Parasitism rate differs between herbivore generations in the univoltine, but not bivoltine, range

With climate change, plant-feeding insects increase their number of annual generations (voltinism). However, to what degree the emergence of a new herbivore generation affects the parasitism rate has not been explored. We performed a field experiment to test whether the parasitism rate differs between the first and the second generations of a specialist leaf miner (Tischeria ekebladella), both in the naturally univoltine and bivoltine parts of the leaf miner's distribution. We found an interactive effect between herbivore generation and geographical range on the parasitism rate. The parasitism rate was higher in the first compared to the second host generation in the part of the range that is naturally univoltine, whereas it did not differ between generations in the bivoltine range. Our experiment highlights that shifts in herbivore voltinism might release top-down control, with potential consequences for natural and applied systems

Mixed-severity natural disturbance regime dominates in an old-growth Norway spruce forest of northwest Russia

Questions: What were the long-term disturbance rates (including variability) and agents in pristine Norway spruce-dominated (Picea abies (L.) Karst.) forests? Have soil moisture conditions influenced disturbance rates across this boreal spruce-dominated forest? Were the temporal recruitment patterns of canopy dominants associated with past disturbance periods? Location: Interfluvial region of Northern Dvina and Pinega rivers, Arkhangelsk, northwest Russia. Methods: We linked dendrochronological data with tree spatial data (n trees=1659) to reconstruct the temporal and spatial patterns of canopy gaps in a 1.8-ha area from 1831-2008, and to develop a growth-release chronology from 1775-2008. Results: No evidence of stand-replacing disturbances was found within selected forest stands over the studied period. Forest dynamics were driven by small- to moderate-scale canopy disturbances, which maintained a multi-cohort age structure. Disturbance peaks were observed in the 1820s, 1920s, 1970s and 2000s, with decadal rates reaching 32% of the stand area disturbed. Conclusions: The overall mean decadal rate was 8.3% canopy area disturbed, which suggests a canopy turnover time of 122yr, with a 95% confidence envelop of 91-186yr. Bark beetle outbreaks (possibly exacerbated by droughts) and wind-storms emerged as the principal disturbance agents. Recruitment of both Norway spruce and downy birch was associated with periods of increased canopy disturbance. Moisture conditions (moist vs mesic stands) were not significantly related to long-term disturbance rates. The studied spruce-dominated boreal forests of this region apparently exhibited long-term forest continuity under this mixed-severity disturbance regime. These disturbances caused considerable structural alterations to forest canopies, but apparently did not result in a pronounced successional shifts in tree species composition, rather occasional minor enrichments of birch in these heavily spruce-dominated stands

Testing for anthropogenic influence on fire regime for a 600-year period in the Jaksha area, Komi Republic, East European Russia

In an attempt to quantitatively evaluate the natural versus anthropogenic signal in site fire histories, the statistical relationship between dendrochronologically dated fire events and tree-ring chronologies (deemed to be an independent proxy for climate variation) was analyzed for 14 sites in a 2600-km(2) area of pine-dominated forests in the Komi Republic (East European Russia) over the period from 1424 to 1954. We developed a cumulative measure of statistical fit between two types of fire events (early- and late-season fires) and ring-width chronologies of Scots pine (Pinus sylvestris L.) (total ring- and latewood-width chronologies). For a given site, the statistical fit between fires and tree-ring data tended to decrease with an increasing proportion of unique fire years. Distance from a site to the nearest village (deemed to be a proxy of human impact) explained 50% of the variation in statistical fit between fires and tree-ring data. The fit decreased in the majority of the sites from the earlier (1424-1700) to the later (1700-1960) periods. We interpret this to be a result of increased human impact on the fire regime since 1700 due to intensified colonization of the area

Species-specific growth responses of black spruce and trembling aspen may enhance resilience of boreal forest to climate change

