Modélisation des pertes de croissances, de densité et de diversité des semis sur un gradient de densité de population des ongulés sauvages

Wild ungulates have determinant impacts on temperate forest regeneration. They causes shifts in species composition, alter forest structure and functioning. Their abundance has strongly increased for the last decades in the forests of the northern hemisphere. Although these effects have already been documented for some particular study cases, a deeper and more general understanding of these effects is needed across varying environmental conditions, and particularly across varying densities of wild ungulates. In 2016, a vast network of 970 enclosure-exclosure experiments was set up in southern Belgium. Each of the 970 plots was established in forest areas where natural generation was present or expected. The plots were composed of one fenced subplot of 6 m2 (control) and two unfenced square subplots of 6 m2 at a 4 meter between centers distance. The heights of the highest seedlings, the number of seedlings of each species, the herbaceous and shrub covers were measured at the installation, and in 2017 and 2018. That network covers a region of 3000 km2 and a wide gradient of abundance of red deer (Cervus elaphus)and wild boar (Sus scrofa). To model the effects of red deer and wild boar on forest regeneration, we fitted non-linear models to predict regeneration height growth, density and diversity in the unfenced plots in response to observations, carried out in the adjacent fenced plot: red deer abundance, wild boar abundance and different environmental conditions as altitude, soil and basal area. A relationship was then established between the severity of ungulates impacts on forest ecosystem and the density of ungulates in different environmental conditions. Such understanding will be particularly helpful to design adequate forest and wild game management guidelines.15. Life on lan

Are wild ungulates a threat for the forest diversity?

Les ongulés sauvages interfèrent avec la dynamique forestière en modifiant les avantages concurrentiels des espèces en cours de régénération. Le monitoring d'un réseau d'enclos-exclos à l'échelle régional dans les hêtraies-chênaies d'Ardenne a permis de mettre en évidence la nature de ces impacts. Les ongulés favorisent les essences climaciques au détriment des essences liées aux stades moins avancés de la succession. Ces impacts vont à l'encontre d'objectifs de gestion visant à diversifier les forêts. Les essences défavorisées ont une régénération déficitaire depuis des décennies. Ces impacts réduisent également la résilience des forêts face à l'augmentation des températures et des épisodes de déficit hydrique.15. Life on lan

Belgique

Bark stripping caused by Red deer is a major economic issue for forest owners and wood transformation sector. Norway spruce is the most abundant tree specie in Wallonia (southern Belgium), and is particularly sensitive to bark stripping and to resulting wood decay. Occurrence of new damages varies strongly between years and sites. Causes of these variations should be better understood to avoid high proportions of damaged trees. Bark stripping occurrence is influenced by numerous factors, often inter-correlated and highly changing in space and time. Dense stands combine good thermic and security cover, poor feeding resources and trees with thinner bark and lighter branches, what could explain high sensitivity of spruce to bark stripping, especially during winters with important snow cover synonyms of temporary food scarcity. Strategies to avoid important damages are unclear since relative importance of forest characteristics and deer abundance is variable between studies. We modeled the yearly proportion of bark stripped Norway spruces based on a 13 years yearly inventory representing 2, 570 km² of forest with a 4 hectares spatial resolution and a half year temporal resolution. That large dataset covered contrasted environmental conditions and allowed to consider a wide number of influent factors concurrently, minimizing misinterpretation risk. We modeled bark stripping probability at the sampling unit scale allowing then to aggregate the evaluation at management area level with a goodness of fit of 42% pseudo R². Our model clearly evidence red deer abundance as the main bark stripping occurence driver. Factors shaping red deer distribution and food abundance such as stand age, season, distance from human infrastructures, distance from roads and number of froze days had also a strong effect on damage occurrence. Since we observed a weak effect of crown closure around stand on damage probability and a negative effect of coniferous proportion around stand, we confirm the combination of cover quality, scarce herbaceous layer and thin bark to be responsible of young coniferous stand sensitivity to bark stripping. Reducing coniferous proportion is not a way to reduce damage occurrence in remnant stands. Favoring herbaceous layer around stand by stronger thinning could only have a moderate effect since red deer would continue to select coniferous stands and is conflicting with wood quality. Deer abundance regulation is thus the main way to modulate damage intensity. We also showed how our model can be used to monitor population changes in time at hunting district scale.L'écorcement causé par le cerf élaphe est un enjeu économique majeur pour les propriétaires forestiers et le secteur de la transformation du bois. L'épicéa commun est l'essence d'arbre la plus abondante en Wallonie et est y est particulièrement sensible, ainsi qu’à la dégradation du bois qui en résulte. L'apparition des dégâts varie fortement d'une année à l'autre et d'un site à l'autre. Les causes de ces variations doivent être mieux comprises pour éviter des proportions élevées d'arbres endommagés. L'écorcement est influencé par de nombreux facteurs, souvent interdépendants et très variables dans l'espace et dans le temps. Les peuplements denses combinent une bonne couverture thermique et de sécurité, de faibles ressources alimentaires et des arbres avec une écorce plus mince et des branches plus légères, ce qui pourrait expliquer la sensibilité élevée de l'épicéa à l'écorcement, surtout durant les hivers avec une importante couverture neigeuse synonyme de pénurie temporaire de nourriture. Les stratégies pour éviter des dommages importants ne sont pas claires puisque l'importance relative des caractéristiques de la forêt et de l'abondance des cerfs varie selon les études. Nous avons modélisé la proportion annuelle d'épicéa écorcés sur la base d'un inventaire annuel de 13 ans représentant 2 570 km² de forêt avec une résolution spatiale de 4 hectares et une résolution temporelle de 5 ans. Ce vaste ensemble de données couvrait des conditions environnementales contrastées et permettait de tenir compte simultanément d'un grand nombre de facteurs influents, minimisant ainsi les risques de mauvaise interprétation. Nous avons modélisé la probabilité d'écorcement à l'échelle de l'unité d'échantillonnage permettant ensuite d'agréger l'évaluation au niveau de l'aire de gestion avec une qualité d'ajustement de 42% pseudo R². Notre modèle montre clairement que l'abondance des cerfs est le principal facteur d'occurrence de l'écorcement. Les facteurs qui déterminent la répartition du cerf et l'abondance de la nourriture, comme l'âge des peuplements, la saison, la distance des infrastructures humaines, la distance des routes et le nombre de jours de gel, ont également eu un effet important sur la fréquence des dégâts. Puisque nous avons observé un faible effet de la fermeture de la canopée autour du peuplement sur la probabilité de dégâts et un effet négatif de la proportion de conifères autour du peuplement, nous confirmons que la combinaison d’un bon couvert, de la rareté de la strate herbacée et d’une fine écorce est responsable de la sensibilité des jeunes résineux à l'écement. La réduction de la proportion de conifères n'est pas un moyen de réduire les dégâts dans les peuplements restants. Favoriser la couche herbacée autour du peuplement par un éclaircissage plus fort ne pourrait avoir qu'un effet modéré puisque le cerf rouge continuerait à sélectionner des peuplements de conifères et est en conflit avec la qualité du bois. La régulation de l'abondance de cerfs est donc le principal moyen de moduler l'intensité des dommages. Nous avons également montré comment notre modèle peut être utilisé pour suivre et comprendre les changements d’abondance de cerfs à l'échelle du district de chasse.Accord-cadre de recherches et vulgarisation forestières 2014-201

