Distinct trait syndromes and plasticity maintain similar performance between seedlings populations of the riparian tree species Populus nigra L

Riparian forests remain largely understudied in the context of climate change in comparison to other forest ecosystems although they serve multiple socio-ecological functions. We evaluated local adaptation and adaptive potential in Populus nigra L., an emblematic tree species of European riparian forests. We set up a reciprocal transplant experiment and measured 17 structural and functional traits among 10 progenies of two genetically differentiated populations. The populations originated from two separate watersheds differing in climate conditions and a total of 1200 seedlings were grown in 1-m3 mesocosms at both sites for one growing season. Traits measured were related to growth, leaf physiology and xylem water transport. The populations showed similar biomass suggesting no local adaptation for overall performance but displayed distinct trait syndromes and plastic abilities. The southern population primarily adjusted through changes in allocation and leaf water-use efficiency while the northern population primarily adjusted through changes in specific leaf area. Genetic variation within populations was most of the times equal or larger than between populations. This combined with the generally moderate to high heritability values and the observed plasticity suggested significant adaptive potential. Altogether, our findings reveal that although evolution may not lead to obvious differentiation between populations in global performance, integrated multi-trait approaches are highly valuable to shed light on how evolution may shape distinct underlying functional strategies among populations resulting in a similar outcome.