Fraser Experimental Forest Research Articles

A watershed-scale multi-approach assessment of design flood discharge estimates used in hydrologic risk analyses for forest road stream crossings and culverts

Flood peak magnitudes and frequency estimates are key components of any effective nationwide flood risk management and flood damage abatement program. In this study, we evaluated normalized peak design discharges (Qp) for 1,387 hydrologic unit code 16 to 20 (HUC16-20) watersheds in the White Mountain National Forest (WMNF), New Hampshire and in five Experimental Forest (EF) regions across the United States managed by USDA Forest Service (USDA-FS). Nonstationary regional frequency analysis (RFA) and single site frequency analysis (FA) with long-term high-resolution observed streamflow data along with the deterministic Rational Method (RM) and semi-empirical United States Geological Survey regional regression equation (USGS-RRE) were used. Additionally, a hydrologic vulnerability assessment was performed for 194 road culverts as a result of extreme precipitation-induced flooding on gauged and ungauged watersheds in the Hubbard Brook EF (HBR) within the WMNF. The RM outperformed the USGS-RRE in predicting Qp in the gauged and ungauged HUC16-20 watersheds of WMNF and in three other small, high-relief forest headwater watersheds—Coweeta Hydrologic Lab EF’s watershed-14, and watershed-27 in North Carolina and HJ Andrews EF’s watershed 8 in Oregon. However, the USGS-RRE performed better for larger watersheds, such as the Fraser EF’s St. Louis watershed in Colorado and the Santee EF’s watershed 80 in South Carolina. About 31 %, 26 %, and 56 % of the culverts at the HBR site could not accommodate the 100-yr Qp estimated by RFA, RM and USGS-RRE, respectively. Based on the chosen RIs and techniques, it is determined that except for one culvert with diameter = 0.91 m (36 in.), none of the culverts with diameter of 0.75 m (30 in.) or larger are hydrologically vulnerable. Our results suggest that the observation based RFA works best where multiple gauges are available to extrapolate information for ungauged watersheds, otherwise, RM is best-suited for smaller headwater watersheds and USGS-RRE for larger watersheds. Results from the hydrologic vulnerability analysis revealed that replacing undersized culverts with new culverts of diameter ≥ 0.75-m will improve flood resiliency, provided that the structure is geomorphologically safe (with minimal effects of debris flow, erosion, and sedimentation) and allows for both bank-full discharge and necessary fish passage within that design limit. This study has implications in managing road culverts and crossings at Forest Service and other forested lands for their resiliency to extreme precipitation and flooding hazards induced by climate change.

Amount and reactivity of dissolved organic matter export are affected by land cover change from old‐growth to second‐growth forests in headwater ecosystems

AbstractHeadwater forest ecosystems of the western USA generate a large portion of the dissolved organic matter (DOM) transported in streams across North America. Land cover changes that alter forest structure and species composition affect the quantity and composition of DOM transferred to aquatic ecosystems. Clear‐cut harvesting affects ~1% of the forest area of North America annually, leaving most western forests in varying stages of regrowth and the total area of old‐growth forest is decreasing. The consequences of this widespread management practice on watershed carbon cycling remain unknown. We investigated the role of land cover change, because of clear‐cut harvesting, from mixed‐species old‐growth to lodgepole pine‐dominated second‐growth forest on the character and reactivity of hillslope DOM exports. We evaluated inputs of DOM from litter leachates and export of DOM collected at the base of trenched hillslopes during a 3‐year period (2016–2018) at the Fraser Experimental Forest in north‐central Colorado, USA. Dissolved organic carbon and total dissolved nitrogen were higher in lateral subsurface flow draining old‐ versus second‐growth forest. Fluorescence spectroscopy showed that the DOM exported from the old‐growth forest was more heterogeneous and aromatic and that proteinaceous, microbially processed DOM components were more prevalent in the second‐growth forest. Biological oxygen demand assays revealed much lower microbial metabolism of DOM in litter leachate and subsurface exports from the old‐growth forest relative to second growth. Old‐growth and second‐growth forests are co‐mingled in managed ecosystems, and our findings demonstrate that land cover change from a mixture of conifer species to lodgepole pine dominance influences DOM inputs that can increase the reactivity of DOM transferred from terrestrial to aquatic ecosystems.

