Forest structure and live aboveground biomass variation along an elevational gradient of tropical Atlantic moist forest (Brazil)

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Abstract

Live aboveground biomass (AGB) is an important source of uncertainty in the carbon balance from the tropical regions in part due scarcity of reliable estimates of live AGB and its variation across landscapes and forest types. Studies of forest structure and biomass stocks of Neotropical forests are biased toward Amazonian and Central American sites. In particular, standardized estimates of aboveground biomass stocks for the Brazilian Atlantic forest are rarely available. Notwithstanding the role of environmental variables that control the distribution and abundance of biomass in tropical lowland forests has been the subject of considerable research, the effect of short, steep elevational gradients on tropical forest structure and carbon dynamics is not well known. In order to evaluate forest structure and live AGB variation along an elevational gradient (0–1100 m a.s.l.) of coastal Atlantic Forest in SE Brazil, we carried out a standard census of woody stems ≥4.8 cm dbh in 13 1-ha permanent plots established on four different sites in 2006–2007. Live AGB ranged from 166.3 Mg ha−1 (bootstrapped 95% CI: 144.4,187.0) to 283.2 Mg ha−1 (bootstrapped 95% CI: 253.0,325.2) and increased with elevation. We found that local-scale topographic variation associated with elevation influences the distribution of trees >50 cm dbh and total live AGB. Across all elevations, we found more stems (64–75%) with limited crown illumination but the largest proportion of the live AGB (68–85%) was stored in stems with highly illuminated or fully exposed crowns. Topography, disturbance and associated changes in light and nutrient supply probably control biomass distribution along this short but representative elevational gradient. Our findings also showed that intact Atlantic forest sites stored substantial amounts of carbon aboveground. The live tree AGB of the stands was found to be lower than Central Amazonian forests, but within the range of Neotropical forests, in particular when compared to Central American forests. Our comparative data suggests that differences in live tree AGB among Neotropical forests are probably related to the heterogeneous distribution of large and medium-sized diameter trees within forests and how the live biomass is partitioned among those size classes, in accordance with general trends found by previous studies. In addition, the elevational variation in live AGB stocks suggests a large spatial variability over coastal Atlantic forests in Brazil, clearly indicating that it is important to consider regional differences in biomass stocks for evaluating the role of this threatened tropical biome in the global carbon cycle.

Introduction

Biomass is a key property of ecosystems (Chapin et al., 2002, Fahey and Knapp, 2007) that results from the mass balance between rates of gain due to productivity and losses due respiration and mortality (Keeling and Phillips, 2007a). In tropical forests, the live aboveground biomass (AGB) pool plays an important role in the global carbon cycle, accounting for a significant fraction of the total carbon pool and nutrient stocks (Brown and Lugo, 1984, Dixon et al., 1994, Brown et al., 1995, Phillips et al., 1998). AGB estimates are still an important source of uncertainty in the carbon balance from the tropical regions in part because of a scarcity of reliable estimates of live AGB and its variation across landscapes and forest types (Houghton, 2007, Saatchi et al., 2007, Houghton et al., 2009). Therefore, improved local and regional AGB estimates provide essential data that enable the extrapolation of biomass stocks to ecosystems or biome-wide carbon cycle modeling, as well as to allow reliable emission estimates from land use change scenarios (Urquiza-Haas et al., 2007, Houghton et al., 2009, Loarie et al., 2009).

Tropical South America represents the greatest concentration of tropical rain forest in the world, extending over the Amazonian region (885 million hectares), and extra-Amazonian areas in the Pacific coast of Colombia and Ecuador, and the Atlantic coast and Iguaçu and Paraná River valleys of Brazil (85 million hectares) (FAO, 2000). The Atlantic Forest Domain (Morellato and Haddad, 2000, Oliveira-Filho and Fontes, 2000), considered as one of the global centers of vascular plant diversity and endemism (Mutke and Barthlott, 2005, Guedes-Bruni et al., 2009, Murray-Smith et al., 2009), and one of the most threatened tropical forest regions in the world (Myers et al., 2000, Laurance, 2009), still covers about 14 million hectares (Galindo-Leal and Camara, 2003), or 16,5% of the total extra-Amazonian tropical rain forest in South America. Despite its extent, forest carbon pools and fluxes of the Atlantic Forest have seldom been studied, and standardized estimates of aboveground stocks of live and dead biomass are rarely available. The only comparable studies published to date are those of Tiepolo et al. (2002) and Rolim et al. (2005) that reported aboveground carbon stocks (Tiepolo et al., 2002) and changes (Rolim et al., 2005) for two Atlantic Forest sites.

Live aboveground biomass stocks vary widely among Neotropical forests due to regional differences in stem size distribution, soil fertility and topography, as well as disturbance (Clark and Clark, 2000, DeWalt and Chave, 2004, Vieira et al., 2004, Rolim et al., 2005, Sarmiento et al., 2005, Castilho et al., 2006, Malhi et al., 2006, Muller-Landau et al., 2006, Urquiza-Haas et al., 2007). Differences in live aboveground biomass among Neotropical forests may also be related to local canopy height variation as well as average wood density and forest composition (Baker et al., 2004, Chave et al., 2005, Vieira et al., 2008, Nogueira et al., 2008). Understanding the role of environmental variables that control the distribution and abundance of biomass in tropical lowland forests has been the subject of considerable research (Clark and Clark, 2000, DeWalt and Chave, 2004, Vieira et al., 2004, Baker et al., 2004). Nonetheless, the effect of short (<2000 m elevational range), steep elevational gradients on tropical forest structure and carbon dynamics is not well known (Lieberman et al., 1996, Bruijnzeel and Veneklaas, 1998, Waide et al., 1998, Takyu et al., 2002, Takyu et al., 2003, Leuschner et al., 2007, Zach et al., 2010).

