2.1 Study site
A field-manipulative warming experiment was performed at the Haibei
Grassland Ecological Monitoring Station (Xihai Town, Qinghai province,
100°51´E, 36°57´N, 3,140 m a.s.l.). The study site has been used as a
winter grazing grassland since 1976, with moderate grazing intensity
during the non-growing season. Based on meteorological records from 1995
to 2013, the mean annual precipitation is 408 mm, and the mean annual
temperature is 1.3 oC
(Chen et al. 2016). Relatively high
air temperature and rainfall occur from mid-April to mid-October
(growing season), while low rainfall (< 5% of annual) and low
temperatures occur during the non-growing season. Additional long-term,
detailed information about the study site can be found in
(Chen et al. 2015b;
Guo et al. 2018;
Li et al. 2019) (Fig. S1). The
dominant species are Stipa krylovii, Poa crymophila, Koeleria
cristata, Medicago ruthenica, and Kobresia humilis (Fig. S1 and
Table S1).
2.2
Experimental design
The study site (200 m × 400 m) was fenced off for three years before the
initiation of experimental warming to exclude disturbance by herbivores.
All mammalian herbivores were completely removed for the whole
experiment duration. In August 2010, the entire study site was divided
into six blocks with 10-m buffer zones between each edge of adjacent
blocks. Each block was then divided into two plots (5 m × 10 m), one of
which was randomly selected for experimental warming. In each of the
experimental warming plots, one OTC (with a base area of 2.1
m2, six OTCs in total) was installed to achieve a
passively warmed environment (Fig. S1). Protocols for the OTCs used in
this study are described in detail for previous studies at this site
(Chen et al. 2017a;
Chen et al. 2017b), as well as at
other sites (Dorji et al. 2013;
Baruah et al. 2017).