Effects of mountain pine beetle-killed forests on source water contributions to streamflow in headwater streams of the Colorado Rocky Mountains

Abstract

Natural or human-influenced disturbances are important to the health and diversity of forests, which in turn, are important to the water quantity and quality exported from a catchment. However, human-induced disturbances (prescribed fire and harvesting) have been decreasing, and natural disturbances (fires and insects) have been increasing in frequency and severity. One such natural disturbance is the mountain pine beetle (MPB), (Dendroctonus ponderosae) an endemic species. A recent epidemic resulted in the mortality of millions of hectares of lodgepole pine (Pinus contorta) forests in Colorado, USA. Beetle-induced tree mortality brings about changes to the hydrologic cycle, including decreased transpiration and interception with the loss of canopy cover. This study examined the effect of the mountain pine beetle kill on source water contributions to streamflow in snowmelt-dominated headwater catchments using stable isotopes (²H and ¹⁸O) as tracers. Study catchments with varying level of beetle-killed forest area (6% to 97%) were sampled for groundwater, surface water, and precipitation. Streams were sampled to assess whether beetle-killed forests have altered source water contributions to streamflow. Groundwater contributions increased with increasing beetle-killed forest area (p=0.008). Both rain and snow contributions were negatively correlated with beetle-killed forest area (p=0.035 and p=0.011, respectively). As the beetle-killed forest area increases, so does fractional groundwater contribution to streamflow.

Graphical abstract

Keywords

mountain pine beetle / isotope tracers / streamflow generation / headwaters

1 Introduction

Forest catchments provide almost two-thirds of the surface freshwater in the conterminous United States (Maxwell, 2009). Colorado and the other mountain states rely on snowmelt for water supplies to sustain agricultural, industrial, recreational, environmental, and municipal sectors. In Colorado, 68% of the average annual water yield from national forest lands is used as drinking water (Maxwell, 2009). There is a direct connection between the health of forested catchments and the health of the waters produced.

Forests are influenced by disturbances from natural and human-influenced activities. Human-influenced activities include prescribed fire, timber harvesting and forest clearing. With aggressive forest fire fighting and decreased timber harvesting, western forests do not have the stand age and structure diversity typically associated with healthy forests. As a result, many forested landscapes in the Rocky Mountain area are composed of overstocked even-aged stands, and larger, more contiguous landscapes in these areas have now become more susceptible to bark beetle outbreaks.

The mountain pine beetle (Dendroctonus ponderosae Hopkins) (MPB) populations in the Rocky Mountain region are at epidemic conditions (Man, 2012). Altered hydrologic processes as a result of MPB-caused tree mortality may potentially affect source water (groundwater, snow, rain) contributions to streamflow. As Colorado experiences increasing demand on water supplies (Colorado Water Conservation Board, 2010), identifying the relationship between beetle-killed forests and streamflow generation processes is important.

1.1 Mountain pine beetle

Bark beetles are significant disturbance agents responsible for millions of ha of tree death in conifer forests, most notably in North America and Europe (Raffa et al., 2008). Bark beetle activity increases when critical thresholds are exceeded, which may be exacerbated by climate change, drought, and warmer winter temperatures (Thomson and Shrimpton, 1984; Raffa et al., 2008). The recent upsurge in bark beetle populations is mostly attributable to climate change, specifically increasing warmth, which accelerates beetle fecundity while also enhancing winter survival rates (Bentz et al., 2010). Climate change is expected to promote range expansion of these insects beyond their historic boundaries, leading to non-linear shifts in forest disturbance regimes in many high latitude and high elevation regions (Weed et al., 2013). The combined effects of climate warming, associated drought, and shifting disturbance regimes places considerable stress on forest ecosystems, which have strong potential to degrade forested landscapes and ultimately lead to undesirable social and ecological outcomes (Carpenter et al., 2009; Seidl et al., 2011; Morris et al., 2016).

The mountain pine beetle is native to the forests of Colorado and western North America. Pine beetles have coevolved with pine trees and are a natural part of the ecosystem (Hazlett and Hansen, 2007). Therefore, it is neither possible nor desirable to eradicate them (Fettig et al., 2007). Normally, with endemic populations, outbreaks occur on the scale of a few trees with low tree mortality and high beetle mortality. Beetle epidemics occur in response to physiological tree stress, caused by site disturbance or drought. By periodically killing off the largest trees, the mountain pine beetle maintains uneven-aged stands and increases succession by climax species. In this manner, MPB are an important factor in maintaining forest health, especially in the absence of fire.

