Azorella selago (Apiaceae) as a model for examining climate change effects in the sub-Antarctic

[摘要] ENGLISH ABSTRACT: There is increasing evidence that the rapid and anomalous changes in climateexperienced in the last century have had widespread ecological impacts. Indeed, sub-Antarctic Marion Island has experienced particularly large increases in temperatureand declines in rainfall. However, the effects of these changes on the island'sextensive fellfield vegetation remain largely unexamined. The aim of this study wasto examine the sensitivity of a dominant and keystone fellfield plant species, thecushion-forming Azorella selago Hook. (Apiaceae), to changes in climate. Threecomplementary approaches (two mensurate, one experimental) were used, and allshowed that A. selago is likely to change in response to further changes in climate.First, the unimodal age class distribution of A. selago suggested that the species'establishment is episodic, and therefore reliant on specific (possibly climatic)conditions. Azorella selago growth rate was related to environmental factors,suggesting that both the establishment and growth rate of the species is likely to besensitive to changes in climate. Second, altitudinal variation in A. selago plantattributes suggested that the species' morphology would be responsive to changes inclimate (assuming that a spatial gradient in climate is a suitable analogue for similarchanges in climate over time). Plant height, leaf size and trichome density differedmost consistently over altitude across the island. The altitudinal range of someepiphyte species, as well as the cover and species richness of epiphytes growing on A.selago, also showed consistent patterns along the altitudinal gradient. These cushionplant and epiphyte attributes appeared to be related to climatic factors, and aretherefore predicted to change in response to further shifts in climate. Finally, A.selago showed a rapid vegetative response to short-term experimental reductions inrainfall and increases in temperature and shading. Reduced rainfall acceleratedautumnal senescence, shortening the species' growing season. Plants were relativelyunaffected by the magnitude of warming imposed, although the foliar nutrientconcentrations of some elements were higher in warmed plants than in control plants.Experimental shading of A. selago (simulating a predicted indirect effect of climatechange: increased cover of the dominant epiphyte species, Agrostis magellanica(Lam.) Vahl (Poaceae)) caused greater stem elongation, and the production of larger,thinner leaves, with lower trichome densities and higher foliar nutrient concentrationsof some elements. Given this sensitivity of A. selago to shading, it is possible that changes in epiphyte load could overshadow the direct effects of changes in climate onthis species. Ongoing changes in climate are predicted for the next century. Based onthe results of this study the following scenarios are proposed. Continued warming anddrying of the island will potentially favour the upslope expansion of A. selago(although also shortening its growing season) and decrease the abundance of itsdominant epiphyte. Under such a scenario fellfield primary production may decline.In contrast, under warming alone, most epiphyte species could increase in abundanceand expand their altitudinal ranges upslope. This would bring about much heaviershading of A. selago plants, leading to a short-term increase in stem growth and leafnutrient concentrations. However, ultimately a decline in A. selago abundance andproduction would also be expected if cushion plants experience stem mortality underlonger-term shading. Nonetheless, monitoring A. selago leaf size, trichome densityand phenology, as well as the altitudinal range of dominant epiphyte species(attributes that this research suggests may be most sensitive to short-term changes inclimate), will indicate the biological consequences of these changes in climate. Thisstudy, therefore, shows that further climate changes on Marion Island will affect A.selago and its epiphytes, with likely repercussions for fellfield communities.

[发布机构] Stellenbosch University