University of Wyoming Laramie, Wyoming, United States
Abstract: Fine root traits have recently been shown to span two independent axes of variation delineating a root economic space. The first axis is a trade-off between root diameter and specific root length, where thick-rooted species are associated with greater mycorrhizal colonization. In contrast, thin-rooted species are associated with a do-it-yourself nutrient acquisition strategy. The second axis is a trade-off between root tissue density and nitrogen concentration, where conservative species exhibit dense longer-lived roots and acquisitive species show faster metabolic rates. These axes influence resource acquisition rates, but many questions remain about how these axes of trait variation affect ecosystem functions, especially rates of decomposition critical to carbon and nutrient cycling. We predicted that low-density tissue with high root nitrogen should lead to faster decomposition rates. In a common garden setting, we measured the decomposition rates of 63 indigenous tree species in Aotearoa, New Zealand. Three replicates per species were placed in mesh bags, and the percentage mass loss was measured after 6 months. Relationships were analyzed using phylogenetic generalized least squares regression.
The decomposition rate was significantly phylogenetically conserved (lambda=0.71, P< 0.0001), exhibiting slower rates in podocarps and southern beech species and faster rates in laurels and asterids. The conservation axes were most strongly correlated with decomposition rates: root tissue density was negatively correlated (P< 0.0001), and root nitrogen was positively correlated with decomposition rate (P< 0.0001). However, a phylogenetically informed regression indicated that root diameter was positively correlated with decomposition rates (P< 0.01). The strongest predictor of decomposition rate was the lignin-to-N and lignin-to-P ratios, where species with the most labile carbon decomposed faster. These results suggest that the decomposition rate is driven by more than just the conservation gradient alone; the collaboration gradient may also influence decomposition, possibly due to large-rooted species obtaining access to nutrients through mycorrhizal collaboration. It is also clear that traits beyond the four core traits that delineate the root economic space must be considered when modeling belowground decomposition rates. The traits of fine roots determine their fate in the afterlife.