Large beetle outbreaks in the Colorado Rocky Mountains have occurred in the past, most notably in the mid-19th century and the 1940s and 1970s (Baker and Veblen, 1990; Veblen et al., 1994; Eisenhart and Veblen, 2000). MPB infestations are cyclical in nature, with return periods between 20–40 years, though the frequency depends on stand location and the rate at which the stand can build up to conditions conducive to the insect life cycle. The epidemic lasts 6 years on average, with peak mortality occurring at year 3. Approximately 85% of large-diameter trees die over the course of the epidemic, though stand structure, amount of phloem, elevation, latitude, and beetle populations may alter this pattern (Morris et al., 2016).

Infestations end for several reasons. Woodpeckers and other insects such as the clerid beetle are natural controls, though they are relatively ineffective during large epidemics (Leatherman, 2008). Cold temperatures can kill beetle populations, especially if they occur in the fall before the beetles have metabolized glycerols or in the spring when they transform into pupae. For the temperature to have a significant effect during the winter, temperature must reach –30°C for 5 days, or there must be a lack of available host trees (Morris et al., 2016).

1.2 Effects of mountain pine beetles on water resources

Annual water yield in the higher elevation forests increases in proportion to the amount of forest canopy removed (often indexed by the basal area removed) (Stednick and Troendle, 2016). In the Rocky Mountain region, as little as 15% basal area removal can generate an observable response in annual water yield (Stednick, 1996). Timber harvesting will remove the forest canopy and increase annual water yield due to the reduction of winter interception and summer transpiration. The increase in water yield decreases as the forest regrows. Beetle-killed forests will have reduced interception and reduced transpiration losses and increased soil water or groundwater, and thus streamflow should respond hydrologically similar as timber harvesting.

In 1939, a wind storm in Colorado, USA created ideal breeding conditions for an Engelmann spruce beetle epidemic (Love, 1955). By 1946, the beetle had killed up to 80% of the forest trees. Using a paired catchment approach, it was determined that average annual water yield increased following the epidemic. Overall, the increased water yield was attributed to greater accumulations of snow in the killed areas (Love, 1955). This was the first study to document increased streamflow from an insect defoliation event and adds to the Colorado history in forest hydrology. Later analysis of streamflow records revealed that the smallest increases on both catchments occurred during the first 5-year period (when the beetle population was multiplying to epidemic proportions); the largest increases occurred 15 years later (Bethlahmy, 1974, 1975).

A mountain pine beetle outbreak in the mid-1970s killed an estimated 35% of the trees in Jack Creek in Southwest Montana, USA. Data analysis indicated an increase in annual water yield. Because of de-synchronization of streamflow peaks, the increased annual water yields did not produce a difference in peak flows (Potts, 1984).

The paired catchment technique was used to assess streamflow changes of Camp Creek in interior British Columbia after clear-cut logging 30% of the catchment area affected by beetle-kill. Hydrometric data for Camp Creek (beetle infested) and those of an adjacent control, Greata Creek were analyzed for both the pre-logging and post-logging periods. Post-logging Camp Creek streamflow showed increased annual water yield (Cheng, 1989). Effects of logging and/or beetle-kill were not separated.

Canopy loss from bark beetle attack results in increased evaporation from the forest floor (Mikkelson et al., 2013a; Biederman et al., 2014a, b). Despite this increase, studies suggest decreases in overall evapotranspiration (Chen et al., 2015) and increases in soil moisture (Clow et al., 2011; Mikkelson et al., 2013a; Pugh and Gordon, 2013). These effects will result in increases in groundwater contribution to streamflow, as less water taken up by vegetation through interception and transpiration. Transpiration rates will increase as trees regenerate and grow, decreasing groundwater or soil moisture (Pugh and Gordon, 2013; Bearup et al., 2014) and water yields (Livneh et al., 2015).

