When is dryland wetland?

Forest and Bird, Volume 22, Issue 3, 1 August 1991, Unnumbered Page

When is dryland wetland?

by

Colin Ogle

| T IS NOT COMMONLY KNOWN that New Zealand has naturally occurring ephemeral (or temporary) wetlands, yet when wetlands appear from time to time in central Australia’s dry basins, the event usually makes the news. For example, severe floods in eastern New South Wales and Queensland over the past two summers created rivers which drained to the west to form vast inland wetlands. Desert depressions, named as "lakes" on the maps of Australia (the largest being Lake Eyre) suddenly conformed to their map identities. Resident plants and animals flourished, after surviving in a dormant state sometimes for years. Mobile water fowl and waders soon made use of the ephemeral habitats, especially for massed breeding on islands and margins of "new"’ lakes. In New Zealand we are all used to seeing birds in temporarily flooded pastures and sportsfields, but these habitats, the result of human activities, are mostly used by adaptable species like mallard duck, gulls, whitefaced heron, and pied stilt, while natural ephemeral wetlands, the main subject of this article, are used by highly specialised plants and animals. Ephemeral wetlands are often the habitat (and sometimes the only habitat) of highly specialised indigenous plants and animals. Like their often more extensive Australian counterparts, our natural ephemeral wetlands can be alternately wet and dry every year, Or may remain either wet or dry for several years in succession. Some are places which lacked trees even when the surrounding land was clothed in forest, and they remain to this day as island refuges for nonforest species, often encircled now by a sea of developed land. For biogeography (the study of species’ distribution patterns, leading to an understanding of past geological events and climates), they can be of immense value.

Although I use the term ‘ephemeral’ to describe such wetlands, other authors have used terms such as temporary or seasonal. There are many types of ephemeral wetlands because they are geographically widespread and occur on a wide range of landforms, and in various climatic zones. Many are small in area, and hence are subject to heavy pressure and modification from surrounding land uses. Domestic stock, wild animals, weeds and vehicles are a few of the modifying agents. The following wetlands are chosen to illustrate their diversity, their special features, and some of the problems for their conservation.

Ephemeral dune wetlands

In January 1989 I was one of a group of botanists who discovered a native gentian relative, Sebaea ovata, in a dry dune hollow at the Whangaehu River mouth, near Wanganui. The species had not been reported in New Zealand for 17 years. Why did it remain

here after disappearing from a string of locations between Hokianga and Lake Ellesmere over the past 150 years? I believe the answer lies in the annual cycle of wet and dry conditions of the dune hollow. Between July and December in 1989 sheets of shallow water lay over the low, flat surfaces, or in channels on uneven ground. Black-fronted dotterel and spur-winged plover waded and dabbled in the shallow waters. From January to June the parallel dune hollows (dune slacks) were surface-dry, though they support many indigenous plants that indicate a continuing high water table: the shrub Coprosma propinqua, cabbage tree, toetoe, sand gunnera, a minute sedge Isolepis basilaris, jointed wire rush (Leptocarpus similis), two small milfoil species (Myriophyllum pedunculatum and M. votschii), New Zealand flax, lady's tress orchid (Spiranthes sinensis), two tiny herbs of the foxglove family, Limosella lineata, and an unnamed Mazus sp, and others. Thus, for half the year the Whangaehu dune hollows are undoubted wetlands; for the other half of the year most observers would describe them as dryland. In all sea-

sons, however, the hollows have a high water table and this determines the ecological character of the community. Periodic flooding not only provides for wetland plants and fauna, it also suppresses weeds. Relatively few adventive species seem to cope with the alternate wet/dry nature of the habitat. Sebaea ovata is an annual plant which needs damp, bare ground for seed establishment every year, but dry ground for growth and flowering. The periodically wet and dry hollows near the Whangaehu River mouth happen to provide these needs. Threatened plants scientist David Given of DSIR Land Resources, rates the plant as a nationally endangered species. DSIR botanist Alan Esler described similar plant communities near Himatangi on the Manawatu coast in 1969. I saw these dune hollows with him in the same year, but the area is now almost unrecognisable through the spread of pampas grass, Yorkshire fog, strawberry clover and other weeds. The natural character may have deteriorated because the water table has dropped, so that the periodically wet areas are no longer inundated as deeply or for as long. In the same

