Response of Stem Respiration of Two Tropical Species to an Imposed Drought

Abstract:

Increased instances of drought are predicted for tropical forests; therefore, it is important to better understand how drought will affect individual aspects of the forest carbon cycle. Through photosynthesis, CO₂ is assimilated into sugars, a dominant portion of which goes to the stems where it is used for growth and cell maintenance. Both processes produce CO₂ through respiration, which leaves the stem through the bark. This investigation focused on how stem CO₂ efflux differs between two tree species in the tropical rainforest biome of Biosphere 2 in Oracle, Arizona—a species of legume (Clitoria racemosa) and a species of non-legume (Phytolacca dioica).

A flexible chamber was strapped to each tree and the CO₂ that diffused across the bark was measured with a LI-7000. A 4-week long drought was imposed in an effort to simulate future conditions resulting from climate change. It was found that C. racemosa had an overall higher CO2 efflux than P. dioica. C. racemosa has thinner bark than P. dioica, which displays a secondary thickening of its stem as a result of successive cambia; therefore, CO₂ could more easily diffuse from the stems of C. racemosa. The results also indicate that decreased soil moisture, as a result of the drought, leads to a significantly lower CO₂ efflux from C. racemosa whereas no significant change was observed in P. dioica. This suggests that C. racemosa is more sensitive to water stress than P. dioica, which may have greater water storage capabilities due to its successive cambia. The differing reactions of C. racemosa and P. dioica to decreased soil moisture could be important for calculating carbon stocks and modeling the response of tropical trees to drought.