Long-term observations of rain forest succession,tree diversity and responses to disturbance.

The relationship between succession and tropical forest diversity hasbeen much debated. A fundamental disagreement hinges on whether high localspecies richness is a transient successional property, albeit one that can bemaintained by disturbance, or is rather a property of stable late successionalcommunities. This paper addresses this controversy employing a series oflong-term permanent sample plot data spanning seven decades. W.J. Eggelingstudied the vegetation of Budongo Forest, Uganda during the 1930s and 1940s. Hedescribed a series of ten plots (1.4 and 1.86 ) as asuccessionalprogression of forest types in which tree species numbers show a unimodalrise-and-fall over time – a pattern best known from Connell'sillustration of his intermediate disturbance hypothesis. Tree communities infive of the original plots have been intermittently re-assessed over thesubsequent decades. One data-series provides observations spanning 54-yearsfromone intact undisturbed old-growth forest plot. The remaining fourplots were assessed before and after controlled disturbances (tree poisoning)executed in the late 1950s and early 1960s, and the resulting data-series spanc. 20 years of pre-disturbance and c.35 years of post-disturbance changes. Theunimodal pattern of species-richness in the original comparative plot-series isparalleled by a similar rise-and-fall in stem-densities, but rarefactionconfirms that the unimodal pattern in richness also holds for fixedstem-counts.The proportion of species occurring in both large and small stem-size-classesincreases across the series. As richness declines in later succession, lowabundance species occur predominantly in larger stem-sizes. All time-seriesshowa rise in species richness ranging from 12 to 177% (over 50–60 years). Each of thedisturbed plots ultimately reaches greater richness than was recorded anywherein Eggeling's original series. Contrary to expectation a small rise wasalso recorded in the undisturbed late successional plot (c.42 species 10 diameter ha^–1, rising to c.47). The lowestspecies density observed in the study is a 1940s record of c. 10 species 10 diameter ha^–1 in monodominantCynometra Caesalpinoidae] forest and thehighestrecord is c. 61 recorded in 1992, in theyoungest vegetation type monitored. These observations indicate both thevolatile nature of tree-richness patterns and the limitations of simple modelsas aids to interpretation when confronted with real patterns of long-termchange.