A broad brush-stroke test of an assumption: does increasing 'Callitris' cover reduce native species richness (species density)?

Does an increase in the cover/abundance of 'Callitris glaucophylla' or 'Callitris endlicheri' affect the number of species recorded in plots (species density) or do other factors such as altitude or logging, fire or grazing history have greater explanatory power? This was tested using survey data from 1351 plots from northern New South Wales. Altitude was found to have the greatest explanatory power in predicting the number of species per plot. Increasing cover/abundance of 'Callitris glaucophylla' was found to be positively correlated with increasing species density. Fire was found to have a minor negative effect on species density in 'Callitris glaucophylla' stands and grazing a small positive correlation in 'Callitris endlicheri' stands.