Charles C. Rhoades

Biogeochemistry of beetle-killed forests: Explaining a weak nitrate response

A current pine beetle infestation has caused extensive mortality of lodgepole pine (Pinus contorta) in forests of Colorado and Wyoming; it is part of an unprecedented multispecies beetle outbreak extending from Mexico to Canada. In United States and European watersheds, where atmospheric deposition of inorganic N is moderate to low (<10 kg⋅ha⋅y), disturbance of forests by timber harvest or violent storms causes an increase in stream nitrate concentration that typically is close to 400% of predisturbance concentrations. In contrast, no significant increase in streamwater nitrate concentrations has occurred following extensive tree mortality caused by the mountain pine beetle in Colorado. A model of nitrate release from Colorado watersheds calibrated with field data indicates that stimulation of nitrate uptake by vegetation components unaffected by beetles accounts for significant nitrate retention in beetle-infested watersheds. The combination of low atmospheric N deposition (<10 kg⋅ha⋅y), tree mortality spread over multiple years, and high compensatory capacity associated with undisturbed residual vegetation and soils explains the ability of these beetle-infested watersheds to retain nitrate despite catastrophic mortality of the dominant canopy tree species.

The influence of wildfire extent and severity on streamwater chemistry, sediment and temperature following the Hayman Fire, Colorado

The 2002 Hayman Fire was the largest fire in recent Colorado history (558km 2 ). The extent of high severity combustion and possible effects on Denvers water supply focussed public attention on the effects of wildfire on water quality.Wemonitoredstreamchemistry,temperatureandsedimentbeforethefireandatmonthlyintervalsfor5yearsafter the fire. The proportional extent of a basin that was burned or that burned at high severity was closely related to post-fire streamwater nitrate and turbidity. Basins that burned at high severity on 445% of their area had twice the streamwater nitrate and four times the turbidity as basins burned to a lower extent; these analytes remained elevated through 5 years post-fire. In those basins, the highest post-fire streamwater nitrate concentrations (23% of USA drinking water standards) were measured during spring, the peak discharge period. Summer streamwater was 4.08C higher in burned streams on average compared with unburned streams; these persistent post-fire stream temperature increases are probably sufficient to alter aquatic habitat suitability. Owing to the slow pace of tree colonisation and forest regrowth, recovery of the watersheds burned by the Hayman Fire will continue for decades.

Clear cutting and burning affect nitrogen supply, phosphorus fractions and seedling growth in soils from a Wyoming lodgepole pine forest

Abstract Timber harvesting, with and without prescribed slash fire, and wild fire are common disturbances in pine forests of western North America. These disturbances can alter soil nitrogen (N) pools and N supply to colonizing vegetation, but their influence remains poorly understood for many forests. We investigated the effects of clear cut harvesting and fire on KCl extractable N pools, net N mineralization rates, phosphorus (P) fractions, seedling N uptake, and seedling growth in mineral soils sampled from a lodgepole pine forest in southern Wyoming. At a site where wild fire burned through a harvested stand of lodgepole pine and the adjacent intact forest, we analyzed mineral soils from the following four treatments: unburned clear cut, burnt clear cut, unburned forest, and burnt forest. Soils from unburned and burnt clear cut treatments had higher concentrations of KCl extractable N and higher net N mineralization rates, and produced larger pine seedlings in bioassays than soils from unburned and burnt intact forest treatments. Further, while seedlings grown in soils from the unburned and burnt forest treatments responded strongly to N fertilization, seedlings grown in clear-cut soils did not respond to fertilization. Taken together, these results suggest that harvesting had increased soil N supply. In comparing clear cut treatments, soils from the unburned clear cut had smaller extractable N and P pools, and lower net N mineralization rates, but produced larger pine seedlings than soils from the burnt clear cut.

Forest biogeochemistry in response to drought

Trees alter their use and allocation of nutrients in response to drought, and changes in soil nutrient cycling and trace gas flux (N2 O and CH4 ) are observed when experimental drought is imposed on forests. In extreme droughts, trees are increasingly susceptible to attack by pests and pathogens, which can lead to major changes in nutrient flux to the soil. Extreme droughts often lead to more common and more intense forest fires, causing dramatic changes in the nutrient storage and loss from forest ecosystems. Changes in the future manifestation of drought will affect carbon uptake and storage in forests, leading to feedbacks to the Earths climate system. We must improve the recognition of drought in nature, our ability to manage our forests in the face of drought, and the parameterization of drought in earth system models for improved predictions of carbon uptake and storage in the worlds forests.

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

Abstract 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.

Rehabilitating Slash Pile Burn Scars in Upper Montane Forests of the Colorado Front Range

ABSTRACT: Slash pile burning is widely conducted by land managers to dispose of unwanted woody fuels, yet this practice typically has undesirable ecological impacts. Simple rehabilitation treatments may be effective at ameliorating some of the negative impacts of pile burning on plants and soils. Here, we investigated: (1) the impacts of slash pile burning on soil nitrogen and understory plant species richness and cover in Colorado Pinus contorta Douglas ex Louden and P. contorta — Populus tremuloides Michx. stands; (2) the effectiveness of woodchip mulch and soil scarification at reversing pile burning impacts on soil nitrogen; and (3) how mulching and scarifying, alone and in conjunction with native grass seeding, promote native plant establishment and discourage exotic invasion in burn scars. We found that pile burning diminished native richness and cover and increased soil nitrogen, particularly in the interior of burn scars where fire severity was greatest. Rehabilitation treatments appear to be useful tools for reversing pile burning impacts on soil and plants. Mulching dampened the increase in soil nitrogen; and scarifying, scarifying plus seeding, and mulching plus seeding were effective at encouraging native plant development while simultaneously minimizing exotic plant colonization.

