Global forest ecosystems are experiencing increasingly frequent and severe insect outbreaks, driven by complex interactions among climate change, land-use alterations, and shifting species distributions. Species that are morphologically indistinguishable - often referred to as cryptic species - can differ significantly in distribution, host use, and susceptibility to natural enemies and might thereby differ in outbreak dynamics. Yet, the contribution of cryptic species to temporal changes in the frequency and severity of insect outbreak dynamics remains poorly understood. Motivated by recent defoliation events in northern European oak forests, we investigated an emerging leaf-miner outbreak in Sweden. Through targeted surveys, rearing from 22 sites and Malaise trapping at 34 sites (56 sites total), we identified a pronounced spatial clustering of outbreaks at higher latitudes. The newly recognised cryptic species Acrocercops andreneli was strongly associated with these outbreaks, whereas sites with only Acrocercops brongniardella never showed outbreaks. Host-parasitoid networks related to the two cryptic moth species were strikingly similar. Our findings demonstrate the importance of cryptic species for outbreak dynamics and their consequences for host plant health in ways that are easily overlooked by traditional taxonomy. Moreover, such outbreak dynamics cannot always be linked to a lack of top-down control by natural enemies.

Theory, manipulation experiments and observational studies on biodiversity and ecosystem functioning largely concur that higher intraspecific diversity may increase the overall productivity of populations, buffer against environmental change and stabilize long-term productivity. However, evidence comes primarily from small and short-lived organisms. We tested for effects of genetic diversity on variation in forest growth by combining long-term data on annual individual growth rate (basal area increment (BAI)) with estimates of intrapopulation genetic variation (based on RAD-seq SNPs) for 18 natural Quercus robur pedunculate oak populations. Higher total or adaptive genetic variability of populations was neither associated with faster average growth nor with increased temporal or spatial stability of growth nor with among-individual asynchrony in growth. However, as expected, we found that greater asynchrony of growth responses within the populations increased their temporal stability. Together, these findings point towards a negligible role of genetic variation in structuring growth patterns in natural populations of tree species. Identifying which environmental factors and phenotypic traits (and its genetic basis) contribute to asynchronous growth responses is an important next step towards a better mechanistic understanding of the causes of temporal stability in tree growth and forest productivity.

Introducing non-native tree species into forest ecosystems is a growing trend, in part as climate change may cause a decline of native species and shifts in species distributions. In European forestry, Quercus rubra (northern red oak) has increasingly been considered a candidate substitute species for native oaks. However, it remains largely unknown how this substitution affects associated biodiversity. This study compares the biodiversity supported by the native oak species Q. petraea (sessile oak) and Q. robur (pedunculate oak) and the invasive Q. rubra in southern Sweden, focusing on both oak-associated organisms and general forest biodiversity. Arthropods were sampled using Malaise traps at the site level. At the same time, vascular plants, leaf herbivory and endophytic insects (leaf miners and gallers) were recorded at the tree level in three sites per oak species. Our results reveal guild-specific effects of oak species on biodiversity. The introduced Q. rubra supported significantly fewer endophytic insects than native oak species. Vascular plant species richness was marginally lower in Q. petraea and Q. rubra sites compared to Q. robur. In contrast, the species richness, abundance, biomass and community composition of arthropods and leaf herbivory did not differ significantly between the three oak species. These findings indicate that the ecological consequences for biodiversity of introduced tree species, such as Q. rubra, are most pronounced for specialised herbivores, including leaf miners and gallers, and suggest that broader forest biodiversity measures may be less responsive to changes in tree species than to local environmental conditions.

1. Movement is fundamental to population persistence and metapopulation dynamics, but robust comparative estimates of dispersal remain scarce for threatened butterflies. Limited quantitative data on movement constrain effective conservation network design.

2. We focused on three threatened butterfly species-Euphydryas aurinia, Parnassius apollo and Phengaris arion-co-occurring on Gotland, Sweden, which together provide an ideal model for comparative dispersal analysis.

