Vegetation Indices Do Not Capture Forest Cover Variation in Upland Siberian Larch Forests

Michael Loranty,Sergey Davydov,Nikita Zimov,Heather Kropp,Heather Alexander,Michelle Mack,Susan Natali

doi:10.3390/rs10111686

Abstract

Boreal forests are changing in response to climate, with potentially important feedbacks to regional and global climate through altered carbon cycle and albedo dynamics. These feedback processes will be affected by vegetation changes, and feedback strengths will largely rely on the spatial extent and timing of vegetation change. Satellite remote sensing is widely used to monitor vegetation dynamics, and vegetation indices (VIs) are frequently used to characterize spatial and temporal trends in vegetation productivity. In this study we combine field observations of larch forest cover across a 25 km2 upland landscape in northeastern Siberia with high-resolution satellite observations to determine how the Normalized Difference Vegetation Index (NDVI) and the Enhanced Vegetation Index (EVI) are related to forest cover. Across 46 forest stands ranging from 0% to 90% larch canopy cover, we find either no change, or declines in NDVI and EVI derived from PlanetScope CubeSat and Landsat data with increasing forest cover. In conjunction with field observations of NDVI, these results indicate that understory vegetation likely exerts a strong influence on vegetation indices in these ecosystems. This suggests that positive decadal trends in NDVI in Siberian larch forests may correspond primarily to increases in understory productivity, or even to declines in forest cover. Consequently, positive NDVI trends may be associated with declines in terrestrial carbon storage and increases in albedo, rather than increases in carbon storage and decreases in albedo that are commonly assumed. Moreover, it is also likely that important ecological changes such as large changes in forest density or variable forest regrowth after fire are not captured by long-term NDVI trends.

Highlights

Boreal forest responses to warming temperatures will feed back to climate in a variety of important ways
We did not observe variation in Normalized Difference Vegetation Index (NDVI) or Enhanced Vegetation Index (EVI) across an upland boreal forest landscape in northeastern Siberia with forest canopy cover ranging from 0% to over 90%
Our results highlight the importance of understory vegetation in boreal vegetation indices (VIs) dynamics, in relation to interpretation of long-term trends

Summary

Introduction

Boreal forest responses to warming temperatures will feed back to climate in a variety of important ways. Boreal ecosystems store globally significant amounts of terrestrial carbon in vegetation and soils. The balance between vegetation uptake and soil carbon losses is important yet represents a large source of uncertainty in high-latitude carbon cycle climate feedback, in the context of permafrost thaw [1,2]. In addition to carbon cycle effects, vegetation change may feed back to climate via altered ecosystem water and energy dynamics. Increasing canopy cover reduces land surface albedo during the spring when vegetation masks snow, leading to surface warming [5]. Changes in light attenuation with canopy composition and density may alter soil temperatures [6], which have important implications for soil carbon cycling. There has been a great deal of effort to understand how boreal forest vegetation is responding to climate [7,8,9,10,11,12]

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Conclusion