Aggregated and complimentary: Fine root distribution patterns in a temperate deciduous forest

Background/Question/Methods

Belowground plant interactions are assumed to have a profound impact on ecosystem processes such as intra-specific competition and fine root biomass (FRB) productivity. However, the lack of studies describing root distributions at the species level has hampered the evaluation of this assumption, particularly in diverse forest ecosystems. Here we studied fine root distributions at the species level in a temperate forest in order to address the following questions: 1) Can the proportional species distribution of FRB be predicted from aboveground species abundance? 2) How segregated are the distributions of tree root systems, and to what extent does segregation correspond to niche partitioning? 3) How do soil resource heterogeneity, phylogenetic diversity, and root functional traits may explain spatial variation in FRB across ecosystems? 4) Are these relationships similar in core areas of ecosystems compared with ecotones near the ecosystem edges?  We addressed these questions using molecular tools to identify and describe morphologically > 700 fine root samples (< 1 mm in diameter) representing 14 coexisting canopy tree species in an Acer-Fagus dominated forest in northeast Ohio, USA. Root biomass, root trait data and species identity were then compared to a detailed description of soil conditions.   

Results/Conclusions

Our findings indicated that total FRB abundance roughly corresponds with aboveground tree abundance. However, ectomycorrhizal (ECM) species as a category and some arbuscular-mycorrhizal (AMF) species were under-represented belowground, possibly reflecting tradeoffs between FRB productivity and root maintenance. Moreover, we found little evidence of spatial niche segregation vertically or horizontally among tree species, indicating extensive overlapping foraging areas among neighboring trees. In fact, FRB and species diversity increased with soil resource availability, suggesting strong competition for soil hotspots and pervasive root interactions in the community. Total FRB spatial variation was better explained by variation in soil resource availability than tree composition, particularly at core ecosystem areas. Nonetheless, phylogenetic diversity was increasingly important explaining FRB variation when ecotone samples were included. These results add evidence to recent studies suggesting that symmetric competition and evolutionary relationships between coexisting species are important factors explaining soil resource use in tree communities. Moreover, it emphasizes the decoupling of environmental factors and plant communities in ecotone areas between ecosystems.