Functional diversity differently shapes growth resilience to drought for co-existing pine species

Questions: What are the differences in tree growth and resilience in response to extreme droughts between three co-existing pine tree species? Are growth-resilience components more influenced by intrinsic (age, tree height) or extrinsic factors (mass effects or functional diversity of the neighbourhood)? Is the tree's vulnerability to drought buffered by the functional diversity of the surrounding plant community? Location: Mediterranean forest, northeast Spain. Methods: Trees from three co-existing pine species with different drought tolerances (Aleppo pine - Pinus halepensis Mill.; Black pine - Pinus nigra subsp. salzmannii (Dunal) Franco; Scots pine - Pinus sylvestris L.) were sampled. The average age of the studed pines was similar to 30years. Radial growth from 1980 to 2015 was measured and the pines' responses to the most severe recent droughts (2005 and 2012) were studied by relating different growth resilience components (Rt, resistance; Rc, recovery; RRs, relative resilience) with intrinsic (age and height) and extrinsic factors of the neighbouring community around each tree. The considered extrinsic factors were CWM and niche complementarity of tree (height), leaf (specific leaf area, leaf dry matter content) and wood (wood specific gravity) traits. The way in which these factors affected growth resilience to drought was assessed. Results: For the whole study period we found lower growth of Scots pine than Aleppo pine, but similar resilience components (except for higher Rt in 2005 for the former species). Despite each drought year resulting in distinct responses, the growth resilience to drought was mostly dependent on tree age for Aleppo pine, while functional diversity modulated a high percentage of the resilience variance of Scots pine. Black pine had an intermediate strategy. CWM explained, in general, less variance of the resilience components than functional diversity. Conclusions" The positive effect of surrounding functional diversity on Scots pine could be related to a greater resistance to drought, likely due to improved resource acquisition favouring its performance at the dry limit of its distribution area. Intrinsic tree features, mass effects and functional diversity differently shape growth resilience to different droughts for co-existing tree species.