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

Le Roux, Peter Christiaan (2004-12)

Thesis (MSc)--Stellenbosch University, 2004.

Thesis

ENGLISH ABSTRACT: There is increasing evidence that the rapid and anomalous changes in climate experienced in the last century have had widespread ecological impacts. Indeed, sub- Antarctic Marion Island has experienced particularly large increases in temperature and declines in rainfall. However, the effects of these changes on the island's extensive fellfield vegetation remain largely unexamined. The aim of this study was to examine the sensitivity of a dominant and keystone fellfield plant species, the cushion-forming Azorella selago Hook. (Apiaceae), to changes in climate. Three complementary approaches (two mensurate, one experimental) were used, and all showed 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 be sensitive to changes in climate. Second, altitudinal variation in A. selago plant attributes suggested that the species' morphology would be responsive to changes in climate (assuming that a spatial gradient in climate is a suitable analogue for similar changes in climate over time). Plant height, leaf size and trichome density differed most consistently over altitude across the island. The altitudinal range of some epiphyte species, as well as the cover and species richness of epiphytes growing on A. selago, also showed consistent patterns along the altitudinal gradient. These cushion plant and epiphyte attributes appeared to be related to climatic factors, and are therefore predicted to change in response to further shifts in climate. Finally, A. selago showed a rapid vegetative response to short-term experimental reductions in rainfall and increases in temperature and shading. Reduced rainfall accelerated autumnal senescence, shortening the species' growing season. Plants were relatively unaffected by the magnitude of warming imposed, although the foliar nutrient concentrations of some elements were higher in warmed plants than in control plants. Experimental shading of A. selago (simulating a predicted indirect effect of climate change: 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 concentrations of 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 on this species. Ongoing changes in climate are predicted for the next century. Based on the results of this study the following scenarios are proposed. Continued warming and drying of the island will potentially favour the upslope expansion of A. selago (although also shortening its growing season) and decrease the abundance of its dominant epiphyte. Under such a scenario fellfield primary production may decline. In contrast, under warming alone, most epiphyte species could increase in abundance and expand their altitudinal ranges upslope. This would bring about much heavier shading of A. selago plants, leading to a short-term increase in stem growth and leaf nutrient concentrations. However, ultimately a decline in A. selago abundance and production would also be expected if cushion plants experience stem mortality under longer-term shading. Nonetheless, monitoring A. selago leaf size, trichome density and phenology, as well as the altitudinal range of dominant epiphyte species (attributes that this research suggests may be most sensitive to short-term changes in climate), will indicate the biological consequences of these changes in climate. This study, therefore, shows that further climate changes on Marion Island will affect A. selago and its epiphytes, with likely repercussions for fellfield communities.

AFRIKAANSE OPSOMMING: Daar is toemende bewys dat die vinnige en onreëlmatige veranderinge in klimaat oor die laaste half-eeu wye ekologiese gevolge gehad het. Inderdaad, sub-Antarktiese Marion Eiland het 'n ook 'n besondere groot toename in temperature en daling in reënval ervaar. Nogtans is die gevolge van hierdie veranderinge op die eiland se uitgebreide dorveld (fellfield) plantegroei nog nie nagevors nie. Die doel van hierdie studie was om die sensitiwiteit van 'n dominante hoeksteen spesie, die kussingvormige Azorella selago Hook. (Apiaceae), aan veranderinge in klimaat te ondersoek. Drie aanvullende metodes (twee waarnemend, een eksperimenteel) was gebruik, en al drie het aangedui dat A. selago waarskynlik sal reageer op verdere veranderinge in klimaat. Eerstens, die enkelpiek-vormige ouderdomsverspreiding van A. selago dui daarop dat die spesie ongereeld vestig, en is daarom afhanklik van spesifieke (dalk klimatiese) toestande. Verder, was A. selago se groeitempo aan omgewingsfaktore verwant. As gevolg hiervan sal die spesie se vestiging en groeitempo vermoedelik sensitief vir klimaatsveranderinge wees. Tweedens, veranderinge in A. selago eienskappe met 'n toename in hoogte bo seespieël (hoogte) dui daarop dat die spesie se morfologie sal reageer op veranderinge in klimaatstoestande (op voorwaarde dat 'n ruimtelike verandering in klimaat goed ooreenstem met 'n soortgelyke verandering in klimaat oor tyd). Planthoogte, blaaroppervlakte en trigoomdigteid het geleidelik met hoogte verander oor die eiland. Die verspreiding en bedekking van sommige epifitiese spesies, asook epifiet spesie rykheid, was ook aan hoogteverwant. Hierdie verwantskap tussen A. selago (en die epifiete) en hoogte is vermoedelik deur klimatiese faktore veroorsaak, en daarom word voorspel dat dit sal verander soos die klimaat verander. Laastens, het A. selago 'n vinnige vegetatiewe reaksie tot korttermyn eksperimentele vermindering in reënval en toename in temperatuur en beskaduwing gewys. 'n Afname in reënval het blaarveroudering versnel, en dus A. selago se groeiseisoen verkort. Plante het min verander as gevolg van hoër temperature, alhoewel die konsentrasie van sommige plantvoedingstowwe hoër was in blare van verwarmde plante as in die wat gewone temperature ervaar het. Eksperimentele beskaduwing van A. selago (wat 'n verwagde indirek effek van klimaatsverandering naboots, naamlik die toename in bedekking van A. selago deur die dominante epifiet spesie, Agrostis magellanica (Lam.) Vahl (Poaceae)) het stingel groei versnel, en veroorsaak dat groter en dunner blare met laer trigoomdigthede en hoër konsentrasies van sommige plantvoedingstowwe op die plante groei. As gevolg van die sensitiwiteit van A. selago op beskaduwing, is dit moontlik dat die gevolge van veranderinge in die bedekking van epifiete belangriker sal wees as die direkte gevolge van klimaatsverandering. Verdere klimaatsveranderinge word vir die volgende eeu voorspel. Gebasseer op die resultate van hierdie navorsing, word twee moontlike toekomstige omstadighede voorgestel. Toenemende verwarming en verdroging van die eiland sal vermoedelik veroorsaak dat A. selago op hoër hoogtes voorkom (alhoewel die spesie se groeiseisoen ook sal verkort), en dat die volopheid van A. magellanica sal afneem. In so 'n geval sal dorveld se plantproduksie waarskynlik effens verminder. In teenstelling, as die eiland slegs verwarm (sonder 'n verandering in reënval) kan die volopheid en verspreiding van epifiet spesies waarskynlik toeneem. Dit sal vermoedelik tot 'n toename in the verskaduwing van A. selago lei, wat tot 'n kort-termyn verhoging van stingel groeitempo en plantvoedingstof konsentrasies sal lei. Alhoewel, uiteindelik, word 'n vermindering van A. selago volopheid en groei verwag as plantstingels van lang-termyn beskaduwing vrek. Nietemin, as die blaargroote, trigoomdigteid en groeiseisoenlengte van A. selago en die hoogte verspreiding van die dominante epifiet spesie gemonitor word (eienskappe wat deur hierdie studie aangedui is as gevoelig aan kort-termyn veranderinge in klimaat), kan die biologiese gevolge van hierdie klimaatsveranderinge aangewys word. Hierdie navorsing bewys dus dat verdere veranderinge in klimaat op Marion Eiland 'n invloed sal hê op A. selago en geassosieerde epifiete, met moontlike gevolge vir die hele dorveld gemeenskap.

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