To understand how the future climate will affect the boreal forest, we studied growth responses to climate variability in black spruce (Picea mariana [Mill.] B.S.P.) and trembling aspen (Populus tremuloides Michx.) two major co-occurring boreal tree species of the eastern Canadian boreal forest. We analysed climate growth interaction during (i) periods of non-anomalous growth and (ii) in years with strong growth anomalies. We utilized paired tree level data for both growth and soil variables, which helped ensure that the studied growth variability was a function of species specific biology, and not of within stand variation in soil conditions. Redundancy analysis conducted on spruce and aspen tree ring chronologies showed that their growth was affected differently by climate. During non-anomalous years, growth of spruce was favoured by cooler temperatures and wetter conditions, while aspen growth was favoured by higher temperatures and drier conditions. Black spruce and trembling aspen also showed an inverse pattern in respect to expression of growth anomalies (pointer years). A negative growth anomaly in spruce tended to be associated with positive ones in aspen and vice versa. This suggested that spruce and aspen had largely contrasting species specific responses to both "average" weather conditions and extreme weather events. Synthesis. Species specific responses to environmental variability imply that tree responses to future climate will likely be not synchronized among species, which may translate into changes in structure and composition of future forest communities. In particular, we speculate that outcome of climate change in respect to relative abundance of black spruce and trembling aspen at the regional levels will be highly dependent on the balance between increasing temperatures and precipitation. Further, species specific responses of trees to annual climate variability may enhance the resilience of mixed forests by constraining variability in their annual biomass accumulation, as compared to pure stands, under periods with high frequency of climatically extreme conditions

Reconstruction of a 253-year long mast record of European beech reveals its association with large scale temperature variability and no long-term trend in mast frequencies

Synchronous production of large seed crops, or mast years (MYs), is a common feature of many Fagus species, which is closely linked to the dynamics of forest ecosystems, including regeneration of canopy trees and changes in animal population densities. To better understand its climatic controls and check for the presence of long-term temporal trends in MY frequencies, we reconstructed MY record of the European beech (Fagus sylvatica L.) for the southern Swedish province of Halland over 1753-2006. We used superimposed epoch analysis (SEA) to relate MY (a) to summer temperature fields over the European subcontinent and (b) to the patterns of 500 mb geopotential heights over the 35-75 degrees N. For the MY reconstruction, we used newly developed regional beech ring-width chronology (1753-2006), an available summer temperature reconstruction, and a discontinuous historical MY record. A Monte Carlo experiment allowed identification of the thresholds in both growth and summer temperature anomalies, indicative of historical MYs, which were verified by dividing data into temporally independent calibration and verification sub-periods.MYs were strongly associated with both the 500 mb height anomalies and average summer temperatures during two years preceding a MY: a mast year (t) followed a cold summer two years (t-2) prior to the mast year and a warm summer one year prior (t-1) to the mast year. During t-2 years, the geographical pattern of 500 mb height anomalies exhibited a strong height depression in the region centered in the Northern Sea and extending toward eastern North America and statistically significant (p<0.05) temperature anomalies covering predominantly southern Scandinavia (area below 60 N) and British Isles. A year immediately preceding a mast year (t-1) was characterized by a strong regional high pressure anomaly centered in southern Scandinavia with significant temperature anomalies extended mostly over southern Scandinavia and Germany.The long-term mean MY return interval was 6.3 years, with 50 and 90% probabilities of MY occurrence corresponding to 6 and 15 years, respectively. Periods with intervals significantly shorter than the long-term mean were observed around 1820-1860 and 1990-2006 (means 3.9 and 3.2 years, respectively). However, the difference in return intervals between two sub-periods themselves was not significant.Geographically large and temporally rapid changes in atmospheric circulation among years, responsible for summer temperature conditions in the Northern Europe, are likely primary environmental drivers of masting phenomenon. However, decadal and centurial variability in MY intervals is difficult to relate directly to temperature variability, suggesting the presence of conditions "canceling" would-be MYs. Long-term MY reconstruction demonstrates high variability of reproductive behavior in European beech and indicates that a period with shorter MY intervals at the end of 20th may be not unique in a multi-century perspective

First Spatial Reconstruction of Past Fires in Temperate Europe Suggests Large Variability of Fire Sizes and an Important Role of Human-Related Ignitions

The spatial component of past forest fires in temperate Europe has been little studied, despite the value of such data in quantifying human and natural factors driving fire activity and associated forest dynamics. Changes in fire regimes reported across a range of ecosystems call for a better understanding of variability in historic fires and may help define reference points that can be relied upon when discussing climate change effects. We provide the first dendrochronological reconstruction of historical fire sizes in Central Europe and analyze the minimum extent of fires during the last four centuries in a 9.2 km(2) (920 ha) conifer-dominated section of Bialowieza Forest, one of the largest continuous lowland forests of the subcontinent. We recorded 82 fires between 1666 and 1946, using 275 sample trees, while 92% of fires (76 out of 82) spread beyond the studied area. Fires varied considerably in size, from events recorded at only one site (1-200 ha) to fires recorded in more than half of the studied area, thus exceeding 500 ha in size. The fire cycle was 11 years over the whole study period, with three distinct periods revealed by the regime shift analysis. In the years 1670-1750, the fire cycle averaged 12 years. It shortened to 7 years between 1755-1840 and increased to 22 years over the 1845-1955 period. In comparison with present day data, the reconstructed fire density of 3.2 fires per 100 km(2) (10 000 ha) and year exceeded lightning ignition density by one to two orders of magnitude, suggesting a significant contribution of human-related ignitions. Our results highlight the important role of fire disturbance in Bialowieza Forest and provide critical baseline information to design biological conservation strategies for European forests

