Long-term influences of introduced deer on the composition and structure of New Zealand Nothofagus forests

Abstract

New Zealand’s Nothofagus (southern beech) forests evolved in the absence of mammalian herbivores. The deliberate introduction of deer in the 19th century caused a rapid decline in the abundance of palatable understorey herbs and shrubs, but the longer-term consequences of deer herbivory remain poorly understood. This study uses data from permanent plots established in two separate surveys in northern and southern Kaimanawa Forest Park, central North Island, New Zealand to relate the impacts of two post-irruptive populations of deer to two decades of change in forest composition and canopy species regeneration. Plots were established on randomly located transects in Nothofagus forest between 1979 and 1982. Some were re-measured in 1987/1988, and all were re-measured between 1998 and 2000. Our objective was to gain a better understanding of the long-term effects of deer on Nothofagus forests. In the northern forests dominated by Nothofagus fusca, N. menziesii and Weinmannia racemosa, there was an expansion in the stem densities (stems ≥20 mm diameter at breast height) of highly unpalatable species of small trees. Numbers of unpalatable Pseudowintera colorata stems increased on north-facing slopes. The unpalatable Neomyrtus pedunculatus and Leucopogon fasciculatus increased in number at many sites. Analysis of seedling densities indicated that two palatable species ( W. racemosa and Griselinia littoralis) were failing to recruit into the >75 cm height class. In the southern Kaimanawa forests of N. solandri var. cliffortioides, understorey composition shifted towards browse-tolerant herbs. This is consistent with deer influencing the competitive interactions of herb and shrub communities, but storm-induced disturbances dominated tree demographics in the southern Kaimanawa, making the effects of deer difficult to ascertain. It is possible, but less likely, that factors other than deer, such as other herbivores and normal successional processes, are responsible for the changes in forest structure and composition that were observed in this study. There is some evidence that deer were affecting gap-phase regeneration of Nothofagus, but not strongly enough to prevent recruitment on a large scale. We believe that there is sufficient evidence of deer browsing shifting the composition of Kaimanawa Forest Park Nothofagus forests for managers to justify increased deer control.