North-facing Sites Research Articles

Long-term measurements of seasonal snowpacks indicate increases in mid-winter snowmelt and earlier snowpack disappearance in the northeastern U.S.

Snowpacks are changing in northeastern North America as the regional climate warms, yet the relative influence of changes in precipitation compared to changes in ablation on snowpacks is poorly understood. We use 56 years of weekly snow water equivalent (SWE) measurements from three locations within a study site which vary in elevation and aspect, paired with adjacent daily climate measurements, to investigate relationships between climate and snowpack onset, maximum, and disappearance. Maximum snowpack size and snowpack duration are shrinking at all sites, at rates ranging from 4.3 days/decade at the coldest site to 9.6 days/decade at the warmest site. The shorter snowpack duration at all sites results from an earlier snowpack disappearance, stemming largely from reduced winter maximum snowpack sizes. Trends in snowpack establishment dates vary, with the south-facing site showing a trend toward later establishment but the two north-facing sites showing no change. The date of the maximum snowpack size varies by aspect and elevation but is not changing at any site. Using a 0° C threshold for frozen vs. liquid precipitation, we only observed a decrease in the proportion of precipitation falling in frozen form at the warmer, south-facing site in the winter period. In contrast, the total weekly snowpack ablation in the winter period has been increasing at least marginally at each site, even at sites which do not show increases in thawing conditions. Ablation increases range from 0.4 cm/decade at the warmest site, to 1.4 and 1.2 cm/decade at the north-facing sites. The south-facing site shows only marginally significant trends in total winter ablation, which we interpret as being limited by the smaller snowpack at this site. Overall, we conclude that rising air temperatures are leading to warmer, more sensitive snowpacks and this change becomes evident before those temperatures lead to changes in precipitation form.

Impact of livestock activity on near-surface ground temperatures in central Mongolian grasslands

Abstract. Grazing by livestock can alter the surface conditions at grassland sites, impacting the transfer of energy between the atmosphere and ground and consequentially ground temperatures. In this study, we investigate surface cover in summer and winter and measure ground surface temperatures over 14 months at sites in central Mongolia that feature different grazing intensities (intensely and ungrazed) and topographic aspects (north- and south-facing). Overall, intense grazing leads to a substantially reduced vegetation cover, altered snow conditions, and lack of surface litter accumulation. Comparing intensely grazed and ungrazed plots shows large seasonal differences in ground surface temperatures, with grazed plots being up to +5.1 °C warmer in summer and −5.4 °C colder in winter at a south-facing site. We also find that the effect of grazing intensity depends on topographic aspect, with smaller seasonal differences of +1.4 °C and −2.5 °C found between grazed and ungrazed plots at a north-facing site. This relates to the lower available solar radiation at north-facing sites, which reduces the differences in vegetation cover between open and fenced plots. For both aspects, the seasonal differences largely offset each other, with both a small net cooling and warming depending on effects in spring and autumn. Our study suggests that livestock management could be used to modify the annual ground temperature dynamics, possibly even influencing local permafrost dynamics.

Forest gap effects on snow storage in the transitional climate of the Eastern Cascade Range, Washington, United States

Forest thinning and gap creation are being implemented across the western United States of America (USA) to reduce wildfire and forest mortality risk as the climate warms. The Eastern Cascades in Washington, USA, is in a transitional zone between maritime and continental climate conditions and represents a data gap in observations describing the relationship between forest density and snowpack. We collected 3 years of snow observations across a range of forest densities to characterize how forest management efforts in this region may influence the magnitude and duration of snow storage. Observations indicate that peak snow storage magnitude in small gaps ranges from the same to over twice that observed in unburned forest plots in the Eastern Cascades. However, differences in snow duration are generally small. Across all Eastern Cascade sites and years, we observed a median difference of snow storage lasting 7 days longer in gaps as compared to nearby forest plots. A notable exception to this pattern occurred at one north-facing site, where snow lasted 30 days longer in the gap. These observations of similar snow storage duration in the Eastern Cascades are attributed to minimal differences in canopy snow interception processes between forests and gaps at some sites, and to higher ablation rates that counterbalance the higher snow accumulation in the gaps at other sites. At the north-facing site, more snow accumulated in the gap, and ablation rates in the open gap were similar to the shaded forest due to the aspect of the site. Thus, snow storage duration was much longer in the gap. Together, these data suggest that prescriptions to reduce forest density through thinning and creating gaps may increase the overall amount of snow storage by reducing loss due to sublimation and melting of canopy-intercepted snow. However, reducing forest density in the Eastern Cascades is unlikely to buffer climate-induced shortening of snow storage duration, with the possible exception of gap creation in north-facing forests. Lastly, these observations fill a spatial and climatic data gap and can be used to support hydrological modeling at spatial and temporal scales that are relevant to forest management decisions.

