I am an ecosystem ecologist specialized in vegetation-climate interactions in northern ecosystems. I like to combine field studies with ecosystem modelling. Current research focuses on how vegetation influences permafrost thawing and vice versa. Together with PhD candidates and thesis students I also investigate the possibility of shrub decline due to small-scale permafrost collapse instead of the widely reported shrub expansion in tundra ecosystems. At a Siberian tundra site we run field experiments in which vegetation or thawing depth is manipulated.

• 1994, MSc. Environmental Sciences, Wageningen University, thesis topics: wetland ecology, ecohydrology, aquatic chemistry
• 2000, PhD, Wageningen University, Efffects of elevated CO2 and increased N deposition on bog vegetation
• 2002, Postdoc, University of Alaska Fairbanks, Talent fellowship
• since 2003, Assistant Professor, Wageningen University
• 2010-2014, Postdoc, Vidi grant

My research has been mainly on vegetation-climate interactions in northern ecosystems, using both experimental and modelling approaches. I have studied the impacts of climate change on vegetation and carbon dynamics in European peatlands and on natural ecosystems in the Netherlands. Vice-versa, I have also studied the effects of changes in vegetation composition on the climate system through exchange of greenhouse gases and heat fluxes in Alaskan boreal forest and Siberian tundra. Vegetation-soil feedbacks and competition between plant species are included in the ecosystem models NUCOM-bog and NUCOM-tundra.

My current research in Siberian tundra, together with Ko van Huissteden (VU Amsterdam), Gabriela Schaepman-Strub (Zurich), Daan Blok (Copenhagen) and PhD candidates Bingxi Li and Peng Wang focuses on feedbacks of vegetation change to permafrost thawing, soil nutrient availability and carbon storage in tundra ecosystems.

Project Shrub decline instead of shrub expansion in arctic lowland tundra?

Shrub expansion is widely observed in the Arctic and is related to climate warming. We found that low shrubs, Betula nana, are important for protecting the underlying frozen ground (permafrost) against thawing. At our field site in Siberian tundra we observe that Betula nana (dwarf birch) can be very dominant, but we also see local drowning of Betula nana due to small-scale permafrost collapse. A possible decline in shrub cover due to more frequent permafrost collapse has not been considered so far.

Questions we attempt to answer are:

• What causes small-scale permafrost degradation (drowning of Betula)?
• What are the consequences of permafrost collapse and the following vegetation succession for the carbon balance?
• Is it possible that shrub area declines in lowland tundra vs. shrub expansion in upland tundra? And for the same reason that the treeline does not shift northward in this wet type of tundra ecosystem?

Our approach to tackle these questions is a combination of field investigations and ecosystem modeling. We will continue our measurements in the removal experiment, in which we removed Betula nana in plots of 10 m diameter, at the Kytalyk field site in north-east Siberia. Five years after the first removal most of the removal plots have turned into thaw ponds.In addition, we will compare locations where Betula is drowning, where Betula is in bad condition, and where Betula is expanding into Sphagnum mats to examine vegetation development, dendrochronologies, nutrient availability, and soil biota. Ecosystem modeling is required for analyzing long-term feedbacks. We are developing a tundra vegetation model which includes interaction with thawing depth from a permafrost module and soil moisture. Such an approach could fill the gap between permafrost models that lack a dynamic vegetation component and dynamic global vegetation models that are not detailed enough for analyzing vegetation-permafrost feedbacks at the ecosystem scale.