Changing environments and land use trends in New Zealand's hill country
Climate change
The Royal Society of London recently reported51
'it is at least 90% certain that temperatures will continue to rise, with average global surface temperature projected to increase by between 1.4 and 5.8 degrees centigrade above 1990 levels by 2100. This increase will be accompanied by rising sea levels, more intense precipitation events in some countries, increased risk of drought in others, and adverse effects on agriculture, health and water balance.'
Our information on climate change comes from CLIMPACTS National Assessment: NIWA Contribution52. The climate change patterns are based on "downscaling" various general circulation model (GCM) simulations. Downscaling is a procedure that allows local scale climate changes to be inferred from the raw data provided by the GCM at a much coarser spatial scale. Downscaled GCM patterns are recorded as a change per degree global warming. The National Assessment focuses on the HadCM2 and CSIRO9 transient GCMs, and we have used data from the former model.
Rainfall
As Fig. 5 shows, intensified westerlies lead to increased rainfall in the west, particularly for the South Island, and decreases in the east. The changes are of the order of 10% per degree global warming, and intensify the existing rainfall gradient (wet in the west, dry in the east) across New Zealand. The CSIRO9 pattern is much more uniform, still giving an increased west-east rainfall gradient in the summer half-year, but much weaker than the HadCM2 pattern.
Figure 5. HadCM2 model downscaled changes per degree global warming, for precipitation (contours in %): Winter (Apr-Sep), left-hand panel; Summer (Oct-Mar), right-hand panel.
Temperature
Climate models for the Australia-New Zealand region predict average temperatures to rise between 0.7°C and 3.1°C over the next 100 years53. According to the HadCM2 model, the temperature scenario shows an east-west gradient, with temperature increases likely to be largest in the winter season and in the north-east of the country. Fewer frosts will occur, with the greatest reduction in the South Island, of over 10 fewer days by 2050 and 20 fewer days by 2100. This would approximately halve the number of frosts in that part of the country. Prediction of increases in maximum temperatures vary, but in Northland the number of days above 25°C could double by 2100, and increase by 30-50% in the far south of New Zealand. An increasing number of very hot days are likely in Gisborne, Hawke's Bay, Wairarapa and Marlborough.
Carbon dioxide levels
Carbon dioxide, generated by burning oil, coal and natural gas, contributes about 64% of the effect of greenhouse gases. The carbon dioxide concentration in the atmosphere is now about 30% higher than it was 200 years ago. If it continues to increase at the same rate, it could nearly double by the year 203553. Apart from its effects on temperature and rainfall, increasing levels of carbon dioxide will directly affect plant growth and especially the interactions between different components of pastures. According to Campbell et al.'s overview54, a doubling of CO2 will lead to a 17% increase in grassland production and a 10% rise in legume content of pastures.
In FACE (Free Air Carbon dioxide Enrichment) field experiments at Flock House in the Rangitikei, total herbage grown was 18% greater at elevated CO2, with most of this response occurring during peak spring growth. The CO2 effect was least marked during autumn and winter. The most responsive species were forbs (broad-leaved herbaceous species), with legumes producing considerably more dry matter. Grazing the sward with sheep further enhanced the CO2 effect on legume growth55.
In the same experiment, Edwards et al.56 (2000) found that elevated CO2 increased seed production by the grasses Anthoxanthum odoratum, Lolium perenne and Poa pratensis, the legumes Trifolium repens and Trifolium subterraneum, and the herbs Hypochaeris radicata and Leontodon taraxacoides. The number of seedlings that emerged and survived to at least 7 months of age was increased by elevated CO2 for H. radicata, L. taraxacoides, T. repens and T. subterraneum in both years and for A. odoratum and L. perenne in the first year.
Adverse events
The frequency and intensity of extreme weather events such as cyclones, floods, droughts and heatwaves will probably tend to increase with climate change, although climate change models still cannot predict exactly how. These effects are likely to have a dramatic effect on the incidence of some weeds. Annual or biennial weeds of pastures, like thistles and barley grass, emerge in autumn in pastures suffering from drought stress, and more extensive droughts will result in more regular and even more dramatic 'thistle years'.
