Complex terrain leads to bidirectional responses of soil respiration to inter-annual water availability

Research on the terrestrial C balance focuses largely on measuring and predicting responses of ecosystem-scale production and respiration to changing temperatures and hydrologic regimes. However, landscape morphology can modify the availability of resources from year to year by imposing physical gradients that redistribute soil water and other biophysical variables within ecosystems. This article demonstrates that the well-established biophysical relationship between soil respiration and soil moisture interacts with topographic structure to create bidirectional (i.e., opposite) responses of soil respiration to inter-annual soil water availability within the landscape. Based on soil respiration measurements taken at a subalpine forest in central Montana, we found that locations with high drainage areas (i.e., lowlands and wet areas of the forest) had higher cumulative soil respiration in dry years, whereas locations with low drainage areas (i.e., uplands and dry areas of the forest) had higher cumulative soil respiration in wet years. Our results indicate that for 80.9% of the forest soil respiration is likely to increase during wet years, whereas for 19.1% of the forest soil respiration is likely to decrease under the same hydrologic conditions. This emergent, bidirectional behavior is generated from the interaction of three relatively simple elements (parabolic soil biophysics, the relative distribution of landscape positions, and inter-annual climate variability), indicating that terrain complexity is an important mediator of the landscape-scale soil C response to climate. These results highlight that evaluating and predicting ecosystem-scale soil C response to climate fluctuation requires detailed characterization of biophysical-topographic interactions in addition to biophysical-climate interactions.