Species Richness of Overstorey Strata in Australian Plant Communities ̵2 the Influence of Overstorey Growth Rates

Abstract

The species richness (number of tree and tall shrub species per hectare) of overstorey strata is examined in tropical, subtropical and temperate climax plant communities of Australia. Species richness (N) is shown to increase as the evaporative coefficient (k) of the sampling site increases from semiarid climates (k = 0.035-0.045) to perhumid climates (k = 0.075-0.100):

Tropical: log N = 17.38k + 0.40.

Subtropical: log N = 25.40k - 0.60.

Temperate: log N = 8.90k + 0.09.

Species richness of overstorey strata is highest in the tropics (N = 138 when k = 0. 100), followed by the subtropics (N = 73 when k = 0. 100), with only a small number of overstorey species being associ- ated with temperate communities (N = 8-10 when k = 0.100).

Species richness of the overstorey is positively related to the annual shoot growth (vertical com- ponent) of the foliage canopy as it regenerates after disturbance. The tendency to dominance of only a few overstorey species in temperate communities may be enhanced by rapid stem growth (current annual growth increment) of the plant community due to lower cellular metabolism and respiration in stems and roots in cooler climates compared with those in the tropics.

Trees and tall shrubs appear to be excluded as overstorey components when the respiratory coefficient (c) approaches 0.030 and the mean annual temperatures (of snow-free localities) are less than 13°C.