Simulated winter browsing may lead to induced susceptibility of willows to beavers in spring

Abstract

Browsing may lead to an induced resistance or susceptibility of the plant to the herbivore. We tested the effect of winter browsing by Eurasian beavers (Castor fiber L., 1758) on food quality of holme willows (Salix dasyclados Wimm.) in and after the following growth season. Shrubs were pruned in February, and new shoots from these (cut) shrubs were compared with those of untreated (uncut) ones in May and November. The shoots were analysed for dry matter, nitrogen, acid detergent fibre, and total phenolics. In May, the leaves from the cut treatment had a better food quality (more water, more nitrogen, and less phenolics) than those from the uncut one. There was in part also a systemic response, with lower total phenolics in both the cut and untreated parts of pruned shrubs (uncut-cut) than in the uncut shrubs. In November, we did not find significant differences in biochemistry of bark among cut, uncut, or uncut-cut treatments. These results are in accordance with a cafeteria experiment in the field: in May the beavers preferred shoots from the cut treatment, but in November they showed no preference. The results suggest that willows invest in compensatory growth rather than a defence response early in the regrowing phase.Résumé : Le broutement peut provoquer chez la plante soit une résistance soit une vulnérabilité à l'herbivore. Nous avons vérifié l'effet du broutement en hiver par les castors d'Europe (Castor fiber L., 1758) sur la qualité de la nourriture fournie par le saule à rameaux velus (Salix dasyclados Wimm.) durant la saison suivante de croissance et par la suite. Nous avons émondé les buissons en février et comparé en mai et en novembre les nouvelles pousses de ces buissons « taillés » à celles de buissons « non taillés ». Nous avons soumis les pousses à des analyses de matière sèche, d'azote, de fibres au détergent acide et de substances phénoliques totales. En mai, les feuilles des buissons taillés représentent une nourriture de meilleure qualité (plus d'eau, plus d'azote, moins de substances phénoliques) que celles des buissons non taillés. Il y a aussi une ré-action systématique partielle, puisque les concentrations de substances phénoliques totales, tant dans les parties émondées que non émondées (« non taillés -taillés ») des buissons ébranchés, sont moins importantes que dans les buissons non traités. En novembre, il n'existe pas de différence biochimique significative entre les écorces des buissons taillés, non taillés et non taillés -taillés. Ces données s'accordent avec les résultats d'une expérience de type cafétéria faite en nature: en mai, les castors préfèrent les pousses provenant de buissons taillés, mais en novembre ils ne montrent aucune préfér-ence. Ces observations indiquent que les saules investissent en une croissance compensatoire plutôt qu'en une réaction de défense durant les premières phases de la régénération.[Traduit par la Rédaction]

Reproductive ecology of the Sitka willow (Salix sitchensis ).

“…This study offers more support to previous findings that beavers interact with and greatly influence riparian plant species (Kindschy 1985;Donkor and Fryxell 1999;Veraart et al 2006;Jones et al 2008). Beavers have been introduced or reintroduced in order to enhance riparian restoration; however, if woody plant species have not sufficiently reestablished beaver activity can limit ecological recovery (Kauffman et al 1997).…”

“…Willows respond to browse, especially beaver browse, by growing larger (compensatory growth), increasing basal area and diameter, as well as producing more stems (Kindschy 1985;Donkor and Fryxell 1999;Veraart et al 2006;Jones et al 2008). Perhaps co-evolution occurred between beavers and willows that resulted in the willow response to browse.…”

“…Animal browse can induce compensatory growth, alter nutrient and secondary compound concentrations, as well as increase basal area, stem diameter and stem production (Kindschy 1985;Donkor and Fryxell 1999;Veraart et al 2006;Jones et al 2008). All of these animal/plant interactions can alter the proportions of plant sexual and asexual reproduction.…”

“…Beaver forage has also been observed to alter the abundance, richness, and composition of plant and fish communities (Donkor and Fryxell 1999;Collen and Gibson 2001;McKinstry et al 2001). The association between willows and beavers is more than spatial proximity; beaver browse has been observed to alter the nutrient and secondary compound concentrations in willows (Veraart et al 2006). Stem basal area and diameter; stem and plant abundance; sapling recruitment; stem regrowth; and plant growth rates have also been observed to increase with beaver foraging (Kindschy 1985;Donkor and Fryxell 1999;Jones et al 2008).…”

“…Beavers have been introduced or reintroduced in order to enhance riparian restoration; however, if woody plant species have not sufficiently reestablished beaver activity can limit ecological recovery (Kauffman et al 1997). The variation seen in riparian species response to beaver browse (Kindschy 1985;Donkor and Fryxell 1999;Veraart et al 2006;Jones et al 2008), while following a similar trend, suggests that the impact of beavers is variable by species. It appears that in order to succeed, restoration plans must consider not only detailed knowledge of the reproductive strategies of common riparian plants, but also how they interact with other riparian species (e.g.…”