Understanding the spatiotemporal distribution of snow refugia in the rain-snow transition zone of north-central Idaho

Knowledge of snow cover distribution and disappearance dates over a wide range of scales is imperative for understanding hydrological dynamics and for habitat management of wildlife species that rely on snow cover. Identification of snow refugia, or places with relatively late snow disappearance dates (SDDs) compared to surrounding areas, is especially important as climate change alters snow cover timing and duration. The purpose of this study was to increase understanding of snow refugia in complex terrain spanning the rain-snow transition zone at fine spatial and temporal scales. To accomplish this objective, we used remote cameras to provide relatively high temporal and spatial resolution measurements on snowpack conditions. We built linear models to relate SDDs at the monitoring sites to topoclimatic and canopy cover metrics. One model to quantify SDDs included elevation, aspect, and an interaction between canopy cover and cold-air pooling potential. High-elevation, north-facing sites in cold-air pools (CAPs) had the latest SDDs, but isolated lower-elevation points also exhibited relatively late potential SDDs. Importantly, canopy cover had a much stronger effect on SDDs in CAPs than in non-CAPs, indicating that best practices in forest management for snow refugia could vary across microtopography. A second model that included in situ hydroclimate observations (December–February (DJF) temperature and March 1 snow depth) indicated that March 1 snow depth had little impact on SDD at the coldest winter temperatures, and that DJF temperatures had a stronger effect on SDD at lower snow depths, implying that the relative importance of snowfall and temperature could vary across hydroclimatic contexts in their impact on snow refugia. This new understanding of factors influencing snow refugia can guide forest management actions to increase snow retention and inform management of snow-dependent wildlife species in complex terrain.

Integration of landscape-level remote sensing and tree-level ecophysiology reveals drought refugia for a rare endemic, bigcone Douglas-fir

For forest species, areas buffered from the rapidly increasing climate stressors and patterns of disturbance — i.e., climate change refugia — are important targets for conservation and protection. Here, we present a novel field survey and remote sensing approach to identification of fine-scalefunctional drought refugiafor bigcone Douglas-fir (Pseudotsuga macrocarpa)-dominated forests. This rare species has been exposed to climate change-exacerbated drought conditions over the past two decades; yet, little is known about its responses to recent drought and how these drought responses vary across local environmental gradients and interact with recent record wildfire seasons. We combined a remote sensing analysis of vegetation condition with field surveys and physiological measurements to better understand bigcone Douglas-fir responses to recent climate trends. We also identified 444 stands exhibiting relatively low response and high resilience to drought — i.e., potential drought refugia. We found that low elevation stands and those in south-facing aspects generally experienced greater levels of seasonal and interannual drought stress. This trend was more pronounced for stands that experienced fire (2007 Zaca Fire) prior to the drought, suggesting that wildfire can increase the importance of topographic mediation of climate conditions in bigcone Douglas-fir forests. Elevation and aspect also interacted to affect physiological acclimation to seasonal drought conditions, with low elevation north-facing sites in particular experiencing a favorable combination of greater climate buffering and greater drought resilience, suggesting that these sites may be important refugia for bigcone Douglas-fir at low elevations. Furthermore, we found that the relationships between topography and drought response were weaker in more coastal sites, possibly due to maritime climate buffering in these sites. Altogether, these results illustrate how topographic mediation of regional drought conditions is critical for the persistence of this rare species in drought and fire-prone landscapes, and offer important insights for the conservation and restoration of this iconic species.

