Woodland grazing toolbox

Woodland grazing toolbox

6.1.4 Relative palatability and resilience of native tree seedlings and saplings to browsing

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 palatabilities will probably remain much the same between different sizes of herbivore.

Palatability

Tree species

1

Aspen, Willow

2

Ash, Rowan

3

Hazel, Oak

4

Scots pine, Juniper, Holly

5

Birch, Hawthorn

6

Beech

7

Alder

Notes on the table:

  • 1 = most preferred species
  • In lowland woodlands aspen may be in palatability class 3
  • 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.

N.B. There is ongoing debate about whether holly and hawthorn should be higher up the list. They are often seen heavily browsed but this may be only when all other species are either more heavily browsed or are not there at all.

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 often 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 are more selective herbivores.

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 might also be protected). 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. 

Resilience

Tree species

1

Eared Willow, Birch, Alder

2

Holly, Juniper

3

Hazel, Oak, Rowan, Ash

4

Scots pine


Note on the table: 1 = most resilient.