The lesson learnt from two long-term precipitation exclusion experiments: how intensity and duration of drought may influence xylem and phloem plasticity

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.), Norway spruce (Picea abies Karst.) and beech (Fagus sylvatica L.) in two throughfall exclusion experiments(TEEs). One TEE is the “Kranzberg Roof Project” (KROOF) in Bavaria and the second is the Sevilleta Long Term Ecological Research (LTER) project (New Mexico, USA). Sampling in Sevilleta was carried out on 1,5m long branches in 4 different experimental plots of Piñon Pines: control (CO), long term 45% rain-off (Legacy, 10 years), short term 45% rain-off (New45, 1 year) and short term 90% rain-off (New90, 1 year). Sampling in Kranzberg Forest was carried out on 1,5m long branches in 2 experimental plots made by mixed stands of spruce and beech: control (CO) and troughfall-excluded (TE, 5 years -70% rain). Experimental results show that the biomass allocation and the ratio between needles and branches did not differ between treatments with a clear isometric relationship for Sevilleta’s Piñon Pines and KROOF’s Norway Spruces. On the contrary F. sylvatica trees allocated proportionally more biomass to leaves than biomass compared to control trees. For Sevilleta the xylem tracheid diameter of the outermost ring is larger only in New90 treatment, but the overall trend in the last decade shows no difference between legacy and control. The same trend is seen in the apical branches of KROOF experiment, where for both species the lumen diameter (or area) of xylem conduits did not differ between TE and CO trees. Phloem cell diameter display larger sieve cell perimeter only in Legacy treatment (Sevilleta) but show no changes in other treatments and in KROOF experiment. These results would suggest only few phenotypic plasticity changes for xylem, phloem, and biomass allocation. Furthermore, these plastic changes appear to be different for each species and treatment intensity, but they all push towards an increased efficiency of the overall hydraulic transport system.