Abstract
Satellite remote sensing of plant phenology provides an important indicator of climate change. However, start of the growing season (SOS) estimates in Northern Hemisphere boreal forest areas are known to be challenged by the presence of seasonal snow cover and limited seasonality in the greenness signal for evergreen needleleaf forests, which can both bias and impede trend estimates of SOS. The newly developed Plant Phenology Index (PPI) was specifically designed to overcome both problems. Here we use Moderate Resolution Imaging Spectroradiometer (MODIS) data (2000–2014) to analyze the ability of PPI for estimating start of season (SOS) in boreal regions of the Northern Hemisphere, in comparison to two other widely applied indices for SOS retrieval: the Normalized Difference Vegetation Index (NDVI) and the Enhanced Vegetation Index (EVI). Satellite-based SOS is evaluated against gross primary production (GPP)-retrieved SOS derived from a network of flux tower observations in boreal areas (a total of 81 site-years analyzed). Spatiotemporal relationships between SOS derived from PPI, EVI and NDVI are furthermore studied for different boreal land cover types and regions. The overall correlation between SOS derived from VIs and ground measurements was rather low, but PPI performed significantly better (r = 0.50, p < 0.01) than EVI and NDVI which both showed a very poor correlation (r = 0.11, p = 0. 16 and r = 0.08, p = 0.24). PPI, EVI and NDVI overall produce similar trends in SOS for the Northern Hemisphere showing an advance in SOS towards earlier dates (0.28, 0.23 and 0.26 days/year), but a pronounced difference in trend estimates between PPI and EVI/NDVI is observed for different land cover types. Deciduous needleleaf forest is characterized by the largest advance in SOS when considering all indices, yet PPI showed less dramatic changes as compared to EVI/NDVI (0.47 days/year as compared to 0.62 and 0.74). PPI SOS trends were found to be higher for deciduous broadleaf forests and savannas (0.54 and 0.56 days/year). Taken together, the findings of this study suggest improved performance of PPI over NDVI and EVI in retrieval of SOS in boreal regions and precautions must be taken when interpreting spatio-temporal patterns of SOS from the latter two indices.
Highlights
Plant Phenology Index (PPI) start of season (SOS) is better correlated with the gross primary production (GPP) (r = 0.50; p < 0.01) phenology as compared to Enhanced Vegetation Index (EVI) (r = 0.11; p = 0. 16) and Normalized Difference Vegetation Index (NDVI) (r = 0.08; p = 0.24), indicating a more accurate capturing of SOS by PPI
The tests, as well as examination of vegetation indices (VI) relation to Normalized Difference Snow Index (NDSI), revealed a pronounced NDVI and EVI association to the changes in snow cover (NDVI was reported to be more sensitive to partly snow-covered surfaces, whereas EVI is more sensitive to high snow depth) [36,71]
This study analyzes the robustness of the satellite derived Plant Phenology Index (PPI) for estimating start of season (SOS) in boreal forests of the Northern Hemisphere (2000–2014), in relation to two other widely applied indices NDVI and EVI
Summary
Growing consensus on severity of the ecological impacts posed by recent global warming and its amplification in the high latitudes of the Northern Hemisphere (NH) increases the importance of understanding the interaction between the climatic changes and terrestrial ecosystems [1,2,3,4,5].Plant cyclical biological events (plant phenology) are strong indicators of the seasonality of the environment, revealing the implications of rising temperatures on vegetation functioning [6,7,8,9].Depending on the vegetation cover and climate zone, plant phenology responds differently to climatic factors, such as air temperature, precipitation and photoperiod [10]. Growing consensus on severity of the ecological impacts posed by recent global warming and its amplification in the high latitudes of the Northern Hemisphere (NH) increases the importance of understanding the interaction between the climatic changes and terrestrial ecosystems [1,2,3,4,5]. Hemisphere, which makes changes in plant phenology a significant indicator of biological responses to increasing temperatures [1,11,12,13,14]. Is plant phenology crucial for understanding the impacts of climate change, but phenology is controlling various processes in the terrestrial ecosystems, such as net primary productivity and seasonal exchanges of CO2 , water and energy between the land surface and the atmosphere [15,16,17,18]. Investigating phenology of boreal forests is important for understanding the impacts of climate change and associated changes in global
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