Elder (Sambucus nigra) as a facilitator of succession, Hoon Hay Valley, Banks Peninsula, New Zealand

Abstract

Native woody plants appear to regenerate more profusely under elder (Sambucus nigra) trees, than in exotic scrub vegetation in Hoon Hay Valley. This observation suggests elder may act as a succession facilitator, speeding up succession from scrub to native forest. The objectives of this study were to: 1) confirm and quantify the increased seedling density under elder canopies in Hoon Hay Valley; 2) test the following explanations for the greater seedling density: I. elder trees receive a greater quantity of native seed rain; II. elder trees are less accessible to grazing animals, and native seedlings therefore less grazed; III. there are more favourable environmental conditions for growth under elder than in adjacent vegetation.

There was a significantly greater density of native woody species (mainly Melicytus ramiflorus) under the 60 elder canopies studied, compared to 60 plots (of equivalent area) in the adjacent (bracken-gorse-broom) scrub. An investigation of the seed rain, using 20 paired seed traps, found a significantly greater quantity of native seed in the elder canopy traps, compared to the traps in the adjacent vegetation. Examination of the soil seed bank revealed a similar pattern, with significantly more native seed germinating from soil samples from under elder trees than from the adjacent scrub plots. The native seedlings, and the species in the seedfall and seed bank, were predominantly fruiting species. Two measures of grazing by sheep (accessibility to animals; and evidence of grazing on seedlings) were not significantly different between the 60 elder and 60 control plots. Nor were either of the measures of grazing correlated with seedling density or seedling height within the elder or control plots. Some significant differences in non-native vegetation and ground cover existed between the elder and control plots. In particular, the elder plots had less bracken and dense litter, but more bare ground and herbaceous species than the control plots. Seedling density was positively correlated with gorse, blackberry and bare ground cover within the elder plots. Within the control plots, seedling density was negatively correlated with bracken, and positively correlated with gorse and blackberry cover.

I conclude that the greater seedfall to the elder canopies - probably due to the attraction of frugivorous birds - was the main cause of the greater seedling densities under elder canopies compared to the adjacent vegetation. A secondary factor influencing the high seedling density under elder may be a more suitable environment, provided by the lack of competition from bracken under elder canopies, and more bare ground. Differential grazing did not appear to contribute to the greater seedling densities under elder canopies.