Shrub encroachment in Mediterranean mountain grasslands: Rate and consequences on plant diversity and forage availability

摘要

Abstract Question Shrub encroachment has been confirmed in the past decades all over the world and is currently viewed as a “global process” threatening many grass‐dominated biomes. In southern Europe has generally been related to rural depopulation, land‐use changes and grazing abandonment. Nevertheless, in several mountain ranges of the Iberian Peninsula with secular pastoralism and high shrub cover, neither stocking rate nor traditional management has substantially altered in the past decades. Within this framework, to deepen our knowledge of shrub encroachment and to adopt, if necessary, appropriate control measures, we aim to discover: (i) the overall expansion rate in the main grassland–shrub communities; (ii) the course of shrub expansion; and (iii) the consequences for grassland floristic composition, plant diversity and frequency of the main forage functional groups throughout the period of woody expansion. Location This study was undertaken in Moncayo Natural Park (Spain), a climate and vegetation crossroads with remarkable presence of four widespread Mediterranean shrubs: Cytisus oromediterraneus, Erinacea anthyllis, Juniperus communis and J. sabina. Methods To determine the expansion rates of those shrubs, we examined four distinct and sufficiently separated grazing areas (and different shrub combinations), each with four different stages of shrub cover, over a 6‐year period. To assess changes in vegetation structure between 2008 and 2014 during the different stages of shrub encroachment, we used a paired t‐test comparing 14 parameters related to the diversity, life‐form spectra and abundance of grazing plants. The influence of year, cover category and zone was jointly assessed using a Linear Mixed Model. Results For the whole territory, we found an increase in average yearly cover of 1.3% (with high variation between the four species), although at the zone scale (areas with a particular shrub dominance) this increase was significant in only half of them. When the four shrubs occur together, Juniperus species show faster expansion rates than the other two species, although in only a few cases was the increase significant over the 6 years of the study. We found a significant decrease in total plant diversity and a significant increase in dominance between cover categories and years. Looking at the vegetation life‐forms, the increase in chamaephytes and phanerophytes, which include invader shrubs, caused a decrease in therophytes and hemicryptophytes. Finally, grasses and leguminous plants, which constitute the main livestock food intake, showed noticeable reductions, resulting in loss of pastoral value. Conclusions Although cessation of grazing has been noted as the main cause of shrub encroachment in Mediterranean grasslands, our results, suggest that encroachment occurs despite the maintenance of stocking rate and livestock management, and leads to a decrease in plant diversity and grassland quality. Focusing on use of the territory and taking into account the prevalence of negative effects after shrub encroachment, additional measures to safeguard ecological and pastoral values in Mediterranean mountain grasslands should be considered, particularly on sites of nature conservation interest with a long grazing history.