Effects on agriculture (modified from MAF NZ, 200053)
Climate change is likely to have both positive and negative effects on agricultural and forestry production in New Zealand. Climate change will tend to increase production risks and require changes in the way farmers and growers select and manage their enterprises.
Major risks include:
- Loss of productivity, loss of income and associated social stresses in some areas.
- Transitional costs of shifting from one land use activity to another more suited to a new climate.
- Increased frequency of extreme weather events will lead to business disruption and infrastructure damage.
- Increased pressure on water resources in some areas.
- Increased pest and disease problems (a significant risk for the horticulture sector), due to increased activity of organisms already present and the increased risk of any new species entering the country being able to survive and reproduce.
Some potential benefits include:
- Expansion of existing crops into new production areas, for example Pinus radiata or kiwifruit or maize, may be able to be grown in areas not currently planted due to climatic limitations.
- The prospect of being able to introduce new crops (and crop related industries) not currently present in New Zealand.
- Rising temperatures and CO2 levels may assist the growth of pastoral plants and forestry species.
- The opportunity to supply products into export markets facing more adverse impacts on production than New Zealand.
Effects on weeds and weed invasions
Dukes and Mooney57 suggested that most of the important elements of global change are likely to increase the prevalence of invasive species. They mentioned that studies in cooler ecosystems have suggested that shrubs may be favoured by increasing temperatures. Ahmed and Wardle20 considered the effects of increasing temperatures on the germination and emergence of nodding thistle, summer grass and spiny emex and deduced that all three weeds could increase in distribution and importance, although nodding thistle may become less important in northern regions. Wardle and Rahman58 assessed the possible effects of climate change on pasture weeds in New Zealand and concluded that climate change was likely to increase pasture productivity and thus restrict weed establishment. However reduced summer rainfall and increased likelihood of summer drought, they suggested, may encourage drought-tolerant weed species. They considered ragwort was not likely to be affected much by a changing climate, but might decline in areas subject to increased summer droughts. Nodding thistle, they thought, might increase in response to drier summers, although it may decline in northern North Island areas because of its need for low temperature vernalisation to promote flowering. Scrub weeds, they suggested, are unlikely to be much affected by climate change, except that warmer-zone species like woolly nightshade might spread further southwards. The same is true of many other warm-zone plant species.
Lower rainfall in eastern areas of New Zealand, combined with greater occurrence of drought events will exacerbate the opening up of pastures over summer, making way for greater invasions of annual and biennial weeds like thistles and barley grass. The pattern of series of dry summers that increase the opportunities for thistles and similar weeds will probably be enhanced, so that thistles will tend to increase in drier, eastern areas, although there will still be 'thistle seasons' following drier summers and 'thistle-free years' following wetter summers. Increasing temperatures and more intense droughts could open the way for different thistle species, like artichoke thistle, a problem in parts of Australia and present in New Zealand gardens but not yet in pastures. Saffron thistle, already present in a few areas in New Zealand, could also become more widespread and a more serious problem as it can be in Australia.
Higher rainfall areas, to the west of the country, will exacerbate the difficulties of grazing control of pasture growth, especially in hill country, and will probably result in scrub weed problems even more serious than they are at present. Warm zone scrub weeds like woolly nightshade and hakea may move further south in response to warmer climates. Poisonous, warm-zone plants like castor oil plant will almost certainly expand into new areas and become more abundant in places they already occupy.
The ongoing southward movement of summer-active grasses like Kikuyu and paspalum may influence the germination, seedling establishment and growth patterns of other pasture weeds. The difficulty of managing these grasses include problems of maintaining a high clover content, and during the autumn change-over period from summer active to winter active grasses. This last phase in particular offers ample opportunity for the germination and establishment of both scrub weed and annual or biennial weed species.
Higher carbon dioxide levels in the atmosphere seem likely to enhance the growth of broad-leaf species, including legumes, at the expense of grasses. This effect will certainly further exacerbate predicted increases in thistles and similar annual or biennial species. It may also increase the establishment of seedlings of scrub weeds.
