It is not surprising that totara and gorse were the dominant regenerating woody species on this pastoral hill-country site in Northland as both are light-demanding pioneer species that readily establish in grass and are relatively unpalatable to grazing stock (e.g., Ebbett and Ogden [1998]; Zabkiewicz [1976]). Successful regeneration of totara has occurred despite intensive cattle grazing. Totara has also been found to colonise pastures elsewhere in the North and South Islands (e.g. Beveridge [1977], Wardle [1974], Miller and Wells [2003]). Regeneration of totara also occurs in open seral vegetation, such as under a light canopy of manuka and kanuka (Wardle [1991]). These and other studies indicate that totara is light-demanding, tolerant of dry exposed sites and that regenerating totara plants are not particularly palatable to livestock.
The pattern of regeneration of totara on farmland in Northland is consistent with the ‘catastrophic’ regeneration mode (Veblen [1992]) where an even-aged stand develops after a massive disturbance such as fire, mass movement, flooding, large-scale windthrow or volcanic eruption. Seedlings of species that adopt this mode are typically shade-intolerant. Ebbett and Ogden ([1998]) showed that totara and kahikatea display the greatest height growth responses to increased light compared with the other major podocarps matai (Prumnopitys taxifolia (D.Don) de Laub.), rimu (Dacrydium cupressinum Lamb.) and miro (Prumnopitys ferruginea (D.Don) Laubenf.). Clearance of forest cover for farming has resulted in similarly catastrophic disturbance and consequently, shade-intolerant species such as totara and kahikatea are successful colonisers on farmed sites (Burns [2000]).
Totara regeneration was associated with steeper slopes and sites with greater diversity of herbaceous species but with lower levels of herbaceous cover. These steeper slopes have less herbaceous vegetation cover with species of lower fertility tolerance, possibly because the soils are less fertile, but also because of footfalls and skidding by cattle when compared with adjacent flatter sites. It is this combination of less grass competition and highly disturbed microsites prone to drought which favours establishment of totara and other woody pioneer species. In contrast, managed highly productive grass swards dominated by perennial ryegrass and clover, which are common on fertile soils throughout the country, are effective in preventing reversion to woody vegetation (Levy [1970]).
Totara regeneration is occurring at this site despite intensive grazing by cattle on a regular basis during the 10-year period prior to the study. Although there was no ungrazed control site for comparison, some reasonable conclusions can be made regarding the role played by grazing cattle in the successful regeneration of totara at this site. It seems likely that grazing has assisted regeneration by keeping grass cover short, thus increasing light levels near ground level for newly germinated seedlings. Miller and Wells ([2003]) cited several New Zealand studies demonstrating the reduced ability of native woody seedlings to establish and survive through rank grass. On grazed land leased from the Department of Conservation in south Westland, Buxton et al. ([2001]) found there were higher numbers of seedlings of a particular variety of totara (Podocarpus totara var. waihoensis Wardle) in grazed plots than in plots within fenced exclosures. They suggested that cattle may be removing the more palatable herbaceous and woody species reducing competition for the less palatable canopy-formers and, to some extent, facilitating their recruitment.
On sloping ground, particularly when wet, footfalls of cattle churn up the soil profile and prevent development of a dense ground cover of grass. Sheath and Carlson ([1998]) found damage to the soil surface was greatest on animal tracks and camps on moderate slopes, with high levels of disturbance to the soil surface due to skid damage on steep inter-tracks in hill country during winter. Although not quantified directly in the current study, considerable skid damage was evident on steep faces between animal tracks, resulting in a greater proportion of bare ground on steep slopes compared with flat sites. Many seedlings had established on the bare ground and less densely vegetated steep faces between formed animal tracks. Although trampling by livestock can severely damage young totara (Bergin and Pardy [1987]), the present study shows that many seedlings are able to survive and develop on the steeper hill faces.
Further evidence of the influence of dense grass competition on germination of woody native species can be seen in direct seeding trials of various indigenous woody species on steep hill country grassland, which indicates the need to control intense competition from existing vegetation, particularly exotic grasses (Davis et al. [2009]). For example, Ledgard et al. ([2008]) found that seedlings of indigenous species germinated from broadcast seeding in direct-seeding trials where soil had been disturbed. Further direct-seeding work indicated seedling establishment was best in ploughed/cultivated treatments, especially where rank grass had been mown and sprayed with herbicide beforehand (Davis et al. [2009]).
Grazing maintains a light cover of grass and, on steep slopes, creates bare ground from animal disturbance. It also eliminates regeneration of palatable species such as hardwood trees and shrubs, some of which would otherwise regenerate quickly on these sites, competing with totara and slowing its establishment. Wardle ([1991]) indicated that seral shrubs and trees can form self-perpetuating communities where grazing eliminates tree seedlings of more palatable species. The unpalatable shrubs manuka and gorse, and the trees kanuka and totara clearly have an advantage on these grazed pasture sites in Northland. No other forest tree species have a chance to regenerate under a regime of continuous grazing.
