Forest ecologists did not believe a pathogen was at work. The focus was on the Colorado drought from 2000 to 2004. The two basic hypotheses were:
- Carbon starvation. Drought drives stomatal (pore) closure and/or other effects on photosynthesis, leading to a negative carbon balance that depletes carbohydrate reserves and ends in tissue-level carbohydrate starvation.
- Hydraulic failure: Drought stresses and impairs the water transport system that supplies leaves.
Researchers at Stanford University and the University of Utah, led by William Anderegg of Stanford, found ''substantial evidence of hydraulic failure of roots and branches linked to landscape patterns of canopy and root mortality in this species.''
Essentially, the research believes the die-off is a delayed reaction to the Colorado drought from 2000 to 2004, which broke down the systems that carry water through aspen stands. The study 1 explains the water column in xylem (wood tissue) conduits ''breaks or cavitates as a result of excessively negative xylem sap pressure, leading to tissue dehydration.''
Anderegg's team studied climate records in 51 different aspen areas in Western Colorado from 1900 to 2009. The 2000 to 2004 drought was the most severe during that 109 year period.
Utah researcher John Sperry found that in SAD trees an average of 70 percent of the vascular system was blocked, noting that such severely compromised trees fought against dehydration for a few years after the drought before dieing.
The researchers, however, found ''no evidence that drought stress led to depletion of carbohydrate reserves.'' The researchers note forest ecosystems store approximately 45 percent of the carbon found in terrestrial ecosystems.
The full study is available at www.pnas.org.
1. William Anderegg, et al. The roles of hydraulic and carbon stress in a widespread climate-induced forest die-off. Proceedings of the National Academy of Sciences 2012 109 (1) 233-237.