Resistance of European tree species to drought stress in mixed versus pure forests: evidence of stress release by inter-specific facilitation.

While previous studies focused on tree growth in pure stands, we reveal that tree resistance and resilience to drought stress can be modified distinctly through species mixing. Our study is based on tree ring measurement on cores from increment boring of 559 trees of Norway spruce (Picea abies [L.] Karst.), European beech (Fagus sylvatica [L.]) and sessile oak (Quercus petraea (Matt.) Liebl.) in South Germany, with half sampled in pure, respectively, mixed stands. Indices for resistance, recovery and resilience were applied for quantifying the tree growth reaction on the episodic drought stress in 1976 and 2003. The following general reaction patterns were found. (i) In pure stands, spruce has the lowest resistance, but the quickest recovery; oak and beech were more resistant, but recover was much slower and they are less resilient. (ii) In mixture, spruce and oak perform as in pure stands, but beech was significantly more resistant and resilient than in monoculture. (iii) Especially when mixed with oak, beech is facilitated. We hypothesise that the revealed water stress release of beech emerges in mixture because of the asynchronous stress reaction pattern of beech and oak and a facilitation of beech by hydraulic lift of water by oak. This facilitation of beech in mixture with oak means a contribution to the frequently reported overyield of beech in mixed versus pure stands. We discuss the far-reaching implications that these differences in stress response under intra- and inter-specific environments have for forest ecosystem dynamics and management under climate change.