Frequent, low-amplitude disturbances drive high tree turnover rates on a remote, cyclone-prone Polynesian island

Please be informed of this newly published journal article by RMBR research associate Professor Edward Webb in the Journal of Biogeography.

Webb, Edward L., J. O. Seamon, S. Fa’aumu (2011) Frequent, low-amplitude disturbances drive high tree turnover rates on a remote, cyclone-prone Polynesian island. Journal of Biogeography, 38: 1240-1252.

Its abstract is as follows:

Aim
How important are frequent, low-intensity disturbances to tree communitydynamics of a cyclone-prone forest? We tested the following hypotheses concerning the ‘inter-cataclysm’ period on a remote Polynesian island: (1) tree turnover would be high and recruitment rates would be significantly higher than mortality; (2) low-intensity disturbance would result in a marginal increase in tree mortality in the short term; (3) turnover would vary among species and would be associated with plant traits linked to differences in life history; and (4) mortality and recruitment events would be spatially non-random.

Location
Tutuila, a volcanic island in the Samoan Archipelago, Polynesia.

Methods
We censused the tree (stem diameter ‡ 10 cm) community in 3.9 ha of tropical forest three times over a 10-year period, 1998–2008. We calculated annual mortality, recruitment and turnover rates for 36 tree species. We tested for non-random spatial patterns and predictors of mortality, and non-random spatial patterns of tree recruitment. A 2004 cyclone passing within 400 km allowed us to measure the effects of a non-cataclysmic disturbance on vital rates.

Results
Annual turnover was 2.8% and annual recruitment was 3.6%; these are some of the highest rates in the tropics, and likely to be a response to a cyclone that passed < 50 km from Tutuila in 1991. Species turnover rates over 10 years were negatively correlated with wood specific gravity, and positively correlated with annual stem diameter increment. Mortality was spatially aggregated, and a function of site, species and an individual’s growth rate. Recruitment was highest on ground with low slope. The low-magnitude cyclone disturbance in 2004 defoliated 29% of all trees, but killed only 1.8% of trees immediately and increased annual mortality over 5 years by 0.7%.

Main conclusions
The inter-cataclysm period on Tutuila is characterized by frequent, low-amplitude disturbances that promote high rates of tree recruitment and create a dynamic, non-equilibrium or disturbed island disequilibrium tree community. Species with low wood density and fast growth rates have enhanced opportunities for recruitment between cataclysms, but also higher probabilities of dying. Our results suggest that increases in the frequency of cyclone activity could shift relative abundances towards disturbance-specialist species and new forest turnover rates.

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