Increasing numbers of tree species worldwide are experiencing reduced recruitment and mammalian consumers are often implicated as drivers of these changes. The situation appears especially acute for oaks, which are reported to exhibit reduced recruitment and regeneration in many parts of the world. Here, we summarize findings from two long-term field experiments conducted in northern California. The first used an eight-year-old exclosure experiment at two sites to evaluate the individual and interactive effects of black-tailed deer (Odocoileus hemionus columbianus) and small mammals (meadow voles Microtus californicus and deer mice Peromyscus maniculatus) on the growth and abundance of seedlings and juveniles for three dominant oak species (Quercus agrifolia, Q. kelloggii and Q. garryana). The second study involved a 14-year-old exclosure experiment that evaluated the impact of herbivory by black-tailed deer on the growth, architecture and survival of coast live oak (Q. agrifolia). This experiment explicitly focused on the fate of juvenile oaks – individuals that had passed the seedling stage but had not yet become saplings – because, like many regions in California, our study area had an abundance of established reproductive trees but near-complete absence of sapling-stage individuals.
Results/Conclusions
Both experiments revealed that mammalian consumers had large impacts on oaks in our system. The first experiment showed that small mammals reduced the height of seedlings and juveniles for all three oak species and reduced seedling abundance for two species (Q. kelloggii and Q. garryana). In contrast, deer by themselves only reduced the size of juvenile Q. agrifolia. However, deer had negative effects on oak abundance and cover when small mammals were present and positive effects when small mammals were absent. Our data suggest that these interactive effects occurred because small mammals reduced the cover of woody understory species and increased the intensity of deer browsing on oaks by reducing protective cover. The second experiment revealed that deer altered tree architecture and caused 3-fold reductions in juvenile oak height and canopy area and 1.5-fold reductions in basal stem diameter, leaf abundance and tree survival. Our research shows that interactions between large and small mammals can alter the intensity and direction of their effects on oaks. Collectively, our results indicate that mammals play crucial roles in limiting tree recruitment by reducing seedling abundance and maintaining oaks in stunted states and preventing them from reaching sapling stages and becoming reproductive.