Tree growth increases through opposing above-ground and below-ground resource strategies

  1. Studying functional traits and their relationships with tree growth has proved a powerful approach for understanding forest structure. These relationships are often expected to follow the classical resource economics perspective, where acquisitive leaves combined with acquisitive roots are expected to enhance resource uptake and tree growth. However, evidence for coordinated leaf and roots trait effects on growth is scarce and it remains poorly understood how these traits together determine tree growth. Here, we tested how leaf and root trait combinations explain tree growth.
  2. We collected data on leaf and root traits of 10 common tree species, and on soil carbon (C) and nitrogen (N) concentrations in a temperate forest in Michigan, US. Tree growth was calculated as the stem diameter increment between three censuses measured across 13,000 trees and modelled as a function of different combinations of leaf and root traits and soil properties.
  3. The two best models explaining tree growth included both specific leaf area (SLA), root diameter and soil C or N concentration, but their effects on growth were contingent on each other: thick roots were associated with high growth rates but only for trees with low SLA, and high SLA was related to fast growth but only for trees with thin roots. Soil C and N% negatively impacted the growth of trees with high SLA or high root diameter.
  4. Synthesis. In this study, resource economics did not explain the relationships between leaf and root traits and tree growth rates. First, for trees with low or intermediate SLA, thick roots may be considered as acquisitive, as they were associated with faster tree growth. Second, trees did not coordinate their leaf and root traits according to plant resource economics but enhanced their growth rates by combining thick (acquisitive) roots with conservative (low SLA) leaves or vice versa. Our study indicates the need to re-evaluate the combined role of leaves and roots to unveil the interacting drivers of tree growth and, ultimately, of forest structure and suggests that different adaptive whole-tree phenotypes coexist.
Monique Weemstra, Jenny Zambrano, David Allen, and María Natalia Umaña
Journal of Ecology