The Control of Leaf Morphology and the Tolerance of Shade by Woody Plants

We compared the rates of stem growth and leaf area per plant of 10 woody dicotyledonous species in full sun and under an artificial shade transmitting about 18% light and did not find any correlation with tolerance to shade (silvicultural classification). Cottonwood, the most intolerant species, grew rapidly in the shade for two seasons but died during the third. The reduction in the shade of leaf thickness (density thickness, measured as fresh weight per unit area) varied between species but did not correlate with tolerance. However, the product of the density thickness and the leaf area (fresh weight per leaf-lamina) was reduced (60%) in the shade much more in the three least tolerant species (cottonwood, catalpa, and American plum) than in the seven more tolerant species (20%). This may be a consequence of shade avoidance by the intolerant species, reducing the net photosynthate available per leaf.