Depending on latitude and regional climate, the elevational rate of change in vegetation structure with elevation varies greatly, resulting in short (<2000 m) or long (>2000 m) gradients (Grubb, 1977, Bruijnzeel and Veneklaas, 1998, Bruijnzeel, 2002). In short elevational gradients vegetation zonation can be compressed (“Massenerhebung” effect; Richards, 1996) allowing the appearance of cloud montane forests in lower elevations (Grubb, 1977). Short elevational gradients in tropical regions may display stronger edaphic discontinuities over short distances due to steep topography and microclimate variation than long elevational gradients (Ashton, 2003, Silver et al., 1999, Daws et al., 2002, Takyu et al., 2003). Air temperature, solar radiation and atmospheric pressure variability are primarily a function of elevation (Korner, 2007), but other environmental conditions, including light availability (photosynthetically active radiation), soil moisture and temperature, and nutrients are expected to co-vary along short elevational gradients in the tropics due to steep topography (Proctor et al., 1983, Silver et al., 1999, Daws et al., 2002, Takyu et al., 2003, Aiba et al., 2004); biotic factors such as rates of microbial decomposition are likely to reflect these features (Lieberman et al., 1996, Abril and Bucher, 2008). Therefore, differences in forest structure and biomass stocks may be found among sites over short distances. The general trends for long tropical elevational gradients are a decline in forest stature and live aboveground biomass while stem density increases with elevation (Grubb, 1977, Raich et al., 1997, Tanner et al., 1998, Waide et al., 1998, Aiba and Kitayama, 1999, Kitayma and Aiba, 2002, Moser et al., 2007). This pattern has been explained as a result of climatic constrains on photosynthesis and transpiration and nutrient uptake with increasing elevation (Bruijnzeel and Veneklaas, 1998, Kitayma and Aiba, 2002, Raich et al., 2006).

Our study is focused on a geologically and topographically complex region located along the Serra do Mar mountain range covered by a floristically diverse old-growth, tropical Atlantic moist forest (Oliveira-Filho and Fontes, 2000, Murray-Smith et al., 2009) in SE Brazil. Given the limited information about live aboveground biomass stocks for the Atlantic Forest, our goals are: (1) to quantify how biomass is spatially distributed along the elevational gradient; (2) to evaluate the spatial pattern of biomass distribution within the landscape in relation to environmental gradients including climate, soil moisture and chemical properties; and (3) to compare this estimate obtained for a large-scale network of permanent plots to those observed across Neotropical forests. To the best of our knowledge no prior studies have investigated the effect of elevational gradients on forest structure and live aboveground biomass in the Atlantic Forest. Therefore, quantifying the complex three-dimensional forest structure of this threatened tropical forest is the first step to understand aboveground biomass partitioning and carbon dynamics (Broadbent et al., 2008).

Section snippets

Study site and sampling design

We assessed forest structure and live aboveground biomass (AGB) variation at four sites along an elevational gradient (0–1000 m asl) of tropical moist forest in São Paulo state, SE Brazil. The elevational gradient includes a network of 13 1-ha permanent plots established in 2005–2006 to study forest diversity and dynamics, and ecosystem functioning of the Brazilian Coastal Atlantic Forest (Joly et al., 2008). The plot network is located within the Serra do Mar State Park (PESM) (23°34′S and

Forest structure and live aboveground biomass (AGB) distribution

On average, we sampled 1576 stems ≥4.8 cm dbh per hectare (bootstrapped 95% CI: 1446, 1707; see Appendix C) distributed across different life forms: trees (83%), palms (16%), and tree ferns (1%). For stems ≥10 cm dbh, we sampled 759 stems ha−1 (bootstrapped 95% CI: 693, 822). Trees were the dominant life form across all forest types, comprising 72–89% of stems and 96–98% of live AGB  4.8 cm; and 71–98% of stems and 97–99.5% of live AGB  10 cm dbh (Table 3). The contribution of palms was highest in the

Forest structure and live aboveground biomass (AGB) distribution

Despite has been shown that the aboveground biomass declines with increasing elevation (Raich et al., 1997, Waide et al., 1998, Aiba and Kitayama, 1999, Kitayma and Aiba, 2002, Moser et al., 2007, Leuschner et al., 2007) we found an opposite pattern for this short elevational gradient of tropical Atlantic moist forest in Brazil. In the cited studies, soil supply of nutrients became progressively more limiting relative to plant demands at higher, cooler sites thereby limiting forest productivity

Conclusion

We provide the first extensive, landscape-scale estimate available for live AGB in the Brazilian Atlantic forest by evaluating forest structure variation along an elevational gradient. The high capacity of intact Atlantic forest sites for carbon storage aboveground in live biomass stocks for the Atlantic forest is within the range of other Neotropical forests, but is lower than central Amazonian forests. Large and very large trees comprised the most important and most variable live biomass

Acknowledgements

This research was supported by the State of São Paulo Research Foundation (FAPESP) as part of the Thematic Project Functional Gradient (FAPESP 03/12595-7 to C. A. Joly and L. A. Martinelli), within the BIOTA/FAPESP Program—The Biodiversity Virtual Institute (http://www.biota.org.br). COTEC/IF 41.065/2005 and IBAMA/CGEN 093/2005 permit. F.A.M. Santos was supported by a grant from the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq, Grant No. 304937/2007-0). We gratefully

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