Understanding mortality in plants is a tangle of proximal and distal as well as competing hypotheses. A recent paper in PNAS tried to disentangle a number of issues for understanding mortality in trembling aspen (Populus tremuloides).
The authors use a mix of gradients and experiments to examine patterns of carbohydrate reserves and hydraulic properties for droughted and non-droughted aspen plants. Plants that were droughted and non-healthy did not have reduced carbohydrate levels in their tissues (leaves or roots). In contrast, dying plants consistently were experiencing loss of hydraulic conductance and cavitation.
What is interesting here is that aspen is the lettuce of trees. It is an isohydric, physiologically drought-intolerant species. The research shows that pot experiments should be pretty good at determining the drought tolerance characteristics of species. Screening experiments (and rated, more involved detailed studies like these) should allow for the type and degree of drought tolerance to be assessed for other species. hence, models of future mortality could be generated for forests across the world.
Anderegg, W. R., J. A. Berry, D. D. Smith, J. S. Sperry, L. D. Anderegg, and C. B. Field. 2012. The roles of hydraulic and carbon stress in a widespread climate-induced forest die-off. Proceedings of the National Academy of Sciences of the United States of America 109:233-237.