DOI: 10.5061/dryad.931zcrjg3
title: The role of fine‐root mass, specific root length and lifespan in tree performance: a whole‐tree exploration

description: 1. The root economics spectrum (RES) hypothesis predicts that fast-growing
 tree species have short-lived roots with high specific root length (SRL)
 to allow rapid resource uptake, and opposite trait expressions for
 slow-growing species. Yet, the mixed support for this hypothesis suggests
 that trees can adopt alternative strategies to increase resource uptake,
 besides an increase in SRL. 2. We combined a novel mechanistic whole-tree
 model and empirical fine-root data of ten tree species to test the effects
 of one of these alternative strategies, notably increasing fine-root mass,
 on the tree’s net C gain (used here as a proxy for tree performance), and
 to assess how fine-root lifespan influences the relative importance of SRL
 and fine-root mass for the C balance of trees. 3. Our results indicate
 that accounting for the short lifespan of high-SRL roots has important
 implications for explaining tree performance and the role of roots herein.
 Without considering their faster turnover, high-SRL roots and low
 fine-root mass resulted in the highest performance as predicted from the
 RES. Yet, when their higher turnover rates were accounted for, a high
 fine-root mass and low SRL lead to the highest performance. Both our model
 outcomes and field data further show a negative relationship between SRL
 and fine-root mass through which species aim to realise a similar root
 length density. This trade-off further indicates how high a SRL and low
 fine-root mass as well as opposite trait values can both lead to a
 positive C balance in a similar environment. 4. Our study may explain why
 high-SRL roots do not necessarily lead to the fastest tree growth as often
 hypothesised and demonstrates the importance of fine-root mass in
 combination with fine-root lifespan for explaining interspecific
 differences in tree performance. More generally, our work demonstrates the
 value of identifying and investigating different belowground strategies
 across species from a whole-plant modelling perspective, and identifies
 the relationship between SRL, fine-root biomass and lifespan as an
 important functional dimension to variation in species’ performance.
description: Fine-root data were
 collected from ten temperate broadleaved tree species (Table 1) growing in
 a common garden forest at the Hollandse Hout, the Netherlands (52°28’N,
 5°26’E), established in the 1960s and 1970s. The forest is subjected to a
 temperate marine climate with mild winters and summers and an annual
 rainfall of approximately 800 mm (KNMI, 2009). The soil is described as a
 calcareous and nutrient-rich clay soil with a homogeneous texture
 throughout the forest, and the groundwater table ranges between 1.6 and
 1.2 m below the soil surface (Weemstra et al., 2013). Fine-root mass and morphology were
 measured in three monospecific forest plots per species. Per plot, three
 soil samples up to 1 m depth and randomly distributed over the plot were
 collected using a 42 mm-diameter root auger, the roots washed out,
 separated into fine and coarse roots based on their diameter (fine roots
 being &lt; 2 mm diameter) and weighed. As roots were sampled with a
 root auger and cut into smaller fragments, we were unable to accurately
 sort roots based on their order. Fine roots were scanned, the scans were
 analysed with WinRhizo (Regent Instruments, Quebec, Canada) to determine
 total fine-root length, mean diameter and mean root volume, and the fine
 roots were oven-dried at 60° C for 48h to determine dry mass. Fine-root
 SRL was calculated by dividing fine-root length by dry weight, fine-root
 tissue density was calculated by dividing fine-root dry weight by
 fine-root volume, and fine-root length density (fine-root length per soil
 volume) was calculated by multiplying SRL and fine-root mass.