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Leaf area index and specific leaf weight : keys to interpreting canopy photosynthesis and stand growth

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Disciplines
  • Biology
  • Chemistry
  • Medicine

Abstract

Two physiological factors are of major importance to tree and stand growth: (1) the photosynthetic rate of foliage and (2) the amount of foliage. If carbohydrate allocation patterns remain constant, stand growth should be related directly to total canopy photosynthesis. From a literature analysis I assess methods of relating photosynthetic rates to biochemical, anatomical, and structural characteristics of foliage. A number of these foliage characteristics were found to be interrelated. Specific leaf weight was shown to be a valuable index for comparing photosynthesis by various parts of a tree canopy over a season or throughout an entire year. Mean annual photosynthetic rate in five separate portions of a spruce canopy was directly proportional to observed differences in specific leaf weight (r2 = 0.99). Annual carbon uptake was a function of total foliage biomass (r2 = 0.96). When foliage biomass at each crown segment was adjusted for differences in specific leaf weight, reflecting differences in photosynthetic rates, the predictive equation further improved C r2 = 0.99). Specific leaf weight is recommended as an index for comparing the relative effects of various silvicultural treatments on photosynthesis. I then evaluated how stand growth and canopy leaf area were related by analyzing 24 years of growth records from a Pinus ponderosa (Laws.) experiment. The experiment included a wide range in initial stocking and partial control of understory vegetation (Barrett 1982). I found that treatment effects on tree growth can be evaluated at low values of stand leaf area from comparison of growth efficiencies (wood produced per unit leaf area) among plots of similar canopy leaf area. By comparing stand growth with stand leaf area, I concluded that the major effect of removing understory vegetation was to speed the development of the canopy. This interpretation was also supported by a comparison of the rate of leaf area development on plots with and without understory vegetation at comparable levels of canopy leaf area. Comparing stands at a similar canopy leaf area is advised for assessing how treatment affects stand development. This is a valuable alternative to analyzing treatment effects at one point in time and helps to explain the results of many fertilization experiments.

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