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

Abstract

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.

How does pedogenesis drive plant diversity?

“…Soil chronosequences, by contrast, enable comparisons between ecosystems within much smaller regions, where factors such as climate and parent material are controlled [50,51]. The available data reveal that total plant species richness usually increases with soil age across soil chronosequences spanning boreal, temperate, subtropical, and Mediterranean climates [4,5,24] (Figure II). Together, these global and smallerscale patterns motivate the search for underlying mechanisms that can explain the greater plant diversity on older soils.…”

“…Soil chronosequences provide some support for the productivity-diversity hypothesis, in that total plant species richness generally increases as ecosystem retrogression proceeds (and productivity declines) [4,5]. By contrast, according to the resource-ratio theory [12], higher plant diversity is maintained under equilibrium conditions if multiple resources limit productivity [14], such that a positive relation is expected between the number of colimiting resources and plant diversity [14].…”

“…By contrast, according to the resource-ratio theory [12], higher plant diversity is maintained under equilibrium conditions if multiple resources limit productivity [14], such that a positive relation is expected between the number of colimiting resources and plant diversity [14]. However, the available evidence suggests that plant species richness along soil chronosequences is not greatest on intermediate-aged soils [5], where multiple nutrient limitation is most likely [10]. For example, evidence for multiple nutrient limitation [N, P, potassium (K), and/or micronutrients] along the Jurien Bay chronosequence is only found on relatively species-poor young to intermediate-aged soils (<7000 years), whereas P is the only limiting nutrient on older soils (>120 000 years) [10], where plant diversity is greatest ( Figure II How to test the productivity-diversity and resourceratio theories?…”

“…All of these soil chronosequences include retrogressive stages [9,30]. Data are from [5], except data from the Jurien Bay chronosequence [10], which are unpublished (Graham Zemunik). Points represent individual plots, which were 0.01 ha in size for the Jurien Bay chronosequence and 0.03 ha for the three others [5].…”

“…Data are from [5], except data from the Jurien Bay chronosequence [10], which are unpublished (Graham Zemunik). Points represent individual plots, which were 0.01 ha in size for the Jurien Bay chronosequence and 0.03 ha for the three others [5]. Meta-analysis of six chronosequences (these four plus two others in [5] for which data were available) reveal an overall highly significant increase in plant species density with soil age (Poisson linear mixed-effect model with logarithmic link, random intercepts per chronosequence and 'stage within chronosequence', random slopes per chronosequence, plot size as an offset; b = 0.120, s = 0.028, z = 4.28, P 0.0001; no evidence for overdispersion: x 2 = 127.2, df = 194, P = 0.999).…”