2.3 Measurements
The main objective is to study the species’ specific phenological responses as well as their links with species dominance. To meet this objective, eight commonly shared plant species were selected and monitored across the three growing seasons, covering all plant functional groups at the study site (three grasses, one sedge, one legume, three forbs). Based on our preliminary field investigations, those eight species were the only common species observed in all experimental plots at the study site (Fig. S1 and Table S1). The total aboveground biomass of those eight species occupied more than 85% of the total community biomass and the total cover of those eight species occupied more than 90% of the total community cover (Table S1) (Chen et al. 2017a; Chen et al. 2018).
To avoid the edges effects caused by OTCs, a quadrat (0.5 m × 0.5 m) was selected from the center area within each OTC as well as from the center area in each ambient plot for the species’ phenology and dominance measurements. To reduce the heterogeneity and uncertainties associated with variations among individual plants, six individuals of each commonly shared species were selected and marked in each quadrat after the first leaf out. These marked individuals for each species in each quadrat were monitored every 2 to 4 days during the whole growing season. Phenological observations of each species in each quadrat were the average of those marked individuals, in case that some individuals got lost or died during the growing seasons. The first flower dates were documented when the flower buds had broken and anthers/stigmas were visible for the marked plants (Suonanet al. 2017). The last flower dates were measured when all petals had dropped off (Iler et al.2013; CaraDonna et al. 2014). Leaf senescence was defined as more than 50% of a plant’s leaves having changed color (Marchin et al.2015). The duration of the growing season was calculated as the difference between leaf out and leaf senescence dates, and the length of flower duration was calculated as the difference between first and last flower dates. Phenological observations for each species in each quadrat were recorded every 2 to 4 days during the growing seasons from 2011 to 2013. All phenological observations were transformed into Julian days for further analysis.
Height, abundance and cover for each species within the quadrats in ambient and warming plots were recorded during the peak biomass period, which was usually in mid-August (apart from Gentiana squarrosa , which is in mid-July). Height for each species was calculated as the mean of the marked individuals. Species abundance was calculated as the total number of each species within the quadrat. A gird frame (0.5 m × 0.5 m) with 25 grid cells (0.1 m × 0.1 m) was placed in each quadrat to help estimating the total areal cover of each focal species in each plot (Penuelas et al. 2004; Damgaard 2014).
Soil temperature during the whole year and soil volumetric moisture during the growing season (frozen during the non-growing season) for each plot were documented using HOBO data loggers at a depth of 10 cm (Onset Computer Company, USA) (Chenet al. 2016). For each plot, three soil cores (0-10 cm) adjacent to the quadrats were collected and combined to make a composite soil sample in mid-August. Soil inorganic N content was measured using a flow injection auto-analyzer (FIAstar 5000 Analyzer, Denmark).