Sources of variability in springwater chemistry in Fool Creek, a high‐elevation catchment of the Rocky Mountains, Colorado, USA

AbstractSprings are the point of origin for most headwater streams and are important regulators of their chemical composition. We analysed solute concentrations of water emerging from 57 springs within the 3 km2 Fool Creek catchment at the Fraser Experimental Forest and considered sources of spatial variation among them and their influence on the chemical composition of downstream water. On average, calcium and acid neutralizing capacity (bicarbonate‐ANC) comprised 50 and 90% of the cation and anion charge respectively, in both spring and stream water. Variation in inorganic chemical composition among springs reflected distinct groundwater sources and catchment geology. Springs emerging through glacial deposits in the upper portion of the catchment were the most dilute and similar to snowmelt, whereas lower elevation springs were more concentrated in cations and ANC. Water emerging from a handful of springs in a geologically faulted portion of the catchment were more concentrated than all others and had a predominant effect on downstream ion concentrations. Chemical similarity indicated that these springs were linked along surface and subsurface flowpaths. This survey shows that springwater chemistry is influenced at nested spatial scales including broad geologic conditions, elevational and spatial attributes and isolated local features. Our results highlight the role of overlapping factors on solute export from headwater catchments.

Temperature-induced iron (III) reduction results in decreased dissolved organic carbon export in subalpine wetland soils, Colorado, USA

Concentrations of dissolved organic carbon (DOC) in surface waters have rapidly increased across Europe and North America over the past few decades. The causes for such trends have yet to be fully understood, although several factors including elevated temperatures and decreased atmospheric deposition of acid, nitrogen and sulfur, are considered responsible for the release of carbon from soils into streams. In order to determine how seasonal temperature patterns and soil physical and chemical characteristics influence iron (Fe)(III) reduction and DOC release to pore-water in subalpine wetland soils, we measured potential Fe(II) and DOC export rates at low (6 °C) and high (18 °C) temperatures using flow-through reactors containing undisturbed wetland soils sampled in the Fraser Experimental Forest (Colorado, USA) in two wetland types and at two depths.Higher temperature, soil organic matter, nitrogen, and extractable Fe concentrations, but lower soil C:N ratios and extractable manganese concentrations promoted Fe(II) export from soil to water. Additionally, Fe(III) reduction was 1.5–5 times more responsive to temperature in shallow, organic matter-rich soils, compared to organic-poor deeper soils. Conversely, DOC export declined at higher temperature in all soils, consistent with a temperature-related increase in carbon respiration by Fe(III)-reducers. While higher temperature is expected to reduce carbon storage in wetland soils, our findings suggest that temperature-related increase in Fe(III) reduction will not generate additional release of DOC from soils to rivers.

Snagfall the first decade after severe bark beetle infestation of high-elevation forests in Colorado, USA.

The persistence and fall rate of snags (standing dead trees) generated during bark beetle outbreaks have consequences for the behavior, effects, and suppression of potential wildfires, hazard tree and timber salvage operations, wildlife habitat, and numerous ecosystem processes. However, post-beetle snagfall dynamics are poorly understood in most forest types. We tagged standing live and dead lodgepole pine (Pinus contorta), subalpine fir (Abies lasiocarpa), and Engelmann spruce (Picea engelmannii), including beetle-killed pine snags following the peak of a recent mountain pine bark beetle outbreak in watersheds at the Fraser Experimental Forest in northcentral Colorado and sampled snagfall 10 and 12 years later. Bark beetle attacks began in 2003, peaked by 2006, and killed 78% of overstory lodgepole pine in 133 plots distributed across a range of stand and site conditions. Of those snags, only 17% fell between 2007 and 2018. Most snags broke at ground level, due to butt rot, and were oriented downhill. In contrast, snags that tipped up or snapped off above the ground were oriented with the prevailing winds. Equal numbers of snags fell singly and in multiple-tree groups, and equal numbers remained elevated rather than in contact with the ground. Lodgepole pine snagfall was 1.6-times higher on steep slopes (>40%) where dead pine density was higher, compared to flatter sites. Based on our findings and previous research, we estimate that one-half the beetle-killed lodgepole pine in high-elevation forests such as those at Fraser may fall within 15-20yr of beetle infestation, but that some pine snags are likely to persist for decades longer. Post-outbreak snagfall dynamics create a multiple-decade legacy of bark beetle outbreaks that will persist longer in high-elevation compared to lower-elevation forests.

