The expansion of forest vegetation within and into the Arctic is one of the profound transformations that the Arctic land surface is likely to undergo in the coming decades. The spread of forest vegetation has significant ramifications for the arctic system, as it is likely to cause both positive and negative feedbacks on climate, and to alter the availability of crucial natural resources. Although forest expansion within and into the Arctic has been widespread in recent decades, there is growing evidence that non-linear responses to warming may prevail within areas of expanding forest. In particular, large areas of 'browning' (declining trends in NDVI) have recently been identified in the southern Arctic. The goal of this research is to assess the relationships among tree growth, climate, and NDVI in order to identify the causes of non-linear responses to temperature (i.e., the 'browning' response) and better understand the likely effects of future warming on the productivity and dynamics of forest vegetation expanding into the Arctic. The proposed research tests three hypotheses. First, it is hypothesized that the 'browning' response in the southern Arctic represents a non-linear response to temperature induced by drought stress. The researchers propose to test this hypothesis by compiling a circum-arctic database of tree-ring chronologies, representing all major Arctic tree species and regions. Most of these chronologies are readily available in an existing data archive; they propose fieldwork in years 2 and 3 of the project to update chronologies from two key areas. They will use regression tree analysis to model the climate-tree growth relationships. Second, they hypothesize that trends in NDVI are consistent with trends in tree growth: they expect that a drought-induced threshold response to temperature explains both tree growth and the NDVI data. They will test this hypothesis by comparing the AVHRR-NDVI record with the tree-ring data. Finally, they hypothesize that temperature and precipitation will continue to influence trends in NDVI and arctic forest productivity in the future. They will test this hypothesis at a limited number of sites using a customized version of the Biome-BGC model, an ecophysiological forest growth model. This modeling exercise will allow them to explore, in a qualitative fashion, the implications of spatially variable tree growth (and NDVI) for C dynamics in the southern Arctic during the next several decades.
The research should yield the first circum-arctic analysis of tree growth response to climate, addressing the critical question of the prevalence (and causes) of non-linear responses to climate in Arctic forests. This will provide important insight into the likely future trajectory of land cover change in the southern Arctic: if non-linear responses to temperature prevail, then future expansion of forests into the Arctic seems far less certain than if responses are primarily linear. Second, the proposed research will link the site-level climate responses to C dynamics, allowing us to determine, at least for a subset of sites, how non-linear climate responses alter forest C flux.
