ASSESSING FOREST RESPONSES TO CLIMATE CHANGE AND RESOLVING PRODUCTIVITY MEASUREMENTS ACROSS SPATIAL SCALES

Informed decisions regarding forest and carbon resources require knowledge of the impacts of environmental changes on forest productivity. We also need to reconcile the diverging productivity estimates that are presently available. This dissertation assembles two publications addressing the impacts of climate change on forest productivity and one exploring the relationship between three estimates of forest productivity. In the first chapter, I evaluated whether forests have responded to recent changes in climatic conditions. Through combining published evidence I show that forests have responded to changes in the patterns of light, water, and temperatures over the last half of the 20th century. Most published studies showed a positive growth trend. Negative growth trends were found for drier study areas. Conclusions on the greening of the world\u27s forests, are difficult due to poor geographical coverage and measurement method disparity. In the second chapter, I compared three productivity estimation methods (two ground-based and one satellite-based) using 166 sites in Austria. Results of disturbance-free projections show the relevance of each method to actual site productivity and their combined usefulness in identifying the most appropriate scale for monitoring climate forcings. Each estimation method provides information on a portion of the underlying actual NPP. In the last chapter, I explore the effect of three IPCC climate change scenarios on forests of the US Northern Rockies. Results show an increase in growing season length and in water stress, and a decrease in snow quantities and in number of days with ground snow for all forests by 2089. Under the driest and warmest scenario, the majority of the sites became carbon sources, and I identify a water/temperature tipping point, past which system stored carbon drastically declines. For these disturbance-free projections, water availability drove the system. In this dissertation, I resolve a otential source of conflict among forest productivity estimates; combined, these estimates lead to a broader understanding of productivity. I also present evidence that forests are already responding to climate change, and that more drastic changes are likely in the future