Non-native mule deer drive post-fire resprout failure of chaparral vegetation on Santa Catalina Island

Background/Question/Methods

High levels of browsing by alien mule deer may reduce survival of post-fire resprouting shrubs on Santa Catalina Island in southern California. This can lead to large-scale vegetation type shifts from evergreen chaparral shrubs to invasive alien grasses and forbs.  Three deer exclosures were established in recently burned areas to protect vegetation from browse.  Unenclosed plots adjacent to exclosures were established for comparison.  Our objectives were to determine the role of deer browse on post-fire resprout success in a dominant chaparral shrub, Heteromeles arbutifolia, and to assess the effect of deer browse on the water and carbon relations of resprouts.  We hypothesized that deer browse would diminish both resprout success and physiological performance. To test our hypotheses, measurements were made on both browsed and non-browsed resprouts, as well as unburned adults as a control.  Measurements included plant mortality, canopy death (%), pre-dawn and mid-day water potential (Y_pd and Y_md), stem elongation (mm/day), plant canopy area (m²), maximum net carbon assimilation (A_nmax), stomatal conductance (g_s), and lignotuber starch content.

Results/Conclusions Browsed resprouts had greater levels of mortality compared to non-browsed resprouts (63.2% and 11.5%, respectively; P < 0.05).  The average percentage of dead canopy for browsed resprouts was also greater than non-browsed resprouts (88% and 18%, respectively; P < 0.05). Stem elongation and plant canopy area for non-browsed resprouts were significantly greater compared to browsed resprouts (P < 0.05).  Net carbon assimilation (A_nmax) was higher for non-browsed resprouts (P < 0.05), but g_s was not significantly different.  Pre-dawn water potentials revealed that both browsed and non-browsed resprouting plants were well hydrated (Y_pd > -0.9 MPa) indicating a lack of water stress for both groups.  Lower values of A_nmax, stem elongation, and canopy area for browsed resprouts suggesting decreased ability to capture CO₂ that could lead to carbon starvation once reserves are depleted.  Consistent with this, browsed plants had low lignotuber starch content.  Alien deer appear to be drivers of a vegetation shift from native shrublands to alien grasses and forbs post-fire.