Abstract

In boreal and tundra ecosystems the freeze state of soils limits rates of photosynthesis and respiration. Here we develop a technique to identify the timing of freeze and thaw transitions of high northern latitude land areas using satellite data from the Scanning Multichannel Microwave Radiometer (SMMR) and Special Sensor Microwave/Imager (SSM/I). Our results indicate that in Eurasia there was a trend toward earlier thaw dates in tundra (−3.3 ± 1.8 days/decade) and larch biomes (−4.5 ± 1.8 days/decade) over the period 1988–2002. In North America there was a trend toward later freeze dates in evergreen conifer forests by 3.1 ± 1.2 days/decade that led, in part, to a lengthening of the growing season by 5.1 ± 2.9 days/decade. The growing season length in North American tundra increased by 5.4 ± 3.1 days/decade. Despite the trend toward earlier thaw dates in Eurasian larch forests, the growing season length did not increase because of parallel changes in timing of the fall freeze (−5.4 ± 2.1 days/decade), which led to a forward shift of the growing season. Thaw timing was negatively correlated with surface air temperatures in the spring, whereas freeze timing was positively correlated with surface air temperatures in the fall, suggesting that surface air temperature is one of several factors that determines the timing of soil thaw and freeze. The high spatial resolution, frequent temporal coverage, and duration of the SMMR and SSM/I satellite records makes them suitable for rigorous time series analysis and change detection in northern terrestrial ecosystems.

Highlights

  • [2] In high northern latitude regions, excluding the North Atlantic and Greenland, mean annual surface air temperature increased at rates of up to $0.7°C per decade during the latter half of the 20th century, with the largest increase occurring during winter and spring months [Chapman and Walsh, 1993; Hansen et al, 1999, 2001]

  • [6] Normalized Difference Vegetation Index (NDVI) provides a means for evaluating regional trends in leaf area, and atmospheric CO2 records allow for analysis of trends in net ecosystem production (NEP), no comparable constraint is available for estimating trends in biome-level soil respiration

  • We used data from the Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR), which operated from late 1978 to mid 1987, and the Defense Meteorological Satellite Program (DMSP) Special Sensor Microwave/Imager (SSM/I), which operated from mid 1987 to present

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Summary

Introduction

[2] In high northern latitude regions, excluding the North Atlantic and Greenland, mean annual surface air temperature increased at rates of up to $0.7°C per decade during the latter half of the 20th century, with the largest increase occurring during winter and spring months [Chapman and Walsh, 1993; Hansen et al, 1999, 2001]. [4] In addition to impacts on soil respiration, increasing air temperatures may lengthen the growing season and enhance rates of net primary production (NPP) in tundra and boreal biomes. [6] NDVI provides a means for evaluating regional trends in leaf area (and NPP), and atmospheric CO2 records allow for analysis of trends in net ecosystem production (NEP), no comparable constraint is available for estimating trends in biome-level soil respiration. The growing season length increased significantly in North American evergreen conifer and tundra biomes, but did not change in Eurasian biomes due to the parallel shift of the spring thaw and fall freeze to an earlier date

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