Causes and consequences of deer browsing on red trillium (Trillium erectum) along an elevational gradient

Models of ecological responses to climate warming predict species' migration towards higher latitudes or elevations. However, models often neglect nonclimatic factors, such as herbivory, which could slow down or prevent geographic range expansion. A previous study in Mont Megantic National Park (Quebec) found that in one year (2016), browsing by white-tailed deer on Trillium erectum L. was substantially higher at high elevations than low elevations. Under the hypothesis that deer herbivory could limit the upper elevational range expansion of T. erectum, here we ask (i) whether herbivory increased with elevation during two additional years (2017 and 2018), (ii) whether the rate of deer visitation increases with elevation, and (iii) whether the effect of herbivory on relative growth rates increases with elevation. Contrary to the earlier study, we did not find a significant trend of herbivory with elevation, although there was a weak positive relationship between deer visitation and elevation. We found a strong negative impact of browsing on relative growth rates, but the magnitude of this effect did not vary with elevation. Our results thus do not support the hypothesis that herbivory limits the range of T. erectum at high elevation, but herbivory could have a negative impact on populations in general if the browsing rate remains high. Key words: distribution limit, prey-predator relationship, altitudinal range, herbivory, population growth, climate change. Les modeles de reponses au rechauffement climatique predisent la migration des especes vers des altitudes ou latitudes plus elevees, negligeant souvent les facteurs non climatiques, comme l'herbivorie, qui pourraient ralentir ou empecher leur migration. Une precedente recherche au parc national du Mont-Megantic (Quebec, 2016) a revele que l'herbivorie de Trillium erectum L. par le cerf etait plus importante a haute qu'a basse elevation. Notre hypothese etant que l'herbivorie par le cerf pourrait limiter l'expansion de T. erectum, nous cherchons a savoir si (i) l'herbivorie augmente avec l'elevation pendant deux annees (2017 et 2018), (ii) le taux de visitation du cerf augmente avec l'elevation, et (iii) l'effet de l'herbivorie sur le taux de croissance relatif augmente avec l'elevation. Contrairement a la precedente recherche, nous n'avons pas detecte une tendance significative entre l'herbivorie et l'elevation, bien qu'il y ait une faible relation positive entre le taux de visitation du cerf et l'elevation. Nous avons detecte un fort impact negatif de l'herbivorie sur le taux de croissance relative, sans variation de magnitude avec l'elevation. Nos resultats ne soutiennent pas l'hypothese que l'herbivorie limite la distribution de T. erectum a haute elevation, mais l'herbivorie pourrait avoir un impact negatif sur les populations en general si le taux d'herbivorie reste haut. Mots-cles: gradient altitudinal, limite de distribution, relation predateur-proie, herbivorie, taux de croissance, rechauffement climatique.
Introduction One of the many effects of climate warming on plant species is a shift of geographic ranges towards polar latitudes or higher altitudes (Thuiller et al. 2005; Parmesan 2006; [...]