The many species of willow are subject to chewing, nibbling, gnawing, and poking by a huge variety of consumers. Here are just of few of the complex interactions.
Most of us here have seen the ‘willow roses’ or rosettes that develop on the twigs or shoots of certain species of willows. The rosettes are galls, induced when a certain tiny fly called a midge lays eggs on the tip of the shoot. The normal elongation of the shoot is suppressed but leaves continue to develop and become crowded together, forming the rosette. The midge’s larva develops inside the rosette, feeding on the bases of the innermost leaves. The larva pupates inside the rosette gall, and the adult emerges the following spring, in time to lay eggs before leaves develop.
The rosette is formed of more leaves than would occur on normal shoots, perhaps forming a wall of defense against enemies of the midge larva (such as parasitoid wasps that would lay eggs on the larva). The inner portions of the rosette also have less photosynthetic capacity and more defensive compounds than the outer portions, which may deter parasitoids and pathogens. The midge larva is presumably is physiologically capable of dealing with the defensive compounds. However, I’ve not found out how well these deterrents work against such enemies. I’ve read that European titmice know how to open the rosettes to gobble up the larva; so of course I now wonder if our chickadees can do the same.
Female midges are quite choosy about where to lay their eggs. Only some species of willow are susceptible to attack by this gall-forming midge; Barclay and Sitka willow are among them here. Experiments in other regions have shown that individual plants of the same species differ genetically in their susceptibility to these gall midges. And I have observed that rosette galls seem to be more common on shoots that are not shaded.
The rosette-bearing, stunted shoots cannot produce catkins, so the reproductive capacity of the willow plant is reduced in proportion to the number of rosettes. Eventually, the rosette kills the shoot, without apparently affecting neighboring twigs. The rosette, however, offers winter protection for spiders and beetles that shelter among the crowded leaves.
Willows are often heavily browsed by snowshoe hares, moose, and reindeer, and this activity can affect the abundance of various kinds of galling insects on the plant. Several studies have shown that some galls can be more abundant on heavily browsed stems. Unfortunately, I have found no such information for the rosette-forming midge specifically.
However, there is evidence for the reciprocal interaction: moose browsing is affected by the presence of rosette galls. Experiments with captive moose in British Columbia showed that moose clearly preferred to eat willow shoots that bore no rosettes. Although they sometimes bit a shoot with a gall, they soon spit out the rosette.
In the absence of rosette galls, browsing by mammalian herbivores, such as moose and hares, can have significant effects on willow growth and reproduction (by removing stems that would bear catkins). Some studies have shown that severe browsing, which leaves little more than a stump, leads to the production of so-called juvenile shoots and leaves. These often have a somewhat different shape from normal leaves and commonly have more defensive compounds, which reduce palatability and nutritional value; this protects the new shoot from further browsing, at least for a year or two. Moose and hares tend to avoid browsing twigs with lots of those defensive compounds.
However, moderate browsing may have very different effects: One study showed that winter browsing by hares on feltleaf willow twigs led to bigger, more nutritious leaves the following spring. In other cases, moderate browsing has elicited compensatory growth of the willow, but this is not feasible in habitats with low nutrient availability and poor growing conditions. The bottom line here is that the interaction between herbivorous mammals and willows varies a lot, depending on severity of browsing, growing conditions, the species of willow, and no doubt many other factors.
It is clear, at least, that herbivores selectively forage on different species of willow; even within a single species of willow, some plants are more palatable than others. Some such differences are genetic, while others have to do with growing conditions, such as the amount of shade. In either case, selective removal of favored kinds of leaves and twigs makes them unavailable for decomposers below the plants. Heavy browsing obviously reduces the amount of litter fall and can change the availability of soil nutrients that result from decomposition. So moose browsing can affect the soils, leading to changes in plant species composition and, potentially, the course of early plant succession below the browsed shrubs.