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).