Abstract

Forests are very diverse all over the world, but they all share the decline in diameter growth rather after an early plateau (moment of full canopy development). Growth of trees and forests may be determined by other factors than carbon supply.

1. Introduction

It’s not the tall trees that slow down in diameter growth but the medium to small trees. They really decline in grwoth rate which reduces the forest’s growth rate. This can be caused at three levels:

  • resource use
  • efficiency
  • partitioning

2. Resource use

  • decrease in LAI:
    • due to collisions of branches, nutrient supply, hydraulic constraints on moving water up
    • LAI reduction by 30% leads to light interception reduction of 10-20% (less self-shading)
    • not enough to explain decreasing stem growth
  • water flow in streams draining old forests smaller than in young forests: declining water use cannot explain declining growth
  • not many studies on age vs nutrient supply
    • big trees could limit nutrient availability of smaller trees
    • fertilization increases growth
    • but: still the same pattern of decreasing growth

3. Efficiency: Declines in growth per unit of resource use

  • efficiency seems more site-dependent than age-dependent

4. Partitioning

  • tempting assumption: older forests have more tissue and respire more, less C available for growth
  • but: most tissue is dead, half of respiration comes from growth, studies show lower respiration in older forests
  • below-ground C flux usually declines in concert with stemwood growth; no universal pattern though

5. Beyond Carbon: Fitness may be about more than photosynthesis and C budgets

  • seemingly: declining PH from declining resource use efficiency, then different partitioning
  • Is this focus on C sufficient?
  • growth often out of synchrony with C assimilation
  • growth not always limited by rate of C uptake
    • trees might not maximize C uptake, might partition elsewhere to dispose of excess C (see Prescott et al., 2021)
    • REDUNDANCY???
  • once hierarchy of trees is established, trees arrange with it
  • older trees use resources less efficient, why?

6. Three key points for the future

  • more data needed for aging forests, managed and unmanaged
  • do trees sustain their established social status?