Riparian forest structure and stream geomorphic condition: implications for flood resilience

Managing riparian corridors for flood resilience requires understanding of linkages between vegetation condition and stream geomorphology. Stream assessment approaches increasingly use channel morphology as an indicator of stream condition, with only cursory examination of riparian vegetation. Our research (i) examines relationships between stream geomorphic condition, as assessed by Rapid Geomorphic Assessment (RGA) scores, and riparian forest structure, and (ii) investigates scale dependencies in the linkages between land cover and stream geomorphology. We sampled vegetation structure and composition and assessed geomorphic condition at 32 stream reaches within the Lake Champlain Basin, USA. RGA scores were modeled as a function of structural attributes using classification and regression trees. Landsat coverages were used to delineate land uses within five nested spatial scales. Generalized linear models (GLM) evaluated relationships between land cover and RGA scores. Standard deviation of basal area partitioned the greatest variability in RGA scores, but dead tree density and basal area (positively) and shrub density (negatively) were also significant predictors. RGA was related to forest and agricultural cover at the two finest scales. Riparian forest structure is highly dynamic in relation to stand development and disturbance history; simple forest cover information does not capture these differences or their influences on stream geomorphic condition.

The role of watershed characteristics, permafrost thaw, and wildfire on dissolved organic carbon biodegradability and water chemistry in Arctic headwater streams

In the Alaskan Arctic, rapid climate change is increasing the frequency of disturbance including wildfire and permafrost collapse. These pulse disturbances may influence the delivery of dissolved organic carbon (DOC) to aquatic ecosystems, however the magnitude of these effects compared to the natural background variability of DOC at the watershed scale is not well known. We measured DOC quantity, composition, and biodegradability from 14 river and stream reaches (watershed sizes ranging from 1.5–167 km2) some of which were impacted by permafrost collapse (thermokarst) and fire. We found that region had a significant impact on quantity and biodegradability of DOC, likely driven by landscape and watershed characteristics such as lithology, soil and vegetation type, elevation, and glacial age. However, contrary to our hypothesis, we found that streams disturbed by thermokarst and fire did not contain significantly altered labile DOC fractions compared to adjacent reference waters, potentially due to rapid ecosystem recovery after fire and thermokarst as well as the limited spatial extent of thermokarst. Overall, biodegradable DOC ranged from 4 to 46 % and contrary to patterns of DOC biodegradability in large Arctic rivers, seasonal variation in DOC biodegradability showed no clear pattern between sites, potentially related to stream geomorphology and position along the river network. While thermokarst and fire can alter DOC quantity and biodegradability at the scale of the feature, we conclude that tundra ecosystems are resilient to these types of disturbance.

The role of watershed characteristics, permafrost thaw, and wildfire on dissolved organic carbon biodegradability and water chemistry in Arctic headwater streams

In the Alaskan Arctic, rapid climate change is increasing the frequency of disturbance including wildfire and permafrost collapse. These pulse disturbances may influence the delivery of dissolved organic carbon (DOC) to aquatic ecosystems, however the magnitude of these effects compared to the natural background variability of DOC at the watershed scale is not well known. We measured DOC quantity, composition, and biodegradability from 14 river and stream reaches (watershed sizes ranging from 1.5–167 km2) some of which were impacted by permafrost collapse (thermokarst) and fire. We found that region had a significant impact on quantity and biodegradability of DOC, likely driven by landscape and watershed characteristics such as lithology, soil and vegetation type, elevation, and glacial age. However, contrary to our hypothesis, we found that streams disturbed by thermokarst and fire did not contain significantly altered labile DOC fractions compared to adjacent reference waters, potentially due to rapid ecosystem recovery after fire and thermokarst as well as the limited spatial extent of thermokarst. Overall, biodegradable DOC ranged from 4 to 46% and contrary to patterns of DOC biodegradability in large Arctic rivers, seasonal variation in DOC biodegradability showed no clear pattern between sites, potentially related to stream geomorphology and position along the river network. While thermokarst and fire can alter DOC quantity and biodegradability at the scale of the feature, we conclude that tundra ecosystems are resilient to these types of disturbance.

Reply to comment by Jackson and Martin on “Does timber harvest influence the dynamics of marine-derived nutrients in Southeast Alaska streams?” 1Original article by Levi et al. appears in Can. J. Fish. Aquat. Sci. 68(8): 1316–1329 and is available at http://www.nrcresearchpress.com/doi/full/10.1139/f2011-067. Comment by Jackson and Martin appears in Can. J. Fish. Aquat. Sci. 69: this issue, and is available at http://www.nrcresearchpress.com/doi/full/10.1139/f2012-104.

The stated goal of Levi et al. (2011, Can. J. Fish. Aquat. Sci. 68: 1316–1329) was to determine the influence of geomorphic complexity on the dynamics of salmon-derived nutrients. We studied seven streams in Southeast Alaska with varying degrees of historical (mid-1900s) timber harvest and, as a result, differences in stream geomorphology. In a comment on our study, Jackson and Martin (2012, Can. J. Fish. Aquat. Sci. 69: this issue) suggest that the geomorphic complexity we ascribe to timber harvest may be due to natural variation in watershed characteristics and offer alternative hypotheses. We sought to reduce the natural variation among our study streams by using a stratified sampling design (i.e., selecting reaches classified as floodplain 4 or 5 by the USDA Forest Service), but acknowledge that, as with any ecological field study, alternative hypotheses may exist to explain observed patterns in ecological responses. We maintain that our study design was sufficiently robust (i.e., 300 m reaches studied in seven streams for 3 years, totaling 21 stream-years) to draw inferences about the influence of salmon on streamwater nutrients and, secondarily, the role of geomorphic variation in mediating nutrient dynamics. Our data also support our finding that the legacy of timber harvest altered nutrient dynamics in salmon-bearing streams via alterations to stream geomorphology that were quantifiable.