DOI: 10.5061/dryad.qjq2bvqm2
title: Overgrowth competition or facilitation from cushion plants: Implication for the role of plant-plant interactions

description: 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.
description: 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 (&gt; 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 &lt;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). <em>Thylacospermum
 caespitosum</em> (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). <em>Caragana versicolor
 </em>(Fabaceae) is the dominant shrub species that forms typical
 shrubland communities in the region (Dvorský et al., 2011 for a description of community types).
 <em>C</em>. <em>versicolor</em> 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 <em>T</em>.
 <em>caespitosum</em> on <em>C. versicolor</em>,
 competitive or facilitative, was inferred from dendrochronological
 analysis. During summer 2017, we sampled 2 twigs per individual
 (duplicates hereafter) for 14 shrubs of <em>C</em>.
 <em>versicolor</em> overgrown by <em>T</em>.
 <em>caespitosum</em> and for 10 adjacent shrubs growing free
 of <em>T</em>. <em>caespitosum</em>. 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 (µm<sup>2</sup>) 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
 <em>T</em>. <em>caespitosum</em> on the growth of
 <em>C</em>. <em>versicolor</em>. Only 2 twigs were
 younger than 10 years (5 and 8 years old). A linear
 mixed model was used to test the effect of <em>T</em>.
 <em>caespitosum</em> on the growth of <em>C</em>.
 <em>versicolor</em> (log-transformed), with “overgrown”
 (overgrown by <em>T</em>. <em>caespitosum</em> 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, R<sup>2</sup> = 0.25, P
 &lt; 0.001).