Sometimes a piece of research changes the way one looks at ecosystems.
Evaporation from tiny pores in the leaves of trees pulls water up from the roots through thousands of slim tubes, called xylem tissue. When water is scarce, these tubes can develop microscopic air bubbles which block these tubes. Too many of these bubbles across the xylem kills the tree.
To judge the state of forests, researchers studied 81 sites spanning from wet tropics to arib shrublands. The study gauged water transport in various tree species, and sought to assess the point at which each species dies from lack of water being delivered to its tissues.
The researchers concluded that 70% of 226 woody species in forests around the world routinely function near the point at which a serious drought would stop sufficient water transport from their roots to their leaves. Even trees in moist, "lush" places operate with slim margins. Flowering tree species (e.g., maples, oaks) are more vulnerable overall to dry conditions than conifers.
What drives this marginal state? Over time, trees have evolved to make a tradeoff between water lost and the capture of carbon dioxide, which is needed for metabolism and growth. When a tree opens its pores, it loses approximately 400 water molecules for each molecule of carbon that is captured. The reseachers thus conclude that trees are maximizing their carbon capture "for food" even though it puts their existence near the margin in terms of water.
The research can be found at: http://www.nature.com/nature/journal/v491/n7426/full/nature11688.html.