period, however, there was also oversowing of pasture plants, and fertiliser was applied. About six years ago the dune hollows were mown and "hay’’-bales were made of jointed wire rush! Further south, between stabilised dunes in Queen Elizabeth Park near Paekakariki, there are ephemeral wetlands surrounded by pasture. They contain regional rarities like tall spike-rush (Eleocharis sphacelata) in its closest occurrence to Wellington, tumble grass (Lachnagrostis filiformis) and Gratiola sexdentata, a small creeping herb with 12mm tubular flowers. When wet, the area is used by waterfowl, including New Zealand shoveler and black swan, and also by dabchick, pukeko, white-faced heron, and shags. After the area was fenced off from grazing in 1982, the native grass Amphibromus fluitans was found. David Given lists this species as having a nationally ‘vulnerable’ status. Although Amphibromus exists in only a few places, nearly all are ephemeral wetlands such as those, discussed below, in the Kaimanawa Mountains and at Lake Wairarapa. Recent problems in protecting wetlands in Queen Elizabeth Park have arisen mostly

from peoples’ different perceptions of ephemeral wetlands. These wetlands had been identified for protection from grazing in the 1982 Management Plan for the Park but after the Queen Elizabeth Park Board came under the control of Kapiti Borough Council in 1984, the fence was used to keep cattle and sheep in the wetland, and horse trialling and crosscountry motor cycle racing were permitted when the wetland was dry. Appeals to the Board over five years by members of conservation groups and, more recently, by staff of the Department of Conservation and Botany Division DSIR, obtained a respite from horses and motor cycles, but not from grazing. Progressive drying out of the wetlands seems to have coincided with the laying of Wellington's natural gas pipeline along the eastern edge of the wetlands in 1984. Clearing drains in other parts of the Park and nearby land may have had effects on the water table but these are unknown. The impact of a well which was installed close to the wetland in 1976 is hotly disputed. The Park’s Management Plan specified that the wetland’s natural values should be protected, but the means of achieving this were not addressed. Weed invasion is so severe now that restoration of the wetland's natural character may be very difficult.

Ephemeral wetlands of mountainlands

Many of our most important ephemeral wetlands are in mountain country. Perhaps this is because such areas have been less modified than lowlands by human activities, or because trees tend to occupy non-forested, low altitude wetlands quite rapidly. In the second case, long-term habitats for species needing ephemeral wetlands would arise only under peculiar conditions, such as on saline soils or where new open sites such as dune hollows are being created over time. Karst (limestone) country in mountain areas has some very significant ephemeral wetlands, created by a combination of gentle slopes, good drainage and high rainfall. Such temporary wetlands usually have hard, sandy or silty substrates with little or no organic soil or peat, because peat accumulation leads to permanent wetlands (usually bogs and tarns). In a sea of red tussock in the north-west Ruahine Range is a mosaic of temporarily wet hollows and also permanent peaty pools, the Makirikiri Tarns, which overlie limestone. The hollows support mats of short stature ‘turf plants, and almost every hollow is floristically different; a 10cm tufted sedge, Carex rubicunda dominating one, and creeping herbs such as Tetrachondra hamiltonii or Hypsela rivalis dominating others. During rain, all the hollows fill with water. Some dry out quite quickly as water drains into the subterranean streams, but others take days or weeks, and some pools remain as permanent water. Vegetation differences probably relate closely to water regimes, but may also result from small differences in substrate, or even from competition between the first plants to arrive and later arrivals which cannot establish in the dense turf. Drier hollows have been invaded recently by mouse-

