Regeneration and structural attributes 49

Regeneration is an important component of stand structure as the flux of seedlings to a juvenile state and then to a mature state determines the population structure of standing plants (Clarke 2000; Parkes et al. 2003; McElhinny et al. 2005).

Regeneration also enables the stand to respond effectively to disturbance events and hence constitutes one of the more important drivers of forest/woodland dynamics (Florence 1981; Ashton 2000). In this study quantity of regeneration was

1. Life forms

The quantity of eucalypt regeneration was strongly correlated with the number of life forms present in a remnant stand. Regenerating stems were included as two separate life form categories (overstorey regeneration <2m height, overstorey regeneration >2m height) in the structural complexity index showing the importance of

regeneration not only as a population process but also as a potential provider of physical structures for microhabitat in more than one vegetation stratum. A lack of regeneration, therefore, also results in a diminished range of physical structures in a stand.

2. Perennial species

The strong correlation between the quantity of eucalypt regeneration and the number of perennial species reflects the much greater amount of regeneration in healthier sites with a wide suite of species compared to the dearth of regeneration in paddock tree sites which had very few species. This supports the proposal by Yates and Hobbs (1997a) and Yates et al. (2000b) that degrading processes (such as fragmentation, grazing changed nutrient fluxes and invasion of exotic species) that reduce the diversity of native plant species in remnant vegetation also affect the presence of eucalypt regeneration. A reduced suite of perennial species also has implications for the diversity of eucalypt pollinator assemblages, which include a variety of insects and birds (Hingston and Mc Quillan 2000). Lack of year round nectar sources provided by multiple plant species may mean reduced pollinator diversity, pollinator populations, pollination services and seed set for eucalypts (Burkle and Alarcon 2011). A reduced complement of host plant species also decreases the diversity of mycorrhizal fungal communities and the inoculum levels of ectomycorrhizal fungi (Tommerup and Bougher 2000). The growth of eucalypts is inhibited in soils without

ectomycorrhizal inoculum (Ellis and Pennington 1992; Close and Davidson 2004) and lack of ectomycorrhizal fungi in degraded woodlands may play a role in limiting eucalypt establishment.

3. Quadratic mean dbh

The strong negative correlation between quadratic mean dbh and regeneration reflects the inclusion of paddock tree sites in the index as these stands had a few large trees with mostly no regeneration, while the healthier sites had more trees in a range of sizes and plentiful regeneration. This result is in agreement with Fischer et al. (2009) and Ottewell et al.(2010) who, working in dry agricultural areas of south- eastern and South Australia, found that scattered paddock tree sites had the greatest mean tree diameter with a symmetrical spread of diameters and no regeneration, while more intact remnant woodlands had smaller mean and median tree diameters, greater numbers of trees and an increased probability of regeneration.

4. Vegetation cover 0.5-6m high

The strong correlation between the quantity of eucalypt regeneration and the percentage cover of vegetation 0.5-6m in height reveals the importance of

regeneration as a component of this vegetation stratum. The presence of eucalypt saplings adds to the physical distribution of foliage at this height along with other shrub and tree species. As the frequency of regeneration reduces (along with the number of large shrub species), the habitat resources provided by this stratum also declines. The richness of woodland bird species have been shown to be particularly sensitive to changes in this stratum of vegetation (MacDonald and Kirkpatrick 2003; Kutt and Martin 2010; Ford 2011; Munro et al. 2011).

5. Dead trees

The association between regeneration and the number of dead trees suggests evidence of gap phase regeneration i.e. regeneration occurring where there is a release from competition in the area around a tree following its death (Grubb 1977; Yates et al. 2000a). Tree death results in a break in the canopy, increasing the amount of light that reaches the forest floor with the likelihood of increased nutrient availability for individuals in the regeneration pool on the forest floor. Most

importantly in dry woodland systems, tree death may also result in increased

underground moisture availability for remaining stems (Stoneman et al. 1995; Yates et al. 2000a). Death of trees can occur not only as a result of episodic disturbance that results in recruitment events (such as fire) but also at other times, thus allowing some previously suppressed regeneration to access increased resources and be released from limiting competition to progress to the canopy as advanced growth (Florence 1996).

