Medium-term dynamics of tree species composition in response to silvicultural intervention intensities in a tropical rain forest
Introduction
Tropical forests are a source of many goods and services, presenting the most complex structures and the highest biological diversity among terrestrial ecosystems (Whitmore, 1998, Ghazoul and Sheil, 2010). The Brazilian Amazon harbors around 20% of the world's species of fauna and flora (Azevedo-Ramos, 2008), and it is estimated that it houses more than 12,000 tree species (ter Steege et al., 2013). Moreover, this region accounts for the largest-continuous area of tropical forest in the world (FAO, 2011). Many anthropogenic processes such as unregulated exploitation and land-use change threaten tropical forest ecosystems and only a small proportion of the biodiversity is contained within protected areas (ITTO, 2009). Thus, conservation strategies will also have to consider forest areas not designated for conservation. Given that logged and secondary forests still harbor a considerable amount of tropical biodiversity (Chazdon et al., 2009, Dent and Wright, 2009, Putz et al., 2012, Burivalova et al., 2014), these anthropogenically disturbed ecosystems are important landscape components for biodiversity maintenance and conservation.
The proportion and identity of species retained in managed forests depend on the applied practices and the ecosystem's resistance and resilience in relation to the disturbance. Considering that managing forests implies manipulating their structures and composition to favor some species and life forms over others, shifts are inevitable (Whitmore, 1998) as well as subsequent impacts on biodiversity (Putz, 2011). Consequently, the intensity and caution employed in silvicultural practices shape the compatibility of timber production and conservation (Putz, 2011). The long-term response of forest biodiversity will depend in particular on the question whether anthropogenic disturbances are compatible with the ecological stability of the system (Swanson et al., 1994). Finally, the maintenance of species diversity and composition will support the continuity of provision of goods and services (Thompson, 2010), and the ecosystem's ability to cope with changing environmental conditions (ITTO, 2009).
Therefore, when managing tropical forests, the challenge is to reduce harmful effects while maintaining productivity (Sheil and van Heist, 2000) and provision of other goods and services. To keep biodiversity, resilience and productivity in managed forests, it is essential to have a detailed understanding of these ecosystems, identifying at which level of use and manipulation thresholds might exist, beyond which the system may lose the capacity to recover from interventions (Thompson, 2011). Furthermore, it is crucial to understand the long-term response to management, since the effects may vary according to the time scale that is being considered.
Tree species diversity, measured either as species richness or as a diversity index, is a common response variable used to assess the impact of forest management interventions. In tropical forests, Clark and Covey (2012) reported that logging impaired tree species diversity, whereas many studies found no negative effect (e.g. Yosi et al., 2011, Baraloto et al., 2012, Carreño-Rocabado et al., 2012) or only a slight influence (e.g. Gourlet-Fleury et al., 2013, Duah-Gyamfi et al., 2014a). However, response variables related to species numbers do not represent species identities per se and therefore may not be suitable to identify relevant impacts of forest interventions (Sheil and van Heist, 2000, Putz, 2011, Putz et al., 2012). Although species diversity in general may be little affected, changes in species composition are likely to occur. If compositional changes through management favor post-disturbance dominance of few widespread species at the expense of rare and specialist species, this may lead to a local biotic homogenization (McKinney and Lockwood, 1999). Such a process will probably lead to lower species richness and higher similarity among formerly distinct communities (Olden et al., 2004).
The evaluation of management impacts on species composition and recovery is challenging (van Kuijk et al., 2009), since the effects may greatly vary among and within tropical forests (Ghazoul and Sheil, 2010). Given our limited understanding of forest ecosystems, it is necessary to make use of all available knowledge to support management decisions (Kimmins, 1997). This can contribute to improve techniques and to sustain tropical biodiversity in managed forests (Meijaard et al., 2005). To make management compatible with conservation of original biodiversity, interventions should not lead to substantial or long-lasting changes in plant species composition and dependent diversity at other trophic levels. So far, there is only scant information on possible thresholds of intervention intensity at which such compositional changes might happen in tropical forests.
In this study, we assessed the effects of different silvicultural intervention intensities on tree species composition and diversity over a period of 30 years. The experiment analyzed here is one of the few long-term studies on forest dynamics following logging and thinning in the Brazilian Amazon or any other lowland tropical rainforest. Specifically, we investigated the medium-term dynamics of tree species composition and diversity compared with the pre-logging condition and to an unlogged control treatment to address the following hypotheses:
- (i)
Logging, damage and thinning affect tree species composition in comparison to the natural forest;
- (ii)
Recovery of pre-logging species composition is negatively related to intervention intensity, measured as reduction in basal area;
- (iii)
Within the range of applied intervention intensities, tree species diversity is not impaired;
- (iv)
If changes in species composition over time are characterized by higher spatial similarity among managed treatments associated with lower species richness, we expected that a local biotic homogenization will occur.
Section snippets
Study site
The study was conducted in the Tapajós National Forest, municipality of Belterra, State of Pará, Brazil (3°18′S to 3°19′S, 54°56′W to 54°57′W). The area was originally chosen for the long-term experiment because it represents a typical forest of the region with a low degree of human impacts (Carvalho, 2002). The topography of the region is flat to slightly undulating and the altitude is around 175 m above sea level. The climate is tropical with one dry season and annual rainfall averages 2100 mm.
Results
Over time, 21,595 trees and 319 tree species were recorded. In the natural forest (C: control), no discontinuities in pole and tree species composition occurred over time. In contrast, silvicultural interventions (L: logging only, LLTI: logging and light thinning intensity, LMTI: logging and medium thinning intensity, LHTI: logging and high thinning intensity) changed species composition in both size classes (see Fig. A2 in Supplementary data).
For poles, logging, harvesting damage and thinning
Discussion
For the community comprising tree-sized individuals (DBH ≥ 10 cm), the species composition was substantially affected by basal area reduction greater than 6.6 m2 ha− 1. This change in composition may lead either to a longer recovery time or to a different state. Conversely, species diversity (species richness and Shannon diversity) was not impaired under disturbed conditions relative to the natural forest and pre-logging conditions, irrespective of intervention intensity.
Conclusions
Our results demonstrate that the composition of tree species (DBH ≥ 10 cm) was substantially affected by high silvicultural intervention intensities (basal area reduction > 6.6 m2 ha− 1) over a period of 30 years after initial selection harvest in a tropical rainforest. Beyond this limit in basal area reduction, changes in tree species composition are still ongoing without signs of recovery towards pre-logging conditions. Strong reductions in basal area through logging damage and follow-up thinning
Acknowledgments
We are especially grateful to Embrapa and all researchers who have been engaged in this project as well as to all colleagues who participated in field inventories and botanical identification. We thank Francis Putz for constructive comments on an earlier version of the manuscript. We also thank the three anonymous reviewers for their valuable comments. The experiment analyzed in this study was financially supported by FAO, PNUD, PPG7, DFID, IBRD, ITTO and CNPq. ALA was financially supported by
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