Functional distance is driven more strongly by environmental factors than by genetic relatedness in Juniperus thurifera L. expanding forest stands

In our study of colonisation of abandoned farmland by Juniperus thurifera L., environmental and geographic distances were stronger drivers of functional differentiation among trees than genetic distance and tree intrinsic features. However, these effects varied both along the colonisation gradient and across predictors’ values, highlighting the complexity of the conditions under which functional diversity of forests is maximised. Forest expansion after land abandonment is changing Mediterranean landscapes, but the underlying ecological and evolutionary processes remain poorly understood. We evaluated whether forest expansion reduces within-stand tree functional distances and whether its environmental, geographical and genetic drivers vary along the expansion gradient. We sampled Juniperus thurifera in mature, transition and expanding stands along a colonisation gradient in Spain. We measured functional traits, genetic relatedness, surrounding vegetation cover and intrinsic features (size, phytosanitary status) on trees. We calculated within-stand pairwise functional distances and quantified the effects of genetic relatedness, geographic distance, vegetation cover and intrinsic tree features. Functional and genetic distances showed no consistent patterns along the expansion gradient. Functional distance was unrelated with genetic distance and strongly driven by geographic distance, likely representing unmeasured environmental factors. Vegetation cover and intrinsic tree features affected functional traits in transition and expanding stands. The effects on functional distance changed across predictors’ values. The role of environmental and geographic factors on tree functional divergence varied along the farmland colonisation gradient, while we did not detect effects of genetic distance. Accounting for this context-dependence and for non-linear effects on functional traits can improve our understanding and management of forest expansions.