How to Calibrate Historical Aerial Photographs: A Change Analysis of Naturally Dynamic Boreal Forest Landscapes

Niko Kulha,Leena Pasanen,Tuomas Aakala

doi:10.3390/f9100631

Niko Kulha, Leena Pasanen + Show 1 more

Open Access

https://doi.org/10.3390/f9100631

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Journal: Forests	Publication Date: Oct 11, 2018
Citations: 6	License type: CC BY 4.0

Affiliation: University of Helsinki, University of Oulu

Abstract

Time series of repeat aerial photographs currently span decades in many regions. However, the lack of calibration data limits their use in forest change analysis. We propose an approach where we combine repeat aerial photography, tree-ring reconstructions, and Bayesian inference to study changes in forests. Using stereopairs of aerial photographs from five boreal forest landscapes, we visually interpreted canopy cover in contiguous 0.1-ha cells at three time points during 1959–2011. We used tree-ring measurements to produce calibration data for the interpretation, and to quantify the bias and error associated with the interpretation. Then, we discerned credible canopy cover changes from the interpretation error noise using Bayesian inference. We underestimated canopy cover using the historical low-quality photographs, and overestimated it using the recent high-quality photographs. Further, due to differences in tree species composition and canopy cover in the cells, the interpretation bias varied between the landscapes. In addition, the random interpretation error varied between and within the landscapes. Due to the varying bias and error, the magnitude of credibly detectable canopy cover change in the 0.1-ha cells depended on the studied time interval and landscape, ranging from −10 to −18 percentage points (decrease), and from +10 to +19 percentage points (increase). Hence, changes occurring at stand scales were detectable, but smaller scale changes could not be separated from the error noise. Besides the abrupt changes, also slow continuous canopy cover changes could be detected with the proposed approach. Given the wide availability of historical aerial photographs, the proposed approach can be applied for forest change analysis in biomes where tree-rings form, while accounting for the bias and error in aerial photo interpretation.

Highlights

Tree growth and senescence occur slowly but continuously over large areas in the boreal forests [1]
The difference between the canopy cover reconstructed using the field and tree-ring measurements, and the canopy cover visually interpreted from the aerial photographs ranged from −4 to 15 percentage points (p.p.) (mean (x) = 5 p.p., standard deviation (SD) = ±5 p.p.) in the Finnish landscapes, and from
Our results suggest that the visual interpretation bias depends on the aerial photo scale and quality, most likely because the individual tree crowns appear larger and are easier to delineate from the recent high-quality aerial photographs

Summary

Introduction

Tree growth and senescence occur slowly but continuously over large areas in the boreal forests [1]. Several approaches have been developed to study forest structural changes which occur at various temporal scales. These include permanent plot measurements, where the same stand is repeatedly measured (e.g., [4]). In seasonal climates, another option is tree-ring based reconstruction, where the information stored in tree rings is used to analyze forest dynamics (e.g., [5]). Another option is tree-ring based reconstruction, where the information stored in tree rings is used to analyze forest dynamics (e.g., [5])

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