Drought response of water use and stem growth dynamics of trees in central Amazonia

Large areas of the central Amazon basin are characterized by a dense rainforest cover and subtle variation in topography, microclimate and edaphic conditions. This leads to pronounced differences in tree species composition with their specific functional traits. Major forest types in the Central Amazon are seasonally flooded riparian forest along blackwater rivers (igapó), mesic forest in small valleys dissecting the terra firmeplateaus (baixios), extremely nutrient-poor forest on white sands (campinas), and upland terra firme forest on plateaus.

We hypothesize that transpiration patterns and stem growth dynamics of these forest types are significantly different in their response to drought. Therefore, we investigate their reaction and adaptation to reduced soil water availability and atmospheric heat stress during extended droughts.

Information on the water status of trees can be derived from monitoring of hourly radial stem changes using high resolution dendrometers. Such changes are mainly due to two physiological processes: irreversible stem expansion due to cambial growth or reversible variations in stem size driven by call hydration or dehydration. Concurrent measurements of stem xylem sap flow, soil water content and atmospheric conditions allow to determine when and to what extent trees undergo water stress. By observing the daily amplitudes of stem contraction, it is possible to recognize whether and how fast trees recover from drought stress and resume cambial stem growth.

During the first phase of this study, we monitored fifteen trees in a terra firme forest to determine both the effects of water scarcity on the seasonality of stem increment and water uptake and the thresholds for stress caused by heat or soil water shortage. First results indicate significantly reduced transpiration in most trees during the dry season, partly due to leaf shedding, while radial stem growth showed very different dynamics.