Abstract
•We compared snow disappearance date (SDD) and tree growth onset timing at the FTE.•There is variability between in situ SDD and remotely sensed SDD estimates.•SDD estimates are confounded by vegetative structure at the FTE.•Comparisons between SDD and tree growth onset timing are site specific.•Other biophysical factors besides SDD must also be considered when estimating tree growth onset.
Tree wood growth is a key physiological process governing the seasonal duration of carbon sequestration, but few methods exist for remotely monitoring the phenology of wood growth. Hence, scalable methods for detecting and monitoring tree growth onset are needed, particularly in climate-sensitive regions like the forest-tundra ecotone (FTE). Because snow disappearance date (SDD) is observable across large spatial scales using satellite remote sensing and may coincide with tree growth onset at high latitudes, we tested the reliability of remotely sensed SDD as a proxy for tree growth onset at the FTE. We hypothesize that: (1) satellite based SDD estimates from the Moderate Resolution Imaging Spectroradiometer (SDDMODIS) and PlanetScope imagery (SDDPlanet) are not statistically different (p > 0.05) from in situ estimates of SDD from soil temperature probes (SDDST), and; (2) estimates of SDDMODIS or SDDPlanet are not significantly different from tree growth onset, thus providing novel monitoring methods for tree wood growth onset at the FTE. We used data across two growing seasons from two field sites - one in Alaska, USA (AK) and one in Northwest Territories, Canada (NWT). Results differed between sites, with remote and in situ SDD estimates in AK occurring simultaneously, while remotely estimated SDD preceded in situ SDD in NWT. All SDD estimates were statistically different from tree growth onset in AK in both years. While comparisons showed possible synchrony between SDD estimates and tree growth onset at NWT, these results were limited and suggested the influence of other biophysical drivers in tree growth onset. These results highlight the phenological heterogeneity of the FTE and the key knowledge gaps remaining in our understanding of processes driving tree growth onset at this ecotone. Thus, there is a clear need for more research into the relationships between tree growth onset and remote sensing information at the FTE.