Exotic pine forestation shifts carbon accumulation to litter detritus and wood along a broad precipitation gradient in Patagonia, Argentina
Graphical abstract
Impacts of pine plantations on C cycles along a precipitation gradient in Patagonia, Argentina. (a) Relative changes (%) in C fluxes (ANPP, as the sum of wood and leaf production, and litter decomposition). Relative changes in C fluxes were calculated as (C flux PP–C flux NV)/C flux NV). (+) refers to positive changes and (−) to negative ones. (b) Absolute (kg C m−2, above) and relative (%, below) changes for each C compartment at each level of precipitation. Relative changes in C stocks were calculated as (C stock PP–C stock NV)/C stock NV). Numbers in blacks refers to net C sequestration in PP; and in red, C-loss.
Introduction
The ecosystem carbon (C) balance in terrestrial ecosystems reflects long-term differences between C inputs and turnover, through photosynthesis and decomposition of organic matter respectively (Amundson, 2001, Chapin et al., 2002, Schlesinger and Bernhardt, 2013), and there is increasing evidence that human activities are altering these C fluxes through multiple pathways (Lal, 2004, Stockmann et al., 2013, Vitousek et al., 1997). Forests are important reservoirs of C (Bonan, 2008) and established and secondary growth forests represent a persistent global C sink (Pan et al., 2011). Accordingly, the introduction of tree species either through afforestation, defined as the conversion of non-tree lands to forest (IPCC, 2014), or reforestation projects of land that was previously forested (Nave et al., 2018) (collectively referred to as forestation projects) are thought to play an important role in efforts to remove carbon dioxide from the atmosphere through biological C sequestration (Fang, 2001, Gliksman et al., 2018, Jackson and Schlesinger, 2004, Lu et al., 2018, Vitousek and Matson, 1991). Recent evidence suggests that even climate could be affected by large-scale afforestation projects in semiarid ecosystems (Yosef et al., 2018). Nevertheless, controversial evidences of the realized C sequestration in tree plantations have arisen (Jackson et al., 2005, Lewandrowski et al., 2014), and alterations in ecosystem attributes and services, such as a reduction in water retention and streamflow (Farley et al., 2005, Jackson et al., 2005), changes in soil chemistry (Berthrong et al., 2009) and nitrogen stocks (Hess and Austin, 2014) have been reported. These trade-offs between C sequestration and ecosystems properties with land-use changes suggest the need for a more complete accounting of nature’s services that could be altered with afforestation (Kim et al., 2016).
Realized C sequestration with intentional planting of fast-growing tree species depends on a number of factors, including climate, stand age, the vegetation being replaced, and the origin of the species. Several studies have demonstrated increased C storage following afforestation of drylands or degraded soils (Bárcena et al., 2014, Grünzweig et al., 2007, Grünzweig et al., 2003, Laganiere et al., 2010; Poeplau et al., 2011), which is consistent with studies in South America that showed that afforestation may increase soil C pools but these C gains decline with increasing rainfall (Berthrong et al., 2012, Eclesia et al., 2012). Plantation age is often positively correlated with increases in biomass C, demonstrated in various studies including China (Francis Justine et al., 2015, Justine et al., 2017), North America (Law et al., 2003) and Korea (Li et al., 2011). While soil C generally diminishes immediately following grassland afforestation (Guo and Gifford, 2002; Paul et al., 2002) or native forest conversion (Chen et al., 2016), these initial losses of soil C with disturbance can be compensated for over time (Berthrong et al., 2012, Eclesia et al., 2012). Finally, the planting of exotic species, particularly Pinus and Eucalpytus species outside their natural range often results in faster growth and more extended geographic distributions than in their native ranges (Procheş et al., 2012), as well as having larger impacts on plant community structure of adjacent ecosystems (Taylor et al., 2016).
Afforestation has been recognized as a tool for sequestering C into both plant biomass and soil (Berthrong et al., 2012). Nevertheless, a key question for forestation projects is to know; a) where the newly fixed C is stored; and b) which pools actually increase as a result of this intentional planting of woody vegetation. Several studies have shown that the second does not occur, with no detectable storage of C in soil pools (Hess and Austin, 2014; Laclau, 2003; Nosetto et al., 2006; Richter et al., 1999). As potential C losses may occur either due to natural disturbances such as fire and pest infestations (Goodale et al., 2002, Lewandrowski et al., 2014) or from wood harvest (Jackson and Schlesinger, 2004), it is important to consider the potential residence time of this new C stored in the afforested ecosystems when it is not sequestered in soil (Farley et al., 2004).
While there has been much interest focused on the quantity of C input and storage in plantations (Berthrong et al., 2012; Laclau, 2003; Nosetto et al., 2006), afforestation projects are not usually evaluated with the simultaneous evaluation of C inputs and turnover, which effectively determine the C balance at the ecosystem scale (Lajtha et al., 2014). Indeed, C inputs from primary production and C turnover from decomposition are controlled by multiple biotic and abiotic factors. Primary productivity correlates positively with mean annual precipitation (MAP) (Jobbágy and Sala, 2000, Knapp and Smith, 2001) and temperature (Del Grosso et al., 2008), while decomposition correlates positively with MAP (Austin and Vitousek, 2000) but also varies with plant cover (Araujo and Austin, 2015, Austin, 2011) and litter quality (Cornelissen et al., 1999, Cornwell et al., 2008). These differential controls on C fluxes suggest that C balance and sequestration with forestation may depend on climate and vegetation traits and attributes of the original and planted species.
