Combined Effects of Simulated Browsing, Warming and Nutrient
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Polar Biology (2019) 42:1561–1570 https://doi.org/10.1007/s00300-019-02543-y ORIGINAL PAPER Combined efects of simulated browsing, warming and nutrient addition on forage availability for migratory caribou in Nunavik, Canada Valérie Saucier1 · Emilie Champagne1,2 · Steeve D. Côté1 · Jean‑Pierre Tremblay1,2 Received: 23 July 2018 / Revised: 29 May 2019 / Accepted: 9 July 2019 / Published online: 30 July 2019 © Springer-Verlag GmbH Germany, part of Springer Nature 2019 Abstract At high population size, migratory caribou (Rangifer tarandus) are regulated by forage abundance in their summer range. Climate warming likely afects forage availability by increasing productivity and advancing phenology of vegetation. Our objective was to investigate the combined efects of browsing and climate warming on the availability of dwarf birch (Betula glandulosa). We simulated direct (warming, with open-top chambers) and indirect (increased nutrient cycling) efects of climate warming in interaction with simulated browsing (leaf stripping) from 2009 to 2013 in Nunavik, Canada. We meas- ured the efect of treatments on dwarf birch biomass and phenology. Moderate and heavy browsing reduced the estimated biomass of birch leaves by 14% and 34%, respectively. Fertilization did not increase the biomass of birch leaves, but increased the biomass of another forage, Poaceae. The warming treatment advanced the opening of birch leaves by 4 days (95% CI: [3, 6]) in 2011 and 7 [5, 8] days in 2013, the two years colder than average. The absence of signifcant phenological shifts in warmed plots during warmer springs suggests that established dwarf birches may have reached a threshold in a limiting resource, likely soil moisture, under which they cannot respond to further warming. Our results demonstrate that browsing can reduce forage biomass, but the variability in caribou populations could provide windows of opportunity for shrub growth. Keywords Betula glandulosa · Climate change · Herbaceous arctic tundra · Phenology · Rangifer tarandus Introduction forage to restore their body condition (Klein 1990) and pro- duce milk (Taillon et al. 2013) that calves rely on for their At high population size, migratory caribou (Rangifer taran- growth (Cebrian et al. 2008; Post et al. 2008). Erect decidu- dus Linnaeus, 1758 subsp caribou) are likely regulated by ous shrubs, such as willows (Salix spp.) and dwarf birches the availability of summer forage (Messier et al. 1988). For- (Betula glandulosa Michx. and B. nana L.), can be a major age availability in early summer is crucial for the recon- source of protein for caribou in summer (Crête et al. 1990; stitution of body mass and growth of caribou (Gunn and Klein 1990). They also are the dominant species involved in Skogland 1997). Females rely on the protein content of the densifcation of erected shrubs observed in Arctic and sub-Arctic ecosystems (Myers-Smith et al. 2011; Elmendorf et al. 2012; Ropars and Boudreau 2012). Understanding how Electronic supplementary material The online version of this forage availability will be modifed by the response of shrubs article (https ://doi.org/10.1007/s0030 0-019-02543 -y) contains to climate changes appears fundamental to our understand- supplementary material, which is available to authorized users. ing of migratory caribou population ecology. * Jean-Pierre Tremblay Shrub densifcation is driven by warming temperatures [email protected] and accelerated by feedback loops (Myers-Smith et al. 1 2011). For example, shrub densifcation can increase snow Département de Biologie, Centre D’Études Nordiques cover and thus soil temperature, consequently increasing and Caribou Ungava, Université Laval, 1045 ave de la Médecine, Pavillon Vachon, Québec, QC G1V 06, Canada microbial activity and nutrient availability further promot- ing shrub growth (Sturm et al. 2005). This positive efect of 2 Centre D’Études de La Forêt, Université Laval, 2405 Rue de la Terrasse, Pavillon Abitibi-Price, Québec, QC G1V 06, climate warming on forage biomass could be counteracted Canada by the browsing of Arctic herbivores (Olofsson et al. 2009; Vol.:(0123456789)1 3 1562 Polar Biology (2019) 42:1561–1570 Ravolainen et al. 2011). For example, biomass and height predicted a positive efect of warming with open top cham- of Betula nana were higher in reindeer (Rangifer tarandus bers and nitrogen fertilization on the biomass of dwarf birch subsp platyrhynchus) and small mammals exclosures (Olof- leaves only under low or no simulated browsing pressures. sson et al. 2009), and cumulative browsing over multiple Because caribou feed extensively on Poaceae before the years reduced radial growth in Betula glandulosa (Morris- opening of shrub leaves (Crête et al. 1990), we also meas- sette-Boileau et al. 2018a). ured the biomass of Poaceae expecting positive responses Climate warming is also known to lengthen the growing independent of browsing pressure in species with a basal season by advancing spring (Linderholm 2006) and delaying meristem. Finally, we expected earlier