Tundra fire and shrub expansion: leveraging remote sensing to explore post-fire vegetation trajectories across Arctic North America

NPS photo, Kaluktavik River fire, Noatak

NAU LEAD

Katie Orndahl

keywords

Tundra; wildfire; shrub expansion; vegetation trajectories; remote sensing; climate change; Landsat

As the Arctic warms rapidly, wildfires are increasing in frequency and intensity. This new tundra fire regime will have important consequences for the fate of Arctic vegetation and carbon storage. Fires combust carbon, but post-fire vegetation recovery can claw back some of these ecosystem carbon losses. Furthermore, compositional changes in post-fire vegetation can have important implications for ecosystem function, for example, by impacting wildlife habitat. Thus, the composition and pace of vegetation recovery is of particular importance. A potential fire-shrub feedback loop has been hypothesized whereby fire thaws permafrost which facilitates shrub expansion, and shrub expansion facilitates additional fires via increased woody fuels. We leveraged 30 m resolution gridded data of burned area, plant functional type cover and vegetation biomass to assess post-fire vegetation trajectories across Arctic North America. We analyzed several hundred fires across the past four decades and found that, on average, post-fire deciduous shrub cover exceeded pre-fire baseline levels, whereas cover of evergreen shrubs, graminoids and lichens did not. Lichen cover was particularly slow to recover, failing to reach pre-fire levels even after ten years. Vegetation recovery was influenced by burn severity, with more severe fires causing larger reductions in plant functional type cover immediately post-fire, but also potentially spurring greater long-term increases in deciduous shrubs. Similarly, fires in areas with continuous and extensive discontinuous permafrost had larger long-term deciduous shrub increases, on average. This work thus lends some credence to the hypothesis that fire induced permafrost thaw facilitates shrub expansion in Arctic tundra, and highlights the importance of understanding trajectories of vegetation change under a novel tundra fire regime and rapid climate warming.

Mapping Pan-Arctic vegetation biomass and biomass change

leadS

Katie Orndahl, Logan Berner

keywords

Pan Arctic; plant biomass; woody plant dominance; vegetation distribution; remote sensing; climate change; Landsat

Arctic ecosystems are warming faster than anywhere else on Earth. This warming influences ecosystem processes and alters carbon balance across the Arctic, placing the region at the forefront of global climate change. Vegetation plays a crucial role in shaping the Arctic’s future by both responding to changes in climate and modulating potential changes. The distribution of vegetation across the landscape also influences the health and behavior of human and wildlife communities. To fully understand the status of Arctic ecosystems and how they are likely to change in the future, quantification of the distribution, extent and structure of Arctic vegetation is critical. Plant biomass is a fundamental ecosystem attribute that is sensitive to changes in climate, closely tied with ecological function and crucial for determining ecosystem carbon storage. This makes it a high priority characteristic for inventory and monitoring.

We are developing the first pan-Arctic, moderate resolution (30 m) maps of total and tree/shrub above-ground plant biomass. We are leveraging the Landsat satellite record to create seasonal synthetic composites of spectral features and using these (together with a suite of additional predictors) alongside a new and unique database of plant biomass harvest data to model biomass using machine learning models. Biomass estimates will be paired with pixel-wise uncertainty values to facilitate judicious use of these products. Initial results suggest we are able to distinguish biomass presence vs. absence with high accuracy and quantify biomass with reasonable accuracy. These maps represent the current status of Arctic plant biomass across the biome and can be used for myriad applications related to wildlife, carbon accounting, disturbances, climate modeling and more.    

Our Pan-Arctic biomass maps are complete! You can read about this work here, and explore the biomass maps via a Google Earth Engine app here.

We are now working on producing maps for previous epochs and analyzing changes in biomass over the past two decades.

Caribou and vegetation structure in a warming Arctic

NAU LEAD

Katie Orndahl

Keywords

Caribou, unmanned aerial vehicles, biomass mapping, herbivory, carbon cycle, arctic, tundra

Arctic regions are experiencing some of the fastest rates of climate warming in the world. These regions are poised for significant changes in vegetation composition and structure, which in turn affect carbon cycling. Vegetation and carbon cycle changes have the potential to impact one of the region’s most ecologically, culturally, and economically important species: barren-ground caribou.   In addition, caribou have the potential to affect tundra vegetation, creating complicated interactions among herbivore assemblages, climate, and vegetation structure and function. Of particular importance is the potential of caribou to impact ecosystem responses to climate change.

As one of only two large herbivores present in the North American Arctic, caribou play an important role in shaping future Arctic climate and vegetation communities. They will also be acutely affected by changes in climate and vegetation. To understand these interactions, and make accurate predictions about Arctic response to climate change, it is important to both quantify these effects, and explicitly include herbivory in models of Arctic climate and carbon – to “animate” the carbon cycle.

Our work seeks to bridge the gap between small-scale experimental research on arctic herbivore-vegetation interactions, and coarse spatial resolution remote sensing efforts examining these same impacts.  We are used a combination of field data, UAV imagery, airborne imagery and satellite imagery to quantify plant functional type biomass at 30 m resolution across barren-ground caribou ranges in Arctic Alaska and Northwest Canada.  Products such as these will be used in conjunction with caribou location data to assess caribou-vegetation interactions.

Read about how shifts in Fortymile Caribou ranges were correlated with vegetation change here!