Salix shrub expansion over the past 62 years in rocky mountain alpine tundra

Global change factors are causing a shift in tundra vegetation in that woody plants are encroaching into herbaceous-dominated communities. This drastic change in functional composition of the tundra has consequences for ecosystem function, notably carbon storage. Quantifying the rates of woody plant encroachment and determining which tundra community types are being invaded is essential for predicting future trajectories in tundra vegetation as well as assessing the implications this will have for the tundra as a carbon sink. This study examines changes in willow (Salix planifolia) shrub cover from 1946-2008 in a 0.18 km² area of alpine tundra on Niwot Ridge, CO using aerial photographs. 

The photo series reveals that shrub cover has increased by almost 5 times (370%) over the 62-year period, and is increasing at a faster rate in recent years. This corresponds to an increase in carbon storage in woody biomass of 130 kg/ha in the study region. Shrubs have expanded into many different community types, including wet, moist, and dry meadow. Shrub expansion may be due to elevated N deposition and may be reinforced by positive snow-N cycling feedbacks generated by the willows themselves, since Niwot is not experiencing warming. Some of this shrub growth response may also be due to cessation of sheep grazing in 1946. This change in shrub biomass will have implications for carbon cycling as well as herbaceous tundra vegetation and physical properties in the alpine.