Abstract

GOLLUSCIO, Rodolfo A et al. Divergences in carrying capacity estimation in North-western Patagonia: conceptual differences and practical consequences. Ecol. austral [online]. 2009, vol.19, n.1, pp.3-18. ISSN 1667-782X.

An effective estimate of the carrying capacity of different vegetation types would contribute to reduce some grazing-induced desertification processes in Patagonia. At present there are several methods in use. However, none can be considered "a priori" as the reference-method. In this study, we compared the estimates obtained by two commonly used in northwestern Patagonia, Aboveground Net Primary Productivity Method (MPPNA) and Pastoral Value Method (MVP). Using both methods, we calculated the carrying capacity for 119 sites encompassing a wide range of ANPP (130 to 2100 kg DM.ha^-1.y^-1). Estimates obtained using MPPNA were generally higher than those obtained using MVP, and the difference widened as ANPP increased. We used Path Analysis to identify the determinants of both estimates and their difference. MPPNA values depend mainly on ANPP, as the other component of the metric (Harvest Index) is also calculated as a function of ANPP. On the other hand, MVP values are more strongly dependent on the value of Forage Consumed prior to measurement than on the Pastoral Value of the stand at that time (VPstand), as the latter is used to estimate the amount of Forage Available at the time of measurement. VPstand, in turn, depends more on Forage Cover than on the VP of the vegetation (VPveg). As expected, the difference between the two carrying capacity estimates widened as ANPP increased and as Forage Consumed prior to measurement decreased. The difference between estimates also broadened as VPstand increased, reflecting an indirect effect of ANPP as sites of high VPstand had also high ANPP and forage cover. The analysis suggests that MPPNA and MVP provide the upper and lower limits, respectively, to carrying capacity estimates. These estimates could be improved by developing more reliable models for the estimation of Harvest Index and Forage Available at the time of measurement, reducing the sensitivity to ANPP of the first variable and increasing the sensitivity to ANPP of the second.

Keywords : Desertification; Grazing; Harvest index; Pastoral value.

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