Browsing often reduces plant growth, survival and/or reproduction, and might also reduce a plant’s ability to tolerate severe drought. Previous tests of the hypothesis that browsing reduces plant performance during drought have been ambiguous, and have typically been done on grasses and forbs, not on woody plants. Deer browse on Ceanothus cuneatus var rigidus shrubs was chronically high at the Fort Ord Natural Reserve before the severe California-wide drought from 2011-2015. In a long-term, ongoing, field experiment established in the mid-1990s, performance of shrubs under continued deer browse (controls) was compared with that of plants in deer exclosures. During the 15 years before the drought, control plants lost biomass (and several died), while most plants inside deer exclosures grew larger and survived better. The higher biomass of protected plants was associated with increased water use efficiency, suggesting they experienced greater water stress. Some plants in deer-exclosures also sustained high damage from woodrats.
Results/Conclusions
Browsed and unbrowsed Ceanothus responded very differently both to 4 years of severe drought and subsequent two years of rainfall. Leaf biomass of control shrubs did not change during the drought, but then nearly doubled in the wet years. In contrast, plants inside deer exclosures lost ~50% of leaf area during the drought and did not recover during the wet years. Deer abundance declined drastically during the drought, and remained low during the wet years, while woodrat abundance and damage remained high on caged shrubs. We conclude/hypothesize: (1) Drought was a weaker stressor for control plants than previous deer browsing; these stressors were substituted temporally during the drought; relaxation of both stressors enabled rapid growth after the drought. (2) Shrubs in deer exclosures were water-stressed before and during the drought, and this, in combination with continuing woodrat browsing, caused significant loss of leaf area and deaths, and inhibited recovery despite increased rainfall – chronic water stress, drought and woodrat browsing were additives stressors. (3) The combination of greatly increased growth and little deer browse after the drought may cause control Ceanothus to becoming water-stressed (similar to exclosure plants), making them particularly vulnerable and less resilient to subsequent droughts.