Abstract:

As extreme climatic events including droughts and heat waves become more common in a changing climate, tree mortality has increased across the globe. In order to determine whether certain species have a competitive advantage over others, we explored the water-relations and leaf-gas exchange of four co-occurring species in a forest in northern Aotearoa-New Zealand. We studied the ecologically and culturally significant foundation species, Agathis australis (a conifer), two additional conifers, Phyllocladus trichomanoides and Podocarpus totara and the angiosperm Knightia excelsa.

We measured sap flow, leaf-gas exchange and xylem water potentials of leaves and terminal branches with concurrent measures of micrometeorological data on days with very few clouds. We derived whole tree hydraulic conductance and instantaneous water-use efficiency (WUE_i) at our remnant forest in west Auckland during February 2015 (southern hemisphere summer).

The four species behaved similarly in their diurnal curves of gas exchange and water potential. Rates of assimilation, stomatal conductance and WUE_i were similar among trees of different species. Whole tree hydraulic conductance was also similar among species. These results indicate functional convergence in water relations, possibly driven by low nutrient soils at the site. Our results suggest that there is no species with a clear adaptive advantage over the others in the context of climate change.

Key words: southern conifers, sap flow, water potential, hydraulic conductance, leaf-gas exchange, functional convergence