Effects of increased N and P availability on biomass allocation and root carbohydrate reserves differ between N‐fixing and non‐N‐fixing savanna tree seedlings

Abstract

In mixed tree‐grass ecosystems, tree recruitment is limited by demographic bottlenecks to seedling establishment arising from inter‐ and intra‐life‐form competition, and disturbances such as fire. Enhanced nutrient availability resulting from anthropogenic nitrogen (N) and phosphorus (P) deposition can alter the nature of these bottlenecks by changing seedling growth and biomass allocation patterns, and lead to longer‐term shifts in tree community composition if different plant functional groups respond differently to increased nutrient availability. However, the extent to which tree functional types characteristic of savannas differ in their responses to increased N and P availability remains unclear. We quantified differences in above‐ and belowground biomass, and root carbohydrate contents in seedlings of multiple N‐fixing and non‐N‐fixing tree species characteristic of Indian savanna and dry forest ecosystems in response to experimental N and P additions. These parameters are known to influence the ability of plants to compete, as well as survive and recover from fires. N‐fixers in our study were co‐limited by N and P availability, while non‐N‐fixers were N limited. Although both functional groups increased biomass production following fertilization, non‐N‐fixers were more responsive and showed greater relative increases in biomass with fertilization than N‐fixers. N‐fixers had greater baseline investment in belowground resources and root carbohydrate stocks, and while fertilization reduced root:shoot ratios in both functional groups, root carbohydrate content only reduced with fertilization in non‐N‐fixers. Our results indicate that, even within a given system, plants belonging to different functional groups can be limited by, and respond differentially to, different nutrients, suggesting that long‐term consequences of nutrient deposition are likely to vary across savannas contingent on the relative amounts of N and P being deposited in sites.

Nutrient deposition enhances post-fire survival in non-N-fixing savanna tree seedlings

“…One reason for greater survival of N-fixers could be their faster growth rates compared to non-N-fixers, which allows them to reach larger seedling sizes before the start of the fire season, which in turn confers them with increased post-fire survival (Trollope 1984;Gignoux et al 1997;Hoffmann and Solbrig 2003;Bond 2008;Lawes et al 2011;Wakeling et al 2011). Further, N-fixing tropical savanna and dry forest tree seedlings have been shown to partition relatively greater amounts of biomass to roots, as well as invest in higher root storage carbohydrate concentrations (Varma et al 2018). This below-ground investment is positively associated with post-fire resprouting (Hoffmann et al 2000;Bell 2001;Bond et al 2003b;Bond and Midgley 2003;Clarke and Knox 2009;Wigley et al 2009;Schutz et al 2009;Clarke et al 2013), which can promote survival in N-fixers.…”

“…Numerous studies have demonstrated that increased availability of these essential plant nutrients can differentially affect plant growth amongst members of a community, bringing about compositional changes in vegetation assemblages (Huante et al 1995a, b;Zavaleta et al 2003;Khurana and Singh 2004;Stevens et al 2004Stevens et al , 2006Stevens et al , 2010Wassen et al 2005;Elser et al 2007;Clark and Tilman 2008;Xia and Wan 2008;Tripathi and Raghubanshi 2014;Verma et al 2014;Powers et al 2015). In addition, increased nutrient availability can also indirectly modify plant communities by affecting properties of local disturbance regimes -which play a ubiquitous role in shaping ecosystems across the globe -and by modifying the susceptibility of plant species to disturbance events (Heil and Diemont 1983;Bobbink and Lamers 2002;Strengbom et al 2002;Bobbink et al 2010; Barbosa et al 2014b;Varma et al 2018). To assess the net effect of nutrient deposition on vegetation communities, we need to therefore consider both direct nutrient-mediated effects on plant growth, as well as indirect effects that arise through alterations in disturbance characteristics and the differential susceptibility of plant species to these altered disturbance regimes.…”

“…In the long-term, nutrient deposition has the potential to alter tree canopy cover in mixed tree-grass ecosystems, both directly, through their effects on tree seedling growth, as well as indirectly via effects on grass growth, which in turn can modify the strength of tree-grass competition and the nature of fire regimes. Tree seedlings in general demonstrate enhanced growth with nutrient availability (Huante et al 1995a;Khurana and Singh 2004;Vadigi and Ward 2013;Varma et al 2018) and may therefore be expected to show increased post-fire survival because larger seedlings survive fires better (Trollope 1984;Gignoux et al 1997;Hoffmann and Solbrig 2003;Bond 2008;Lawes et al 2011;Wakeling et al 2011). However, nutrientmediated increases in seedling growth are also typically associated with reduced relative investment in root biomass, as well as root carbohydrates (Tilman 1988;Khurana and Singh 2004;Knox and Clarke 2005;Hermans et al 2006;Wang et al 2015;Varma et al 2018), which in turn could negatively influence post-fire seedling survival and resprouting ability (Hoffmann et al 2000;Bell 2001;Bond et al 2003b;Lamont and Wiens 2003;Vesk and Westoby 2004;Clarke and Knox 2009;Wigley et al 2009;Clarke et al 2013) .…”

“…Tree seedlings in general demonstrate enhanced growth with nutrient availability (Huante et al 1995a;Khurana and Singh 2004;Vadigi and Ward 2013;Varma et al 2018) and may therefore be expected to show increased post-fire survival because larger seedlings survive fires better (Trollope 1984;Gignoux et al 1997;Hoffmann and Solbrig 2003;Bond 2008;Lawes et al 2011;Wakeling et al 2011). However, nutrientmediated increases in seedling growth are also typically associated with reduced relative investment in root biomass, as well as root carbohydrates (Tilman 1988;Khurana and Singh 2004;Knox and Clarke 2005;Hermans et al 2006;Wang et al 2015;Varma et al 2018), which in turn could negatively influence post-fire seedling survival and resprouting ability (Hoffmann et al 2000;Bell 2001;Bond et al 2003b;Lamont and Wiens 2003;Vesk and Westoby 2004;Clarke and Knox 2009;Wigley et al 2009;Clarke et al 2013) . In addition, increases in grass productivity with fertilisation (Fynn and O'Connor 2005;Lee et al 2010) has the potential to negatively affect tree seedling growth and survival, both as a result of increased grass competition (Higgins et al 2000;Kraaij and Ward 2006;Cramer et al 2010;Kambatuku et al 2012;Cramer and Bond 2013;Ward 2013, 2014), as well as more intense and frequent grass-fuelled fires (Williams et al 1999;Higgins et al 2000;Ryan and Williams 2011;Hoffmann et al...…”

“…N-fixers and non-N-fixers. N-fixers and non-N-fixers have contrasting nutrient demands and acquisition strategies (Vitousek et al , 2013Pearson and Vitousek 2002) which can result in differing growth responses of these functional groups to nutrient enrichment (Khurana and Singh 2004;Cramer et al 2007;Barbosa et al 2014a;Tripathi and Raghubanshi 2014;Varma et al 2018). Furthermore, differences between functional groups in their ability to withstand grass competition (Cramer et al 2007(Cramer et al , 2010, and in biomass allocation patterns to above-versus below-ground organs and root carbohydrate reserves (Khurana and Singh 2004;Tripathi and Raghubanshi 2014;Varma et al 2018) can result in differing post-fire resprouting and survival with N and P fertilisation.…”