Regeneration of Scots pine (Pinus sylvestris L.) under drought

Abstract

Drought-induced tree mortality is a phenomenon affecting many forest ecosystems and is predicted to increase under ongoing climate change. Forest stability partly depends on regeneration: the process of renewing mature forest with subsequent generations. As seedlings are more susceptible to drought effects than mature trees, mortality of the seedling bank can represent a major bottleneck controlling forest structure and species composition. Scots Pine (Pinus sylvestris L.) is the most widely distributed of the Pinus species, covering a broad latitudinal gradient of ecological conditions. The thesis aims to deepen understanding of drought-induced mortality, while analysing intra-specific variation in the phenotypic and metabolic profile of Pinus sylvestris seedlings subjected to drought stress. I also consider the relevance of the results to the broader conceptual framework of drought-induced mortality. The experiments utilise seeds from different populations of origin (provenances) across the north-south axis of the European range of Pinus sylvestris, in order to determine the extent of regeneration capacity in this species under drought. Seeds were collected from different populations (provenances) that, along with other climatic and edaphic differences, span a gradient of water availability: from wet (Scotland) to intermediate (Austria, Poland) to dry (Spain). In Chapter 2, the effects of osmotic stress on the initial seedling establishment stage were studied by comparing phenotypic responses across provenances. Seedling germination, early growth, osmotic stress tolerance and survival were investigated using a polyethylene glycol irrigation treatment as a proxy for rapid and severe drought. Treatment, provenance and interaction effects were found for rate of germination, final proportion of seeds germinated, seedling size, and superoxide dismutase activity (an antioxidant enzyme). Root investment was affected by both provenance and time to germination. Although there was no significant effect of provenance on survival, a trend towards increased probability of survival under osmotic stress was indicated for the southernmost (driest) as compared with the northernmost (wettest) provenance. Chapter 3 investigates the responses of older seedlings (at 10 months) to a drying down of soil moisture for 40 days. Morphological and physiological data were collected to assess intra-specific and intra-population variation in the seedling stress response under drought. A metabolomics analysis using Ultra performance Liquid chromatography followed by mass spectrometry (UPLC/MS) was carried out to investigate whether metabolic markers could be identified that are suggestive of heightened oxidative stress and whether populations in different climatic and edaphic environments show variation in metabolic activity under drought. Preliminary results suggest large intra-population variability yet clear differentiation in metabolic responses to drought over the time course of the experiment. Univariate and multivariate analyses indicated that among the most significant increases in response to drought were those involved in osmoprotective and antioxidant capabilities, including the free amino acid proline and a quercetin derivative (a flavonoid). Interestingly, provenances, either under experimental drought or not, did not show significantly different metabolite profiles, even though provenance and its interaction with drought treatment did significantly affect seedling biomass and photochemical efficiency. In Chapter 4 the effects of provenance, maternal parentage and seed weight on germination rate, final germination percentage, as well as seedling drought responses in biomass allocation and the expression of selected antioxidant genes were analysed. Seed weights were measured individually and seed weight was found to have a strong positive effect on: germination rate, seedling dry weights, and number of needles. Expression of two antioxidant enzymes increased under drought. Seed weight was strongly determined by provenance and maternal parentage as well as their interaction. However, root to shoot biomass allocation depended on provenance and maternal effects that were not mediated by seed weight effects. Principal component analysis indicated that the Spanish provenances could be characterised by a higher root to shoot ratio and stem weight. Specific leaf area was also found to be lowest for the Spanish provenances.