Retention of stumps on wet ground at stump-harvest and its effects on saproxylic insects

Low stumps represent on their own up to 80% of the dead wood remaining on clear cuts and therefore supply suitable habitat for saproxylic insects i.e. insects depending on dead wood for their survival. Recent stump harvesting activities threaten this substrate of ecological importance and increase the anthropogenic negative impacts on these species. Because of technical and environmental reasons (nutrient leakage, erosion) guidelines for stump harvesting recommend to retain stumps standing in wet parts of clear cuts. However, stumps in wet positions might not be a satisfactory substrate for saproxylic insects and therefore might not be as much used as stumps in dry positions. To test this hypothesis, a total of 100 stumps (50 spruces and 50 birches) were collected on four clear cuts between 5 and 7 years old near Uppsala, Sweden. Stump samples were paired to get a balanced dataset, each pair containing two stumps of the same tree species, diameter and sun-exposure, one dry and one wet. Each sample was placed in a rearing box for 2 months. All emerging insects were sorted out down to order, family, genus or species level according to their importance in the project. Species richness, abundance and composition in each type of stumps were analyzed. Proportion of stumps inhabited, density and Shannon’s diversity and evenness were calculated for each insect order. A canonical correspondence analysis was performed to investigate the possible connections existing between insect species and the tree species and dryness of the stumps. In total, 17065 insects were collected representing 114 species out of which 96 were considered as saproxylic. An overall of 76 and 55 saproxylic species were collected on birch and spruce stumps respectively. Species richness was higher in birch stumps with 11 species in average per sample and only 4 in spruce ones .The results show that the tree species was the only factor significantly affecting both the species richness and the species abundance of a stump. The variable “Dryness” had significant effect on the samples’ species diversity with dry samples harboring a higher number of species. The proportion of stump types used by different orders clearly shows that the tree species was again the main factor influencing the species abundance of a stump. More insects were found in birch stumps, regardless of their dryness, than spruce. Wet spruce and dry birch stumps were respectively the least (7.9%) and the most used (34.1%) substrates. Coleoptera beetles were more numerous in birch stumps but did not show any preference concerning the moisture level of the stumps. The other orders showed a similar pattern with Hymenoptera, Lepidoptera insects however favoring dry birch stumps and Dipterans wet ones. Arhopalus rusticus and Curtimorda maculosa were the only species to show a significant correspondence to a substrate and were associated to spruce stumps. The results show that both birch and spruce support the life cycle of many different insect species and not only saproxylic. As wet spruce stumps presented the lowest biodiversity, it could be thus advised, in a context of biodiversity conservation, to set aside in priority birch and more generally broadleaves stumps.Avverkningsstubbar svarar för upp till 80 % av den döda ved som lämnas kvar vid slutavverkning. De förser många vedlevande insekter (insekter som är beroende av död ved för att överleva) med lämpliga habitat att försöka sig i. Stubbrytning, som i ökande takt görs efter slutavverkningar för att utvinna bioenergi, minskar därför deras livsutrymme. De vedlevande insekterna har sedan tidigare identifierats som en grupp som hotas av intensivt skogsbruk och stubbrytning riskerar att ökar denna negativa påverkan. På grund av tekniska svårigheter och miljöhänsyn, till näringsurlakning och erosion, är skogsstyrelsens riktlinjer för stubbrytning att man bör lämna stubbar i blötare partier på hyggena. Det finns dock anledning att tro att stubbar i dessa partier inte är lämpliga substrat för vedlevande insekter. Målet med detta projekt var därför att testa om de blött stående stubbarna är fattigare på vedlevande skalbaggsarter än stubbar på torrare mark. Jag testade även om mönstret var detsamma för gran- och björkstubbar. Skalbaggsfaunan provtogs genom att kläcka fram dem ur vedprover. Prover från 100 stubbar sågades ut och samlades in (50 gran och 50 björk) från fyra slutavverkningsbestånd, 5 till 7 år gamla, utanför Uppsala i Mellansverige. Stubbarna provtogs i par, med en blött och en torrt placerad stubbe. Inom paren var trädslag, diameter och solexponering desamma. Varje prov placerades i två månader i en låda för att skalbaggarna skulle kläcka fram. Vedlevande skalbaggar och fjärilar artbestämdes, andra grupper bestämdes till ordning eller familj. Totalt hittades 17065 insekter från 114 arter, varav 97 ansågs vara vedlevande. Totalt samlades 76 respektive 55 vedlevande arter in från björk- respektive granstubbar. Artrikedomen per stubbe var också högre i björk (genomsnitt 11 arter) än i gran (4 arter). För granstubbarna var det signifikant fler arter i de torra stubbarna än i de blöta, medan det för björk inte fanns någon skillnad. En analys av artsammansättningen (ordination) kunde dock inte detektera någon skillnad mellan torra och blöta stubbar. Slutsatsen av studien är att de granstubbar som lämnas i blöta lägen på hyggena är ett sämre habitat för vedlevande skalbaggar än stubbar i torra lägen. Att enbart lämna stubbar i blöta lägen är sålunda ingen fullvärdig miljöhänsyn för den vedlevande mångfalden

