Droughts have a significant & long-lasting change on tree and liana regeneration in a monodominant Amazon forest

Monodominant tropical forests, especially those not associated with flooded environments, are rare and still poorly understood. In the transition between Cerrado and the Amazon rainforest biomes in Brazil, lies patches of monodominant forests of “Pau-Brasil” or Bloodwood cacique (Brosimum rubescens, Figure 1). The structure of these forests have trees of different sizes and represents about 80% of above-ground biomass (Marimon et al., 2001).

Figure 1: Monodominant forest of Brosimum rubescens in Cerrado-Amazonia transition, Brazil. Credits: Beatriz S. Marimon.

The mechanisms behind the dominance of few species and their temporal maintenance in tropical forests are varied and often distinctive (Elias et al., 2018). Analysis how regeneration occurs especially looking at the regrowth of younger and non-established individuals within a community (Figure 2), has been an effective way of finding answers to this occurrence of monodominance as well as helping us to understand the diversity (alpha and beta) found within tropical forests. The diversity seen in tropical forests is normally maintained by balancing or compensatory mechanisms that benefit rare species. As such, when monodominance occur, these regeneration mechanisms uncharacteristically favour common species and tree diversity remains low. Thus, evaluation of long-term regeneration dynamics should be able to determine whether changes in diversity occur with changes in disturbances, such as droughts, logging and anthropogenic climate change — all of which impact tree mortality, recruitment, carbon sequestration and species composition in Amazonia (Esquivel‐Muelbert et al., 2019; Phillips et al., 2009).

Figure 2: Young individuals from the natural regeneration of Brosimum rubescens monodominant forest. Credits: Paulo S. Morandi.

In our study published in Plant Ecology (Marimon et al., 2020), we analysed the number of different tree and liana species (species richness and alpha-diversity) and how they changed annually between 1996 and 2017 in a monodominant forest of Brosimum rubescens in the Cerrado-Amazonia transition in Brazil. The study area is marked by high annual climatic seasonality (~6 months), and in the last years the intensity of seasonal droughts and average temperature are increasing.

Twenty-one years of tree and liana regeneration were evaluated in four censuses in 30 plots. Within these subplots we nested smaller plots to sample vegetation in different size classes, totalling 30 per class: 1 m × 1 m (seedlings: height ≤ 30 cm), 2 m × 2 m (saplings: > 30 cm to ≤ 60 cm), 5 m × 5 m (poles or young stems: > 60 cm to ≤ 200 cm) and 10 m × 10 m (treelets: height > 200 cm and diameter < 5 cm). Our study represents by far the longest-running assessment of regeneration in monodominant forests to date anywhere in the Tropics, and is the first attempt in such system to explicitly account for the long-term regeneration of woody lianas and trees.

Our results show that stem density, species richness, tree diversity and evenness changed between regenerative classes over time. Removing the lianas from the analysis, in all regenerative size classes, tree species richness was highest in 2002 and declined substantially after 2007–2008 and 2015–2016 drought events. In all regenerative size classes the proportion of liana (species and abundance) increased, especially among seedlings and saplings. In 1997 lianas contributed less than 1% of the woody plant seedlings, but by 2018 they represented more than 50% of all woody plant seedlings (Figure 3). The abundance of B. rubescens seedlings declined markedly, from 85% in 1997 to 29% in 2018, after the most intense El Niño-driven drought, whereas the fraction contributed by other tree species changed little (Fig. 3). The regeneration community experienced a high rate of species turnover, with changes in the overall richness and species diversity determined principally by lianas and not trees.

Figure 3: Variation in seedlings stem density (lianas and B. rubescens) and climate over the time series (1996-2018). Credits: Dr. Fernando Elias.

In conclusion, we found unexpectedly large changes, including a dramatic decline in dominance of the smaller size classes of B. rubescens, and a compensatory shift towards dominance of the seedlings community by lianas. The nature and the timing of the shifts and the length of the observation window is consistent with drought events, especially the increase in temperature associated with water deficit experienced over the last two decades, and that may induce long-term shifts in the structure and floristic composition of forest regeneration (seedlings, saplings, poles and treelets), particularly via increases in liana richness and abundance, as observed for adult trees in other studies (Esquivel‐Muelbert et al., 2019; Phillips et al., 2009).

We suggest that if these trends (increase in lianas, hotter and more variable climate, and decline in B. rubescens regeneration) continue the structure and the floristic composition of this tropical monodominant forest will experience large changes, potentially becoming transformed into a mixed forest. Since ongoing land-use change, climatic changes, and increases in lianas appear to be almost ubiquitous among tropical forests of the Americas, our results suggest a high level of threat to the survival and maintenance of remaining Brosimum rubescens monodominant forests.

About the Author: Dr. Fernando Elias is a biologist and Postdoctoral Research Fellow in the Sustainable Amazon Network (www.rasnetwork.org) at Embrapa Amazônia Oriental in Brazil. He holds a PhD in Ecology at Universidade Federal do Pará/Embrapa Amazônia Oriental, and Master degree in Ecology and Conservation at Universidade do Estado de Mato Grosso. He has experience with forest ecology and restoration related to the assessment of carbon dynamics, biodiversity recovery, phytogeography and conservation biology. Twitter: @feliasbio. E-mail: fernandoeliasbio@gmail.com. ORCID: orcid.org/0000-0001-9190-1733

References

Elias, F., Marimon, B. S., Marimon-Junior, B. H., Budke, J. C., Esquivel-Muelbert, A., Morandi, P. S., … Phillips, O. L. (2018). Idiosyncratic soil-tree species associations and their relationships with drought in a monodominant Amazon forest. Acta Oecologica, 91(July), 127–136. https://doi.org/10.1016/j.actao.2018.07.004

Esquivel-Muelbert, A., Baker, T. R., Dexter, K. G., Lewis, S. L., Brienen, R. J. W., Feldpausch, T. R., … Phillips, O. L. (2019). Compositional response of Amazon forests to climate change. Global Change Biology, 25(1), 39–56. https://doi.org/10.1111/gcb.14413

Marimon, B. S., Felfili, J. M., & Haridasan, M. (2001). Studies in monodominant forests in Eastern Mato Grosso, Brazil: I. a Forest of Brosimum rubescens Taub. Edinburgh Journal of Botany, 58(1), 123–137. https://doi.org/10.1017/S096042860100049X

Phillips, O. L., Aragao, L. E. O. C., Lewis, S. L., Fisher, J. B., Lloyd, J., Lopez-Gonzalez, G., … Torres-Lezama, A. (2009). Drought Sensitivity of the Amazon Rainforest. Science, 323(5919), 1344–1347. https://doi.org/10.1126/science.1164033

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