Wet and dry tropical forests show opposite pathways in forest recovery

Tropical forests are being deforested at an alarming rate for agricultural use and pastureland, but fortunately they can regrow naturally after agricultural fields are abandoned. Over time, the vegetation gradually builds up (called “succession”), leading to changes in environmental conditions at the forest floor. Because species differ in their growing strategies, these changes in environmental conditions lead to shifts in species composition over time. However, so far we have a poor understanding of forest succession across broad spatial scales. This study presents a large collaboration effort of the 2ndFOR research network (www.2ndFOR.org) that assesses forest succession across Latin America.
Published in Ecology & Evolution

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Young secondary forest (in front) and older secondary forest (in back) that recover after manioc cultivation in Tefé, Brazil. (photo: F. Bongers)

The 2ndFOR network is a large team of ecologists from Latin America, United States, Australia and Europe working on tropical forest succession. By working together, we could analyze forest succession at an unprecedented spatial scale, using original data from 50 sites, 1,400 plots and >16,000 trees from tropical forests across Latin America. We found that wet and dry forests show opposite successional pathways, probably because they differ in environmental conditions just after land abandonment, and they differ in how environmental conditions change during succession. To understand this process, we evaluated changes in the characteristics of the species.

Locations of the 50 study sites and their climatic water deficit (a measure for water lost by the environment for the months when evapotranspiration exceeds precipitation, with high values indicating areas with a strong dry season and low values indicating areas with a weak or no dry season). The size of the circle indicates the average wood density of a 5 year old secondary forest.

Species with different characteristics thrive under different environmental conditions. A key characteristic of tree species is their stem wood density; the stem dry mass per unit of green stem volume (in g cm-3). Species that produce soft (and therefore cheap) 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. On the other hand, species that produce durable (and therefore expensive) wood can persist for a very long time, especially under adverse conditions. However, this strategy comes at the expense of a reduced and slow growth.

Anatomical picture of the wood of Heliocarpus americanus (left), an early-successional species from moist forest with wood density of 0.3 g cm-3, and of Amburana cearensis (right), a late-successional species from dry tropical forest with wood density of 0.6 g cm-3. The holes are water transporting vessels, the white cells (right) are aliform axial parenchyma, which is a useful trait to identify wood from the Fabaceae family, and the horizontal lines are annual growth rings that are typical for many seasonally dry deciduous tropical trees. (photo left: I. McDonald; photo right: G. Colletta)

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. Our results show that in wet forest we indeed see a shift from soft- to hard-wooded species over time, in line with current forest successional theory. However, in dry forest we see an opposite shift from hard- to soft-wooded species. In wet forests, resources (e.g. light) are indeed high at early succession but decline later during succession, explaining the shift from soft-wooded to hard-wooded species. In dry forests, however, initial conditions are very harsh (i.e. dry and hot), and get milder over time (e.g. cooler because of a thicker forest cover). For that reason, we see that dry forests are initially dominated by hard-wooded species and over time get more soft-wooded species. In sum, we see tough species at tough times; for wet forests this is late in succession and for dry forests this is early in succession.

The ecological insights of our study can be used to improve species selection for restoration. When surrounding forest cover is sufficiently high, then restoration can rely on natural regeneration, which is cheaper, technically easier and results in a more natural and biodiverse forest. However, in fragmented or degraded areas, the planting of suitable native species can greatly enhance forest recovery rate. Our findings suggest that in areas with intense dry season, restoration projects should prioritize planting species with high wood density because those have higher chances of survival. In wet forests, however, a mix of soft- and hard-wooded species can be successfully planted at the onset; the fast-growing, soft-wooded species will rapidly establish a protecting forest cover, while the slow-growing, hard-wooded species will form the basis of a long-term stable forest.

To learn more about our paper, see:

Wet and dry tropical forests show opposite successional pathways in wood density but converge over time. 2019. Nature Ecology and Evolution. DOI: http://dx.doi.org/10.1038/s41559-019-0882-6

To follow our work, check us out on Twitter: @2ndFOR1, and on our webpage: www.2ndfor.org.

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