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Plant species may be subject to periodic plagues of predators or diseases.
Widespread dieback and death of mature trees has occurred on a large scale in many NSW woodland areas, particularly on the Tablelands, and has been related to buildup of leafeating insects, and fewer, often isolated old remnant trees and little natural replacement due to grazing of seedlings, as well as increased pasture nutrients which have favoured larval stages of insects.
Crown canopy dieback of eucalypts
Crown canopy dieback of eucalypts is periodically noted in Cumberland Plain Woodland remnants in western Sydney, but has not been associated with widespread tree death. perhaps as regeneration of trees in many areas has occurred and there are many young trees. Such periods of leaf attack rarely last more than one or two seasons.
Bell Miner associated dieback
A more localised form of eucalypt decline is Bell Miner associated dieback.
Bell Miners, Manorina melanophrys, are insectivorous, territorial birds that live in large colonies covering several hectares. Eucalypt stands colonised by Bell Miners often exhibit symptoms of crown decline, and colonisation by Bell Miners correlates with high densities of leaf damaging psyllids in tree crowns. There are currently no Bell Miner colonies in the woodland at Mount Annan Botanic Garden though they occur in other places in western Sydney.
Impacts of plagues and diseases are most obvious on tree canopy species because of their size and importance, but have doubtless affected other smaller species, but which have not been recorded. Signs of soil-borne disease such as Phytophthora have not been noted in the woodland, but it is unlikely that many of the native species will be susceptible.
Invasion by exotic species
Natural communities may be invaded by species that do not occur naturally in the area, but have been introduced either deliberately or accidentally by humans. Such species may be plants or animals and are referred to as exotic. While some introduced species may remain in low numbers on the margins of the natural community, others may establish vigorously, changing local conditions and outcompeting native species.
While our Mount Annan woodland contains a high proportion of exotic plant species, many of these are agricultural weeds that were introduced during the 19th and 20th centuries. In the absence of agricultural practices involving ongoing large scale soil disturbance, and intensive stock grazing, most of these species have become less abundant over the last 20 years, or remain restricted to disturbed sites such as roadside margins or periodically mown areas e.g. *Lolium perenne.
The conservation-management conditions of the last two decades have benefited a few species however, and these pose the main weed threats to the longterm survival of the woodland. The main threat is the African Olive *Olea europea subsp. cuspidata. Other threats are posed by Chilean Needle Grass *Nasella neesiana, which was only noted on the woodland in 1991, St Johns Wort, *Hypericum perforatum and Heliotrope, *Heliotropium amplexicaule. Unlike the agricultural exotics, these species have all invaded the woodland and have spread in the last 20 years; active management is needed to control them. Rhodes Grass *Chloris gayana is persistent and may be difficult to eradicate in grassland areas, but not generally invasive.
Interestingly there are some genera that have both weed and native species in the woodland e.g. *Sida rhombifolia and Sida corrugata, *Solanum nigrum and Solanum cinereum, *Chloris gayana and Chloris ventricosa, *Sporobolus africanus and Sporobolus creber.
Introduced animals include rabbits, hares and Garden snails (which may impact on native species through Predation - grazing and herbivory), foxes (through weed seed dispersal), and Honey bees (possible pollination impacts).
Storms and lightning
Storms can cause damage to trees particularly by loss of tree branches. This may expose hollows in the trunks providing nesting sites and habitat for wildlife. Lightning strikes can kill individual trees, and some of the plants growing around their base. Again the dead trees may provide wildlife habitat. The main benefit of storms is the occasional heavy rainfall that may occur, though in a short intense rainfall event much of this will runoff rather than penetrating the soil.
In the Sydney area future climate change appears likely to lead to higher temperatures and less rainfall. The drought tolerant adaptations of many of the native plant species in our Mount Annan woodland are likely to allow them to persist for some time under drier conditions. However in the long-term most populations do need some periodic seedling recruitment, and adequate wet periods will be necessary to allow germination and establishment. If such periods become less frequent, it will be important that the seedling recruitment and growth to maturity are successful whenever those less-frequent conditions occur. Given adequate rainfall events, other threats that are likely to impact on the recruitment of species include inadequate microsites for germination, and increased seedling predation, particularly by exotic species such as rabbits, though artificially high macropod species numbers may also be a threat at that time.
Competition from similarly drought-adapted exotic species may also threaten native species, though other exotic species may be destroyed by extended dry periods.
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