The lesson learnt from two long-term precipitation exclusion experiments: xylem/phloem plasticity is not a real option in trees

The last decades have been characterized by an increased number of extreme events, such as droughts and heatwaves which had a clear impact on plants, with effects on tree status, vigor decline and mortality. Mechanism of acclimation and adaptation to drought are still to be fully understood, but they play a critical role in the evaluation of climate change effects on vegetation’s survival. Throughfall exclusion experiments may give great insights about all the possible reactions to drought in standing vegetation. We investigated xylem and phloem anatomical traits and leaf/branch biomass from mature Piñon Pine (Pinus edulis Engelm.) in a throughfall exclusion experiment at the Sevilleta Long Term Ecological Research (LTER) project (New Mexico, USA). Sampling was carried out on 1,5m long branches in 4 different experimental plots : control (CO), long term 45% rain-off (Legacy), short term 45% rain-off (New45) and short term 90% rain-off (New90). Preliminary results show that the biomass allocation and the ratio between needles and branches did not differ between treatments with a clear isometric relationship. The xylem tracheid diameter of the outermost ring is larger in Legacy and New90 treatments. Phloem cell diameter follows the same pattern, with larger sieve cell perimeter in Legacy treatment, but appears lower in the New45 treatment. These results would suggest an adaptation to drought by a modification of the vascular structure, without any significant change in biomass allocation for needles and branches.