Windthrow

The forests of 473 Italian Alpine municipalities were severely damaged by a strong wind at the end of October 2018. The affected forest area covers 42,500 ha. The president of one of the damaged regions asked for help from the TESAF department of the University of Padua. 26 international scientists (listed: 25; anonymous: 1) responded to the appeal and collectively wrote this article. At first the value of ramial chipped wood was discussed; then of leaving or not the forest to its natural evolution; there was no lack of bark beetles; the biodegradation times of fallen trees were estimated according to the on-site forms of humus; and ends also in political and social considerations. After eight months of discussion, with various reworkings and cuts, a controversial text was born, complete and practical at the same time. There are several ways to read an article that seems too long: a) focus on Conclusions. In this article there are two types of them: 1) Part Three “Conclusions”: a letter to the Governor of the Region, with applied considerations; 2) Part Two “Actions”, in chapter "2.2.1. Silviculture on 75% of the damaged area - Synthetic plan", with the practical things to do in the field; b) having a little more time: pass through titles of all paragraphs and jump here and there inside if attracted by interesting issues; crucial piece: figure 7; c) while discussing, both traders and artists did not stop working. Just look at figures 8 and 9, or listen to Vaia’s scream. Do not hesitate to contact us for clarification or to continue the debate.

• This review summarizes early stand-scale studies of pristine forest structures, disturbance regimes and successional patterns carried out in boreal Eurasia. We attempt to reveal, characterize and classify stand dynamic types that can be used as templates for nature-based forest management. • The studies reviewed demonstrate multiple successional pathways in stand development in all types of pristine forests. All-aged stands driven by small-scale disturbances are formed over successional development of several hundreds of years. This endogenous development can be interrupted by stand-replacing or partial disturbances leading to successions with even-aged or cohort-structured stands, respectively. In Western Europe, the most common disturbances are windthrows, surface fires and fluctuations in moisture regime; in Eastern Europe and Siberia, the most common disturbances are crown and surface fires and insect outbreaks. Type, return interval and severity of disturbances are strongly influenced by the site conditions and successional stage of a stand. • Based on characteristics of forest stands and disturbance regime, four main types of pristine boreal forest stand dynamics can be distinguished: (1) even-aged, compositional change dynamics, (2) even-aged, mono-dominant dynamics, (3) cohort dynamics and (4) fine-scale gap dynamics. These types can be mimicked in developing scenarios of ecological sustainable forest management in Eurasian boreal forests. • Cette revue bibliographique résume les résultats de nombreuses études anciennes sur la structure des peuplements forestiers, sur les régimes de perturbation et sur les dynamiques de succession en forêt boréale eurasienne. Une typologie des modes de succession est proposée pour servir de cadre à une gestion forestière proche de la nature. • Les études analysées montrent l’existence de multiples modes de succession et de développement des peuplements dans tous les types de forêts vierges analysées. Des peuplements mélangés comportant des arbres de tous âges se développent en réponse à des perturbations locales avec des dynamiques de plusieurs siècles. Ce développement endogène peut être interrompu par des renouvellements massifs ou des perturbations partielles qui conduisent à des successions basées sur des peuplements équiennes ou structurés en cohortes, respectivement. En Europe occidentale, les perturbations les plus fréquentes sont les chablis, les incendies de surface et des fluctuations des régimes hydriques ; en Europe oriental et en Sibérie, ce sont les incendies de surface et de canopée, et les attaques massives par des insectes. • En se basant sur les caractéristiques des peuplements et des types de perturbations, nous avons pu identifier quatre types principaux de dynamiques forestières : (1) une dynamique de changement de composition spécifique avec une structure équienne ; (2) une dynamique de domination par une espèce avec une structure équienne ; (3) une dynamique de cohortes ; et (4) une dynamique de trouées locales. Ces types de succession peuvent servir de base pour le développement de scénarios de gestion forestière durable de ces forêts boréales eurasiennes.

Windthrow is a recurring disturbance process in regions that are influenced by maritime climates, including boreal forests. Within the circumboreal region, forest management approaches have been adapted to emulate natural disturbance in order to promote biodiversity and ecosystem resilience. Few studies have evaluated windthrow outcomes in natural or managed old-growth boreal mixed-species stands, and these studies are primarily empirical. The objective of this study was to adapt the hybrid empirical-mechanistic ForestGALES BC model, to investigate windthrow dynamics in natural and managed boreal old-growth stands, under various wind regimes. ForestGALES BC was updated by adding biomechanical data for balsam fir (Abies balsamea (L.) Mill.) and black spruce (Picea mariana (Mill.) B.S.P.). The model has the ability to simulate damage propagation during wind events by recalculating wind loading on a subject tree after failure of upwind trees. The number of iterations of this recalculation provides insights into the potential for propagation during longer duration storms. A simulation space made up of 500, 20 m × 20 m cells, was created using tree-lists from silviculture systems experiments in north eastern Quebec. The tree-lists for cells within the simulation space were edited to represent the plot conditions at the Quebec experimental sites, allowing simulation of a range of cell-(plot) and matrix-level (landscape) partial harvesting regimes. Above-canopy winds of various speeds were applied in order to test the initiation and propagation of damage within the simulated forest. Simulated outcomes were compared to observations of windthrow 6–7 years after partial harvesting at the field experimental site. Observed damage at the experimental site ranged from 1 to 40% of plot total basal area (mean = 14%, se = 1.96). For above-canopy wind speeds within the range expected for the experimental site, simulated windthrow levels in partial cuts ranged from 7 to 32% (mean = 12%, se = 1.94). When the combined effects of wind speed and number of iterations (∼event duration) were investigated, event duration was more important for wind speeds over 20 m/s. By enabling simulation of outcomes at the tree, plot (cell) and stand (simulation space) levels, the model allows investigation of a wide range of harvesting strategies, and sets the stage for inclusion of wind disturbance in ecosystem succession models and other stand or landscape-level decision-support tools for forests in windy climates.

