Spatial and Temporal Changes in Vegetation Phenology at Middle and High Latitudes of the Northern Hemisphere over the Past Three Decades

Abstract

Vegetation phenology is a key biological indicator for monitoring terrestrial ecosystems and global change, and regions with the most obvious phenological changes in vegetation are primarily located at high latitudes and altitudes. Over the past three decades, investigations of obvious phenological changes in vegetation at middle and high latitudes in the Northern Hemisphere have provided significant contributions to understanding global climate change. In this study, phenological parameters were extracted from the Global Inventory Modeling and Mapping Studies (GIMMS) Normalized Difference Vegetation Index (NDVI3g) to analyze the spatial and temporal characteristics of vegetation phenological changes above 40°N in the Northern Hemisphere from 1982-2013. The results showed that the start of season (SOS) was significantly advanced (−2.2 ± 0.6 days· decade −1 , p < 0.05) and that the end of season (EOS) was slightly delayed (0.78 ± 0.6 days· decade −1 , p = 0.21) over the entire study area in the initial 21 years . When the time scale was extended to 2013, the change rate of the SOS and EOS was significantly reduced; in addition, the SOS was delayed (3.2 ± 1.7 days· decade −1 , p < 0.05), and the EOS was advanced (4.5 ± 0.9 days· decade −1 , p < 0.05) over the entire study area in the last 11 years (2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013). The trends of advanced SOS and delayed EOS over the past three decades OPEN ACCESSRemote Sens. 2015, 7 10974 were slower than those over the initial two decades on a hemispheric scale. The change trends showed obvious variability with different vegetation types and were greater for woody plants than for herbaceous plants. For broad-leaved forest, the SOS was significantly advanced (2 ± 0.5 days· decade −1 , p < 0.05) and the EOS was significantly delayed (2.7 ± 0.6 days· decade −1 , p < 0.05) from 1982-2013. The trend of delayed EOS was greater than that of advanced SOS for different vegetation types. With respect to the spatial distribution of phenological trends in the Northern Hemisphere, the trends of advanced SOS and delayed EOS were strongest in Europe followed by North America, and the trends were least significant in Asia. Coniferous forest, shrub forest, grassland, and the entire study area have the same change trends for the two time periods (1982-2002 and 2003-2013), and the increased rate of the phenology parameters has decelerated over the most recent decade. The length of season (LOS) of broad-leaved forest and mixed forest over the past 32 years shows a strong increased trend, and simultaneously, the SOS and EOS show an advanced trend and a delayed trend, respectively.

Temporal Changes in Coupled Vegetation Phenology and Productivity are Biome-Specific in the Northern Hemisphere

“…The calculated trends in LOS, including SOS and EOS (Figure 2A-C), are consistent with the spatial patterns of phenological trends in Zhao et al [52], who extracted vegetation phenological changes above 40°N of the NH during 1982-2013 from GIMMS3g NDVI data. They also employed the TIMESAT approach, but with different parameter settings as compared to this study, indicating the robustness of the method for phenology retrieval and consequently the results obtained in this study.…”

“…From such approach, both increasing LOS and vegetation greenness have been shown to be slowing down or have experienced abrupt changes around 2000 in the NH using break point analysis techniques [6,7,26,52,[58][59][60]. Nevertheless, a recent study by Wang et al [61] suggested that the advancing SOS is unlikely to have slowed down or have abruptly changed at the hemispheric scale of the NH over the last three decades, although it could have happened at a local/regional scale because of changes in fire regimes or winter chilling.…”

Evaluation of the Plant Phenology Index (PPI), NDVI and EVI for Start-of-Season Trend Analysis of the Northern Hemisphere Boreal Zone

“…Boreal forests-the dominating biome of the NH, are representing 32% (272 ± 23 PgC) of the global forest C stock, hence playing a crucial role in the global C cycle, either as a sink or source [21,22]. Investigating phenology of boreal forests is therefore important for understanding the impacts of climate change and associated changes in global C balance [23].…”

“…Satellite-based observations are becoming essential for boreal region plant phenology retrieval, since it allows for remotely capturing phenological variations both at small and large scales, while ground observations of the Northern mid and high latitude plant phenology are limited and represent local conditions [7,16,[23][24][25][26]. Existing satellite-based research in plant phenology over the boreal zone of the NH has been mainly carried out by employing spectral vegetation indices (VI), calculated from the reflectance acquired from optical remote sensing using satellites [27,28].…”

Non-Gaussian data assimilation of satellite-based leaf area index observations with an individual-based dynamic global vegetation model

“…Here, [a, b] denotes random draws from the uniform distribution between a and b. These initial perturbation sizes are based on the previous studies (Kolari et., 2006;Zeng et al, 2011;Zhao et al, 2015;Takagi et al, 2015). We ran 8000 parallel simulations for 103 years for spin-up from the bare ground using the same climate forcing data as the nature run.…”