3. To quantify and compare movement patterns among species using extended capture-mark-recapture (CMR) data, test for random-walk behaviour, and identify the best-fitting dispersal kernels.

4. CMR datasets for E. aurinia and P. apollo were extended to 2024 and combined with comprehensive data for P. arion, yielding 9670 net-displacement observations (the distance between the first and last captures) collected from 2017 to 2024. One movement value per individual was used to avoid pseudoreplication, detection probabilities were estimated with Cormack-Jolly-Seber models, and four dispersal kernels were evaluated.

5. Median net displacement differed significantly among species (chi(2) = 450.14, p < 0.001): E. aurinia showed the lowest value (0.135 km), while P. apollo (0.253 km) and P. arion (0.252 km) were similar. Movements deviated from random-walk expectations (log-log slope = 0.431 versus 0.5 expected, p < 0.001), indicating area-restricted movement. Lognormal kernels best described E. aurinia and P. apollo, whereas an exponential distribution fitted P. arion best, with maximum displacements of 8.19, 10.69 and 4.31 km, respectively.

6. Even butterflies traditionally regarded as sedentary exhibit substantial dispersal capacity. Species-specific movement strategies influence metapopulation connectivity, and the derived parameters provide essential inputs for designing habitat networks within each species's dispersal range.

Fermentative yeasts play important roles in both ecological and industrial processes, but their distribution and abundance in natural environments are not well understood. We investigated the diversity of yeasts at the northern range limit of their oak tree hosts (Quercus spp.) in Sweden, and identified climatic and ecological conditions governing their distribution. Yeasts were isolated from bark samples from 28 forests and identified to the species level using DNA metabarcoding. Most communities were dominated by species in the Saccharomycetaceae family, especially by species of Saccharomyces, Kluyveromyces and Pichia. Each genus showed a distinct latitudinal and longitudinal distribution, and both temperature and precipitation metrics predicted significant variation in their abundance. Consistent with this, laboratory assays revealed significant effects of temperature on the growth of strains collected from different longitudes and latitudes. We found that older trees harbour more diverse and more balanced fermentative yeast communities with more evenly distributed species abundances. Communities across trees were more similar when sharing a common dominant species. This work provides a baseline for future studies on the impact of climate change on the fermentative yeast biodiversity of temperate forests in northern latitudes and contributes to a growing collection of wild isolates for potential biotechnological applications.

Leaf phenology of trees responds to temperature and photoperiod cues, mediated by underlying genes and plasticity. However, uncertainties remain regarding how smaller-scale phenological variation in cold-limited regions has been affected by modified selection pressures from herbivores, pathogens, and climate conditions, and whether this leaves genetic signatures allowing for projections of future responses. We investigated environmental correlates and genetic variation putatively associated with spring and autumn leaf phenology in northern range margin oak (Quercus robur L.) populations in Sweden (55.6 degrees N-60.8 degrees N). Results suggested that budburst occurred later at higher latitudes and in locations with colder spring (April) temperatures, whereas leaf senescence occurred earlier at higher latitudes. Several candidate loci associated with phenology were identified (n = 40 for budburst and 47 for leaf senescence), and significant associations between these loci and latitude were detected. Functions associated with some of the candidate loci, as identified in previous studies, included host defence and heat stress tolerance. The proportion of polymorphic candidate loci associated with budburst decreased with increasing latitude, towards the range margin. Overall, the Swedish oak population seems to comprise genetic diversity in phenology-related traits that may provide resilience to a rapidly changing climate.

