Causes of stability in the alpine treeline in the Snowy Mountains of Australia – a natural experiment

Abstract

Large areas of treeline in the Snowy Mountains of south-eastern Australia burnt in wildfires in 2003, providing the opportunity to quantify seedling establishment at the treeline after disturbance, by comparing burnt and unburnt sites. Eucalyptus pauciflora niphophila (Maiden and Blakely) L. Johnson and Blakely, (snowgum) generally responds to fire by resprouting from lignotubers rather than by the death of the tree, hence the location of the pre-fire treeline was unaffected. Burnt and unburnt sites along the treeline therefore differed in the removal of immediate competition from understorey plants and loss of a protective canopy shelter. Five years after these fires, 27 sites were examined to determine whether the resulting conditions led to increased establishment of snowgums above the treeline. Paired plots 15 m wide were established, extending a distance of 15 m above and below the treeline in 15 unburnt and 12 burnt sites. There were significantly more seedlings in burnt than unburnt plots below the treeline. However, even in burnt areas, there were significantly fewer seedlings found above the treeline than below the treeline. Although conditions for seedling establishment at the treeline were good at burnt sites (as indicated by seedling recruitment within 15 m of the treeline), this major disturbance by fires resulted in no pulse of seedling establishment above the treeline. It is concluded that stability in the treeline of the Snowy Mountains is likely to be due to the failure of seeds to disperse uphill.