Forgotten GRASSLANDS

Forest and Bird, Issue 280, 1 May 1996, Unnumbered Page

Forgotten GRASSLANDS

GEOFF

ROGERS

Ebb and flow in the North Island tussock country

the South Island. Yet the issues surrounding

their welfare are no less complex. Be ees as

looks at conflicts in multiple land

use, unsustainable farming practices and natural vegetation change in these grasslands and. shows how invasion by native shrubs and weeds, inadequate a es

N THE eastern slopes of the Tongariro volcanoes below the treeline and on the high plateau immediately to the southeast, lies the last stronghold of the North Island’s tussock grasslands, a tussock estate that once stretched north as far as the lakelands of Rotorua. Travellers on the Desert Road, about Waiouru and in the Rangipo depression, get a good feel for these grasslands that today are very much the junior relation of the 3.4 million hectares of pastoral high country in the central and eastern South Island. However, even a casual glance from the car window along the Desert Road will suggest dramatic variations in their condition. Dense stands of tall tussock contrast with areas where introduced pasture grasses have almost entirely replaced the tussocks, and other areas where thickets of native shrubs and trees, such as monoao, manuka, and kanuka, have replaced the previous tussock communities. The changing status of North Island tussock grasslands has spurred quite a body of recent research that has unravelled insights into their origins, land use history, natural values and threats to their condition. It is a story of changing fortunes, mostly declining, but not all the result of insensitive, exploitative land use.

Weeds and tussocks

HEIR GROWTH form and occupation of dry soils have rendered tussocks below the treeline quite vulnerable to invasion by other plants. When one adds the stress caused by burning and the loss of foliage from stock grazing between burns, it is little wonder that nutrient depletion is primarily driving the tall tussock system toward mat vegetation. As an analogy, imagine the dramatic rundown of a native forest ecosystem if it was burnt every 10 to 20 years and selectively cropped in between. Many introduced plants such as Hieracium and introduced grasses have a superior capacity to capture resources as tussock soils become drier and depleted of their nutrients. Furthermore, many weeds such as heather, broom, and gorse can grow taller, capturing most of the available light. The spread of heather since 1912 through mostly red tussock grassland in northern Tongariro National Park is well documented (see Forest ¢& Bird May 1994). Its spread there is a portent of its likely performance in the tussock grasslands of the Army holding at Waiouru. Already dominating 140 hectares on the eastern side of the Desert Road, heather has also established numerous vigorous outliers in the Army country up

to 27 kilometres away. Patches of it, some up to 100 metres across, dot the open vegetation of the Rangipo depression. The weed is performing best in those open tussock grasslands where native shrubs have disappeared after a long history of burning. Hopefully the recent release of the heather beetle as a biological control will reduce the vigour and dominance of the weed. Another huge weed problem for the

Army with a similar smothering capacity to heather, is Pinus contorta or lodgepole pine. The last decade has seen a systematic and labour intensive scheme to eradicate it from the Army country and from the slopes of Ruapehu. Individual stands are cut at intervals less than the four to five years required for this pine to seed. The approach has had considerable success but, while seed sources remain in the nearby plantations, infestation will continue.

Before humans arrived, there were virtually no extensive tussock grasslands below the treeline in the North Island, save for small areas of tussocks mixed with shrubs in natural non-forest sites. These areas were restricted mostly to the floors of deep river valleys and basins, to peat bogs, to cliffs and to outwash fans on the lahar ringplains of the Tongariro volcanoes. The total area of these sites was very small among the virtually unbroken tracts of forest below the treeline. Bristle tussock (Rytidosperma setifolium), silver tussock (Poa cita), hard tussock (Festuca novae-zelandiae) and red tussock (Chionochloa rubra) all had their special habitats in these clearings. Only in the Moawhango River headwaters in the southern Kaimanawa Mountains had volcanic eruptions or natural lightninginduced fires led to some long-term forest clearance and its replacement with tussock grasses and shrubs. We now know this from the record of vegetation history preserved in charcoal and fossil pollen in peat bogs. This record indicates that small

