Aim

The Climate Variability Hypothesis (CVH) predicts that locations with reduced seasonal temperature variation select for species with narrower thermal ranges. Here we (1) test the CVH by assessing the effect of latitude and elevation on the thermal ranges of Andean vascular plant species and communities, and (2) assess tropical alpine plants vulnerability to warming based on their thermal traits.

Location

Tropical Andes

Taxon

Vascular plants

Methods

Temperature data for 505 vascular plant species from alpine communities on 49 summits, were extracted from 29,627 geo-referenced occurrences. Species thermal niche traits (TNTs) were estimated using bootstrapping for: minimum temperature, optimum (mean) temperature, and breadth (maximum-minimum). Plant community-weighted scores were estimated using the TNTs of their constituent species. CVH was tested for species, biogeographic species groups and communities. Vulnerability to global warming was assessed for species, biogeographic species groups and communities.

Results

Species restricted to the equator showed narrower thermal niche breadth than species whose ranges stretch far from the equator, however, no difference in niche breadth was found across summits’ elevation. Biogeographic species groups distributed close to the equator and restricted to alpine regions showed narrower niche breadth than those with broader ranges. Community weighted-scores of thermal niche breadth were positively related to distance from equator but not to elevation. Based on their TNTs, species restricted to equatorial latitudes and plant communities dominated by these species were identified as the most vulnerable to the projected 1.5 °C warming, due to a potentially higher risk of losing thermal niche space.

Main conclusions

Our study confirms that the CVH applies to high tropical Andean plant species and communities, where latitude had a strong effect on the thermal niche breadth. TNTs are identified as suitable indicators of species’ vulnerability to warming and are suggested to be included in long-term biodiversity monitoring in the Andes.

Thermal niche traits (optimum temperature, minimum temperature and niche breadth) were estimated for each species. We used all available geo-referenced records of each species to extract monthly mean temperatures for each location record. We collected 29,627 species records from on-line databases and South American herbaria, including 1,388 records from the summit sites. Mean monthly temperature maps were obtained from WorldClim gridded datasets (Hijmans, Cameron, Parra, Jones, & Jarvis, 2005) and then were downscaled to a 90 m pixel resolution. For this we used the Shuttle Radar Topography Mission Digital Elevation Model (SRTM DEM; http://www2.jpl.nasa.gov/srtm/) with a lapse rate of 0.54 °C 100 m^-1 (Bush, Silman, & Urrego, 2004).

To estimate the species TNTs, we first calculated the mean, minimum and maximum temperature of each location record only using the months of the growing period. The length of the growing period (active season) varies along the latitudinal gradient (Körner, Paulsen, & Spehn, 2011). Around the equator the growing season is all year round. Moreover, significant inactive seasons in terms of prolonged periods with low temperatures are exceptional for most of the entire tropical area involved. For transitional areas (i.e. 15-18 °N; 23-27 °S) and outside of the tropics (> 27 °S), the ‘inactive season’ was defined here as the months where mean air temperature is equal or below 0 °C.

Second, we used the estimated values of mean, minimum and maximum temperatures of all records per species to obtain 1,000 bootstrapped observations of the mean, minimum and maximum temperatures per species. Then we estimated the mean of the bootstrapped observations to obtain unique values for each of the three temperature values per species and obtained the following species thermal niche traits: optimum temperature (mean), minimum temperature and niche breadth (maximum – minimum temperature).