Effects of Experimental Defoliation on a Population of Pedunculate Oak (Quercus Robur L.)

Summary An experiment simulating defoliation at three levels of severity was conducted on 196 young oaks in a plot at Wolverhampton. In comparison with control plants, defoliation caused (1) earlier production of growth spurts (which are normal in undefoliated trees) and formation of more lateral branches, which were susceptible to winter frost damage; (2) production of more abundant, smaller leaves; (3) reduced diameters of main stems, from which can be calculated growth rates, varying with both defoliation treatment and growing conditions over the year, and returning to normal values as soon as defoliation ceased; (4) formation throughout the season of xylem of early wood type having a lower proportion of xylem fibres. With light defoliation, most of these responses were scarcely apparent, but with more severe defoliation, morphological effects dominated. The most noticeable effects of total defoliation were low growth rates and effects on wood anatomy. The results suggest that substantial natural defoliation could cause failure of regeneration, although in un shaded plots of young trees leaf loss results in retarded growth rather than death.