The research in this 5-yr project will extend the longest-running study of how tropical-forest productivity is controlled by climatic variation. In a network of 18 0.5-ha plots across a Costa Rican rain forest, each year's forest productivity (new wood, leaves, fruits and flowers) will be measured, as well as yearly changes in forest structure caused by tree death and in-growth. The measurements from the first 11 years showed that forest growth decreased dramatically in hotter and drier years. Extending the record a further 5 years will provide a globally-unique measurement series for assessing how a tropical rain forest is responding to on-going climatic and atmospheric changes.
Tropical rain forest plants continuously absorb vast amounts of carbon dioxide from the atmosphere through the process of photosynthesis, while at the same time re-emitting large amounts of carbon dioxide in respiration. A decline in tropical-forest productivity, the balance between these two processes, would increase the rate of greenhouse-gas accumulation in the atmosphere, thus speeding global warming. It is therefore critical to monitor these processes in tropical forests, but the existing data records are sparse. By extending the long-term measurements in this Costa Rican rain forest to span 16 years, this study will produce a unique basis for refining current scientific understanding of how tropical forests are responding to, and contributing to, global climate change.
The CARBONO project is a long-term effort to understand how climate controls the productivity and diversity of Tropical Rain Forests (TRF). The project pioneered the concept of studying an entire TRF landscape with replicated sample sites (Fig. 1) and of gearing measurement intervals to the natural annual and subannual rhythms of these forests. These concepts were novel when the project began in 1997, and have subsequently been adapted by numerous investigators around the world. The current paradigm for the effect of increasing atmospheric CO2 on TRF productivity is that more CO2 has led to increased productivity. These analyses have been based almost entirely on samples from sites widely separated in space and time, as well as on ecological models. In the CARBONO project, in contrast, we have intensively studied one old-growth landscape through time. A major result from this work is that this TRF is negatively affected by even small increases in temperature or decreases in rainfall (Fig. 2). While a small positive effect of increasing CO2 on wood production was finally observed (it wasn't detectable until year 10 of the study), this positive effect is insignificant in relation to the effects of temperature and rainfall (Fig. 3). These results have major implications for the future of global TRF. Global temperatures will continue to rise for the foreseeable future. Assuming that other TRF landscapes function like the one we study does, TRF productivity is expected to decrease. Some species will be driven into climates where they cannot survive, and others will migrate or increase in local density as more temperature-sensitive species recede. The final results are difficult to predict exactly, in part due to the very large number of tree species in tropical forests, and in large part due to the absence of more data such as those provided by this project, which is currently unique in the length and intensity of the data produced. But the clear implications of these results are that globally TRF productivity will decrease in an ever-warmer climate. All life depends on plant productivity, and the decrease in TRF productivity will certainly have cascading effects on animal biodiversity. In tropical ecology it is extremely difficult to maintain intensive long-term research on fundamental ecological processes. Money is always short, and novel or popular research questions attract funding more readily than sustained research on basic ecology. In the CARBONO Project we have been fortunate to maintain for 16 years an uninterrupted research program on the core elements of TRF structure, dynamics, and productivity. Because of this intensive focus on one landscape, we were able to demonstrate with statistically-powerful field data the major roles of temperature and rainfall on forest productivity, as well as the much smaller role of CO2. These results were only possible due the relatively stable funding of the National Science Foundation's program in long-term ecological research (the LTREB program). There are many useful approaches to studying TRF, but our results demonstrate the power of sustained intensive research on key ecological processes over one landscape.
