Changes in Vegetation Phenology and Productivity in Alaska Over the Past Two Decades

Christopher Potter,Olivia Alexander

doi:10.3390/rs12101546

Christopher Potter, Olivia Alexander

Open Access

https://doi.org/10.3390/rs12101546

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Journal: Remote sensing	Publication Date: May 13, 2020
Citations: 14	License type: CC BY 4.0

Affiliation: Ames Research Center, San Jose State University

Abstract

Understanding trends in vegetation phenology and growing season productivity at a regional scale is important for global change studies, particularly as linkages can be made between climate shifts and the vegetation’s potential to sequester or release carbon into the atmosphere. Trends and geographic patterns of change in vegetation growth and phenology from the MODerate resolution Imaging Spectroradiometer (MODIS) satellite data sets were analyzed for the state of Alaska over the period 2000 to 2018. Phenology metrics derived from the MODIS Normalized Difference Vegetation Index (NDVI) time-series at 250 m resolution tracked changes in the total integrated greenness cover (TIN), maximum annual NDVI (MAXN), and start of the season timing (SOST) date over the past two decades. SOST trends showed significantly earlier seasonal vegetation greening (at more than one day per year) across the northeastern Brooks Range Mountains, on the Yukon-Kuskokwim coastal plain, and in the southern coastal areas of Alaska. TIN and MAXN have increased significantly across the western Arctic Coastal Plain and within the perimeters of most large wildfires of the Interior boreal region that burned since the year 2000, whereas TIN and MAXN have decreased notably in watersheds of Bristol Bay and in the Cook Inlet lowlands of southwestern Alaska, in the same regions where earlier-trending SOST was also detected. Mapping results from this MODIS time-series analysis have identified a new database of localized study locations across Alaska where vegetation phenology has recently shifted notably, and where land cover types and ecosystem processes could be changing rapidly.

Highlights

Recent studies have shown that many regions of Alaska are experiencing a high level of inter-annual variability in the seasonality of vegetation growth [1,2,3]
We found that trends in the start of the season timing (SOST) over the period of 2000 to 2018 showed significantly (p < 0.05) earlier seasonal vegetation greening across sections of the northeastern region of the Brooks Range Mountains into the Richardson Mountains and Mackenzie River Delta of Canada, on the Yukon-Kuskokwim coastal plain, and most notably in the Ahklun Mountains across to Bristol Bay and the Cook Inlet lowlands in the southern coastal areas (Figure 1a,b)
Using an improved physically-based plant phenology index (PPI) with a nearly linear relationship to green leaf area index (LAI), it was reported by Karkauskaite et al [64] that PPI SOST preceded Normalized Difference Vegetation Index (NDVI) SOST by several days in most high-latitude shrublands and herbaceous cover, where the strongest trends in earlier SOST for Alaska were detected in the MODerate resolution Imaging Spectroradiometer (MODIS) data we have presented in this study (Figure 1b)

Summary

Introduction

Recent studies have shown that many regions of Alaska are experiencing a high level of inter-annual variability in the seasonality of vegetation growth [1,2,3]. The northern and interior Brooks Range Mountains have experienced the greatest year-to-year variation in snowmelt timing dates of any sub-region of Alaska over the past 20 years [4]. High magnitudes of inter-annual variability in springtime snow-free conditions may be having strong impacts on many aspects of ecosystem functioning [6], including the dynamics of Alaskan wildlife populations [7] and on the area of tundra severely burned each year by wildfires [8]. Over the past 30 years, the expansion of woody shrub cover in northern Alaska has been reported from both remote sensing and field studies [9,10,11,12,13]. In some boreal forest sub-regions of the state, evergreen tree cover has decreased, while deciduous tree cover has increased, along with the expansion of shrub and herbaceous vegetation above tree lines [13], patterns which have been confirmed by field measurements of shrub encroachment and greening trends [12]

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