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Possible constraints in degraded systems include disturbance regime; physical (abiotic) conditions; changed above and below ground interactions; herbivory; propagule limitation; and regional environmental change (Tommerup and Bougher 2000; Suding et al. 2004).

Disturbance regime: In agreement with other studies in south east Australia and in

the south west of Western Australia, the work described in Chapter 2 found that the intensity of livestock grazing was one of the most important degrading influences on eucalypt regeneration in Tasmanian remnant woodlands and that lack of fire was another important barrier to recruitment. It is generally accepted that the frequency of fire has decreased, and the intensity and scale of fire has increased since European settlement in temperate areas (Bowman 1998; Close et al. 2009a). A shift in mean fire frequency can induce shifts in vegetation composition and structure and soil physical and nutritional characteristics and this has consequences for adult tree health, fuel accumulation and eucalypt recruitment (Jackson 1968; Close et al. 2009a).

Tasmanian households are estimated to use over half a million tons of firewood annually (RPDC 2006a). Firewood collection is undertaken almost exclusively in dry sclerophyll forests and woodland remnants in Tasmania, often on private land and with Eucalyptus amygdalina extensively targeted (Miller 2001; RPDC 2006a). This

represents a major degrading influence in these remnants and removes heavy fuels which would otherwise contribute to hot spot fires.

Abiotic conditions: Nutrient enriched and compacted soils are legacies of past

applications of fertilizers and heavy grazing (Yates et al. 2000b; Prober et al. 2002; Close et al. 2008; Duncan et al. 2008). A study by Close et al. (2008) in remnants in the current study area showed that recently heavily grazed remnants had raised levels of Nitrate N and Colwell P and significantly less penetrable soils than in healthier remnants and that this impacted on adult tree health. The C/N ratios in the heavily grazed remnants were in the range of typical agricultural soils managed with fertilisers while those in lightly grazed remnants were typical of soils of healthy temperate eucalypt woodlands and forests (Granger et al. 1994). Fertiliser drift from adjacent agricultural land also increases nutrient enrichment, particularly in small to medium sized remnants and around the edges of large remnants and this could impact seedling regeneration (Duncan et al. 2008). However, one of the remnants surveyed in the recruitment niche study was nutrient enriched with significantly higher N, P, K and S than the three other nature reserve sites studied and yet it contained plentiful regeneration in the form of seedlings and lignotuberous sprouts (Barton Farm site, reported in Chapter 3). This suggests that nutrient enrichment alone may not be a barrier to regeneration but it is implicated in its interactions with biotic components of degraded ecosystems (Close et al. 2008; Skinner et al. 2010). Compaction of soils by livestock and machinery leads to decreased penetrability, increased bulk density, reduced water infiltration and increased runoff (King and Hobbs 2006), all of which has implications for water availability and root penetration for germinating seeds and growing seedlings (Passioura 1991; Yates et al. 2000b; Skinner et al. 2009). Skinner et al. (2009) showed that eucalypt seedlings were more

susceptible to surface drying in compacted soils and thus at greater risk of

desiccation following germination. Lack of surface roughness in both compacted and uncompacted soils may also affect water availability and suitability as a seed bed (Battaglia and Reid 1993; Yates et al. 2000b). A disturbed seed bed is required as eucalypt seed is small, there is limited or no endosperm and the radicle needs to find a way into moist soil rapidly in order to take up water and nutrients (Florence 1996). Soil water repellency is another potential barrier to eucalypt recruitment, both in healthy and degraded sites, as discussed in the previous chapter. Spatial and temporal heterogeneity in soil water repellency influences where and when seeds are able to germinate and seedlings establish. Unburnt soils in eucalypt woodlands and forests can be severely hydrophobic (Chapter 4, Doerr et al. 2004). Subsequent preferential flow of water as well as lack of wettable patches (initiated by intense fire in coarse woody debris) may restrict eucalypt recruitment.

Coarse woody debris was also shown to be a vital element of the seedling recruitment niche with functions in the provision of shelter, microclimate

amelioration, moisture sinks and protection from browsing (Chapter 3). Intermediate sites measured in the study reported in Chapter 2 had, on average, two thirds the length of logs ≥10cm in diameter found in healthy sites (Table 2-9). Lack of nurse structures such as logs and branches that facilitate a favourable microsite condition for establishment of seedlings may represent another barrier to recruitment.

