Clark (University of Missouri-St. Louis), Jim Kellner (Brown University)