Evaluation of methods for resolving tree rings in a tropical Hawaiian tree species, Psydrax odorata, and applications for water resource studies

Tropical dry forests on the Hawaiian Islands have declined in recent decades because of human impacts. Blackmore and Vitousek (2000) found that a dry forest area in the Kona region of the island of Hawaii declined by 62% within 40 years. It is believed that invasion by Pennisetum setaceum , a non-native grass, is partially responsible for this decline. The presence of Pennisetum may alter water availability to native plants by sequestering the majority of the surface water, thereby hindering growth and survival. To evaluate this hypothesis the δ18O and δ13C ratios from the native tree, Psydrax odorata, were measured in trees taken from an invaded and non-invaded site. To compare trees between sites over time, a method of determining tree age is required. I assessed three methods: dendrochrology, wood anatomy, and digital imaging. Only digital imaging yielded accurate results when compared to dates determined by radiocarbon analysis. The δ 13C and δ18O ratios did not have the resolution to resolve whether Pennisetum was competing with Psydrax for water. In addition, correlation of δ18O and precipitation data coupled with reduced major analysis indicated that Psydrax odorata may not be suitable for climate reconstruction without additional analysis.