areas in the region — a region subject to very dry summers — were deforested by lightning-strike fires about 3,000 years ago. Subsequent fires maintained a secondary vegetation of tussocks and shrubs. HE ARRIVAL OF early Maori in the central North Island resulted in widespread removal of forest by burning. Between 430 and 700 years ago, almost all the forest surrounding Lake Taupo and that on the extensive ignimbrite plateau to the east (Kaingaroa), much of the forest on the northern and eastern flanks of the Tongariro volcanoes and the extensive kaikawaka and beech forests on the high plateaux east of Tongariro disappeared. Tussocks and shrubs spread to occupy the burnt out areas and continued periodic burning of this vegetation by Maori progressively selected tussocks ahead of shrubs. Silver tussock and hard tussock spread to create extensive short tussock grasslands on the ignimbrite plateau around Lake Taupo, which if not repeatedly burnt, were progressively invaded by bracken fern, monoao, and manuka. On the wetter, colder and more leached soils of the Tongariro ringplain and the Moawhango headwaters red tussock colonised the previous forest soils. So when Europeans arrived in the central North Island they were greeted with extensive areas of tussock-dominated open vegetation. There was so little forest that Charles Bidwill on his 1839 exploratory trip from the Rotorua lakes to Lake Taupo had trouble finding enough wood among the tussocks on the Kaingaroa Plains to boil his billy. Nevertheless, the 660,000 hectares of tussock grasslands stretching from Waiotapu near Rotorua in the north to the northwest Ruahine Range in the south appeared to be ideal sheep pasture, and by 1880 large run holdings had commandeered most of them. The farming fortunes of the runs varied. By 1917 the large run covering the northern Desert Road area had been abandoned entirely, due to stock losses from wild dogs, few fences, harsh winters, and from shrub invasion of the previous tussock grassland. Likewise the large run covering the country about and north of Waiouru was mostly unprofitable for a succession of owners and was progressively acquired by the Army for a training ground from 1939. Areas, such as Karioi on the southeast slopes of Mt Ruapehu, the plateaux surrounding Lake Taupo and the Kaingaroa Plain were planted with pines, while farmers, spurred on by Land Development Encouragement Loans,

North Island tussock grasslands

converted other large areas to improved pasture. There were also subtle and dramatic shifts in the composition of the surviving grasslands. Pastoral use led to the gradual disappearance of palatable native grasses such as silver tussock, Deschampsia caespitosa, and species of Elymus. After Europeans arrived, burning was apparently less frequent than before, and manuka and other shrubs spread to dominate extensive areas of previous tussock — the northern area of the Rangipo depression between Mt Tongariro and the Kaimanawa Mountains is a prominent example. Heather was introduced to the northern ringplain of the Tongariro volcanoes early this century and now dominates some 52 square kilometres of former tussockland. Today, only about 65,000 hectares or 10 percent of the previous tussockdominated vegetation remains, concentrated in the southern Rangipo depression and in the Moawhango headwaters (see map). Virtually nothing remains of the 440,000 hectares of hard tussock and silver tussock grasslands that covered the ignimbrite plateau about Lake Taupo — only remnants of the flora exist in waste places along forestry roads and in isolated "frost flats". Of the original 41,000 hectares around the Tongariro volcanoes, 2,500 hectares are left on the summit of Mt Hauhungatahi and on the eastern ringplain. Heather has smothered a large area on the northern ringplain and large areas have succumbed to the inexorable spread of native shrubs and trees once burning fell from favour in the 1950s. The Moawhango area has fared a little better — of the original 166,000 hectares, 57,000

remains, the great bulk of it in the Army country at Waiouru. Only 10 percent of all that remains is formally protected in reserves — approximately 2,000 hectares each in Tongariro National Park, the Kaimanawa Conservation Park, and Moawhango Ecological District. If the military had not acquired the Waiouru grasslands even less would remain, because in private ownership the fate of these grasslands would have been conversion to improved pasture, as happened in adjacent privately owned blocks. By and large, Army stewardship, despite the overt damage caused by artillery and armour, has had important conservation spinoffs not only on their

grasslands, but also their shrublands and forests. Several land management policies target tussock welfare or rehabilitation. Their weed and pest control budget — mainly removal of Pinus contorta and rabbit poisoning — often extends to $500,000 a year (far more than DoC could afford to spend in a similar area). Several grazing leases over its tussock grasslands have recently been cancelled to encourage tussock recovery. The Army has also actively supported moves to eliminate feral horses from areas of high natural values in the north, and to containing numbers elsewhere on their land to levels compatible with improving grassland condition. Some of the higher altitude red tussock grasslands in the southern Kaimanawa Mountains and the northwest Ruahine Range are in Maori ownership. The individual blocks earn little revenue for their numerous owners, but they all represent valuable assets for nature conservation. Interminably long negotiations between the Awarua-Aorangi Trust Board and the Department of Conservation to secure some form of formal protection for the Mangaohane plateau grasslands (including Reporoa Bog and Makirikiri Tarns) appear to have recently stalled. LTHOUGH the days of periodic deliberate burning are over, accidental burning is very much an issue. About 18,000 hectares of tussock and scrub were burnt in one wildfire in the southern Kaimanawa Mountains in 1983. Explosions in rabbit populations