bud burst and leaf fall (Jeong et al. 2011). Populations of migratory caribou opening of dwarf birch in warmed plots independent of other are closely related to plant phenology, mainly because calf factors. recruitment depends on protein-rich forage (Post and Klein 1999). From fall to the following spring, caribou forage mainly on lichens with a low protein content (Klein 1990), Materials and methods leading to a negative protein balance for 7 months a year (Gerhart et al. 1996). Thus, an advance in plants phenol- Study area ogy could lead to a mismatch between the availability of protein-rich forage and protein requirements in spring and Our study site was located near Deception Bay (N62.08′41″, early summer (Stenseth 2002; Post et al. 2008; Gauthier W74.41′52″; Québec, Canada; Fig. 1), within the summer et al. 2013). A mismatch has been shown to decrease the range of the Rivière-aux-Feuilles migratory caribou herd recruitment of calves in caribou (Post et al. 2008). (Taillon et al. 2012). The Rivière-aux-Feuilles herd reached Our objective was to investigate the combined efects of 1,193,000 ± 47.5% individuals in 2001 (Couturier et al. browsing, warming and fertilization on the availability of 2004) but was estimated at 430,000 ± 23% individuals at Betula glandulosa (hereafter dwarf birch) in summer for the time of this study (2011; Québec Government aerial caribou. Dwarf birch can be a main constituent of the diet survey, unpubl. data). The herd is still declining and was of caribou (Crête et al. 1990) and its protein content when estimated at 199,000 ± 8% in 2016 (Québec Government leaves open is higher than in many other plants (Klein 1990), aerial survey, unpubl. data). The bedrock geology consists although it is high in triterpenes and could be less digest- of foliated granite and granitic gneiss (Agriculture and Agri- ible than other resources (Bryant et al. 2014). We hypoth- Food Canada 2013) and the surfcial material is composed esized that browsing by caribou can counteract the expected of till veneer (Fulton 1995) and is underlain by continuous positive efects of climate warming on forage availability, permafrost. The vegetation at the study site is typical of the through longer growing seasons and more fertile soils, as Arctic herbaceous tundra bioclimatic region (Ministère des it removes photosynthetic tissues on the shrubs. In addi- Ressources Naturelles et de la Faune 2003) is essentially tion, warming and/or fertilization could advance the tim- composed of bryophytes, Poaceae (mainly Calamagrostis ing of dwarf birch leaves development in early summer. We lapponica (Wahlenb.) Hartm., Hierochloë alpina alpina Fig. 1 Location of the study area at Deception Bay, Nunavik, Canada (Base map credits: Esri, HERE, Garmin, OpenStreetMap contributors, and the GIS user community) 1 3 Polar Biology (2019) 42:1561–1570 1563 (Sw. ex Willd.) Roem. & Schult, and Eriophorum angus- 2014). We applied the browsing treatment to all distal twigs tifolium subsp. angustifolium Honck.), evergreen shrubs in a 1 × 1 m subplot within 4 × 4 m plots when ca. 75% of (mainly Vaccinium vitis-idaea L. and Vaccinium uligino- birch leaves had opened, which approximately corresponds sum L.) and deciduous shrubs (mainly B. glandulosa and to the moment when caribou were observed stripping ramets Salix spp.; Walker et al. 2005). Gauthier et al. (1989) found (Online resource 1; Bergerud et al. 2008). B. glandulosa leaves in 25% of rumen from the George- River herd caribou sampled in June, while Crête et al. (1990) Forage biomass reported that shrub leaves and twigs constituted respectively 54% (SE = 2) and 12% (SE = 1) of rumen content in July for We estimated the biomass of dwarf birch leaves in the sub- the Rivière-aux-Feuilles herd. In spring, mean temperatures plots in early August of each year using the point intercept are between − 2 and − 4 °C and mean annual precipitation method with 25 systematic points within a 75 × 75 cm frame ranges from 100 to 150 mm for the 1981–2010 period (April on the 1 × 1 m browsing subplot (Jonasson 1988; Bråthen to June; Environment Canada 2014). and Hagberg 2004). To validate the method, we applied the point intercept protocol to 32 plots outside of the treated Experimental design blocks and destructively harvested birch leaves within those plots. We dried these samples and correlated the number In 2009, we implemented a simulation experiment to manip- of intercepts on dwarf birch leaves with their dry biomass. ulate temperature (warming) and soil nutrient (fertilization) The number of intercepts on birch leaves explained a large in interaction with caribou browsing pressure on dwarf proportion of the variance in leaf biomass (R2 = 0.80) con- birch. The split-plot experimental design included two levels frming that the number of intercepts is an acceptable proxy of fertilization in 4 × 24 m main plots where we randomly of the biomass of birch leaves (Online resource 2). Because allocated two levels of warming crossed with three levels of caribou feed extensively on Poaceae before the opening of simulated summer browsing in 4 × 4 m plots (Morrissette- shrub leaves (Crête et al.