Bark stripping inventories to monitor forest-large fauna balance

Bark stripping damages on Norway spruces are an important economic issue, in particular in Wallonia (southern Belgium) where it is the main tree specie, with 12% of standing volume damaged. As Norway spruces are sensitive to wood decay when debarked, financial losses of standing volume of a damaged standing spruce is approximately 30% of the tree value. Based on a systematic inventory measured since 2004 at regional scale, we could model probability of bark stripping occurrence on contrasted environmental conditions, with consideration of numerous factors. We showed that bark stripping probability is mainly driven by red deer abundance, but also influenced by numerous factors shaping red deer spatial distribution, food resource availability, and stand sensitivity. The adjusted model allows to correct bark stripping rates variations caused to climate changes what helps to understand changes caused by other factors, in particular deer abundance.Accord-cadre de recherches et vulgarisation forestières 2014-201

Enclosure-exclosure to monitor forest-wild ungulates balance

Wild ungulates strongly affect forest regeneration and can alter forest species composition and functioning. Their impact is difficult to measure since forest regeneration is also affected by a multitude of other factors such as light, soil characteristics or competition between species in close interaction and difficult to quantify. Enclosures-exclosures experiments allow isolating the effect of ungulates from other factors. Vegetation changes can be observed without ungulate impact (in enclosures) and compared to vegetation changes in natural conditions (exclosures). Forty-eight enclosures-exclosures pairs where monitored during 4 years in a beech forest of southern Belgium. They were installed in small forest gaps favorable for seedlings and sapling development. Beech and spruce Saplings were the most abundant in the two compartments. Growth of all species was higher in enclosures than in exclosures. Species with the most important growth were beech and spruces whereas all other species nearly didn’t grew in exclosures. In enclosures, growth of birch and willow was highly faster than other species. Oaks had the slowest growth but it was greater than 5 centimeters by year in more than half of the enclosures whereas it was null in more than half of the cases in exclosures when present. These results highlight the difficulty to regenerate a diversified forest due to ungulates in the forest of Florenville. Similar analysis will soon be performed on a network of 900 pairs of enclosures-exclosures installed and monitored since 2016 by the DEMNA (Département d’étude du milieu naturel et agricole), DNF (Division nature et forêt) and SRFB (Société royale forestière de Belgique) and should allow to asses impact of ungulates on regeneration at a regional scale.Accord-cadre de recherches et vulgarisation forestières 2014-201