Soil Collembola communities were investigated in spruce forest stands of the High Tatra Mts that had been heavily damaged by a windstorm in November 2004. The study focused on the effect two different forest practices had on collembolan community distribution and structure in the three years after the windstorm. Three different treatments were selected for the study: intact forest stands (REF), clear-cut windthrown stands (EXT), and non-extracted windthrown stands (NEX), with each treatment having three study stands. From a total 6829 individuals, 65 species were identified. The highest Collembola abundance means were recorded in NEX stands, with the same parameter being lower in REF stands and the lowest in EXT stands. Although there was a significant increase in abundance over time observed in EXT stands, this increase in abundance over time was much higher in NEX stands. The ordination method used demonstrated a significant influence of both treatment and sampling date on the abundance of Collembola. The present study shows that clearing windthrown spruce forests following a catastrophic storm is less favourable for communities of soil collembolans and slow down the recovery process.

The forestry area was analyzed by first masking out the nonforest land cover classes and the spectral indices were standardized around the scene mean forest value to obtain better results. The main workflow steps of windthrow analysis include: (i) calculate the spectral index (ii) detecting the disturbance pixels; (iii) filtering training data and disturbances pixel through a number of statistical filters; (iv) mapping forest disturbance.

Canopy disturbances such as windthrow events have obvious impacts on forest structure and composition aboveground, but changes in soil microbial communities and the consequences of these changes are less understood. We characterized the densities of a soil-borne pathogenic oomycete (Pythium) and a common saprotrophic zygomycete (Mortierella) in nine pairs of forest gaps created by windthrows and adjacent forest understories. We determined the levels of Pythium necessary to cause disease by performing pathogenicity experiments using two Pythium species, a range of Pythium densities, and two common tree species (Acer rubrum and Prunus serotina) from the study sites. Three years post-disturbance, densities of Mortierella remained suppressed in soil from forest gaps compared to levels in intact forest understories while varying across sites and sampling dates. Pythium were infrequently detected likely because of soil handling effects. Expression of disease symptoms increased with increasing inoculum density for seedlings of P. serotina with each Pythium spp. having a similar effect on this species. Conversely, A. rubrum appeared resistant to the two species of Pythium. These results suggest that Pythium densities at sites where they were detected are sufficient to cause disease and possibly affect establishment of susceptible species like P. serotina. Because early seral environments have lower loads of the saprotrophic Mortierella, pathogen loads may follow a similar pattern, causing susceptible species to establish more frequently in those habitats than in late-seral forests. Forest disturbances that alter the disease landscape may provide an additional mechanism for explaining succession of temperate forests in addition to the shade-tolerance paradigm.

Boreal ecosystem functioning is largely controlled by disturbance dynamics. There have been efforts at adapting forest management approaches to emulate natural disturbance effects, as this is expected to maintain ecosystem resilience. In many
instances, this involves resorting to partial cutting strategies that are likely to increase windthrow losses. The objective of this study was to determine the effects of alternative silvicultural practices on windthrow damage and how these effects vary with the scale of treatment. The study was conducted in the Quebec North Shore region (Canada), an area dominated by balsam fir (Abies balsamea (L.) Mill.) and accompanied by black spruce (Picea mariana (Mill) B.S.P.). Four different silvicultural treatments (overstory removal, heavy partial cutting, and two patterns of selection cutting) and control areas were implemented in 2004 and 2005. The experiment used a nested approach where treatment at the plot level was independent and yet nested within the block-level treatment. At the block level, treatments were applied over 10–20 ha units, leaving a small portion of the block for
a smaller application of each treatment (plot scale, 2500 m2). Inventory was carried out before harvesting and monitoring was done yearly after harvesting, with the aim to better understand the plot- and block-level factors that drive windthrow damage
levels and the effects of alternative silvicultural treatments. Results after 6–7 years show that basal area proportion windthrown differs substantially between treatments, as well as between treated sites and control sites. Windthrow levels were higher under heavy cuts relative to selection cuts and also increased with balsam fir proportion. Windthrow proportions were better correlated
to block-level treatment than plot-level treatment, showing that the environment surrounding the treated plot can have an important effect on windthrow losses. Overall, the selection cutting system, particularly SC2, retains the most green-tree basal area and thus best meets the management objective of retaining old-growth attributes. A simple empirical model was calibrated that could aid in hazard rating.