Changes in Vegetation Phenology and Productivity in Alaska Over the Past Two Decades

Abstract

Understanding trends in vegetation phenology and growing season productivity at a regional scale is important for global change studies, particularly as linkages can be made between climate shifts and the vegetation’s potential to sequester or release carbon into the atmosphere. Trends and geographic patterns of change in vegetation growth and phenology from the MODerate resolution Imaging Spectroradiometer (MODIS) satellite data sets were analyzed for the state of Alaska over the period 2000 to 2018. Phenology metrics derived from the MODIS Normalized Difference Vegetation Index (NDVI) time-series at 250 m resolution tracked changes in the total integrated greenness cover (TIN), maximum annual NDVI (MAXN), and start of the season timing (SOST) date over the past two decades. SOST trends showed significantly earlier seasonal vegetation greening (at more than one day per year) across the northeastern Brooks Range Mountains, on the Yukon-Kuskokwim coastal plain, and in the southern coastal areas of Alaska. TIN and MAXN have increased significantly across the western Arctic Coastal Plain and within the perimeters of most large wildfires of the Interior boreal region that burned since the year 2000, whereas TIN and MAXN have decreased notably in watersheds of Bristol Bay and in the Cook Inlet lowlands of southwestern Alaska, in the same regions where earlier-trending SOST was also detected. Mapping results from this MODIS time-series analysis have identified a new database of localized study locations across Alaska where vegetation phenology has recently shifted notably, and where land cover types and ecosystem processes could be changing rapidly.

Remotely sensed trends in vegetation productivity and phenology during population decline of the Bathurst caribou (Rangifer tarandus groenlandicus) herd

“…There is some evidence from both tundra and boreal systems that increases in peak productivity correspond with longer growing seasons and earlier green-up dates (Arndt et al 2019, Bergeron et al 2007). However, other studies have found spatial mismatches between greening and earlier start of season dates similar to what we found in the Bathurst range (Raynolds et al 2013, Potter andAlexander 2020). One potential explanation for this is that earlier and more rapid green-up in the spring depletes soil moisture reserves earlier in the growing season, which decreases maximum productivity later in the summer (Gonsamo et al 2019).…”

“…In any case, the spatial mismatch between the two trends in the range of the Bathurst herd explains why the prevalence of significant time integrated EVI trends (which integrates the two) was so much greater than it was for either one individually. Other studies that have examined additive or time integrated vegetation indices have found similarly high prevalence of significant positive trends in the tundra (Park et al 2016, Potter andAlexander 2020).…”

“…wooded areas), it is difficult to say how maximum EVI would have changed in the absence of fire. However, it is worth noting that boreal browning is most prevalent in the drier boreal forests of western Canada and Alaska (Sulla-Menashe et al 2018, Potter andAlexander 2020), and that at least some of the boreal browning detected using AVHRR datasets (Beck and Goetz 2012;Goetz et al 2005) is not present when trends are derived using Landsat data (e.g. Ju and Masek 2016).…”

Habitat selection by Dall’s sheep is influenced by multiple factors including direct and indirect climate effects

“…Snow-free periods are 5–6 days longer per decade [ 4 , 8 ], and precipitation has increased about 1% per decade over the past 100 years [ 7 ], which influences not only depth, extent and persistence of snow cover [ 4 ], but also snow density and occurrence of rain-on-snow events [ 9 , 10 ]. Climate change also can indirectly affect wildlife populations at high latitudes by altering the composition of ecological communities [ 11 ], including expansion of shrubs into tundra and alpine habitats [ 12 , 13 ] and changes in plant phenology in arctic communities [ 14 , 15 ]. Understanding of the effects of climate change on arctic-adapted wildlife species via both direct and indirect pathways is advancing.…”

Drone data reveal heterogeneity in tundra greenness and phenology not captured by satellites

“…Satellite observations indicate a 'greening' of tundra ecosystems (13,(15)(16)(17)(18)(19)(20) and shifts in growing season phenology over recent decades (21)(22)(23)(24) . Observations of increasing tundra greenness are often reported from surface-reflectance-derived Normalised Difference Vegetation Index (NDVI) (16,18,25,26) .…”