The influence of seasonal precipitation patterns on transpiration in pinon-juniper woodlands

There is a growing realization of the importance of seasonality of precipitation in structuring moisture resources in water-limited environments and the effects this might have on the structure and function of plant communities. The season in which precipitation falls impacts the timing, depth and longevity of soil moisture resources. As plant species differ in their ability to utilize these different moisture resources, seasonality of precipitation may directly influence floral composition and productivity of an ecosystem.; Anthropogenic climate change is likely to alter the patterns of precipitation availability globally, through changes to the Earth's energy budget. The consequent changes in seasonal moisture availability may have a large impact on species composition and productivity in water-limited ecosystems, where small changes in moisture availability may have large ecological impacts.; This thesis is an investigation into the importance of seasonal moisture availability on a widespread vegetation type, pinon-juniper woodlands, in the semi-arid southwestern United States.; This thesis demonstrates that pinon-juniper woodlands are highly sensitive to seasonal variations in moisture availability. Total woodland transpiration varied considerably over 3 years of measurement in direct relationship to interannual and seasonal variations in moisture. The two species composing the woodland, Pinus edulis and Juniperus osteosperma , differed in their transpiration patterns and their sensitivity to seasonality of precipitation. P. edulis was capable of higher maximal transpiration rates than J. osteosperma, but was more sensitive to drought. Additionally, P. edulis utilized summer precipitation to a greater extent than J. osteosperma. Ecophysiological investigations indicated that the differential responses of these species could be explained by differences in (1) the hydraulic properties of xylem, (2) the regulation of water potential and (3) the ability to refill embolized root xylem (P. edulis) following a moisture pulse.; The different sensitivities of the species to seasonal moisture availability were correlated with their geographical distributions. This suggested that ecophysiological adaptations to seasonality of precipitation play an important role in species biogeography in the Intermountain West. Additionally, pinon-juniper woodlands are unlikely to respond uniformly to changes in seasonality of precipitation. Rather, the composite species will be independently affected as has been suggested by the paleo-ecological record.