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Natural Regeneration in Irish Woodlands: Seedling Establishment

Natural Regeneration in Irish Woodlands: Seedling Establishment

The fourth and final in a series of articles on natural regeneration in Irish woodlands, Teagasc Forestry Development Officer, Kevin O’Connell examines the principal of seedling establishment.

The establishment and survival of seedlings following germination will depend on environmental and biological variables. These usually include soil type and condition, moisture availability, vegetation competition, temperature, light and predation extent (Harmer, 1995).

Soil

Natural regeneration of trees species occurs on a variety of soil types (Joyce, 1998).  It is often more difficult on moisture-retaining heavy and fertile soils than on those which are light, dry and infertile (Harmer et al, 1995). 

Tree seeds germinate best on soils which are well drained at the surface, with plenty of leaf mould to suppress other growth (Agate, 2002).  The depth and quantity of leaf mould is important. A deep layer of un-decayed humus is unsuitable for natural regeneration (James, 1989).  A common cause of the whole scale deaths of young seedlings is the inability of their roots to penetrate heavy litter layers (Anderson, 1953).  Suitable ground preparation can be beneficial for oak and beech on sites with deep layers of litter (Harmer et al, 1995). 

Anderson (1953) maintains that the chemical nature of the soil, particularly the presence of soluble calcium, prevents the accumulation of raw humus and, on the more fertile soils, allows a more favourable mixture of organic matter with mineral matter, which favours germination and the subsequent growth of the seedlings.

Species such as oak and beech have the capacity to grow well under a wide range of soil conditions (Joyce 1998).

Moisture

Moisture availability is the most important determinant of germination and subsequent survival of all plants (Harper et al., 1965).  Mineral soils need to be able to retain moisture near the surface.  Ovington and MacRae (1960) found that 80% of acorns buried at a depth of 2.5cm produced seedlings compared with <1% for acorns left on the surface.  The high mortality rate of the unburied acorns appears to be due to desiccation of the root.  Watt (1919) states that if acorns fall on a bare surface, the evaporation would be twice that of a similar soil covered with leaf mould and that germination would be inhibited.

Soil moisture is not only affected by the soil’s composition and structure but also by external variables, like vegetation type and density, shade etc.  The amount of light and soil water is one of the main limiting factors for beech regeneration (Madsen, 1995).  The roots of larger trees can remove water from the soil.  Species like beech, which produce dense shallow layers of fibrous roots, will steal all the moisture from young plants (Anderson 1953).

Too much moisture in the soil, when seedlings are emerging can lead to ‘damping off’ resulting in high mortality rates (Aldhous 1975).

Vegetation competition

Ground flora can affect seedlings in a variety of ways, such as competing for moisture and soil nutrients, creating shade, physical smothering and providing suitable habitats for small mammals and other animals that can damage seedlings (Harmer et al, 2005). 

Natural regeneration requires sparse ground vegetation.  Seeds will not germinate easily in thick grass, bramble or dense ground vegetation and seedlings will be suppressed by such growth.  For this reason, natural colonisation by trees is often associated with poor stony substrates, sandy soils and other sites where grass and dense ground vegetation cannot survive (Agate, 2002). 

However when tree seeds germinate concurrently with vegetation colonisation, the seedlings are able to compete (McNeill & Thompson, 1982).  One of most challenging forms of vegetation is dense grasses forming compact swards, whose roots consume all the available moisture in the upper soil layers (Anderson, 1953).  The density of the ground flora depends broadly on the canopy species and site.  Woods of ash and oak, which cast relatively little shade, tend to have denser ground flora than woods of beech and sycamore, which cast more shade (Harmer, 1994).

Oak is vulnerable to competition from a range of herb, shrub and tree species (Kelly, 2002).    Grasses or perennial herbs can smother some of the more tender pioneer species, by growing vigorously on the same food supplies (Anderson 1953).  Successful regeneration of beech is more likely where the ground vegetation consists of low herbs such as wood sorrel (Oxalis acetosella), yellow dead nettle (Lamiastrum malculatum), wood melick (Melica uniflora), wood millet (Milium effusum) and creeping soft-grass (holcus mollis).  Where bramble and coarser herbs are luxuriant, regeneration is more difficult (Jones, 1952).  Trees differ in their abilities to compete with other vegetation, oak is more able to establish in closed vegetation than beech, ash or sycamore (Jones, 1959). 

Light

Lack of sufficient light is one of the main reasons for the later failure of germinated seed (Anderson, 1953).  Different tree species can survive under different light intensities.  For example, beech and sycamore are shade tolerant species that will grow under dense tree canopies, whereas oak is thought to be intolerant of shade throughout its life (Harmer, 1995). 

Kelly (2002) noted in Killarney National Park that oak regeneration was precluded by the presence of a closed oak canopy accompanied by a patchy evergreen understorey.  This, he concluded, was due to insufficient penetration of photosynthetially active radiation.  Oak species, although often heavily dominant in the canopy, are often replaced within one generation by tree species in the understorey that are much more shade tolerant (Lorimar et al., 1994).

Jarvis (1964) however found in semi-natural oak woodland, the light intensity under normal tree spacing and in small clearings was sufficient for oak regeneration provided that bracken (Pteridium aquilium) was not present.  In France, where beech and oak occur naturally together, the beech will always dominate the oak, unless there is some active intervention to prevent it (Woolsey, 1920). Although beech withstands considerable shade and benefits from sheltered conditions, less than full daylight does reduce total growth, though height increment may be enhanced (Evans, 1984).  Intense shading, less than 25% daylight, causes flattening of side branches (Brown, 1952).  Beech seedlings require moderate light levels to grow vertically and even under these conditions are initially slow growing (Mountford et al., 2006).  Natural regeneration of beech tends to initiate best around the edges of medium to large gaps at intermediate levels of canopy openness, bramble cover and leaf litter depth (Mountford et al., 2006).

References with author

Also read: Natural Regeneration in Irish Woodlands: Germination

Also read: Natural Regeneration in Irish Woodlands: Seed predation

Also read: Natural Regeneration in Irish Woodlands: Seed production