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Biotic disturbances that overlap in space and time may result in important shifts in forest structure and composition, with potential effects on many ecosystem services. Starting in the late 1990s, outbreaks of multiple bark beetle species caused widespread mortality of three co‐occurring conifer species in the ca. 40,000‐km2 subalpine zone of the southern Rocky Mountains (SRM), USA. To better understand the implications of such outbreaks, our goal was to determine if overlapping outbreaks of multiple bark beetle species caused greater tree mortality than single‐species outbreaks in stands with multiple susceptible host tree species. We mapped stand susceptibility to outbreaks of spruce beetle (SB, Dendroctonus rufipennis), mountain pine beetle (MPB, Dendroctonus ponderosae), and western balsam bark beetle (WBBB, Dryocoetes confusus) by combining aerial survey data and forest composition variables in a random forest modeling framework. Then, we used existing maps of cumulative forest mortality from bark beetles to investigate the extent and severity of overlapping outbreaks from 1999 to 2019. We found that 46% of stands with two or more of the three studied hosts species—Engelmann spruce (Picea engelmannii), lodgepole pine (Pinus contorta var. latifolia), or subalpine fir (Abies lasiocarpa)—were susceptible to overlapping outbreaks (25% of all sampled stands). Of those stands, 31% experienced outbreaks of two or more beetle species. Stands affected by outbreaks of both MPB and SB had higher tree mortality than stands affected by one species alone, though stands susceptible to both MPB and SB were uncommon (<4% of all sampled stands). No other combinations of beetle outbreaks increased tree mortality above levels caused by single‐species outbreaks. Thus, contrary to expectations, overlapping outbreaks were rarely more severe than single‐species outbreaks in the SRM. This suggests that diverse forest communities may buffer against the most severe effects of bark beetle outbreaks, even during warm, dry conditions. [ABSTRACT FROM AUTHOR]

The spatial overlap of multiple ecological disturbances in close succession has the capacity to alter trajectories of ecosystem recovery. Widespread bark beetle outbreaks and wildfire have affected many forests in western North America in the past two decades in areas of important habitat for native ungulates. Bark beetle outbreaks prior to fire may deplete seed supply of the host species, and differences in fire‐related regeneration strategies among species may shift the species composition and structure of the initial forest trajectory. Subsequent browsing of postfire tree regeneration by large ungulates, such as elk (Cervus canadensis), may limit the capacity for regeneration to grow above the browse zone to form the next forest canopy. Five stand‐replacing wildfires burned ~60,000 ha of subalpine forest that had previously been affected by severe (>90% mortality) outbreaks of spruce beetle (SB, Dendroctonus rufipennis) in Engelmann spruce (Picea engelmannii) in 2012–2013 in southwestern Colorado. Here we examine the drivers of variability in abundance of newly established conifer tree seedlings [spruce and subalpine fir (Abies lasiocarpa)] and resprouts of quaking aspen (Populus tremuloides) following the short‐interval sequence of SB outbreaks and wildfire (2–8 yr between SB outbreak and fire) at sites where we previously reconstructed severities of SB and fire. We then examine the implications of ungulate browsing for forest recovery. We found that abundances of postfire spruce seedling establishment decreased substantially in areas of severe SB outbreak. Prolific aspen resprouting in stands with live aspen prior to fire will favor an initial postfire forest trajectory dominated by aspen. However, preferential browsing of postfire aspen resprouts by ungulates will likely slow the rate of canopy recovery but browsing is unlikely to alter the species composition of the future forest canopy. Collectively, our results show that SB outbreak prior to fire increases the vulnerability of spruce–fir forests to shifts in forest type (conifer to aspen) and physiognomic community type (conifer forest to non‐forest). By identifying where compounded disturbance interactions are likely to limit recovery of forests or tree species, our findings are useful for developing adaptive management strategies in the context of warming climate and shifting disturbance regimes. [ABSTRACT FROM AUTHOR]

