Märtha Wallgren

Balancing moose Alces alces populations with forest production objectives remains a major challenge in Swedish forestry. We developed a mechanistic, data‐driven model of browse availability in Scots pine Pinus sylvestris saplings by linking shoot‐level allometry, crown architecture, and tree height development. Empirical data were collected from nine unbrowsed trees, for which current‐year shoots were measured by branch whorl, including shoot length, shoot number, and the wet/dry biomass ratio. Shoot scaling with whorl position was analysed to quantify crown allometry and distribution. Crown architecture was coupled with a height–age growth model to simulate vertical crown development and estimate shoot biomass accessible to moose browsing within a defined browsing height for moose. Mathematical model simulations of browse availability were conducted with varied site productivity classes and scaled to the stand level. Individual shoot biomass, length and diameter increased with tree height and declined exponentially with distance from the apical top shoot by 50, 33 and 13% per whorl, respectively. The vertical distribution of both total shoot length and shoot biomass among whorls was unimodal, with peaks occurring close to the midpoint of the crown. Simulations showed that browse availability increased rapidly with tree growth, peaked at tree heights around 3.5 m (15–25 years depending on site productivity), and subsequently declined. Peak shoot biomass ranged from 330 to 450 g dry weight per tree, with high‐productivity sites yielding up to 35% more biomass. When scaled to stands ≤ 6.0 m in height at 2000 stems ha ⁻¹ , with a browsing height limit of 2.5 m, simulated browse availability was 100–200 kg dry weight ha ⁻¹ . These results show that crown architecture in young Scots pine follows consistent scaling rules that can be expressed as functions of tree height and whorl position. The model provides a mechanistic connection between tree growth and herbivore‐accessible forage.

Moose Alces alces is a culturally and economically important game species in Sweden, but their browsing on regenerating Scots pine trees Pinus sylvestris often causes extensive damage to the production and quality of timber. Forest and wildlife managers are faced with the dilemma of how to reduce damage to timber trees while also supporting moose populations and hunting opportunities. The proportion of damaged trees can be reduced by decreasing the number of moose, but also by increasing the number of pines. However, the relative effectiveness of these two approaches is debated and has not been conclusively determined. Here, we addressed this question by analyzing the effects of moose and pine density on pine damage based on yearly data from almost all of Sweden's moose management areas (MMAs) over a 10‐year period (2015–2024; 718 observations). We developed a mechanistic model to realistically represent the browsing process and used regression with mixed models to account for variable vulnerability (variation in damage risk at a fixed moose per pine ratio) among MMAs in the statistical analysis. The model explained 53% of the variation in the proportion of damaged trees and showed that, on average, the relative damage reduction effect of a decreased moose population was ~ 1.5× larger (25%) than the effect of increased pine density (17%). Vulnerability to browsing varied substantially among MMAs and between years within each MMA, especially in areas with low pine density. This variability prevents reliable predictions of management effects at the individual MMA level for most MMAs. Such local predictions may be improved in the future by incorporating longer time series of observations and additional variables, such as alternative forage sources, browsing by other deer species, and snow cover and duration.

Cervid (Cervidae) populations that are overabundant with respect to their food resources are expected to show declining physiological and reproductive fitness. A proactive solution to such declines is to integrate the monitoring of food resources with animal harvesting strategies, but there are few studies available to guide managers regarding which food resources to monitor and how to do so. In this study, we used a large, rare data set that included detailed absolute measures of available food quantities and browsing intensity from field inventories, to test their relationship with fitness indices of moose Alces alces in 24 management units in four regions across Norway. We found that calf body mass and calves seen per cow during the autumn hunt were strongly and positively related to the availability of tree forage, especially the species most selected for by the study moose (e.g., rowan [ Sorbus aucuparia ] and sallow [ Salix caprea ]). The strength of the correlations varied between regions, apparently being stronger where the moose were closer to being overabundant or had a legacy of past overabundance. As expected, the intensity of browsing on the three most common tree species, that is, birch ( Betula spp.), rowan, and pine ( Pinus sylvestris ), was also negatively and strongly related to the fitness. We discuss how our approach to food monitoring can facilitate a management that proactively adjusts densities of moose, and possibly other cervids, to trends in food availability and browsing intensity, thereby avoiding detrimental effects of overabundance.

