Regeneration of Scots pine (Pinus sylvestris L.) under drought
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Date
28/11/2016Author
MacAllister, Sarah Louise
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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.