Conversion of total to projected leaf area index in conifers

Several definitions of leaf area index (LAI) presently exist in the literature but the relationships among them are not clear. To compare the results of various studies, there is a need to convert from one definition to another. Factors for converting among four definitions of LAI are presented for six conifer species: Abies grandis (DougI. ex D. Don) Lindl., Thuja plicata Donn ex D. Don., Tsuga heterophylla (Raf.) Sarg., Picea sitchensis (Bong.) Carr.), Pinus contorta Dougl., and Pseudotsuga menziesii (Mirb) France). Among the four definitions of LAI, the two extremes involve (i) the total area of the leaf and (ii) the projected area of nonhorizontal leaves, as they occur on the tree. If leaves are randomly oriented in space, then the conversion factor between definitions i and ii should be 0.25. Four of the six species have conversion factors very close to this value, and three of these four are relatively shade-intolerant. The remaining two species, A. grandis and Thuja plicata, have conversion factors of approximately 0.35, owing to the approximately horizontal orientation of their leaves. These two species are both relatively shade-tolerant, and the trend toward horizontal leaves might be an adaptation to assist in shade tolerance. A sensitivity analysis indicated that the foliage of most of the species maximized the amount of light gathered when the light was coming from almost straight overhead, as is the case with many shaded forest trees.