Hiking up to the next plot…
Examines how bark beetle disturbance and conifer needle litter decomposition across elevation gradients affect biogeochemical processes and downstream drinking water quality in Colorado watersheds.
Forests in the Rocky Mountains do far more than provide habitat and timber — they regulate the timing, quantity, and quality of water flowing from headwater catchments to millions of downstream users. In the Gunnison Basin, lodgepole pine (Pinus contorta) and Engelmann spruce dominate large swaths of subalpine forest, and what happens in their canopies, needle litter, and rhizospheres ultimately shapes the chemistry of creeks like the East River, Coal Creek, and Slate Creek. Over the past two decades, this coupled tree-to-tap system has come under stress from warming temperatures, earlier snowmelt, and large outbreaks of bark beetles, making it an urgent topic for both researchers and water managers.
A few key concepts are essential for understanding the findings below. Bark beetle disturbance refers to the widespread tree mortality caused by native beetles (such as the mountain pine beetle) whose populations have surged with warmer winters; when beetles kill a stand, needles drop in a pulse, canopy cover is lost, and the belowground activity of living roots ceases. These shifts can alter how carbon and nitrogen cycle through soils and how much organic matter washes into streams. Dissolved organic matter that leaves forested watersheds matters not just ecologically but also for human health: when chlorine-based treatment is applied to drinking water containing this organic material, it reacts to form disinfection byproducts (DBPs) — regulated compounds such as haloacetic acids (HAA5) and trihalomethanes that can pose health risks at elevated concentrations. Researchers measure a water sample's DBP formation potential by applying a standardized chlorination procedure in the lab, which reveals how reactive the organic matter is before it ever reaches a treatment plant.
Finally, two place-based references appear throughout this work. The Pitch Project refers to a historic mining-impacted site in the basin that provides context for watershed contaminant studies, and the East River and Coal Creek catchments near Crested Butte and Gothic serve as the primary outdoor laboratories where tree-scale experiments are scaled up to whole-watershed measurements.
Early research on how forest disturbance alters water chemistry in Colorado was anchored by a regional study of mountain pine beetle impacts on soil and stream chemistry in Grand County (Clow et al., 2011). That work established baseline expectations that beetle-driven mortality could shift nutrient export from forested catchments, and it set the stage for more mechanistic studies in the Gunnison Basin. Around the same time, a watershed-modeling effort in the East River headwaters used the ParFlow integrated hydrologic model at 10-meter resolution to project how climate warming and vegetation succession would propagate through snowpack, soil moisture, and streamflow , framing the headwaters of the upper Colorado River as a bellwether for downstream water supply.
Standard litterbag method deployed across elevation gradient to study litter decomposition rates and microbial succession. Involves placing known mass...
Fourier Transform Infrared spectroscopy analysis of ground needle samples to quantify functional group concentrations including aromatics, ethers, ami...
Standardized chlorination procedure to simulate drinking water treatment and quantify the potential for organic matter to form regulated disinfection ...
Spatially distributed sampling campaign conducted during peak hydrologic conditions to identify locations of enhanced dissolved organic carbon export ...
This dataset is associated with the publication “Disinfection byproducts formed during drinking water treatment reveal an export control point f...
This data package contains text files that describe geochemical measurements collected from 2017-2019 during isolated conifer needle decomposition fie...
Complementary ecological foundations came from comparative vegetation studies of lodgepole and aspen stands (Gray & Nilson, 1999) and from work on the wildlife that depends on conifer patches, including American red squirrels whose middens track the distribution and quality of cone-bearing stands (Peterson-Trujillo, 2016). Together, these studies linked forest structure, species composition, and disturbance to both ecosystem function and downstream water resources.
A central insight from recent Gunnison Basin research is that tree species identity, more than beetle status per se, governs how needle litter influences soil biogeochemistry. In a controlled decomposition experiment, lodgepole needles proved less recalcitrant than spruce needles (lower carbon-to-nitrogen ratio), produced the highest CO2 flux during wet periods, and shaped distinct soil microbial communities (Leonard et al., 2020). Importantly, needles from beetle-killed spruce decomposed almost identically to needles from healthy spruce, suggesting that the biogeochemical fingerprint of bark beetle disturbance is driven less by changes in needle chemistry itself and more by the loss of canopy cover and root activity (Leonard et al., 2020).
