Stem respiration in tropical forests along an elevation gradient in the Amazon and Andes

Abstract

AbstractAutotrophic respiration involves the use of fixed carbon by plants for their own metabolism, resulting in the release of carbon dioxide as a by‐product. Little is known of how autotrophic respiration components vary across environmental gradients, particularly in tropical ecosystems. Here, we present stem CO2 efflux data measured across an elevation transect spanning ca. 2800 m in the Peruvian Amazon and Andes. Forest plots from five elevations were studied: 194, 210, 1000, 1500, and 3025 m asl Stem CO2 efflux (Rs) values from each plot were extrapolated to the 1‐ha plot level. Mean Rs per unit stem surface area declined significantly with elevation, from 1.14±0.12 at 210 m elevation to 0.62±0.09 μmol C m−2 s−1 at 3025 m elevation. When adjusted for changing forest structure with elevation, this is equivalent to 6.45±1.12 Mg C ha−1 yr−1 at 210 m elevation to 2.94±0.19 Mg C ha−1 yr−1 at 3025 m elevation. We attempted to partition stem respiration into growth and maintenance respiration components for each site. Both growth and maintenance respiration rates per unit stem showed similar, moderately significant absolute declines with elevation, but the proportional decline in growth respiration rates was much greater. Stem area index (SAI) showed little trend along the transect, with declining tree stature at higher elevations being offset by an increased number of small trees. This trend in SAI is sensitive to changes in forest stature or size structure. In the context of rapid regional warming over the 21st century, such indirect, ecosystem‐level temperature responses are likely to be as important as the direct effects of temperature on maintenance respiration rates.