Abstract
Since their emergence onto land, terrestrial plants have developed diverse strategies to acquire soil resources. However, we lack a framework that adequately captures how these strategies vary among species. Observations from around the world now allow us to quantify the variation observed in commonly-measured fine-root traits but it is unclear how root traits are interrelated and whether they fall along an “economic” spectrum of acquisitive to conservative strategies. We assessed root trait variation and mycorrhizal colonization rates by leveraging the largest global database of fine-root traits (the Fine-Root Ecology Database; FRED). We also developed a heuristic model to explore the role of mycorrhizal fungi in defining belowground exploration efficiency across a gradient of thin- to thick-diameter roots. In support of the expectations of the “root economic spectrum,” we found that root diameter was negatively related to specific root length (Pearson’s r=-0.76). However, we found an unexpected negative relationship between root diameter and root tissue density (Pearson’s r = -0.40), and we further observed that root nitrogen content was largely unrelated to other economic traits. Mycorrhizal colonization was most closely associated with root diameter (Pearson’s r = 0.62) and was unrelated to root tissue density and root nitrogen. The heuristic model demonstrated that while thinner roots have inherently greater capacity to encounter soil resources based on higher surface area per unit mass, the potential for increased associations with mycorrhizal fungi in thicker roots, combined with greater hyphal growth, can result in equally acquisitive strategies for both thin- and thick roots. Taken together, our assessments of root trait variation, trade-offs with mycorrhizal fungi, and broader connections to root longevity allowed us to propose a series of fundamental constraints on belowground resource acquisition strategies. Physical tradeoffs based on root construction (i.e., economic traits) and functional limitations related to the capacity of a root to encounter and acquire soil resources combine to limit the two-dimensional belowground trait space. Within this trait space there remains a diversity of additional variation in root traits that facilitates a wide range of belowground resource acquisition strategies.
Highlights
Plants depend on roots to perform multiple functions, including anchorage, storage, and most critically, the exploration of soil and acquisition of soil resources by narrow-diameter “fine” roots located at the most distal portions of belowground root systems
Root diameter was negatively related to root tissue density (RTD), resulting in lower densities on average among thicker roots compared to thinner roots (r = -0.40; Figure 2)
Mycorrhizal colonization was strongly and positively related to root diameter (r = 0.62) and negatively related to specific root length (SRL) (r = -0.61), though in both cases this was most consistent among species associating with arbuscular mycorrhizal fungi and not for ectomycorrhizal associates (Figure S2)
Summary
Plants depend on roots to perform multiple functions, including anchorage, storage, and most critically, the exploration of soil and acquisition of soil resources by narrow-diameter “fine” roots located at the most distal portions of belowground root systems Since their emergence onto land over 400 million years ago (Morris et al, 2018), plants have evolved a diverse suites of physical and chemical traits that affect both plant success and terrestrial biogeochemical processes (Raven and Edwards, 2001; Comas et al, 2012; Ma et al, 2018). Aboveground, variation in leaf traits is often distilled to an “economic” spectrum of acquisitive to conservative strategies, where cheaply-constructed, fast-growing, and short-lived leaves are considered acquisitive and more resource-intensive and longer-lived leaves are conservative This “fast-slow” conceptualization has helped to explain leaf trait variation and to link this variation to leaf-level and whole-plant strategies from local to global scales (Westoby and Wright, 2006; Reich, 2014; Díaz et al, 2016; Messier et al, 2017). It should be of little surprise that substantial effort to understand and predict root trait variation has focused on the identification of an equivalent belowground economic spectrum of acquisitive to conservative strategies
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