Le taux d'écorcement pour le suivi des populations du cerf élaphe

Wild ungulates, including red deer, have become abundant in Wallonia (Belgium) as well as in many temperate European forests. Their ecological and economical impacts, notably bark-stripping, are widely documented. Instead of inaccurate and time consuming census of deer populations, forest managers argue for sets of validated indicators to monitor their impact on the environment, including the damages on timber production. Number of bark stripped trees may vary considerably from year to year and between sites and the reasons of these variations are not yet fully understood. This study aimed to identify and assess the effects of determinant factors on damage probability. A systematic sampling inventory was realised from 2004 to 2016 on sensitive stands (8–36 years old stands of Picea abies (L.) H. Karst). More than 52,000 trees and 3,500 sampling plots where observed each year in 82 kha of forest divided into 64 blocks facing contrasted red deer densities. Annual climate, red deer population densities and landscape indicators were computed at block level based on pre-existing data sources. Yearly barkstripping probability was estimated using a generalized linear mixed model. Population densities, number of froze days, of heavy precipitations and snow cover proportion showed significant and positive effect on barkstripping. The availability of close agricultural lands was the only landsape variable having significant effect but the error structure had to be handle by application of a random effect. Standardisation of climate effect on bark peeling rate produced a more stable indicator over time than raw bark-stripping rate. That indice was strongly correlated (R² = 0.67) with abundance indices derived from spotlight counts in a forest block where red deer numbers have been significantly decreased during the study period. Our findings suggested that bark stripping rate can provide useful informations not only on economic damages but also on changes in deer populations.Accord-cadre recherche et vulgarisation forestière

How to reduce the bark stripping rate caused by red deer on spruce, the first forest species of Wallonia?

L’épicéa est la principale essence de Wallonie et parmi les plus importante d’Europe. Elle est malheureusement particulièrement touchée par les écorcements de cerf. Ce dégât rend une part importante du volume des arbres impropre à des transformations de haute valeur ajoutée, réduit la croissance et rend les peuplements sensibles aux tempêtes et aux stress climatiques. Ses répercussions dépassent donc de loin la seule perte de valeur marchande. L’écorcement est devenu un problème déterminant pour la très importante filière du bois résineux dans toute l’Europe avec l’accroissement des populations de cerfs depuis la fin du 20e siècle. Il peut contribuer à rendre problématique l’approvisionnement de la filière en s’additionnant aux pertes de volume causés par des épisodes de sécheresse qui seront plus intenses et fréquents dans les années à venir. D’autre part, le cerf est une espèce emblématique, qui a failli disparaître de nombreuses régions d’Europe Occidentale. Cette espèce présente aussi un enjeu important car elle contribue à la valeur des territoires de chasse. Cette étude a valorisé un inventaire remarquable par la superficie, la résolution spatiale et temporelle et la durée du suivi. En croisant ces données avec de multiples variables à fine résolution spatiale et temporelle, l’effet des principaux facteurs influençant l’écorcement a pu être évalué simultanément. Les résultats obtenus donnent des indications précieuses car ils mettent en évidence les meilleures stratégies pour réduire les taux d’écorcement pour lesquelles il n’existait pas encore de consensus.Spruce is the main forest species in Wallonia and among the most important in Europe. Unfortunately, high proportions of spruces are affected by bark stripping caused by red deer. This damage makes a large part of the tree volume unsuitable for high value-added processing, reduces growth and makes the stands sensitive to storms and climatic stress. Its repercussions therefore far exceed the loss of market value alone. Debarking has become a key issue for the very important softwood sector throughout Europe with the increase of deer populations since the end of the 20th century. The reduction of timber production due to bark stripping adds up to the reductions caused by droughts which will be more intense and frequent in the coming years. On the other hand, red deer is an emblematic species, which has almost disappeared from many parts of Western Europe. This species also presents an important challenge because it contributes to the value of hunting territories. Finding alternative options to the red deer abundance reduction is thus highly desirable. This study has valued a remarkable inventory in terms of surface area, spatial and temporal resolution and duration of monitoring. By cross-referencing these data with multiple variables of fine spatial and temporal resolution, the effect of the main factors influencing barking could be assessed simultaneously. The results obtained provide valuable insights as they highlight the best strategies for reducing debarking rates for which there was not yet a consensus.Plan quinquennal de recherches et vulgarisation forestière

Mitigation of bark stripping on spruce: the need for red deer population control

Norway spruce (Picea abies (L.) Karst.) is a major production tree species for the European wood industry. However, it is highly sensitive to bark stripping (BS) by red deer (Cervus elaphus L.), which causes large timber losses. Because the red deer population has increased over the last decades, a better understanding of the underlying causes driving BS is urgently needed. BS outbreaks are multifactorial: winter food shortage, local and regional deer abundance and stand properties (thermal and visual cover, and scarcity of herbaceous layers) have been found to drive BS rates. Although the influence of each of these factors is well studied, there is still no consensus on their relative contributions to BS or what mitigation strategy is best. In this study, we made use of a long-term, large-scale BS inventory (13 years over 2570 km2) to fill these gaps. We modeled winter and summer BS rates in response to the most explanatory variables, selected from a large set of environmental variables. Our results highlight the prevalence of deer abundance combined with high sensitivity of coniferous thickets and pronounced habitat selection by deer for these stands. Our findings show that the control of red deer populations is a key measure for reducing BS in Norway spruce stands