The role of roots in the resource economics spectrum

The fact that plant traits are correlated in functional trait syndromes within and across biomes has gained considerable importance in ecology (Wright et al., 2004). For example, leaf lifespan is negatively related to specific leaf area (SLA), and further linked via nitrogen (N) content per leaf area to photosynthetic characteristics such as Rubisco, maximum photosynthesis and respiration (Reich et al., 1992). Plants can thus achieve fast growth by rapid assimilation, driven by high leaf N content and a large leaf area with low biomass investment. These large, thin leaves are less mechanically robust and more prone to herbivory than thick leaves and are, therefore, short-lived (Reich et al., 1992). This functional leaf-trait syndrome helps to explain growth–survival tradeoffs, light niche separation and co-existence in tropical forests (Poorter Withington et al., 2006; Freschet et al., 2010). As biomass allocated to roots varies from 20% in tropical rainforests to 70% in temperate grasslands (Poorter et al., 2012), root traits should be fully integrated into the resource economics spectrum. More specifically, as root production and root lifespan are major drivers in nutrient cycling and water use, our understanding of how these ecosystem processes are shaped will be enhanced by understanding the role of species and their root traits. The study of McCormack et al. (pp. 823–831) in this issue of New Phytologist, is an important contribution to this area. ‘Clearly, the resource economics syndromes that have been widely observed in leaves cannot directly be extrapolated to roots, or may at least not be directly comparable between roots and shoots.’ McCormack and coworkers investigated root lifespan of 12 temperate tree species, among which three Acer species, two Quercus species, Juglans nigra, Populus tremuloides and two Pinus species – growing in monoculture stands in a common garden in Pennsylvania, USA over 4 yr. Minirhizotron tubes (clear acrylic tubes, 3 cm diameter and 45 cm long, installed in the soil at an angle of 30° from vertical, with a camera connected to image analysis software) were used to regularly scan root birth and root death during the growing season. Median root lifespans of individual roots of the 12 tree species were compared with a range of fine root traits determined from destructive harvests, including specific root length (SRL, m root length g−1 dry root mass), root diameter, N, carbon (C) and calcium contents in root tissues, and root respiration. Root lifespans were also correlated to general tree growth parameters such as diameter at breast height (DBH) at a stand age of 10 yr and wood tissue density. Variation in root lifespan ranged from 95 to 336 d among species and a large fraction of the variation was explained by three traits only: DBH, root diameter and N : C ratio (Fig. 1). This finding suggests that relatively simple parameters can be used to estimate an important ecosystem process like root turnover. In contrast to this simplicity, however, are other studies of Withington et al. (2006, on 11 mature tree species in Poland) and Tjoelker et al. (2005, on seedlings of 37 herbaceous and two tree species) that show that a putative functional root trait syndrome may be more complex. These two studies did observe a general correlation between root N (Tjoelker et al., 2005) or N:C content (Withington et al., 2006) and root lifespan, but not with several other root traits, including root diameter and lifespan. Diagram showing the root, stem and leaf traits to be measured in order to develop a whole plant resource economy perspective. The significant traits measured by McCormack et al. (pp. 823–831) in this issue of New Phytologist are presented in red. SLA, specific leaf area; Amax, maximum assimilation rate; Rd, respiration rate; DBH, diameter at breast height; SRL, specific root length. The lack of consistency among studies may be explained by the use of different methodologies in the different studies, as some investigated seedling growth stage (Reich et al., 1998; Tjoelker et al., 2005) and others mature trees (Withington et al., 2006; McCormack et al.). Also, root lifespans have been based on investigations with ingrowth cores (Tjoelker et al., 2005), or minirhizotrons, either calculating root lifespan from the fate of individual fragments (Withington et al., 2006; McCormack et al.) or using the root fragment length as the independent variable (Ferguson Lei et al., 2012). One of the main explanations for overyielding has been spatial niche differentiation belowground: for example, certain species will root in deeper soil layers in mixtures as compared with monocultures, thus exploring larger soil territories. Belowground overyielding is generally observed in grasslands (Cardinale et al., 2007). However, evidence for niche differentiation as a driving force in grasslands is limited and an experiment with four grassland species even detected greater shallow rather than deeper rooting of the community in mixtures compared with monocultures (Mommer et al., 2010). Other studies have shown that N uptake patterns in grassland mixtures showed less variation than expected based on niche differentiation theory (Von Felten et al., 2012). Experimental studies on forest sites also often, but not always, report belowground overyielding (Brassard et al., 2011; Lei et al., 2012), but evidence for spatial niche differentiation belowground seems more prominent in these studies (Schmid Brassard et al., 2011). Future studies will need to reveal the exact differences between grasslands and forests in this context, but the larger overall rooting depths of trees compared with grasslands may be key to this difference. If there is the potential for trees to explore over a larger soil depth than grasses, there may be more opportunity for spatial niche differentiation within a mixed forest stand than in a grassland. In summary, the study of McCormack et al. demonstrated significant root trait correlations leading to the possibility of a root trait syndrome similar to what has been observed for leaf traits; however, it is too early to conclude on its generality. We hope that McCormack et al.’s study will stimulate more research effort in this important, yet underexplored research area of functional root trait economics. Information on root lifespan is important to link roots to the leaf economics spectrum of forests, but also to better understand how species richness drives ecosystem functioning in terms of productivity, nutrient cycling and C storage.