The Arctic is a particularly sensitive ecosystem which vegetation has responded to environmental changes, anthropogenically-induced or other, during the past decades. With regard to arctic tundra, remote sensing and normalized difference vegetation index (NDVI, a proxy to photosynthetic activity), field observations, and simulation modeling, all concur that there is a generally widespread “greening” of tundra vegetation.
Epstein et al (2012) found an average circumpolar increase in aboveground tundra biomass of 19.8% between 1982 and 2010. This increase was accentuated in the mid- to southern tundra subzones (20–26% increase), yet it was substantially less in the more northern tundra (2–7%).
A major but not the only one component of this increase in photosynthetic vegetation is the proliferation of shrubs, largely in the more southern subzones of arctic tundra. Globally, all studies about arctic tundra also indicate that shrub expansion and other tundra vegetation changes are highly spatially variable, and that certain species and landscape types are likely more sensitive to changes than others.
Despite this variability, recent works showed that the warming and greening trends over pan-Arctic tundra have slowed down and become more heterogeneous. Decline in greenness has recently been detected especially during the last 3–4 years (Epstein et al. 2016). Negative trends for maximum normalized difference vegetation index (MaxNDVI, the peak NDVI for the year) and time-integrated NDVI (TI-NDVI, an indicator of vegetation productivity) were more common during 1999–2015 compared to 1982–1998 (Bhatt et al, 2017).
TI-NDVI trends from 1999–2015 displayed significant negative trends in May and the first half of June, suggesting that there are processes delaying green-up. Changes in timing of spring snow melt, permafrost degradation, killing frosts due to mid-winter or early-spring snow melt, or vegetation shift from graminoids to deciduous shrubs are all possible reasons for arctic tundra browning.
For the boreal forest, remote sensing studies continue to support the “browning” of forest vegetation (1982–2008) with increasing drought stress as the most probable driver. However, this reduction in photosynthesizing vegetation appears to be related to the fractions of evergreen trees and deciduous trees on the landscape – with greater declines in evergreen-dominated areas (Miles and Esau, 2016). Changes towards greening or browning appear here as well highly variable, both in time and space.
WinterNet is a virtual network established to facilitate communication between researchers interested in the impacts of winter change on Arctic and Alpine ecosystems, plant and soil responses, and consequences for biogeochemical cycling. The “Event Drivers of Arctic Browning” workshop, organized this May in Sheffield, UK in connection to WinterNet, was gathering international researchers working on the topic to discuss the increasing appreciation of the diversity of different events that can cause Arctic browning.
The workshop should result in an opinion paper to be published in the near future, proposing a definition of Arctic browning, in opposition to Arctic greening, discussing its drivers, subsequent consequences and future research priorities to focus on.