Soil characteristics mediate the distribution and response of boreal trees to climatic variability

We studied the effects of the soil organic layer (SOL) accumulation on growth and distribution of black spruce (Picea mariana (Mill.) BSP) and trembling aspen (Populus tremuloides Michx.) within the Quebec Clay Belt. At the landscape scale, spruce was present over a much larger gradient in SOL thickness (similar to 1 to 100 cm) than aspen (similar to 1 to 30 cm). For trees between 60 and 100 years old, SOL thickness had no effect on the basal area increment (BAI) of spruce but showed a strong and negative correlation with BAI in aspen. Radial growth of black spruce was favored by higher precipitation in June of the previous growing season, higher temperatures in early winter and in spring, and by low temperatures in summer. SOL thickness had statistically significant but moderate effects on the climate-growth relationships in spruce, apparently affecting root insulation during the dormant period and water availability during the growing period. In aspen, current-year June temperature was the most important factor positively correlated with growth. The SOL thickness affected the relationship between the aspen growth and (i) January temperature and (ii) June-August monthly drought code. We predict that the response of black spruce to climate change should be rather uniform across the study region, while the response of aspen is likely to be strongly mediated by SOL thickness

Scandinavian Forest Fire Activity Correlates with Proxies of the Baffin Bay Ice Cover

Understanding factors driving fire activity helps reveal the degree and geographical variability in the resilience of boreal vegetation to large scale climate forces. We studied the association between sea ice cover in the Baffin Bay and the Labrador Sea and observational records of forest fires in two Nordic countries (Norway and Sweden) over 1913-2017. We found a positive correlation between ice proxies and regional fire activity records suggesting that the Arctic climate and the associated changes in North Atlantic circulation exercise an important control on the levels of fire activity in Scandinavia. Changes in the sea cover are likely correlated with the dynamic of the North Atlantic Current. These dynamics may favor the development of the drought conditions in Scandinavia through promoting persistent high-pressure systems over the Scandinavian boreal zone during the spring and summer. These periods are, in turn, associated with an increased water deficit in forest fuels, leading to a regionally increased fire hazard. The Arctic climate will likely be an important future control of the boreal fire activity in the Nordic region

Contrasting Root System Structure and Belowground Interactions between Black Spruce (Picea mariana (Mill.) B.S.P) and Trembling Aspen (Populus tremuloides Michx) in Boreal Mixedwoods of Eastern Canada

This study explored the underground interactions between black spruce and trembling aspen in pure and mixed stands to understand how their soil resource use help these species coexist in the boreal mixedwoods of Western Quebec. We analyzed species-specific fine root foraging strategies (root biomass and root tissue density) along three soil layers (organic, top 0-15 cm, and bottom 15-30 cm mineral soil), using 180 soil cores. We collected cores in three sites, each containing three 20 × 50 m2 plots of pure spruce, pure aspen, and mixed spruce and aspen stands. Spruce had a shallow rooting, whereas aspen had a deep rooting in both types of stands. Compared to pure spruce stands, spruce had a lower fine root biomass (FRB) and a higher root tissue density (RTD) in the organic layer of mixed stands. Both patterns were indicative of spruce's more intensive resource use strategy and competitive advantage over aspen in that layer. Aspen FRB in the organic soil did not differ significantly between pure and mixed stands, but increased in the mineral soil of mixed stands. Since we did not observe a significant difference in the nutrient content of the mineral soil layer between pure aspen and mixed stands, we concluded that aspen may experience competitive exclusion in the organic layer by spruce. Aspen exhibited an extensive nutrient uptake strategy in the organic layer of mixed stands: higher FRB and lower RTD than spruce. In mixed stands, the differences in aspen rooting patterns between the organic and mineral layers suggested the use of contrasting nutrient uptake strategies along the soil profile. We speculate that the stronger spatial separation of the roots of spruce and aspen in mixed stands likely contribute to a higher partitioning of their nutrient uptake along the soil profile. These results indicate the competitive exclusion of aspen by spruce in boreal mixedwoods, which likely occurs in the soil organic layer