Patrick J. McHale

Modeling the Effects of Urban Vegetation on Air Pollution

Urban vegetation can directly and indirectly affect local and regional air quality by altering the urban atmospheric environment. Trees affect local air temperature by transpiring water through their leaves, by blocking solar radiation (tree shade), which reduces radiation absorption and heat storage by various anthropogenic surfaces (e. g., buildings, roads), and by altering wind characteristics that affect air dispersion. During the summertime, trees predominantly reduce local air temperatures, but may increase within- and below-canopy air temperature due to reduced turbulent exchange with above-canopy air (Heisler et al., 1995). Reduced air temperature due to trees can improve air quality because the emission of many pollutants and/or precursor chemicals are temperature dependent. Decreased air temperature can also reduce ozone (O3) formation (Cardelino and Chameides, 1990).

Seasonal pattern of dissolved organic matter (DOM) in watershed sources: influence of hydrologic flow paths and autumn leaf fall

Seasonal patterns of dissolved organic matter (DOM) were evaluated for multiple watershed sources and stream water during baseflow and stormflow to investigate the influence of hydrologic flow paths and key phenological events. Watershed sources sampled were throughfall, litter leachate, soil water, and deep groundwater. DOM data for a 4-year period (2008–2011) included: DOC concentrations and spectrofluorometric indices such as a254, humification index, protein-like and humic-like DOM. Seasons were defined as—winter (December–February), spring (March–May), summer (June–September) and autumn (October and November). Seasonal differences in DOM were most pronounced for surficial flow paths (e.g., stormflow, litter leachate, throughfall and soil water) but muted or absent for groundwater and baseflow. This was attributed to the loss of DOM by sorption on mineral soil surfaces and/or microbial breakdown. DOM in summer stormflow had higher DOC concentrations and was more humic in character versus DOM in spring and winter runoff. Storm events in early autumn produced a sharp increase in DOC concentrations and % protein-like DOM for stream waters and litter leachate. Elevated DOC concentrations for early spring throughfall were attributed to leaching of organic exudates associated with leaf emergence. Our results underscore that watershed and ecosystem studies need to pay a greater attention to surficial flow paths and runoff sources (including stormflow) for understanding seasonal patterns of DOM. Understanding the influence of phenological episodes such as autumn leaf-fall for DOM is important considering that these transitional events may be especially affected by climate change.

Effects of Soil Warming on Carbon and Nitrogen Cycling

Atmospheric concentrations of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) have increased dramatically since the beginning of the industrial revolution, largely due to human activities such as fossil fuel combustion and land use change. These gases are currently accumulating in the atmosphere at the rate of approximately 0.40% yr-1 for CO2, 0.60% yr-1 for CH4, and 0.25% yr-1 for N2O (Houghton et al., 1996). Because these gases have the capacity to retain heat in the atmosphere by trapping infrared radiation, considerable concern has been raised that increased concentrations of these gases will result in higher mean global temperatures, or the “greenhouse effect.” Currently, the radiative forcing of climate by greenhouse gases is predicted to increase global mean annual temperature by 1 to 3.5°C in the next 100 years, with a greater warming occurring in the higher latitudes than at the equator (Houghton et al., 1996).

Disturbance effects on soil solution chemistry due to heating cable installation

Installation of heating cables for warming soil was used to evaluate the effect of disturbance on soil solution chemistry within a northern hardwood forest (Adirondack Mountains, New York). Differences in response among treatments suggested the importance of both the depth and timing of cable installation. There were increases (p>0.05) in many solutes within pilot study plots in which “surrogate cable” was installed at 15 cm depth. Most notably, mean nitrate concentrations for the 1st year following disturbance were 744 μeq l-1 at 15 cm depth compared to 7 μeq l-1 for the non-disturbed control. A comparison of pilot plots with 5 cm cable depth and an unheated soil-warming control plot with the same cable disturbance showed that the seasonality of soil disturbance may have a key role in response to disturbance. The soil solution response was diminished if installation occurred during the spring, a period of rapid uptake of nitrogen by vegetation. Mean nitrate concentrations were 176 μeq l-1 for 5-cm pilot plots (installed in fall 1991) versus 6 μeq l-1 for 5-cm, unheated soil-warming control plots (installed in spring 1992). Disturbance effects were attenuated over time and not generally apparent 1 year after installation.

