Reasons for the absence of trees on flat valley-bottoms in the subalpine regions of south-eastern Australia were studied at Seventeen Flat near Currango and other sites in the Snowy Mountains of New South Wales.

Temperature changes with height above terrain were measured at grassland and woodland sites by thermistors attached to a hydrogen-filled balloon. The occurrence of nocturnal temperature inversions above grassland was established.

Minimum temperatures at 80 cm above ground were recorded for thirty-one weeks at thirteen stations along a transect across the grassland and extending into the woodland on either side. Minima were lower and frequency of temperatures below—9°C were higher in grassland than in woodland.

Seedlings of subalpine woodland species and other eucalypts were planted at both grassland and woodland stations. At grassland stations seedlings of tree-line species showed frost injury within a short time from planting and none survived two winters.

The death of planted trees at grassland stations, the high frequency of temperature inversions and the low minima recorded on grassland compared with woodland suggest that temperature is a primary cause of the absence of trees in grasslands below subalpine woodlands.