Abstract
The latitudinal gradient of the start of the growing season (SOS) and the end of the growing season (EOS) were quantified in Alaska (61°N to 71°N) using satellite-based and ground-based datasets. The Alaskan evergreen needleleaf forests are sparse and the understory vegetation has a substantial impact on the satellite signal. We evaluated SOS and EOS of understory and tundra vegetation using time-lapse camera images. From the comparison of three SOS algorithms for determining SOS from two satellite datasets (SPOT-VEGETATION and Terra-MODIS), we found that the satellite-based SOS timing was consistent with the leaf emergence of the forest understory and tundra vegetation. The ensemble average of SOS over all satellite algorithms can be used as a measure of spring leaf emergence for understory and tundra vegetation. In contrast, the relationship between the ground-based and satellite-based EOSs was not as strong as that of SOS both for boreal forest and tundra sites because of the large biases between those two EOSs (19 to 26 days). The satellite-based EOS was more relevant to snowfall events than the senescence of understory or tundra. The plant canopy radiative transfer simulation suggested that 84–86% of the NDVI seasonal amplitude could be a reasonable threshold for the EOS determination. The latitudinal gradients of SOS and EOS evaluated by the satellite and ground data were consistent and the satellite-derived SOS and EOS were 3.5 to 5.7 days degree− 1 and − 2.3 to − 2.7 days degree− 1, which corresponded to the spring (May) temperature sensitivity of − 2.5 to − 3.9 days °C− 1 in SOS and the autumn (August and September) temperature sensitivity of 3.0 to 4.6 days °C− 1 in EOS. This demonstrates the possible impact of phenology in spruce forest understory and tundra ecosystems in response to climate change in the warming Artic and sub-Arctic regions.
Highlights
In the Arctic and sub-Arctic regions, including Alaska, warming trends have been accelerating and the increased trend in surface temperature in the region over the past decade is twofold higher than that in the whole northern hemisphere (Bekryaev, Polyakov, & Alexeev, 2010; Hinzman et al, 2013; IPCC, 2013)
The date of ground-based start of the growing season (SOS) and end of the growing season (EOS) were determined by the analysis shown (Section 3.1)
Despite their numerous spatial and trend analyses in SOS and EOS, there is still little information on how the latitudinal gradient of satellite-based SOS and EOS are related to changes in seasonal surface condition, plant phenology, and satellite observation conditions in the Arctic and sub-Arctic regions
Summary
In the Arctic and sub-Arctic regions, including Alaska, warming trends have been accelerating and the increased trend in surface temperature in the region over the past decade is twofold higher than that in the whole northern hemisphere (Bekryaev, Polyakov, & Alexeev, 2010; Hinzman et al, 2013; IPCC, 2013). The environmental factors (e.g. photoperiods and temperature changes) that control the changes in EOS are not clearly understood (Delpierre et al, 2009; Jeong & Medvigy, 2014; Richardson et al, 2013a)
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