Changes in plant functional types in response to goat and sheep grazing in two semi-arid shrublands of SE Spain

Abstract

In Mediterranean plant communities, grazing induces severe floristic changes affecting the life histories of grazed and non-grazed species. Alteration of the grazing regimen causes important changes in the structure and dynamics of the plant community and ecosystem stability. To determine the susceptibility of different plant functional types to landscape management, we measured changes in Plant Functional Types (PFTs) in response to grazing by goat and sheep in an inland dwarf-palm matorral and a marine-exposed thorny-shrub matorral in Cabo de Gata Natural Park (SE Spain). We classified the major life forms into PFTs, and identified six PFT shrubs (dwarf-palms, sclerophyllous small trees, xeric thorny-shrubs, spiny legumes, glaucous dwarf-shrubs, and xeric half-shrubs), four PFT forbs (leafy stem herbs, xeric prostrate herbs, rosette herbs, and clonal spiny herbs), and two PFT grasses (steppe and short grasses). Morphological traits measured include sclerophilly, leaf presence, leaf size, shape of leaf margins, hairiness, position of dormant buds (growth form), clonality, plant coverage, canopy structure, phenological deciduousness (drought resistance), and regeneration (reproduction type, pollination type, inflorescence position, and seed size). There was a higher correlation within and between morphological growth forms, leaf and phenological traits, than within regenerative traits (only seed size was correlated with main dispersal type). We analysed the importance of these PFTs at several sites of the two communities, which were subjected to different livestock rates. In inland and marine-exposed communities, the same PFTs decreased in response to medium-high grazing: sclerophyllous small trees ( Quercus coccifera, Olea europaea var. sylvestris), glaucous dwarf-shrubs ( Phlomis and Cistus spp.) and short grasses ( Brachypodium retussum). In both communities, the decrease of these grazing-susceptible PFTs was widely associated with an increase in steppe grasses ( Stipa tenacissima, “alfa-grass”) and xeric prostrate herbs ( Fagonia cretica, Paronichia sufruticosa), the latter of which is a reliable indicator of degradation in semi-arid systems. Instead, different PFTs behave as either grazing-averse and/or grazing-tolerant in each community: Dwarf-palms ( Chamaerops humilis) and xeric thorny shrubs ( Periploca laevigata) in the marine-exposed community, and xeric half-shrubs ( Thymus hiemalys, Sideritis osteoxylla, Teucrium spp., Artemisia herba-alba) in the inland community. The latter functional group resists disturbances, such as medium-moderate grazing and drought, in semi-arid zones and is an indicator of long-term degradation.