Computational analysis of the effects of light gradients and neighbouring species on foliar nitrogen

Pekka Kaitaniemi,Anna Lintunen,Risto Sievänen,Jari Perttunen

doi:10.1016/j.ecoinf.2018.09.009

Abstract

Foliar nitrogen is one of the key traits determining the photosynthetic capacity of trees. It is influenced by many environmental factors that are often confounded with the photosynthetic photon flux density (PPFD), which alone strongly modifies the nitrogen content and other foliar traits. We combined field measurements and computational estimates of light transmittance in 3D stands with different combinations of Scots pine (Pinus sylvestris) and silver birch (Betula pendula) to decouple the effect of PPFD from other potential effects exerted by the species of neighbouring trees on the leaf nitrogen content per unit leaf area (Narea) and leaf mass per area (LMA). Independent of the level of PPFD, silver birch had a significantly lower Narea and LMA when Scots pine was abundant in its neighbourhood compared with the presence of conspecific neighbours. In Scots pine, Narea and LMA were only dependent on PPFD and the branching order of shoots. In both species, the relationships between PPFD and Narea or LMA were nonlinear, especially at intermediate levels of PPFD. The levels of PPFD did not show any dependence on the species of the neighbouring trees. The responses of silver birch suggest that the species composition of the surrounding stand can influence foliar nitrogen, independent of the level of PPFD within the canopy.

Highlights

The mass, area, nitrogen content, age and longevity of leaves are suitable as proxies to describe the photosynthetic capacity of foliage in growth models, because they are simple to measure and show tight coupling with directly measured photosynthetic productivity (Evans, 1989; Niinemets, 2016; Sakschewski et al, 2015)
The neighbouring species (χ12 = 5.85, P = .02) and photosynthetic photon flux density (PPFD) (χ12 = 14.4, P = .0001) both had a significant effect on nitrogen content per unit leaf area (Narea), which was generally higher with silver birch neighbours and increased along with PPFDs (Fig. 3a)
Our study demonstrated how a 3D stand model can be used to decouple the amount of shading from other potential effects that neighbouring trees may have had on target trees in a field experiment

Summary

Introduction

The mass, area, nitrogen content, age and longevity of leaves are suitable as proxies to describe the photosynthetic capacity of foliage in growth models, because they are simple to measure and show tight coupling with directly measured photosynthetic productivity (Evans, 1989; Niinemets, 2016; Sakschewski et al, 2015). It is known that the coupling of foliar traits with light is not as complete as theoretical models for optimising photosynthesis suggest. Genotypic differences may indicate individual differences in crown structure and self- shading, the removal of foliage by herbivores may alter shading within the crown, the forest type may impose limits on the structure and light transmission capacity of the surrounding vegetation, and needle age may reflect proximity to incoming light. As substitutes to avoid confounding, Hilbert and Messier (1996) proposed artificially constructed trees to control for individual plant differences in light transmission, and Louarn et al (2015) demonstrated the use of a model to separate the effect of local withinplant light availability from the effect of whole-plant nitrogen status. Tang et al (2015) provide an example of using 3D tree models to analyse the light interception efficiency of variable crown forms

Methods

Results

Conclusion