Resilience and successional trends of woody vegetation in seasonally dry tropical forests

Abstract

Understanding the recovery time of vegetation structure and species composition to estimate the resilience of seasonally deciduous tropical forest (SDTF) is fundamental for conservation actions. Related information is still scarce for the semi-arid Brazilian SDTF (locally called Caatinga), which covers more than 800 000 km2 in the northeast of Brazil. We aimed to estimate how long it takes for 11 attributes of vegetation structure and species composition, separately and aggregated, to become similar to an old-growth forest (OGF). The attributes were floristic composition, species richness, Shannon diversity index, plant density, median and maximal diameters and height, basal area, volume and aboveground biomass. The attributes’ regeneration ratios were aggregated as an arithmetic average and a geometric average, based on a variance–covariance analysis. The ratios were determined in chronosequences (5, 15, 25 and 35 regeneration years and an OGF) of SDTF, in three different locations in Ceará state. Plant density, which had a unimodal tendency, reached a peak at 15 years and then decreased, although it was still greater than that of the OGF after 35 years. Richness and diversity, with monotonic trends, reached values similar to the OGF. All other attributes, especially floristic composition, volume and biomass, had lower values than the OGF. Based on the aggregate set of attributes, the SDTF was 79 per cent recovered after 35 years of regeneration. The variance–covariance analysis indicated that species richness and aboveground biomass contributed the most to the aggregate ratio of regeneration, while maximum diameter and basal area contributed the least. Recovery of this semi-arid forest takes longer than recovery of other SDTF and requires more than 35 years of regeneration to return to a composition and structure similar to the original one. Therefore, management plans should restrict destructive uses before at least 35 years of regeneration have been reached.

Resilience of Aboveground Biomass of Secondary Forests Following the Abandonment of Gold Mining Activity in the Southeastern Peruvian Amazon

“…This supports our results in SEM calculations and could explain why the AGB is higher in suction pumping mining forest stands. The positive relationship between AGB and tree species diversity found in suction pumping mining forest stands of the present study was reported by other studies in secondary forests [60,83,84] and old-growth forests [24,58,76,80,85]. Because of forest woody productivity, it is usually positively associated with tree species diversity [85].…”

“…In our study, the dynamics of the slow recovery of species follow the "initial floristic composition model" hypothesis [100], because the dominance (IVI) is relayed from one floristic group (species, genus or family) to another throughout the chronosequence [97]. This hypothesis states that pioneer species colonize early successional stages which remain for some decades [76], and soon they make suitable conditions for colonization by other species more typical of later successional stages [97]. A similar pattern was found in other secondary successions post mining in the Peruvian Amazon [41,41].…”

“…In this study, we found a significant difference in species composition between the two types of mining activities. This may be related to differences in landscape fragmentation and proximity to the sources of propagules [82], as has been observed for other tropical forests [76,79] and secondary forests after mining in the Peruvian Amazon [41]. This difference in species composition and stand structure may have implications for biomass accumulation [30,99], due to the turnover of high wood density species [7,44,60] and the increase in species diversity [24,79,97].…”

“…This result indicates that these relationships are context-dependent [24], because the effect of the distance from the forest matters significantly in heavy machinery mining but not in suction pumping. This suggests that heavy machinery mining is severely impacting AGB resilience because the AGB accumulation may depend on stand forest age [76], land-use history [37], forest cover [30], soil properties [77,78], proportion of wind-dispersed species [79], and type and severity of disturbance [21,80]. In contrast, the small-scale disturbance caused by suction pumping mining and a relative short distance from a propagule source, in some sites of the study, promotes the establishment of species and increasing species diversity [41,81] and facilitating a faster AGB recovery [80,82] in areas recovering from suction pumping mining.…”

“…and Vismia spp.) along the chronosequence, remoteness from existing forest edges [41,76,97], and a lower soil fertility than suction pumping [54]. In addition, different woody plant species can become monodominant due to smaller natural regeneration community size and may generate stochastic effects on species diversity [98].…”