Our data reports the annual growth and age of the shrub Caragana versicolor overgrown or free of the cushion plant Thylacospermum ceaspitosum. The observations and sampling were performed at ~5000 m.a.s.l., on the East facing slope of the Korzok range, on the North-Eastern side of Lake Tso Moriri above Korzok village (4522 m a.s.l.). Our study demonstrates that the growth of Caragana versicolor is repressed by the competitive effect of the cushion plant Thylacospermum ceaspitosum, and shows no evidence of facilitation between the two co-occurring species.

Study area and target species

The study was conducted in Ladakh, NW India.  Overall the climate in the region is cold and arid due to its high elevation (> 4500 m a.s.l.) and its position in the rain shadow of the Himalaya Range. The study site where the observations and sampling were performed was located at ~5000 m.a.s.l., on the East facing slope of the Korzok range, on the North-Eastern side of Lake Tso Moriri above Korzok village (4522 m a.s.l., 32.968125°N 78.2639885). Annual precipitation is <250 mm per year around the lake (mostly during the Indian summer monsoon), and temperatures in the study area vary from 0˚C to 30˚C in summer and from -40˚C to -10˚C in winter (Liancourt et al. 2017 and references therein). Thylacospermum caespitosum (Caryophyllaceae) is a prominent cushion plant in the Himalayas, which can be found from 4800–5850 m.a.s.l. in the region. It forms a very dense and solid cushion that does not seem to facilitate other species (de Bello et al. 2011, Dvorský et al. 2013 but see Jiang et al. 2018). Yet, it can be colonized by functionally diverse (Pinguet et al. 2018) but functionally similar competitive graminoids and herbaceous perennials from alpine grasslands (Dolezal et al. 2019). Caragana versicolor (Fabaceae) is the dominant shrub species that forms typical shrubland communities in the region (Dvorský et al., 2011 for a description of community types). C. versicolor is also a cushion-forming plant that has been described to facilitate other species (Liancourt et al. 2017).

Sampling and Data analyses

The effect of T. caespitosum on C. versicolor, competitive or facilitative, was inferred from dendrochronological analysis. During summer 2017, we sampled 2 twigs per individual (duplicates hereafter) for 14 shrubs of C. versicolor overgrown by T. caespitosum and for 10 adjacent shrubs growing free of T. caespitosum. Samples were stored in 40 % ethanol to keep them soft and prevent mold. From all twigs, cross sections were cut using a sledge microtome, stained with Astra Blue and Safranin and embedded in Canada Balsam (Gärtner & Schweingruber 2013). Microscopic images of these sections were taken using Olympus BX53 microscope, Olympus DP73 camera and cellSense Entry 1.9 software. This enabled us both to count the number of rings and to determine the age of the twigs and their annual growth (Dolezal et al., 2018). Annual growth (µm²) was calculated as the difference in area for the twig for two consecutive years. Altogether 47 twigs (one twig had to be discarded) corresponded to 24 individuals and a total of 1110 individual rings were measured. We averaged the annual growth of the twigs over the last past 10 years for testing the effect of T. caespitosum on the growth of C. versicolor. Only 2 twigs were younger than 10 years (5 and 8 years old).

A linear mixed model was used to test the effect of T. caespitosum on the growth of C. versicolor (log-transformed), with “overgrown” (overgrown by T. caespitosum or free) as a fixed factor, “duplicate” as a random factor and “age” as a continuous covariate to control for the positive relationship between age and growth of the twigs (n = 47, R² = 0.25, P < 0.001).