Short‐term effects of summer warming on caribou forage quality are mitigated by long‐term warming

Abstract

Abstract Rapid Arctic climate change is leading to woody plant‐dominated ecosystems with potential consequences for caribou foraging and nutritional ecology. While warming has been clearly linked to shrub expansion, the influence of higher temperatures on variables linked to the leaf‐level quality of caribou forage is equivocal. Moreover, warming results in a complex set of ecosystem changes that operate on different timescales such as not only rapidly accelerating phenology, but also slowly increasing thaw depth and plant access to soil resources. Here, we compare changes in leaf nitrogen (N) concentration, digestibility, and protein‐precipitating capacity (PPC) in short‐term (i.e., <1–2 summers) and long‐term (approximately 25 years) experimental warming plots with ambient temperature plots for three species commonly included in caribou summer diets: Salix pulchra (diamond‐leaf willow), Betula nana (dwarf birch), and Eriophorum vaginatum (cottongrass). Short‐term warming modestly decreased leaf N concentration in B. nana. Long‐term and short‐term warming slightly increased the digestibility of S. pulchra, but only short‐term warming increased digestibility in B. nana. Greater dry matter digestibility in both shrubs occurred through reductions in the lignin and cutin quantity in plant cells. Long‐term warming had no impact on PPC and equivocal impact on digestible protein of B. nana. Overall, we found short‐term warming to be more impactful on forage quality than long‐term warming at Toolik Lake, Alaska. Apart from a long‐term warming reduction of approximately 13% in acid detergent lignin in S. pulchra and B. nana, other differences were only observed in the short‐term warming plots. Hence, our results indicate acclimation of plants to long‐term warming or possible negative feedback in the system to reduce warming effects. We suggest that warming summers may have a lesser effect on caribou forage than changes in winter precipitation or the influence of climate change on the abundance of critical species in the caribou diet.

Keywords

• digestibility
• leaf N concentration
• long‐term warming
• Rangifer tarandus
• secondary compounds
• short‐term warming