Patterns of variability in Azorella selago Hook. (Apiaceae) on sub-Antarctic Marion Island : climate change implications

[摘要] ENGLISH ABSTRACT: Understanding the responses of species to climate change is a scientificproblem that requires urgent attention, especially under current conditions ofglobal climate change. The large and rapid rates of climate change reportedfor sub-Antarctic Marion Island makes the island highly suitable for studyingthe biotic consequences of climate change. Furthermore, the extremeenvironments on the island result in a close coupling of the biotic (e.g.population dynamics) and abiotic (e.g. climate) factors. Therefore, examiningthe response of the dominant and keystone plant species on the island,Azorella selago Hook. (Apiaceae), to climate-associated environmentalchange (e.g. temperature) may provide insight into how A. selago and theassociated species communities will be affected by climate change. Thisstudy described the variability in microclimate temperatures associated with A.selago across altitudinal gradient and between the eastern and western sidesof Marion Island. Microclimate temperatures were also compared to theisland's Meteorological data to determine variation between temperaturesexperienced by A. selago cushion-plants in the field and those recorded at theisland's Meteorological Station. Temperature variation inside and outside A.selago cushions was also examined. Azorella selago cushions were found tohave a buffering effect on temperature, such that species occurringepiphytically on A. selago experience more moderate temperatures than thesurrounding environment. However, A. selago were found to experience moreextreme temperatures than temperatures recorded at the MeteorologicalStation. Therefore, A. selago may possibly experience greater environmentalwarming than recorded by the Meteorological Station. While temperaturesdecline with altitude, temperature conditions on the western side of the islandwere more temperate than the eastern side. This presents the first record oftemperature conditions on the western side of the island. This study alsoquantified fine-scale (e.g. within-site) and broad-scale (e.g. island-wide)variability patterns of A. selago (morphology, phenology, and epiphyte load)across Marion Island. Altitudinal gradient and climatic exposure at differentsides of the island were used to understand the likely effects of climateassociated environmental change on this dominant component of the fellfield habitat. Site-specific processes were found to determine the spatial structureof A. selago characteristics at fine-scales. However, broad-scale observationsestablished strong responses of A. selago characteristics to altitudinalgradients and different sides of the island. Azorella selago morphologicalfeatures (e.g. plant size and leaf size) were found to be more responsive todifferences between the eastern and western sides of the island than toaltitudinal gradient. Azorella selago micro-morphological features (e.g. leaftrichomes and stomatal densities) were also found to be more responsive toclimatic exposure at different sides of the island than to altitudinal gradient.However, differences in A. selago epiphyte density (e.g. Agrostis magellanica)and phenology resembled microclimate temperatures in that they were moreresponsive to altitudinal gradient than to side of the island differences. Fromthese results it can therefore be predicted that the A. selago of Marion Islandis likely to be morphologically fairly resilient to moderate climatic shifts,although at lower altitudes and on the eastern side of the island, it may beoutcompeted by the epiphytic grass, Agrostis magellanica. The results alsosuggest that the warming climate of Marion Island may result in an earlyoccurrence of phenological processes particularly at lower altitudes and theeastern side. Azorella selago at lower altitudes and on the eastern side ofMarion Island are therefore expected to largely show more symptoms ofclimate change (e.g. warming) on this species. Azorella selago is alsopredicted to move up altitudinal gradients in response to warming.

[发布机构] Stellenbosch University