Strong Gradients in Forest Sensitivity to Climate Change Revealed by Dynamics of Forest Fire Cycles in the Post Little Ice Age Era

The length of the fire cycle is a critical factor affecting the vegetation cover in boreal and temperate regions. However, its responses to climate change remain poorly understood. We reanalyzed data from earlier studies of forest age structures at the landscape level, in order to map the evolution of regional fire cycles across Eastern North American boreal and temperate forests, following the termination of the Little Ice Age (LIA). We demonstrated a well-defined spatial pattern of post-LIA changes in the length of fire cycles toward lower fire activity during the 1800s and 1900s. The western section of Eastern North America (west of 77°W) experienced a decline in fire activity as early as the first half of the 1800s. By contrast, the eastern section showed these declines as late as the early 1900s. During a regionally fire-prone period of the 1910s–1920s, forests in the western section of Eastern boreal North America burned more than forests in the eastern section. The climate appeared to dominate over vegetation composition and human impacts in shaping the geographical pattern of the post-LIA change in fire activity. Changes in the atmospheric circulation patterns following the termination of the LIA, specifically changes in Arctic Oscillation and the strengthening of the Continental Polar Trough, were likely drivers of the regional fire dynamics

Post-1980 shifts in the sensitivity of boreal tree growth to North Atlantic Ocean dynamics and seasonal climate

The mid-20th century changes in North Atlantic Ocean dynamics, e.g. slow-down of the Atlantic meridional overturning thermohaline circulation (AMOC), have been considered as early signs of tipping points in the Earth climate system. We hypothesized that these changes have significantly altered boreal forest growth dynamics in northeastern North America (NA) and northern Europe (NE), two areas geographically adjacent to the North Atlantic Ocean. To test our hypothesis, we investigated tree growth responses to seasonal large-scale oceanic and atmospheric indices (the AMOC, North Atlantic Oscillation (NAO), and Arctic Oscillation (AO)) and climate (temperature and precipitation) from 1950 onwards, both at the regional and local levels. We developed a network of 6876 black spruce (NA) and 14437 Norway spruce (NE) tree-ring width series, extracted from forest inventory databases. Analyses revealed post-1980 shifts from insignificant to significant tree growth responses to summer oceanic and atmospheric dynamics both in NA (negative responses to NAO and AO indices) and NE (positive response to NAO and AMOC indices). The strength and sign of these responses varied, however, through space with stronger responses in western and central boreal Quebec and in central and northern boreal Sweden, and across scales with stronger responses at the regional level than at the local level. Emerging post-1980 associations with North Atlantic Ocean dynamics synchronized with stronger tree growth responses to local seasonal climate, particularly to winter temperatures. Our results suggest that ongoing and future anomalies in oceanic and atmospheric dynamics may impact forest growth and carbon sequestration to a greater extent than previously thought. Cross-scale differences in responses to North Atlantic Ocean dynamics highlight complex interplays in the effects of local climate and ocean-atmosphere dynamics on tree growth processes and advocate for the use of different spatial scales in climate-growth research to better understand factors controlling tree growth

Saproxylic insect fauna in stumps on wet and dry soil: Implications for stump harvest

