Palatability

Palatability is the innate attractiveness of the species to being browsed and is likely to be a function of digestibility and toxicity. This will vary with the condition of the tree (as affected by many factors including season, site type and disease status) and also with the type, and condition, of the herbivore.

Large herbivores can, in general, extract more energy from low digestibility food than can small herbivores, however, the ranking of palatability will probably remain much the same between different sizes of herbivore.

Notes on the table: 1 = most palatable, 6 = least palatable.

• Relative palatability is based largely on observations by the authors, and others, supported by limited published information. Note that relative palatability can change with season and can differ, to some degree, between sites
• Aspen may be relatively unpalatable to roe deer
• Elder is unpalatable to rabbits
• Scots pine, juniper and holly are more preferred in winter than summer because they are evergreen, however young holly shoots, before the leaves have hardened, are also often taken
• Alder is slightly more palatable than rhododendron and Sitka spruce

Preference

Preference is the tendency of the herbivore actually to eat trees of a given species in the field in relation to how much is available.

If there were no preferences then we would expect trees to be eaten in proportion to their relative abundance. In practice, this is not the case. Expressed preferences depend on the relative palatability of each species as well as the apparency of individual trees i.e. how visible they are. Thus, saplings below the vegetation height are often not browsed, unless they are browsed along with the surrounding ground vegetation. Cattle are more likely to do this than herbivores that are more selective.

‘Natural protection’ can allow trees to get away. Examples of ‘natural protection’ are: cherry coming up in a clump of juniper, birch coming up in a block that is so thick that the trees in the middle are protected by those on the outside (other species such as Scots pine and oak might be protected by the birch as well). Hawthorn, blackthorn (both the latter are light demanding so might only occur in an open wood), gorse and dead wood can also protect seedlings and saplings of more vulnerable species from being browsed.

Resilience

Resilience is the ability of the seedling or sapling to survive being browsed and to continue to grow. Resilience is affected by the ability to survive and grow under browsing and as well as by the speed of growth i.e. the rate at which the tree can reach a height at which it is no longer susceptible to browsing.

In general, conifers are less able to survive browsing than are broadleaved trees because they store more of their nutrients in their leaves. Slow growth means that seedlings and saplings are more likely to be grazed to a standstill. Growth rate is dependent on site type and climatic conditions as well as on the age of the seedling or sapling and its species.

Under favourable conditions, birch and alder are very fast growing. Ash, oak, hazel and rowan can be fast growing under favourable conditions but they are more choosy about what is favourable.

More shade-tolerant species e.g. holly, ash, oak and rowan, are likely to be more able to survive browsing than are more light-demanding species since they build up nutrient stores in their roots that enable them to withstand both low light levels and defoliation. Once released from shade or browsing such old saplings can then grow rapidly. 

Note on the table: 1 = most resilient 4 = least resilient.