Abstract
Understanding and predicting the effects of deer (Cervidae) on forest ecosystems present significant challenges in ecosystem ecology. Deer herbivory can cause large changes in the biomass and species composition of forest understory plant communities, including early life‐cycle trees (i.e., seedlings and saplings). Such changes can impact juvenile to adult transitions and the future age structure and species composition of mature forests. Changes to understory vegetation also impact flow of energy and nutrients in forest ecosystems. Studies examining the influence of deer on mature trees, however, are rare and rely on extrapolating effects from early life cycle stages of trees. We tested the hypothesis that the absence of deer would result in an increase in the growth rate of mature trees by examining the impact of white‐tailed deer (Odocoileus virginianus) on mature canopy trees. We examined incremental growth in individuals of Quercus rubra, an important component of temperate deciduous forests in North America, inside and outside 16‐year deer exclosures in eastern U.S. deciduous forests. We found that adult trees inside exclosures grew less than those directly exposed to deer. Our findings highlight the indirect effects of white‐tailed deer on the growth of adult individuals of Q. rubra in a way opposite of what would be expected from previous studies based on immature or understory tree populations. We suggest the increased growth of adult trees in the presence of deer may be explained by increased nutrient inputs through deer fecal and urine deposits and the alteration of the competitive environment belowground through the reduction of understory vegetation by browsing. Underscoring the ecological and demographic importance of adult trees in forest ecosystems, results from this study suggest the direct and indirect effects of deer on mature trees should not be overlooked.
Highlights
In recent years, deer (Cervidae) populations have increased dramatically throughout many areas of the world, including North America (Horsley et al 2003), Europe (Apollonio et al 2010), Asia (Tsujino and Yumoto 2004, Suzuki et al 2008), and Oceania (Platt et al 2004, Husheer et al 2006)
Site comparisons The mature tree community structure was similar among the four sites and between deer treatments, with no differences in total basal area or overall tree density of all trees having a dbh .10 cm (Appendix: Table B1)
The species composition of mature trees differed somewhat among sites, Q. rubra was consistently a dominant species, and its density in particular did not differ among sites or between deer treatments (Appendix: Table B1)
Summary
Deer (Cervidae) populations have increased dramatically throughout many areas of the world, including North America (Horsley et al 2003), Europe (Apollonio et al 2010), Asia (Tsujino and Yumoto 2004, Suzuki et al 2008), and Oceania (Platt et al 2004, Husheer et al 2006). As deer populations have grown, they have increasingly impacted the biomass and species composition of understory woody plant communities in forest ecosystems. Many studies have shown that in forest ecosystems, deer have contributed to the decreased abundance and diversity of palatable herbs and tree seedlings (Gill and Beardall 2001, Kuiters and Slim 2002, McGraw and Furedi 2005, Knight et al 2009, Heckel et al 2010). Deer browsing activity modifies the flow of energy and nutrients in forest ecosystems (Hairston et al 1960, Pastor et al 1993, McNeil and Cushman 2005, Pastor et al 2006, Stritar et al 2010)
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