Response of different plant functional types to environmental variability on Marion Island : quantifying diurnal patterns over a seasonal cycle using the photochemical reflectance index

[摘要] ENGLISH ABSTRACT: Marion Island forms part of the Prince Edward Island group, situated near the latitude of 47° South. Seasonal and temporal variability in climate on Marion Island has been described as muted, due to the thermal buffering of the surrounding ocean. This is thought in turn to lead to an extended growing season. However, empirical in situ measurements of net primary production (NPP), are lower than estimations based on annual temperature and precipitation.The aim of this study was to explore which potentially limiting environmental factors exert control over photosynthetic behaviour at a range of sub-annual temporal scales, in order to better understand what limits production in plant functional types (PFTs) typical of Marion Island. The three main PFTs selected for study were lower plants, cushion plants and grasses.Spectral reflectance sensors were used in situ to investigate the diurnal and seasonal patterns of physiological stress and inferred photosynthetic behaviour. The Photochemical Reflectance Index (PRI) is calculated from a ratio of reflected versus incoming light wavebands, that are influenced by a change in carotenoid ratios, indicating photosynthetic efficacy through the activity of the xanthophyll cycle. The xanthophyll cycle is closely linked to photosystem II and thus an important component of the non-photochemical quenching (NPQ) process that acts as a photo-protection mechanism.PRI measurements require careful interpretation in the absence of any independent confirmatory measurements. Repeated ancillary measurements of leaf chlorophyll fluorescence and leaf chlorophyll content via independent instrumentation provided support for the PRI measurements as an indicator of physiological stress. This approach was also used to confirm that the point monitoring of individual canopies was representative of surrounding vegetation.Contrary to the assumption that climate variability is muted, fine temporal scale monitoring revealed remarkably high temporal climate variability on Marion Island. Although seasons sensu stricto could not be clearly defined, a shift in climate can be seen between 'winter and 'summer months, most notably by a replacement of cold, calm days by warm, windy days. PRI data revealed that different PFTs (and to an extent, individual species) showed somewhat distinct optimum growing seasons, with the seasonal shift in climate affecting PFTs differently.The three main PFTs showed distinct PRI patterns. Lower plants showed the deepest daily PRI depression, almost regardless of environmental conditions, confirming for thefirst time over an entire annual cycle their previously proposed low light adaptive characteristics. Cushion plants only showed a midday PRI depression on days withhigh temperatures, revealing their optimal adaptation to cooler diurnal conditions. Grasses had the highest PRI values, responding positively on days with higher temperatures, and revealing their more efficient performance under warmer and brighter conditions, in distinct contrast to the other two PFTs.Environmental drivers of stress varied significantly between PFTs. Lower plants were strongly influenced by moisture regimes and experienced significant stress during days of decreased habitat moisture levels. Cushion plants experienced less stress in colder temperatures, and responded positively to environmental variables that decreased canopy temperatures. Plant responses to changes in environmental variables were also clearly reflected in the seasonal PRI measurements. Grasses showed a decreases in stress during the 'summer months, while cushion plants experienced significantly more stress during the 'summer months. The lower plant species did not have a significant decrease or increase in PRI measurements between the different 'seasons.There is therefore no common or general driver of diurnal or seasonal stress response across different PFTs and species on Marion Island. This suggests that distinct PFTs would respond differentially as the climate regime continues to shift on Marion Island due to anthropogenic climate change. The in situ approach shows great promise for unlocking a deeper understanding of the environmental controls on this extraordinary ecosystem. The techniques described in this thesis would provide an extremely valuable set of tools to achieve this relatively inexpensively, while also providing a detailed picture of how this ecosystem is responding to climate change.

[发布机构] Stellenbosch University