Irena F. Creed

Regulation of nitrate-N release from temperate forests: A test of the N flushing hypothesis

During the past decade, significant spatial and temporal variability in the release of nitrate-nitrogen (N) from catchments in a sugar maple forest in central Ontario was observed. To explain this variability, we tested the flushing hypothesis [Hornberger et al., 1994], where, when the soil saturation deficit is high, N accumulates in the upper layers of the soil and, as the soil saturation deficit decreases, the formation of a saturated subsurface layer flushes N from the upper layers of the soil into the stream. We used the Regional Hydro-Ecological Simulation System to simulate water, carbon, and N dynamics. A N flushing index was modeled as S/S30, the ratio of the current day saturation deficit to the previous 30-day average saturation deficit. A N source index was modeled as the ratio of N supply/demand. The relationship between the simulated N indices and the observed release of N indicated two mechanisms for the release of N from catchments: (1) a N flushing mechanism, where the N-enriched upper layer of the soil is flushed, after a period of low demand for N by the forest (e.g., during spring snowmelt and autumn stormflow, the water table rising into previously unsaturated parts of a N-enriched soil profile) or after a period of high demand for N by the forest (e.g., during summer droughts, the water table rising into previously saturated parts of a N-impoverished soil profile following a period of enhanced rates of nitrification); and (2) a N draining mechanism, where spring snowmelt recharge of the groundwater translocates N from the upper layer of the soil into deeper hydrological flow pathways that are released slowly over the year.

Vegetation class dependent errors in lidar ground elevation and canopy height estimates in a boreal wetland environment

An airborne scanning light detection and ranging (lidar) survey using a discrete pulse return airborne laser terrain mapper (ALTM) was conducted over the Utikuma boreal wetland area of northern Alberta in August 2002. These data were analysed to quantify vegetation class dependent errors in lidar ground surface elevation and vegetation canopy surface height. The sensitivity of lidar-derived land-cover frictional parameters to these height errors was also investigated. Aquatic vegetation was associated with the largest error in lidar ground surface definition (+0.15 m, SD = 0.22, probability of no difference in height P < 0.01), likely a result of saturated ground conditions. The largest absolute errors in lidar canopy surface height were associated with tall vegetation classes; however, the largest relative errors were associated with low shrub (63%, –0.52 m, P < 0.01) and aquatic vegetation (54%, –0.24 m, P < 0.01) classes. The openness and orientation of vegetation foliage (i.e., minimal projection of horizontal area) were thought to enhance laser pulse canopy surface penetration in these two classes. Raster canopy height models (CHMs) underestimated field heights by between 3% (aspens and black spruce) and 64% (aquatic vegetation). Lidar canopy surface height errors led to hydraulic Darcy–Weisbach friction factor underestimates of 10%–49% for short (<2 m) vegetation classes and overestimates of 12%–41% for taller vegetation classes.

Exploring functional similarity in the export of Nitrate‐N from forested catchments: A mechanistic modeling approach

Functional similarity of catchments implies that we are able to identify the combination of processes that creates a similar response of a specific characteristic of a catchment. We applied the concept of functional similarity to the export of NO3−-N from catchments situated within the Turkey Lakes Watershed, a temperate forest in central Ontario, Canada. Despite the homogeneous nature of the forest, these catchments exhibit substantial variability in the concentrations of NO3−-N in discharge waters, over both time and space. We hypothesized that functional similarity in the export of NO3−-N can be expressed as a function of topographic complexity as topography regulates both the formation and flushing of NO3−-N within the catchment. We tested this hypothesis by exploring whether topographically based similarity indices of the formation and flushing of NO3−-N capture the observed export of NO3−-N over a set of topographically diverse catchments. For catchments with no elevated base concentrations of NO3−-N the similarity indices explained up to 58% of the variance in the export of NO3−-N. For catchments with elevated base concentrations of NO3−-N, prediction of the export of NO3−-N may have been complicated by the fact that hydrology was governed by a two-component till, with an ablation till overlying a basal till. While the similarity indices captured peak NO3−-N concentrations exported from shallow flow paths emanating from the ablation till, they did not capture base NO3−-N concentrations exported from deep flow paths emanating from the basal till, emphasizing the importance of including shallow and deep flow paths in future similarity indices. The strength of the similarity indices is their potential ability to enable us to discriminate catchments that have visually similar surface characteristics but show distinct NO3−-N export responses and, conversely, to group catchments that have visually dissimilar surface characteristics but are functionally similar. Furthermore, the similarity indices provide a potentially powerful method to scale and generalize NO3−-N export responses to other regions.

