Wind-induced leaning (toppling) in young Pinus radiata plantations in Chile

The toppling of young trees considerably reduces the value of the first log in Pinus radiata plantations. Although wind is the main cause of toppling, the quality and type of plants, planting techniques, soil fertility and soil preparation may play an important role in the susceptibility of the species to toppling. This article analyzes the influence of ripping and soil type on the probability of P. radiata toppling after severe windstorms in central south Chile. To this aim, pairs of similar trees with and without toppling were selected in 2001 in six soil types with and without soil ripping. Root system differences in toppled versus straight trees were compared to gain better insight into the causes of the phenomenon. Among all root measurements, the quality of the tap root measured through the Menzies scale was significantly correlated with the probability of toppling. Trees with a strong, dominant and well-developed tap root (Menzies value equal to zero) showed a toppling probability of 0.34 (1 of each 3 trees being damaged). At the other extreme, trees with a horizontal tap root or with no tap root (Menzies value equal to 10) showed a toppling probability of 0.72 (i.e., three of each four trees were damaged). A toppling probability of 0.50 (or damage observed in one of each two trees) was found for trees with a tap root distinctly hooked but functional (Menzies value equal to 4). Ripping reduced the toppling probability from 0.56 to 0.44 by improving the quality of the tap root consistently across all soil types from Menzies 5.7 to Menzies 2.5. The worst tap root qualities were found for sandy soils (Menzies equal to 7.0, toppling probability of 0.62), followed by metamorphic soils (Menzies equal to 4.5, toppling probability of 0.52). Alluvial, granitic, red clay and volcanic ash soils did not differ in tap root quality (Menzies equal to 3.3, toppling probability of 0.47). In conclusion, the study suggests that all measures favoring a strong, dominant and well-developed tap root, in the nursery and in the field, are likely to considerably reduce toppling damage in young P. radiata plantations in Chile.