Contrasting impacts of short- and long-term large herbivore exclusion on understory net CO2 exchange in a boreal forest

Across boreal forests, trees are the main living biomass carbon (C) stock, but the understory vegetation can contribute significantly to the C cycling and net forest carbon dioxide (CO2) balance. The patchy understory vegetation which consists of sunlit (i.e., lichen-like) and shaded habitats (i.e., dwarf shrub-like) is often altered by ungulate grazers. Grazers may influence understory CO2 exchange, and consequently, the forest CO2 balance. Grazing affects differently the biomass of slow growing lichens compared to the faster growing mosses and dwarf shrubs, and therefore the effects of grazing on CO2 exchange in the patchy understory vegetation could vary temporally. We studied how excluding grazing for short- and long-term affects the CO2 exchange and vegetation biomass in the understory of an oligotrophic Scots pine forest. We measured growing season (2019, 2020) CO2 exchange across sunlit and shaded habitats inside fences that had excluded large grazers for 0–1 and 25–26 years and in the adjacent grazed area. In addition, we measured the height of understory vegetation. We found that short-term grazer exclusion increased ecosystem CO2 source fluxes only in the shaded habitats. However, long-term exclusion of grazing decreased CO2 net release regardless of the habitat type. Furthermore, grazer exclusion increased moss depth immediately which coincided with an abrupt intensification of CO2 net release. Considering the impacts of grazing over both short- and long-term may help to forecast C fluxes more accurately which may be relevant for informed climate solutions regionally and even on a larger scale.

Methods
CO2 fluxes were measured on two understory vegetation types in short- and long-term large herbivore exclosures and in an adjacent grazed area with manual and automated chamber over the growing seasons of 2019 and 2020. The manual CO2 measurements were accompained with soil temperature measurements. In addition, vegetation height was measured across study plots in 2020. This data consist of four different data sets as follows: automated CO2, manual CO2, vegetation height, and soil temperature. 

Automated CO2 data

The automated system measured CO2 fluxes at an interval of 1h and continued for 24h day-1 from June until October in 2019 and 2020. Each measurement consisted of a 2-min pre-purge period, a 3-min measurement period and a 60 sec post-purge period. The system (model LI-8100A, Li-Cor Inc., Lincoln, NE, USA) consisted of an analyzer control unit (LI-8100A), a multiplexer (LI-8150), and eight automated clear chambers (8100-104C). NEE was calculated from the exponential change of the CO2 concentration during chamber closure excluding the 45-second ‘dead band’.

Manual CO2 data

The manual measurements were taken 1-3 per week from May until October in 2019 and 2020. The measurement protocol consisted of two measurements at each study plot: net ecosystem exchange (NEE) under full light, then the chamber was vented, placed over the ground, and covered with an opaque chamber for measuring ecosystem respiration (ER). The system consisted of a custom-made chamber (2-mm thick transparent polycarbonate, diam. 30 cm × height 39 cm) equipped with a fan, a CO2 sensor (model GMP343, Vaisala Inc., Vantaa, Finland), and air temperature and humidity sensor (model HMP75, Vaisala Inc.). CO2 concentration (ppm), relative humidity (%), and temperature (℃) during each manual measurement were logged every 5 sec for 2 min with a handheld logger (model MI70, Vaisala Inc.). NEE and ER were calculated from the linear change of the CO2 concentration in the chamber by the ideal gas equation, and gross ecosystem productivity (GEP) was obtained by subtracting ER from NEE.

Vegetation height

Vegetation heights were measured at two locations within each study plot: one in the CO2 study plot and the other in an undisturbed monitoring plot. For undisturbed monitoring plots, the height of six dwarf shrubs, lichens, and mosses (if available) were measured and then average height for each vegetation group was calculated. In CO2 study plot, the height of three dwarf shrubs, lichens, and mosses (if available) were measured and these values were used to calculate the average height for each vegetation group.

Soil temperature

Soil temperature was measured manually from three points in each short-term exclosure CO2 study plot. The values were used to calculate the average soil temperature for each CO2 study plot. Model: Lollipop™ Traceable Thermometer, Traceable® Products Inc., Webster, TX, USA.