Abstract
Underplanting tree seedlings in areas where natural regeneration is limited may offer a tool by which desired overstory composition can be maintained or restored in forests. However, invasive plant species and ungulate browsing may limit the effectiveness of underplanting, and in-turn, the successful restoration of forest ecosystems. Individually, the invasive shrub Lonicera maackii and browsing by white-tailed deer (Odocoileus virginianus) have been found to negatively affect the regeneration of native tree species in the Midwestern United States, but few studies have examined their interactive or cumulative effects. Using exclosures and shrub removal at five sites, we examined the effects of white-tailed deer and L. maackii both on underplanted seedlings of Castanea dentata and Quercus rubra and on the composition, species richness and diversity of naturally regenerated native tree seedlings. Individually, both deer and L. maackii had negative effects on the survival of underplanted seedlings, but we identified no interactive effects. The presence of L. maackii or deer alone resulted in similar declines in the survivorship of Q. rubra seedlings, but the presence of deer alone resulted in lower survival of C. dentata seedlings than the presence of L. maackii alone. Lonicera maackii reduced light levels, increased seedling moisture stress and decreased relative basal diameter growth for Q. rubra seedlings. Deer reduced the relative growth in height of underplanted C. dentata and Q. rubra seedlings and increased moisture stress of C. dentata seedlings. No effects of L. maackii or deer were found on soil or foliar nitrogen or the overall abundance, species richness and diversity of naturally regenerated seedlings. However, L. maackii and white-tailed deer did affect the abundance of individual tree species, shifting composition of the regeneration layer towards shade tolerant and unpalatable and/or browse tolerant species.
Highlights
Successful regeneration of overstory species is integral to maintaining forest systems in a time of ecological change (Reyer et al 2015)
For C. dentata, the second highest survival rate was in areas in which L. maackii was present and deer were excluded, countering our prediction that L. maackii alone would have a greater impact on survival than deer alone
The presence or absence of L. maackii outside of the deer exclosures did not significantly impact the number of underplanted seedlings of either species browsed by the end of the study, a finding that did not support our prediction that L. maackii would protect seedlings from browsing (Table 2)
Summary
Successful regeneration of overstory species is integral to maintaining forest systems in a time of ecological change (Reyer et al 2015). Within North America, the Midwestern United States offers an archetype of a fragmented landscape altered by invasive plants (Luken 1997; Oswalt et al 2015) and a frequently overabundant ungulate species (white-tailed deer; Anderson 1997; Hurley et al 2012). White-tailed deer preferentially browse certain species, altering forest dynamics over time by shifting composition towards species that are unpalatable or browse-tolerant (Rooney and Waller 2003; Rossell et al 2005). White-tailed deer alter nutrient cycling by preferentially browsing plants that have nutrient-rich tissue, over time, increasing the abundance of nutrient-poor species which decompose more slowly Composition and structure of forests are altered as heavily browsed species such as oak (Quercus spp.) are lost, and the regeneration layer is dominated by less-preferred or browse-tolerant species such as Fraxinus americana or Prunus serotina (Tilghman 1989; Rossell et al 2005)
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