ear hawkweed (Hieracium pilosella) which could be a serious competitor with native species. Dr Geoff Rogers, of Forest Research Institute (FRI) at Rotorua, reports that Acaena rorida, a bidibidi species of these hollows which is endemic to the north-west Ruahines, is seriously threatened by hawkweed. This bidibidi is rated as ‘rare’ by David Given. After many years without livestock, land around Makirikiri has been recently grazed by cattle. Physical damage by cattle hooves opening the turf also assists the entry of weeds, such as hawkweeds and pasture grasses. Dr Gillian Rapson of Massey University describes (pers. comm.) similar impacts of cattle in string bogs and associated ephemeral wetlands of the Lammerlaw Range in Otago. A very similar situation occurs around the highest parts of the Maungaharuru and Te Waka Ranges in inland Hawkes Bay. Turf mats on hard substrates resist the trampling of cattle, sheep and goats which graze the surrounding pasture (at over 1,800m altitude). Soft-bottomed, seasonally wet areas are badly trampled and adventive plants (for example jointed rush, Isolepis setacea, and Carex ovalis) have invaded them. Searches in 1989 and 1990 failed to re-find a New Zealand endemic asphodel, Iphigenia novae-zelan-diae, on Te Waka Range, the site of its only North Island record. Wild animals can also have considerable impacts on wetlands. For example, in periodically wet areas of the Moawhango River, southern Kaimanawa Ranges, wild horses are modifying the habitat of nationally threatened plants. On-going research by Geoff Rogers has already shown that the present high numbers of horses are incompatible with the conservation of the diverse native flora of these wetlands. One remarkable site on the shoulder of a ridge above Awapatu Stream can be empty or full of water at any time of the year because its water comes from the overflowing of an adjoining creek. Plants at risk there include Gnaphalium ensifer in its only North Island site, and Amphibromus fluitans. Ironically, the horses are protected under the Wildlife Act but the threatened native plants have no formal protection. The majority of the southern Kaimanawa wetlands are "flushes" in tussockland — permanently damp areas which have trickling surface water during wet weather. Carex berggrenii occurs only here in the North Island. Both the buttercup Ranunculus recens (brown hairy form) and a fine-leaved, small sedge, Carex uncifolia are here and in only one other North Island site each. The original population of the latter plant was wiped out during road construction by the army. Fortunately, it had been taken into cultivation by botanist Tony Druce, from whom Geoff Rogers obtained material to replant it recently in several places with similar habitat nearby.

Ephemeral saline wetlands

The Protected Natural Areas (PNA) programme was set up to identify and protect the best remaining examples of all types of natural area in each ecological district. However, PNA surveys have sometimes failed to recognise that ephemeral wetlands exist as, for example, ephemeral saline wetlands in Old Man Ecological District.

Inland saline areas of Central Otago were comprehensively surveyed and described by Department of Conservation (DoC) scientist Brian Patrick in 1989. A subsequent article in Forest & Bird (February 1990) gave some of the highlights. Although these areas have some similarities in flora and fauna to coastal salt marshes, long periods of isolation from the coast have produced unique ecosystems with endemic biota. Agriculture has eliminated many sites, and adventive plants are invading those that remain. Brian Patrick makes a strong plea for the conservation of remaining saline areas, but he stresses the need for management to retain or restore their natural character. One area of 125 hec-

tares, including Sutton Salt Lake, has been purchased recently as a reserve by Doc.

Ephemeral wetlands of lake shores

Other types of ephemeral wetlands can be found on lake shores, such as DoC scientists Peter Moore, Kevin Moynihan and I described for Lake Wairarapa in 1984. On the eastern

shore of Lake Wairarapa, sand dune formation has isolated parts of the lake bed, and produced quite different habitats from the lake shore itself, One example is Boggy Pond Reserve, a wetland isolated by stopbanks. The Reserve comprises a large pond and several shallow basins which become discrete ephemeral wetlands when water levels drop. One of these basins we dubbed "Wader Pond" in 1983 because, as water levels dropped in January, Peter Moore found it was used regularly by pied stilt, banded dotterel, white-faced heron, sharp-tailed sandpiper, pectoral sandpiper, and, less often, wrybill and lesser yellowlegs. By March, Wader Pond was dry and the

native turf plant community was found to contain an aquatic fern called pillwort (Pilularia novae-zelandiae), and again that characteristic grass of ephemeral wetlands, Amphibromus _fluitans. Shortly after, gamebird managers cut through the stopbank to allow water from an adjoining reserve, Matthews Lagoon, to "topup" Boggy Pond. Matthews Lagoon received

water pumped from nearby farmland at that time, and its water level fluctuated less than in Boggy Pond. Its water would also have been more nutrient-rich, and certainly the raupo and willows of Boggy Pond are now more similar in size and vigour to those of Matthews Lagoon. Sedges which thrive in high nutrient waters, such as the tall sum-mer-green Bolboschoneus fluviatilis, soft grassy Carex maorica and tall spike rush were in Matthews Lagoon only, but in March 1989 I found a bed of B. fluviatilis on the edge of Wader Pond nearest the cut in the stopbank. Introduced Mercer grass now appears to be spreading in the habitat of the native Amphibromus, and pillwort has not been seen for several years. A range of wading birds such as seen in 1983 has not been seen since. The end result of this attempt to enhance Boggy Pond for waterfowl shooting was a

loss of wetland diversity; Boggy Pond became more like Matthews Lagoon in both its water regime and water quality and, as a result, in its flora and vegetation. The shores of some inland lakes of Canterbury and Otago are similar to those at Lake Wairarapa. The only South Island record of Amphibromus fluitans was on the shore of Lake Tekapo in 1935, before the lake was raised for electricity generation.

