I have been observing beaver behaviour in our area, and I believe I have noticed a tendency which is that of felling larger trees rather than foraging on saplings (except for willows).
In our area, the river is rather wide and deep (at least is had been up until the past years when droughts have altered the flow regime) and winters have become rather mild thereby beavers mostly do not build lodges, dams and they do not even stack food piles (apart from perhaps gathering some resources in the riverside burrow).
Tree felling would thereby occur in order to access branches and xylem for feeding purposes.
During the past years of observation, it appears to me that beavers tend to fell trees rather than foraging on the nearby saplings of the same species (the term ‘sapling’, on this occasion, referring to a young tree from the seedling/sucker stage to <5 cm in diameter at breast height).
I began wondering whether there could be reasons why beavers prefer to feed on felled trees rather than very young trees.
In order to discuss the possible reasons, I should briefly describe the species availability.
Willow is the most abundant riparian woody plant that is also highly palatable to beavers.
Willow is also foraged upon during regrowth (small stem diameter) stage and in sites where willows have attained a larger trunk diameter, beavers do not necessarily fell the entire willow but rather cut off branches or offshoots (although thicker branches are the more common forage type).
There are no/very few riparian aspens close enough to the shoreline (another beaver-preferred species).
There are birches, black alders, ashes, hazels and oaks, however.
While foraging on these species, it seems to the me that the beavers prefer to fell the entire tree and to generally focus on mature trees rather than on saplings (or in the case of hazels, they gnaw off thicker stems).
Perhaps it is important for the beaver to include xylem in their diet which might explain why they prefer trees with a larger circumference.
If the species I have listed are of secondary preference (and in many places willows are scarce, as well), xylem might provide some nutrients that the current riparian plant communities are deficient in.
I believe access to xylem and the role of xylem in beaver diet might be of importance because lately I have seen beaver-felled trees that are extremely tall and provide very scarce canopy (having grown in conditions where achieving extraordinary height has been crucial to ensure access to light).
These trees were not felled in areas where other options did not exist (in fact, there were some shorter trees of the same species (perhaps ash or alder) with more plentiful branches closer to the water).
It is possible that the beaver (having a poor vision) is not capable of properly assessing the actual canopy abundance of the tree that the beaver is felling because the canopy is too distant.
I suppose, however, that beavers would have evolved other mechanisms to estimate the canopy density (e.g., through light and shadow conditions on the ground level, through tree density, through the location of the tree with respect to edges etc.).
(This would be also interesting to study – whether beavers make estimates of the trees they are felling with respect to forage amount the particular tree can offer relative to other trees.)
The choice of a tall tree with a small, compact canopy might indicate that the beaver valued the xylem of this tree over the branches (in this particular place, no materials were being gathered for building purposes).
Xylem might be of important and beavers might choose to fell trees specifically for access to their xylem but I was also wondering about plant defensive mechanisms (secondary compounds).
For example, beavers still choose willows that are of current regrowth and that are of a very small diameter.
I have read that willows do not invest in defensive secondary compounds upon regrowth, at least not to the extent other species do, and, as a result, young willows are more palatable than young saplings/offshoots of other tree species.
It is also possible that beavers have learned about the shoot production in willows which is why they avoid to fell the entire willow because the trunk itself would produce new growth that would be soon available for browsing (while felling the entire willow might result in reduced forage availability).
Perhaps beavers do not select saplings or younger shoots (despite their nutritional value) of species that produce plenty of secondary metabolites.
However, I would assume that beavers – a woody plant foraging specialist – would be equipped to process such secondary compounds.
Still, there might be some costs to dealing with plant chemical defense that the beaver attempts to avoid if possible.
I was also wondering if felled trees are somehow altered chemically in order to benefit the beaver.
For example, as the beaver wounds the tree, the plant might direct its chemical defense at the injury and once the tree has been felled, the concentrations of chemical defense compounds might be lower – further away from the wound.
Thus, by wounding trees, beavers might redistribute unwanted chemicals in the tree.
There might be other changes that occur in the tree after it has been severed from the nutrient circulation.
While I find it easier to imagine that beavers would prefer fresh leaves and twigs (especially, because I have observed windthrown or broken trees that are not foraged upon as they begin drying out), perhaps wilting in some tree species is also beneficial to a herbivore.
I am curious if there is, indeed, such tendency for beavers to fell mature trees rather than cutting down saplings of the same tree species and why beavers would make such choices.
Maybe it is also a mechanism to ensure regeneration of trees in the riparian areas (avoidance to fell trees before they are mature in order for these saplings to grow into taller trees that are later removed to facilitate the growth of the next generation canopy individuals).
Notes by Ieva 