In British Columbia, Canada, plot level studies suggested changes in the hydrological processes, particularly increased snow pack accumulation under beetle-killed trees (British Columbia Ministry of Forests, 1995) increased ablation under the dead trees (Boon, 2007, 2009; Teti, 2007) and subsequent increased melt rates (Spittlehouse, 2007; Winkler, 2007). Snowpack snow water equivalent increases after canopy loss resulting from the bark beetle kill (Mikkelson et al., 2013b) because the snowpack will lose less to canopy sublimation (Boon, 2012; Pugh and Small, 2014a). However, wind energy, which can increase surface sublimation rates, will increase as the canopy and stand thins, increasing snowpack ablation (Bergen, 1971) and could offset increased snow accumulation in some cases (Biederman et al., 2012; Pugh and Small, 2012).

This literature review shows variable results and variable interpretations on the effects of beetle-killed forests on water yield. Differing mechanisms are proposed for interpreting the change or lack of change in annual water yield, and only a few studies examined changes in source water contributions to streamflow. Our study hypothesis is that source water contributions, as identified by isotopic signature, will change with increased beetle-killed forest area, specifically that these headwater snowmelt hydrographs will show increases in groundwater contributions to streamflow with increasing beetle-killed catchment area.

2 Methods

This study analyzed source water contributions to streamflow in 25 catchments in the north-central Rocky Mountains of Colorado (Table 1). This area is dominantly lodgepole pine forest with varying degrees of MPB infestation and has a typical cold-snow zone hydrograph, where the seasonal snowpack melt generates the annual peak flow. Summer precipitation events typically do not result in streamflow responses. Study catchments were largely on United States Department of Agriculture Forest Service (USFS) land, and thus had little land use activity. Study catchment selections were based on representing a range of beetle-killed area (%) and the availability of United States Department of Interior Geological Survey stream gauging records.

Few of the study catchments had precipitation gauges and only one catchment had snowpack monitoring. Lack of precipitation data precluded a water balance approach to assess source water changes as related to annual precipitation, snowpack accumulation and melt, or other streamflow metrics. Nonetheless, the range in beetle-killed area represented in the study catchments can identify potential changes in source water contributions to streamflow.

A Geographic Information System (GIS) was used to calculate catchment area, elevation, forest layers, and MPB kill layers (Fig. 1). Elevation was obtained from a 10-meter resolution digital elevation model (USGS National Elevation Dataset, 2013). The mountain pine beetle-killed layer for 2002 to 2015 was obtained from the USFS Aerial Detection Survey (USDA, 2014), and the forests layer was obtained from the 2005 National Land Cover Data set (USGS Land Cover Institute, 2005).

Distinct isotopic signatures in source waters can be used to identify their contribution to streamflow. The isotopic composition of precipitation is based on temperature and original vapor source, so the stable isotope composition of rain differs from snow. While rain and snow both fall on the meteoric water line, rain is more enriched in heavy isotopes (²H and ¹⁸O) due to preferential condensation of heavy isotopes at warmer temperatures (Ingraham, 1998).

Groundwater displays a distinct isotope composition compared to precipitation and stream water due to evaporation. When evaporation occurs, deviations from the meteoric water line occur, as ²H evaporates more readily than ¹⁸O. Groundwater is also more enriched in both heavy isotopes than surface water (Ingraham, 1998). The distinct composition of each source water allows for source identification.

Surface and groundwater samples were collected from each catchment April to October from 2011 to 2015. The seasonal snowpack limited site access to this time frame. Precipitation samples were collected in most catchments, but due to limited winter site access, sampling frequency was limited, and precipitation isotope composition was assumed to be homogenous across the study area. In total, 222 stream samples, 117 groundwater samples, 80 rain samples, and 68 snow samples were collected. Samples collected during this time represent the snowmelt and summer low flow portions of the annual hydrograph. For surface water samples, 20 mL polypropylene scintillation bottles were submerged and capped under water to avoid air entrapment, which would alter isotope composition. Groundwater samples were obtained using polyvinylchloride or stainless steel piezometers and a vacuum pump. Piezometers were placed (<1 m deep) in the riparian area.

Precipitation (rain and snow) samples were periodically taken in most catchments. Snow samples were collected from the snowpack in a plastic bag and allowed to melt before filling the sample bottle. Rain samples were collected using the International Atomic Energy Agency precipitation sampler of an open-top brown one-liter polypropylene bottle, one meter above ground level with a thin layer of paraffin oil added to prevent evapoconcentration (IAEA, 1997). Paraffin oil was used to prevent evaporation and isotopic enrichment of the precipitation sample. All samples were analyzed at the University of Wyoming Stable Isotope Facility (UWYSIF, 2015). Isotope composition results are accurate to 0.2‰ for ¹⁸O measurements and 1‰ for ²H measurements (UWYSIF, 2015).

