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1 We evaluate the effects of large-scale disturbance on tropical tree communities by examining the population dynamics of all individuals > 4.9 cm in diameter at breast height (d.b.h.) of 12 tree species over 30 years (1964–94) in lowland tropical rain forest on Kolombangara, Solomon Islands.
2 During the study period Kolombangara was struck by four cyclones between 1967 and 1970. The last cyclone caused most damage to canopy structure. Mortality in the 6-month interval spanning the first cyclone was 7.0% of all trees, while mean annual mortality for all other intervals (including those spanning other cyclones) was 1.4–2.2% year−1. Mortality varied between species but was independent of topography and geographical location.
3 Recruitment increased from very low rates (median 0.0% year−1) before the first cyclone to median values of 1.6–3.2% year−1 during 1971–79, i.e. following a lag period of 3.5–8 years after the first cyclone. Recruitment rates were higher on plots showing greater mortality rates during this cyclone. Recruitment and mortality rates were still higher in 1994 than they had been before the 1967–70 cyclones.
4 Mean annual mortality rates were positively correlated with mean annual recruitment rates across species. This relationship reflects a continuum of life-history characteristics and contributes to constancy in the relative abundance of the 12 species when the same sets of plots are compared over all measurement intervals up to 30 years.
5 We conclude that cyclone impacts have only short-term effects on the relative abundance of common tree species on Kolombangara, and do not therefore prevent the establishment of an equilibrium rank abundance hierarchy or create spatial variation in tree species composition. Differences in forest composition across Kolombangara are more likely to have been caused by differential anthropogenic disturbance linked to settlement patterns.
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Studies of damage and mortality to trees by the catastrophic windstorms in the Pacific and Caribbean variously known as cyclones and hurricanes (Whitmore 1974, 1989; Walker et al. 1991; Everham & Brokaw 1996), as well as studies of the impacts of drought, fire, landslides and earthquakes in a variety of tropical forests (reviewed by Whitmore & Burslem 1998), all suggest that tropical rain forests are non-equilibrium plant communities in which background tree mortality rates are high (1–2% year−1), and that community composition may be influenced strongly by rare, but large-scale, disturbance events.
Cyclones or hurricanes, which occur in two belts 10–15° north and south of the equator, cause massive canopy damage to forests (reviewed by Everham & Brokaw 1996). The heavy rain and high winds associated with these catastrophic windstorms often cause high rates of defoliation, uprooting and snapping of stems and branches to trees in their path (e.g. Wadsworth & Englerth 1959; Unwin et al. 1988; Brokaw & Walker 1991; Bellingham et al. 1994; Zimmerman et al. 1994). However, their impacts on tree population dynamics and community composition are less clear, and therefore more controversial, because the evidence needed to assess these processes can come only from long-term monitoring of tree populations, and long-term data are mostly lacking. In tropical forests, short-term mortality in response to catastrophic windstorms is relatively low (1–25%, Everham & Brokaw 1996), compared with the impacts of other natural landscape-level disturbance factors such as fire or landslides. However, comparisons between studies are complicated by the differing intervals after disturbance during which mortality has been recorded and the (mostly unquantified) importance of delayed mortality of damaged trees (Walker 1995).
Recovery of forest structure after a severe windstorm may occur by one or more mechanisms. These are re-sprouting of damaged stems or crowns, recruitment of new individuals from seed arriving after the disturbance or previously buried in the soil, and release of seedlings and saplings that are present in the forest understorey. At one extreme it is possible that the open, defoliated canopy conditions created by severe windstorms allow mass germination of the seeds of pioneer species (sensuSwaine & Whitmore 1988) which become a recognizable cohort of larger trees within a short period. Then, as in the process of forest regeneration on abandoned agricultural fields, shade-tolerant species might become established and grow up beneath the canopy of the pioneers, and the forest gradually revert to its original composition. This mechanism of forest recovery, however, seems rare, although the early stages have been described in Nicaragua, where a pulse of recruitment of Cecropia spp. followed hurricane Joan in 1988 (Ferguson et al. 1995).
A more general outcome is that short-term recovery of forest structure takes place by re-sprouting of damaged stems and branches (Walker 1991; Yih et al. 1991; Bellingham et al. 1994; Zimmerman et al. 1994). The prevalence of re-sprouts among the stems damaged by hurricane Joan in Nicaragua led Yih et al. (1991) and Boucher et al. (1994) to propose a ‘direct regeneration’ model of forest recovery, in which ‘species dominant in the first years after the disturbance will be the same as the species which were dominant before the disturbance’ (Boucher et al. 1994). If validated, this model would challenge the view that massive disturbances prevent the establishment of an equilibrium species composition.
Observations on two of the species affected by hurricane Joan have supported the ‘direct regeneration’ model (Boucher et al. 1994). However, rigorous validation requires much longer-term data on a wider range of species because of the considerable time-lag between seedling establishment after disturbance and the first record of a stem above the minimum size used in tree population surveys (usually 5 or 10 cm diameter), and because different species in the community may behave differently.
The mechanism by which tropical forests recover from catastrophic windstorms is important because it determines the species composition of the ensuing forest and hence the long-term response of the community to disturbance. Where forests recover primarily by regrowth of damaged stems or release of shade-tolerant seedlings pre-existing under the canopy, then relative abundance hierarchies and forest floristic composition are less likely to change in response to disturbance than when regrowth occurs by recruitment of pioneer species (Vandermeer et al. 1996). A number of studies in tropical forests have found that windstorms do not have major impacts on tree species composition, but again long-term post-disturbance data are lacking (Dittus 1985; Bellingham et al. 1995). The one study with decades of data on post-disturbance tropical forest recovery from a hurricane (Crow 1980 working in Puerto Rico) is complicated by additional human impacts on the forest. Thus it is usually impossible to determine the relative importance of delayed mortality of damaged stems (including those which have re-sprouted), regrowth by sprouting and recruitment by individuals which established in the disturbed forest shortly after the storm.
Populations of the 12 most common big tree species have been studied on natural forest plots on Kolombangara in the Solomon Islands since 1964 (Whitmore 1974, 1989). There is circumstantial evidence that some of these plots occur in forests that have grown up on land close to human settlements that were abandoned approximately 100 years ago (Burslem & Whitmore 1999). In this paper we present data on disturbance history and tree population dynamics over 30 years, during which all the forests were subjected to four cyclones in the period 1967–70. Our focus here is to answer the following questions:
What are the short-term effects of a cyclone on canopy structure and tree mortality?
How long does it take for stem density and basal area to recover to pre-cyclone levels?
Is the relative abundance of the common big tree species influenced in the long term by the impact of cyclones?
What are the relative contributions of anthropogenic disturbance and cyclone impact to the determination of variation in tree species composition across Kolombangara?