Climate Warming Decreases Plant Diversity but Increases Community Biomass in High-Altitude Grasslands

The rugged Himalayan landscape results in large variations in site conditions that regulate plant response to warming. There is lack of deeper understanding on plant response to warming under differing site characteristics in the mountainous terrain. A 2-yr experiment was conducted in the high mountains of Bhutan. The objective was to investigate the effects of short-term artificial warming on high-altitude grassland vegetation at north- and south-facing sites. An artificially warmed environment was simulated using open-top chambers (OTCs) that were compared with control chambers experiencing ambient conditions. Variables measured were species diversity, species richness, proportions of plant functional groups, forage dry matter, and forage quality. Generally at the north-facing site, OTC treatment showed a lower species diversity (OTC treatment H' ≈2.35; Control treatment H' ≈2.75), species richness (OTC treatment MI ≈1.38; Control treatment MI ≈1.45), sedge abundance (OTC treatment sedge cover ≈14.5%; Control treatment sedge cover ≈25%), and crude protein content (OTC treatment CP ≈6.60%; Control treatment CP ≈7.70%). On both sites, OTC treatment had a higher grass abundance (OTC treatment grass cover ≈24.0%; Control treatment grass cover ≈17.0%) and higher dry matter content (OTC treatment DM ≈1.70 t ha−1; Control treatment DM ≈1.50 t ha−1). The study suggests that climate warming triggers shifts in vegetation characteristics of high-altitude grasslands in the rugged mountainous terrain, but the magnitude of shift varies according to site characteristics. Under warming, the north-facing site could experience greater vegetation change, characterized by reduced species diversity, species richness, proportion of sedge, and crude protein content.

The decline of Algerian Cedrus atlantica forests is driven by a climate shift towards drier conditions

Some north-African Atlas cedar (Cedrus atlantica) forests are in decline, following decades of anthropogenic pressure and repeated drought events. We investigated if the recent decline episodes of these forests are linked to precipitation and temperature shifts, leading to a reduction in tree radial growth and climate-growth uncouplings. Tree-ring width chronologies of Atlas cedar in north-western Algeria allow the identification of climate and growth shifts in these vulnerable Mediterranean forests. Such chronologies, built for six sites, showed common patterns of year-to-year variability during the period 1910-2006. The growth at north-facing sites declined from the 1980s until 2006, whereas the growth at mid-elevation sites declined from the early 20th century until the 1940s, remained stable until the 1980s and then declined until 2006. Cool and wet spring conditions enhanced cedar growth. Sites with fast-growing trees, where growth was strongly reduced by dry summer conditions, showed the most-pronounced negative trends. However, a clear climate-growth uncoupling was observed after the 1970s, when the climate rapidly warmed. We also detected a negative growth shift in the 1980s, when mortality increased. This coincided with changes in early-warning signals of the growth series, such as an increase in the first-order autocorrelation of tree-ring width. All these lines of evidence indicate that the 1980s climate shift towards warmer and drier conditions triggered a shift in cedar growth. The use of radial-growth series as early-warning signals should be further investigated in this species and in other drought-sensitive conifers, given the aridification trends expected for the Mediterranean Basin.

Historical Climate Warming in the White Mountains of New Hampshire (USA): Implications for Snowmaking Water Needs at Ski Areas

The objectives of this study were to examine changing snowmaking conditions in the New Hampshire White Mountains and how changes in snowmaking operations have compared with winter warming. We analyzed three 50-year high-quality daily temperature records representing different elevations and aspects to assess changes in snowmaking conditions during important snowmaking periods. The analysis provides context for discussing the historical relationships between temperatures, water, and snowmaking infrastructure. There was significant warming of winter temperatures over the 50-year record, notably strongest at the early portion of the snowmaking season, especially in the weeks between 1 December and 25 December. While the rates of warming were comparable on both north- and south-facing aspects, the implied reduction in days suitable for snowmaking in each period was always lowest on the north-facing aspect as the mean temperatures on this aspect were farther below the snowmaking threshold. Daily average temperatures of –2°C and –5°C were both explored as thresholds for snowmaking. The implied reduction in snowmaking opportunity during the 1 November to 25 December period using a –2°C snowmaking threshold at the north-facing site was 20%, while the implied reduction for the entire season for that site was 8.5%.This decrease in opportunity for snowmaking, especially in the economically important early season, suggests an increasing need for large volumes of water to make snow in less time, given that holiday vacations are fixed in time. Analysis of snowmaking operations at Loon Mountain Resort suggest that modern snowmaking investments there have outpaced the pressure from climate warming to date, but this has concentrated demand for water into smaller time frames.