Ephemeral wetlands on glacial debris

Moraines and outwash gravel surfaces resulting from glaciation contain yet other types of ephemeral wetlands. These deserve more specific survey and recognition than they have received in the parts of the PNA programme. The 1984 PNA survey report on Mackenzie Ecological Region called these "kettlehole tarns", or simply "tarns’ and identified them almost entirely as waterbird habitat, especially for black stilts. Where their ephemeral character was mentioned at all, the seasonal absence of water was, by implication, a blemish on their biological importance. However, native turf communities on the tarn beds are distinctive enough to warrant protection, with or

without bird usage.

Ephemeral wetlands on pumice

Central North Island "frost flats’ can contain ephemeral wetlands, but they show why it can be difficult to classify communities as "wetland" or "dryland". The few frost flats that have native vegetation remaining after extensive exotic pine plantings and pastoral farming are characteristically covered by low heath scrub or short tussocks. Native forest species are absent, or are very slow to occupy the sites for a number of reasons. The micro-climate of the broad flats or ba-sin-shaped surfaces is frosty because of cold air ponding, the areas are drought-prone and have a history of fires, and the soils are often of low fertility. Although Taupo pumice in the soil is excessively free-draining, other less permeable tephra deposits and buried soils

can impede drainage for shorter or longer periods. Areas with hard tussock and monoao (Dracophyllum subulatum), which are wet underfoot only during heavy rain, seem to be undoubted drylands, but these grade into progressively wetter areas with wetland species such as the sedges Schoenus pauciflorus and Baumea spp., tanglefern (Gleichenia dicarpa), a shrub daisy Olearia virgata, and bog pine. Between these extremes of dry and wet sites are those which are wet for some weeks or months — the truly ephemeral wetlands. Mark Smale of FRI has recently pointed out that the natural character of frost flats is threatened by the spread of lodgepole pine (Pinus contorta) and mouse-ear hawkweed. In similar habitats in Tongariro Forest, heather (Calluna vulgaris) is another invader.

Conservation needs

Identification: People do not generally appreciate the ecological importance and distinctiveness of ephemeral wetlands, their national rarity, their great range of types, their dynamic nature and their vulnerability to disturbance. Even PNA survey has sometimes failed to identify them or at least to distinguish their special qualities.

Nature conservation tends to fare badly in competing interests for use of wetlands, and especially of those that are not permanently wet. The important first step in the conservation of ephemeral wetlands is to recognise their existence. This should be followed by recording their flora and fauna, in all seasons, and mapping to achieve an understanding of what changes are happening. Their dynamics can be better appreciated by study of earlier maps and aerial photographs, and talking with landowners and others with long-term local knowledge. Management: In the past, formal reservation of ephemeral wetlands has not always led to their protection. No attempt should be made to "improve" an ephemeral wetland which has natural values until the possible consequences of change have been considered. Biologists from a range of disciplines should be consulted, as well as people with local knowledge. A primary aim of wetland management should be to retain and, where necessary, enhance, natural diversity. This applies as much to ephemeral wetlands as other types. It must be understood that there are risks in deliberately modifying a wetland. As an example, disturbance of the native vegetation and substrate by livestock or vehicles allows the entry of weeds. Once weeds are established it can be very difficult to eradicate them, and restoration of the natural condition is likely to require much more than merely fencing to exclude animals or vehicles. In certain circumstances, controlled grazing may be necessary to suppress weed growth. Carefully designed experiments and monitoring should be part of every attempt to manage disturbed wetlands, and the results made widely known to wetland managers.

Acknowledgements

My special thanks to Tony Druce and Geoff Rogers for field discussions in many of the sites discussed above, and for their valuable comments on a draft of this script. My thanks also to others who commented on the script, including Department of Conservation staff Brian Patrick, Susan Timmins, Don Ravine, Hugh Robertson and Jan Heine. Colin Ogle is a botanist and conservancy advisory scientist with the Department of Conservation in Wanganui. #&