To burn or not to burn

AND MANAGERS of Tongariro National Park now face the dilemma of preserving the last red tussock grasslands that extended throughout the northern and eastern parts of the park earlier this century. The representativeness of the vegetation and the ecological diversity of the park are declining due to the rapid spread of heather and native shrubs such as monoao, inaka and manuka. Differing attitudes to grasslands flavour the debate on whether fire is either proper or useful as a management tool. One school of thought maintains that vegetation change is a natural process and should not be interrupted. Others, however, argue that secondary tussock grasslands are mostly caused by burning by humans

in the first place and intervention is therefore justified. Yet there is no provision in conservation legislation for the deliberate burning of tussock grassland and shrubland in protected areas. Protocols and methods for the use of fire as a management tool in tussock grasslands will need to be developed. Perhaps a lack of these and lack of a full appreciation of the dynamics of these grasslands prevent reserve managers, as well as the Army at Waiouru, from using fire to contain the spread of shrubs and trees. Deliberate burning has ongoing costs and potential problems, not only with controlling burn patterns but because of uncertainties with weed behaviour, possible ecosystem degradation and vulnerability of the recovering vegetation to weed infestation.

and the proliferation of Hieracium or hawkweed partly reflect subsequent insensitive land use. Another fire burnt tussock grassland from the Desert Road up onto the flanks of Mt Ruapehu in the Waihohonu area of Tongariro National Park in 1988. And about 30 to 100 hectares are accidentally burnt each year in the Army country from artillery practice. As occurred in the period of Maori burning, each fire interrupts the progressive invasion of tussock grasslands by native shrubs and trees by selecting tussocks, which survive burning better, ahead of non-adapted shrubs and trees. But today the big difference is that fire also leads to a proliferation of weeds, especially introduced grasses. The "invadibility" of grasslands reflects what is essentially a wet forest climate throughout the central North Island and the abundant seed sources for shrubs and trees in gullies and on humid southern aspects where they were naturally protected from fires. Inaka and mountain toatoa invade the higher altitude and wetter grasslands and monoao, manuka and kanuka the lower altitude and somewhat drier areas. With burning removed, shrubs exceed the cover of tussocks some 25 to 55 years after the last fire. There is no better example of the profound changes that occur after the end of burning than in the northern area of the Desert Road. When

this area was abandoned as a sheep station 75 years ago the vegetation was tussock grassland and shrubland, but today scrub or low forest of manuka and kanuka with scattered young beech trees dominate. Two to three centuries of uninterrupted change will see a reconstitution of the original beech forest in most areas. The

tussock grassland flora will be reduced to wetlands and frosty valleys, and to natural non-forest sites above the regional treeline. The diversity of present plant community and landscapes will be lost, at least locally, along with those species that depend on some degree of disturbance. But what happens if a too-frequent fire

regime continues? When you also have grazing animals and associated nutrient depletion, and a suite of weeds ( Hieracium, exotic grasses, heather and Pinus contorta) brought to the area by Europeans, the system degrades from tall tussocks to short tussocks to a mat vegetation of introduced grasses and Hieracium. Examples of this

trend are far less numerous in North Island tussocks than in the South Island, yet there are prominent examples. Herein lies a management dilemma. While the tussock grasslands owe their origins and early maintenance to humaninduced fire, the unsustainability of the practice in combination with grazing

animals and the likely invasion of weeds makes it an unacceptable management practice today (see box page 22). There are a few areas where stock still use unimproved tussock grassland, and all display typical decay symptoms — progressive loss of tussocks and spread of exotic grasses and Hieracium.

ESPITE THEIR RELICT status, [) the remaining tussock grasslands still contain wide ecological diversity, camouflaged perhaps by the apparent sameness of rolling tawny landscapes. They occur in an area with a wide span of rainfall (900 to 2200 mm a year), altitude (800 to 1250 metres), landforms and geology (volcanic and greywacke to soft sediment), and occupy soils from fertile limestone to highly acidic peat bogs. This has produced a number of very different plant communities. Often concealed within the grasslands are special ecosystems with associated rare plants — flush zones fed by nutrient-rich seepage from limestone outcrops, tarns in peat bogs, gravel levees alongside meandering streams, and ephemeral wetlands. Most of these rare plants, while also found in parts of the South Island, are restricted in the North Island to small sites within the tussock grasslands. A few are endemic to the North Island tussocklands only. One bidibidi (Acaena rorida) is restricted

to one small basin within the grasslands of the Mangaohane Plateau. Rare plant habitats are intimately linked with the grasslands through hydrology, sedimentation and microclimate. Many of these important ecosystems remain outside the reserves network although both the Army country and unfarmed Maori land have de facto protection. Future changes in attitudes to and the economics of tussock use, however, could change this. But given the inevitability of shrub invasion and a reluctance by land managers to deliberately burn open vegetation, another 150 years could see tussock grasslands a very minor component of central North Island landscapes. @

Dr GeorF Rocers, formerly with Landcare Research, is now based in Dunedin, studying plant rarity and biodiversity for the Department of Conservation.