The generation and redistribution of soil cations in high elevation catenas in the Fraser Experimental Forest, Colorado, U.S.

Pedogenic processes imprint their signature on soils over the course of thousands to millions of years in most soil systems. Variation in soil forming processes – such as parent material weathering, organic material additions, hydrologic processes, and atmospheric additions – account for the distribution and sourcing of cations in ecosystems, and hence exert a strong influence on ecosystem productivity. Soil nutrient dynamics of cations also provide an indication of the dominant soil forming processes at work in a particular system. To gain insight into the generation and distribution of the soil cation pool in the Fraser Experimental Forest (FEF), we combined geochemical mass balance techniques and isotopic analyses of soil geochemical data to pedons across eight soil catenas in complex mountain terrain typical of the central Rocky Mountains. We found that mass gains in the FEF soils are primarily attributable to pedogenic additions of Ca to the soil mantle via atmospheric dust, and specifically that soil catenas on the summit landscapes were most enriched in Ca. Our data also show that atmospheric deposition contributions (calculated using Sr isotope ratios) to soils is as high as 82% (±3% SD), and that this isotopic signature in A-horizons and subsurface soil horizons diverges along a soil catena, due to both vertical and lateral hydrologic redistribution processes. Our results suggest that long term soil development and associated chemical signatures at the FEF are principally driven by the coupling of landscape scale cation supply processes, snow distribution, and snowmelt dynamics. Soil development models describing pedogenesis across catenas in montane ecosystems must pay special attention to atmospheric inputs and their redistribution. Any changes to these dynamics will affect productivity and soil/water chemistry in such ecosystems as investigated here.

A Preliminary Report on Aerophytic Cyanoprokaryota and Chlorophyta from Rocky Mountain National Park and Fraser Experimental Forest, Colorado, USA

This paper reports the results of a study of the aerophytic algae from Fraser Research Forest (Fraser, Colorado) and the Loch Vale site in Rocky Mountain National Park, Colorado. Ten bark samples from two trees (Engelmann spruce and Douglas fir) were collected from both study sites. A total of 16 species of aerophytic algae were recovered from these samples. Cyanoprokaryota were the dominant group, with 13 species identified (81.2%) and green algae had three species at 18.8%. There were nine species identified from Fraser Experimental Forest and 13 from Loch Vale. There were 14 species identified on Douglas fir and 8 on Engelmann spruce. There was a slight positive trend for higher numbers of species at higher elevations.

Snowmobile impacts on snowpack physical and mechanical properties

Abstract. Snowmobile use is a popular form of winter recreation in Colorado, particularly on public lands. To examine the effects of differing levels of use on snowpack properties, experiments were performed at two different areas, Rabbit Ears Pass near Steamboat Springs and at Fraser Experimental Forest near Fraser, Colorado USA. Differences between no use and varying degrees of snowmobile use (low, medium and high) on shallow (the operational standard of 30 cm) and deeper snowpacks (120 cm) were quantified and statistically assessed using measurements of snow density, temperature, stratigraphy, hardness, and ram resistance from snow pit profiles. A simple model was explored that estimated snow density changes from snowmobile use based on experimental results. Snowpack property changes were more pronounced for thinner snow accumulations. When snowmobile use started in deeper snow conditions, there was less difference in density, hardness, and ram resistance compared to the control case of no snowmobile use. These results have implications for the management of snowmobile use in times and places of shallower snow conditions where underlying natural resources could be affected by denser and harder snowpacks.

Initial evidence for simultaneous, bi-directional sap flow in roots of interconnected aspen ramets (Populus tremuloides)

Approximately 40% of the world’s currently known plant species exhibit some form of clonality. Yet, little is known about the extent of clonal integration (e.g. the sharing and translocation of resources among ramets), especially in woody species. Aspen (Populus tremuloides Michx.), a widespread clonal tree species of high ecological value and conservation concern, is an excellent model species to study clonal integration. We used sap flow sensors on the root system of aspen located in the Fraser Experimental Forest (Colorado) to quantify water fluxes, assess directionality of flow and assess responses to a root severing experiment. Our results indicate simultaneous, bi-directional flow in roots with 3–4 times more flow in one direction. Flow towards ramets that were subjected to severing of roots (except the measured root) decreased considerably, and an increase in root flow ‘down the line’ towards connected, untreated ramets suggests complex interactions within the root system. Our results are intriguing and provide a first account of directionality of flow and distribution of water in an interconnected root system of a clonal tree species. Based on these findings, we formulate a set of further research questions and discuss methodology and experiments to test them.

