Climate change effects on understory plant phenology: implications for large herbivore forage availability

Abstract

Consistent with a warming climate, the timing of key phenological phases (i.e. phenophases) for many plant species is shifting, but the direction and extent of these shifts remain unclear. For large herbivores such as ungulates, altered plant phenology can have important nutritional and demographic consequences. We used two multi-year datasets collected during 1992–1996 and 2015–2019 of understory plant phenology in semi-arid forested rangelands in northeastern Oregon, United States, to test whether the duration of phenophases for forage species has changed over time for three plant functional groups (forbs, graminoids, and shrubs). Duration of spring green-up was approximately 2 weeks shorter in the later years for forbs (19 ± 3.8 d) and graminoids (13.2 ± 2.8 d), and senescence was 3 weeks longer for graminoids (25.1 ± 5.1) and shrubs (22.0 ± 4.6). Average peak flowering date was 3.1 ± 0.2 d earlier per decade for understory forage species with approximately 1/3 of the species (35%) exhibiting earlier peak flowering dates over time. Variation in late-winter precipitation had the greatest effect on the duration of understory green-up, whereas variation in summer precipitation had a greater effect on duration of the senescent period. Collectively, these results indicate climate-related progression towards shorter periods of peak plant productivity, and earlier and longer periods of plant senescence, the combination of which substantially reduces the temporal window of forage available in growing forms most usable to herbivores. This work adds a needed component to the climate change literature, by describing links between shifting climate variables, multiple phases of understory plant phenology, and possible nutritional consequences for herbivores under a warming climate.