The hydrological and ecophysiological effects of simulated climate warming on the soil and vegetation of the Nama Karoo

Thesis (MSc)--Stellenbosch University, 2018.ENGLISH ABSTRACT: Climate change is increasingly recognised as threat to individual species performance and survival. Manipulative field experiments, such as warming experiments, are crucial to understanding the potential future effects of a warming climate on plant function and survival. In warm, semi-arid ecosystems, increasing temperatures are expected to have detrimental effects on plant growth and survival due to exceedance of optimum temperatures for growth and functional processes such as photosynthesis. However, such detrimental effects may not be felt by all plant functional types. Most notably, C3 and C4 species are expected to respond in different ways; C4 species are expected to show a more positive growth response to increasing temperatures than are C3 species. Experimental studies globally have found variable effects of warming on plants, but relatively little is known about how warm, semi-arid ecosystems will respond to warming, especially in southern Africa. Many regions of the world are expected to become drier under climate change but the prediction of future rainfall changes has far higher uncertainty than the prediction of future temperatures. I investigated the effect of warming on plant growth by establishing a simulated climate warming experiment near Middelburg in the Nama Karoo (Eastern Upper Karoo, Mucina and Rutherford 2006), a warm, semi-arid region with vegetation comprised of both C4 grass and C3 shrub species. A variety of warming systems have been used for experimental manipulation in the field, but the most commonly used is the open-topped chamber (OTC) system. I designed an experiment using a modified version of an OTC system first employed by Godfree et al (2011) in central NSW Australia, using water-filled PVC pipes, painted black, as a thermal buffer within the chambers. Six control and six heated plots were set up, and growth of a selected shrub species (Eriocephalus ericoides (L.f.) Druce) and grasses was analysed during a single growing season, and up until the end of the dry season. Soil water content and plant water potential measurements were made to determine the effect of warming on the soil and Eriocephalus ericoides water balance in treatment plots. The OTC system successfully raised temperatures during the day and night, with mean daytime increases of 0.89°C and night-time increases of 1.03°C over the treatment period. The addition of top panels during the winter months would likely have attributed to the continued increase of temperatures during these cold months, and the system likely reduced frost incidence and severity in warmed plots. The diurnal warming pattern was more even during winter than during summer months. There did not appear to be a rain shadow effect due to the chambers, except for one incidence in July. Warmed Eriocephalus ericoides had higher growth rates than control plots when soil moisture content was above 3.55%. Warming had no impact on grass growth, perhaps due to decreased water availability in treatment plots prohibiting the grasses from responding to higher temperatures. Daily measures of volumetric soil water content revealed significantly lower soil water content in heated plots, likely due to increased evapotranspiration. Values of gravimetric soil water content, measured monthly, differed between treatment and months (water content was lower in warmed plots) while plant water potential sampled at the same times showed no effect of warming treatment but also differed over time. Soil water content and shrub water potential were positively related in a curvilinear relationship best described by a polynomial function that suggested maximum shrub water potential above gravimetric soil moisture levels of 5%. Overall, both the positive effect of warming on the growth of the C3 shrub Eriocephalus ericoides during the growing season and the lack of response of the C4 grasses were unexpected. For Eriocephalus ericoides, this suggests that the Middelburg site temperatures may be sub-optimal for growth, highlighting the need for better projections regarding the climate change impacts on the woody shrub flora of this region. It is possible that low rainfall prohibited any response to warming in the C4 grass component, indicating a need for further manipulative experiments including water addition in order to better understand this response. Future warming in this region may well lead to greater ecological success of C3 shrubs in this region; but if this is combined with increased rainfall in this region, as some models have projected, the response of the C4 grasses remain uncertain. The use of a passive OTC system with