| The objective of my dissertation is to evaluate climatic and human influences on global NPP. In the first part of my study, I evaluated the different approaches used in global terrestrial biospheric models to introduce water budget limitations on NPP and tested the assumption that moisture availability is a primary driver of the NPP estimated by the current global models. Three methods to restrict NPP by water availability were distinguished; (1) physiological control on evapotranspiration through canopy conductance; (2) supply/demand constraints on ecosystem productivity; (3) water limitation inferred from satellite data. A water balance coefficient, calculated as the difference of mean annual precipitation and potential evapotranspiration, has been compared to NPP for each grid cell in each of fourteen models. While correlation plots revealed similar boundary lines for most global models, there was high variability in these distributions related to other environmental controls on NPP.;In the second part, I assessed relative importance of climatic controls (temperature, water availability, and radiation) in limiting NPP in the array of climatic combinations found globally. The degree of limitation on NPP by climatic controls was defined using an empirical membership function. Results showed that temperature or water availability limited NPP over larger land areas (31% and 52% respectively) than did radiation limitation (5%). Climatic controls appeared to be important in limiting productivity in most vegetation biomes, except for evergreen broadleaf forests. There were areas of the globe (12%) where none of the climatic factors appeared to limit NPP.;In the third part, measurements of extracted timber and modeled forest productivity were used to investigate the relationship between harvested timber and natural forest productivity for current conditions, under doubled CO 2, and climate change scenario. The analysis was confined to coniferous forests and countries that have coniferous forests within their territories. The results of this study suggested that global coniferous forests currently produce more wood than people consume, but this gap would narrow in the future. Wood extraction may reach forest regrowth by the middle of the next century, even though most coniferous forests are located in high latitudes and would have an accelerated stem growth associated with the joint effect of climate change and elevated carbon dioxide concentration in the atmosphere. |
