Barking up the right tree? : development of ecological integrity in restoration plantings

Abstract

New Zealand has lost much of its original forest cover to pasture, but restoration programmes are now attempting to mitigate this loss through revegetation. Programmes typically involve planting seedlings of pioneer species to initiate natural forest succession towards a self-sustaining stand of planted forest with similar species composition and function to a stand of naturally occurring mature native forest. My study examines this plant community development over time through the concept of ecological integrity, defined as the capability of an ecosystem to maintain a community comparable to that of a natural habitat for a given region. Twenty nine stands of different aged plantings and nine stands of naturally occurring forest in the Auckland Region of New Zealand were surveyed for vegetation composition and structure. Experimental translocations of mature forest duff were also undertaken into four of these plantings to examine effects on the germination and diversity of native understorey species. The structure and species composition of planted sites converged on that of remnant forest with increasing age of planting. Native species richness also increased with planting age, and exotic species richness, original planting composition and distance to remnant forest had no significant effects on species recruitment to planted sites. Factors shown to have a significant effect on community composition included planting age, stand area, stand shape, canopy height and species, mean tree diameter, and slope. Functional group analysis across different aged plantings showed climbers, epiphytes, and ground ferns were most underrepresented in planted stands compared to remnant forest, suggesting they need to be specifically targeted if desired in habitat restoration. Duff translocation was effective at improving germination and diversity of desired late successional and interstitial plant species. Overall, the sequential recruitment of native species with planting age shows promise for the success of restoration plantings in Auckland. Continuation of current planting approaches is generally recommended, however, potential applications of the findings of this research in improving conception and management of restoration planting programmes are also discussed.