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

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.