Assessing cross-taxon congruence is vital for effective forest conservation, because different taxonomic groups may respond inconsistently to key habitat variables such as stand age. We examined six taxonomic groups-insects, arachnids, springtails, epiphytic lichens, bryophytes, and vascular plants-across 25 Swedish oak stands ranging from 19 to 165 years old to determine whether species richness correlated among groups (cross-taxon congruence) and how it related to stand age. In total, we identified 22,276 unique taxa (with on average 4,128 per stand) using COI metabarcoding for arthropods and field surveys for lichens, bryophytes, vascular plants. Associations of species richness in each taxonomic group with richness in the others were weak, indicating low cross-taxon congruence. Only lichens showed a significant, positive relationship of species richness with stand age, while springtails exhibited a unimodal pattern, and the other four groups were unaffected by stand age. Although species composition in four groups changed with stand age, the explanatory power was generally low. Overall, the heterogeneous responses of different groups indicated by our findings caution against the use of single taxonomic groups or environmental variables as indicators and keys to successful protection of biodiversity. Instead, forest management strategies should adopt multi-taxon assessments and recognize the value of both younger and older stands to safeguard biodiversity in oak-dominated landscapes.

Amid accelerating global biodiversity loss, reintroducing and translocating endangered species have become pivotal conservation strategies. This study used radio telemetry to investigate the dispersal and movement patterns of the reintroduced Great Capricorn Beetle Cerambyx cerdo (Coleoptera: Cerambycidae) L. within Tromt & ouml; Nature Reserve, Sweden. We tracked 50 beetles (29 females, 21 males) from 17 June 2022 to 17 July 2022, quantified their movements within the landscape and recorded the distances moved across oak-rich areas at the northern edge of the species' range. Female beetles were significantly larger and heavier than males, with longer antennae. The beetles were relocated an average of 9.5 times. Total movement distances ranged from 2.8 to 822.2 m, with no significant association of movement distance or movement speed with sex, body size or mating status. We conclude that C. cerdo can reach oaks several hundred meters away and that dispersal distances were not associated with sex, body size or mating status. We recommend future studies employing more advanced telemetry techniques to refine estimates of long-distance dispersal and habitat use.

We examined latitudinal gradients in species richness and body size of saproxylic beetles across 28 veteran oak forest sites spanning from Israel to Norway. Focusing on 425 species from 11 taxonomic families and five trophic groups, we tested three hypotheses to elucidate: (i) family-specific richness responses to latitude, (ii) trophic mediation of richness patterns, (iii) whether body size follows Bergmann-like clines. We found significant family-level variations in richness-latitude relationships. These non-uniform patterns highlight the importance of taxonomic resolution in capturing macroecological diversity gradients. Body size analyses revealed significant latitude associations, indicating that both phylogenetic constraints and trophic group modulate latitudinal size patterns among saproxylic beetles. Taken together, our findings emphasize that macroecological patterns in saproxylic beetles are shaped by a synthesis of phylogenetic history and functional traits. Conservation strategies should, therefore, account for family-level and trophic-group heterogeneity, particularly as climate warming and shifting resource distributions may differentially affect lineages with distinct thermoregulatory and life-history constraints. These results underscore the need for taxon-specific approaches when predicting and managing biodiversity in changing oak forest ecosystems.

Insects are pivotal to ecosystem diversity and functionality, yet they face increasing threats from anthropogenic climate change impacts. A growing body of studies reports the effects of changing temperature and precipitation patterns, but relatively few studies focus on the consequences for insect populations because of extreme weather events. Here, we examine population growth responses to temperature, precipitation and the extreme summer drought in Sweden in 2018. For this purpose, we used longitudinal data for 54 moth species collected between 2005 and 2023 using light traps at three sites in southeastern Sweden. We found a positive relationship between temperature and population growth rates across all study sites, while precipitation showed a positive relationship at two sites and no effect at the third. The results indicated a negative time-lag effect on population growth, at two of the sites, of precipitation the previous year, while there were no significant effects of temperature the previous year. Despite the extreme drought in 2018, moth populations remained resilient, with no dramatic decline in population growth between 2018 and 2019. Our results contrast earlier studies reporting severe declines in population growth in response to extreme drought events. The discrepancy may reflect a combination of region-specific effects of extreme weather events and that selected species in this study predominantly consist of range-expanding and migratory species, better able to withstand adverse conditions due to a higher climatic tolerance and being habitat and food plant generalists. Our findings underscore the importance of an increased knowledge of site-specific responses and effects of extreme weather events such as droughts when outlining conservation efforts.