Predicting export of dissolved organic carbon from forested catchments in glaciated landscapes with shallow soils

[1] This study presents a simple model of dissolved organic carbon (DOC) loading to surface waters that is applicable to headwater catchments in forested regions on glaciated landscapes. Average annual DOC export was highly variable among the 33 experimental catchments along an east-west transect, ranging from 0.90 to 13.74 g C/m 2 /a. It was hypothesized that the proportion of wetlands within the catchments would explain the majority of variation in average annual DOC export. To test this hypothesis, digital terrain analysis was used to derive wetlands automatically under both open and closed forest canopies by identifying the probability of a grid cell being a depression and/or flat. Using a 10 m digital elevation model (DEM) derived from readily available sources, the proportion of wetlands explained 63% of the variance in average annual DOC export among the 33 experimental catchments. Inclusion of regional climatic indicators, including the number of growing degree days (with a base of 10C) and the runoff coefficient, increased explanation of variance from 63% to 89%, once catchments with lakes (>5% of catchment area) adjacent to the catchment outlets were removed. This study shows that DOC export can be predicted accurately from headwater catchments in forested regions on glaciated landscapes using a simple model based on the proportion of wetlands and easily calculated climatic variables.

Ecosystem Processes and Human Influences Regulate Streamflow Response to Climate Change at Long-Term Ecological Research Sites

Analyses of long-term records at 35 headwater basins in the United States and Canada indicate that climate change effects on streamflow are not as clear as might be expected, perhaps because of ecosystem processes and human influences. Evapotranspiration was higher than was predicted by temperature in water-surplus ecosystems and lower than was predicted in water-deficit ecosystems. Streamflow was correlated with climate variability indices (e.g., the El Niño—Southern Oscillation, the Pacific Decadal Oscillation, the North Atlantic Oscillation), especially in seasons when vegetation influences are limited. Air temperature increased significantly at 17 of the 19 sites with 20- to 60-year records, but streamflow trends were directly related to climate trends (through changes in ice and snow) at only 7 sites. Past and present human and natural disturbance, vegetation succession, and human water use can mimic, exacerbate, counteract, or mask the effects of climate change on streamflow, even in reference basins. Long-term ecological research sites are ideal places to disentangle these processes.

Towards a universal lidar canopy height indicator

A light detection and ranging (lidar) canopy height study was conducted with 13 datasets collected using four different models of airborne laser terrain mapper (ALTM) sensors over 13 widely variable vegetation types ranging in average height from <1 m to 24 m at five sites across Canada between 2000 and 2005. The study demonstrates that the vertical standard deviation of all topographically detrended first and last laser pulse returns (LSD) is a robust estimator of canopy height (Ht) for a wide variety of vegetation types and heights and lidar survey configurations. After regressing Ht against LSD for 77 plots and transects, it was found that Ht could be predicted as a simple multiplication (M) of LSD (M = 2.5, coefficient of determination (r2) = 0.95, root mean square error (RMSE) = 1.8 m, tail probability (p) < 0.01). For forest plots only, LSD was found to better predict average tree height (r2 = 0.80, RMSE = 2.1 m, p < 0.01) than Loreys height (r2 = 0.59, RMSE = 3.0 m, p < 0.01). A test of the LSD canopy height model was performed using stand heights (HtFRI) from an independent forest resource inventory (FRI) for four vegetation classes. Results from the raw FRI and modelled stand height comparison displayed close to a 1:1 relationship (HtFRI = 0.97HtLSD, r2 = 0.73, RMSE = 4.7 m, p < 0.01, n = 38). All plot and transect canopy heights were also compared with the localized maxima of laser pulse returns (Lmax). For individual surveys over homogeneous vegetation types, Lmax generally provides a better canopy height indicator. Across all surveys and site types, however, LSD was almost always shown to have a more consistent relationship with actual canopy height. The only observed exception was in the case of forest plot level Loreys mean tree height. The advantages of using a multiplier of LSD to estimate canopy height are its apparent insensitivity to survey configuration and its demonstrated applicability to a range of vegetation types and height classes.