Chemical and microbiological changes in Norway spruce deadwood during the early stage of decomposition as a function of exposure in an alpine setting

ABSTRACT Alpine ecosystems are vulnerable to ever-changing environmental conditions, leading to shifts in vegetation distribution and composition with implications on soil functionality and carbon (C) turnover. Although deadwood represents an important global C stock, scarce information is available on how slope exposure influences the wood-inhabiting microbiota throughout the decomposition process in an Alpine setting. We therefore evaluated the impact of slope exposure (north- vs. south-facing sites) on physicochemical and microbiological properties (microbial abundance based on real-time PCR: fungal 18S rRNA, dinitrogen reductase [nifH]; microbial biomass: double strand DNA; and microbial activity: hydrolytic enzyme activities of the main nutrient cycles) of Picea abies wood blocks and the underlying soil in a field experiment in the Italian Alps during a three-year period. Overall, a higher abundance of fungi and nitrogen-fixing bacteria was recorded in the soil at the north-facing site where cooler and moister conditions were observed. In contrast, no exposure effects were found for these two microbial groups in the wood blocks, while their abundance increased over time, accompanied by more acidic conditions with progressing decay. The impact of exposure was also enzyme specific and time dependent for both the P. abies wood blocks and the underlying soil.

Divergent growth rates of alpine larch trees (Larix lyallii Parl.) in response to microenvironmental variability

ABSTRACT In this study we explore radial growth rates and climatic responses of alpine larch trees (Larix lyallii Parl.) growing in high elevations of the northern Rocky Mountains of Montana, USA. We examine responses between two stands of alpine larch that are separated by less than one kilometer and are growing at similar elevations, but with different aspects. Radial growth rates from trees sampled on the southern aspect of Trapper Peak (TPS) were largely controlled by January snow-water equivalent, while summer maximum temperature was the principal radial-growth driver for trees sampled on the northern aspect of Trapper Peak (TPN). Following the coldest summer (1993) in the century-long instrumental climate record, the radial growth at TPN became greater than at TPS and was the reverse of what occurred pre-1993. We posit that an upward trend in maximum summer temperature is preferentially benefitting the trees growing on the north-facing TPN site by extending the growing season and causing earlier snowmelt, and this has caused the growth rate divergence during the past two decades. As such, our study illustrates that the growth-divergence phenomenon noted in other high-elevation species, whereby macroenvironmental changes are eliciting responses at the microenvironmental level, occurs within stands of alpine larch growing in western Montana.

Changes in humus forms, soil invertebrate communities and soil functioning with forest dynamics

Three field studies have been performed, in order to assess changes in the composition of soil animal communities, soil physical and chemical features and mineralization processes with Norway spruce (Picea abies, L.) phases of the forest cycle. These field studies were conducted in three sites that differed in the acidity level of their bedrock (acidic, sub-acidic, and basic). The influence of exposure, through modifying microclimatic conditions, was also taken in consideration by comparing north and south exposures at each site. The data issued from each of these studies have already been published separately, and the aim of the present paper is to confront the three series of data in order to assess: 1) The general trend of changes in soil invertebrate communities, humus forms and mineralization processes with the age of spruce; 2) The impact of the type of bedrock and exposure on these changes.The results indicate that deep modifications occurred in animal communities, humus forms and soil functioning among clearing, regeneration and mature tree stands. The changes consist in an increase of Humus Index and the density of mites, especially oribatids, with a decrease in mineralization rate and animal diversity, from clearing to mature stands.Regeneration stands occupy an intermediate level as regards soil features but cumulates highest densities of most groups, highest levels of zoological diversity and mineralization activity. This higher level of richness and functional activity in regeneration stands could be explained by a more heterogeneous habitat in this phase of the forest cycle. Our results thus support the hypothesis that forest dynamics drives soil functioning and diversity, at least during the phase of intense growth of trees.The evolution of humus forms and animal communities along the developmental phases of forest stands were considerably more pronounced in south-facing sites than in north-facing sites. Unexpectedly, no increase in animal diversity was observed from the more to the less acidic bedrock. Suggestions for forest management are proposed.

