Four Decades of Land-Cover Change on the Kenai Peninsula, Alaska: Detecting Disturbance-Influenced Vegetation Shifts Using Landsat Legacy Data

Carson Baughman,Lisa Saperstein,Dawn Magness,Rosemary Sherriff,Rachel Loehman

doi:10.3390/land9100382

Carson Baughman, Lisa Saperstein + Show 3 more

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https://doi.org/10.3390/land9100382

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Abstract

Across Alaska’s Kenai Peninsula, disturbance events have removed large areas of forest over the last half century. Simultaneously, succession and landscape evolution have facilitated forest regrowth and expansion. Detecting forest loss within known pulse disturbance events is often straightforward given that reduction in tree cover is a readily detectable and measurable land-cover change. Land-cover change is more difficult to quantify when disturbance events are unknown, remote, or environmental response is slow in relation to human observation. While disturbance events and related land-cover change are relatively instant, assessing patterns of post-disturbance succession requires long term monitoring. Here, we describe a method for classifying land cover and quantifying land-cover change over time, using Landsat legacy imagery for three historical eras on the western Kenai Peninsula: 1973–2002, 2002–2017, and 1973–2017. Scenes from numerous Landsat sensors, including summer and winter seasons, were acquired between 1973 and 2017 and used to classify vegetation cover using a random forest classifier. Land-cover type was summarized by era and combined to produce a dataset capturing spatially explicit land-cover change at a moderate 30-m resolution. Our results document large-scale forest loss across the study area that can be attributed to known disturbance events including beetle kill and wildfire. Despite numerous and extensive disturbances resulting in forest loss, we estimate that the study area has experienced net forest gain over the duration of our study period due to reforestation within large fire events that predate this study. Transition between forest and graminoid non-forest land cover including wetlands and herbaceous uplands is the most common land-cover change—representing recruitment of a graminoid dominated understory following forest loss and the return of forest canopy given sufficient time post-disturbance.

Highlights

The boreal forest is a global resource in terms of biodiversity, silviculture production [1], and global carbon stocks [2]
Our analysis indicates that substantial land-cover change has occurred across the Kenai Peninsula over the last 44 years with the exception of water extent, which remained stable between 1973 and with a slight peak in 2002 (Figure 3)
Deforestation events related to spruce bark beetle outbreaks, wildfires, and timber harvest have occurred on the Kenai Peninsula over the past four decades [24,36,49], our classifications suggest that the peninsula has become more forested over the period of analysis, because significant and prolonged forest loss in the southern half of the peninsula (Figure 5) has been offset by forest regrowth in the northern half

Summary

Introduction

The boreal forest is a global resource in terms of biodiversity, silviculture production [1], and global carbon stocks [2]. Additional observed changes over the last two decades include greening and browning trends [9], tree line expansion into tundra environments [10], tree mortality from insect outbreaks and drought [11,12], more frequent and extreme wildfires [13,14], and the recruitment of novel post-disturbance lifeforms and species [15] These processes, many of which are projected to increase with ongoing and increasing climate changes and can be linked via interactions and feedbacks [16,17,18], may impact broad patterns of land cover and, the provisioning of ecosystem services critical for natural and human communities [19]

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Conclusion