Extent and spatial patterns of grass bald land cov

Authors: Harold S J Zald

Publish Date: 2008/10/02

Volume: 201, Issue: 2, Pages: 517-529

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Abstract

Globally temperate grasslands and meadows have sharply declined in spatial extent Loss and fragmentation of grasslands and meadows may impact biodiversity carbon storage energy balance and climate change In the Pacific Northwest region of North America grasslands and meadows have declined in extent over the past century Largely undocumented in this regional decline are the grass balds of the Oregon Coast Range isolated grasslands in a landscape dominated by coniferous forests This study was conducted to quantify the spatial extent and patterns of grass bald change Five balds in the Oregon Coast Range were evaluated using historical aerial photographs and recent digital orthophoto quadrangles DOQ Over the time period of study 1948/1953 to 1994/2000 bald area declined by 66 primarily from forest encroachment The number and average size of bald vegetation patches declined while edge density increased Tree encroachment into balds was inversely related to distance from nearest potential parent trees Spatial patterns of bald loss may result from a forest to bald gradient of unfavorable environmental conditions for tree establishment and/or seed dispersal limitation Species dependent on balds may be at risk from loss of bald area and increased fragmentation although metrics of habitat fragmentation may not reflect speciesspecific habitat requirements Tree encroachment patterns and increased bald edge densities suggest increasing rates of bald loss in the future The remote sensing nature of this study cannot determine the fundamental causes of bald decline although prior research suggests climate change cessation of native burning successional changes in response to prior wildfires and cessation of livestock grazing all may have potential influenceThe author acknowledges Colin Kelly at the University of Oregon Map Library for assistance locating and organizing hardcopy historical aerial photographs Special thanks to Jonathan Thompson for helpful advice regarding image enhancement and spatial statistics Funding for this study was provide by the US Forest Service Pacific Northwest Forest Inventory and Analysis Program Additional thanks to Tom Spies Andy Gray Rob Pabst and two anonymous reviewers for providing thoughtful comments and suggestions on prior versions of this manuscriptHistorical panchromatic 112000 scale aerial photographs were acquired from the University of Oregon Map Library in Eugene OR All historical aerial photographs were taken during the summer of 1948 except Mount Hebo taken during the summer of 1953 Historical aerial photographs were scanned on a flatbed scanner at 600 dpi Scanned historical images had 8bit radiometric resolution and 063–089 m postrectification spatial resolution resampled to 1 m pixel size after georectification using nearest neighbor interpolation Recent images of the study areas are panchromatic digital orthophoto quadrangles DOQs DOQs were downloaded from TerraServerUSA http//terraserverusacom/defaultaspx The Mount Hebo Marys Peak Grass Mountain and Prairie Peak DOQs were taken in the summer of 1994 except Bald Mountain taken during the summer of 2000 DOQs were in Universal Transverse Mercator Projection UTM Zone 10N using the Geodetic Reference System Spheroid of 1980 and the North American Datum of 1983 All DOQs had 8bit radiometric resolution and 1 m spatial resolution

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