Topoclimate effects on growing season length and montane conifer growth in complex terrain

Spatial variability in the topoclimate-driven linkage between forest phenology and tree growth in complex terrain is poorly understood, limiting our understanding of how ecosystems function as a whole. To characterize the influence of topoclimate on phenology and growth, we determined the start, end, and length of the growing season (GSstart, GSend, and GSL, respectively) using the correlation between transpiration and evaporative demand, measured with sapflow. We then compared these metrics with stem relative basal area increment (relative BAI) at seven sites among elevation and aspects in a Colorado montane forest. As elevation increased, we found shorter GSL (−50 d km−1) due to later GSstart (40 d km−1) and earlier GSend (−10 d km−1). North-facing sites had a 21 d shorter GSL than south-facing sites at similar elevations (i.e. equal to 200 m elevation difference on a given aspect). Growing season length was positively correlated with relative BAI, explaining 83% of the variance. This study shows that topography exerts strong environmental controls on GSL and thus forest growth. Given the climate-related dependencies of these controls, the results presented here have important implications for ecosystem responses to changes in climate and highlight the need for improved phenology representation in complex terrain.

Interacting effects of topography, vegetation, human activities and wildland-urban interfaces on wildfire ignition risk

Effective fire prevention requires a better understanding of the patterns and causes of fire ignition. In this study, we focus on the interacting factors known to influence fire ignition risk, such as the type of vegetation, topographical features and the wildland-urban interface (WUI; i.e. where urban development meet or intermingle with wildland). We also analyze the human activities and motivations related to fires and whether they differ depending on the type of vegetation and the location within/outside WUI. There were significant interactions between topography, type of vegetation and location within/outside WUI. The risk of ignition was in general higher at lower elevations, and this tendency was more marked in forested land covers (all plantations and open woodlands), with the noticeable exception of native forests. North-facing sites had lower fire ignition risk outside the WUI, especially in native forests, while southern aspects showed higher fire ignition risk, especially in open shrublands. However, this effect of the aspect was only significant outside WUI areas. In relation to causes, there were also interactions between human activities/motivations related to fires, the type of vegetation and the location within/outside WUI. All forestry plantations appeared clustered in relation to fire causes, especially in the WUI, with high incidence of deliberately caused fires related to violent or mentally ill people and rekindle fires. In contrast, native forests, despite structural similarities with forestry plantations, showed more similarity with agricultural areas and open woodlands in relation to fire causes. In shrublands, there was a relatively high incidence of fires related to ranching, especially outside the WUI. This pattern of interactions depicts a complex scenario in relation to fire ignition risk and prompts to the importance of taking this complexity into account in order to adjust fire management measures for improved effectiveness.

Physico-chemical and microbiological evidence of exposure effects on Picea abies – Coarse woody debris at different stages of decay

Although slope aspect determines the amount of solar irradiation, with implications on the functioning of forest ecosystems, little is known yet about how this affects the aboveground deadwood decomposition dynamics. Therefore, we set up a climosequence case study to evaluate the impact of slope exposure (north- vs. south-facing sites) on the physico-chemical and microbiological properties of Picea abies coarse woody debris (CWD) at different stages of natural decay (decay classes, DCls 1–5) in an Italian Alpine setting. Variations in bacterial, fungal and archaeal abundances were assessed by real-time PCR in the extra- and intracellular DNA fractions (eDNA vs. iDNA) of the total deadwood DNA pool. Physico-chemical wood properties (macro- and micronutrients; lignin and cellulose content; 3D structure via X-ray microtomography) were also performed along with the determination of key enzymatic activities involved in the main nutrient cycles. Overall, higher microbial abundances were registered in Picea abies CWD samples at the cooler, more acidic and moister north-facing site, which are favourable conditions especially for fungal wood decomposers. This thermal signal (N>S) was more evident for the advanced decay stages (DCls 4 and 5), being wood pH the most determinant factor for discriminating between both slopes. We also found that the impact of exposure was enzyme-specific and strongly dependent on the decay class, except for those enzymes involved in the P cycle. In addition, the eDNA/iDNA ratio provided a simple yet powerful index of microbial activity in terms of exposure, with lower values at the north-facing slope indicative of a higher microbial activity. This is in line with the more pronounced physical wood damage detected at this slope by the X-ray microtomography. A higher microbial activity at the cooler north-facing site rather seems surprising – a circumstance that probably is not due to temperature itself but due to increased moisture availability at this slope.

