Oskars Krišāns


1st day: “Tree mechanical stability under wind loading”

2nd day: “Tree susceptibility to wind damage: results of static tree-pulling tests in Latvia”

Abstract of presentation:

Tree mechanical stability under wind loading. The effect of wind loading on trees can result in various responses ranging from nearly noticeable leaf waving to high frequency swaying and to internal wood damages that could lead to a fatal failure such as stem breakage or uprooting regardless of the growing environment. Tree resistance to wind loading is a function consisting of the wind force applied on the above-ground parts and the strength of resistance of stem and soil-root anchorage. This determines the limit of wind loading that a tree can bear, and thus the maximum wind speed that either stem or root system can resist. The presence of secondary disturbances, such as older mechanical damages either in roots or above-ground parts, pests, pathogens, water deficit or anthropogenic stresses can substantially decrease the mechanical stability of trees, leading to lower wind loads required to cause a fatal failure. This is of great importance to both urban (also peri-urban) and commercial forests, although the type of consequences differ between them. In commercial forests, wind can cause catastrophic damages to wood production, while in urban areas tree failure during strong winds is a serious threat to infrastructure, as well as to the health or lives of people. Therefore, the evaluation of mechanical stability of trees is of great importance for managing safe and pleasant surroundings in both residential and public areas of urbanized environments.

Tree susceptibility to wind damage: results of static tree-pulling tests in Latvia

During the last decades, the prevalence of wind disturbances has increased, causing substantial socio-economic and ecological impacts. Both frequency and magnitude of wind disturbances are amplified by the ongoing climate warming and heterogeneity of precipitation, under which some tree species might decline in both urban and peri-urban forests. Moreover, amplified severity of post-storm legacy effects is projected to decrease tree resistance to pathogens, pests and water deficit, increasing tree susceptibility to wind damage and resulting as a negative feedback loop. An insight into tree susceptibility to storms can be provided by the assessment of tree loading resistance via a static tree-pulling test.  

Since 2017, in total nearly 500 trees of Norway spruce, Scots pine, silver birch, common aspen, pedunculate oak and European beech have been tested destructively, acquiring information on the strength of soil-root anchorage and stem under various growing conditions. Taking into account also such aspects as – soil type and moisture content, as well as freezing of soil, presence of mechanical damages, invasion of pathogens, etc.

Tree resistance to static loading differs among species and soil types, with oak and beech growing on moraine soils being the strongest. Spruce and pine on deep peat soils have lower resistance to static loading than on well drained mineral soils. Birch has explicit plasticity in adaptation to loading resistance on moist and less stable soils, implying a high potential in increasing wind resistance of stands under such conditions. The application of destructive testing on tree mechanical stability has provided valuable data that can be used in the evaluation of methods and thresholds typically applied in commercial tree stability assessments. This has a high potential in facilitating substantial improvements of non-destructive testing methods for estimation of tree wind resistance.

Biography: Oskars Krišāns is a certified arborist and a researcher of the Latvian State Forest Research Institute "Silava". His current research looks at the loading resistance of trees and causes of wind damage to forests in the context of climate change, employing static tree-pulling and dynamic tests. Since 2020, in total 16 research articles on tree wind resistance, as well as assessment methods of the strength of tree anchorage have been published.