Influence of Bedrock Geology on Water Chemistry of Slope Wetlands and Headwater Streams in the Southern Rocky Mountains

We characterized the water chemistry of nine slope wetlands and adjacent headwater streams in Colorado subalpine forests and compared sites in basins formed on crystalline bedrock with those formed in basins with a mixture of crystalline and sedimentary bedrock. The pH, Ca2+, Mg2+, NH4+, acid neutralizing capacity, and electrical conductivity of wetland porewater and streamwater were higher in the basins with mixed geology. Bryophyte cover was higher in lower pH, crystalline basins, and vascular plant cover was higher in the mixed bedrock basins. On average, wetland porewater had lower pH and higher concentrations of dissolved organic carbon (DOC) and nitrogen and several other ions than streamwater; however, because discharge from these small wetlands is low, their direct influence on stream solute concentrations was generally undetectable. Dilution altered stream solute concentrations during peak flow in both basin types, but had little effect on wetland chemistry. In contrast to other solutes, the concentration of DOC in streams increased marginally during peak runoff and its concentration in wetland porewater was stable throughout the year. These findings further knowledge of the influence of watershed characteristics on wetland and stream chemistry and will inform future decisions regarding conservation and management in headwater basins.

Examining the Potential of Forest Residue-Based Amendments for Post-Wildfire Rehabilitation in Colorado, USA

Wildfire is a natural disturbance, though elemental losses and changes that occur during combustion and post-fire erosion can have long-term impacts on soil properties, ecosystem productivity, and watershed condition. Here we evaluate the potential of forest residue-based materials to rehabilitate burned soils. We compare soil nutrient and water availability, and plant recovery after application of 37 t ha−1 of wood mulch, 20 t ha−1 of biochar, and the combination of the two amendments with untreated, burned soils. We also conducted a greenhouse trial to examine how biochar influenced soil nutrient and water content under two wetting regimes. The effects of wood mulch on plant-available soil N and water content were significant and seasonally consistent during the three-year field study. Biochar applied alone had few effects under field conditions, but significantly increased soil pH, Ca, P, and water in the greenhouse. The mulched biochar treatment had the greatest effects on soil N and water availability and increased cover of the most abundant native plant. We found that rehabilitation treatments consisting of forest residue-based products have potential to enhance soil N and water dynamics and plant recovery following severe wildfire and may be justified where erosion risk or water supply protection are crucial.

Evaluating Controls on Nutrient Retention and Export in Wide and Narrow Valley Segments of a Mountain River Corridor

Over the past few decades, nitrate-nitrogen (NO3-N) concentrations have increased within streams of the central Rockies, a pattern linked to regional N deposition trends. As NO3-N concentrations increase, in-stream biological demand may become saturated and stream N export may increase. In mountain landscapes, streams generally flow through steep, narrow valleys with limited riparian area and strong stream-hillslope connectivity. Interspersed between the narrow valleys are wide segments where substantial floodplain riparian areas can develop. Here, we coupled measures of stream reach NO3-N flux balances with nutrient enrichment experiments along two stream reaches of contrasting valley morphology in Rocky Mountain National Park. The stream reaches were (1) a narrow valley segment with limited floodplain riparian area and (2) a longitudinally adjacent (directly downstream) wide valley segment with extensive floodplain riparian area. We found that in-stream biological uptake of added NO3-N was limited in both segments, presumably as a consequence of saturating conditions. Assessment of mass flux indicated that the narrow valley segment was a consistent source of water and NO3-N across flow states, while the wide segment was a sink at high flow and a source at low flow. Due to low in-stream biological retention, gross gains and losses of water and NO3-N to and from the stream exerted primary constraint on segment mass balances. Our results suggest that the exchange of water and nutrients between the stream and adjacent landscape can exert strong control on reach-scale nutrient export, particularly in streams experiencing or approaching N saturation.

The role of experimental forests and ranges in the development of ecosystem science and biogeochemical cycling research [Chapter 17]

Forest Service watershed-based Experimental Forests and Ranges (EFRs) have significantly advanced scientific knowledge on ecosystem structure and function through long-term monitoring and experimental research on hydrologic and biogeochemical cycling processes. Research conducted in the 1940s and 1950s began as “classic” paired watershed studies. The emergence of the concept of ecosystem science in the 1950s and 1960s, the passage of the Clean Air Act and Clean Water Act in the 1970s, the nonpoint source pollution provision enacted in the Federal Water Pollution Control Act, and various other forces led to an increased interest in biogeochemical cycling processes. The ecosystem concept recognized that water, nutrient, and carbon cycles were tightly linked, and interdisciplinary approaches that examined the roles of soil, vegetation, and associated biota, as well as the atmospheric environment, were needed to understand these linkages. In addition to providing a basic understanding, several watershed-based EFRs have been at the core of the development and application of watershed ecosystem analysis to ecosystem management, and they continue to provide science to land managers and policy makers. The relevance and usefulness of watershed-based EFRs will only increase in the coming years. Stressors such as climate change and increased climate variability, invasive and noninvasive insects and diseases, and the pressures of population growth and land-use change increase the value of long-term records for detecting resultant changes in ecosystem structure and function.