THE POLICY FORUM BY J. D. GLOVER ET AL. ("INCREASED FOOD AND ecosystem security via perennial grains," 25 June, p. 1638) highlights environmental advantages of perennial relative to annual bioenergy crop systems but omits potentially important consequences related to hydrology and climate. They categorize greater perennial leaf area index and rooting depth (relative to annual crops) as "utiliz[ing] more precipitation," but the work cited provides no evidence for increased rainfall recycling.

The direct climate impact of land-use change associated with bioenergy expansion (such as a shift from annual to perennial cropping systems) has received little attention. The impacts of changing fundamental biogeophysical surface properties associated with bioenergy crops may have significant implications for local and regional climate. Changes to local hydrology caused by large-scale perennial systems may be complex, and thus require careful evaluation. For example, the drawdown of soil water and enhanced evapotranspiration from perennial relative to annual cropping systems could lead to long-term depletion of the soil-water column, as well as changes in clouds and rainfall in downwind locales. Quantifying local and remote consequences for hydrology and climate resulting from a shift from annual to perennial bioenergy crops is therefore required if longterm sustainability of biomass production is to be attained.