Severity of a mountain pine beetle outbreak across a range of stand conditions in Fraser Experimental Forest, Colorado, United States

The recent mountain pine beetle (Dendroctonus ponderosae Hopkins) outbreaks had unprecedented effects on lodgepole pine (Pinus contorta var. latifolia) in western North America. We used data from 165 forest inventory plots to analyze stand conditions that regulate lodgepole pine mortality across a wide range of stand structure and species composition at the Fraser Experimental Forest in Colorado, USA. Forest inventory data were then combined with Landsat 7 Enhanced Thematic Mapper Plus (ETM+) imagery and boosted regression trees modeling to map outbreak severity (proportion of basal area killed). The outbreak severity map was evaluated with training (pseudo-R2=0.63, RMSE=0.13) and independent test plots (pseudo-R2=0.42, RMSE=0.27). This map was used to compare pine mortality in regenerating clearcuts and mature stands, which would have been problematic otherwise since regenerating clearcuts were underrepresented in the forest inventory data. Mortality spanned from 0 to 99% of stand basal area, proportional to the abundance of pine in surveyed stands. During the outbreak, mortality was highest for larger-diameter trees; however, contrary to earlier outbreaks, beetles also attacked younger stands. Pine mortality was lower in stands regenerating from clearcut harvests conducted between 1954 and 1985 than in mature stands and was more closely related to topographic factors than stand age or clearcut size; mortality was highest on southerly aspects and lower elevation sites, favorable to lodgepole pine. The best predictors for mapping outbreak severity were the change in the Normalized Difference Moisture Index between pre- and end-of-outbreak imagery and the end-of-outbreak ETM+ band 5. A better understanding of mortality patterns relative to forest management can inform management planning and assessment of the influence of bark beetle outbreaks on ecosystem processes.

A Decade of Streamwater Nitrogen and Forest Dynamics after a Mountain Pine Beetle Outbreak at the Fraser Experimental Forest, Colorado

Forests of western North America are currently experiencing extensive tree mortality from a variety of bark beetle species, and insect outbreaks are projected to increase under warmer, drier climates. Unlike the abrupt biogeochemical changes typical after wildfire and timber harvesting, the outcomes of insect outbreaks are poorly understood. The mountain pine bark beetle (Dendroctonus ponderosae) began to attack lodgepole pine (Pinus contorta) at the Fraser Experimental Forest in 2002 and spread throughout the research area by 2007. We compared streamwater nitrogen (N) from 2003 through 2012 with data from the previous two decades in four watersheds with distinct forest management histories, stand structures, and responses to the beetle outbreak. Watersheds dominated by old-growth had larger trees and lost 85% of overstory pine and 44% of total basal area to bark beetles. In contrast, managed watersheds containing a mixture of second-growth (30–60 year old) and old-growth (250- to 350-year old) had higher density of subcanopy trees, smaller mean tree diameter, and lower bark beetle-induced mortality (~26% of total basal area). Streamwater nitrate concentrations were significantly higher in old-growth watersheds during the outbreak than pre-outbreak levels during snowmelt and base flow seasons. In mixed-age stands, streamwater nitrate concentrations were unaffected by the outbreak. Beetle outbreak elevated inorganic N export 43 and 74% in two old-growth watersheds though the amounts of N released in streamwater were low (0.04 and 0.15 kg N ha−1) relative to atmospheric inputs (<2% of annual N deposition). Increased height, diameter, and foliar N of measured in residual live trees augmented demand for N, far in excess of the change in N export during the outbreak. Reallocation of soil resources released after pine mortality to overstory and understory vegetation helps explain high nutrient retention in watersheds affected by bark beetle outbreaks.

