June 2004
All Ecosystems Are Equally Productive Under Drought Conditions
Travis Huxman, University of Arizona, Department of Ecology
and Evolutionary Biology
A new study published in Nature shows that diverse biomes, such as forests, shrublands, grasslands, and deserts, all converge to a common, rain-use efficiency during droughts. In other words, when push comes to shove, all ecosystems have the same maximum rain-use efficiency (RUE), a measure of total plant growth per unit of precipitation. In general, RUE varies greatly between ecosystems, and decreases as precipitation increases. That is, the more rain that falls, the more ecosystems can afford to waste. But under drought conditions, tropical forests apparently can be as efficient at using water as desert ecosystems.
In the early 1970s, ecologists tried to answer the question of how water affects ecosystem processes by fanning out over the globe and measuring plant productivity in a wide array of ecosystems during a single year. The project, called the International Biological Program (IBP), lacked temporal perspective and failed to consider the influence of variability. Ecologists realized that they needed data collected in many different ecosystems over many years, the kind of data now available for Long-Term Ecological Research Sites (http://lternet.edu) that were inspired by IBP. The Nature paper was the product of a series of workshops held at the National Center for Ecological Synthesis in Santa Barbara, California.
Travis Huxman and his colleagues selected fourteen sites with at least six years of precipitation and annual above ground net primary production (ANPP) measurements. The sites included temperate forest in New England and boreal forest in Alaska, Mojave Desert at the Nevada Test Site, wet tropical forest in Panama's Barro Colorado Island, and Patagonian steppe in southern Argentina.
Huxman and colleagues surmise that water limitation imposes a common constraint on primary production across diverse biomes. Ecosystems have the same potential, maximum RUE despite differences in sensitivities of primary production to precipitation, physiognomy, climate history, hydrology, and phylogenetic origin of the representative flora. As precipitation increases, limitations in resources other than water (in other words, biogeochemstry) constrain primary production. This paper points out the importance for continued long-term monitoring and manipulative experiments to test how various types of ecosystems respond to different levels of precipitation.
Huxman TE, Smith, MD, Fay PA, Knapp AK, Shaw MR, Loik ME, Smith SD, Tissue DT, Zak JC, Weltzin JF, Pockman WT, Sala OE, Haddad BM, Harte J, Koch GW, Schwinning S, Small EE, Williams DG (2004). Convergence across biomes to a common rain-use efficiency. Nature 429, 651-654.
For more information contact Travis Huxman at huxman@email.arizona.edu.