Shrub encroachment shifts the bioclimatic limit between marcescent and sclerophyllous oaks along an elevation gradient in west-central Spain

Questions (1) Does shrub encroachment affect the regeneration of two contrasting species of oak along an elevation gradient in the west-central Iberian Peninsula? (2) Do different nurse shrubs have any relevant species-specific effects along the gradient? (3) Does shrub encroachment affect the location of the bioclimatic limit between these two oak species along the elevation gradient? Location Twelve sites distributed from the Arribes del Duero plains to the Sistema Central mountains (Salamanca province, Spain). Methods We measured the regeneration of two contrasting oak tree species (Quercus ilex subsp. ballota Samp - sclerophyllous, Q.pyrenaica Willd - marcescent) as the relative percentage cover of seedlings and saplings (<1.3-m high) in plots 10m in diameter distributed in stands of different age. We fixed the bioclimatic limit at 50% relative proportion of species in each plot, i.e. whether one species was more abundant than the other in a plot. Shrub stand age was estimated by counting growth rings in the principal stems of the oldest shrubs in a plot. We fitted generalized linear mixed models to analyse the effects of elevation, specific nurse shrub and shrub stand age on regeneration and the probability of one species being more abundant than the other. Results The regeneration and relative proportion of Q.ilex (sclerophyllous) decreased with elevation. Regeneration did not vary with shrub encroachment, although the relative proportion increased notably in the youngest stands. In turn, regeneration and relative proportion of Q.pyrenaica (deciduous) significantly increased towards the upper sites and mature shrub stands. We found no evidence of specific nurse shrub effects on regeneration or the probability of one species being more abundant than the other in either of the two species of oak. Conclusions The bioclimatic limit between the two contrasting species of oak shifted with shrub stand age along the elevation gradient. Land practices preventing shrub encroachment can thus indirectly shift this limit towards higher elevations. The effects of shrubs may be critical in resolving the climatic and land-use effects on elevation shifts of species under changing conditions in bioclimatic transitions.