In relatively low densities, squirrels can be beneficial to our woodlands; they play an important role in the spread, regeneration and structure of the tree species and, in particular the Red squirrel, aid the dispersal of several species of fungi (see Woodland Regeneration). Squirrels can, nonetheless, also be a significant pest to forestry, particularly when densities exceed about five per hectare (500 per sq-km or 1,300 per sq-mile).
Today it is the Grey squirrel that is the focus of persecution from forestry owners, but historically Reds have been considered pests and were targeted by squirrel ‘destruction societies’. Indeed, in his 1930 appraisal of the Red squirrel in Britain, Adrian Middleton noted how in the New Forest, Hampshire, between 1880 and 1927 some 21,352 Red squirrels were shot as forestry pests; 2,281 of these were killed in 1889 alone. To the best of my knowledge, there are no figures for the number of Greys shot annually in the UK.
Probably the most significant problem that squirrels cause forestry owners is tree damage caused by bark stripping. Both species will strip bark, but Greys are considered a more significant pest because they are more widespread and live at higher densities than Reds, meaning that they have the potential to do more damage in a given woodland. Squirrels will strip off bark to eat and to use in the construction of their dreys, but in many cases it appears that the bark is removed in order to satisfy a nutritional need. During the winter, Red squirrels will also strip bark from dead or dying oak trees in order to harvest the Vuilleminia fungal mycelium (the body of the fungi that spreads masses of filaments out into its food source) underneath. Sycamore and beech appear to be at greatest risk, followed by oak, while cherry and hazel are seldom targeted.
In their chapter on Grey squirrel bark stripping behaviour in The Grey Squirrel, Christopher Nichols and Robin Gill note that, on the trunk at least, it is generally accepted that trees between 10 and 40 years of age are most susceptible; younger trees appear unappealing while beyond the upper age the bark is apparently too thick to allow damage. Indeed, during his study of bark stripping at Lady Park Wood in the Wye Valley on the England-Wales border, Edward Mountford found that stands of 40-50 year old trees were most at risk, while those more than 100 years old remained largely unscathed. Nichols and Gill are quick to point out, however, that the picture may be more complicated and that ease of bark removal is not correlated with tree species preferred by Greys. They mention that the bark stripping “season” typically runs from late April until late July, although can continue into September – most economic damage is done to the timber crop in June and July.
Squirrels will strip patches of bark from a tree trunk or branch, which essentially creates a wound that leaves the tree open to attack from insects and infection by fungus. Perhaps a more significant form of the behaviour, however, is so-called ring-barking or girdling, where they chew away a narrow ring of bark tissue from a branch. Bark, as it is phytologically defined, is an inclusive term for the part of the tree comprising four tissues: the cork; cork cambium; phelloderm; and the phloem. The tissue of interest here is phloem. Basically, a plant’s vascular system is composed of two “conducting” tissues: the phloem and the xylem.
The xylem is a dead tissue made up of tubular cells that carry water and minerals up from the roots, while the phloem is a similarly structured but living tissue that distributes sugars, amino acids and various other organic nutrients throughout the plant. Consequently, bark stripping involves the removal of the phloem tissue and causes a break in the food transport system to the affected limb. This process is something akin to wearing a very tight ring on your finger; after a while the finger begins to turn purple because the blood vessels that usually supply the tissues with oxygen and food, and remove metabolic waste, have been vastly restricted by the ring – if the ring isn’t removed the finger will die. The same process happens in ring-barked trees: the branch dies and either falls to the ground or is snapped off by the wind – either way there is a dieback of the tree. During their study of bark stripping damage in the Forest of Dean, south-west England, Brenda Mayle and colleagues observed that between 2% and 17% of trees were ring barked in any given year, with most damage concentrated above four metres (10 ft.) up the main trunk.
Why strip bark?
The reasons for this debarking behaviour are poorly understood. It has been suggested that squirrels may strip bark as a means of grinding down their incisors, which grow continuously throughout their lifetime, but in such cases one might expect incidents of bark stripping to correlate with availability of other hard foods and it does not. Captive squirrels, provided with food ad libitum, still strip bark, suggesting it is not a response to hunger. Observations of Grey squirrels at Wytham Woods in Oxford suggest that bark may be an important source of nutrition to squirrels at certain times of the year – here adult females and spring-born young were seen returning to the same trees each day to feed on the inner layers of the bark. Indeed, it may be a particularly rich source of calcium for squirrels.
