The interaction between a keystone plant species and its dominant epiphyte on Marion Island : climate change implications

[摘要] ENGLISH ABSTRACT: Climate has shown some remarkable changes over the past century, especially at thepolar and sub-polar regions. Southern Ocean Islands provide good models for studiesrelated to climate change effects, since effects may be evident in the short term and mayalso be clearer. Marion Island is an example of such a system with a harsh abioticenvironment, and low species richness often vulnerable to change. Climate change ispredicted, and also reported, to have biological consequences on plant communities,affecting the phenology, morphology, and the interaction between individuals andspecies. This study examines the association between the keystone plant species, Azorellaselago Hook. (Apiaceace), and its dominant epiphyte Agrostis magellanica Lam.(Poaceae). Two complimentary approaches were used, one observational and the otherexperimental. The main objective for the observational study was to quantify bioticallyrelevantmicroclimate temperature, as well as the morphology, epiphyte load andphenology of A. selago at three different altitude sites on Marion Island. This providedinformation on baseline variation for understanding specific variability in plant responseto the experimental part of this study, against which future patterns arising frombiological monitoring can be compared. Studying plants at different altitudes provides apossible analogue for temperature-related climate change consequences for the ecologyof A. selago, and its interaction with A. magellanica. The microclimate temperatureassociated with A. selago differed between the three sites examined. This difference wasrelated to local topographic conditions and altitude differences. Cushion size differeddistinctively between the three altitude sites, with this difference related to environmentalheterogeneity such as differences in age and substrate structure. Azorella selago annualgrowth rate was estimated through stem length and the number of leaves on both exposedand shaded stems. Within-site variability, as well as epiphyte cover were found to be thecontributing factors on A. selago annual growth rate. This highlights the importance ofsite-specificity when estimating growth rate within and between different altitude sites.Leaf characteristics differed between the sites, with this attributed to local habitatconditions, such as topography, as well as epiphyte cover. As expected, the difference inleaf size between exposed and shaded leaves demonstrated a larger specific leaf area on leaves shaded by A. magellanica. Agrostis magellanica abundance and density werealtitudinally related, with A. selago demonstrating facilitation effects on A. magellanica.The trend shown in this study suggests that in spite of general facilitative effect of A.selago on A. magellanica towards higher altitudes, the abiotic environmental thresholdfor A. magellanica occurs at lower altitudes than it does for A. selago. Phenologicaldifferences were also apparent between the three sites. The objectives of the experimentalpart of this study were to quantify the effect of the dominant epiphyte, A. magellanica onbiotically-relevant microclimatic temperatures, as well as on the phenology and physicalcondition of A. selago. Different treatments were applied to cushions at the three altitudesites to examine the shading effect of epiphytic A. magellanica on cushion plants, as wellas the effect of treatment-related disturbance. Azorella selago microclimate temperatureshowed no significant difference between treatments, suggesting that on averageepiphytic A. magellanica cover has no effect on cushion microclimate temperature. Thepercentage of flower budding and flowering of A. selago was negatively related toepiphyte cover. Cushion vitality was also responsive to epiphyte cover, with highervitality scores on low grass covered cushions than on high grass cushions. This showsthat A. magellanica competes with A. selago, while A. selago facilitates A. magellanica.Heavy epiphyte numbers impose negative effects on A. selago vegetative andreproductive performance, as well as cushion vitality. Therefore, the results of thisresearch show that the vegetative and reproductive performance of A. selago and cushionvitality are likely to be negatively affected under ongoing climate change on MarionIsland if this brings about heavier epiphyte loads on this keystone cushion plant species.

[发布机构] Stellenbosch University