Slow recovery of tropical old‐field rainforest regrowth and the value and limitations of active restoration

Abstract

There is current debate about the potential for secondary regrowth to rescue tropical forests from an otherwise inevitable cascade of biodiversity loss due to land clearing and scant evidence to test how well active restoration may accelerate recovery. We used site chronosequences to compare developmental trajectories of vegetation between self-organized (i.e., spontaneous) forest regrowth and biodiversity plantings (established for ecological restoration, with many locally native tree species at high density) in the Australian wet tropics uplands. Across 28 regrowth sites aged 1-59 years, some structural attributes reached reference rainforest levels within 40 years, whereas wood volume and most tested components of native plant species richness (classified by species' origins, family, and ecological functions) reached less than 50% of reference rainforest values. Development of native tree and shrub richness was particularly slow among species that were wind dispersed or animal dispersed with large (>10 mm) seeds. Many species with animal-dispersed seeds were from near-basal evolutionary lineages that contribute to recognized World Heritage values of the study region. Faster recovery was recorded in 25 biodiversity plantings of 1-25 years in which wood volume developed more rapidly; native woody plant species richness reached values similar to reference rainforest and was better represented across all dispersal modes; and species from near-basal plant families were better (although incompletely) represented. Plantings and regrowth showed slow recovery in species richness of vines and epiphytes and in overall resemblance to forest in species composition. Our results can inform decision making about when and where to invest in active restoration and provide strong evidence that protecting old-growth forest is crucially important for sustaining tropical biodiversity.

Restoration of soil microbes and organic matter through tropical reforestation

“…In this study, C retention differences between litter types appeared to occur to a large extent through microbial recomposition, 34 indicating perhaps that decomposition was more strongly regulated by microbial composition than directly by substrate chemistry. We found a similar result regarding influence of litter treatments on microbial efficiency, and our study is thereby consonant with work by in indicating that substrate quality may most strongly regulate SOM by means of recomposing the soil microbial community, which in turn alters C dynamics (but see (Don et al, 2011) and a long lag-time for floristic composition to converge on that of oldgrowth forest during secondary succession (Curran et al, 2014;Finegan, 1996), including within the study area (Shoo et al, 2016). Further investigation of the coupling between diversity and composition of litter and soil microbial community may be an important step towards identifying land use interventions that best promote SOM recovery.…”

“…These two conditions are met by the studied plantings. Indeed aboveground native woody plant richness of local plantings typically reach values similar to reference forest within 25 years (Shoo et al, 2016) and strict selection criteria in our study 59 avoided sites with a history of tillage. Despite the theoretically optimised conditions, stable SOM did not measurably increase in the restoration plantings and enzyme efficiency was seemingly unchanged relative to baseline pasture.…”

“…All sites were located across uplands in tropical north-eastern .65 E). The study region consists of a mosaic of pasture, cropland, and small patches of plantations, secondary forest, and remnant complex notophyll and mesophyll forest (Shoo et al, 2016). Nineteen 0.3 hectare plots distributed across eight sites were sampled for soil microbial traits and SOM content.…”

“…Global carbon (C) cycling and biodiversity are perturbed by the changes in forest cover (Gibson et al, 2011;Pan et al, 2011), but the extent of perturbation remains under debate, largely because of contention over the capacity of plantations and secondary forests for C sequestration and biodiversity conservation (Bonner et al, 2013;Gibson et al, 2011;Shoo et al, 2016). Belowground processes remain poorly studied and are a major knowledge gap that hinders the holistic understanding of land use conversion, despite their significant effects across local, landscape and global scales.…”

“…Ecological restoration plantations appear to balance aboveground biodiversity and C sequestration goals, often demonstrating a strong capacity for reinstating aboveground rainforest structure and species richness within a few decades of establishment (Kanowski et al, 2003;Kanowski and Catterall, 2010;Rodrigues et al, 2009;Shoo et al, 2016), although floristic composition is difficult to recover even with such directed plantations (Rodrigues et al, 2009;Shoo et al, 2016). The efficacy of ecological plantings for soil restoration is less understood.…”