The overall effects of climate change on weed invasions seem to bring bad news for farmers. More species already present in the country will be able to become naturalised in the wild, species not yet in the country may be able to gain a foot-hold. Warmer, drier conditions in the north and east of both Islands, coupled with increased frequency of adverse events, and rising carbon dioxide levels, means that annual or biennial forbs like thistles will increase in range and importance. Warmer, wetter western areas are likely to see expansion of current areas of scrub weeds, and an increasing southward drift of northern species. New weeds will continue to emerge from the naturalised species already present in the country.
Changing land use trends
The many dramatic changes taking place in the rural environments of New Zealand are discussed briefly in this section.
Sheep depopulation
According to MAF's December 2000 SONZAF Update59:
Sheep numbers are estimated at 45.51 million at June 2000, down slightly on the previous year's final estimate by Statistics NZ of 45.680 million. This reflects continued land use conversions to deer and forestry, and policy shifts to cattle by farmers. Sheep numbers are expected to fall to 40.88 million by June 2004, due to continued land use changes to deer, forestry, and, in the South Island, dairy.
Total beef cattle numbers at June 2000 are estimated at 4.68 million, up 1% on Statistics NZ's estimate of 4.62 million at June 1999. This reflects a response to increasing beef prices. At June 2004, beef cattle numbers are projected to be 4.79 million due to favourable beef prices.
Recent and projected changes in stock units (one breeding ewe = one stock unit) in New Zealand are shown in Fig. 6.
Figure 6. Trends in New Zealand stock units, 1994-200459
These changes in sheep numbers will probably mean relative destocking of steeper hill country, and the concentration of sheep on kinder, rolling and less steep hill country.
Sheep/cattle ratios
The changing ratios between national flock sizes of sheep and cattle will probably impact most on hill country pastures, where cattle to sheep ratios will continue to increase as more consistently profitable cattle replace sheep.
Fertiliser inputs and land management
Fertiliser application to sheep and beef farms declined dramatically after 1980 and only now is fertiliser applied per hectare returning to its former levels (Fig. 7). The proportion of hill country to which fertiliser was being applied fell from an average of 38 % between 1975 and 1985 to 22% between 1985 and 1992, and then recovered to its former level.
Figure 7. Fertiliser application to North Island hill country and hard hill country, and to South Island hill country (Meat and Wool Economic Service 2001).
Gillingham et al.60, O'Connor et al.61 and Lambert et al.62 explored the effects of withholding phosphate application to hill country. At Whatawhata, reductions in pasture production were significant only on easier country, but the percentage of moss, weeds and dead matter in the sward tended to be higher than where maintenance fertiliser application was continued. At Te Kuiti, production declined from year 2 onwards, and moss and weeds increased, although no ingress of scrub weeds was observed. At Ballantrae pasture production decreased over the course of the trial but pasture composition changed only where fertiliser input had previously been high, with a greater proportion of other species, mostly 'flatweeds'.
Although fertiliser application to sheep and beef pastures has been increasing in recent years, productivity and composition of hill country pastures has declined over the last 20 years, with lower productivity resulting partly from a shift to low-fertility tolerant grasses like browntop and to a higher weed content.
Application of herbicides commonly used for brush weed control (like clopyralid, dicamba, picloram and triclopyr) has declined over the past 10 years. According to Holland and Rahman63 this has probably occurred as a result of economic factors, although such herbicides have also been supplanted to some extent by metsulfuron and glyphosate. These statistics suggest that farmers are using less scrub weed herbicides and, therefore, presumably controlling scrub weeds on a smaller proportion of their farms.
The decline in rabbit numbers in some areas of the country, brought about by rabbit haemorrhagic disease, has led to an apparent increase in the number of scrub weeds64, an effect noted previously as a response to reduction in rabbit numbers both in New Zealand and in Britain. Although rabbit numbers are now rising again, many scrub weeds will have increased the areas they cover, and removing them again would be expensive and possibly uneconomic.
