Forest ecosystems store large amounts of carbon (C) globally, and presently mitigate anthropogenic climate change by acting as a persistent sink for atmospheric CO2. Yet forest C density (i.e., the amount of C stored per unit area) varies greatly in space, both globally and at landscape scales, and the multi-scale drivers of this variation are not yet fully understood. In the context of climate change mitigation it is pivotal to consider the sensitivity of C stocks to climate. Furthermore, understanding the role of stand dynamics is fundamental to gauging the potential for forest management to influence C stocks. To gain insight into the contributions of environmental drivers and stand dynamics processes to spatial variability in C density we studied a 2191 ha old-growth forest landscape in the western Cascades of Oregon, USA. Using airborne light detection and ranging (Lidar) and a novel high resolution landscape model (iLand) we tested the hypotheses that environmental (i.e., climatic and soil) variation is the primary driver of spatial C patterns in the complex mountainous terrain of our study landscape, while processes of stand dynamics have little influence in forests within the same seral stage of stand development.
Notwithstanding the distinct environmental gradients in our mountainous study landscape only 55% of the variation in C density was explained by environmental drivers, with radiation and soil physical properties having a more prominent influence than temperature and precipitation. The remaining variation in C stocks was largely attributable to stand dynamics, despite our focus on forests which are in the same seral stage of stand development. Community processes such as competition and dispersal thus contributed in the same order of magnitude to landscape-scale C heterogeneity as the influence of varying environmental drivers on physiological processes. We found that not only density- and size-related indicators of stand structure but also diversity in composition and structure were positively related to C on the landscape, pointing at complementary resource use and response diversity as major processes responsible for the large and persistent C stocks in the old-growth forests of the Pacific Northwest. In the context of managing forests for climate change mitigation our findings suggest that fostering complexity (e.g., in stand structure and composition) can promote long-term landscape-scale forest C storage.