6. Litter

There was a significant positive correlation between the amount of litter and abundance of eucalypt regeneration which reflects the lower amount of litter near isolated paddock trees in pastures and in degraded remnants compared to healthy remnants which had higher stem densities. Generally, litter loads and types are highly variable spatially and temporally (Howell et al. 2006; McElhinny et al. 2010) and tend to be sparser in drier forests and woodlands compared to wet forests (Facelli and Pickett 1991; Facelli et al. 1999). The substantial litter loads in wet forests can have negative effects on germination and survival of seedlings through reduced light availability, being a mechanical barrier to emergence, damping off of seedlings by fungi and leaching of chemical germination inhibiters (Facelli and Pickett 1991).

However, in drier forests and woodlands where accumulation of litter is patchier and may not reach detrimental levels, litter may favour seedling establishment by

reducing evaporative water loss from the soil, thus enhancing germination and/or seedling survivorship (Enright and Lamont 1989; Facelli and Pickett 1991; Howell et al. 2006). Litter patches may also suppress the emergence and growth of herbaceous and grass competitors (Facelli and Pickett 1991).

7. Trees >40cm dbh

Healthy sites had significantly more large trees and regeneration than poor sites and regeneration was positively correlated with the number of large trees. Woodland fragments containing few trees or a low density of paddock trees have a high likelihood of increased rates of inbreeding (Hardner et al. 1996) which impacts on seed set and offspring fitness (Burrows 2000; Mimura et al. 2009), reducing the likelihood of recruitment success. Poor seed set compounds recruitment problems (Turnbull et al. 2000). Trees within small woodland fragments may also set less seed because they are stressed. Isolated trees are more severely stressed physiologically than trees aggregated in remnant patches because edge trees are more frequently and more intensively exposed to environmental extremes (frost, drought, wind), soil compaction by stock, nutrient enrichment and attack by insects and mammalian herbivores (Landsberg and Wylie 1983; Davidson et al. 2007; Duncan et al. 2008) and may have low levels of ectomycorrhizal infection (Ellis and Pennington 1992; Close and Davidson 2004).

8. Total length of logs

The final attribute that was significantly correlated with quantity of regeneration was the total length of fallen logs. With aggregated length of logs up to 2km per hectare in healthy sites, the contribution of coarse woody debris (CWD) to the functioning of

dry woodland ecosystems should not be underestimated. Numerous studies have shown the importance of CWD in the provision of habitat for animals, birds, reptiles, insects, bryophytes and fungi (see Lindenmayer et al.(2002) for a review also (Yee et al. 2001; Mac Nally et al. 2002; Wardlaw et al. 2009; Brown et al. 2011) and the current study also implies a role of CWD in the regeneration process of the

overstorey plant species. Studies overseas showed that CWD can protect seedlings from browsing animals (Milchunas and Noy-Meir 2002; Kupferschmid and

Bugmann 2005; de Chantal and Granstrom 2007) while a local study has highlighted the role of logs as substrates for seedling germination in wet forests (McKenny and Kirkpatrick 1999). The role of CWD in the regeneration of eucalypts in woodlands will be explored further in the following chapters of this thesis.

The four structural attributes that were not correlated with abundance of eucalypt regeneration (length of large logs, % vegetation cover <0.5m height, stand basal area and number of hollow bearing trees), did not vary strongly along the gradient of degradation from healthy to poor sites as did the attributes that were correlated with regeneration abundance. While still adding to the ability of the structural complexity index to distinguish between individual sites, the length of large logs and stand basal area attributes did not significantly differ among condition groups (where

regeneration abundance did). Cover of vegetation < 50cm high was also similar across all condition groups (although floristic composition was quite different) and was not useful in distinguishing between sites. There were less hollow bearing trees in intermediate sites than in healthy or paddock tree sites as a product of tree size, maturity and age. The number of hollow bearing trees per hectare did therefore not correlate with the number of regenerating stems.

2.4.2 Effect of remnant vegetation condition, fire and disturbance