Dinámica de la vegetación bajo alteración por el pastoreo. El modelo de estado-y-transición para las estepas Patagónicas

Resumen

Plant communities ín Patagonia have a low plant cover (10-60%) and are dominated by two main functional groups: shrubs and perennial grasses. Domestic grazing reduces total plant cover and favors the replacement of species within functional groups (grasses or shrubs) and the replacement of entire functional groups (grasses for shrubs). We assessed the dynamics of vegetation in grazed ecosystems of Patagonia within the conceptual framework of the state-and-transition model, which allowed us to identify and arrange vegetation states induced by grazing along a sequence of physiognomic types. State-and-transition models for different Patagonian ecosystems indicate that changes are greater in the most humid ecosystems. In areas with annual precipitation above 250 mm, grazing induces a high number of stable states (4 to 7) with marked changes in physiognomy, from grass steppes to dwarf shrublands. Conversely, in areas with annual precipitation below 250 mm, grazing induces a low number of stable states (3 to 5) with less marked physiognomical differences, from shrub-grass steppes to dwarf shrub steppes. These changes were associated with specific steps in soil degradation and in the status of the soil seed bank. The intrusion of shrubs (a more xeric plant functional group than grasses) following soil disturbance is a key transition to an alternate stable state with strongly different structural and functional properties. Our results suggest that, the more humid areas possibly deserve special attention because they suffer more drastic changes than arid areas. In the future, stopping degradation and restoring ecosystems will need more intensive interventions than the sole regulation of stocking rates. Other strategies, like manipulating surface soil, adding nutrients or seeds, and planting target species in seasons or years with favorable precipitation and under grazing exclusion, should be designed and experimentally tested.