Ant‐hemipteran mutualisms are keystone interactions that can be variously affected by warming: these mutualisms can be strengthened or weakened, or the species can transition to new mutualist partners. We examined the effects of elevated temperatures on an ant‐aphid mutualism in the subalpine zone of the Rocky Mountains in Colorado, USA. In this system, inflorescences of the host plant, Ligusticum porteri Coult. & Rose (Apiaceae), are colonized by the ant‐tended aphid Aphis asclepiadis Fitch or less frequently by the non‐ant tended aphid Cavariella aegopodii (Scopoli) (both Hemiptera: Aphididae). Using an 8‐year observational study, we tested for two key mechanisms by which ant‐hemipteran mutualisms may be altered by climate change: shifts in species identity and phenological mismatch. Whereas the aphid species colonizing the host plant is not changing in response to year‐to‐year variation in temperature, we found evidence that a phenological mismatch between ants and aphids could occur. In warmer years, colonization of host plant inflorescences by ants is decreased, whereas for A. asclepiadis aphids, host plant colonization is mostly responsive to date of snowmelt. We also experimentally established A. asclepiadis colonies on replicate host plants at ambient and elevated temperatures. Ant abundance did not differ between aphid colonies at ambient vs. elevated temperatures, but ants were less likely to engage in tending behaviors on aphid colonies at elevated temperatures. Sugar composition of aphid honeydew was also altered by experimental warming. Despite reduced tending by ants, aphid colonies at elevated temperatures had fewer intraguild predators. Altogether, our results suggest that higher temperatures may disrupt this ant‐aphid mutualism through both phenological mismatch and by altering benefits exchanged in the interaction. [ABSTRACT FROM AUTHOR]

Recent large and severe outbreaks of native bark beetles have raised concern among the general public and land managers about potential for amplified fire activity in western North America. To date, the majority of studies examining bark beetle outbreaks and subsequent fire severity in the U.S. Rocky Mountains have focused on outbreaks of mountain pine beetle ( MPB; Dendroctonus ponderosae) in lodgepole pine ( Pinus contorta) forests, but few studies, particularly field studies, have addressed the effects of the severity of spruce beetle ( Dendroctonus rufipennis Kirby) infestation on subsequent fire severity in subalpine Engelmann spruce ( Picea engelmannii) and subalpine fir ( Abies lasiocarpa) forests. In Colorado, the annual area infested by spruce beetle outbreaks is rapidly rising, while MPB outbreaks are subsiding; therefore understanding this relationship is of growing importance. We collected extensive field data in subalpine forests in the eastern San Juan Mountains, southwestern Colorado, USA, to investigate whether a gray-stage (<5 yr from outbreak to time of fire) spruce beetle infestation affected fire severity. Contrary to the expectation that bark beetle infestation alters subsequent fire severity, correlation and multivariate generalized linear regression analysis revealed no influence of pre-fire spruce beetle severity on nearly all field or remotely sensed measurements of fire severity. Findings were consistent across moderate and extreme burning conditions. In comparison to severity of the pre-fire beetle outbreak, we found that topography, pre-outbreak basal area, and weather conditions exerted a stronger effect on fire severity. Our finding that beetle infestation did not alter fire severity is consistent with previous retrospective studies examining fire activity following other bark beetle outbreaks and reiterates the overriding influence of climate that creates conditions conducive to large, high-severity fires in the subalpine zone of Colorado. Both bark beetle outbreaks and wildfires have increased autonomously due to recent climate variability, but this study does not support the expectation that post-beetle outbreak forests will alter fire severity, a result that has important implications for management and policy decisions. [ABSTRACT FROM AUTHOR]

