Below the organic horizon: investigating the effects of shrubification on mineral soil organic matter in a subarctic tundra-forest transition in northern Sweden

Abstract

Shrub range expansion, also known as shrubification, across arctic and subarctic ecosystems is expected to increase with climate change, with implications for vegetation structure, biodiversity, and soil carbon dynamics. Previous research has found a shrubification-induced decrease in organic matter stocks from tundra to forest in the organic horizon, prompting further investigation into whether this pattern persists in the mineral soil. This study examines soil, vegetation, and microbial changes along a subarctic forest-tundra ecotone near Abisko, Sweden, focusing on how shrubification affects carbon stocks in both particulate organic matter (POM) and mineral-associated organic matter (MAOM) soil fractions in the mineral horizon. In this study, across seven sites spanning tundra to birch forest, increasing shrub and tree cover was linked to more acidic soils, reduced microbial diversity, and greater fungal dominance. Despite these biotic and abiotic shifts, mineral soil carbon stocks showed no consistent trend linked to shrubification, and were not associated with the measured microbial, vegetation, and edaphic parameters. However, fungal community composition, especially ericoid mycorrhizal taxa, strongly predicted the C:N ratio, suggesting a role in nutrient cycling via nitrogen mining, which did not seem to be associated with a change in organic matter stocks. These results suggest that while shrubification alters vegetation and microbial communities, mineral soil C may be buffered from rapid change. The weak predictability of mineral soil C stocks highlights the need to consider additional factors in assessing carbon cycle responses to subarctic vegetation shifts in order to improve global carbon cycle models.

Popular Abstract

Northern Shrubs on the March

Climate change is helping woody shrubs and small birch trees creep northward and uphill in the northern Swedish mountains. That movement of woody plants, called shrubification, raises big questions: will new plants lock away more carbon in the soil, or will they disturb the vast underground store that keeps huge amounts of carbon out of the air? We know from previous studies that in the top layer of soil, called the organic horizon, this increase in shrubs and trees causes a decrease in the underground stocks of carbon. But does this hold true in the deeper soil underneath? To find out, I looked at a stretch of land near Abisko in northern Sweden that runs from open tundra to full birch forest, treating it like a glimpse into the future.

At seven sites along this stretch of land, I identified all of the plants, measured the soil’s moisture and temperature, and collected the deeper “mineral” soil for lab tests. Those tests involved quantifying the soil microbes and seeing what kinds of microbes were found where, in addition to measuring how much carbon was stored in the soil.

We found that soils beneath shrubs and trees were more acidic. Moreover, when the land had more birch trees and shrubs, fungi flourished, while the total variety of microbes fell. Surprisingly, deeper‑soil carbon stayed roughly the same from bare tundra to forest, even though surface layers in other studies often lose carbon when shrubs move in. We also found some evidence that certain fungi, while getting nutrients for the plants they work together with, raised the carbon‑to‑nitrogen ratio of the soil, but without changing the overall carbon stocks.

These findings suggest the deeper soil is more resilient than the top layer to aboveground vegetation shifts, at least as far as carbon storage goes. But why is this? Local small-scale differences such as snow drifts and grazing reindeer may matter as much as the march of shrubs.  Pinning down those extra factors will help scientists sharpen global models that predict how much carbon these ecosystems can store. So what should be done? Removing shrubs or increasing grazing might protect open‑tundra plants and microbial diversity, but it probably will not change much in regards to the carbon stores in the deeper soil. Over the long haul, curbing greenhouse‑gas emissions remains the surest way to keep both arctic and subarctic landscapes and their complex soils in balance.

Master’s Degree Project in Conservation Biology, 45 credits, 2025
Department of Biology, Lund University
Advisor: François Maillard
Functional Ecology, Lund University

• author: Francis, Hanna
• supervisor: Francois Maillard
• organization: Degree Projects in Biology
• course: BION02 20242
• year: 2025
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 9212836
• date added to LUP: 2025-09-19 10:12:32
• date last changed: 2025-09-19 10:12:32