Natural Regeneration in Irish Woodlands: Seed Predation
In a series of articles, Teagasc Forestry Development Officer, Kevin O’Connell, focuses on four important principals governing natural regeneration in Irish woodlands; seed production, seed predation, germination and establishment. In this article, Kevin examines seed predation.
Natural regeneration in woodlands is susceptible to damage from invertebrates, mammals and birds through predation of seeds and young seedlings. Birds and small mammals may aid the dispersal of species such as oak, holly, and whitethorn, but the effects of these on regeneration of most trees are predominantly harmful, with seeds being eaten and young trees damaged by browsing and fraying (Harmer & Kerr, 1995). There are a number of different mammals and birds reported to eat seed: mice (Apodemus spp), rabbits (Oryctolagus cuniculus) and birds such as wood pigeons (Columba palumbus), pheasants (Phasianus colchicus), jays (Garrulus glandaruis) and rooks (Corvus frugilegus) (Wardle, 1959), (Tanton 1965), (Linnard 1987), (Shaw 1968), (Kelly, 2002). The effect of seed predation varies with species, the larger seeds of beech and oak being a more desirable source of food than ash and sycamore (Harmer 1994).
The majority of seed predation takes place after dispersal; some however may take place within the canopy (Tanton 1965). Shaw (1968) found that the majority of acorns disappeared before the end of December and were probably removed by small mammals such as mice and voles, which took c. 99% of acorns from unenclosed plots. Approximately 1% of the acorns produced germinated to form first year seedlings. Linnard (1987) found that in autumn, 40% of beech seed disappeared within one month of falling and by March, less than 3% of full seed remained. Wardle (1959) found that mice and voles may eat 49-75% of ash seed.
Pigeons and squirrels (Sciuris spp) are generally the main agents of acorn destruction both on the tree and on the ground. Tanton (1965) found that in the canopy of a mature enclosed oak stand, 92.5% of the crop was lost from the trees. Linnard (1987) states that, beech natural regeneration in French forests generally fails unless rodents and birds are controlled.
If seed is covered by leaf litter, soil or humus on reaching the ground its chances of survival are increased. Watt (1919) noted that oak continues to shed its leaves long after the acorns have fallen and this assists in the concealment of the acorns. He also noticed that if wind has access to the woodland floor, leaves may collect in certain sheltered spots and in these areas the acorns are effectively concealed. Reade (1965) has reported that in Belgian forests, the soil is rotavated to create a seedbed. As soon as the seed have fallen, the ground is harrowed to cover the seed and hide them from pigeons. The old management technique of allowing pigs into the forest in early winter for pannage, led to successful regeneration, as some acorns were buried by the pigs (Watt, 1919). Although animals and birds do more damage than good to seeds, some may inadvertently assist in seed survival. Mice, voles and rooks are known to bury acorns and a certain proportion is forgotten and may grow to produce seedlings. Acorns buried by grey squirrels will not germinate as their embryos are usually destroyed before burial (Harmer & Kerr, 1995).
The seedlings produced from the seed that has survived are further subjected to predation. Slugs (Deroceras reticulatum) and leatherjackets, the larva of the cranefly (Tipula spp) can cause problems during the first growing season after seeding. High population levels result in browsing at or below ground level of the seedlings, sometimes leading to tree death (Harmer & Kerr, 1995). Other pests such as cut worms (Agrotis segetum) or short snouted weevils (Otiorrhynchus spp., Phyllobius spp) can cause problems to young first year tree seedlings (Aldhous, 1975). Various lepidopteron species are also known to defoliate oak seedlings (Humphrey & Swaine, 1997).
Rabbits (Oryctolagus cuniculus) and hares (Lepus timidus hibernius), although not woodland dwellers, will eat young seedlings. Rabbits are the biggest nuisance (Gill, 1992). The bank vole (Clethrionomys glareolus) is locally common in the south west of the Ireland (Sterry, 2004). They can cause severe localised damage, particularly when population numbers are high (Gill, 1992).
Deer can do considerable damage to both seedlings and saplings through browsing, stripping and fraying (Gill, 1992). Of the four species of deer found in Ireland, only fallow deer (Dama dama) are widespread and are usually associated with old estates. The major populations of fallow deer are in Laois/Offaly, Tipperary/Waterford, Clare, Galway, Wicklow, Monaghan/Louth/Armagh, Fermanagh, Down, Roscommon and Sligo/Leitrim. Red deer (Cervus elaphus) are mainly confined to Donegal, Wicklow, Galway and Kerry, while significant populations of Sika deer (Cervus nippon) are found in Wicklow, Kerry/Cork and Donegal/Tyrone (Sterry, 2004, Carden et al., 2011).
