The response of plant diversity to ecosystem retrogression: evidence from contrasting long-term chronosequences

Following catastrophic disturbances, succession and vegetation development occur, but in the prolonged absence of these disturbances a decline (retrogressive) phase follows in which nutrient availability and tree biomass declines considerably. We measured plant diversity across six long-term chronosequences that each included retrogressive stages in Australia, New Zealand, Alaska, Hawaii and Sweden. In contrast to theories predicting negative or hump-shaped responses of tree diversity to biomass or soil fertility, tree species richness often peaked coincidentally with tree basal area (a surrogate of tree biomass), and declined during retrogression. Similar patterns were found regardless of whether or not species richness estimates were rarefraction-adjusted to correct for variation in stem densities across plots. The Shannon-Weiner diversity index sometimes showed the same pattern, but in two chronosequences was least when tree basal area peaked; this was driven by the domination of total basal area by single tree species in both cases. The decline in tree diversity during retrogression was often associated with reduced relative amounts of total phosphorus in soil. In contrast, total vascular plant species richness often increased during retrogression. These results demonstrate that forests with high tree diversity and biomass do not persist indefinitely in the long-term absence of catastrophic disturbance, and that similar patterns occur across the boreal, temperate and subtropical zones.