Climate warming as a driver of tundra shrubline advance

Abstract

Abstract Climate warming is predicted to alter ecological boundaries in high‐latitude ecosystems including the elevational or latitudinal extent of tall shrubs in Arctic and alpine tundra. Over 60 studies from 128 locations around the tundra biome have investigated shrub expansion in tundra ecosystems; however, only six studies test whether shrublines are actually advancing up hill‐slopes or northward into tundra where tall shrubs are currently absent. We test the hypothesis that willow shrublines have expanded to higher elevations in relation to climate across a 50 × 50 km area in the Kluane Region of the southwest Yukon Territory, Canada by surveying of 379 shrubs at 14 sites and sampling of 297 of the surveyed shrubs at 10 sites. We compared growth and recruitment to climate variables to test the climate sensitivity of shrub increase using annual radial growth analysis, age distributions and repeat field surveys to estimate the current rate of shrubline advance. We found consistent and increasing rates of recruitment of alpine willows, with estimates of faster advancing shrublines on shallower hill‐slopes. Mortality was extremely low across the elevation gradient. Aspect, elevation and species identity did not explain variation in recruitment patterns, suggesting a regional factor, such as climate, as the driver of the observed shrubline advance. Annual radial growth of willows was best explained by variation in summer temperatures, and recruitment pulses by winter temperatures. Measured recruitment rates are ~20 ± 5 individuals per hectare per decade (M ± SE) and measured rates of increased shrub cover of ~5 ± 1% per decade (M ± SE) measured at the Pika Camp site between field surveys in 2009 and 2013. Our results suggest that shrubline will continue to advance over the next 50 years, if growing conditions remain suitable. However, if future conditions differ between summer and winter seasons, this could lead to contrasting trajectories for recruitment vs. growth, and influence the vegetation change observed on the landscape. Synthesis. Our findings in the context of a review of the existing literature indicate that elevational and latitudinal shrublines, like treelines, are advancing in response to climate warming; however, the trajectories of change will depend on the climate drivers controlling recruitment vs. growth.