The distribution of epiphytic Astelia and their role in habitat formation

Abstract

The question of how abiotic and biotic filters contribute to community assembly across multiple scales is central to contemporary ecology. The stress-gradient hypothesis (SGH) predicts that as abiotic stress increases the net direction of biotic interactions will shift from negative (competitive) to positive (facilitative). Epiphytes are subject to stressful abiotic conditions, with light, water and nutrient-limitations across multiple scales; this makes epiphyte communities ideal taxa for studying community assembly. Moreover, epiphytes are important components of ecosystems as they create and modify habitats for a variety of other flora and fauna. Epiphyte assemblages in New Zealand are nested, which has been attributed to the high proportion of obligate, rather than facultative, epiphytes, and the presence of epiphytic Astelia spp. that trap litter, which may facilitate other vascular epiphytes. My thesis aimed to: (i) identify the abiotic conditions correlated with Astelia spp. distributions across three hierarchically nested spatial extents (regional, forest and host), (ii) assess whether the Astelia spp. exhibited niche differentiation at any of these scales, and (iii) whether Astelia spp. facilitated other vascular epiphytes. I employed presence-only species distribution models (SDMs) to characterise the three species distributions across New Zealand. At three forest sites in northern New Zealand representing different points along environmental gradients identified as important by the SDMs, I conducted ground-based surveys to: (i) evaluate the SDMs' predictions and (ii) assess if local-scale abiotic correlates of Astelia distributions differed from those at the regional-scale. I used drones to determine differences in radial, vertical and structural preferences in epiphytic Astelia spp. on individual hosts. In addition, I examined the associations of other vascular epiphytes with Astelia spp. along gradients of abiotic stress among forest sites, within forest sites and on individual host trees. I found evidence that epiphytic Astelia spp. differ in their drought-/shade-tolerance, with tolerance of low temperatures defining southern range limits. Similarly, at the host-scale niche partitioning was observed along vertical gradients of light/water stress on host trees. Other vascular epiphyte associations with Astelia spp. were species-specific and varied along gradients of light and water availability. These associations tended to be negative for 'pioneering' species of the epiphyte community (i.e. Pyrrosia eleagnifolia) and positive for later successional species (i.e. shrub epiphytes). These results suggest that Astelia spp. form habitat critical for the success of vascular epiphytes particularly at high abiotic stress, consistent with the SGH.