I have paid quite a bit of notice to beaver foraging behaviour this year (summer 2023 up to now – Feb, 2024) and I believe there is a tendency for beavers to avoid trees that have been felled by humans but especially trees that have been sawed into smaller chunks.
They even seem to avoid windthrows if those have been later altered by humans.
On one hand, it might be that beavers do not avoid the specific resources but they avoid approaching resources that exhibit signs (scent) of human activity.
However, beavers seem to avoid these logs even after a prolonged period when rain etc. would have washed away all scents and under circumstances humans have not returned to continue working the materials.
Namely, also in areas where the trees have, for example, fallen across a footpath and they have not been removed entirely but they have been sectioned in order to free the trail and later they have been left alone for months (or forever), beavers still choose other boles which have never been touched by humans.
It is not ‘a law’ without exceptions because I have seen beaver foraging marks on some of the larger logs which have been separated into pieces and this has mostly occurred relatively shortly after the tree has fallen/been felled while other logs/boles are utilized also after a longer period of having died.
Smaller logs that would normally have some foraging activity associated to them if they had been felled by the beaver or by the wind; but that have been worked by humans instead, are left untouched by the beaver despite their convenient location (close to the shore and also very close to other beaver foraging sites) and despite their ‘ready delivery’ (the beaver would not have had to chew it down by itself).
In fact, some smaller trees that have been chainsawed into logs are untouched by a beaver despite the beaver having gnawed on nearby trees (within 5 metres from the human-altered logs) of the same species or even of less preferred species (e.g., alder or ash vs. oak).
I wonder if this effect truly exists and if it does, how it might inform us on beaver diet needs and preferences.
On one hand, I should suppose that trunks which have not been severed from the roots immediately (which is often the case when humans have gone at it) might stay ‘fresh’ longer.
It is overall curious to try and understand the ‘food expiration dates’ on beaver-foraged items.
I have seen windthrown trees (attached to root system) utilized after several months (perhaps 4) after they have died.
Accordingly, I should assume that there are still nutrients available in the phloem and cambium.
Perhaps the phloem is the first ‘to expire’ because its most essential nutrients are probably contained within the sap that dries out once the leaves have wilted and stopped photosynthesizing.
This process might be slower in boles that are attached to roots if the roots do not die at once and if they keep delivering water slightly prolonging the carbohydrate production in the leaves.
Cambium, on the other hand, might not expire as easily and, subsequently, the degradation rates by microorganisms and macrovertebrates might be the factors further determining the ‘expiration date’ for these resources.
Cambium and phloem might be the first targets by fungi etc. as they have lower lignin content.
I should expect that phloem is more appealing than cambium, especially, for fungi that can produce hyphae in order to get under the bark and exploit the sap layers.
However, I am not knowledgeable regarding the decomposition rates of these materials (and these rates are probably also varied among different tree species).
When logs have been partitioned into smaller segments by humans, it is possible that the decomposition rate is increased and some potential nutrients are also released as gaseous compounds as well as leached from the log.
Thus, it is likely that human-altered logs have ‘expiration dates’ set sooner than simply wind-fallen trees.
Perhaps logs that have been severed from the root but that have not been cut into smaller segments are more viable for longer than logs that have been severed from the root and also further partitioned.
I would also like to note that it is curious what constitutes ‘freshness’ in these resources with respect to the other organisms involved in their decay.
This could also concern beaver-felled or recently wind-fallen trees that have been formerly damaged and that have been impacted upon by the many organisms that colonize scars, cavities, loose bark etc.
More specifically, do beavers consume wood material that has been ‘contaminated’ by certain fungi, bacteria, algae etc.?
It appears to me that beavers mainly cut younger trees or trees that are mature but not injured yet.
However, it might not be the result of avoidance of these colonizing organisms but simply a preference for diameter size that is convenient for the beaver to fell and to handle.
Still, it is interesting to inquire whether beavers turn down certain infections in the tree or perhaps sometimes even choose them because they might provide additional nutrients.
I have never really observed beaver foraging marks on those type of boles that have been decaying for long enough to exhibit significant stages of wood decomposition, to begin accommodating bryophytes, lichens and so on.
Is it because the nutrients that beavers are looking for are no longer available (they have been the first to become decomposed) or do beavers also observe some ‘sanitary norms’?
For example, most of the mature trees that have been felled (and larger trees might become felled more often in places where beavers build lodges and dams which they do not in my area) have at least some infections.
It would be interesting to see whether beavers avoid certain patches on a freshly-felled tree if those patches have specific colonizers.
I also wonder whether beavers eat up mosses, lichens etc. that grow on the branches of the trees they have felled.
Do these vegetation resources become a contribution to beaver diet or are they discarded in the beaver foraging patches?
I will attempt to pay closer attention but, unfortunately, in my area beavers mainly select younger, smaller stems.
It is also sort of interesting (but I am not certain regarding the validity of this observation) that it would appear beavers utilize the canopy section and the lower trunk section of the bole but they do not generally seem to touch the middle part of the trunk.
Of course, I am not stating that beavers never consume the bark in the middle section.
In fact, I have seen, for example, an extremely tall, beaver-felled alder the bark of which has been chipped off by beavers from top to bottom.
This tree was not very big in diameter but it was very tall – it had been growing in a thick cluster of alders by the riverside and, in order to reach light, apparently the tree had invested in height over girth or even canopy (rather few branches at the top and no branches lower down along the trunk which perhaps might be significant as branches probably introduce structural changes in the conductive tissues and removal of the bark of a tree which has many branches might be more complicated).
The bole was also still attached to its roots, and it was consumed during winter (fallen in autumn).
Additionally, I have seen middle sections of trunk foraged for cambium and phloem where other parts of the tree are inaccessible (e.g., the tree has fallen inconveniently across a very steep river bank and the middle section becomes accessible when the river freezes over).
Perhaps the consumption of bark of large fallen trees is different in my area because beavers do not use large trees here as materials for the building of dams and lodges, and rarely caches are formed.
However, if, indeed, beavers prefer parts of tree closer to roots and canopy, this could be one of the reasons why human-cut logs are little utilized because humans often remove the branches and then they leave these middle sections which might be less appealing to the beaver.
But why are they less appealing (if it is so)?
Is there a difference in the chemical composition between the trunk areas close to the roots, well above the root but below the branches and above the branches?
I hope to come across some answers in my readings.
Notes by Ieva 