Growth reduction after defoliation is independent of CO 2 supply in deciduous and evergreen young oaks

Summary Reduced productivity of trees after defoliation might be caused by limited carbon (C) availability. We investigated the combined effect of different atmospheric CO 2 concentrations (160, 280 and 560 ppm) and early season defoliation on the growth and C reserves (nonstructural carbohydrates ( NSC )) of saplings of two oak species with different leaf habits (deciduous Quercus petraea and evergreen Quercus ilex ). In both species, higher CO 2 supply significantly enhanced growth. Defoliation had a strong negative impact on growth (stronger for Q. ilex ), but the relative reduction of growth caused by defoliation within each CO 2 treatment was very similar across all three CO 2 concentrations. Low CO 2 and defoliation led to decreased NSC tissue concentrations mainly in the middle of the growing season in Q. ilex , but not in Q. petraea . However, also in Q. ilex , NSC increased in woody tissues in defoliated and low‐ CO 2 saplings towards the end of the growing season. Although the saplings were C limited under these specific experimental conditions, growth reduction after defoliation was not directly caused by C limitation. Rather, growth of trees followed a strong allometric relationship between total leaf area and conductive woody tissue, which did not change across species, CO 2 concentrations and defoliation treatments.