Stable Isotope Analysis in R (SIAR) requires isotopic data of a target and its sources and fits a Bayesian model to potential sources based on a Gaussian likelihood (Parnell and Jackson, 2015). The model requires an input (target) file of streamflow isotope composition and a source file of isotope compositions. The target file includes isotope measurements for each sample of the stream water (for both ²H and ¹⁸O), while the source file includes the mean and standard deviation for all of the measurements of each source’s isotope compositions (for both ²H and ¹⁸O). The model runs a Markov Chain Monte Carlo simulation with a dirichlet prior distribution on the mean. SIAR was run separately for each study catchment. Default settings on the program were used, with 500,000 iterations and the first 50,000 were discarded (Inger et al., nd). The result was a mean fractional contribution of each source to each stream over the study period (2011–2015) and a 95% Bayesian credibility interval. The mean fractional contributions for each of the source waters to streamflow during the study period were plotted against percent catchment area of beetle-killed forest. Statistical significance was assessed using Spearman’s rank order.

An end-member mixing model (EMMA) was attempted using solution specific conductivity and chloride concentrations. Variability in chloride and specific conductivity measurements voided the model (Wehner, 2016).

3 Results

A local meteoric water line (LMWL) was plotted using precipitation isotope data from the Colorado Rocky Mountains from 2011–2015 and fitting a line (d²H= 7.9*d¹⁸O+ 8.9). Measurements of isotopic signature in source waters (groundwater, rain, and snow) showed distinct separations (Fig. 2). Differences in source water composition were used to determine the fractional contribution of each source to streamflow. Mean and standard deviation of the isotope composition for each source over the study period were used as input values in the model. Snow was depleted relative to rain and groundwater in ²H and ¹⁸O, while rainwaters were enriched relative to groundwater and snow, which is consistent with the formation temperature of rain versus snow.

Most of the groundwater and surface water samples were below the LMWL. Evaporation brings about enrichment in heavy isotopes of hydrogen and oxygen, as the lighter isotopes evaporate more readily. In addition, preferential evaporation of ²H in subsurface water brings about depletion in ²H, relative to ¹⁸O, or a lower d-excess value (Gat, 2010).

Results from the source separation for the study period were plotted against beetle-killed forest area and Spearman rank order correlation (r) determined (Fig. 3). The area of beetle-killed forest ranged from 6% to 97% of the catchment area. The MPB epidemic peaked in 2012 and beetle-killed area changed little afterwards. Too few samples were collected to analyze inter-annual variability. Groundwater contributions had a correlation coefficient of 0.515 with beetle-killed forest area (p=0.008). Groundwater contributions ranged from a low of 30% to a high of 95% (Fig. 3). The amount of snow as a source of streamflow decreased with increased beetle killed area with a correlation of –0.507 (p=0.011). Rain contributions to streamflow were low (up to 20%) and were negatively correlated to beetle-killed area, with a correlation coefficient of −0.425 (p=0.035).

4 Discussion

Distinct isotope compositions of source waters and stream water were used to determine a range of probable contribution from each source. The evolution of isotopic composition of precipitation over the season was not considered in the source separation, although variability (expressed as the standard deviation) was included in the model. Rain and snow showed distinct isotopic compositions from each other, as rain was more enriched in heavy isotopes (Fig. 3). Precipitation isotope data were used to develop a LMWL and groundwater samples, tended to be below the line (Menger, 2015). Waters that fall below the LMWL are enriched with ¹⁸O compared with meteoric water due to preferential evaporation of ²H (Ingraham, 1998) (Fig. 3). For a typical snowmelt hydrograph (Fig. 4), the snow pack melts in May and June, infiltrates and displaces older groundwater and contributes the most to streamflow generation. The contribution from precipitation as rain is relatively low.

An increasing groundwater contribution to streamflow after beetle kill is consistent with other studies. An increase in groundwater contribution to streamflow was driven by the cessation of transpiration in killed trees (Pugh and Gordon, 2013). A paired catchment study using specific conductivity and ¹⁸O as tracers detected an increase in groundwater contribution to streamflow corresponding with the onset of beetle kill, and increases were most apparent during the receding limb of the hydrograph, when groundwater is a higher fraction of total runoff (Bearup et al., 2014).

