Amburana cearensis wood anatomyAmburana cearensis is a late-successional species from dry tropical forest with wood density of 0.6 g cm-3. The holes indicate the water transporting vessels, the white cells indicate aliform axial parenchyma, a useful trait to identify wood from the Fabaceae family, and the horizontal lines indicate the annual growth rings, that are typical for many seasonally dry deciduous tropical trees
Tropical forests are being deforested at an alarming rate for agricultural use and pastureland, but the good news is that they can also regrow naturally after agricultural fields are abandoned. This regrowing process is called “succession”, which is one of the most widespread and fundamental processes in nature. During succession the vegetation gradually builds up, leading to changes in environmental conditions at the forest floor, and because species differ in their growing strategies this leads to shifts in species composition over time. Understanding how succession works is crucial to improve forest restoration initiatives and to select the best species for planting.
A large team of ecologists from Latin America, United States, Australia and Europe published this week an article in Nature Ecology and Evolution. They followed recovery of successional tropical forests in 50 locations across 10 Latin American countries. They found that wet and dry forests show actually opposite successional pathways, which implies a paradigm shift in ecology, with large consequences for forest restoration.
Prof. Lourens Poorter from Wageningen University and lead author of the study, says: “Species with different characteristics thrive under different environmental conditions. A key characteristic of tree species is their stem wood density. Species that produce cheap and soft wood have the ability to grow very fast when light and water are abundant. However, this soft wood comes at the expense of a reduced survival, especially under suboptimal conditions like shade and drought. As a result, soft-wooded species have a ‘rock-and-roll’ life style; they peak early in life, live fast and die young.
Prof. Mark Westoby, Macquarie University says: “On the other hand, species that produce expensive and durable wood can persist for a very long time, especially under adverse conditions. But this strategy comes at the expense of a reduced and slow growth”. He adds that this is an important step to understand the shift in species composition during forest succession: “Successional theory predicts that early in succession light and water resources are in abundant supply, which leads to the dominance of “fast” pioneer species with soft wood, whereas late in succession resource availability declines, leading to the dominance of “slow” late-successional species with hard wood”.
To evaluate successional changes in wood density, the 2ndFOR research team analysed forest recovery at an unprecedented spatial scale, using original data from 50 sites, 1,400 plots and >16,000 trees from tropical forests across Latin America. Co-author Dr. Danae Rozendaal says: “Our results show that in wet forest we indeed see a shift from soft- to hard-wooded species over time, which is in line with successional theory. However, in dry forest we see an opposite shift from hard- to softwooded species s” She explains that “This opposite trend happens because in wet forests, resources (e.g. light) decline during succession, whereas in dry forests initial conditions are very harsh, dry and hot. Only hard-wooded species can tolerate these extreme conditions. When they grow they create a milder micro environment, which paves the way for the establishment of soft-wooded species. Intriguingly, the result is that wet and dry forests start out very differently but become more similar over time in terms of microclimate and species wood density, when vegetation cover builds up.”
Professor Dr. Frans Bongers from Wageningen University assures: “The importance of our study goes beyond the scientific value of testing established theories”. He explains how the ecological insights can be used to improve species selection for restoration: “Our findings suggest that forest restoration in areas with intense dry season should prioritize planting species with high wood density, because those have higher chances of surviving the dry period. This is different for wet forests, though, where a mix of soft and hard wood species can be successfully planted at the onset; the fast soft-wooded species rapidly establish a functioning vegetation, and shelter the slow hard-wooded species so that they can take over at the longer term.”
Dr. Ima Vieira, a co-author of the study, from Goeldi Museum in the Brazilian Amazon, argues “It must be recognized that planting trees should not be a priority in restoration projects, but rather the use of natural regeneration. In landscapes that have not been severely degraded and still hold enough forest cover, natural regeneration is the cheapest and technically easier alternative for forest restoration, as previous 2ndFOR studies enphasizes.” And she adds that “The recovery time can, in some cases, be accelerated by planting native species. In such situations, the recommendations from this study on species selection are key for restoration success.”