Soil water availability and wet degree days in pinyon and juniper communities

Background/Question/Methods Resource availability is associated with plant community distribution, productivity, and susceptibility to invasion by weedy species.  Yet, comparative resource availability data are limited in extent and intensity for wildland plant communities.  The cold deserts of western North America are characterized by short seasonal periods when available soil moisture and growing temperatures are in phase.  What is the role of annual, seasonal, and spatial patterns in available soil moisture and soil temperatures in the distribution, production, and invasibility of sagebrush steppe communities to weeds?  For example, are locations with more temporal variability in availability of resources more susceptible to annual weed invasion, as stated by the fluctuating resource hypothesis?  Soil moisture and temperature are being monitored at 4 depths, 4 micosites, 2-3 understory/overstory ratios, and 3-4 vegetation disturbance treatments for 4 bunchgrass and 9 woodland-invaded sagebrush steppe locations across the Great Basin.   Seasonal resource availability variables include number of wet days, wet degree days (sums of hourly temperatures above 0 °C when the soil is wet) , number of wet periods, soil frost-free days, total degree days, and maximum wet and dry periods and their starting dates. 

Results/Conclusions Initial data indicate high seasonal and annual variability for resources, limited differences in resource availability for different understory/overstory ratios, and greater resource availability after fire which kills woody species.  The scope and intensity of these ongoing measurements should allow us to discover patterns of resource availability associated with vegetation changes over time that have not been obvious in the past, as well as model germination and growth responses of associated species.