Insect and other pests that damage pasture species can also affect weed populations. The cyclical nature of damage caused by insect pests like grass grub have been well documented, and pasture damage often leads to increased weed populations. New and spreading pasture pests like the clover root weevil (Sitona lepidus) can affect the clover content of pastures and this may encourage farmers to apply nitrogen fertilisers to promote grass growth. This in turn can affect pasture composition, and may encourage some species at the expense of others57.
In relation to new insect pests, Green65 points out that the deliberate introduction of a new and superior pollinating species (like leaf-cutting bees to pollinate lucerne) could turn benign alien plants into invasive weeds.
Rural depopulation
The rural downturn that began in the early 1980s has caused a major drift of population now from country to town and city. Farms are bigger and use less labour. Southland, with a higher than average rural population, has been particularly affected by this depopulation. Southland region's employment (both part time and full time) in the rural area declined by 14% for males and 1% for females from 1986-1991. Over the same period, New Zealand rural areas overall declined by 9% for the males and increased by 6% for females.
Rural depopulation and its effects - lack of availability of labour, loss of services and limited opportunities for the new generation - have a demoralising effect on farmers66. This in turn may lead to less intense and careful management of pastures, especially because of lower availability of labour.
Increasing diversity of land use- 'ten-acre blocks', forestry, conservation
With the decline in the financial viability of pastoral farming over the past 20 years, especially in hill country, other enterprises have found alternative uses for the land. Foremost among these alternatives is forestry. Areas of farmland taken over by forestry have increased as the perceived value of forestry has increased against that of agriculture. Although there is no strong evidence for increased forestry leading to further rural depopulation, forestry is of concern to many farmers, who tend to see it in a negative light66.
Some weed species (gorse and broom for example) cause serious problems both in plantation forests and in pasture and either land use can act as a reservoir of plant species for the other.
Over the last 20 or so years, farmers have come to realise the benefits of biodiversity and have increasingly preserved and often fenced off areas of remnant bush on their properties. Sometimes these reserved areas may serve as reservoirs of weed seeds to re-infest the farm itself with weeds. State-owned conservation areas have long been treated with suspicion by neighbouring land owners because of the weeds they harbour and their role as a source of seed, especially for species like ragwort or thistles that have wind-borne seed. One of the respondents in our survey commented on retired land around waterways serving as a source of weed seeds.
'Lifestyle' blocks, often of 10 acres or so, have become more prevalent around cities in the last 30 years, and sometimes occupy hill country. Their owners are often town-dwellers who may not be familiar with weeds and their importance. Sometimes these blocks are developed 'organically', with limited use of weed management options, and the blocks become a potential source of weed seeds.
Reversion
Lower stocking rates, lower fertiliser rates and reduced profitability of hill country farming may lead to farmers abandoning pastoral land, especially on steeper country where scrub weeds are already encroaching. This reversion may be beneficial if the reversion is to native species like manuka, or even to gorse, which can act as a nurse crop for native species67. However in some regions and circumstances other scrub weeds may increase, and there is always the opportunity for the uncontrolled spread of weed species that were previously scarce in the area.
Effects on weeds and weed populations
Many of the changes in farm management over the last 20 years have affected to a greater or lesser degree the occurrence of weeds and their importance. The trends we have noted for example in relation to declining sheep and rural populations, seem likely to continue. Others, like fertiliser and herbicide use, will fluctuate along with the fortunes of farming and politics. The long-term changes will have the greatest impact in marginal hill country.
Increases in scrub weed cover brought about by lower sheep numbers, reduced fertiliser use and a smaller rabbit population will probably be irreversible without a considerable input of capital.
Reductions in the rural population, and lower numbers of Biosecurity Officers means fewer people in the countryside likely to recognise and take action on newly emerging weeds. Bigger farms and fewer hands means less time that can be devoted to grubbing or spot spraying isolated patches of weeds.
All of these factors allow existing weeds to increase both their range and the amount of ground they occupy. Increased cattle to sheep ratios in hill country almost certainly mean an increase in the pasture weeds that cattle ignore, like ragwort and buttercups.
Contact for Enquiries
MAF Information Services
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PO Box 2526
Wellington, NEW ZEALAND
Fax: +64 4 894 0721
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