Physiological responses and statistical models of the environmental niche: a comparative study of two co-occurring Eucalyptus species

1. Our objectives were as follows: (i) to quantify the realized environmental niche of two co-occurring species and their differences and (ii) to examine whether differences in the species niches relate to differences in water and light use efficiency and growth responses to light and water. 2. Statistical models of the environmental niches of two Eucalyptus species were derived from presence/absence records and from environmental variables (mean annual temperature, mean summer rainfall, mean summer daily radiation) and three factors (topographic position, lithology and a nutrient index) for 10 577 plots from south-eastern New South Wales, Australia. Generalized additive models (GAM) were fitted in a backwards stepwise procedure using Aikake's Information Criterion (AIC) for model selection. 3. Seedlings were used in two experiments: (i) a water supply gradient with 20 treatments from 10 mL per day to 1000 mL per day and (ii) a light gradient with five treatments from 3% to 100% (full) sunlight. 4. After controlling for lithology and nutrients, the principal differences in niche between species were as follows: (i) Eucalyptus mannifera was insensitive to mean summer daily radiation, (ii) it had a higher probability of occurrence in gullies and (iii) the two species differed in response shape and breadth of response to temperature, and temperature of maximum occurrence. Both species had a similar response to mean summer rainfall. 5. Eucalyptus mannifera showed a monotonic increase in total biomass along the water supply gradient, consistently outperforming Eucalyptus dives, which showed little further increase above 440 mL per day. Consistent differences between species were found for leaf area, specific leaf area and allocation to leaves and stems. 6. Diurnal measurements of conductance, transpiration, carbon dioxide assimilation and water use efficiency (WUE) showed differences between species along both the water and light gradients. Eucalyptus dives exhibits conservative responses to increasing resources and relatively higher WUE while E. mannifera has lower WUE. 7. Synthesis. Species differences in ecophysiological strategies are consistent with their co-occurrence and with their realized environmental niche responses except for their temperature responses. The combination of statistical niche models and ecophysiological gradient experiments focuses attention on important unresolved environmental processes controlling distributions.