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Using aerial photographs to remotely assess tree hollow availability

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Abstract

Tree hollows provide critical habitat for many species worldwide. The conservation of hollow-bearing trees presents a particular challenge for forest managers, partly due to difficulties in predicting their occurrence across a landscape. We trialled a novel approach for assessing relative hollow availability, by remotely estimating mature crown cover and senescence from aerial photographs in Tasmania, Australia. These estimates were tested against plot-based field assessments of actual occurrence of hollow-bearing trees. In dry forest we conducted ground-based surveys of hollows for all mature trees (>50 cm dbh) in 37 half-hectare plots. In wet forest, we conducted helicopter-based surveys of hollows for all mature trees in 45 oldgrowth plots (0.29–4.63 ha). Aerial photographs (1:10,000–1:25,000) were used to classify the senescence and cover of mature crowns in each plot. Regression analysis showed that, in dry forest, hollow-bearing tree densities were strongly related to the remote assessment of mature crown cover, with an 8% increase in variability explained if senescence was also included (R 2 = 0.50). In wet forest, mature crown cover alone was the best model (R 2 = 0.53 when outliers were removed). Assessing senescence was less important in dense wet forests than dry forest because trees take longer to form mature-shaped crowns and so mature-shaped crowns are more likely to have hollows. These results suggest that, with skilled photo-interpretation, aerial photographs can be useful for remotely assessing the relative density of hollow-bearing trees. This approach has the potential to greatly improve conservation planning for hollows and hollow-dependent fauna.

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Acknowledgments

Thanks to M. Yee and Forestry Tasmania for providing the SenCode and photo-interpreted data and for discussions on the construction of the layer. We are grateful to K. Wotherspoon for field assistance. Thanks to S. Maxwell and G. Tomes (Forestry Tasmania) for assessing the aerial photos, and to D. Livingston (Forest Practices Authority) for GIS assistance in study site selection and map construction. Thanks to the landowners, including Gunns Ltd., for allowing access onto their properties. Thanks to S. Munks, F. Duncan, P. MacIntosh and unknown reviewers for comments on the manuscript.

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Correspondence to Amelia J. Koch.

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Koch, A.J., Baker, S.C. Using aerial photographs to remotely assess tree hollow availability. Biodivers Conserv 20, 1089–1101 (2011). https://doi.org/10.1007/s10531-011-0018-z

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