Global Change Affects Large Herbivore Forage Biomass Through Gradual Successional Shifts and Abrupt Disturbances

Forage availability is a key factor regulating large herbivore populations. Global changes in land use and climate may affect the spatiotemporal distribution of forage across the ranges of large herbivores, especially in mountain ecosystems. We test two synergistic hypotheses for how landscape and climate changes from 2001 to 2023 have affected forb and graminoid biomass at the peak of the growing season within and across ecoregions of the eastern slopes of the Rocky Mountains in Alberta, Canada. The successional change hypothesis posits that the encroachment of woody vegetation into previously herbaceous communities has reduced forage biomass. The disturbance hypothesis proposes that abrupt community shifts caused by fire and timber harvesting have increased forage biomass. Using remote sensing, we quantify temporal changes in land cover and disturbances, NDVI greenness and phenology indices, and spring climate. We then used in situ vegetation data to parameterize generalized linear and gradient boosted regression tree models of forb and graminoid biomass to predict annual peak forb and graminoid biomass. Herbaceous land cover declined while shrub and forest area increased, and the percent of annual biomass within herbaceous areas declined from woody encroachment. Disturbance effects varied, with rising forage biomass in conjunction with increased area of logged forests in the foothills ecoregion, while burned areas declined and had a reduced contribution to the percent of annual biomass. Additionally, spring became warmer across the study area and ended earlier in the alpine, suggesting the effects of long-term climatic shifts may be strongest at higher ecoregions. Disturbance frequency, succession, and climate together shaped forage biomass in space and time. Increased prescribed fire and other ecological restoration actions may be needed to ensure that shifts in forage biomass do not threaten large herbivore persistence in the face of global change.