Intra-specific variation in growth and wood density traits under water-limited conditions: Long-term-, short-term-, and sudden responses of four conifer tree species.

Consequences of climate change will severely affect forest ecosystems in the near future, yet our understanding of how and why trees are responding to their abiotic environment is still limited. Intra-specific variation (ITV) in the growth response of trees to warming and drought has been widely neglected so far, but could play a key role for adapting forests to future climate conditions. We analyzed tree rings from four conifers (Picea abies, Abies alba, Larix decidua, Pseudotsuga menziesii) regarding their intra-specific adaptation potential when trees are growing at the warm and dry margins of species distributions. Our study comprises data from four common garden experiments (45 provenances and a total of 743 trees) and assessed growth response at different temporal scales from decades (long-term) to only a few event years (short-term) and finally for density fluctuations within one year (sudden response). We observed significant variation among provenances at all time-scales, but with varying degree among species. However, variation in short-term response (drought years) was remarkably unstable across all species, when the seasonal variation of drought occurrence was considered. Silver-fir and Douglas-fir showed significant associations between seed-source climate and growth response as well as trade-offs between early- and latewood growth reaction which strongly suggests that growth responses are adaptive. Intra-specific variation in conifers in response to drought will probably be sufficient to mitigate climate change consequences on forest growth, but growth-environment interactions as well as dependencies between temporal scales could create major pitfalls for adaptive forest management in the future.