An interesting framework to study the relative importance of forestation and climate conditions on C fluxes and storage at regional scale is using a state factor approach (Jenny, 1980), where climate, organisms, topography, geological substrate and time are state factors controlling ecosystem functioning. Through the identification of a suite of sites where most of these factors are kept constant except the one of interest (i.e., precipitation), which varies broadly, there is great potential to understand how ecosystem process respond to this single variable (Austin, 2002, Jenny, 1980, Vitousek, 1994). In this sense, the Patagonian region in temperate South America provides an ideal scenario to study ecosystem functioning along natural climatic gradients (Austin and Sala, 2002, Bertiller et al., 2006, Mazzarino et al., 1998). Westerly winds from the Pacific Ocean bring abundant rainfall to the Chilean side of the Andes and as clouds move in an eastern direction, rainfall declines exponentially (Jobbágy and Sala, 2000). Changes in rainfall are reflected in large changes in natural vegetation life forms, which varies from grass-shrub steppe to closed-canopy forest in a distance of <100 km (Araujo and Austin, 2015, Austin and Sala, 2002).
Exotic conifers from North America were introduced to the region beginning in 1970 (Licata et al., 2008) and currently occupy nearly 70,000 ha in the region. During the 1970s, tree planting with exotic conifers was strongly supported by the Argentine government through tax subsidies, with a rapid expansion of pine plantations in northwest Patagonia, replacing natural ecosystems along a broad range of annual rainfall (Laclau, 2003). The planted species derived almost exclusively from populations in the Pacific Northwest of North America because of the geographical and climatic similarities with Patagonia (Licata et al., 2008). The original purpose of these plantations was for commercial wood production (Schlichter and Laclau, 1998) and original planting density was similar across the region. Nevertheless, due to the changes in the government subsidy structure after 5 years, many of the plantations were left with minimal management and subsequent rotations were not implemented. However, for the purpose of our ecological study, this land-use change along a natural rainfall gradient provides a unique opportunity to explore the effects of forestation on ecosystem processes (Araujo and Austin, 2015, Hess and Austin, 2017, Hess and Austin, 2014). The specific objective in this study was to advance our mechanistic understanding of potential C sequestration with forestation of an exotic species, evaluating its impact on ecosystem C fluxes and stocks using a paired site approach along a broad precipitation gradient. We hypothesized that exotic pine plantations would respond differently than their natural vegetation counterparts to changes in water availability, thus affecting C sequestration potential. We predicted that an increase in C inputs (production), coupled with a simultaneous decrease in C turnover (decomposition) in plantations would increase C storage, and that this increased C storage would diminish as annual rainfall increases.
Section snippets
Study sites
We conducted our study in the Patagonian region of Argentina, located 40° to 55° S latitude and 65° to 73° W in South America. The combination of climate and topography and simultaneous introduction of exotic pines provide an ideal system to evaluate how tree plantations affect C fluxes and stocks with climate variation. There is a strong and predictable gradient of rainfall, varying from 200 mm to 2200 mm MAP in 100 km east–west and occurring primarily during winter (Austin and Sala, 2002,
C fluxes
ANPP increased linearly with MAP in both NV (r2 = 0.90, P = 0.01) and PP sites (r2 = 0.71, P = 0.07), although this relationship was marginally significant in PP. ANPP in NV sites varied between 43 (±1.1) and 839 g m−2 yr−1 with increasing MAP, while ANPP in PP sites varied between 381 (±53.9) and 1045 (±100.6) g m−2 year−1 (mean of two years, n = 2). Slopes did not differ between paired sites, but ANPP was consistently higher in PP as intercepts were significantly different (F1,3 = 30.43;
Discussion
A growing body of evidence indicates that tree plantations alter the C cycle through diverse mechanisms, including changes in C allocation (Eclesia et al., 2012; Laclau, 2003; Nosetto et al., 2006), nitrogen cycling (Hess and Austin, 2014), soil and rhizosphere enzymatic activity (Hess and Austin, 2017), soil pH (Hong et al., 2018), and the quantity and quality of litter inputs (Araujo and Austin, 2015, Post and Kwon, 2000). This study provides novel insights regarding the impacts of a land-use
Contributions
PIA and ATA conceived the ideas, designed methodology and collected the data; PIA analyzed data; PIA and ATA wrote the paper, contributed critically to the drafts and gave final approval for publication.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We thank A. González-Arzac, M.L. Martinez, A. Grasso, M. Gonzalez-Polo, L. Vivanco, J. Moyano, C. Fariña, J. Landesmann, L. Hess, S. Mendez, A. Tornese and W. De Nicolo for field and laboratory assistance. We are very grateful to M. Zimmerman, S. Focarazzo, P. Pesce, H. Brockerof, J. Casado, R. Pizales and E. Coliqueo for permission to conduct the experiments within their property. We also thank the staff of Parque Nacional Lanín for permission to conduct this study within the park. P. Laclau
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