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

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Video in TIB AV-Portal: Remote sensing of interannual boreal forest NDVI in relation to climatic conditions in interior Alaska

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Remote sensing of interannual boreal forest NDVI in relation to climatic conditions in interior Alaska
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CC Attribution 3.0 Unported:
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2015
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English

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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.
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welcome to my backyard in Alaska's interior boreal forest a vast mosaic of wetlands shrub lands wetlands and spruce aspen and birch
forests I'm David Bella professor at the
University of Alaska-Fairbanks and a scientist with the Bonanza Creek long-term he got a research program the boreal forest
of interior Alaska has an extreme continental climate science rebounded by massive mountain ranges missiles East in North due to our continental climate waiters are typically cold sometimes extremely cold and summers can be warm and dry during summer
drought daily temperatures can exceed 26 degrees centigrade and total summer precipitation can be less than 50 mM with daylight exceeding 20 hours each day in
Fairbanks the mean summer temperature now exceeds that of the previous 200 years so summer warming has dramatically occurred especially in the past few decades in this paper
user remotely sensed index of photosynthetic activity called the normalized difference vegetation index or NDB I in this remotely sensed index captures the contrast between were the red spectral region which is being absorbed for photosynthesis in the near infrared spectra region which is highly reflective in photo synthetic material and the nice thing about nd EDI is all natural surfaces that are not vegetated such as soil or water will have values that are negative or near 0 well vegetated surface will increase in and EDI as the leaf area index in photosynthetic activity increases there's been a dramatic
decline in peak and EDI associated with climate warming and this has been documented by several so remote sensing studies using several different senses and it's been term the browning trend in this paper I investigate the interannual variability of this peak and EDI and how that is related to a variety of climatic factors in interior boreal so the question is what are the most important climatic factors we have spring snowpack which contributes to soil moisture and then we have the length of the growing season so is it an early spring versus a late spring and that would also contribute to soil moisture so the last day of spring snow or ice out or the day of spring but burst and then the 4th climatic factor that I investigate is a summer moisture index so as you read this paper keep in mind that although above ground it can be hot and
dry below ground it can be fairly cool so for example here's the temperature profile from a typical black spruce stand and it at 20 centimeters depth it does not get above freezing until the beginning of July so what are the consequences of these cold boreal soils a shallow rooting zone especially on cold moist sites so keep that in mind as you read this paper thank you
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