The hanging swamps of the Blue Mountains are a unique and interesting ecological community, but many are situated on the urban fringe, so they are not always in the best condition. However there has been a strong increase in bush regen activity in recent years, due in a large part to “Blue Mountains Urban Runoff Control Program” which has directed a decent amount of State Government funding to local bush regen projects, including hanging swamps. Our understanding of how to protect and repair swamps is increasing – although there is still some way to go, and that knowledge is not always being implemented.
What is a Hanging Swamp?
The Environmental Study for the Blue Mountains Environmental Management Plan: 1989 (EMP) explains as follows: “Hanging swamps are an important part of the character of the upper and central Blue Mountains. There are two broad types of hanging swamp – sedge swamp and shrub swamp. Sedge swamps are a common vegetation community occurring in a broad band from Bell east through all townships to Woodford. Shrub swamps generally occur in the Bell through to Leura areas, where they are locally common. In some situations, particularly in the upper mountains, the two types intergrade”.
“Hanging swamps develop at moderate to high altitudes on sloping rock faces composed of Narrabeen Sandstone which are subject to a constant supply of water – especially groundwater but also surface runoff. Rainfall infiltrates the sandy soils and the permeable sandstone below. At intervals in the sandstone are narrow layers of claystone, or tightly cemented sandstone which are impervious to groundwater. The water then travels laterally on top of this impervious barrier until it reaches the edge of the impervious layer, which has been exposed by geological erosion. The groundwater then seeps out over the broad sandstone rock face below the impervious layer. These rock faces are in effect the foundations of the swamp structure and are often many hectares in area. Slopes vary widely but commonly exceed 1 in 3 (33%). The condition of constant moisture allows a range of plant species to gradually colonise these bare-rock sites over long periods of time…..” Common species are Button Grass Gymnoschoenus sphaerocephalus, Lepidosperma limicola, Xyris ustulata and Baeckea linifolia.
“The constant saturation creates anaerobic (oxygen starved) conditions in the soil, which inhibit the microbial breakdown and decomposition of dead plant material. This organic matter accumulates in a partly decomposed state as peat. Peat has an extraordinary ability to absorb water, and so the swamp soil acts as a sponge, retaining much rainwater for later slow release.”
“Streams fed by hanging swamps will continue to run for weeks after rainfall. These assured water supplies maintain the flow in creeks and the numerous escarpment waterfalls”
“Swamps have an intrinsic visual quality but they also contribute to the colour and texture in the landscape panorama as a whole. The low yellow-green of these swamps forms a contrasting mosaic with the tall olive green of surrounding Eucalypt forests”.
The Blue Mountains Water Skink, Eulampres leuraensis is endemic the Blue Mountains Hanging Swamps. And according to the EMP, swamps are a “particularly rich habitat for marsupial mice and other small native rodents”.
Response to fire
Swamp vegetation does burn in bushfires, but is able to recover if fires are not too frequent.
In the past swamps have been destroyed by development, and many are in private ownership. Since 1991 hanging swamps have been included in Blue mountains Councils environmental protection zone. However some are closely adjacent to developed areas and as the group saw, are vulnerable to weed invasion, urban runoff, and physical disturbance.
In pristine swamps the peat layer will evenly dissipate water, but when drainage patterns are altered, water sometimes concentrates and incises channels through the swamp. This leads to erosion and sedimentation and facilitate the spread of weeds. Elevated nutrient levels can also be a problem.
The moist sandy peat of the hanging swamps provides an excellent substrate for a variety of weeds. Many of the swamps near urban areas have been infiltrated by; Buttercup Ranunculus repens, Honeysuckle Lonicera japonica, Blackberry Rubus fruticosus spp agg, Montbretia crocosmia x crocosmiifolia, as well as some of the cool climate specialists such as Tutsan Hypericum androsaenum, Broom Cytisus scoparius , Gorse. Ulex europaeus. Holly Ilex aquifolium, and Cherry laurel Prunus laurocerasus. The woody weeds Gorse and Broom produce massive quantities of viable seed and have the potential to radically alter the nature of the swamps by forming a dense canopy over them. However, they are both noxious weeds and local authorities put a significant effort into controlling them, including serving notices on private landholders.
The cold conditions of the Upper Mountains mean that natural regeneration is generally slow, as is response to herbicide. In some Urban Run-off Control Program regen sites Gleichenia, Blechnum, and Microlaena so far have regenerated, but getting important swamp species such as button grass to germinate is proving elusive (is fire part of the equation ?).
Hanging swamp vegetation is low but very dense and this makes it difficult to physically access all parts of the swamp, although it is possible for a person (if not too heavy) to walk on top a button grass swathe without sinking through. The close packed sedges and ferns also presents a problem for control of weeds such as blackberry and honeysuckle, because it is difficult to locate and treat all the growing points. (These are usually cut and scraped., whereas the woody weeds are cut and painted).
Sites and issues
A few years ago, swamps in Minnehaha Falls Reserve at Katoomba were becoming infested with broom and other weeds. They are now in much better condition due the efforts of the Minnehaha Falls Landcare Group, Council noxious weed control programs and recent regen contracts. However other degrading factors were in evidence, such as formation of new tracks, and erosion due to concentrated water flow.
Hanging swamps are sometimes in the path of services such as sewer pipes, and in the past this has inevitably involved severe physical disturbance and the establishment of weeds. At Frederica St, Lawson the group was able to see a sharp delineation between the weed infested parts of the swamp in places disturbed during construction by Sydney Water, and weed free areas adjacent. Fortunately some of Sydney Water’s more recent work shows that it is possible to greatly reduce the damage by minimising use of machinery, clearing the vegetation by hand, stockpiling and replacing topsoil, and brushmatting.
Sydney Water also deserves a mention for helping to repair at least one past mistake. After requests by Gordon Falls Bushcare Group and Blue Mountains National Park they provided funds for a bush regeneration contract in a hanging swamp at Leura, where degradation was clearly the result of previous sewer construction.
Virginia Bear (and various helpers)