Follow-up work across a 700-meter elevation gradient near Crested Butte found that needle mass loss and chemistry were remarkably consistent regardless of elevation or soil type, with three-year mass losses of about 21 percent for lodgepole and 16 percent for spruce (Leonard et al., 2021). Soil bacterial communities differed across elevations and gained diversity during dry periods — a sign of microbial resilience — while fungal communities remained more stable (Leonard et al., 2021). Experimentally accelerating snowmelt by 2-3 weeks using a permeable black geotextile fabric, a low-cost protocol that reliably advances melt timing by 14-23 days (Leonard et al., 2020), produced surprisingly modest effects on decomposition rates and microbial composition, though it did increase dissolved organic carbon in lodgepole porewater (Leonard et al., 2021).
At the watershed scale, these tree- and soil-level processes translate directly into drinking water quality. A 15-year record from the Crested Butte water treatment facility, which draws from Coal Creek, revealed a long-term rise in haloacetic acid formation and repeated annual peaks in disinfection byproducts coinciding with spring snowmelt and dissolved organic carbon mobilization (Leonard et al., 2022). Synoptic sampling across the watershed pinpointed a biogeochemical hotspot for organic carbon export downstream of the Ohio fork, carrying a distinctive fulvic acid-like fluorescence signature, and laboratory chlorination experiments identified lignin-like and condensed hydrocarbon-like molecules as the chemical classes most reactive with chlorine (Leonard et al., 2022). The forest-to-faucet linkage is also reinforced by the recognition that tree species and stand structure control how much snow is intercepted and sublimated in the canopy before it ever reaches the soil (Beutler, 2024), and that habitat patch size and shape determine where keystone seed predators like red squirrels persist (Peterson-Trujillo, 2016).
Early work in the 1990s and 2000s established that conifer stands differ in understory composition (Gray & Nilson, 1999) and that beetle outbreaks measurably alter water chemistry (Clow et al., 2011). Research since 2020 has shifted toward integrating tree-scale process experiments with watershed-scale water quality monitoring and drinking water treatment outcomes. The synthesis dissertation by Leonard (Leonard, 2021) explicitly framed this "tree to tap" trajectory, arguing that biogeochemical shifts in beetle-impacted forests are mediated more by canopy and rhizosphere loss than by needle chemistry alone.
The most recent work is pushing into new measurement domains. Terrestrial laser scanning is being used to quantify canopy snow interception across multiple conifer species and aspen stands, expanding observations from individual trees to whole forest patches (Beutler, 2024). Standardized DBP formation potential assays and high-resolution mass spectrometry (FTICR-MS) are revealing which molecular classes drive water treatment risks (Leonard et al., 2022), while accelerated-snowmelt field protocols offer a reproducible way to simulate future climate conditions in situ (Leonard et al., 2020).
Several important questions remain. How will compounding disturbances — beetle mortality, drought, earlier snowmelt, and wildfire — interact to determine the timing and magnitude of dissolved organic matter export, and can biogeochemical hotspots like the one identified on Coal Creek be predicted before they emerge? Why do fungal communities appear more stable than bacterial communities under environmental stress, and what does that mean for long-term decomposition under continued warming? How much of the canopy snowpack will be lost to sublimation as forest composition shifts from conifer to aspen or to beetle-killed openings, and what are the downstream consequences for streamflow timing? Finally, can land managers and water utilities use early-season organic matter signatures to anticipate disinfection byproduct exceedances, closing the loop between forest health monitoring and public health protection? Addressing these questions will require sustained, cross-scale collaboration between forest ecologists, hydrologists, biogeochemists, and the communities that depend on Gunnison Basin water.
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Gray, J. & Nilson, K. (1999). Comparison of the understory of an isolated stand of lodgepole pine, Pinus contorta, to the understory of an adjacent stand of Populus tremuloides. →
Leonard, L. T. (2021). From tree to tap: The impacts of climate change on biogeochemical processes during conifer needle distribution and broader implications for water quality in Colorado. →
Leonard, L. T. et al. (2020). A comparison of lodgepole and spruce needle chemistry impacts on terrestrial biogeochemical processes during isolated decomposition. PeerJ. →
Leonard, L. T. et al. (2020). Accelerated snowmelt protocol to simulate climate change induced impacts on snowpack dependent ecosystems. Bio-protocol. →
Leonard, L. T. et al. (2021). Effect of elevation, season and accelerated snowmelt on biogeochemical processes during isolated conifer needle litter decomposition. PeerJ. →
Leonard, L. T. et al. (2022). Disinfection byproducts formed during drinking water treatment reveal an export control point for dissolved organic matter in a subalpine headwater stream. Water Research X. →
Peterson-Trujillo, K. (2016). Habitat patch use, density, and territoriality of American Red Squirrels (Tamiasciurus hudsonicus) in the southwestern Rocky Mountains, Colorado. →
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