1. 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 carbon partitioning. 2. 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 and soil fertility. We tested the allometric relationships among FRB, FRP, aboveground (leaf) biomass and functional traits, and examined how allometry, size-dependent growth strategies, the mass-ratio and complementary effects affected FRB and FRP directly and indirectly. 3. There is stable allometric relationship between FRP and FRB at both the soil-core and plot levels, and the former supported the predicted exponent for leaves (=1) of the metabolic scaling theory. Contrary to common observations, plot-scale FRB and FRP showed negative (or non-significant) relationships with aboveground (leaves) biomass. Instead, higher aboveground biomass 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 (complementary effect) and soil fertility revealed weak effects. FRP was strongly driven by allometry (FRB) and soil nitrogen, while functional traits and diversity affected FRP via FRB instead of directly. 4. Our results do not conflict with the positive correlations of FRB (FRP) with aboveground (leaf) biomass reported by large-scale studies, but together suggest changes of growth strategies with tree size vs. climate, which may affect aboveground-root relationship simultaneously. Thus, we suggest to carefully test allometric relationships to better understand how biodiversity, traits and stand factors affect fine-root dynamics.

1.Ectomycorrhizal (ECM) roots are evolutionary strategies of plants for effective nutrient uptake under varying abiotic conditions. Formation and morphological differentiations of ECM roots are important strategies in foraging environments. However, little is known on how such strategies mediate the nutrients of the below- and aboveground tissues and the balances among nutrient elements across environmental gradients. 2.We studied the function of ECM symbiosis in Abies faxoniana across its distributional range in Southwest China. The effects of differential ECM strategies, i.e. the contact exploration type, the short-distance exploration type, and the medium-distance exploration type, and root tips functional traits, etc., on root and foliar N and P and N:P ratio were examined across natural environmental gradients. 3.The ECM symbionts preferentially facilitated P uptake in A. faxoniana under both N and P limitations. The uptakes of N and P were primarily promoted by the effectiveness of ECM roots, e.g. ECM root tips per unit biomass, superficial area of ECM root tips, the ratio of living and dead root tips, but negatively related to the ECM proliferations and morphological differentiations. Generally, plant N and P nutrients were always promoted by the contact exploration type, while negatively affected by the short-distance exploration type in A. faxoniana. Root and foliar N and P nutrients were expected to be affected by the medium-distance exploration type in dynamics. Especially, root P limitation could be relieved when the frequency of medium-distance exploration type up to c.15%, whilst root N limitation was strengthen when the frequency of medium-distance exploration type over 20%. 4.We suggest that both below- and above-ground nutritional traits of host tree species could be strongly affected by ECM symbiosis in natural environments. The ECM strategies responding to environmental conditions significantly affect the plant nutrient uptakes and trade-offs. ECM soil exploration types are the great supplementary mechanisms for plant nutrient uptake.

In natural forests, it is increasingly suggested that stand factors are far more important for community biomass and productivity than biodiversity, but the relative importance of stand factors vs. diversity on ecosystem stability, and how their relative roles change with grain size, still remain unclear. Using inventory data from tropical forest plots in southwestern China from 2004 to 2010, we found that stand factors were clearly more stronger drivers than diversity for forest biomass and productivity (at each grain size from 400 m2 to 0.25 ha), while diversity was predominate for temporal stability of biomass and productivity. The effect of diversity on biomass and productivity increased with increasing grain size, but did not change clearly for ecosystem stability. Functional diversity was more important for ecosystem functions and stability than taxonomic and phylogenetic diversity, and richness was more important than the other two diversity components (evenness and divergence). Our results reconcile the recent debate on the relative importance of diversity vs. stand factors on ecosystem properties, and suggest that forest management to adjust stand structure is an effective way to increase forest carbon storage rapidly, but biodiversity conservation may be crucial for long-term ecosystem stability under climate change.