water-filled PVC pipes as a thermal buffer provided an inexpensive yet effective method of applying warming in the field, with the only suspected experimental artefact due to a reduction in wind speed. Further work in this region could attempt to quantify the effect of this artefact on plant growth, possibly via the use of physical wind-breaks.AFRIKAANSE OPSOMMING: Klimaatsverandering is ‘n voortdurende bedreiging vir spesie funksie en voortlewing, en eksperimentele studies is ‘n belangrike agent om toekomstige gevolge van klimaatsverwarming op die voortlewing en funksionering van plante te verstaan. In warm, droë ekosisteme word dit verwag dat verwarming ‘n negatiewe effek op plantegroei sal hê as gevolg van ‘n oorskreiding van optimale temperature vir fotosintese en groei. Hierdie effekte sal moontlik nie vir alle plant funksionele tipes realiseer nie. Dit word veral verwag dat C3 en C4 spesies nie op dieselfde manier sal reageer nie. Die verwagting is dat C3 plantegroei nie dieselfde grootte reaksie tot temperatuur stygings as C4 spesies sal toon nie. Wêreldwye eksperimentele studies vind wisselvallige effekte van verwarming op plante, maar daar is relatief min inligting beskikbaar oor hoe warm, droë eskosisteme sal reageer op klimaatsverwarming, veral in suidelike Afrika. Daar word verwag dat baie streke van die wêreld droër sal word as gevolg van klimaatsverandering, maar die voorspelling van hierdie veranderinge is meer onseker as die voorspelling van toekomstige styging van temperature. ‘n Klimaatsverwarming eksperiment is in ‘n area van die Nama Karoo met C4 grasse en C3 struike opgestel om die effek van verwarming op plantegroei te ondersoek. Vele verwarmingssisteme is al in die verlede gebruik, maar die mees gewilde sisteem is die kas waarvan die bokant nie bedek is nie. Ses verwarmde en ses kontrole gedeeltes is in die veld uitgesit en die groei van die grasse en ‘n teiken struik (Eriocephalus ericoides (L.f.) Druce) is geanaliseer vir twee maande van die groeiseisoen en vier verdere maande tot aan die einde van die droë seisoen. Grondwaterinhoud- en plantwaterpotensiaalmetings is geneem om die effek van verwarming op die waterbalans van die grond en plante van die eksperimentele terrein te ondersoek. Die omgewing binne die kas is bedags en snags suksesvol verwarm; gemiddelde dag temperature was 0.89°C hoër binne die kaste en gemiddelde nag temperature was 1.03°C hoër binne die kaste regdeur die eksperimentele tydperk. Die bybevoeging van die boonste panele het waarskynlik bygedra tot die aanhoudende verwarming gedurende die kouer maande en het waarskynlik die intensiteit en voorkoms van ryp binne-in die kaste verminder. Die daaglikse patroon van verwarming is meer eweredig gedurende die winter as gedurende die somer, en dit lyk nie asof daar ‘n reën-skadu effek is nie, behalwe vir een insident in Julie. Verwarmde struike het vinniger gegroei wanneer die grondwaterinhoud hoër is as 3.55%, maar die verwarming het geen effek op die groei van die grasse gehad nie, dalk omdat laer grondwaterinhoud tydens die groeiseisoen die grasse verhinder om op warmer temperature te reageer. Daaglikse metings van volumetriese grondwaterinhoud het ‘n betekenisvolle verskil tussen verwarmde en kontrole gedeeltes getoon, waarskynlik as gevolg van hoër verdamping en transpirasie. Maandelikse metings van grafimetriese grondwaterinhoud het ‘n verskil tussen verwarmde en kontrole gedeeltes asook ‘n verskil tussen verkillende maande getoon, terwyl maandelikse metings van plantwaterpotensiaal net verskil het tussen maande. Grondwaterinhoud en plantwaterpotensiaal was positief verwant in ‘n kromlynige verhouding wat dui op maksimale plantwaterpotensiaal wanneer grondwaterinhoud waardes hoër is as 5%. Algeheel was beide die reaksie van die C3 struik en die gebrek van ‘n reaksie van die C4 grasse onverwags. Die reaksie van E. ericoides dui aan dat die groei van hierdie struik gedeeltlik beperk word deur sub-optimale temperature tydens die groeiseisoen en dat meer inligting nodig is om die toekomstige reaksie van houtagtige struike in die Nama Karoo te kan verstaan. Vir die gras komponent dui dit aan dat verdere navorsing nodig is om die verwantskap tussen groei en waterbeskikbaarheid te verstaan. Toekomstige klimaatsverwarming in hierdie streek kan dalk tot hoër ekologiese sukses van C3 struike in hierdie omgewing lei, maar as verwarming met verhoogde reënval saamval, is die reaksie van die C4 grasse ‘n belangrike onbekende. Die gebruik van ‘n passiewe verwarmingsisteem met ‘n termiese buffer bied ‘n lae-tegnologie en goedkoper manier om temperature in die veld te verhoog, met die effek van windskuiling as die enigste vermoede artefak. Verdere studie in hierdie tipe ekosisteem kan lei tot die kwantifisering van hierdie artefak, moontlik deur fisiese windskuilings te gebruik