Growth and Mortality of Hybrid Poplar Short Rotation Culture (AF8 Clone) in Response to Clostera anastomosis L. (Lepidoptera: Notodontidae) Defoliation

The increasing worldwide interest in renewable energy and carbon storage has led to the development of relatively fast solutions to obtain wood biomass. The cultivation of fast-growing tree species in short-rotation crops, such as hybrid poplar clones, is one such solution, at least in temperate areas. Sometimes these monocultures are affected by disturbing factors, including severe insect defoliation, with strong destructive effects. The impact of defoliation on the growth and productivity of poplar crops is often estimated in the context of artificial defoliation. There have been few studies in which the effect of defoliation was calculated after natural defoliation. Among defoliating insect species, Clostera anastomosis L. is one of the most important defoliators of young poplars. This species developed severe defoliation in a 4-year-old poplar clone AF8 crop, from the northeastern part of Romania, in the spring and summer of 2017. The study aimed to assess the impact of defoliation both on the growth and mortality of defoliated trees and the productivity of the affected crop. To reach this goal, the height and radial growth of 150 trees with different defoliation rates (50 non-defoliated, 50 partially defoliated, and 50 completely defoliated) were measured and the defoliation symptoms and mortality were also evaluated for 4780 poplars (10% of the analyzed population). The defoliation caused a significant decrease in the annual height growth (28.6% of the partially defoliated trees and 38.5% of the completely defoliated trees) and a severe decrease in the radial growth, between 82.2% and 90.4%, respectively, depending on the defoliation rate. These strong decreases led to a significant decline in tree-level biomass accumulation, approximately 74.8% for partially defoliated trees and 83.1% for completely defoliated trees, for the year of defoliation. As a result, the loss in total biomass (for the four years of the rotation) was between 28.1% and 34.6%, respectively, depending on the defoliation rate. Therefore, the total biomass loss was 5 t·ha−1, representing approximately 70% of the forecasted production for 2017 only (approximately 7 t·ha−1).