Precipitation changes the physical and biological characteristics of an ecosystem. Using a precipitation-based conditional sampling technique and a 14 year dataset from a 25m micrometeorological tower in a high-elevation subalpine forest, we examined how warm-season precipitation effected the above-canopy diel cycle of wind and turbulence, net radiation Rnet, ecosystem eddy covariance fluxes (sensible heat H, latent heat LE, and CO2 net ecosystem exchange NEE) and vertical profiles of scalars (air temperature Ta, specific humidity q, and CO2 dry mole fraction χc). This analysis allowed us to examine how precipitation modified these variables from hourly (i.e., the diel cycle) to multi-day time-scales (i.e., typical of a weather-system frontal passage). During mid-day we found: (i) even though precipitation caused mean changes on the order of 50-70% to Rnet, H, and LE, the surface energy balance (SEB) was relatively insensitive to precipitation with mid-day closure values ranging between 70-80 %, and (ii) compared to a typical dry day, a day following a rainy day was characterized by increased ecosystem uptake of CO2 (NEE increased by ≈ 10 %), enhanced evaporative cooling (mid-day LE increased by ≈ 30W m-2), and a smaller amount of sensible heat transfer (mid-day H decreased by ≈ 70 W m-2). Based on the mean diel cycle, the evaporative contribution to total evapotranspiration was, on average, around 6% in dry conditions and 20% in wet conditions. Furthermore, increased LE lasted at least 18 h following a rain event. At night, precipitation (and accompanying clouds) reduced Rnet and increased LE. Any effect of precipitation on the nocturnal SEB closure and NEE was overshadowed by atmospheric phenomena such as horizontal advection and decoupling that create measurement difficulties. Above-canopy mean χc during wet conditions was found to be about 2-3 μmol mol-1 larger than χc on dry days. This difference was fairly constant over the full diel cycle suggesting that it was due to synoptic weather patterns (different air masses and/or effects of barometric pressure). In the evening hours during wet conditions, weakly stable conditions resulted in smaller vertical χc differences compared to those in dry conditions. Finally, the effect of clouds on the timing and magnitude of daytime ecosystem fluxes is described. [ABSTRACT FROM AUTHOR]

Disturbances have a strong role in the carbon balance of many ecosystems, and the cycle of vegetation growth, disturbance, and recovery is very important in determining the net carbon balance of terrestrial biomes. Compound disturbances are phenomena of growing concern which can impact ecosystems in novel ways, altering disturbance intensity, severity, and recovery trajectories. This research focuses on carbon stocks in a compound disturbance environment, with special attention on black carbon (charcoal), a potential source of long-term carbon sequestration. We report on a well-studied compound disturbance event (wind, logging, and severe fire) in a Colorado, USA subalpine forest that was extensively surveyed for impacts on carbon, black carbon, and regeneration. All major pools were considered, including organic and mineral soil (10 cm depth), and contrasted with neighboring undisturbed forests as a reference. The disturbances had an additive effect on carbon loss, with increasing numbers of disturbances resulting in progressively decreasing carbon/black carbon stocks. This resulted from lower substrate availability and higher fire intensity. Surprisingly, there was no significant difference between reference and burned plots in terms of total black carbon. It appears that high-intensity fires do not significantly increase net black carbon in these forests (over the entire fire-return interval), with additional disturbances potentially resulting in a net loss. Disturbances, and their interactions, will have long-lasting legacies for carbon and black carbon. [ABSTRACT FROM AUTHOR]