Do geographically isolated wetlands influence landscape functions

Geographically isolated wetlands (GIWs), those surrounded by uplands, exchange materials, energy, and organisms with other elements in hydrological and habitat networks, contributing to landscape functions, such as flow generation, nutrient and sediment retention, and biodiversity support. GIWs constitute most of the wetlands in many North American landscapes, provide a disproportionately large fraction of wetland edges where many functions are enhanced, and form complexes with other water bodies to create spatial and temporal heterogeneity in the timing, flow paths, and magnitude of network connectivity. These attributes signal a critical role for GIWs in sustaining a portfolio of landscape functions, but legal protections remain weak despite preferential loss from many landscapes. GIWs lack persistent surface water connections, but this condition does not imply the absence of hydrological, biogeochemical, and biological exchanges with nearby and downstream waters. Although hydrological and biogeochemical connectivity is often episodic or slow (e.g., via groundwater), hydrologic continuity and limited evaporative solute enrichment suggest both flow generation and solute and sediment retention. Similarly, whereas biological connectivity usually requires overland dispersal, numerous organisms, including many rare or threatened species, use both GIWs and downstream waters at different times or life stages, suggesting that GIWs are critical elements of landscape habitat mosaics. Indeed, weaker hydrologic connectivity with downstream waters and constrained biological connectivity with other landscape elements are precisely what enhances some GIW functions and enables others. Based on analysis of wetland geography and synthesis of wetland functions, we argue that sustaining landscape functions requires conserving the entire continuum of wetland connectivity, including GIWs.

Advances in Canadian forest hydrology, 1995-1998

Approximately 42% of Canada is covered by forests, which in turn can be subdivided into nine distinct forest ecozones. Many forested ecozones are located in northern Canada, where cold winters and cool summers provide forest environments that are less well-understood that those in more temperate locations. A number of major developments in recent years have stressed the need for enhanced understanding of hydrological processes in these forest landscapes. These include an increased emphasis on sustainable forest management in Canada as well as major scientific initiatives (e.g. BOREAS) examining water, carbon and energy fluxes in forest ecosystems, with a particular focus on boreal and subarctic forests. Recent progress in our understanding of forest hydrology across Canada is reviewed. Studies of hydrological processes across the spectrum of forest ecozones are highlighted, as well as work on hydrological responses to forest disturbance and recovery. Links between studies of hydrological processes in Canadas forests and other fields of research are examined, with particular attention paid to ongoing efforts to model hydrological impacts and interactions with the climate, biogeochemistry, geomorphology and ecology of forested landscapes.

Distinguishing actual and artefact depressions in digital elevation data

Abstract Topographic depressions in digital elevation models (DEMs) are frequently a combination of artefacts and actual features. It is common practice to remove all digital depressions, from DEMs that are used in hydrogeomorphic applications. This practice is inappropriate because actual depressions affect many of the environmental phenomena at study. Nonetheless, indiscriminate depression removal persists because of an inability to distinguish artefacts from actual depressions. Five potential approaches for distinguishing artefacts from actual depressions in DEMs are described in this paper: ground inspection, examining the source data, classification approaches, knowledge-based approaches, and modelling approaches. Of the five methods, ground inspection was the only approach that actually confirms the existence of digital depressions. The other four methods that were identified operate by establishing justification for why a digital depression is likely to be an artefact or actual depression. A comparison of the depression validation approaches for a small sub-catchment on the Canadian Shield showed that the modelling approach performed slightly better than the other methods. While being highly automated and applicable to all landscape types, this approach also explicitly handles DEM uncertainty. By applying the Monte Carlo method, this approach estimates the likelihood of a digital depression actually occurring in the landscape given the degree of uncertainty in local topography. After artefact and actual depressions are identified, it is then possible to remove the artefacts and to preserve the real features for incorporation into modelling.

Changing forest water yields in response to climate warming: results from long-term experimental watershed sites across North America.

Climate warming is projected to affect forest water yields but the effects are expected to vary. We investigated how forest type and age affect water yield resilience to climate warming. To answer this question, we examined the variability in historical water yields at long-term experimental catchments across Canada and the United States over 5-year cool and warm periods. Using the theoretical framework of the Budyko curve, we calculated the effects of climate warming on the annual partitioning of precipitation (P) into evapotranspiration (ET) and water yield. Deviation (d) was defined as a catchments change in actual ET divided by P [AET/P; evaporative index (EI)] coincident with a shift from a cool to a warm period – a positive d indicates an upward shift in EI and smaller than expected water yields, and a negative d indicates a downward shift in EI and larger than expected water yields. Elasticity was defined as the ratio of interannual variation in potential ET divided by P (PET/P; dryness index) to interannual variation in the EI – high elasticity indicates low d despite large range in drying index (i.e., resilient water yields), low elasticity indicates high d despite small range in drying index (i.e., nonresilient water yields). Although the data needed to fully evaluate ecosystems based on these metrics are limited, we were able to identify some characteristics of response among forest types. Alpine sites showed the greatest sensitivity to climate warming with any warming leading to increased water yields. Conifer forests included catchments with lowest elasticity and stable to larger water yields. Deciduous forests included catchments with intermediate elasticity and stable to smaller water yields. Mixed coniferous/deciduous forests included catchments with highest elasticity and stable water yields. Forest type appeared to influence the resilience of catchment water yields to climate warming, with conifer and deciduous catchments more susceptible to climate warming than the more diverse mixed forest catchments.