Water-filled tree holes: Freshwater in the tree tops

Tree holes are a unique and patchily-distributed habitat, providing important resources for many vertebrate and invertebrate species in both temperate and tropical environments, but very little is known about these systems in New Zealand. Previous surveys in New Zealand forests have shown a surprisingly high frequency of tree holes in native tree species, such as red beech (Nothofagus fusca), although hole size and frequency varies significantly among tree species. Despite being regularly exposed to rainwater, surprisingly few natural tree holes retain water for significant periods of time. Those that do, provide habitat for numerous species, including the endemic, forest-dwelling mosquito, Maorigoeldia agyropus. We hypothesised that tree holes in New Zealand forests provide habitat that supports unique invertebrate communities. To experimentally test our hypothesis, we placed 150 artificial water-filled tree-hole containers of varying sizes on the ground and in the canopy of mixed broadleaf-podocarp rainforest, on the West Coast, New Zealand. These were rapidly colonised by a number of invertebrate taxa, with both abundance and diversity steadily increasing over a 12 month period. Subtle height and container-size effects were detected, with some species being rarely encountered in large, canopy containers while others regularly inhabited medium-large sized containers on the ground. Similar experimental containers were placed across a 1458 m edge-gradient extending from within the forest interior out into the surrounding pasture matrix to determine the effect of forest edges on the tree-hole fauna. Preliminary results indicate a weak trend of decreasing abundance and increasing diversity from the forest interior, across the edge gradient. Emerging adult insects were also collected from these experimental water-filled containers and the similarities with those collected from naturally occurring tree holes will be discussed. These studies highlight the importance of tree holes as unique and relatively undiscovered habitat for insect communities within New Zealand's native forests.