Contrasting Dependencies of Photosynthetic Capacity on Leaf Nitrogen in Early- and Late-Successional Tropical Montane Tree Species

Camille Ziegler,Etienne Zibera,Brigitte Nyirambangutse,Johan Uddling,Mirindi Eric Dusenge,Göran Wallin

doi:10.3389/fpls.2020.500479

Abstract

Differences in photosynthetic capacity among tree species and tree functional types are currently assumed to be largely driven by variation in leaf nutrient content, particularly nitrogen (N). However, recent studies indicate that leaf N content is often a poor predictor of variation in photosynthetic capacity in tropical trees. In this study, we explored the relative importance of area-based total leaf N content (Ntot) and within-leaf N allocation to photosynthetic capacity versus light-harvesting in controlling the variation in photosynthetic capacity (i.e. V cmax, J max) among mature trees of 12 species belonging to either early (ES) or late successional (LS) groups growing in a tropical montane rainforest in Rwanda, Central Africa. Photosynthetic capacity at a common leaf temperature of 25˚C (i.e. maximum rates of Rubisco carboxylation, V cmax25 and of electron transport, J max25) was higher in ES than in LS species (+ 58% and 68% for V cmax25 and J max25, respectively). While Ntot did not significantly differ between successional groups, the photosynthetic dependency on Ntot was markedly different. In ES species, V cmax25 was strongly and positively related to Ntot but this was not the case in LS species. However, there was no significant trade-off between relative leaf N investments in compounds maximizing photosynthetic capacity versus compounds maximizing light harvesting. Both leaf dark respiration at 25˚C (+ 33%) and, more surprisingly, apparent photosynthetic quantum yield (+ 35%) was higher in ES than in LS species. Moreover, Rd25 was positively related to Ntot for both ES and LS species. Our results imply that efforts to quantify carbon fluxes of tropical montane rainforests would be improved if they considered contrasting within-leaf N allocation and photosynthetic Ntot dependencies between species with different successional strategies.

Highlights

Tropical forests play an important role in controlling the global carbon cycle and, the rate of ongoing climate change (Lewis, 2006; Stocker et al, 2014)
With the overall aim to explore the controls of interspecific variation in photosynthetic capacity in tropical montane rainforest trees, we investigated physiological, morphological and chemical leaf traits in mature trees of 12 Central African montane rainforest tree species with contrasting light requirements
The results highlight a marked difference in photosynthetic N dependency between different successional groups, with photosynthetic capacity strongly related to total areabased leaf N content (i.e. nitrogen content (Ntot)) in ES species but not in late successional (LS) species

Summary

Introduction

Tropical forests play an important role in controlling the global carbon cycle and, the rate of ongoing climate change (Lewis, 2006; Stocker et al, 2014). Several studies in tropical rainforests have found that area-based leaf nutrient content (i.e. N, phosphorous) is often a poor predictor of the large interspecific variation in photosynthetic capacity (Coste et al, 2005; van de Weg et al, 2012; Houter and Pons, 2014; Dusenge et al, 2015; Bahar et al, 2016; Hasper et al, 2017). Leaf P content and photosynthetic N use efficiency (Vcmax per unit leaf N) were not correlated in a large study of Andean and Amazonian rainforest species (Bahar et al, 2016)

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