Esler ([1967]) regarded the effects of grazing in the conversion of grassland to shrubland on Kapiti Island as diverse. Grazing either accelerates or retards succession of woody species depending on certain other site factors and can also determine which species are likely to succeed. He found that small areas of grassland remained on parts of Kapiti Island where heavy grazing occurred or where there was no grazing at all. Intensive grazing of pasture on fertile soils retarded the appearance of woody species but on other sites the invasion of manuka and kanuka ceased when sheep and goats were removed due the development of a dense deep turf. In management of hill country for pasture, intensity of grazing is a major factor in succession of woody species (Levy [1970]). Sheep and cattle grazing have different effects on reversion of hill country to woody species. Intense close grazing by sheep leads to invasion of manuka, hard fern and rushes whereas controlled grazing with cattle will maintain a permanent sward provided that clovers are kept growing strongly by applications of phosphate (ibid).
The current farm-management practices at the study site of intensive cattle grazing (break-feeding at 35-day rotations during summer and 70-day rotations during winter) and annual topdressing with fertiliser (and lime every four years) are effectively maintaining the clover-perennial ryegrass dominated pastures on the lower angle slopes and flats. The steeper slopes are, however, dominated by less fertility-demanding grass species and with grazing, this open cover is allowing regeneration of totara and other non-palatable seral species such as manuka and gorse.
Manuka and kanuka were only minor components of the regeneration at this site. However, observations of many farmland sites throughout Northland indicate that regeneration of totara can be in pure stands or in mixtures with manuka and kanuka as well as with gorse. Manuka in particular has long been known to be effective in colonising a wide range of soils where sites have been subject to forest clearance, grazing animals and fires (Cockayne [1928]). Large-scale pasture reversion to dense manuka and kanuka communities in hill county on the East Coast of the North Island has been occurring over several decades (Bergin et al. [1995]). Extensive regenerating manuka and kanuka following abandonment of previously farmed land in the Waitakere Range have also developed where there has been a long history of grazing and trampling by cattle (Esler and Astridge [1974]). The lack of widespread kanuka or manuka regeneration at this site may simply reflect the specific environmental and climatic conditions during the 10 years prior to the study. Although manuka and kanuka have different ecologies, both have relatively light wind-dispersed seed. In contrast, the seed of totara (which is dispersed by birds when seed trees are present (Beveridge [1964])), is produced most years and can be formed on relatively young trees. It is therefore not surprising that the proportion of these three species can vary widely within regenerating areas.
Apart from totara, kahikatea was the only other podocarp species found regenerating on this Northland pasture site. Seed-producing trees of both totara and kahikatea were in close proximity to the study site. Differences in site requirements between the two species may explain the lesser abundance of kahikatea, with kahikatea favouring moister sites than totara (e.g. Wardle [1991]). Differences in palatability to stock may also be a contributing factor.
The benefits of reforesting steeply sloping marginal farmland to reduce soil loss and flood damage, and to increase biodiversity, are well known (Davis et al. [2009]). The present study demonstrates that totara-dominated stands can develop naturally in grassland and that totara readily establishes on slopes greater than 20° without affecting flatter country. Such steeper slopes are where the risk of erosion is greatest and would therefore benefit most from conversion from grassland to woody vegetation. This study demonstrates there may be no need to completely retire such areas from grazing. In fact grazing appears to have assisted in the regeneration process at this site.
A major requirement is an adequate local seed source from established totara trees. Where this is not available, a long-term option for landowners is to establish banks of seed trees throughout areas where totara regeneration may be desired. Well-managed totara established on reasonable sites will produce viable seed within 10 years of planting (Bergin et al. [2008]). The role of totara seed sources and the intensity of seedfall required to allow the species to regenerate naturally in grassland, including the potential to establish totara seed sources on key sites across marginal pastoral hill country, should be the subject of further research.
On some sites, periodic control of aggressive persistent weeds such as pampas (Cortaderia spp.) could assist regeneration and development of totara. Other early successional weed species (if present), such as gorse, are likely to be ultimately succeeded by totara (e.g., Wilson [1994]). If a fully-stocked totara stand was desirable, large gaps within regenerating totara stands could be in-filled by planting nursery-raised seedlings or trialling the efficacy of transplanting small seedlings from adjacent over-stocked areas. Nursery-raised totara seedlings are likely to be more susceptible to browsing than hardened naturally regenerating seedlings so may need temporary protection from grazing animals. When seedlings are well established, areas could be fenced off to encourage a more diverse range of native plants including more shade tolerant and palatable broadleaved tree species. If desired, developing totara stands can be managed by thinning and pruning at appropriate stages, potentially providing a valuable resource of timber for future generations.