The calcium connection
In a recent paper to the journal Forest Ecology and Management, a team at the Royal Veterinary College investigated the idea that squirrels may strip bark to get at the calcium contained within the phloem. The team, led by Chris Nichols at the RVC, reviewed the literature on bark stripping in light of particular phases of the Grey squirrel’s reproductive cycle and the minerals contained both in the phloem and other food sources during different seasons. They found that the peak period of bark stripping, May to July, coincides with newly-weaned squirrel kittens gaining their independence. These young squirrels are undergoing their main period of bone growth, while the adult females are recovering from the bodily calcium drain caused by lactation. Furthermore, during these months the trees themselves are growing (the amount of calcium in the phloem during spring/summer can be 40% greater than that present during winter) and the tree species favoured for de-barking, oak and beech, both have a high percentage of calcium and exceptionally high calcium to phosphate ratio.
It has long been known that squirrels will gnaw at limestone rocks and other calcium-rich objects, such as skulls and deer antlers, and calcium-seeking behaviour during and after lactation is well documented in other mammals, such as rats. Indeed, in lab rats we know that the calcium content of their bones declines by up to a third during peak lactation. The squirrels’ normal food is relatively low in calcium; a hazel nut, for example, contains about 0.1% calcium, whereas oak bark contains almost 3%, so thirty times more.
Nichols and his colleagues also noted how trees vary in their calcium levels, both within and between individual trees, with branches often having higher concentrations than the trunk – this might explain why some trees are more vulnerable to de-barking than others, and why branches tend to be afflicted most frequently. More work needs to be done, particularly looking at whether the form that calcium takes in tree bark, the inert compound calcium oxalate, can be absorbed by squirrels, but this review lends considerable support to the so-called ‘Calcium Hypothesis’ as an explanation for bark stripping behaviour in squirrels.
Another popular theory is that the squirrels are after the tree sap; probably phloem sap, which is predominantly sucrose. This theory helps explain why some species and ages of tree are targeted more than others; the apparent preference of Greys for beech and sycamore during their most intense growth phase, for example. If the squirrels are attacking trees in order to gain access to the sap, it is presumably quantity of sap rather than quality that they are looking for: squirrels do not appear to target trees with the highest sap sugar content, but the peak bark stripping period coincides with peak sap production and width of the phloem tissue itself.
In a study of 30 English woodlands in the Midlands between Stamford, Luton, Reading and Oxford, during the 1980s, for example, Robert Kenward and Tim Parish found that bark stripping by Greys was most consistently correlated with phloem width – trees with wider phloem (and therefore greater sap volume) were targeted most often. In their paper to the Journal of Zoology in 1986, Kenward and Parish noted that:
“Although squirrels were not responding significantly to sugar concentration, they were obtaining most sugar per unit area from the most extensively stripped trees.”
If we think of sap as a can of sugary drink, this is akin to saying that, overall, the squirrels can consume more sugar by going for a less concentrated drink that can be drunk through a wide straw than a more concentrated one that can only be drunk with a narrow straw. That said, it has also been suggested that other factors, such as tannins, may reduce the palatability of sap of some trees and that affects the squirrel’s choice of tree, although this has yet to be confirmed.
Finally, it has been conjectured that bark stripping may be an agnostic or displacement behaviour, because it frequently occurs when squirrel densities are high. Interestingly, during her Berkshire-based study on Grey squirrel behaviour, Jan Taylor found that bark stripping was most prevalent just after oestrus at the points in her study area where two or more female ranges overlapped, suggesting that females were largely responsible and it was in some way connected to areas where territory holders meet. Similarly, a study led by Scottish Natural Heritage’s Jenny Bryce, published in 1997, looked at bark damage caused by Red squirrels in a pine forest in Fife, Scotland and found the compartments with the highest squirrel density also suffered the greatest bark damage. In The Eurasian Red Squirrel, Stefan Bosch and Peter Lurz note:
“The scale of the damage is influenced by the local density of squirrels and may be linked to agonistic behaviour. Aggressive interactions with other squirrels may trigger displacement activity such as gnawing on bark when densities of squirrels, and especially juveniles from spring litters, are high.”
