Timescale of Perennial Grass Recovery in Desertified Arid Grasslands Following Livestock Removal

: Over the past two centuries, perennial grass cover has declined and shrub density has increased in many arid grasslands. These changes in vegetation, characteristic of desertification, are thought to have occurred often following prolonged periods of intense grazing by domestic livestock. At many such sites, however, the subsequent removal of livestock grazing for up to 20 years has not resulted in increased grass cover. The apparent stability of vegetation following the cessation of livestock grazing has led to the hypothesis that desertified arid grasslands exist in alternate stable states of either grassland or shrubland over timescales relevant to management. To better understand the timescale of grass recovery in historic arid grasslands dominated by shrubs, we examined the vegetation at two nearby desertified sites that differed in the length of time since livestock removal. There was little difference between the site ungrazed for 20 years and the shrub-dominated vegetation on the other side of the exclusion fence. At a site ungrazed for 39 years there was significantly higher perennial grass cover inside the exclusion fence than outside, and nearly all the increase had occurred over the past 20 years. These data suggest that there may be time lags of 20 years or more in the response of perennial grasses to removal of livestock in historic grassland ecosystems dominated by shrubs. Resumen: En los ultimos dos siglos, la cobertura de pasto perenne ha disminuido y la densidad de arbustos ha incrementado en muchos pastizales aridos. Se considera que estos cambios en la vegetacion, caracteristicos de la desertificacion, frecuentemente han ocurrido despues de periodos prolongados de pastoreo intensivo por ganado domestico. En muchos sitios, sin embargo, la subsiguiente remocion de ganado por hasta 20 anos no ha resultado en un incremento en la cobertura del pasto. La aparente estabilidad de la cobertura vegetal que queda despues del cese del pastoreo ha conducido a la hipotesis que establece que los pastizales aridos desertificados existen en estados alternos estables tanto de pastizal, como de matorral por escalas de tiempo relevantes para el manejo. Para entender mejor la escala de tiempo de la recuperacion del pasto en pastizales aridos historicamente dominados por arbustos, examinamos la vegetacion en dos sitios desertificados cercanos que difieren en la longitud del tiempo que ha transcurrido desde que se removio el ganado. Hubo una pequena diferencia entre el sitio que no ha sido pastoreado por 20 anos y la vegetacion dominada por arbustos que se encuentra del otro lado del cerco de exclusion. En un sitio sin pastoreo por 39 anos hubo significativamente mas cobertura de pastos dentro del cerco de exclusion que en la parte de afuera y casi todos los incrementos se dieron en los ultimos 20 anos. Estos datos sugieren que posiblemente existan lapsos de tiempo de 20 anos o mas en la respuesta de los pastos a la remocion de ganado en ecosistemas historicos de pastizal dominados por arbustos.

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