Impact of Urbanization on Large Wood Sizes and Associated Recruitment Zones

Matthew T Allen,Philippe G Vidon

doi:10.3390/hydrology7040089

Abstract

Although the presence of large wood (LW) has long been recognized to enhance watershed function, land use impact on LW remains poorly understood. Using a series of six watersheds, we investigate the relationships between LW recruitment zones, LW size, and LW jam occurrence and land use. Although the results in general show urban land use to severely limit LW, they also stress that agricultural land use may be positively correlated to LW. Occurrence of potentially productive LW recruitment zones is nevertheless best correlated to total forest land cover and forested riparian area. However, the lack of mature forest due to previous widespread deforestation linked to historical agricultural land use is likely a limiting legacy effect. Since the pattern of land use seen in the study area is typical of much of the developed world, our results suggest the limiting of LW may be a major way in which watersheds are impacted in many regions. Accordingly, reintroduction of LW represents a significant opportunity to restore watersheds on a broad scale. Specifically, we propose a mix of passive conservation and active restoration of LW sources and that the targeting of these tactics be planned using the spatial analysis methods of this study.

Highlights

Large wood has long been recognized as having an important influence on physical and ecological processes in stream channels and watersheds of all orders and gradients [1,2,3,4]
Agricultural land use has been tied in many cases to watershed degradation owing to, among other things, excessive nutrient and pesticide losses to streams [36,37], our results suggest that, in contrast to urban/suburban land use, agricultural land use as it is in the study area may not necessarily negatively impact Large wood (LW) recruitment nor Aquatic Habitat Viability Rating
The results linked high population density, elevated percentage of impervious surface, and high degree of urbanization to a lack of potentially functional recruitment zones for LW, they reveal that agricultural land use in the study area is positively correlated to the occurrence of forested riparian areas at the watershed scale

Summary

Introduction

Large wood ( known in the literature as large woody debris or LWD) has long been recognized as having an important influence on physical and ecological processes in stream channels and watersheds of all orders and gradients [1,2,3,4]. The presence of LW allows for the accumulation and storing of coarse organic matter, which serves as a vital ecological energy base and enhances biogeochemical cycling of nutrients and other constituents in watershed ecosystems [3,7,11,12]. By directing flow into the streambed, LW can increase hyporheic exchange, which is known to enhance water quality [15,16]. Jams can initiate lateral channel migration by increasing the elevation of the streambed and water surface. The presence and frequency of jams is thought to be an indicator of stream wood transport capacity [3,6]

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Results

Discussion

Conclusion