During their study, Kenward and Parish also found that bark stripping always occurred when juvenile densities were high and they noticed a tendency for damage to reoccur in the same location during subsequent years, suggesting that squirrels may learn the behaviour. They concluded:
“The results are consistent with bark-stripping being initiated by juvenile squirrels, or by older squirrels which had learned the habit …”
Some authors have suggested that bark stripping is a UK phenomenon and not something that Greys do in their native North America. In fact, this is a misconception Greys do strip bark in the USA, although it is much less common. The North American sugar maple (Acer saccharinum), which is a source of maple syrup, suffers significant bark stripping in some areas of the US. Nichols and Gill point out that economically important bark stripping occurs when the width of the phloem exceeds 0.3 cubic centimetres under each square centimetre of bark. They also note that, based on a study of 16 woodland sites in the USA where the Grey is native, the trees had low phloem widths. Low phloem widths and the observation that the bark of many of the tree species thickened more quickly with age than their UK equivalents are thought to be the main reason bark stripping by Greys in North America is uncommon.
Whatever the root cause(s) for bark stripping, this behaviour can cause considerable damage to a plantation. Damage from bark stripping is particularly apparent in orchards, market gardens and arable crop plantations that are inopportunely located in, or peripheral to, prime squirrel habitat. As a paper in the Proceedings of the National Academy of Sciences back in 1993 pointed out, however, bark stripping might also have important positive ecological consequences for native species. Branches that die, or become infected with fungal growth, provide important habitat for some invertebrate species and consequently their predators – woodpeckers will, for example, feed on saproxylic (i.e. wood-eating) invertebrates found living in rotting branches and stems.
Removal of leaves
In addition to debarking, squirrels can also cause defoliation, although this may not always be problematic. In 1934, Assistant Professor of Plant Physiology at Yale University, Carl Deuber, wrote a brief article to The Scientific Monthly journal detailing some defoliation activities of Grey squirrels in American elm trees (Ulmus americana) on the university campus. From early May, Dueber observed squirrels feeding on elm seeds, during which time they would remove small twigs, strip off the seed cluster (or dexterously remove the oily kernel) and discard the branch and associated leaves.
Dueber recognised that the loss of leaves represented a physiological loss to the trees and began monitoring the squirrel’s behaviour. The squirrels were observed to cut twigs rapidly (almost four per minute by one individual) and two squirrels cut 517 twigs, with 2,685 leaves adding up to a surface area of nearly seven square-metres (75 sq-ft)—roughly equivalent to a 2.5m x 3m (8 x 9 ft.) rug—in only one day. Defoliation was marginally reduced on cloudy and wet days—just fewer than six square-metres (63 sq-ft)—while on one very warm and sunny day, defoliation increased to more than 12 sq-m (131 sq-ft).
Cumulatively, Dueber estimated that these two squirrels removed more than 46 sq-m (500 sq-ft) from a single tree during the ten days over which he observed them – Dueber calculated that this represented a decline of some 4% in the tree’s tissue and food generating capacity. Far from being disastrous for the elms, however, Dueber suggested that the squirrels might be providing a service to the trees. The theory was that, because American elms are not native to the campus region, they often showed signs of water stress (i.e. dehydration) and one way they could combat this would be to reduce the amount of foliage. In other words, defoliate to a level that can be sustained by the water extraction capacity of the roots. Considering that Grey squirrels only targeted the most vigorously growing trees, for their abundant seed crops, Dueber concluded that this brief (roughly two-week) stint of defoliation was “about as beneficial as destructive so far as the well-being of the tree is concerned”.
There are few cost estimates for the damage squirrels inflict on timber crops but, in their chapter on economic damage in The Grey Squirrel, Jonathan Derbridge and colleagues suggest that high quality timber from the main stem of trees can sell for around £15,000 (US$ 19,000) per hectare at 2015 prices, while wood severely damaged by bark stripping, which develops calluses, is suitable only for firewood and sells at about £4,500 (US$5,600) per hectare – a 70% drop in value. Based on historical surveys indicating that squirrel damage affected 28% of beech, 24% of sycamore and 7% of oak, the estimated loss on the current British timber crop rotation was £10 million (US$12.6 m).
Even setting aside any economic impact for foresters, bark stripping may hamper woodland regeneration. Indeed, over the 10 years of his study at Lady Park Wood, Mountford observed that “squirrels had critically affected the successional development of the wood”. Similarly, an article published in The Guardian in April 2004 noted that Greys remove the leading shoots of broadleaved trees, inhibiting their growth, and in the Cotswolds and Chilterns, young planted beech were failing to develop as a result of such damage.