Are fine roots ‘leaves underground' in terms of allometry? A test in a tropical forest successional series in southwest China

Abstract

Fine roots have been hypothesized to be ‘leaves underground' in terms of vascular network, but this hypothesis has rarely been tested within the framework of metabolic scaling theory (MST). We measured average fine‐root (diameter < 1 mm) mass (M), surface area (A), volume (V), diameter (D) and length (L) for 216 soil cores from 24 plots across four successional stages in tropical forests of Xishuangbanna (southwest China), and examined eight scaling relationships between these variables at the individual root scale. We tested whether fine‐root allometries conformed to MST's model for leaf (MSTl) or model (MSTw) for woody organs (e.g. trunk). We also assessed the relative effects of environmental factors, tree size, species composition and diversity, and stand structural factors on allometric relationships using structural equation models (SEMs). Our results showed that: 1) fine‐root scaling exponents rarely conformed to MSTl's predictions. 2) The scaling exponents between fine‐root M, A, V and D all conformed to MSTw's predictions in later successional forests, but showed greater deviation towards early successional stage. 3) The scaling exponents associated with fine‐root length differed markedly from MSTw's predictions. 4) Changes of some fine‐root scaling exponents across successional stage were mainly affected by tree size or soil fertility, and species composition affected allometry only indirectly via tree size. Our results suggested that the allometries of individual fine roots largely conform to the scaling rules governing woody organs instead of leaves, probably because leaves are nearly two‐dimensional objects while the other two are three‐dimensional. We showed that MSTw can well predict some fine‐root allometries in later successional forests, suggesting great potential of utilizing MSTw to better estimate fine‐root biomass and productivity. However, the present MSTw still needs to be improved for predicting the scaling relationships concerning fine‐root length, and also for better quantifying allometric exponents in earlier successional forests.

Special Issue: Root traits and functioning: from individual plants to ecosystems

“…Reference in this Special Issue Temperate Alonso-Crespo et al 2023, Klimešová and Herben 2023, Mafa-Attoye et al 2023, Mao et al 2023, Schuster et al 2023, Slette et al 2023, Yang et al 2023(Sub)Tropical Dallstream et al 2023, Dai et al 2023,Gagliardi et al 2023, Guo et al 2023, Leroy et al 2023, Kotowska et al 2023 a large variety of fine-root systems coexist in hyper-diverse tropical plant communities. Besides adding information from underexplored biomes, this Special Issue further highlights the importance of incorporating additional traits covering all belowground trait categories (McCormack et al 2017) -albeit to different extents -in the context of a RES (Table 1).…”

“…1). Guo et al (2023) further showed that allometric relationships (and thus covariations) among root traits also changed as a function of tree size in a tropical forest community. The position of species within the RES may thus change with time (at least, along the collaboration axis), but whether these shifts indeed reflect changes in the degree of mycorrhizal dependency is still controversial (Leroy et al 2023).…”

Size-dependent growth strategy and allometry, but not diversity, as major drivers of fine-root biomass and productivity across warm-temperate forest types

“…2a). However, Guo et al (2022) found that multiple scaling relationships between fine-root area, volume, diameter and mass all conform to MST's predictions for tree woody organs instead of leaves. Their results and ours are not exclusive because they examined allometries among traits related to root morphology, while we examined the FRP-FRB allometry concerning root productivity.…”

“…Their results and ours are not exclusive because they examined allometries among traits related to root morphology, while we examined the FRP-FRB allometry concerning root productivity. Fine roots have important structural differences from leaves, including a key difference that leaves are nearly two-dimensional while fine roots are three-dimensional objects similar to woody organs (Price and Enquist 2007), which can lead to markedly difference in morphological allometries (Guo et al 2022). Thus, our results, together with Guo et al (2022), suggest that fine roots may be "leaves underground" in terms of productivity-related allometries, but differ from leaves for morphology-related allometries.…”

“…These key features are similar to leaves but differs markedly from woody organs (e.g. stem and coarse root), which may lead some (but clearly not all, see Guo et al 2022) fine-root allometries to be similar to leaves (Chen et al 2019, Deng et al 2020. MST predicts that the allometric exponent between leaf productivity (gross photosynthesis) and biomass was 1 instead of 3/4 (Enquist et al 2007), we tested whether this is true for fine roots.…”