Considerable research has recently been devoted to understanding biogeochemical processes under winter snow cover, leading to enhanced appreciation of the importance of many winter ecological processes. In this study, a comprehensive investigation of the stable carbon isotope composition (δ13C) of CO2 within a high-elevation subalpine forest snowpack was conducted. Our goals were to study the δ13C of biological soil respiration under snow in winter, and to assess the relative importance of diffusion and advection (ventilation by wind) for gas transport within snow. In agreement with other studies, we found evidence of an active microbial community under a roughly 1-m deep snowpack during winter and into spring as it melted. Under-snow CO2 mole fractions were observed up to 3,500 μmol mol−1, and δ13C of CO2 varied from ~−22 to ~−8‰. The δ13C of soil respiration calculated from mixing relationships was −26 to −24‰, and although it varied in time, it was generally close to that of the bulk organic horizon (−26.0‰). Subnivean CO2 and δ13C were quite dynamic in response to changes in soil temperature, liquid water availability, and wind events. No clear biologically-induced isotopic changes were observed during periods when microbial activity and root/rhizosphere activity were expected to vary, although such changes cannot be eliminated. There was clear evidence of isotopic enrichment associated with diffusive transport as predicted by theory, but simple diffusive enrichment (4.4‰) was not observed. Instead, ventilation of the snowpack by sustained wind events in the forest canopy led to changes in the diffusively-enriched gas profile. The isotopic influence of diffusion on gases in the snowpack and litter was greatest at greater depths, due to the decreased relative contribution of advection at depth. There were highly significant correlations between the apparent isotopic content of respiration from the soil with wind speed and pressure. In summary, physical factors influencing gas transport substantially modified and potentially obscured biological factors in their effects on δ13C of CO2 within this subalpine forest snowpack. [ABSTRACT FROM AUTHOR]

The effect of snow cover on surface-atmosphere exchanges of nitrogen oxides (nitrogen oxide (NO) + nitrogen dioxide (NO2); note, here ‘NO2’ is used as surrogate for a series of oxidized nitrogen gases that were detected by the used monitor in this analysis mode) was investigated at the high elevation, subalpine (3,340 m asl) Soddie site, at Niwot Ridge, Colorado. Vertical (NO + NO2) concentration gradient measurements in interstitial air in the deep (up to ~2.5 m) snowpack were conducted with an automated sampling and analysis system that allowed for continuous observations throughout the snow-covered season. These measurements revealed sustained, highly elevated (NO + NO2) mixing ratios inside the snow. Nitrogen oxide concentrations were highest at the bottom of the snowpack, reaching levels of up to 15 ppbv during mid-winter. Decreasing mixing ratios with increasing distance from the soil–snow interface were indicative of an upwards flux of NO from the soil through the snowpack, and out of the snow into the atmosphere, and imply that biogeochemical processes in the subnival soil are the predominant NO source. Nitrogen dioxide reached maximum levels of ~3 ppbv in the upper layers of the snowpack, i.e., ~20–40 cm below the surface. This behavior suggests that a significant fraction of NO is converted to NO2 during its diffusive transport through the snowpack. Ozone showed the opposite behavior, with rapidly declining levels below the snow surface. The mirroring of vertical profiles of ozone and the NO2/(NO + NO2) ratio suggest that titration of ozone by NO in the snowpack contributes to the ozone reaction in the snow and to the ozone surface deposition flux. However, this surface efflux of (NO + NO2) can only account for a minor fraction of ozone deposition flux over snow that has been reported at other mid-latitude sites. Neither (NO + NO2) nor ozone levels in the interstitial air showed a clear dependence on incident solar irradiance, much in contrast to observations in polar snow. Comparisons with findings from polar snow studies reveal a much different (NO + NO2) and ozone snow chemistry in this alpine environment. Snowpack concentration gradients and diffusion theory were applied to estimate an average, wintertime (NO + NO2) flux of 0.005–0.008 nmol m−2 s−1, which is of similar magnitude as reported (NO + NO2) fluxes from polar snow. While fluxes are similar, there is strong evidence that processes controlling (NO + NO2) fluxes in these environments are very different, as subnivial soil at Niwot Ridge appears to be the main source of the (NO + NO2) efflux, whereas in polar snow (NO + NO2) has been found to be primarily produced from photochemical de-nitrification of snow nitrate. [ABSTRACT FROM AUTHOR]