The presence of Muntjac deer (Muntiacus reevesi) was confirmed in Wicklow and Kildare. Anecdotal but unsubstantiated sightings were reported from Counties Wexford, Longford, Leitrim, Sligo, Roscommon and Donegal (Carden et al., 2011).
Deer are selective browsers whose diet varies with the season and in general most damage to seedlings occurs during spring and early summer when newly flushed shoots of broadleaves are young and most palatable (Harmer & Gill, 2000). Fallow deer, in comparison to Red and Sika deer, tend to be more grazers than browsers, although they will eat the young shoots of oak and beech and positively seek out ash and elm (McCurdy, 1989). Feral goats (Capra hircus) can also cause localised damage. Domestic stock, particularly sheep, will also damage young seedlings. Natural regeneration was reported as virtually precluded where woods are subjected to heavy grazing pressure (Perrin et al, 2006). In Killarney National Park, oak saplings were reported as largely restricted to the proximity of roads and tracks, where browsing animals are more or less deterred by frequent human disturbance (Kelly, 2002).
It must be remembered that low intensity grazing and browsing is a natural feature of woodlands which helps to maintain diversity in composition and structure (Anon, 1994). Late summer would appear to be the most suitable time to graze as ground poaching will not be as severe as in the winter months. Any sensitive vernal species will have flowered and set seed and the buds and foliage of tree saplings will be at their least palatable. Many of the species which dominate the woodland, such as bramble (Rubus fruticosus) and many grass species will also still be thriving at this stage and grazing and trampling are likely to have a significant effect on their growth spread and control (McEvoy et al, 2006).
Read the previous article in this series: Natural Regeneration in Irish Woodlands: Seed Production
References
Anon (1994) the Management of Semi-natural Woodlands: 5 Upland Oak Woods, Practice Guide; Forestry Commission Edinburgh
Distribution and range expansion of deer in Ireland
Carden, Ruth F., Carlin, Caitríona M., Marnell, F., McElholm, D., Hetherington, J. & Gammell, Martin P. (2011) Distribution and range expansion of deer in Ireland. Mammal Review, 41, pp 313–325.
Gill, R.M. (1992) A Review of Damage by Mammals in North Temperate Forests, 1 Deer, Forestry Vol. 65 No. 2 pp 112-116
Harmer, R & Gill, R.M (2000) Natural Regeneration in Broadleaved Woodlands, Deer Browsing and the Establishment of Advanced Regeneration, Information Note, Forestry Commission, HMSO, London
Harmer, R & Kerr, G (1995) Natural Regeneration of Broadleaved Trees, Research information note 275, Research Division, Forestry Commission
Harmer, R. (1994) Natural Regeneration of Broadleaved Trees in Britain: III Germination and Establishment, Forestry Vol. 68 No.1 pp 2-9
Humphrey, J.W. & Swaine, M.D (1997) Factors affecting the Natural Regeneration of Quercus in Scottish Woodlands, II Insect Defoliation of Trees and Seedlings, Journal of Applied Ecology, 34 pp 585-593
Kelly, D.L. (2002) The Regeneration of Quercus petraea (Sessile oak) in South West Ireland: A 25 year experimental study, Forest Ecology and Management Vol. 166 pp 207-226
Linnard, S. (1987). The Fate of Beech Mast Quarterly Journal of Forestry No.81, pp 37-41
McEvoy, P, Flexen, M & McAdam, J (2006) The effects of livestock grazing on ground flora in broadleaved woodlands in Northern Ireland, Forest Ecology and Management Vol. 225 pp 39-50
McCurdy, R.J. (1989) Deer in Northern Ireland Forests: Distribution, impact and Management, Irish Forestry, 46 pp 112-116
Perrin, P.M, Kelly, D.L & Mitchell, F (2006) Long Term Deer Exclusion in Yew wood and oak wood Habitats in South West Ireland: Natural Regeneration and Stand Dynamics, Forest Ecology and Management, article in press, (available on line)
Reade, M.G (1965) Natural Regeneration of Beech, Quarterly Journal of Forestry No.59 pp 152 - 161
Shaw, M. W (1968) Factors affecting the Natural Regeneration of Sessile oak (Quercus petraea) in North Wales., the Journal of Ecology Vol.56, No. 3 I pp 565-583, II pp 647-660
Sterry, P. (2004) Complete Irish Wildlife, Harber Collins
Tanton, M.T. (1965) Acorn destruction of small mammals and birds in British Woodlands, Quarterly Journal of Forestry No.59, pp 1-5
Wardle, P (1959) The Regeneration of Fraxinus excelsior in woods with a field layer of Mercurialis perennis, Journal of Ecology, No. 47, pp 483-497
Watt, A.S. (1919) on the cause of failure of natural regeneration in British Oak woods, Journal of Ecology, 17 pp 173-203