Effects of Drought on Carbon Sequestration of Tropical Dry Forest

Abstract:

The lack of knowledge associated with the controls on CO₂ exchange and responses to climate change of tropical dry forests (TDF) represent substantial uncertainties in global climate models. These ecosystems contain large stores of biomass, which lead to rapid cycling of carbon pools through photosynthesis and respiration. The projected increase in severe drought in a climate change environment is expected to affect the mechanisms and total amounts of carbon sequestered by TDFs. Using eddy covariance CO₂ and H₂O flux measurements in Santa Rosa National Park, Costa Rica, we investigated the relationship between phenology and productivity in normal year, followed by two sequential drought years, in terms of net and gross ecosystem exchange (NEE and GEE, respectively). Eddy covariance measurements were used to derive phenologic indicators. Inflection points displaying the transition points between CO₂ source and sink were determined using NEE flux measurements as well as the onset of phenologic stages (green-up, maturity, senescence and dormancy). Onset of green-up was significantly delayed due to drought; however, the total green-up period remained unchanged in length, compared to the normal year. Thus, the integrated carbon sequestration during green-up was significantly lower during drought. To compensate for this, in drought, the length of maturity and senescence were extended, allowing the ecosystem to reach similar carbon accumulation levels as observed during normal conditions. An increase in physiological activity seems to have also occurred. However, as drought persists in subsequent years and resources become limiting, we expect this activity to decrease, resulting in a reduction of total ecosystem carbon sequestration. The highly dynamic changes occurring in TDFs, make these ecosystems significantly important as we try to better understand the global carbon cycle in a climate change environment.