Topographic influences on shoot litter and root decomposition in semiarid hilly grasslands

Topography has strong effects on microclimates; thus may influence the decomposition of organic matter, a key process determines soil nutrient availability and carbon fluxes in terrestrial ecosystems. Yet, little is known if and how topographic factors influence litter decomposition. We studied the effects of slope aspect (south- vs. north-facing slopes) and position (base and middle positions) on plant shoot litter and root decomposition. We analyzed dynamics of litter mass loss in a 442-day period, and soil and vegetation characteristics in a typical semiarid hilly grassland. Our results showed that the decomposition of roots was faster at south-facing than at north-facing sites, which can be explained by the 2°C higher soil temperature at south-facing sites. Decomposition rate of shoot litter were not different between slope aspects. North-facing sites had 76% higher aboveground biomass and 80% higher belowground biomass than those at south-facing sites. Accordingly, plant N and C storages at north-facing sites were 67% and 76% higher than those at south-facing sites, respectively. The smaller plant carbon and nitrogen stocks and the faster root decomposition at south-facing slopes suggest higher proportions of plant C and N are lost from the ecosystem than that at north-facing slopes. This work highlights the necessity of taking slope aspect into account in carbon and nitrogen cycling studies in hilly grasslands.

Soil attributes and microclimate are important drivers of initial deadwood decay in sub-alpine Norway spruce forests

Deadwood is known to significantly contribute to global terrestrial carbon stocks and carbon cycling, but its decay dynamics are still not thoroughly understood. Although the chemistry of deadwood has been studied as a function of decay stage in temperate to subalpine environments, it has generally not been related to time. We therefore studied the decay (mass of deadwood, cellulose and lignin) of equal-sized blocks of Picea abies wood in soil-mesocosms over two years in the Italian Alps. The 8 sites selected were along an altitudinal sequence, reflecting different climate zones. In addition, the effect of exposure (north- and south-facing slopes) was taken into account. The decay dynamics of the mass of deadwood, cellulose and lignin were related to soil parameters (pH, soil texture, moisture, temperature) and climatic data. The decay rate constants of Picea abies deadwood were low (on average between 0.039 and 0.040y−1) and of lignin close to zero (or not detectable), while cellulose reacted much faster with average decay rate constants between 0.110 and 0.117y−1. Our field experiments showed that local scale factors, such as soil parameters and topographic properties, influenced the decay process: higher soil moisture and clay content along with a lower pH seemed to accelerate wood decay. Interestingly, air temperature negatively correlated with decay rates or positively with the amount of wood components on south-facing sites. It exerted its influence rather on moisture availability, i.e. the lower the temperature the higher the moisture availability. Topographic features were also relevant with generally slower decay processes on south-facing sites than on north-facing sites owing to the drier conditions, the higher pH and the lower weathering state of the soils (less clay minerals). This study highlights the importance of a multifactorial consideration of edaphic parameters to unravel the complex dynamics of initial wood decay.

Air temperature variability in a high-elevation Himalayan catchment

AbstractAir temperature is a key control of processes affecting snow and glaciers in high-elevation catchments, including melt, snowfall and sublimation. It is therefore a key input variable to models of land–surface–atmosphere interaction. Despite this importance, its spatial variability is poorly understood and simple assumptions are made to extrapolate it from point observations to the catchment scale. We use a dataset of 2.75 years of air temperature measurements (from May 2012 to November 2014) at a network of up to 27 locations in the Langtang River, Nepal, catchment to investigate air temperature seasonality and consistency between years. We use observations from high elevations and from the easternmost section of the basin to corroborate previous findings of shallow lapse rates. Seasonal variability is strong, with shallowest lapse rates during the monsoon season. Diurnal variability is also strong and should be taken into account since processes such as melt have a pronounced diurnal variability. Use of seasonal lapse rates seems crucial for glacio-hydrological modelling, but seasonal lapse rates seem stable over the 2–3 years investigated. Lateral variability at transects across valley is high and dominated by aspect, with south-facing sites being warmer than north-facing sites and deviations from the fitted lapse rates of up to several degrees. Local factors (e.g. topographic shading) can reduce or enhance this effect. The interplay of radiation, aspect and elevation should be further investigated with high-elevation transects.

