The ongoing climate warming is promoting shrub abundance in high latitudes, but the effect of this phenomenon on ecosystem functioning is expected to depend on whether deciduous or evergreen species increase in response to warming.

To explore effects of long-term warming on shrubs and further on ecosystem functioning, we analyzed vegetation and ecosystem CO₂ exchange after 20 years of warming in the forest-tundra ecotone in sub-arctic Sweden. A previous study conducted nine years earlier had found increased evergreen Empetrum nigrum ssp. hermaphroditum in the forest and increased deciduous Betula nana in the tundra.

Following current understanding, we expected a continued increase in shrub abundance that would be stronger in tundra than in forest. We expected warming to increase ecosystem respiration (R_e) and gross primary productivity (GPP), with a greater increase in R_e in tundra due to increased deciduous shrub abundance, leading to a less negative net ecosystem exchange (NEE) and reduced ecosystem C sink strength.

As predicted, vascular plant abundances were higher in the warmed plots with a stronger response in tundra than in forest. However, whereas B. nana had increased in abundance since the last survey, E. hermaphroditum abundance had declined due to several moth and rodent outbreaks during the past decade. In contrast to predictions, R_e was significantly lower in the warmed plots irrespective of habitat, and GPP increased marginally only in the forest. The lower R_e and a higher GPP under warming in the forest together led to increased net C sink. R_e was negatively associated with the total vascular plant abundance.

Our results highlight the importance of disturbance regimes for vegetation responses to warming. Climate warming may promote species with both a high capacity to grow under warmer conditions and a resilience towards herbivore outbreaks. Negative correlation between R_e and total vascular plant abundance further indicates that the indirect impacts of increased plants on soil microclimate may become increasingly important for ecosystem CO₂ exchange in the long run, which adds to the different mechanisms that link warming and CO₂ fluxes in northern ecosystems.

Vegetation analyses

The plant community composition was earlier recorded in 1999 and 2009 in five control plots and five OTCs in both habitats (Kaarlejärvi et al. 2012). We used the same plots during the present investigation and analyzed the composition of vegetation in July 2018 with the point intercept method: in OTCs, a total of 87 pins was systematically distributed among three diagonals of the hexagons, 29 pins per diagonal. For each pin, the total number of hits as well as the height of the highest hit were recorded for each plant. Only one hit for each species was counted at the ground layer for each pin. The same method was applied to control plots. Later the total number of hits was normalized to hits per 100 pins. Data from one forest plot was discarded because of poor plot condition.

Ecosystem carbon flux analyses

For the ecosystem carbon flux analyses, we included a few additional plots to have seven plots per treatment in the forest and eight plots per treatment in the tundra. The fluxes were analyzed at two-week intervals throughout the growing season 2018 (from 5^th of June to 19^th of August) using a closed system composed of a custom-built acrylic chamber (diameter 146 cm, height 60 cm) coupled to a Vaisala Carbon Dioxide Probe GMP343, Vaisala Humidity and Temperature Probe HMP75 and Vaisala Measurement Indicator MI70. Measurements included four consecutive measures of gradually changing light intensity: ambient light, 35% and 60% shading, and darkness to reveal ecosystem respiration, R_e. Shading was implemented using hoods made of single- and double-layer white mosquito nets while darkness was obtained by covering the chamber with an opaque white hood. The chamber was vented before each measurement and placed carefully on top of the study plot so that the leakage of air from beneath the chamber was minimized. Photosynthetically active radiation (PAR) within the chamber was recorded using an HD 9021 Quantum-Photo-Radiometer. The CO₂ concentration, temperature, and humidity within chambers were logged at 5-s intervals for 90 s. The CO₂ flux was calculated using CO₂ and the chamber microclimate data and corrected for changes in temperature and water vapor pressure (Hooper et al., 2002). The net CO₂ flux with light intensity above zero was regarded as NEE. For NEE, negative fluxes indicate a net uptake of CO₂ from the atmosphere, whereas positive fluxes indicate a net release of CO₂ into the atmosphere. 

For the comparison of daily CO₂ flux measurements between the treatments and control plots we normalized GPP to the PAR level of 600 µmol m^-2 s^-1. The GPP was calculated from the NEE and R_e as:

GPP = NEE - R_e

Daily plot-specific GPP values were fitted to their corresponding PAR levels using the nonlinear least squares (nls) function from stats package in R software environment  as:

GPPij = A_maxPAR / k + PAR

where i stands for i^th plot and j for j^th date, A_max is the maximum GPP rate when saturated to light (mg CO₂ m⁻² h⁻¹) and k is the half-saturation light constant (μmol m⁻² s⁻¹). Subsequently, the GPP₆₀₀ was calculated for each plot and day at the light level of 600 μmol m⁻² s⁻¹ using Eq. 2 with PAR set to 600.