International audienc

Spatiotemporal variation in the relationship between boreal forest productivity proxies and climate data

The impacts of climate change on high-latitude forest ecosystems are still uncertain. Divergent forest productivity trends have recently been reported both at the local and regional level challenging the projections of boreal tree growth dynamics. The present study investigated (i) the responses of different forest productivity proxies to monthly climate (temperature and precipitation) through space and time; and (ii) the local coherency between these proxies through time at four high-latitude boreal Scots pine sites (coastal and inland) in Norway. Forest productivity proxies consisted of two proxies representing stem growth dynamics (radial and height growth) and one proxy representing canopy dynamics (cumulative May-to-September Normalized Difference Vegetation Index (NDVI)). Between-proxy and climate-proxy correlations were computed over the 1982-2011 period and over two 15-yr sub-periods. Over the entire period, radial growth significantly correlated with current year July temperature, and height growth and cumulative NDVI significantly correlated with previous and current growing season temperatures. Significant climate responses were quite similar across sites, despite some higher sensitivity to non-growing season climate at inland sites. Significant climate-proxy correlations identified over the entire period were temporarily unstable. Local coherency between proxies was generally insignificant. The spatiotemporal instability in climate-proxy correlations observed for all proxies underlines evolving responses to climate and challenges the modelling of forest productivity. The general lack of local coherency between proxies at our four study sites suggests that forest productivity estimations based on a single proxy should be considered with great caution. The combined use of different forest growth metrics may help circumvent uncertainties in capturing responses of forest productivity to climate variability and improve estimations of carbon sequestration by forest ecosystems.acceptedVersio

Previous season climate controls the occurrence of black spruce growth anomalies in boreal forests of Eastern Canada

To better understand climatic origins of annual tree-growth anomalies in boreal forests, we analysed 895 black spruce (Picea mariana [Mill.] B.S.P.) tree-growth series from 46 xeric sites situated along three latitudinal transects in Eastern Canada. We identified inter-annual (based on comparison to previous year growth) and multi-decadal (based on the entire tree-ring width distribution) growth anomalies between 1901 and 2001 at site and transect levels. Growth anomalies occurred mainly at site level and seldom at larger spatial scales. Both positive inter-annual and multi-decadal growth anomalies were strongly associated with below-average temperatures and above-average precipitation during the previous growing season (Junet-1-Augustt-1). The climatic signature of negative inter-annual and multi-decadal growth anomalies was more complex and mainly associated with current year climatic anomalies. Between the early and late 20The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

Spatiotemporal variation in the relationship between boreal forest productivity proxies and climate data

The impacts of climate change on high-latitude forest ecosystems are still uncertain. Divergent forest productivity trends have recently been reported both at the local and regional level challenging the projections of boreal tree growth dynamics. The present study investigated (i) the responses of different forest productivity proxies to monthly climate (temperature and precipitation) through space and time; and (ii) the local coherency between these proxies through time at four high-latitude boreal Scots pine sites (coastal and inland) in Norway. Forest productivity proxies consisted of two proxies representing stem growth dynamics (radial and height growth) and one proxy representing canopy dynamics (cumulative May-to-September Normalized Difference Vegetation Index (NDVI)). Between-proxy and climate-proxy correlations were computed over the 1982-2011 period and over two 15-yr sub-periods. Over the entire period, radial growth significantly correlated with current year July temperature, and height growth and cumulative NDVI significantly correlated with previous and current growing season temperatures. Significant climate responses were quite similar across sites, despite some higher sensitivity to non-growing season climate at inland sites. Significant climate-proxy correlations identified over the entire period were temporarily unstable. Local coherency between proxies was generally insignificant. The spatiotemporal instability in climate-proxy correlations observed for all proxies underlines evolving responses to climate and challenges the modelling of forest productivity. The general lack of local coherency between proxies at our four study sites suggests that forest productivity estimations based on a single proxy should be considered with great caution. The combined use of different forest growth metrics may help circumvent uncertainties in capturing responses of forest productivity to climate variability and improve estimations of carbon sequestration by forest ecosystems

Monitoring climate sensitivity shifts in tree-rings of eastern boreal North America using model-data comparison: Shifts in tree growth sensitivity to climate

