Tamm Review: Insights gained from light use and leaf growth efficiency indices

- Explains why LUE is non-linear on a daily basis but linear at monthly to longer time steps.
- Explains why LUE decreases if trees differ in size and uniformity in spacing.
- Discovery that NPP/GPP approaches a constant simplifies stand growth models.
- As trees grow in size, the ratio of foliage to branch mass decreases exponentially.
- Remote sensing holds promise of validating regional shifts in LUE.

In this Tamm review, we trace the origin and application of two production indices: Light Use Efficiency (LUE) and (Leaf) Growth Efficiency (GE). Light Use Efficiency usually denoted (ε) was originally defined by John Monteith in the 1970s as the rate that dry matter is accumulated by plants in relation to the amount of solar radiation absorbed by leaves; the concept has been a corner-stone of the field of production ecology ever since. Although highly variable at daily intervals, LUE becomes linear at longer intervals, providing a major simplification to the construction and application of process-based models. A further simplification in model construction became possible when it was found that the ratio between total dry mass production and gross photosynthesis is approximately constant (≈0.5). Simplified process-based models provide a means of estimating the maximum productivity of a species growing inside or outside its native range, and help to identify constraints on production in current and projected environments. Consequently, models that incorporate LUE have expanded from research tools to practical ways of assessing silvicultural options in the management of individual forests as well as for measuring and forecasting global trends in forest productivity. The Leaf Growth Efficiency (GE) index, defined as annual growth in stemwood per unit of leaf area, has become widely adopted as a means of identifying the spatial variation among trees, which affects stand growth and LUE. GE was originally used to assess the vulnerability of individual trees to attack by bark beetles but, combined with structural and physiological analyses it has been found useful for interpreting and predicting stand growth responses to tree spacing, aging, and defoliation. Challenges remaining in the field of forest production ecology include prediction of the effects of fast-changing climatic conditions across the globe on the growth and survival of species, and their interactions with continually rising atmospheric concentrations of CO2.