Population Structure, Spatial Pattern and Seedling Establishment of the Grey Mangrove, Avicennia marina var. australasica, in New Zealand

Abstract

Avicennia marina (Forsk.) Vierh. var. australasica (Walp.) Moldenke occurs as monospecific stands in the North Island of New Zealand. Using 5 m wide strip transects, A. marina adult plants and seedlings were mapped out in eight distinctive mangrove forests. All forests showed reversed ‘J’ size distributions of adult plants. In most cases, seedling density increased with increasing distance from the seaward edge of the mangrove forests. Mean plant size and density with respect to tidal position showed no consistent pattern across forests. Plant size seemed correlated with latitudinal gradient, with taller trees in relatively warmer regions and shorter, stunted, dwarf-like types in colder areas. The survival and growth of (a) naturally occurring seedlings in and out of tree-fall gaps and at various distances from the seaward edge of the forest, and (b) transplanted seedlings of different sizes (small, medium and large propagules: ≤ 10 cm, 10–20 cm and 21–40 cm tall, respectively) and densities (2, 5 and 9 seedlings m^-2) in three delineated (low, mid and high) intertidal positions were monitored over 18-month periods. Establishment of both natural and transplanted seedlings varied significantly between locations, canopy light conditions, intertidal levels and seedling sizes, but not with density. Generally, seedling survival and growth were better in gaps than under closed canopy, irrespective of tidal position. Intertidal level significantly affected survival of transplanted seedlings, but did so only marginally for the natural ones, with survival being greatest in the high intertidal zone. By contrast, increases in plant height and leaf production were best promoted in the low intertidal position of the forest floor. Survival of the transplanted seedlings was of the order: large > small ≥ medium-sized. However, the small seedlings grew best both in terms of height and accumulation of new leaves. These patterns of survival and growth were consistent across intertidal position and seedling density. The implication of these results for the conservation and possible restoration of mangrove forests, especially in New Zealand, are discussed.