A Static Pulling Test Is a Suitable Method for Comparison of the Loading Resistance of Silver Birch (Betula pendula Roth.) between Urban and Peri-Urban Forests

Abstract

In urbanized areas, wind disturbances can be intensified by anthropogenic stresses under which trees may become hazardous, creating serious threats and damages to nearby targets. Therefore, species with notably lower both wood mechanical properties and compartmentalization, such as pioneers, are considered to have higher wind damage risk if subjected to unfavorable growing conditions. Eurasian aspen (Populus tremula L.) and silver birch (Betula pendula Roth.), are frequently found in both urban and peri-urban forests in Northeastern and Central parts of Europe, which strengthen the necessity for the evaluation of mechanical stability of such species. Therefore, static pulling tests were performed to compare the mechanical stability of the studied species in both urban and peri-urban forests. The loading resistance of the studied species differed, with birch being more stable than aspen, indicating aspen to be more prone to wind damage. Additionally, the mechanical stability of birch did not differ between trees growing in urban and peri-urban forests, suggesting static pulling tests are a suitable method for comparing trees from completely different growing conditions.

The Destructive Static Tree-Pulling Test Provides Reliable Estimates of the Soil–Root Plate of Eastern Baltic Silver Birch (Betula pendula Roth.)

“…Additionally, due to a slower course of uprooting under static loading, root breakage occurred at smaller diameters [ 25 ], hence at a greater distance from the stem, thus explaining the explicit overestimation of soil–root plate width. Alternatively, the overestimation of the dimensions of the soil–root plate might be partially related to the specific selection of sample trees for the static tree-pulling, as vital, dominant trees without visual signs of mechanical damage were tested [ 18 , 22 ]. In contrast, the set of studied windthrown trees likely contained the weakest individuals in the stands, probably due to various reasons, such as lower vitality, which might include pathogen infestation [ 26 , 27 ].…”

“…Data for the soil–root plates overturned by static tree-pulling were acquired from earlier studies assessing the mechanical stability of Eastern Baltic silver birch [ 18 , 22 ] in the hemiboreal forests of the Eastern Baltic region in Latvia (55°56′–57°27′ N and 21°40′–26°23′ E; Table 3 ). The climate in the studied region is humid continental [ 36 ], influenced by the dominant westerlies from the North Atlantic [ 37 ], under which continentality increases eastwards [ 38 ].…”

“…In total, data for 115 birch trees were collected in the destructive static tree-pulling tests ( Table 3 ), which were conducted during 2019–2021 [ 18 , 22 ]. During the tests, the dimensions of the stem and crown were recorded and the BBM of the stem at the failure was estimated.…”

“…Similar to pulling tests, trees growing on the edges of openings were avoided. The surveyed trees represented 15 stands similar to those sampled during the static tree-pulling [ 18 , 22 ]. Canopy trees were surveyed, which represented the mean dimensions of stands and all of them formed the canopy.…”

“…Although the properties of the soil–root plate can be assessed for naturally windthrown trees, such trees can rarely form a reasonable sample size to evaluate the effects of management, as wind damage is largely stochastic [ 15 ]. Another option is experimental overthrowing by static tree-pulling tests, which have been widely applied to assess the tree mechanical stability [ 16 , 17 , 18 ]. Under static pulling, the tree is subjected to a continuous loading until a fatal failure as stem breakage or uprooting occurs; however, such an evaluation might result in biased estimates of the properties of the soil–root plate due to the lack of dynamic components of stability, such as inertia and swaying [ 19 ].…”