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The Adventive Flora of “Riccarton Bush” (or Deans Bush)—a Remnant of Modified Indigenous Swamp Forest in Lowland Canterbury
Perhaps the most studied and the most written of botanical site in lowland Canterbury, this small but very well-known forest remnant is the last remnant of indigenous lowland forest on the Canterbury Plains in the vicinity of Christchurch.
This forest remnant is situated on the south bank of the Avon River at Riccarton, being about three miles from the centre of the city of Christchurch. The site (altitude ca. 50ft) was very swampy in its original state, but as a consequence of drainage associated with settlement about the remnant the habitat has become progressively more modified, and the ground surface level has sunk a foot or more.
Originally (ca. 1843) about 55 acres in extent, half of the forest was given in 1850 to the Canterbury settlers for firewood and building purposes, while the balance was retained by the Deans family as a reserve, a belt of English trees being planted round it to protect the forest from the heavy winds. The present day area of actual forest is 15 acres, with an adjacent four acres partly planted to indigenous trees and shrubs.
Riccarton Bush was constituted a permanent reserve by the Riccarton Bush Act, 1914, and is administered by a Board of Trustees representative of the Deans family, the Christchurch City Council, Riccarton Borough Council, Waimairi County Council, Paparua County Council, Heathcote County Council, and the Royal Society of New Zealand.
The land itself was a gift of the Deans family. The finance for administration and maintenance depends on annual allocations from the local bodies mentioned.
This forest remnant has been studied from about 1870 onwards, and a number of botanists—Armstrong (1870), Cockayne (1906, 1914), Bird (1916), Wall (1922, 1923, 1953), Murray (1924, 1950)—have described the vegetation, and a number have listed the indigenous species present. The last enumeration of species by Murray (1950: 31—34) lists 96 indigenous species.
A small number of species, possibly in part indigenous, in part adventive—e.g., Gnaphalium luteo-album L. have been included in the several lists of indigenous species, but to date the adventive species have never been enumerated, nor has their progressive significance in the vegetation been mentioned other than incidentally.
Cockayne (1914: 21) implies the presence of adventive species when he states that, “In order to restore the bush to its primitive condition all foreign species should
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be gradually removed ….” Wall (1922: 20, 1953: 40) is much more definite on the status of the adventive species when he writes: “Ferns and other plants of the forest-floor are still fairly abundant, but this part of the reserve is not at all in a satisfactory state. Far too great a number of aliens have got a footing, and the very existence of the native ground-flora is threatened by them.”
The first mention of the actual species occurring is by Deans (1924: 10) when he states: “When the bush was taken over by the Trustees, it was found that parts of it were overrun with elderberry [Sambucus], and that wild cherries [prunes] and certain exotic trees had invaded other parts.…”
As it stands at present, this forest reserve is in effect a collection of micro-habitats—(a) forest, (b) forest margins, (c) forest clearings, (d) bare tramways, (e) grassed tramways, (f) grassy waste land and (g) cultivated land.
It is apparent that over the years a number of introduced woody and herbaceous species have been deliberately planted, some having reproduced and spread subsequently; other species have come in as garden escapes and garden outcasts from adjacent residential sections, while the remaining species have come in incidentally and accidentally by animal, wind or other agency.
The present vegetation of this remnant is not that which existed when the first observations were made; it is a new vegetation, a blend of indigenous and adventive elements with the adventive element now very significant in the shrub and herb layers.
It is unfortunate that over the years that the remnant has been studied there has been no enumeration of the adventive species, no account of their introduction and of their increasingly significant role in the vegetation, but, be that as it may, it is considered that even a preliminary listing of the adventive species at this time is desirable.
The list of species which follows results from personal observation and recordings over a period of eighteen months, and while it is not presented as complete, it will serve as a checklist of species known to occur at the present time, and will serve as a basis for comparison in the future.
The assemblage of adventive species must, I suggest, be regarded as unusual, perhaps not duplicated elsewhere in the world, considering the limited area involved, and the geographic regions represented in the adventive flora.
The species are listed alphabetically with comment restricted to those significant in the vegetation, or noteworthy for some other special reason.