Low phenotypic plasticity of Mediterranean Pinus sylvestris seedlings questions the benefit of assisted colonization for forest persistence at the Central Alpine forest-steppe ecotone under climate change

Background/Question/Methods

Increased summer drought will exacerbate the regeneration of many tree species at their lower latitudinal and altitudinal distribution limits. In the European Central Alps, where forests are essential for the protection of human settlements and infrastructure against natural hazards, introduction of more drought-tolerant provenances or species is currently considered to accelerate tree species migration and facilitate forest persistence at the forest-steppe ecotone. Trade-offs between drought adaptation and growth plasticity might, however, limit the effectiveness of assisted colonization, especially if introductions focus on provenances or species from different climatic regions. In two common garden experiments, we tested the performance of Pinus sylvestris from the continental Central Alps under extended summer drought, and compared phenotypic plasticity of 6-months old seedlings and 3-year old saplings to that of P. sylvestris from a Mediterranean seed source at the southernmost distribution limit of the species in Europe. Relative distance plasticity index (RDPI) was used to calculate phenotypic plasticity in biomass allocation between provenances.

Results/Conclusions

In the Central Alpine provenance, a 20% reduction in long-term summer precipitation resulted in a 50% reduction of seedling survival 6 months after seedling emergence. At the same time, higher emergence and lower mortality resulted in twice as many Mediterranean than Central Alpine seedlings present under dry conditions. By contrast, Mediterranean seedlings and saplings exhibited lower phenotypic plasticity in terms of biomass allocation, which led to higher investments into roots irrespective of water conditions. This corroborates the theory that limited phenotypic plasticity is an adaptation to stressful environments such as the Mediterranean. As a result, Mediterranean saplings were two times smaller and had four times lower aboveground biomass than Central Alpine saplings both under wet and dry conditions. In the longer term, limited aboveground growth capacity is likely to compromise the competitive ability of Mediterranean P. sylvestris provenances in Central Alpine forests since competition for light and nutrients from understory vegetation is substantial in temperate forests during seedling establishment. Even though the Mediterranean provenance showed higher emergence and lower mortality during early stages of tree regeneration under dry conditions, its low growth plasticity during the sapling stage questions the benefit of assisted colonization of Mediterranean provenances to facilitate forest persistence at the Central Alpine forest-steppe ecotone.