Spatial characterization of roughness elements in high‐gradient channels of the Fraser Experimental Forest, Colorado, USA

AbstractWe collected high‐resolution LiDAR‐based spatial and reach‐average flow resistance data at a range of flows in headwater stream channels of the Fraser Experimental Forest, Colorado, USA. Using these data, we implemented a random field approach for assessing the variability of detrended bed elevations and flow depths for both the entire channel width and the thalweg‐centered 50% of the channel width (to exclude bank effects). The spatial characteristics of these channels, due to bedforms, large clasts and instream wood, were compared with Darcy‐Weisbach f and stream type through the use of the first four probability density function moments (mean, variance, skewness, kurtosis). The standard deviation of the bed elevations (σz) combined with depth (h), as relative bedform submergence (h/σz), was well correlated with f (R2 = 0.81) for the 50% of channel width. The explained variance decreased substantially (R2 = 0.69) when accounting for the entire width, indicating lesser contribution of channel edges to flow resistance. The flow depth skew also explained a substantial amount of the variance in f (R2 = 0.78). A spectrum of channel types is evident in depth plots of skew versus kurtosis, with channel types ranging from plane bed, transitional, step pool/cascade, to cascade. These results varied when bank effects were included or excluded, although definitive patterns were observed for both analyses. Random field analyses may be valuable for developing tools for predicting flow resistance, as well as for quantifying the spectrum of morphologic change in high‐gradient channel types, from plane bed through cascade.

Influence of large wood on channel morphology and sediment storage in headwater mountain streams, Fraser Experimental Forest, Colorado

Large fallen wood can have a significant impact on channel form and process in forested mountain streams. In this study, four small channels on the Fraser Experimental Forest near Fraser, Colorado, USA, were surveyed for channel geometries and large wood loading, including the size, source, and characteristics of individual pieces. The study is part of a larger effort to understand the impact of mountain pine beetle infestation on a suite of watershed properties. Here, we present baseline data collected at the onset of widespread tree mortality. Channels range from 1.5 to 2m in width, with slopes ranging from 3 to >10%. Median (D50) streambed particle sizes range from gravel to very coarse gravel. Channels are characterized as cascade, step-pool, and plane bed over varying scales. Large wood loads ranged from about 0.4 to 1.0 piece per meter length of channel, which is comparable to values reported for other Colorado sites. Much of the wood showed indications of being in place for long periods of time (decayed/rotten, broken into ramps, and partially buried in beds and banks). Nearly all surveyed reaches contained steps formed from small boulders and/or logs. Significant portions of the elevation drop in some of the reaches were made up by log steps, though the percentages varied (0 to 60%). Individual log steps trap a portion of the coarse sediment moved as bedload, forming wedge-shaped accumulations upstream of the logs. The particle size distributions for measured bedload and step accumulations largely overlapped, but more so for the coarse ends of the distributions, suggesting a trapping inefficiency for the finer component of bedload. Estimates of the total volume of sediment stored behind log steps were approximately an order of magnitude greater than the mean sediment volume exported on an annual basis, as determined from measured accumulations in weir ponds. The particle size distribution of sediment in the ponds – ranging from sand to medium gravel – is considerably finer than sediment stored in steps. The series of comparisons between storage volumes, particle size distributions, and sediment export suggests that log steps effectively trap coarse sediment moved in these small streams and act as a series of check dams that inhibit channel erosion, but may be less effective at trapping finer sediment (sand and small gravel).

Characterizing spatial variability in velocity and turbulence intensity using 3-D acoustic Doppler velocimeter data in a plane-bed reach of East St. Louis Creek, Colorado, USA

