Stem Diameter (and Not Length) Limits Twig Leaf Biomass.

Jun Sun,Quanlin Zhong,Mantang Wang,Min Lyu,Man Li,Karl J. Niklas,Dongliang Cheng

doi:10.3389/fpls.2019.00185

Abstract

The relationship between leaf and stem biomass as well as the relationship between leaf biomass and stem length and diameter are important to our understanding of a broad range of important plant scaling relationship because of their relationship to photosynthesis and thus growth. To understand how twig architecture (i.e., current year leaves, and stem diameter and length) affects stem diameter and length, and leaf number and biomass, we examined the twigs of 64 woody species collected from three forest types along an elevational gradient in the Wuyi Mountains, Jiangxi Province, China. We also compared the scaling relationships we observed with biomass allocation patterns reported at the whole tree level. Our results revealed isometric relationship between leaf and stem biomass on twigs despite differences in forest communities and despite changes in environmental factors along an elevational gradient. Across the 64 species, from twigs to individual trees, leaf biomass scaled approximately as the 2.0-power of stem diameter (but not for stem length or leaf number). These results help to identify a general rule that operates at two different levels of biological organization (twigs and whole trees). The scaling relationship between leaf biomass and stem diameter in twigs is insensitive to differences in species composition, elevation, or forest type. We speculate that this rule emerges because stem diameter serves as a proxy for the amount of resources supplied per unit cross section to developing leaves and for the flow of photosynthates from mature leaves to the rest of the plant body.

Highlights

Regardless of their form, size, or longevity, the leaves on current-year shoots provide the photosynthetic machinery that drives annual growth, whereas the stems of twigs sustain the static and dynamic mechanical forces leaves experience by gravity and wind (Niklas, 1992a)
In order to clarify the scaling relationships of twigs along the elevational gradient, we developed a mathematical model for the scaling of critical twig functional traits and we tested the model using data gathered from 915 twigs from 64 woody species in three different forest-types along an elevational gradient in the Wuyi Mountains
The scaling of stem biomass vs. total twig biomass across the three different forest communities was consistent with our model, i.e., the scaling had a common slope of 1.08 (Figure 1A)

Summary

Introduction

Regardless of their form, size, or longevity, the leaves on current-year shoots (i.e., twigs) provide the photosynthetic machinery that drives annual growth, whereas the stems of twigs sustain the static and dynamic mechanical forces leaves experience by gravity and wind (i.e., self-loading and wind induced drag forces, respectively) (Niklas, 1992a). The allocation of biomass to leaves, stems, and roots can be described using a scaling function (Enquist and Niklas, 2002), which takes the general mathematical formula Ma = βMbα, where Ma and Mb are the biomass of different organs, β is the normalization constant, and α is the scaling exponent Prior investigations using this formula have focused at the whole-plant level (Enquist and Niklas, 2002; Cheng and Niklas, 2007; Chave et al, 2014; Paul et al, 2016), and at the level of individual twigs (e.g., Westoby and Wright, 2003; Sun et al, 2006; Xiang et al, 2009a; Yang et al, 2015). When viewed collectively, there is considerable ambiguity about how the key functional traits of twigs (e.g., leaf and stem biomass, and stem diameter and length) scale with respect to one another, especially for different plant communities growing along environmental gradients, such as an elevational gradient

Methods

Results

Discussion

Conclusion