The effect of shrub cover on conifer water-use patterns, growth and response to precipitation variability in the southern Sierra Nevada.

This article investigates how shrub cover influences water uptake patterns, growth, and carbon-use efficiency of juvenile white fir (Abies concolor) and Jeffrey pine (Pinus jeffreyi) in the southern Sierra Nevada of California, a region experiencing increased wildfire activity and shrub expansion. Using stable isotope analysis of xylem and soil water, the study found that shrub presence does not significantly alter the depth or source of soil water used by these conifers, indicating limited competition for water between shrubs and trees. Tree-ring analyses revealed that trees growing near shrubs exhibited higher growth rates and lower carbon isotope discrimination (Δ13C), suggesting enhanced photosynthetic assimilation potentially due to increased nitrogen availability from the nitrogen-fixing shrub Ceanothus cordulatus. Overall, the findings suggest that shrub cover may facilitate rather than hinder juvenile conifer growth in this fire-affected ecosystem, although results are specific to the studied species and site.