Precipitation (both rain and snow) contributions to streamflow decrease with increasing beetle-killed forest area. This may result partially from the increasing fractional groundwater contribution, given that if groundwater increases in proportional contribution, the precipitation contribution must decrease. This observation may also be explained by process-based phenomena. Decreased canopy can lead to increased snow ablation rates (Boon, 2007, 2009; Teti, 2007) and increased surface evaporation (Biederman et al., 2015). If exposure to sunlight increases surface evaporation of rain and snow, the fractional contribution of these inputs to streamflow would decrease.

In order to explore other variables that may be affecting stream source composition, catchment characteristics were used as exploratory variables to help explain the different source water contributions to streamflow. Catchment slope and catchment area were both insignificantly correlated to groundwater contributions to streamflow (r= −0.141, p= 0.502; r=0.071, p= 0.735, respectively). Only mean catchment elevation (r= −0.533, p= 0.006) was significantly correlated with groundwater contribution. Mean catchment elevation may complicate results, as higher elevation catchments tend to receive more precipitation (Alpert, 1986) and the highest total snow water equivalents are at elevations between 2000 m and 3000 m (Fassnacht et al., 2003).

Our results indicate that changes in source water contributions to streamflow as a result of mountain pine beetle-killed forests do occur. Because bark beetle epidemics are increasing in severity and frequency with increasing winter temperatures (Raffa et al., 2008), it is important to understand the possible implications to the health of water resources. Catchments affected by the mountain pine beetle may experience higher relative groundwater inputs to streamflow compared with rain and snow. Changing source composition may bring about changes in stream water quality, as the chemistry of rain, snow, and groundwater differ from each other, although research suggests that the rapid forest regeneration often associated with beetle kill compared with timber harvesting allows the forest to retain nutrients (Rhoades et al., 2013). Changes to soil water, groundwater, and stream water chemistry may be specific to location, as climate, spatial distribution of mortality, and sub-surface flow paths play a role in biogeochemical cycles in the forest (Mikkelson et al., 2013a). A suggestion is to continue water quality monitoring as the forest regenerates to determine potential water quality changes.

Due to limitations on site access, mean values across the study area for precipitation were used in our analysis. However, it may be useful to sample with higher frequency in order to examine the spatial and temporal variability of precipitation isotope composition. Higher condensation temperatures, a progressively enriched vapor source, and increased rainfall evaporation result in rain becoming more enriched in heavier isotopes of oxygen and hydrogen throughout the summer (Ingraham, 1998). Therefore, it is ideal to account for the variation in isotopic composition of rain over a season. The isotopic composition of snowmelt also changes over the season (Taylor et al., 2002), but was not sampled here due to site access limitations. While our analysis used a mean value of d ¹⁸O, the isotopic composition of snowmelt can differ from that of the snowpack (Hermann and Stichler, 1981; Taylor et al., 2001). The snowpack becomes more enriched in ¹⁸O as snowmelt exits the snowpack. The ¹⁸O enrichment can increase from 1‰–4‰ during snowmelt progression (Feng et al., 2002). It is recommended that future studies incorporate measurements of snowpack and snowmelt isotopic evolution.

This study sampled water during the peak snowmelt and recession into summer low flows on approximately a monthly basis. A catchment specific study with an increased sampling frequency might provide more insight into the source water contributions to streamflow over the entire hydrograph. Sampling with higher frequency may better define source water composition and increase confidence in their fractional source contributions.

5 Conclusions

In this study, samples for isotopic analysis of streamflow and source waters of rain, snow, and groundwater were collected from 25 catchments with varying levels of beetle-killed forest area. It was determined that as the area of beetle-killed forest increased, the contribution of groundwater to streamflow increased. Groundwater contributions had a correlation coefficient of 0.515 with beetle-killed forest area (p=0.008). Groundwater contributions ranged from 30% to 95%. Previous studies conducted during the recent bark beetle epidemic in the north-central Rocky Mountains of Colorado suggest little streamflow response from beetle killed forests other than an increased groundwater contribution to streamflow. The amount of snow as a source of streamflow decreased with increased beetle-killed area with a correlation of –0.507 (p=0.011). Streamflow contribution from precipitation as rain were low (up to 20%) and were negatively correlated to beetle-killed area (r=−0.425, p =0.035). Annual water yield changes are not expected from the change in source water contributions, and downstream water yield should not be affected by the bark beetle epidemic.

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