We investigated the influence on flow resistance of flow structure and turbulence at the reach scale in a mountain channel using 3-D velocity measurements and geostatistical analysis to understand the complexity of the flow structure in a reach with limited bed irregularities. The increase in flow resistance at low flows in a plane-bed reach was not fully explained by grain resistance, therefore detailed 3-D velocity measurements were made to investigate spatial variability in velocity and turbulence components and potential controls on flow resistance. One plane-bed reach was surveyed over two stages in Fraser Experimental Forest, Colorado, using a combination of a total station, LiDAR (Light Detection and Ranging), and a SonTek Flowtracker handheld ADV (acoustic Doppler velocimeter). LiDAR was used to capture bank and channel geometry at low flows, whereas the water surface and bed data were collected with the total station at all flows. We used the standard deviation of bed elevation (σb) within a moving window as an index of roughness height (ks) and calculated the relative submergence of the bed at different stages as h/ks, where h is the local flow depth. ADV measurements were collected on a grid with a 0.3m to 0.5m spacing. Geostatistical analysis of the velocity data indicated that the flow was highly three-dimensional and varied based on stage, demonstrating that even small irregularities in the bed have a significant influence on the flow characteristics. The streamwise component was the largest at both low and high flow, but varied more throughout the reach at low flow. At high flow, the greatest streamwise velocities were located within the thalweg. Areas of upwelling and downwelling also varied based on stage, with this component being strongly influenced by small changes in the morphology at high flow, and by protuberant grains at low flows. The cross-stream velocity and turbulence components were controlled by the flow structure and less by the roughness of the bed. The turbulence intensity is significant when considering hydraulics for predicting sediment transport and for habitat assessment.The plane-bed flow patterns were compared to results from a pool-riffle reach in the North Fork Cache La Poudre River, a step-pool reach in East St. Louis Creek, and a step-pool reach in Italy on the Rio Cordon. The comparison of the 3-D velocities among channel types showed that the plane-bed reach has higher streamwise velocities, but similar values of cross-stream and transverse velocities. Streamwise turbulence intensities were similar in both the plane-bed and step-pool reach in East St. Louis Creek. The analysis revealed that the connection between the flow characteristics and the channel was not only related to the gross morphology and location of larger clasts, but also to the development of shear layers from the convergence of flow. Therefore, it is essential to understand how water moving in one direction can alter the characteristics of another component of flow and how this interaction is connected to the bed morphology.

Comparative analysis of bed resistance partitioning in high‐gradient streams

Total flow resistance can be partitioned into its components of grain (ffgrain), form (ffstep), wood (ffwood), and spill (ffspill) resistance. Methods for partitioning flow resistance developed for low‐gradient streams are commonly applied to high‐gradient systems. We examined the most widely used methods for calculating each component of resistance, along with the limitations of these methods, using data gathered from 15 high‐gradient (0.02 &lt; S0 &lt; 0.195) step‐pool, cascade, and plane‐bed reaches in Fraser Experimental Forest. We calculated grain resistance using three equations that relate relative submergence (R/Dm) to ffgrain as well as using an additive drag approach. The drag approach was also used for calculating ffwood and ffstep. The ffgrain contributed the smallest amount toward all reaches at all flows, although the value varied with the method used. The Parker and Peterson (1980) equation using D90 best represented ffgrain at high flows, whereas the Keulegan (1938) equation using D50 best characterized ffgrain at base flows, giving a lower bound for grain resistance. This suggests that ffgrain may be better represented if two grain sizes are used to calculate this component of resistance. The drag approach, which is used to calculate wood resistance, overestimated the significance of individual logs in the channel. The contribution of ffspill was reduced at higher flows when form drag around the step is accounted for at higher flows. We propose a method for evaluating the contribution of ffstep that accounts for form drag around the steps once they are submerged at higher flows. We evaluated the potential sources of error for the estimation of each component of resistance. Determination of the drag coefficient was one of the major sources of error when calculating drag around wood, steps, or boulders.

Tree regeneration and future stand development after bark beetle infestation and harvesting in Colorado lodgepole pine stands

In the southern Rocky Mountains, current mountain pine beetle ( Dendroctonus ponderosae Hopkins) outbreaks and associated harvesting have set millions of hectares of lodgepole pine ( Pinus contorta var. latifolia Engelm. ex Wats.) forest onto new stand development trajectories. Information about immediate, post-disturbance tree regeneration will provide insight on dynamics of future stand composition and structure. We compared tree regeneration in eight paired harvested and untreated lodgepole pine stands in the Fraser Experimental Forest that experienced more than 70% overstory mortality due to beetles. New seedlings colonized both harvested and untreated stands in the first years after the beetle outbreak. In harvested areas the density of new seedlings, predominantly lodgepole pine and aspen, was four times higher than in untreated stands. Annual height growth of pine and fir advance regeneration ( e.g., trees established prior to the onset of the outbreak) has doubled following overstory mortality in untreated stands. Growth simulations based on our regeneration data suggest that stand basal area and stem density will return to pre-beetle levels in untreated and harvested stands within 80–105 years. Furthermore, lodgepole pine will remain the dominant species in harvested stands over the next century, but subalpine fir will become the most abundant species in untreated areas. Owing to terrain, economic and administrative limitations, active management will treat a small fraction (<15%) of the forests killed by pine beetle. Our findings suggest that the long-term consequences of the outbreak will be most dramatic in untreated forests where the shift in tree species composition will influence timber and water production, wildfire behavior, wildlife habitat and other forest attributes.

