Demographic response to transplantation explained by traits
To investigate if traits explained demographic responses to
transplantation, we quantified species’ position in multivariate trait
space by performing a Principal Component Analysis (“PCA”). We
included all 27 traits measured at 2000 m (“+0°C”) and six plasticity
indices for those traits that showed significant differences between
sites on average (WUEinst, GSnight,
Enight, δ18O enrich,
δ13C and depth, Figures S4-S7). To test if species’
locations in the trait space related to their demographic responses to
transplantation to lower elevation, we built linear mixed effects models
(LMMs) with either 2020 cover (cm2) or frequency
(number of quadrants where species was present) as the response
variable. We used both species cover and frequency, because we expected
these to correspond to different demographic processes: while changes in
frequency depend on species’ survival and recruitment, changes in cover
are also influenced by vegetative growth (and, in contrast to frequency,
potentially by errors in cover estimations). The response was modeled as
a function of the initial cover or frequency of the species per site in
2017, the mean summer temperature at each site, the first four PC trait
axes and the two-way interactions between the trait axes and
temperature. The significance of fixed effects was assessed by dropping
one explanatory variable at a time, starting from a full model. The
change in the model fit was assessed using likelihood ratio tests
(Crawley 2007). For responses showing a significant trait axis ×
treatment interaction, we evaluated the amount of variation explained by
calculating pseudo-R2 (Nakagawa & Schielzeth 2013).
To estimate the usefulness of traits as predictors, we compared the
amount of variation explained by traits to that explained by species
identity. To do so, we compared variation explained by models with or
without species or traits as fixed effects (see Table S3 for details).
To further investigate which traits were associated with demographic
response to climate, we tested if warming resulted in communities of
species possessing particular trait values. We calculated
community-weighted means for each trait per each replicate turf across
the four sites, based on the 2020 percent cover of all species and their
2000 m (+0°C) trait value. The only exceptions were
δ18O and δ13C, for which we used
both the site-specific trait values, and the values as measured at 2000
m (+0°C). For these two traits, we evaluated the degree to which changes
in community-weighted trait means were due to within-species trait
change versus species turnover. Differences in community-weighted mean
trait values between sites were tested with linear models.