Abstract:In this paper, we examine the linkage between woody plants and the water budget for three important woody plant communities in Texas, USA: saltcedar (Tamarix chinensis, Tamarix ramosissima), Ashe juniper (Juniperus ashei Buchholz), and mesquite (Prosopis glandulosa Torr. var. glandulosa). In most cases, these species are found in distinct physiographic and soil settings. Saltcedar is restricted to stream channels and floodplains; Ashe juniper is found mostly on karst limestone outcrops with shallow soils; and mesquite is found on deep soils. Because of these differences, changes in woody plant cover in each community will have a different effect on the water budget. For each type, we review the available literature and explicitly report the scale of observation (tree, stand, catchment, or landscape). A simple framework called the shrub-streamflow framework, which recognizes differences in response due to differences in physiographic setting, climate, and potential for deep drainage or subsurface flow, enables us to generalize the results. The fundamental premise of the framework is simple: for shrublands to be hydrologically sensitive to changes in woody plant cover, soil water or groundwater must be accessible to deep-rooting plants but too deep for shallow-rooting ones. Such a situation exists if groundwater is close to the surface (within 3-5 m and/or if deep drainage occurs (because of either high precipitation input or bypass flow in the soil). We argue that on an area basis, conversion of saltcedar stands to herbaceous plants in riparian regions has a much greater potential for increasing water yield than does conversion of woodlands to grasslands in upland regions where deep drainage does not occur. On upland sites where deep drainage does occur, conversion from woody to herbaceous vegetation may result in a savings of 40-80 mm year 1 of water. But such savings have been observed only up to the small-catchment scale, and until further work is done it is uncertain whether they can be achieved at larger scales.

Multiple fires conducted in spring (March-April) and summer (June-July) on blue grama (Bouteha gmdis [H. B. K. Lag.]) grassland near Corona, N.M. were used to relate broom snakeweed (Gutierreziu sarohrue [Pursb] Britt & Rusby) control to pre-burn vegetation, weather, and fire conditions. Spring fws moved faster and burned cooler than summer fires as indicated by measurements from thermocouples giving the fire's rate of spread, temperature, and beat. In spring, broom snakeweed was in the bud stage with little green foliage and fves provided less average crown destruction (8 %) and shrub mortality (65 %) compared to summer fws (66% crown destruction, 92% mortality) when the shrub was growing actively. Air temperature and total fuel biomass positively influenced fire temperatures, and duration of beat above 6O'C resulting in high broom snakeweed mortality. Conversely, as relative humidity, wind speed, and fuel moisture increased, fue beat decreased, resulting iu less broom snakeweed mortality. Attempts to conduct spring or summer fires over a 6-year period in central New Mexico were complicated aud often unsuccessful because of unsuitable weather and fuel conditions. We concluded ideal weather conditions must converge before, during, and after a prescribed burning event in order to maximii broom snakeweed control and forage growth on these grasslands.

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