Demography of Grazed Tussock Grass Populations in Patagonia

Abstract

The cover of Festuca gracillima (coiró n fueguino), a native tussock grass that dominates grass steppes of Southern Patagonia and Tierra del Fuego, has diminished under continuous sheep grazing. This loss is a concern, because it also reduces forage availability in winter, biodiversity, and soil stability. In the present study, the hypothesis that tussock grass birth and mortality rates are balanced only under moderate-grazing or exclusion regimes was tested with two 5-year records of demographic data obtained from 3 sheep grazing regimes: exclosure (no grazing), low (0.0348 AU ha À1 y À1 , where AU represents animal units equivalent to the consumption of a 450-kg cow), and high (0.1043 AU ha À1 y À1). Tussocks were outlined in photographs and marked in the field at two 5-year intervals. The initial total number of plants for the 3 grazing regimes (n ¼ 358) increased to 384 plants at the end of the 10-year period. No recruitment from seed was observed; plant number changed as a balance of yearly rates of tussock mortality (1.48%), amalgamation (0.75%), and subdivision (2.04%). Intensely grazed populations showed greater (2.13%) mortality rates than ungrazed (1.20%) or moderately grazed (0.78%) populations. Tussocks in intensely grazed populations were smaller (167 cm 2 plant À1) than those in moderately grazed (197 cm 2 plant À1) or ungrazed (300 cm 2 plant À1) populations. Transition matrices showed eigenvalues of 0.701 (high grazing), 0.794 (exclosure), and 0.876 (low grazing). All growth rates of demographic models were negative; the largest rate of population decrease was found under high-intensity grazing, for which projections show that half the tussocks would be lost in 37 years. Under low-intensity grazing and exclosure, a similar tussock loss would take place in 87 and 74 years, respectively. Results show 1) the importance of vegetative processes for tussock demography, 2) the extremely slow dynamics of population changes, and 3) that tussocks may suffer increased mortality as a consequence of subdivision or fragmentation, a process that can be viewed as a small-scale example of the generalized effect of patch subdivision under grazing. Resumen Festuca gracillima (coiró n fueguino) es una gramínea mediana dominante en los pastizales del sur de la Patagonia y norte de Tierra del Fuego que ha ido perdiendo cobertura bajo pastoreo continuo. El proceso implica también una reducció n en el forraje disponible en invierno, en la estabilidad del suelo y en la biodiversidad. El presente trabajo analiza 10 añ os de demografía de poblaciones de coirones sujetos a pastoreo controlado con tres intensidades: clausura (sin pastoreo), alta (0.1043 AU ha À1 añ o À1 , AU son unidades animales equivalentes al consumo de una vaca de 450 kg), y baja (0.0348 AU ha À1 añ o À1). Se puso a prueba la hipó tesis de que las tasas de natalidad y mortalidad llegan al equilibrio solamente bajo pastoreo moderado o en exclusió n. Los coirones fueron identificados en mapas fotográficos y marcados en el campo en dos períodos consecutivos ...

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