Radial growth changes in Norway spruce montane and subalpine forests after strip cutting in the Swiss Alps

New forest edges are continuously being created by forest management. In the Swiss Alps, silvicultural treatments have partly changed from the selection cutting widespread two decades ago to a more intensive strip cutting. However, little is known about the impact of such harvesting on tree growth and on the structural development of Alpine forest stands dominated by Norway spruce (Picea abies (L.) Karst.), which have high economic and protective value.We therefore investigated the effect of strip cutting in four Alpine spruce stands differing in site and stand conditions through a dendrochronological analysis of 134 tree stems. The change in growth rate was assessed for the 10-year period before and after the cutting year, and rate changes in edge and non-edge trees were compared. The relative change in Hegyi’s competition index before and after the cut was used as a proxy for the change in space and related resources. A linear model was developed to assess the effects of biotic and abiotic variables on changes in growth after strip cutting.Radial growth responses varied greatly between the stands, with a significant increase only in edge trees in the two north-facing sites, i.e. 12% and 60%. Changes in tree competition had the strongest impact on tree growth, followed by site effects. With the same relative change in competition index, the radial growth of edge trees increased more strongly in reaction to cutting than that of non-edge trees. Additionally, small-diameter trees growing near edges benefited more from the strip cutting than larger trees.Our results suggest that strip cutting on north-facing slopes can boost the growth of trees on the east and north-east-facing forest edges. Small spruce trees growing along newly created forest edges can be kept to enhance stand yield. As cutting often leads to long forest edges and may thus affect the growth of a significant proportion of the forest area, such effects should be considered in planning cutting layouts.

Subalpine fires: the roles of vegetation, climate and, ultimately, land uses

Forest fires are controlled by local (vegetation, aspect, elevation, land uses) and regional (climate) factors. The role of these factors remains poorly assessed on centennial to millennial time scales. Here, we investigate fire and vegetation history in five subalpine sites over the past 8000 years to determine when, how, and why there were differences in fire regimes. We focus on time and aspect separately and account for vegetation composition. There are no significant differences in the fire return intervals among sites (~290 years between two fires). The rise of fire frequency from its lowest values (1.5 fires.1000 yr−1) at 8000–7000 years before present (yrBP) to its highest at 4000-3000 yrBP (5 fires-1000 yr−1) is well explained by the high temperatures and low precipitation of the Holocene, both leading to better conditions for fire ignition and spread. From 8000 yrBP, climate is likely the main factor driving the fire frequency on the north-facing sites, which contained mixed Larix decidua-Pinus cembra vegetation; whereas south-facing sites, dominated by the flammable P. cembra and broadleaved trees, appear to be driven by local features, including land use. A shift in the main drivers occurs at 4000 yrBP, when local processes start to control burning activity at all sites. Forest clearance for land use might have directly increased fire frequency, but has also helped to suppress fires through fuel limitation. During the last four millennia, land use likely altered the natural climate-vegetation-fire system mainly by converting forests to grassland by pastoral practices, thus leading to less fire and making the fire-vegetation-climate relationships more complex.

Spatial and temporal dynamics of Aroga moth (Lepidoptera: Gelechiidae) populations and damage to sagebrush in shrub steppe across varying elevation.

Spatial and temporal variation in the density of the Aroga moth, Aroga websteri Clarke (Lepidoptera: Gelechiidae), and in its damage to its host plant, big sagebrush (Artemisia tridentata Nuttall), were examined at 38 sites across a shrub steppe landscape in mountain foothills of northern Utah. Sites were sampled from 2008 to 2012 during and after an outbreak of the moth, to assess whether and how local variation in moth abundance, survivorship, and damage to the host plant was accounted for by sagebrush cover, elevation, slope, aspect, or incident solar radiation. As moth numbers declined from a peak in 2009, individual sites had a consistent tendency in subsequent years to support more or fewer defoliator larvae. Local moth abundance was not correlated with sagebrush cover, which declined with elevation, and moth survivorship was highest at intermediate elevations (1,800-2,000 m). North-facing stands of sagebrush, characterized by lower values of incident solar radiation, were found to be especially suitable local habitats for the Aroga moth, as reflected in measures of both abundance and feeding damage. This high habitat suitability may result from favorable microclimate, both in its direct effects on the Aroga moth and in indirect effects through associated vegetative responses. North-facing sites also supported taller and more voluminous sagebrush plants in comparison to south-facing sites. Thus, the moth is reasonably predictable in the sites at which it is likely to occur in greatest numbers, and such sites may be those that in fact have most potential to recover from feeding damage.