Seasonal water dynamics of a sky island subalpine forest in semi-arid southwestern United States

This paper describes the first results from a hydro-micrometeorological study designed to characterize the surface exchanges of water, and CO2, at a sky island forest site in the Santa Catalina Mountains. Data collected between June 2002 and December 2004 is used to describe the seasonal water/carbon dynamics of this ecosystem. During the severely dry pre-monsoon period (May–June) when vapor pressure deficit exceeded 2 kPa, daytime relative humidity was below 20%, and soil moisture was less than 10%, the predominantly Douglas Fir/Pine forest severely reduced or curtailed its photosynthetic activity. At the onset of the monsoon season in July, the vegetation responded rapidly to the precipitation input, as soil moisture increased. The trees then remained relatively active throughout the fall and winter, particularly if there was adequate winter precipitation which promoted very high productivity into early spring. Given the relative importance of mountain island forest in this semi-arid landscape in terms of its contribution to area-average surface water and carbon exchange, the strong and somewhat unusual coupled seasonal cycles, this forest ecosystem likely has hitherto unrecognized implication for the seasonality of the regional-scale water and carbon cycle.