Patterns in solute chemistry of six inlet streams to Lake Hövsgöl, Mongolia

A number of characteristics of the Lake Hovsgol watershed, such as the lakes location at the edge of the Central Asian continuous permafrost zone, provide a unique opportunity to evaluate possible anthropogenic impacts in this remote area in northern Mongolia. In this study, we compared stream solute concentrations in six sub-watersheds in the Lake Hovsgol watershed. Water samples were collected during the summer months between 2003 and 2005. Concentrations concentration at sampling stations with minimal animal grazing ranged from 66 to 294 μmol/L. Average dissolved organic carbon (DOC) concentrations ranged from 642 to 1,180 μmol C/L. We did not find sta - tistically significant differences in DOC concentrations among the six streams, although DOC concentrations tended to be higher in the two northernmost streams, possibly related to differences in the active layer above the permafrost. Dis- solved organic nitrogen (DON) concentrations were correlated with DOC concentration, and followed the same spatial pattern as those for DOC. In streams in this remote watershed, total dissolved nitrogen was made up of mostly organic N, as has been found for other regions distant from anthropogenic N sources. Overall, these results suggest that future research on the dynamics of DOC and DON in this watershed will be especially insightful in helping to understand how changes in climate and land use patterns will affect transformations, retention, and export of dissolved organic matter within these sub-watersheds in the Lake Hovsgol region.

Calcium and aluminum cycling in a temperate broadleaved deciduous forest of the eastern USA: relative impacts of tree species, canopy state, and flux type

Ca/Al molar ratios are commonly used to assess the extent of aluminum stress in forests. This is among the first studies to quantify Ca/Al molar ratios for stemflow. Ca/Al molar ratios in bulk precipitation, throughfall, stemflow, litter leachate, near-trunk soil solution, and soil water were quantified for a deciduous forest in northeastern MD, USA. Data were collected over a 3-year period. The Ca/Al molar ratios in this study were above the threshold for aluminum stress (<1). Fagus grandifolia Ehrh. (American beech) had a median annual stemflow Ca/Al molar ratio of 15.7, with the leafed and leafless values of 12.4 and 19.2, respectively. The corresponding Ca/Al molar ratios for Liriodendron tulipifera L. (yellow poplar) were 11.9 at the annual time scale and 11.9 and 13.6 for leafed and leafless periods, respectively. Bayesian statistical analysis showed no significant effect of canopy state (leafed, leafless) on Ca/Al molar ratios. DOC was consistently an important predictor of calcium, aluminum, and Ca/Al ratios. pH was occasionally an important predictor of calcium and aluminum concentrations, but was not a good predictor of Ca/Al ratio in any of the best-fit models (of >500 examined). This study supplies new data on Ca/Al molar ratios for stemflow from two common deciduous tree species. Future work should examine Ca/Al molar ratios in stemflow of other species and examine both inorganic and organic aluminum species to better gauge the potential for, and understand the dynamics of, aluminum toxicity in the proximal area around tree boles.

Effects of cold storage on anion, ammonium, and total nitrogen concentrations in soil water.

Abstract Researchers frequently must store water samples >24 h after collection until chemical analyses can be accomplished. Samples are commonly stored in darkness at near‐freezing temperatures until analysis, but effects of this storage method on soil water chemistry are not well‐documented. Soil water samples were collected from eastern hemlock [Tsuga canadensis (L.) Carriere] stands and analyzed initially and over time to determine if nitrate (NC3 ‐), chloride (Cl‐), sulfate (SO4 2‐), ammonium (NH4 +), and/or total nitrogen (TN) concentrations changed when samples were stored in darkness at 2–4°C. Subsamples were analyzed <24 h after collection and reanalyzed five times in a 12 to 24 wk period following sample collection. Nitrate concentrations remained stable for at least one wk and were only slightly lower than initial concentrations after 3–16 wk of storage. Chloride and SO4 2‐ remained stable for 16 wk, while TN remained stable for 8 wk and decreased slightly after 24 wk. Most NH4 + concentrations were below the limit of quantification, but concentrations of samples with quantifiable NH4 + decreased in the first week of storage. Overall, cold storage in darkness was effective for preserving NO3 ‐, Cl‐, SO4 2‐, and TN concentrations in soil water, but NH4 + analyses should probably be performed as soon as possible after sample collection.