Leaf turnover and herbivory in the tall tussock grass Festuca orthophylla in the Andean Altiplano

Abstract

Tall grass tussocks dominate the Andean highlands above 3,800 m a.s.l., with often no other life form contributing significantly to standing crop biomass and productivity, thus, representing the dominant source of fodder for camelid grazing. To assess the productivity of these semi-arid tropical rangelands (350 mm of annual rainfall confined to a 4–5 months rainy season) is challenging, given the ‘evergreen’ nature of leaves and the unknown periodicity of leaf growth. The aim of this study was to explore the seasonal course of new foliage formation, foliage duration, and the response of leaf elongation and emergence to simulated llama grazing (clipping). The repeated census of Festuca orthophylla leaves in the Sajama National Park at 4,250 m elevation (Bolivia) provided clear evidence that leaf production is not confined to the rainy season, but also occurs during the dry season. Our data revealed a mean leaf longevity of 141 days for peripheral control tillers (unclipped) and 169 days for central control tillers. Leaves grow slower and reach shorter length during the dry season compared to the rainy season, and peripheral tillers are always shorter but more vigorous than central tillers (shorter intervals between the emergence of two leaves). There was no stimulation of leaf growth in tillers that were clipped (no overcompensation). Overall, this study suggests ca. two-fold replacement of foliage per year. We explain the continued growth in the dry season by, on average, six times greater soil area occupied by roots than by the leaf canopy, and thus, much greater water availability per tussock than meteorological data would suggest. Wide spacing of tussocks and a large root-sphere mitigate the impact of periodic drought on tissue formation, providing year-round forage for llamas.