Above and below ground interactions: Cramer et al. (2006) suggest that the legacy

of agricultural practices shifts establishment niche availability to favour the

development of novel plant communities comprised of species adapted to the altered environmental conditions. Often these novel communities are made of a “recalcitrant

understorey layer” that includes invasive native species and/ or unwanted exotics (Royo and Carson 2006). In the Midlands Lomandra longifolia and Pteridium esculentum (bracken fern) are two native species that have traits that have enabled them to adapt and thrive in nutrient enriched agricultural environments (Cramer et al. 2006; Mokany et al. 2006). They have become regarded as pest species by farmers as they are relatively unpalatable to livestock, can blanket the understorey of remnant tree stands and invade pasture (Mokany et al. 2006; McWhirter and Kemp 2010). These species are local examples of what Young et al. (2010) call “widespread and pervasive modifiers of ecosystems and disruptors of forest regeneration” that may also act as ecological filters allowing some species to regenerate and not others (George and Bazzaz 1999). The widespread presence of these species in the understorey of degraded remnants (as was the case in many of the remnants of intermediate condition in the study described in Chapter 2) could constitute a biotic barrier to eucalypt recruitment.

Other persistent species in the understorey in degraded remnants in the Midlands and elsewhere that may compete for space and resources with eucalypt seedlings include exotic grass, herbaceous and woody weeds. Nutrient enrichment in remnants can favour the establishment, competitive ability and persistence of these exotic species with the possibility of positive feedbacks maintaining their populations in preference to native species (Prober et al. 2002; Suding et al. 2004; Cramer et al. 2006; Standish et al. 2008).

Woodland fungi are very susceptible to disturbance (Tommerup and Bougher 2000). Ectomycorrhizal fungi associated with roots of eucalypts are known to improve early growth by increasing the uptake of water and of phosphorus (P), especially where P is limiting (Bougher et al. 1990). The formation of ectomycorrhizae in eucalypts and

their positive effects on nutrient uptake and plant growth are strongly inhibited by excess amounts of available N and P (Mason et al. 2000). Other adverse disturbance effects such as loss or alteration of top soil, litter and organic matter and a reduced complement of host plant species 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 therefore play a role in limiting eucalypt establishment.

Herbivory: The Tasmanian Midlands have large populations of native brushtail

possums (Trichosurus velpecula) with their numbers having steadily increased since the collapse of the fur trade in the 1980’s (Neyland 1996). Possums are generalist herbivores that have been implicated in the decline in adult eucalypt tree health (Neyland 1996; Kirkpatrick et al. 2000) and are one of a number of marsupial browsers that cause significant damage to eucalypt seedlings in forestry operations (Miller et al. 2008). Large populations of mammal herbivores (including feral animals such as deer, hares and rabbits) could constitute a significant barrier to the persistence of eucalypt seedlings in degraded woodland remnants. Insect herbivory has also been implicated in the decline in health of mature trees in nutrient enriched remnants elsewhere (Landsberg et al. 1990) and may also constrain the survival and growth of seedlings in the Midlands.

Propagule limitation: Eucalypts are weakly serotinous with a continuous, low level

of seed release occurring throughout the year that is accelerated by hot dry conditions or fire. This leads to a temporally variable seed rain. Dispersal of eucalypt seed is

usually prevents the formation of a soil seed bank (Yates et al. 1995) and may substantially reduce seed available for germination (Clarke 2000; Clarke and Davidson 2001). Seed availability is therefore spatially and temporally variable (Yates et al. 1994a) and may constrain natural eucalypt regeneration (Clarke and Davidson 2001).

Regional environmental change: Meteorological data indicate that average annual

rainfall over the past 30 years in the Midlands of Tasmania (Oatlands 42°18’S, 147°22’E) has been 498 mm compared to the long-term average of 551 mm. There has been a change in year to year variability in rainfall since 1975 and the seasonal pattern of rainfall has also changed, from a more even distribution to a pronounced dry period in late summer/autumn (Kirkpatrick et al. 2000; ACECRC 2010). Tasmanian temperatures have risen since the 1950s, but at a slower rate than in mainland Australia (ACECRC 2010). These changes have serious consequences for moisture availability for seedlings over summer and suggest that seasonal moisture deficit may be an important limiting factor in eucalypt recruitment.

5.1.2 Potential methods of addressing constraints on