Plant and environmental factors associated with drought-induced mortality in two facultative phreatophytic trees

Background and Aims
Drought-induced mortality of tree species is increasing globally. We aimed to investigate spatial patterns and size dependence of mortality of two dominant tree species (Banksia menziesii R. Br and B. attenuata R. Br) capable of accessing shallow watertables in a Banksia-Allocasuarina-Eucalyptus woodland.

Methods
Living and dead trees were mapped within two plots: a high site (55 m to watertable) and a low site (9–20 m to watertable). Diameter at breast height (DBH) (cm) was measured and year of death estimated for deceased trees.

Results
Tree mortality was higher for most species in the high site. Across sites mortality was greatest during 2011 for most species including Banksia trees following the 2010 drought. Species differences in mortality were observed between B. attenuata and B. menziesii in the high site only. A greater number of large dead Banksias was observed in the high site. Spatial analysis indicated that local scale competition did not contribute to the death of these Banksias, however stand-level competition may have occurred.

Conclusions
We conclude that drought-induced mortality of Banksia trees is more prevalent in landscape regions where trees cannot access the watertable and due to greater water demands of larger trees, mortality is more frequent in these individuals.