Abstract: Aims Australian alpine ecosystems currently experience high precipitation in the snow-free season, but they are predicted to experience drier conditions under climate change. We observed high mortality of the dominant alpine grasses following drought in 2007. Our aims were as follows: to test the involvement of plant-available water (PAW) and other environmental variables in grass mortality in the field; to detect possible species differences in drought response and to link soil moisture to precipitation using soil properties and climate data.
Methods The dominant tussock grasses of the Australian alpine zone, Poa hothamensis var. hothamensis N.G. Walsh, P oa hiemata Vickery and P oa phillipsiana Vickery (Poaceae), all exhibited mortality following drought in the Bogong High Plains, Victoria, Australia in 2007. PAW was calculated using soil water potential measurements, and past drought occurrence was modelled using climate data. We then tested the effects of PAW and soil depth on grass survival both at a large spatial scale spanning the elevational range of the alpine zone and at a smaller scale. Poa hothamensis and P. phillipsiana were compared in a common-garden experiment to test drought tolerance.
Important findings Poa hothamensis survival was predicted by dry-season PAW at the small spatial scale; at the large scale, soil depth and elevation were more important predictors of P. hothamensis survival, but dry-season PAW predicted P. hiemata survival. Common-garden experiments supported field observations that P. hothamensis is more drought-sensitive than is P. phillipsiana. We also present a simple polynomial relationship between rainfall and field soil moisture, which predicts that the alpine soils dry below wilting point several times a year. We suggest the timing of long rain-free periods may be more important than their duration.

Key words: climate change, disturbance, drought adaptation, grassland