Temporal and geographic physiological variation in Fremont cottonwood

Climatic conditions can have a large impact on the physiological ecology of all species. In the western United States the availability of water is both scarce and unpredictable through time. This variation in availability of water is responsible for the physiological patterns observed in desert plants. Riparian plants, those that inhabit the narrow corridor surrounding rivers, receive significantly more water than those in the desert but still experience dramatic variation in water availability. Physiological responses to variation in water availability are well understood in desert plants but little research has been performed on riparian species. This study examines climate/physiology relationships in Populus fremontii, the dominate riparian tree of lowland southwestern U.S. river systems, on multiple spatial and temporal scales. I examined diurnal patterns of physiological activity and found that physiological activity is highest during the morning and declines as temperature and vapor pressure deficit (VPD) increases. Water use efficiency (WUE), the ratio of carbon gained to water used during gas exchange, is highest in the early morning and late evening. P. fremontii individuals responded to a rain event with increased physiological activity and higher WUE. Physiological activity and efficiency was also observed to be highest during the hot, dry mid-summer. Overall, physiological activity was found to be tightly coupled to VPD. Long-term measurements of physiological activity derived from carbon isotope composition (delta13C) measurements demonstrates that physiology fluctuates on an annual basis and that water availability from stream flow and precipitation is partially responsible for this variation. These data suggest that WUE is highest when water is limiting in the environment. Long-term measurements of physiology throughout the range of this species indicates that WUE is highest within populations that experience high water availability while short-term measurements of physiology throughout the range of this species indicates the opposite. Vulnerability curves constructed from P. fremontii individuals suggests that this species is extremely vulnerable to xylem blockage by air under conditions of low water availability. These data demonstrate that the climate/physiology relationship in this species is scale-dependent and may change in response to environmental variation.