Experimental warming alters migratory caribou forage quality

Abstract

Abstract. Global declines in caribou and reindeer (Rangifer) populations have drawn attention to the myriad of stressors that these Arctic and boreal forest herbivores currently face. Arctic warming has resulted in increased tundra shrub growth and therefore Rangifer forage quantity. However, its effects on forage quality have not yet been addressed although they may be critical to Rangifer body condition and fecundity. We investigated the impact of 8 yrs of summer warming on the quality of forage available to the Bathurst caribou herd using experimental greenhouses (n = 5) located in mesic birch hummock tundra in the central Canadian Low Arctic. Leaf forage quality and digestibility characteristics associated with nutrients (nitrogen and phosphorus), phenolics, and fiber were measured on the deciduous shrub Betula glandulosa (an important Rangifer diet component) at six time points through the growing season, and on five other very common vascular plant and lichen species in late summer. Experimental warming reduced B. glandulosa leaf nitrogen concentrations by~10% in both late June and mid-July, but not afterwards. It also reduced late summer forage quality of the graminoid Eriophorum vaginatum by increasing phenolic concentrations 38%. Warming had mixed effects on forage quality of the lichen Cetraria cucullata in that it increased nutrient concentrations and tended to decrease fiber contents, but it also increased phenolics. Altogether, these warming-induced changes in forage quality over the growing season, and response differences among species, highlight the importance of Rangifer adaptability in diet selection. Furthermore, the early season reduction in B. glandulosa nitrogen content is a particular concern given the importance of this time for calf growth. Overall, our demonstration of the potential for significant warming impacts on forage quality at critical times for these animals underscores the importance of effective Rangifer range conservation to ensure sufficient appropriate habitat to support adaptability in forage selection in a rapidly changing environment.

NDVI exhibits mixed success in predicting spatiotemporal variation in caribou summer forage quality and quantity

“…2), dependent on species-and sitespecific changes in fiber, lignin, and phenolic concentrations (Zamin et al 2017, Barboza et al 2018. As plants first initiate their spring growth, forage abundance and quality simultaneously increase, driving stronger correlations between forage components and NDVI.…”

“…To better visualize this relationship, we used our LMMs to project values of N on the coastal plain across the growing season for different dates of peak NDVI (Fig. Warmer summer temperatures and accelerated phenology have been associated with increases in plant biomass, but reductions in concentrations of forage nitrogen (J onsd ottir et al 2005(J onsd ottir et al , Doiron et al 2014, although the responses of individual species are mixed (Welker et al 2005) and can exhibit complex dynamics (Zamin et al 2017). Earlier dates of peak NDVI were associated with earlier seasonal declines in forage N, resulting in particularly low N values late in the summer when female caribou need to amass body stores for future reproduction Parker 2008, Gustine et al 2017).…”

“…Some investigators have suggested that advanced spring phenology is likely to improve summer foraging opportunities for caribou due to earlier availability of high quality resources (Cebrian et al 2008). driven nitrogen dilution, plant secondary compounds, winter snow conditions, and insect harassment (Turunen et al 2009, Culler et al 2015, Zamin et al 2017). 5.…”

The nitrogen window for arctic herbivores: plant phenology and protein gain of migratory caribou (Rangifer tarandus)

“…Although this species of birch does not make up a large part of North Slope caribou diets at present, it may be eaten by caribou more frequently in the future because it is increasing in abundance throughout the Arctic (Sturm et al 2001, Myers-Smith et al 2011, Kaarlejarvi et al 2012). Indeed, another birch, B. glandulosa, makes up a significant part of caribou diets in Quebec (Crete et al 1990, Zamin et al 2017.…”

Tactical departures and strategic arrivals: Divergent effects of climate and weather on caribou spring migrations

“…, Zamin et al. ) or can lead to increased insect harassment (Walsh et al. ), which has significant impacts on caribou physical condition (Helle and Tarvainen , Helle and Kojola , Walsh et al.…”