Phenotypic plasticity versus ecotypic adaptation to recurrent summer drought in two drought-tolerant pine species

Christoph Bachofen, Swiss Federal Institute for Forest, Snow and Landscape Research WSL, Lausanne, Switzerland; School of Architecture, Civil and Environmental Engineering ENAC, École Polytechnique Fédérale de Lausanne EPFL, Lausanne, Switzerland, Anouchka Perret-Gentil, Swiss Federal Research Institute for Forest, Snow and Landscape Research WSL, Birmensdorf, Switzerland, Thomas Wohlgemuth, Forest Dynamics, Swiss Federal Research Institute for Forest, Snow and Landscape Research WSL, Birmensdorf, Switzerland, Pierre Vollenweider, Swiss Federal Institute for Forest, Snow and Landscape Research WSL, Birmensdorf, Switzerland and Barbara Moser, Disturbance Ecology, Swiss Federal Research Institute for Forest, Snow and Landscape Research WSL, Birmensdorf, Switzerland

Background/Question/Methods
Despite worldwide reports of high tree mortality, growing evidence indicates that many tree species are well adapted to survive repeated dry spells. The drought resilience of trees is related to the phenotypic plasticity and ecotypic variation of functional traits. However, there is still little evidence on which combination of traits allows trees to acclimate to specific drought stress situations.
We assessed the phenotypic plasticity of seedlings grown for three years in a common garden and subjected to repeated summer drought by analysing shoot and needle morphology, needle anatomy and foliar macronutrients. We determined the ecotypic variation of these traits by comparing seedlings of four populations of Pinus sylvestris and Pinus nigra that originated along a gradient of growing season water deficit (GSWD) in Central and Southern Europe.
Results/Conclusions
Foliar phenotypic plasticity in response to recurrent summer drought was surprisingly low, with the needle length and the fraction of mesophyll and phloem tissue adjusting to some extent. Predominant responses to the drought stress included shorter apical and lateral shoots, both of which correlated with GSWD at the seed origin. In contrast, between-population variation of foliar morphological, anatomical and chemical traits was unrelated to the GSWD at the seed origin.
Conclusions: High phenotypic plasticity of shoot morphology allowed both P. sylvestris and P. nigra to rapidly acclimate to recurrent and long-lasting dry-spells. Surprisingly, the high plasticity and ecotypic adaptation occurred in traits of the same level of organisation. While higher needle xeromorphy of P. nigra might allow this species to withstand more intensive summer droughts than P. sylvestris, both species predominantly acclimate to aggravating summer drought events with changes in whole-plant morphology.