In North America, ecosystems supporting ponderosa pine

from a moist, maritime Mediterranean climate (in the Klamath Mountains of northwestern California) to subalpine condi-tions (in the Rocky Mountains) to semi-arid woodlands (in the Southwest). Soils are commonly coarse textured and often nu-trient deficient. In such cases, the presence of an understory of nitrogen-fixing shrubs can make an important contribution to site productivity. Ponderosa pine is a thick-barked species that is tolerant of surface fires. Historically, light, periodic ground fires were common in ponderosa pine forests (Bork 1985) and resulted in the formation of park-like stands. Over the last century, how-ever, the policy of protecting forests from wildfire has resulted in the development of a dense understory of suppressed pines and other conifers (McCune 1988), which has increased the risk of conflagrations by allowing fuels to accumulate. As plant competition has increased, so have impacts associated with human activities. Large recreational demands are made on ponderosa pine forests, and fossil fuel use has enriched the atmosphere not only with CO2, but with ozone and other pol-lutants. In response to increases in these atmospheric trace gases, ponderosa pine ecosystems are probably experiencing changes in climatic conditions and atmospheric chemistry greater than any to which they have previously been exposed. As a consequence, empirical predictions of growth based on past management experience may no longer apply. In these circumstances, physiological ecology, which studies the un-derlying environmental interactions controlling biological processes, provides an avenue for assessing both the present status of ponderosa pine forests and the way in which their growth, distribution, and susceptibility to various stresses may be affected by future changes in management, climate and en-vironmental chemistry. Interest in the integrated physiological response of ponder-osa pine to variation in climate, soil, atmospheric concentra-tions of CO2 and ozone, and competing vegetation gave rise to a workshop “The Ponderosa Pine Ecosystem and Environ-mental Stress: Past, Present and Future, ” which was held a