The summer and, to a lesser degree, early fall of 2023 was characterized by a few heavy storms that resulted in a large number of windthrows and windsnaps.
These resources have been exploited by the beaver and during the warmer seasons I observed beaver foraging marks on windthrows close to the river and even on trees submerged in the river.
I have made some remarks regarding this exploitation in a previous post – Observation of colonization by beavers after a prolonged intense rainfall period.
In this post, I wished to briefly discuss an observation regarding consumption of the bark of the canopy parts of the trees (higher branches and the top of the trunk) which might be more palatable due to their age and thinness of the bark that has to be removed to access cambium but which are also smaller in circumference and therefore might be less profitable (the beaver perhaps might not choose to access them if it involves too great of risk).
Some of the trees have fallen over the river.
The river in this area has been modified and its banks have eroded (or, more precisely, its channel has eroded becoming incised) which is why many of the windthrows do not end up in the river.
The root/lower trunk stays fixed on the bank while the canopy is rather overhanging the river – often too high to be reached by the beaver from the channel bottom (even in places and/or periods when the stream conditions would allow foraging from the more central parts of the channel).
Interestingly, when the river froze over (not entirely but along the banks), I have observed foraging marks on these difficult-to-access but probably highly palatable canopy parts because ice offers little platforms on which the beaver can step in order to access the resource.
Some of these platforms are tiny.
In fact, the river has had a lot of water this autumn/early winter and the current is rather strong.
As a result, ice only forms along the slower-flowing parts (such as in meanders) or near obstacles (such as the windthrown trees and large woody debris aggregations themselves) where the stream is impeded creating more lentic conditions.
Some of these platforms are still very small (ca. 1 m2) but they appear to be used by the beaver as the ‘stools to get to the top shelves in the pantry’.
I believe that this foraging behaviour which only emerged after the prolonged period of low temperatures is informative of several interactions between:
- beavers and stream channel erosion;
- beavers and climate change;
- beavers and seasonal exploitation of resources.
Many streams and rivers have become incised with their banks looming high above the channel.
As a result, beavers can mainly exploit the parts of riparian windthrows that are accessible from the terrestrial level (which might be less palatable and which also exposes them to danger due to the necessity to tread on land).
The energy costs of beavers inhabiting historical stream conditions might have been lower as they could have been better able to exploit windthrows that fall right into the stream.
In areas with significant and regular storms this might have been of importance regarding beaver survival and perhaps reproduction.
Curiously, it does not seem that the beavers would attempt to gnaw through the section in the fallen bole which lies on the verge of the bank even if this section is accessible and even if gnawing through it would result in the rest of the trunk ending up in the water.
Perhaps it is suggestive of the lack of need of such foraging adaptations throughout the most part of the beaver’s evolutionary history (they have not figured out that this effort might provision them with easily and safely accessible resources or their calculations have not yet been adapted to prognosticate whether the canopy part would actually end up in the river or if the effort would be in vain, e.g., if the canopy is stuck on the other side of the river and only the thick end of the bole would fall into the stream).
Ice can offer access during the winter season and thereby the foraging patterns on windthrows and windsnaps might differ based on season (but also on climate and weather because ice may or may not form and other variables such as precipitation prior to ice formation can affect the extent of ice cover).
Climate change would also reduce accessibility of windthrow canopies to beavers if they are to be reached from ice platforms.
I do not believe that beavers rely on such opportunistic resources greatly because they do not seem focused on exploiting them primarily or exclusively even when they become available.
However, the additional resources provided by heavier storms might have an impact of beaver demography and colonization rates and it might be worthwhile to study the effects of, e.g., erosion and change in ice formation regarding forage accessibility and forage use patterns in beavers.
Some of the beaver-felled trees, as well, can become subject to the same accessibility issues as windthrows.
For example, if beavers have figured out that trees felled too close to the river might not end up in the river, they might be forced to fell river further from the bank which increases energetic costs and also increases predation exposure.
It might be interesting to study whether beavers fell trees differently in the proximity of highly incised streams and rivers with high, overhanging banks vs. laterally connected rivers.
Notes by Ieva 