Larger investment in roots in southern range-edge populations of Scots pine is associated with increased growth and seedling resistance to extreme drought in response to simulated climate change

Abstract

Extreme climate events such as heat waves or severe drought have the potential to fundamentally alter species dynamics, and are expected to increase in frequency in the coming decades. Early life stages of plant species are highly sensitive to these variations, with the potential for profound consequences for plant communities. In order to explore the response of Scots pine (Pinus sylvestris L.) seedlings to predicted increases in temperature and extreme drought, we performed a controlled-conditions experiment using growth chambers to simulate 1) current and future temperatures expected at the southernmost edge of the range, 2) current and drier growing seasons before an acute summer drought, and 3) drought alleviation by sporadic rainy events during summer. We analysed the response of seedlings by assessing survival, growth, biomass allocation and isotopic discrimination.Southern range-edge seedlings were compared under same conditions with those from the northern range edge, which experience cooler and wetter conditions throughout the year.The combination of extreme drought and the predicted temperature rise severely reduced overall survival for both provenances. A reduction in precipitation before the onset of drought diminished the survival and final biomass of seedlings, while the interruption of summer drought drastically increased survival probabilities. Southern seedlings invested a higher proportion of their biomass in roots, which conferred on them higher growth, higher survival probabilities, better nutritional status and lower drought stress. These results help us to understand the mechanisms of local adaptation at the southern range edge and indicate southern populations as a valuable genetic resource to buffer the response of Scots pine against such extreme climatic events.

Morphological and physiological responses to drought stress of European provenances of Scots pine

“…Provenance comparisons crossed with a controlled water-stress experiment represent therefore a very powerful approach to detect population-level differences in drought susceptibility. In a greenhouse experiment conducted in Germany, provenances from central Italy and Spain were genetically differentiated from populations from central Europe and showed smaller reductions in shoot growth in response to drought conditions and a more plastic behaviour (Taeger et al 2013(Taeger et al , 2015, a result paralleled by a comparison of Spanish rear-edge populations against Finnish front-edge populations in a greenhouse study in Scotland (Matías et al 2014). In a greenhouse comparison of 12 interior and coastal provenances, central Asian seed sources from dry sites survived longer under drought, grew less, had greater biomass allocation to roots and higher water use efficiency than more mesic central European seed sources, a clinal longitudinal gradient possibly driven by continentality (Cregg and Zhang 2001).…”

“…Havaux et al 1988;Reich et al 1996;Oleksyn et al 2003;Bigras 2005). However, only a few have examined the interaction of intra-specific genetic variability with other stress factors, such as drought (in Scots pine, Taeger et al 2013Taeger et al , 2015Matías et al 2014;Seidel and Menzel 2016). Our drought stress treatment was carried out with the objective of exposing plants to a period of prolonged water stress.…”

“…While Cregg and Zhang (2001) could not find any drought-induced variation in mortality rates of P. sylvestris seedlings from several Eastern European and Central Asian origins, other studies found substantial population differences in survival rates after droughts. In one case (Matías et al 2014), population-level differences in drought-induced mortality rates were explained by differences in allocation to above-versus below-ground tissues. In a second study (Seidel and Menzel 2016), provenance differences in probabilities of survival after drought were found to remain significant even after accounting for size differences.…”

“…In Scots pine, structural acclimation to drought involving increased biomass allocation to roots relative to shoots (Taeger et al 2013(Taeger et al , 2015Matías et al 2014;Seidel and Menzel 2016) is complemented by tight stomatal control of plant transpiration and photosynthesis and regulation of the ratio between leaf and sapwood areas (Mencuccini and Grace 1995;Poyatos et al 2007;Martínez-Vilalta et al 2009). Nonetheless, mortality of Scots pine has been observed at the xeric border of its distribution in the Mediterranean area, e.g.…”

Integrating plant ecological responses to climate extremes from individual to ecosystem levels

“…There is also increasing evidence for high intraspecific variability in species responses to extremes, which has been observed to be of equal or even greater magnitude than interspecific variation [51]. High intraspecific variability may be influenced by ecotypic and genetic variation within a species ( [37,52,53], but see [54]). High intraspecific trait variation has thus been posited to contribute to post-extreme shifts in community-weighted trait means irrespective of gains or losses of species from the community [55].…”