Resampling alpine herbarium records reveals changes in plant traits over space and time

Abstract

Abstract Climate warming causes upward shifts in plant species distributions, resulting in an influx of species from lower elevations into alpine plant communities. Plant functional trait changes along elevation gradients and over time may reflect these changing conditions. Intraspecific trait variation measured from herbarium records offers a way to observe such changes in trait values over time. We selected four species: Poa alpina and Polygonum viviparum found in alpine grasslands, and Cardamine resedifolia and Ranunculus glacialis found in high‐alpine to subnival scree habitats. We measured several functional traits from (i) herbarium records collected between 1880 and 1950 and from (ii) individuals resampled in 2014 along an elevation gradient covering >1500 m within the same study region in the Swiss Alps. By comparing (i) against (ii) for each species separately, we analysed temporal changes in the distribution of traits along the studied elevation gradient. After a century of climate warming, the change in the relationship linking plant functional traits with elevation was species dependent. Size‐related and reproductive functional trait values for P. viviparum increased over time, increasing at lower, but not higher elevations. Poa alpina's size‐related traits increased consistently with time along the elevation gradient. Most of C. resedifolia's size‐related and flowering traits decreased over time at lower elevations and converged at higher elevations. Finally, R. glacialis traits did not respond to time alone—reproductive traits decreased over time at lower, and increased at higher elevations, reversing their historical trait distributions. The negative trend for vegetative trait values with elevation did not change over time, however. In 2014, at lower elevations, all species mainly occurred on their typical microhabitat types, but occurrence on other microhabitats increased with elevation for all species. Synthesis: Contrasting temporal changes in the distribution of growth and reproductive trait values between alpine grassland and alpine‐subnival scree species, especially at lower elevations, suggest that climate warming effects vary among species. Additionally, species' physiological constraints and availability of suitable microhabitats may further impact species' distribution changes. Further warming may confine the distribution of high‐alpine plant species to even higher elevations, or microclimates currently difficult to colonise by lower‐alpine species.