An experimental system for sampling trace gas fluxes through seasonal snowpack was deployed at a subalpine site near treeline at Niwot Ridge, Colorado. The sampling manifold was in place throughout the entire snow-covered season for continuous air sampling with minimal disturbance to the snowpack. A series of gases (carbon dioxide, water vapor, nitrous oxide, nitric oxide, ozone, volatile organic compounds) was determined in interstitial air withdrawn at eight heights in and above the snowpack at ~hourly intervals. In this paper, carbon dioxide data from 2007 were used for evaluation of this technique. Ancillary data recorded inlcuded snow physical properties, i.e., temperature, pressure, and density. Various vertical concentration gradients were determined from the multiple height measurements, which allowed calculation of vertical gas fluxes through the snowpack using Fick’s 1st law of diffusion. Comparison of flux results obtained from different height inlet combinations show that under most conditions fluxes derived from individual gradient intervals agree with the overall median of all data within a factor of 1.5. Winds were found to significantly influence gas concentration and gradients in the snowpack. Under the highest observed wind conditions, concentration gradients and calculated fluxes dropped to as low as 13% of non-wind conditions. Measured differential pressure amplitude exhibited a linear relationship with wind speed. This suggests that wind speed is a sound proxy for assessing advection transport in the snow. Neglecting the wind-pumping effect resulted in considerable underestimation of gas fluxes. An analysis of dependency of fluxes on wind speeds during a 3-week period in mid-winter determined that over this period actual gas fluxes were most likely 57% higher than fluxes calculated by the diffusion method, which omits the wind pumping dependency. [ABSTRACT FROM AUTHOR]

The soil emission rates (fluxes) of nitrous oxide (N2O) and nitrogen oxides (NO + NO2 = NO x) through a seasonal snowpack were determined by a flux gradient method from near-continuous 2-year measurements using an automated system for sampling interstitial air at various heights within the snowpack from a subalpine site at Niwot Ridge, Colorado. The winter seasonal-averaged N2O fluxes of 0.047–0.069 nmol m−2 s−1 were ~15 times higher than observed NO x fluxes of 0.0030–0.0067 nmol m−2 s−1. During spring N2O emissions first peaked and then dropped sharply as the soil water content increased from the release of snowpack meltwater, while other gases, including NO x and CO2 did not show this behavior. To compare and contrast the winter fluxes with snow-free conditions, N2O fluxes were also measured at the same site in the summers of 2006 and 2007 using a closed soil chamber method. Summer N2O fluxes followed a decreasing trend during the dry-out period after snowmelt, interrupted by higher values related to precipitation events. These peaks were up to 2–3 times higher than the background summer levels. The integrated N2O-N loss over the summer period was calculated to be 1.1–2.4 kg N ha−1, compared to ~0.24–0.34 kg N ha−1 for the winter season. These wintertime N2O fluxes from subniveal soil are generally higher than the few previously published data. These results are of the same order of magnitude as data from more productive ecosystems such as fertilized grasslands and high-N-cycling forests, most likely because of a combination of the relatively well-developed soils and the fact that subnivean biogeochemical processes are promoted by the deep, insulating snowpack. Hence, microbially mediated oxidized nitrogen emissions occurring during the winter can be a significant part of the N-cycle in seasonally snow-covered subalpine ecosystems. [ABSTRACT FROM AUTHOR]

Eddy covariance measurements of the surface energy balance and carbon dioxide exchange above high-elevation (3,480 m above sea level) alpine tundra located near Niwot Ridge, Colorado, were compared to simultaneous measurements made over an adjacent subalpine forest over two summers and one winter, from June 9, 2007 to July 3, 2008. The surface energy balance closure at the alpine site averaged 71 and 91%, winter and summer, respectively, due to the high wind speeds, short turbulent flux footprint, and relatively flat ridge-top location of the measurement site. Throughout the year, the alpine site was cooler with higher relative humidity, and had a higher horizontal wind speed, especially in winter, compared to the forest site. Wind direction was persistently downslope at the alpine site (summer and winter, day and night), whereas upslope winds were common at the forest site during summer daytime periods. The latent and sensible heat fluxes were consistently larger in magnitude at the forest site, with the largest differences during summer. The horizontal advective flux of CO2 at the alpine site averaged 6% of the net ecosystem exchange ( NEE) during summer nights (5% during summer daytime), and was small in relation to the high wind speeds, relatively flat site, and weak sources of CO2 upwind of the site. The magnitudes and diurnal behavior of the alpine NEE calculated using three methods; eddy-covariance, friction velocity filter, and with advection and storage calculations, gave similar results. The period of net CO2 uptake (negative NEE) was 100 days at the alpine site with a net uptake of 16 g C m−2, compared to 208 days at the forest site with a net uptake of 108 g C m−2, with initiation of net uptake coinciding with air temperatures reaching +10°C. Winter respiration loss at the alpine site was 164 g C m−2 over 271 days, compared to 52 g C m−2 over 175 days at the forest site, with the initiation of net loss coinciding with air temperatures reaching −10°C at each site. [ABSTRACT FROM AUTHOR]

