PS 23-33 - High xylem conductivity and plasticity increase survival in Mediterranean pine trees

Background/Question/Methods

Increasing aridity threatens forests worldwide. In the Mediterranean Basin, where forests are particularly sensitive to more frequent and intense droughts, forest mortality events have increased over the last few decades. Trees adjust their functional traits, including wood anatomical traits, in response to changing environmental conditions. Different adjusting capacities may lead to changes in mortality rates and shifts in species dominance or abundance. Here, we analyse potential causes and dynamics of differential decline in a mixed Mediterranean pine forest where the dominant species P. pinaster shows landscape-scale decline i.e. mortality, defoliation, mistletoe infection and lack of regeneration, not observed in other more drought-tolerant species like P. pinea which are increasingly substituting P. pinaster. We explore responses of xylem traits to water stress in these two pine species by analysing annual series of wood cell anatomical traits, carbon discrimination (D¹³C) and intrinsic water use efficiency (WUEi) in trees with different health status (healthy, declining and dead) for 1978-2017. Additionally, we analysed sapwood non-structural carbohydrates (NSCs) in 2017.

Results/Conclusions

In both pine species, healthy trees had tracheids with larger lumens, which implied higher hydraulic conductivity, thicker cell walls in the latewood and tracheids with a higher resistance to implosion than dead trees. Xylem response to interannual climatic variability was more plastic in healthy than in non-healthy trees. In response to drought, healthy pines developed higher xylem safety by either reducing lumen area (P. pinaster) or by reinforcing cell-walls (P. pinea). Carbon isotopic discrimination was similar in trees of all health status, suggesting that low stomatal control could have led to an excessive water loss in defoliated declining and dead trees. These results in xylem plasticity, WUEi and similarity in starch content between pine trees of different health status suggest that hydraulic failure may be the leading cause of tree die-back in our study area. Our study highlights the major role that xylem adjustment and plasticity may play in differential tree survival during drought events.