Controls on at‐a‐station hydraulic geometry in steep headwater streams, Colorado, USA

AbstractDetailed hydraulic measurements were made in nine step‐pool, five cascade and one plane‐bed reach in Fraser Experimental Forest, Colorado to better understand at‐a‐station hydraulic geometry (AHG) relations in these channel types. Average values for AHG exponents, m (0·49), f (0·39), and b (0·16), were well within the range found by other researchers working in steep gradient channels. A principal component analysis (PCA) was used to compare the combined variations in all three exponents against five potential control variables: wood, D84, grain‐size distribution (σ), coefficient of variation of pool volume, average roughness‐area (projected wetted area) and bed gradient. The gradient and average roughness‐area were found to be significantly related to the PCA axis scores, indicating that both driving and resisting forces influence the rates of change of velocity, depth and width with discharge. Further analysis of the exponents showed that reaches with m &gt; b + f are most likely dominated by grain resistance and reaches below this value (m &lt; b + f) are dominated by form resistance. Copyright © 2010 John Wiley &amp; Sons, Ltd.

The Influence of an Extensive Dust Event on Snow Chemistry in the Southern Rocky Mountains

In mid-February 2006, windstorms in Arizona, Utah, and western Colorado generated a dust cloud that distributed a layer of dust across the surface of the snowpack throughout much of the Colorado Rockies; it remained visible throughout the winter. We compared the chemical composition of snowfall and snowpack collected during and after the dust deposition event with pre-event snow at 17 sites extending from central Colorado into southern Wyoming. The chemistry of dust-event snowfall and the post-event snowpack were compared to long-term wetfall precipitation and snowpack chemistry at the Fraser Experimental Forest (FEF). The pH of the snowpack formed during the dust event was 1.5 units higher, calcium was 10-fold higher, and acid neutralizing capacity (ANC) was 100-fold higher than levels measured in either early or late-season snow. Elevated ANC was attributable to dust-derived increase in carbonate/bicarbonate ions, likely from soluble calcium carbonate inclusions in dust material. The single dust event contributed carbonate/bicarbonate-derived ANC equivalent to the sum of strong acids deposited during the entire winter at FEF. Effects appeared more pronounced at upper elevations and in the sparse forest near treeline, compared to densely forested lower elevation sites. Monthly snowpack solute analysis at FEF has not documented an event of similar magnitude during its 17-year period of record. The solute composition of post-event snow is similar to average Fraser streamwater, however. To assess the effect of dust on the timing and composition of water exported from high-elevation ecosystems, future work should consider both the contributions of eolian deposition on soil development, plant communities, and nutrient and water relations plus the impact of individual deposition events on snowpack chemistry and duration.

Similarity of nutrient uptake and root dimensions of Engelmann spruce and subalpine fir at two contrasting sites in Colorado