Fluxes of CO2 during the snow-covered season contribute to annual carbon budgets, but our understanding of the mechanisms controlling the seasonal pattern and magnitude of carbon emissions in seasonally snow-covered areas is still developing. In a subalpine meadow on Niwot Ridge, Colorado, soil CO2 fluxes were quantified with the gradient method through the snowpack in winter 2006 and 2007 and with chamber measurements during summer 2007. The CO2 fluxes of 0.71 μmol m−2 s−1 in 2006 and 0.86 μmol m−2 s−1 in 2007 are among the highest reported for snow-covered ecosystems in the literature. These fluxes resulted in 156 and 189 g C m−2 emitted over the winter, ~30% of the annual soil CO2 efflux at this site. In general, the CO2 flux increased during the winter as soil moisture increased. A conceptual model was developed with distinct snow cover zones to describe this as well as the three other reported temporal patterns in CO2 flux from seasonally snow-covered soils. As snow depth and duration increase, the factor controlling the CO2 flux shifts from freeze–thaw cycles (zone I) to soil temperature (zone II) to soil moisture (zone III) to carbon availability (zone IV). The temporal pattern in CO2 flux in each zone changes from periodic pulses of CO2 during thaw events (zone I), to CO2 fluxes reaching a minimum when soil temperatures are lowest in mid-winter (zone II), to CO2 fluxes increasing gradually as soil moisture increases (zone III), to CO2 fluxes decreasing as available carbon is consumed. This model predicts that interannual variability in snow cover or directional shifts in climate may result in dramatically different seasonal patterns of CO2 flux from seasonally snow-covered soils. [ABSTRACT FROM AUTHOR]

Much of the research on the chemistry of snow and surface waters of the western US, Europe, and Asia has been conducted in high-elevation catchments above treeline. Here we provide information on the solute content of the seasonal snowpack at the Soddie site on Niwot Ridge, Colorado, a subalpine site near treeline. We focus on the storage and release of both inorganic and organic solutes to the soils underneath the snowpack, and subsequent effects on the chemical and nutrient content of the underlying soil solution and the adjacent headwater stream. The concentration of inorganic nitrogen (N) stored in the seasonal snowpack at the Soddie site of about 11 μeq L−1 was on the upper end of values reported for the northern hemisphere when compared to most areas of the Alps, Himalayas, and Tien Shan mountain ranges, but consistent with other reports of snowpacks in the Rocky Mountains. The storage of inorganic N in the snowpack at maximum accumulation averaged about 17 meq m−2, or 170 eq ha−1 (on the order of 2 kg-N ha−1). Solutes were released from storage in the form of an ionic pulse, with a maximum concentration factor of about four. In contrast to the seasonal snowpack, the dominant form of N in the soil solution was dissolved organic N. Thus, soils underlying the seasonal snowpack appear to assimilate inorganic N released from storage in the snowpack and convert it to organic N. A two component mixing model suggests that the majority of streamflow was this year’s snowmelt that had infiltrated the subsurface and undergone subsequent biological and geochemical reactions. The inorganic N in surface waters at the headwaters of Como creek were always near or below detection limits, suggesting that this area at treeline is still N-limited. [ABSTRACT FROM AUTHOR]