Among-population variation in seed and leaf traits : implications for Western Australian banksia r.Br (proteaceae) in the context of climate change

Abstract

Plant traits are fundamental components of the ecological strategies of plants, influencing how plants acquire and use resources. Selection pressures along environmental gradients often give rise to predictable variation leading to phenotypic variability. This variability can result from genetic differences or environmentally-induced phenotypic plasticity. Phenotypic plasticity is a key mechanism for rapid adjustment to environmental heterogeneity, potentially providing species with a greater capacity to respond to global change. This thesis explored variation in commonly measured functional traits in populations of congeneric species (Banksia baxteri, B. coccinea, B. media and B. quercifolia R.Br.: Proteaceae) along a rainfall gradient in the bio-diverse but highly threatened Mediterranean-climate ecosystem of South-Western Australia. In a series of empirical experiments I investigated the response of morphological, physiological and allocational plant traits to heat and/or drought stress. My hypothesis was that populations at the warm, dry end of the gradient would respond more favourably to heat and drought stress as demonstrated by greater homeostasis of growth and performance relative to populations from the cool, wet end of the gradient. Using a temperature gradient system species differed in their temperature dimensions for germination, optimal temperatures stimulating most rapid and complete germination and the slope of germination decline above optimal temperatures. Overall, a sharp reduction in final percentage germination occurred outside the optimal temperature range, which coincided with germination delays relative to the optimum. The temperatures causing these responses varied among species and populations. Banksia media was least vulnerable to temperature stress, B. coccinea most vulnerable. Variation in tolerance to drought in three populations of the four species was assessed using different osmotic solutions. Overall, the threshold water potential value for a significant decline, and delay, in germination was -0.25 MPa. Banksia media appeared less vulnerable to germination failure under predicted changes in rainfall patterns, whilst B. coccinea seemed most vulnerable. In a common garden, warmer soils generally resulted in significant delays and reductions in seedling emergence and reduced leaf-stem biomass allocation. Banksia quercifolia showed the greatest reproductive decline under warmed conditions, but also the smallest vegetative shift; B. coccinea exhibited the smallest reproductive decline but showed a relatively large vegetative shift. Exposing B. baxteri and B. coccinea to warm/dry conditions in a glasshouse resulted in significant declines in seedling growth and fitness-related traits relative to phenotypes grown under cool/wet conditions. The data revealed the presence of divergent ecological strategies that may lead to current co-existence in these sympatric species, however, warm/dry conditions may lead to a shift in their interactions. These investigations demonstrate the species- and population-specific nature of plant responses to gradients of environmental change. Some common responses occurred across experiments: cross-species patterns generally upheld along the gradient, but decoupling of patterns occurred at the local level. Against expectations, population variation was not reliably associated with geographical location on the rainfall gradient, suggesting that selection for local adaptation in response to water availability or other climate factors has been minimal. Nonetheless, species expressing phenotypic variation along environmental gradients may have greater capacity to respond to global change.