Fast Responses of Root Dynamics to Increased Snow Deposition and Summer Air Temperature in an Arctic Wetland

Abstract

In wet tundra ecosystems, covering vast areas of the Arctic, the belowground plant biomass exceeds the aboveground, making root dynamics a crucial component of the nutrient cycling and the carbon (C) budget of the Arctic. In response to the projected climatic scenarios for the Arctic, namely increased temperature and changes in precipitation patterns, root dynamics may be altered leading to significant changes in the net ecosystem C budget. Here, we quantify the single and combined effects of 1 year of increased winter snow deposition by snow fences and summer warming by open-top chambers (OTCs) on root dynamics in a wetland at Disko Island (West Greenland). Based on ingrowth bags, snow accumulation decreased root productivity by 42% in the 0-15 cm soil depth compared to ambient conditions. Over the growing season 2014, minirhizotron observations showed that root growth continued until mid-September in all treatments, and it peaked between the end of July and mid-August. During the season, plots exposed to experimental warming showed a significant increase in root number during September (between 39 and 53%) and a 39% increase in root length by the beginning of September. In addition, a significant reduction of root diameter (14%) was observed in plots with increased snow accumulation. Along the soil profile (0-40 cm) summer warming by OTCs significantly increased the total root length (54%), root number (41%) and the root growth in the 20-30 cm soil depth (71%). These results indicate a fast response of this ecosystem to changes in air temperature and precipitation. Hence, on a short-term, summer warming may lead to increased root depth and belowground C allocation, whereas increased winter snow precipitation may reduce root production or favor specific plant species by means of reduced growing season length or increased nutrient cycling. Knowledge on belowground root dynamics is therefore critical to improve the estimation of the C balance of the Arctic.

Keywords: arctic tundra; minirhizotrons; open top chambers (OTC); root dynamics; snow fence; warming; wetland.

FIGURE 1

Continuous measurements of (A) air temperature within the canopy (2 cm above soil surface) and air temperature at 2 m height, (B) soil temperature and (C) soil moisture and precipitation at the site in 2013–2014 (n = 3). Daily averages of the main factors ± snow accumulation and ± warming with OTCs are shown. Dotted vertical lines indicate the study period. A common legend to all three graphs is reported in (A).

FIGURE 2

Root biomass (mean ± SE) based on soil samples from all plots in July 2013 at the time of minirhizotron installation (n = 24).

FIGURE 3

Relative depth-specific fine root productivity (mean ± SE) estimated from ingrowth bags placed at control and at snow accumulation plots (n = 6). Significant treatment effects are indicated by a ^∗P ≤ 0.05.

FIGURE 4

Root parameters measured over the growing season 2014 (mean ± SE). (A) Total root number by tube. (B) Total root length per tube area. (C) Total root surface area per tube area. (D) Average root diameter by tube. Tendencies and significant repeated treatment effects are reported for each parameter and indicated by ^†P ≤ 0.1, ^∗P ≤ 0.05, ^∗∗P ≤ 0.01, and ^∗∗∗P ≤ 0.001. In legend: control (C), warming by OTCs (W), snow accumulation (S), and snow + warming by OTCs (SW).

FIGURE 5

Increment of cumulative root length growth between consecutive measurement campaigns (mean ± SE) estimated at each treatment plot: control (C), warming by OTCs (W), snow accumulation (S), and snow + warming by OTCs (SW). The significant effect of day of year (DOY) is indicated by ^∗∗∗P ≤ 0.001.

FIGURE 6

Averaged values estimated for each treatment (mean ± SE) at each soil depth interval of maximum root growth per tube area. Significant treatment effects are indicated by a ^∗P≤ 0.05 and ^∗∗∗P< 0.001.