Primary and secondary growth of Pinus halepensis are more sensitive to inter-annual drought variability than to 14 years of rainfall exclusion in a Mediterranean forest.

Background And Aims: With ongoing climate change, the impact of droughts of increasing intensity on forest functioning is of critical concern. While the adverse effects of drought on tree secondary growth have been largely documented both at the tree and stand scales, our understanding of how primary growth morphological traits, which control crown development, respond to drought remains limited, especially in the long term.

Methods: Based on 14 years of monitoring of four primary growth morphological traits (e.g. shoot elongation, polycyclism rate, branching and needle length) and stem secondary growth in a rainfall exclusion experiment, we investigated (1) the climatic drivers of above-ground growth and (2) the effect of long-term exacerbated drought conditions on the growth response to drought in a mature Pinus halepensis stand.

Key Results: Above-ground growth was strongly and negatively impacted by drought duration during the current year (stem secondary growth), the previous year (polycyclism) and both years (branching, shoot length), and by drought during spring (needle length). While excluding 30 % of the incoming rainfall did not significantly affect the number of ramifications, polycyclism rate or stem secondary growth, it reduced needle and shoot lengths by 14.3 and 7.7 % over the entire study period, respectively. However, this effect was significant only in the first years after the treatment was established. Such acclimation to exacerbated drought conditions is also reported in the drought-growth relationships which are similar among treatments, except for needles that were slightly shorter under a similar level of drought stress in the exclusion.

Conclusions: Our study highlights the key acclimation capacity in the primary and secondary growth response of P. halepensis to drought. In addition to tree structural adjustments, the relatively limited effect of the 30 % rainfall exclusion may also be caused by (1) the substantial inter-annual rainfall variability typical of Mediterranean climates, which modulates the exclusion effect on drought duration, and (2) the inherent inter-individual variability in drought sensitivity.