Remote sensing of interannual boreal forest NDVI in relation to climatic conditions in interior Alaska

David Verbyla

doi:10.1088/1748-9326/10/12/125016

Abstract

Climate has warmed substantially in interior Alaska and several remote sensing studies have documented a decadal-scale decline in the normalized difference vegetation index (NDVI) termed a ‘browning trend’. Reduced summer soil moisture due to changing climatic factors such as earlier springs, less snowpack, and summer drought may reduce boreal productivity and NDVI. However, the relative importance of these climatic factors is poorly understood in boreal interior Alaska. In this study, I used the remotely sensed peak summer NDVI as an index of boreal productivity at 250 m pixel size from 2000 to 2014. Maximum summer NDVI was related to last day of spring snow, early spring snow water equivalent (SWE), and a summer moisture index. There was no significant correlation between early spring SWE and peak summer NDVI. There was a significant correlation between the last day of spring snow and peak summer NDVI, but only for a few higher elevation stations. This was likely due to snowmelt occurring later at higher elevations, thus having a greater effect on summer soil moisture relative to lower elevation sites. For most of boreal interior Alaska, summer drought was likely the dominant control on peak summer NDVI and this effect may persist for several years. Peak summer NDVI declined at all 26 stations after the 2004 drought, and the decline persisted for 2 years at all stations. Due to the shallow rooting zone of most boreal plants, even cool and moist sites at lower elevations are likely vulnerable to drought. For example the peak summer NDVI response following the 2004 drought was similar for adjacent cold and warm watershed basins. Thus, if frequent and severe summer droughts continue, moisture stress effects are likely to be widespread and prolonged throughout most of interior boreal Alaska, including relatively cool, moist sites regardless of spring snowpack conditions or spring phenology.

Highlights

Over the past 50 years, the highest rate of climate warming in North America has occurred in Alaska and Northwest Canada (Clegg and Hu 2010)
One consequence of a warming boreal climate has been regional drought stress leading to reduced tree growth (Barber et al 2000), regional tree mortality (Peng et al 2011, Williams et al 2012) and a decline in the remotely sensed normalized difference vegetation index (NDVI), termed a ‘browning tend’ (Goetz et al 2005, Lloyd and Bunn 2007)
The browning trend may be due to temperature-induced summer drought stress (Barber et al 2000, Lloyd and Bunn 2007) as the optimal temperature for boreal plant growth is exceeded (D’Arrigo et al 2004, Lloyd et al 2013, Juday et al 2014)

Summary

Introduction

Over the past 50 years, the highest rate of climate warming in North America has occurred in Alaska and Northwest Canada (Clegg and Hu 2010). This warming has led to substantial physical and biological changes in Alaska including record sea-ice retreat and autumn warming (Wendler et al 2010), record summer warmth (Barber et al 2004), record wildfire extent (Kasischke et al 2010) and wildfire frequency (Kelly et al 2013), accelerated permafrost thawing (Jorgenson et al 2010), shrinking boreal lakes (Roach et al 2011, Jepsen et al 2013), shrinking mountain glaciers (Arendt et al 2002, Das et al 2014) and a longer unfrozen period (Wendler and Suhlski 2009).

Methods

Results

Conclusion