Browsing by Author "Louw, Anche"
Now showing 1 - 1 of 1
Results Per Page
Sort Options
- ItemPlant functional types on Marion Island(Stellenbosch : Stellenbosch University, 2016-03) Louw, Anche; Smith, Valdon R.; Stellenbosch University. Faculty of Science. Dept. of Botany and ZoologyENGLISH ABSTRACT: A database was compiled of structural and physiological traits for 25 vascular species and 17 bryophyte species on Marion Island (sub-Antarctic). The structural traits included leaf, stem and root characteristics and the physiological traits are all associated with photosynthetic responses to light and were measured using chlorophyll fluorescence quenching analysis. The data were subjected to principal component analysis and clustering analysis to construct a suite of plant functional types (PFTs). The correspondences between the PFTs and plant habit/taxonomy (forb, graminoid, fern, moss and liverwort), status (native to the island or introduced alien) and habitat (oligotrophic, manured or saline) were investigated using correspondence analysis. There were significant differences in most of the structural traits, but in only one of the photosynthetic traits, between sites at the same altitude and between altitudes. The between-site differences could not be explained since site characteristics were not measured, but the between site differences were often species-dependent; a particular species might show its lowest value for a particular trait at the same site where another species showed the highest value for that trait. The between-altitude differences in structural trait values could be ascribed to the effect of greater wind speed at higher altitude (lower stature, tougher leaves and stems). High altitude plants have greater specific root length, probably a response to low soil nutrient status and hence a need for foraging roots. However, plants in saline coastal habitats also show a high specific root length, probably in response to the inhibitory effect of high salt concentration on nutrient and water uptake. All the species except Azorella selago, the archetypical vascular species of high altitude were more stunted and showed greater signs of stress at high altitude than at low altitude. Native species tend to show greater values for those traits indicative of structural strength (tough, thick leaves, strong stems), and allocate a greater proportion of their biomass aboveground, than human-introduced alien species. Alien graminoids also have higher stomatal densities (but lower chlorophyll concentrations on a leaf area basis) than native species. There are no consistent differences in photosynthetic capacity between natives and aliens, except that native species tend to show a sharper photosynthetic response to increasing light at low levels, possibly an adaptation to the consistently low light regime at the island. Maximum photosynthetic electron transport rate varies greatly (by an order of magnitude) between the island’s vascular species and by almost an order of magnitude between the bryophyte species. Species with high electron transport rate also tend to have a high effective quantum yield and show electron transport saturation at high light – all indicative of high photosynthetic capacity sun species, most of which are forbs. The shade-adapted, lowest photosynthetic capacity species are mostly ferns, mosses and liverworts. Graminoids tend to be moderate photosynthetic capacity species but some of the forb species also have moderate, and even low, photosynthetic capacity. Only two structural traits were measured on bryophytes so in grouping both the vascular and bryophyte species together into functional types only the photosynthetic traits were considered. This yielded eight photosynthetic functional groups, together representing three levels of photosynthetic capacity - high, moderate or low capacity. Forbs predominate in the two highest photosynthetic capacity groups, graminoids (with one hepatic) in the moderate to moderately high capacity groups, graminoids, ferns and some bryophytes in the low to moderate capacity groups and bryophytes and shade-adapted ferns in the very low to low capacity groups. Mosses tend to have a higher capacity than hepatics although four of the six species in the very lowest capacity group are mosses. At each level of photosynthetic capacity – the high, moderate or low capacity groups – the species are divided into subgroups based on their capability for photoprotection at high light and how sharply photosynthesis responds to increasing light at low levels. Previous studies at the island have unequivocally shown that habitats influenced by seal and seabird manuring have higher soil and plant nutrient status, greater plant vitality and higher productivity. It was expected that species restricted to, or that attain maximum cover in manured habitats would be in the group with highest photosynthetic capacity. However, a surprising finding of the study is that those species occur in all the photosynthetic capacity groups except the highest capacity one. Further research into plant functional traits and functional types at the island should consider phenological, reproductive, and a wider suite of physiological, traits, especially the temperature responses of, and desiccation effects on, photosynthesis, photorespiration and respiration. Since wind is such a dominant factor at the island, its effects on plant morphology, architecture, growth and physiology also need to be addressed.