Managers reduce piñon (Pinus spp.) and juniper (Juniperus spp.) trees that are encroaching on sagebrush (Artemisia spp.) communities to lower fuel loads and increase cover of desirable understory species. All plant species in these communities depend on soil water held at > −1.5 MPa matric potential in the upper 0.3 m of soil for nutrient diffusion to roots and major growth in spring (resource growth pool). We measured soil water matric potentials and temperatures using gypsum blocks and thermocouples buried at 0.01–0.3 m on tree, shrub, and interspace microsites to characterize the seasonal soil climate of 13 tree-encroached sites across the Great Basin. We also tested the effects of initial tree infilling phase and tree control treatments of prescribed fire, tree cutting, and tree shredding on time of available water and soil temperature of the resource growth pool on nine sites. Both prescribed fire and mechanical tree reduction similarly increased the time that soil water was available (matric potential > −1.5 MPa) in spring, but this increase was greatest (up to 26 d) when treatments were applied at high tree dominance. As plant cover increased with time since treatment, the additional time of available water decreased. However, even in the fourth year after treatment, available water was 8.6 d and 18 d longer on treatments applied at mid and high tree dominance compared to untreated plots, indicating ongoing water availability to support continued increases in residual plants or annual invaders in the future. To increase resistance to invasive annual grasses managers should either treat at lower or mid tree dominance when there is still high cover of desirable residual vegetation or seed desirable species to use increased resources from tree reduction. This strategy is especially critical on warmer sites, which have high climate suitability to invasive species such as cheatgrass (Bromus tectorum L.)
Piñon–juniper (Pinus spp.–Juniperus spp.) tree encroachment and subsequent infilling in former sagebrush (Artemisia spp.) communities results in loss of understory cover, increase in woody fuel loads, and greater risk for high-severity, large-scale wildfire (Miller and Tausch 2001). Increased runoff and erosion associated with bare and water-repellent soils (Pierson et al. 2010; Urgeghe et al. 2010; Madsen et al. 2011) and dominance by annual weeds such as cheatgrass (Bromus tectorum L.; Brooks et al. 2004) may follow. Land managers reduce live tree dominance by prescribed fire and various mechanical means such as manual or hydraulic cut-and-drop or by shredding standing trees. To restore or maintain resilient ecosystems, managers should treat infilling areas well in advance of a suspected ecological threshold of tree cover (Bates et al. 2013; Roundy et al. 2014). This threshold tree cover has conceptually been considered to be an upper ratio of tree to total perennial cover beyond which fuel loads are high and understory residual plants (e.g., shrubs and perennial herbaceous plants) and seed banks are so limited that invasive annuals are much more likely than desirable perennials to dominate after fire or fuel-control disturbances (Miller et al. 2005). Infilling phases based on cover of trees relative to cover of shrubs and herbs (Miller et al. 2005) are relevant ecologically because they represent relative competitive demand for soil water and nutrients.
The annual climatic pattern in the Great Basin consists of soil water recharge in fall, winter, and spring, and short spring periods when warm soil temperatures and water availability coincide to support rapid growth (Caldwell 1985; Smith and Nowak 1990; Leffler and Ryel 2012). Growth is dependent on soil water availability at relatively shallow soil depths (< 0.3–0.5 m) in what Ryel et al. (2008) and Leffler and Ryel (2012) have identified as the resource growth pool. The resource growth pool is defined by high enough soil water matric potentials (> −1.5 MPa) to support nutrient mass flow and diffusion to roots, and root uptake of nutrients in solution. Invasive annuals such as cheatgrass are highly dependent on the shallow resource growth pool for growth and seed production (Ryel et al. 2010). Residual perennials, especially perennial grasses with root systems that deplete the soil water resource growth pool