Browsing ecology involving ungulates and their food resource is complex as consumption may reflect responses to various factors, such as quantity and quality of browse species. This study investigated winter browsing by moose Alces alces in a mountainous landscape of west–central Sweden, characterized by Norway spruce Picea abies production forests with high inclusion of deciduous species. Field data on tree species abundances as well as fresh and previous browsing were collected along moose tracks and control transects, to answer our questions regarding browse selection in relation to availability. The data were analyzed using mainly modelling (generalized linear mixed models) approaches. The results revealed greater amounts of available food trees (< 5 m) in young compared to older forests and along moose tracks compared to control transects. Downy birch Betula pubescens was the most abundant independent of forest age class and transect type (i.e. tracks versus controls). Quantitatively, the key winter browse in the study area was a mix of deciduous species, primarily downy birch and willows (Salix spp.) complemented by Eurasian aspen Populus tremula and grey alder Alnus incana. In young forest, aspen, rowan Sorbus aucuparia and willows were the most selected species, while common juniper Juniperus communis was the least selected. In older forest, alder was most selected, followed by willows and rowan, while aspen and downy birch were the least selected, except for spruce and lodgepole pine Pinus contorta. According to Jacobs selectivity index, downy birch, was underused while aspen, rowan and willows were overused in relation to their frequencies. The latter are often limited in today's cultivated forest landscape and thus an increased amount of those species may be utilized to a higher degree by moose than an increase of downy birch. No effect from snow depth was detected and browsing on dwarf shrubs was not observed.

Climate change causes far‐reaching disruption in nature, where tolerance thresholds already have been exceeded for some plants and animals. In the short term, deer may respond to climate through individual physiological and behavioral responses. Over time, individual responses can aggregate to the population level and ultimately lead to evolutionary adaptations. We systematically reviewed the literature (published 2000–2022) to summarize the effect of temperature, rainfall, snow, combined measures (e.g., the North Atlantic Oscillation), and extreme events, on deer species inhabiting boreal and temperate forests in terms of their physiology, spatial use, and population dynamics. We targeted deer species that inhabit relevant biomes in North America, Europe, and Asia: moose, roe deer, wapiti, red deer, sika deer, fallow deer, white‐tailed deer, mule deer, caribou, and reindeer. Our review (218 papers) shows that many deer populations will likely benefit in part from warmer winters, but hotter and drier summers may exceed their physiological tolerances. We found support for deer expressing both morphological, physiological, and behavioral plasticity in response to climate variability. For example, some deer species can limit the effects of harsh weather conditions by modifying habitat use and daily activity patterns, while the physiological responses of female deer can lead to long‐lasting effects on population dynamics. We identified 20 patterns, among which some illustrate antagonistic pathways, suggesting that detrimental effects will cancel out some of the benefits of climate change. Our findings highlight the influence of local variables (e.g., population density and predation) on how deer will respond to climatic conditions. We identified several knowledge gaps, such as studies regarding the potential impact on these animals of extreme weather events, snow type, and wetter autumns. The patterns we have identified in this literature review should help managers understand how populations of deer may be affected by regionally projected futures regarding temperature, rainfall, and snow.

