Bridging gaps in permafrost-shrub understanding

Permafrost, permanently frozen ground which underlies much of the Arctic and sub-Arctic, is influenced by deciduous shrubs, yet our understanding of permafrost-shrub interactions is limited.This is largely due to a lack of widespread, long-term, high-quality observations across Arctic and sub-Arctic systems, which are difficult to study due to their remote locations.Shrubs are rapidly expanding in many areas of Arctic tundra and can either amplify or inhibit permafrost thaw [1,2], making it crucial that we can understand and predict permafrost-shrub interactions.As we have limited time and resources to make observations, we must design our field campaigns to be as impactful as possible.Below we outline the current state of knowledge and give suggestions about how we can maximize our fieldwork's impact.From field studies conducted around the Arctic, we know that shrubs generally have a cooling effect on permafrost in the summer, largely caused by the shrub canopy shading the soil.However, summertime cooling can be counteracted by a warming effect in winter, caused by the trapping of blowing snow by shrubs, which leads to increased snow depths [3].In addition to these commonly observed effects, both observations and models [4] show that other local factors such as climate, soil, snow, and disturbances can interact to cause contrasting dominant effects on permafrost temperatures.For example, shrubs that protrude through the snowpack have been observed to cool permafrost in winter via thermal bridging between the air above the snow and the soil below [5].However, at other research sites, protruding shrubs lower the surface albedo and cause earlier snowmelt in spring, amplifying permafrost thaw by exposing the soil to thawing earlier [2].Protruding shrubs have also been observed to induce snow melt in autumn, creating ice layers at the snow surface that prevent further snow accumulation and eliminate the ability of shrubs to trap blowing snow, thereby reducing the winter warming effect [6].While we understand the physical processes that lead to these effects, we cannot explain why we observe different effects between sites.By synthesizing literature of past field studies, researchers have been able to qualitatively compare the dominant effects shrubs can have on permafrost conditions, generating diagrams that illustrate the interactions between shrubs, permafrost and other environmental components [3,7].As we continue to discover dominant effects between shrubs and permafrost, we should design our field measurements not only with the intent to describe how shrubs and permafrost interact, but also with the goal that our measurements generate knowledge that can be used to quantify and predict how shrubs are influencing permafrost, using shrub properties and other local factors we have found to be important.