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

Fine roots play a pivotal role in terrestrial carbon and nutrient cycling. However, our knowledge on drivers of fine-root biomass (FRB) and productivity (FRP) focus on functional traits, biodiversity and abiotic factors, while less attention on allometric constraints, an indispensable driver of organism biomass allocation. We measured FRB (FRP) for 24 plots using 216 soil cores (ingrowth cores) from four forest types (birch, oak, larch and pine) on a warm-temperate mountain of north China, and investigated leaf, stem and fine-root functional traits, stand factors, diversity indices and soil property. We tested the allometric relationships among FRB, FRP, aboveground biomass (AGB), leaf biomass and functional traits, and examined how allometry, size-dependent growth strategy, the mass-ratio and complementarity effects affected FRB and FRP directly and indirectly. There was a stable allometric relationship between FRP and FRB at both the soil-core and plot levels, and the former supporting the predicted exponent for leaves (= 1) of the metabolic scaling theory. Contrary to common observations, both FRB and FRP at plot scale showed negative (or non-significant) relationships with aboveground (or leaf) biomass. Instead, higher AGB led to more conservative growth strategies, which led to lower FRB, and thus lower FRP due to allometric constraints. Root traits (mass-ratio effect) showed the strongest direct effect on FRB, while diversity index (complementarity effect) and soil fertility revealed relatively weak effects. FRP was strongly driven by allometry (FRB) and soil nitrogen, while functional traits and diversity index affected FRP via FRB instead of directly. The complementarity effect on FRP may be overestimated if FRP–FRB allometry was not considered in multivariate analyses. Our results are not conflict with the positive correlations of FRB (FRP) with AGB or leaf biomass reported by large-scale studies, but together suggest contrasting changes of growth strategies with tree size versus climate, which may affect aboveground–root relationship simultaneously. We also suggest to carefully test allometric relationships to better understand how complementarity versus mass-ratio effect, stand factors and environment, together affect fine-root dynamics.