Nutrient uptake capacity is an important parameter in modeling nutrient uptake by plants. Researchers commonly assume that uptake capacity measured for a species can be used across sites. We tested this assumption by measuring the nutrient uptake capacity of intact roots of Engelmann spruce ( Picea engelmanni Parry) and subalpine fir ( Abies lasiocarpa (Hook.) Nutt.) at Loch Vale Watershed and Fraser Experimental Forest in the Rocky Mountains of central Colorado. Roots still attached to the tree were exposed to one of three concentrations of nutrient solutions for time periods ranging from 1 to 96 h, and solutions were analyzed for ammonium, nitrate, calcium, magnesium, and potassium. Surprisingly, the two species were indistinguishable in nutrient uptake within site for all nutrients ( P > 0.25), but uptake rates differed by site. In general, nutrient uptake was higher at Fraser ( P = 0.01, 0.15, 0.03, and 0.18 for NH 4 +, NO 3 −, Ca 2+, and K +, respectively), which is west of the Continental Divide and has lower atmospheric deposition of N than Loch Vale. Mean uptake rates by site for ambient solution concentrations were 0.12 μ mol N H 4 + g fwt − 1 h − 1 , 0.02 μ mol N O 3 − g fwt − 1 h − 1 , 0.21 μ mol C a 2+ g fwt − 1 h − 1 , and 0.01 μ mol M g 2+ g fwt − 1 h − 1 at Loch Vale, and 0.21 μ mol N H 4 + g fwt − 1 h − 1 , 0.04 μ mol N O 3 − g fwt − 1 h − 1 , 0.51 μ mol C a 2+ g fwt − 1 h − 1 , and 0.07 μ mol M g 2+ g fwt − 1 h − 1 at Fraser. The importance of site conditions in determining uptake capacity should not be overlooked when parameterizing nutrient uptake models. We also characterized the root morphology of these two species and compared them to other tree species we have measured at various sites in the northeastern USA. Engelmann spruce and subalpine fir were indistinguishable in specific root length and diameter distribution, while most of the other 10 species had statistically distinct diameter distributions across five diameter classes < 2 mm. Based on specific root length, subalpine fir and Engelmann spruce had significantly coarser roots than red pine ( Pinus resinosa Soland), yellow birch ( Betula allegheniensis Britt.), sugar maple ( Acer saccharum Marsh.), chestnut oak ( Quercus prinus L.), black cherry ( Prunus serotina Ehrh.), and red spruce ( Picea rubens Sarg.). White oak ( Quercus alba L.), balsam fir ( Abies balsamea (L.) Mill.), American beech ( Fagus grandifolia Ehrh.) and loblolly pine ( Pinus taeda L.) were intermediate in SRL (indistinguishable from Engelmann spruce and subalpine fir by ANOVA). Species that differ more in physiology and morphology than the two species we compared would likely show dissimilar uptake characteristics even at the same site.

The Impact of Snow Model Complexity at Three CLPX Sites

Abstract Many studies have developed snow process understanding by exploring the impact of snow model complexity on simulation performance. This paper revisits this topic using several recently developed land surface models, including the Simplified Simple Biosphere Model (SSiB); Noah; Variable Infiltration Capacity (VIC); Community Land Model, version 3 (CLM3); Snow Thermal Model (SNTHERM); and new field measurements from the Cold Land Processes Field Experiment (CLPX). Offline snow cover simulations using these five snow models with different physical complexity are performed for the Rabbit Ears Buffalo Pass (RB), Fraser Experimental Forest headquarters (FHQ), and Fraser Alpine (FA) sites between 20 September 2002 and 1 October 2003. These models simulate the snow accumulation and snowpack ablation with varying skill when forced with the same meteorological observations, initial conditions, and similar soil and vegetation parameters. All five models capture the basic features of snow cover dynamics but show remarkable discrepancy in depicting snow accumulation and ablation, which could result from uncertain model physics and/or biased forcing. The simulated snow depth in SSiB during the snow accumulation period is consistent with the more complicated CLM3 and SNTHERM; however, early runoff is noted, owing to neglected water retention within the snowpack. Noah is consistent with SSiB in simulating snow accumulation and ablation at RB and FA, but at FHQ, Noah underestimates snow depth and snow water equivalent (SWE) as a result of a higher net shortwave radiation at the surface, resulting from the use of a small predefined maximum snow albedo. VIC and SNTHERM are in good agreement with each other, and they realistically reproduce snow density and net radiation. CLM3 is consistent with VIC and SNTHERM during snow accumulation, but it shows early snow disappearance at FHQ and FA. It is also noted that VIC, CLM3, and SNTHERM are unable to capture the observed runoff timing, even though the water storage and refreezing effects are included in their physics. A set of sensitivity experiments suggest that Noah’s snow simulation is improved with a higher maximum albedo and that VIC exhibits little improvement with a larger fresh snow albedo. There are remarkable differences in the vegetation impact on snow simulation for each snow model. In the presence of forest cover, SSiB shows a substantial increase in snow depth and SWE, Noah and VIC show a slight change though VIC experiences a later onset of snowmelt, and CLM3 has a reduction in its snow depth. Finally, we observe that a refined precipitation dataset significantly improves snow simulation, emphasizing the importance of accurate meteorological forcing for land surface modeling.