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.

Varun Varma,Arockia M Catherin,Mahesh Sankaran

doi:10.1002/ece3.4289

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.

Highlights

The structure and functioning of mixed tree-grass ecosystems, such as savannas, are governed by both bottom-up and top-down drivers (Ekblom & Gillson, 2010; February, Higgins, Newton, & West, 2007; Frost et al, 1986; Higgins, Bond, & Trollope, 2000; Sankaran, Ratnam, & Hanan, 2004, 2008; Scholes & Archer, 1997)
Given that savannas and dry forest ecosystems are anticipated to be vulnerable to future global change drivers including nutrient deposition (Sala et al, 2000), understanding the impacts of enhanced nutrient availability on vegetation dynamics of mixed tree-grass systems is important, both to assess their future trajectories and to develop appropriate management strategies
Our results indicate that differences in the nature of nutrient limitation between N-fixing and non-N-fixing tropical dry forest and savanna tree seedlings can lead to contrasting responses in the two functional groups to atmospheric nutrient deposition

Summary

| INTRODUCTION

The structure and functioning of mixed tree-grass ecosystems, such as savannas, are governed by both bottom-up (e.g., water and nutrient availability) and top-down drivers (e.g., fire and herbivory) (Ekblom & Gillson, 2010; February, Higgins, Newton, & West, 2007; Frost et al, 1986; Higgins, Bond, & Trollope, 2000; Sankaran, Ratnam, & Hanan, 2004, 2008; Scholes & Archer, 1997). Besides altering growth, increased nutrient availability can influence how tree seedlings respond to fire, a major determinant of vegetation structure in mixed tree-grass ecosystems The ability of both tree seedlings and juveniles to survive recurring fires by resprouting is key to their persistence and eventual recruitment into the canopy as reproductively mature adults (Bond, 2008; Bond & van Wilgen, 1996; Hanan, Sea, Dangelmayr, & Govender, 2008; Higgins et al, 2000; Sankaran et al, 2004, 2005; Schutz, Bond, & Cramer, 2009). Differences between species and functional groups in allocation to belowground resources and root carbohydrate reserves with increasing nutrient availability can significantly alter postfire survival and the composition of the regenerating community Such functional group level differences have previously been quantified, for example, between evergreen and deciduous savanna tree species (Tomlinson et al, 2013), but few studies have far evaluated how N-fixers and non-N-fixers differ in their allocation patterns with N and P addition. We expected: (a) enhanced nutrient availability to lead to increased biomass accumulation in both functional groups, but the magnitude of increase to be greater amongst non-N-fixers, (b) N- fixers to have greater baseline R:S ratios, as well as root carbohydrate content compared to non-N-fixers, and (c) nutrient addition to reduce R:S ratios and root carbohydrate content in both functional groups, with greater reductions in non-N-fixers

| MATERIALS AND METHODS

Findings

| DISCUSSION