The vulnerability of canopy soil to accelerated carbon losses under climate change

Jessica G. Murray and John M. Stark, Department of Biology, Utah State University, Logan, UT, Bonnie G. Waring, Grantham Institute, Imperial College of London, London, United Kingdom

Background/Question/Methods
Canopy soils form via the accumulation of decomposed organic matter on tree branches in wet forests. These unique soils support highly diverse epiphyte communities and play an important role in forest nutrient and water cycling. Like other Histosols that are comprised mostly of organic matter, canopy soil decomposition may be particularly sensitive to subtle changes in the environment. Climate change will likely alter patterns of temperature and precipitation in regions where canopy soil is found. These changes may influence canopy soil decomposition directly or indirectly by changing the structure of the epiphyte community and, subsequently, the chemistry of litter inputs entering the canopy soil. Utilizing static chambers and CO2 sensors to quantify canopy soil carbon dioxide (CO2) respiration in situ, we examined the response of canopy soil carbon losses to natural temperature and moisture variability as well as in response to nutrient and 13C-glucose additions. To capture a range of temperature and moisture conditions and determine whether canopy soil from different sites responds differently to changes in the biotic and abiotic environment, the study was conducted in two primary forests in Costa Rica: a tropical lowland rainforest (TLR) and a tropical montane cloud forest (TMCF).
Results/Conclusions
Overall, canopy soil CO2 respiration was lower in the TMCF. The effect of soil moisture on CO2 respiration depended on site (p<0.001), with a positive relationship at the TLC and a negative relationship at the TMCF. This indicates that the response of canopy soil decomposition to changing precipitation patterns may vary across forest types. CO2 respiration did not respond to nitrogen or phosphorus addition, indicating that nutrient availability does not limit canopy soil decomposition. Glucose addition elicited contrasting responses between the sites: at the TLR, glucose addition initially suppressed CO2 respiration but later stimulated CO2 respiration 24 hours after addition (p=0.06). On the contrary, glucose addition at the TMCF site resulted in an immediate spike in CO2 respiration followed by a subsequent return to baseline after 24 hours (p=0.09). At both sites, the addition of glucose may have “primed” the canopy soil organic matter by relieving carbon limitations to decomposition. These preliminary results are based solely on CO2 sensor data, which only measures gross respiration rates. Results from laboratory analyses of 12C and 13C-CO2 headspace, which we expect to present in our talk, will provide more detail on the responses of canopy soil decomposition processes to these experimental additions.