Falling seed size in forest fruits after Amazonian fires

Author Joseph Hawes discusses recently published Journal of Ecology article: A large-scale assessment of plant dispersal mode and seed traits across human-modified Amazonian forests by Hawes et al. 

Find out more about the effects of disturbance on functional traits related to seed dispersal in a tropical forest and the implications of this for the future.

Forest fires

As we hear of fires burning through tropical forests, our thoughts are often foremost concerned with the immediate threats from habitat loss and rightly so. Although there are different types of fire, with varying impacts, fires are often linked to deforestation. The effects of deforestation and fragmentation are devastating for tropical biodiversity.

Other lower-intensity fires can remain in the understory, leaving the canopy intact. The effects of these fires and other human disturbances, such as selective logging, can also have severe impacts for forest species, even if forest cover is not lost.

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Disturbed tropical forest in the Brazilian Amazon. Photo: Adam Ronan, Rede Amazônia Sustentável

Examining the effects of ecosystem disturbance

There is now increasing attention on developing a better understanding of the more complex, longer-term consequences of these impacts for ecosystem function, including processes such as seed dispersal. To assess these cascade effects of disturbance on functional traits related to seed dispersal, we worked in two areas of the eastern Brazilian Amazon. These were chosen to cover landscapes with a wide range of human disturbance. Pressure from agricultural expansion, including cattle ranching and mechanised farming (e.g. soya) is high in the eastern Amazon but this pressure is not uniform and some areas are more affected than others. This is also the case for other pressures, such as from silviculture plantations, selective logging and fire.

We used a network of 230 permanent tree plots in the eastern Brazilian Amazon that form part of wider investigations by the Sustainable Amazon Network (Rede Amazônia Sustentável, RAS). Our plots covered a steady gradient of increasing disturbance in forests that had been logged, burned or logged-and-burned, as well as undisturbed forests. In addition, we included secondary forests in various stages of recovery from previous clear-felling. This way, we were able to assess the impact of both disturbance and recovery on tree communities; in particular the characteristics of fruits and seeds found in these different forests.

We combined field data from these plots with information on the fruits and seeds of the tree species through measurements made in Brazilian and online herbaria, as well as other measurements extracted from literature sources. We then conducted analyses to see how the type of seed dispersal and seed size in tree communities was affected by the level of disturbance from logging and/or fires, and the level of recovery from clear felling in secondary forests compared to undisturbed primary forests.

Left: A fire burns through a forest reserve in Monte Dourado in the state of Pará in 2004. Photo: Jos Barlow. Centre: Fruits on the forest floor. Photo: Marizilda Cruppe/Rede Amazônia Sustentável. Right: Disturbed tropical forest in the Brazilian Amazon. Photo: Adam Ronan/Rede Amazônia Sustentável.

Shifts in seed dispersal

We found that, in addition to reduced tree diversity, disturbance increased the proportion of lower wood density and small-seeded tree species in study plots. We also found that disturbance increased the proportion of stems with seeds that are ingested by animals and reduced those dispersed by other mechanisms (e.g. wind). Older secondary forests had functionally similar plant communities to the most heavily disturbed primary forests.

Our results add to the evidence that tropical forest disturbance has pervasive ecosystem effects, which extend beyond the loss of species richness. Such long-term changes in fruit and seed traits have major implications for the community of frugivorous animals that depend on them and that act as seed dispersers in the complex web of interactions found in an intact ecosystem.

hawes - Fruits-on-the-forest-floor-Marizilda-Cruppe-Rede-Amazônia-Sustentável
Fruits on the forest floor. Photo: Marizilda Cruppe, Rede Amazônia Sustentável

Looking to the future

Many smaller-seeded trees are also pioneer species, which grow quickly in disturbed forests and absorb carbon as they grow. However, these species are also often those with lower wood densities and can therefore store less carbon in total. The dominance of these species in secondary and disturbed forests could have long-term implications for carbon storage in human-modified tropical forests.

To help reduce the disruption to ecosystem functions, such as seed dispersal and carbon cycling, we need to continue improving our understanding of the interactions between plants and animals in tropical forests. Our study also highlights the importance of developing comprehensive plant traits databases, such as TRY, that include reproductive traits as well as leaf and stem traits.

The priority remains the avoidance of deforestation and forest degradation from fire and logging.

Joseph Hawes Postdoctoral Fellow, Norwegian University of Life Sciences, Norway & Visiting Scientist at Anglia Ruskin University, UK

Read the full Open Access research article online: A large‐scale assessment of plant dispersal mode and seed traits across human‐modified Amazonian forests

You can also read the Press Release for this paper on the British Ecological Society News page: Amazon forest disturbance is changing how plants are dispersed

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