Woody debris in treefall gaps shelters palatable plant species from deer browsing, in an old-growth temperate forest

Abstract

A major issue in the management of New Zealand’s remaining native forests is the extent to which the presence of introduced deer is compatible with national conservation and ecosystem restoration goals. Deer greatly reduce the abundance of their preferred plant species in forest understories, even when present at low densities. Recent work in other temperate regions has found that coarse woody debris resulting from treefalls shelters regeneration of species that are heavily browsed elsewhere in the forest. We asked if a similar effect occurs in lowland New Zealand forest dominated by Beilschmiedia tawa, comparing sapling abundance of preferred tree species in various microenvironments in two old-growth forests <30 km apart growing in near-identical physical environments—one inhabited by introduced fallow deer (Dama dama), the other from which ungulates and other introduced mammals were excluded c. 13 years earlier. Species composition of sapling communities growing within fallen tree crowns and on woody debris in treefall gaps did not differ significantly between the two forests; in contrast, the species composition of sapling communities elsewhere in treefall gaps and on understorey plots differed strongly between sites with and without deer. Saplings of species preferred by deer were equally abundant within fallen tree crowns in the two forests, demonstrating the effectiveness of tree crowns in sheltering palatable species from deer browsing; saplings growing on coarse woody debris in treefall gaps were also of similar abundance in the two forests. However, abundance of preferred species’ saplings on understorey plots was 10-fold lower at sites with deer, and their abundance in parts of treefall gaps unprotected by woody debris was 4-fold lower. Saplings of most other species did not differ significantly in abundance between sites with and without deer, in either understorey or treefall gap environments. Results suggest that in old-growth forest dominated by B. tawa, coarse woody debris associated with treefall gaps enables persistence of woody species that deer largely remove from the rest of the forest mosaic. Useful follow-up research could include modelling of the long-term impact of this effect on forest composition, and determination of the extent of its occurrence in other forest types.