International audienceThe growth of high-latitude temperature-limited boreal forest ecosystems is projected to become more constrained by soil water availability with continued warming. The purpose of this study was to document ongoing shifts in tree growth sensitivity to the evolving local climate in unmanaged black spruce (Picea mariana (Miller) B.S.P.) forests of eastern boreal North America (49°N-52°N, 58°W-82°W) using a comparative study of field and modeled data. We investigated growth relationships to climate (gridded monthly data) from observed (50 site tree-ring width chronologies) and simulated growth data (stand-level forest growth model) over 1908-2013. No clear strengthening of moisture control over tree growth in recent decades was detected. Despite climate warming, photosynthesis (main driver of the forest growth model) and xylem production (main driver of radial growth) have remained temperature-limited. Analyses revealed, however, a weakening of the influence of growing season temperature on growth during the mid- to late-20th century in the observed data, particularly in high-latitude (> 51.5 ºN) mountainous sites. This shift was absent from simulated data, which resulted in clear model-data desynchronization. Thorough investigations revealed that desynchronization was mostly linked to the quality of climate data, with precipitation data being of particular concern. The scarce network of weather stations over eastern boreal North America (> 51.5 °N) affects the accuracy of estimated local climate variability and critically limits our ability to detect climate change effects on high-latitude ecosystems, especially when drought severity is projected to rise. Climate estimates from remote sensing could help address some of these issues in the future

Multi-century reconstruction suggests complex interactions of climate and human controls of forest fire activity in a Karelian boreal landscape, North-West Russia

International audienc

Effects of Thinning Practice, High Pruning and Slash Management on Crop Tree and Stand Growth in Young Even-Aged Stands of Planted Silver Birch (Betula pendula Roth)

The objective was to quantify the influence of thinning, high pruning and slash management on crop tree and stand growth in young even-aged stands of planted silver birch (Betula pendula Roth). This study was based on two field experiments, aged six and eleven years at initiation and re-measured after six and eight years, respectively. Treatments included the unthinned control, moderate thinning mainly from below (removing 28–33% of standing volume), point thinning to favor 300 trees per ha and with no thinning elsewhere in the plot (removing 16–25%), and heavy thinning leaving 600 evenly distributed potential future crop trees per ha (removing 64–75%). Slash management (extraction or retention) was applied to heavily thinned plots. High pruning removing 30–70% of the green crown was carried out in some plots with point or heavy thinning on 300 or 600 trees per ha, respectively. Stand volume growth increased with increasing pre-treatment mean annual volume increment and decreased with increasing thinning intensity as compared to the unthinned control. LS-means estimates indicated a reduction for moderate thinning by 14%, for point thinning by 12% and for heavy thinning (combined with pruning) by 62%. However, in the youngest experiment, heavy thinning (without pruning) reduced growth by 54%. Combining these results with results from a similar experiment in Canada, the reduction in stand volume growth (RedIv%) depending on thinning removal (RemV%), both expressed as a percentage of the unthinned control, was quantified as RedIv% = −23.67 + 1.16·RemV% (calibration range: 30–83%). For heavy thinning (large quantities of slash), slash extraction resulted in no reduction in stand volume growth as compared to slash retention. The instantaneous numeric reduction in the average stem diameter of the 300 thickest trees per ha (D300) due to thinning was 3.5, 15–21% and 955–11% with moderate, point and heavy thinning, respectively. The subsequent average annual increase in D300 during the observation period was 8.5%, 25 and 18%, respectively. In the youngest experiment, pruning in unthinned plots led to a reduction in the annual increase of D300 by 14%, and heavy thinning in unpruned plots led to an increase by 30%. The growth of pre-selected potential future crop trees increased with increasing thinning intensity. In heavily thinned plots, pruning reduced growth increasingly with increasing pruning severity; LS-means estimates indicated 21% larger growth on stem diameter for unpruned trees and 3% for pruned trees. As an adverse side effect, heavily thinned plots with only 600 trees per ha were at increased risk of windthrow for some years after the thinning intervention. In the oldest experiment, 95–21% of the trees in these plots were damaged by wind