Alterations in vessel size in twigs of Quercus robur and Q. petraea upon defoliation and consequences for water transport under drought

Abstract

In the complex of natural factors responsible for damage to the Central European oak species Quercus robur and Q. petraea, repeated defoliation and drought are considered the most important ones. To investigate the impact of these factors on xylem anatomy and hydraulic conductance, saplings of both species were manually defoliated in the spring of two consecutive years, and equal fractions of defoliated and control saplings were subjected to drought stress in the third year of the study. Defoliation did not significantly reduce the annual ring width of the twigs, but in the twigs of Q. robur it resulted in a significant reduction of the cross-sectional area of early- and latewood vessels, and in a shift in the distribution of the earlywood vessel cross-sectional areas towards smaller size classes in the year after the first defoliation. In the early wood of Q. petraea twigs, repeated defoliation led to a significant increase in the cross-sectional vessel areas and to a shift in their distribution towards larger size classes. No significant differences in the percentage loss of hydraulic conductance (PLC) occurred between trees that had been defoliated in two successive years prior to the drought experiment and control trees. However, PLC was significantly increased by drought. The different responses of the oak species are attributed to their different capability to recover from the applied stress factors.