Since the 1950s, more than 200 000 km of roads have been built in Sweden’s forests, making them easily accessible and open to multiple uses. The aim of this study was to review the impacts of forest roads in Sweden from a broad perspective encompassing social, ecological, and environmental factors. The Swedish case is interesting because it has an extensive network of permanent forest roads which were built primarily for forestry-related transportation but are currently used by many other stakeholders for many different purposes. Forest roads not only facilitate transportation of wood, machinery, personnel, and equipment into and out of the forest but also enable emergency response to wildfires and support berry and mushroom picking, hunting, recreation, tourism, and access to second homes. The roads increase the opportunities for members of the public to experience forests in various ways. Conflicts arise when different interests collide, for example when the interests of the forest owner clash with those of commercial berry-picking companies, tourism entrepreneurs, or reindeer (Rangifer tarandus L.) herding. Forest roads may have ecological impacts such as barrier and disturbance effects, fragmentation or loss of habitats, altering fauna movement patterns, and changing the composition of plant and insect species. The environmental impacts of forest roads relate to, among other things, hydrology, water quality, and erosion. Predicted changes in the climate are likely to place new demands on Swedish forest roads but, despite their extent, this review shows that there is only a small amount of rather fragmented research on their social, ecological, and environmental consequences. Overall, few studies appear to cover both social and ecological/environmental factors and their interactions, either in Sweden or elsewhere. This review provides examples of such interactions in the case of Sweden, and suggests that more research into these and the specific social, ecological, and environmental factors involved is warranted.

The moose Alces alces is the largest herbivore in the boreal forest biome, where it can have dramatic impacts on ecosystem structure and dynamics. Despite the importance of the boreal forest biome in global carbon cycling, the impacts of moose have only been studied in disparate regional exclosure experiments, leading to calls for common analyses across a biome‐wide network of moose exclosures. In this study, we use airborne laser scanning (ALS) to analyse forest canopy responses to moose across 100 paired exclosure‐control experimental plots distributed across the boreal biome, including sites in the United States (Isle Royale), Canada (Quebec, Newfoundland), Norway, Sweden and Finland. We test the hypotheses that canopy height, vertical complexity and above‐ground biomass (AGB) are all reduced by moose and that the impacts vary with moose density, productivity, temperature and pulse disturbances such as logging and insect outbreaks. We find a surprising convergence in forest canopy response to moose. Moose had negative impacts on canopy height, complexity and AGB as expected. The responses of canopy complexity and AGB were consistent across regions and did not vary along environmental gradients. The difference in canopy height between exclosures and open plots was on average 6 cm per year since the start of exclosure treatment (±2.1 SD). This rate increased with temperature, but only when moose density was high. The difference in AGB between moose exclosures and open plots was 0.306 Mg ha⁻¹ year⁻¹ (±0.079). In browsed plots, stand AGB was 32% of that in the exclosures, a difference of 2.09 Mg ha⁻¹. The uniform response allows scaling of the estimate to a biome‐wide impact of moose of the loss of 448 (±115) Tg per year, or 224 Tg of carbon. Synthesis: Analysis of ALS data from distributed exclosure experiments identified a largely uniform response of forest canopies to moose across regions, facilitating scaling of moose impacts across the whole biome. This is an important step towards incorporating the effect of the largest boreal herbivore on the carbon cycling of one of the world's largest terrestrial biomes.

Reducing browsing damages from cervids (Cervidae) on economically valuable tree species is a challenging task in many countries. Apart from cervid abundance, landscape characteristics, such as forest composition, land use, forage availability and climatic conditions, may affect the degree of browsing through both direct and indirect effects. A better understanding of basic mechanisms in this complex system is needed to design efficient and convincing management strategies. Focusing on Sweden as a case, which has been widely studied using regression analyses only, we applied path analysis to test the validity of a model on the indirect and direct links between landscape characteristics, cervid populations, and browsing damages on Scots pine (Pinus sylvestris). Our results validated the tested model in which moose (Alces alces) density and pine availability directly influence browsing damages. Increasing amounts of pine forests, preferred deciduous trees, and young forest had positive direct effects on moose densities and thereby indirectly contributed to increased browsing damages. The density of smaller deer species showed no direct effect on browsing damages on pine. Path analysis corroborated our attempt to disentangle direct and indirect potential causal drivers of browsing damages and shows that the choice of statistical method may alter the understanding of mechanistic driving forces.