All the soils are azonal, - immature and poorly developed.

Transcription

1 -45- VEGETATION TRANSECTS IN RANGITOTO CRATER By.. Lidgard INTRODUCTION Among the many volcanic cones of Auckland, Rangitoto, the most recent, still bears a complete and naturally developed plant cover. The following account has resulted from a brief botanical survey of the vegetation of the crater of this island. The crater lies within the steep-sided scoria cone capping the volcano. The substrate of the crater contrasts strongly with that of the lava fields of the lower slopes. In 1956, the writer with the valuable help of Mr Rewi Moynihan, made north to south transects within the crater. In 1959, strip transects of selected sections were made on the east and west-facing slopes, located radially with respect to the centre of the crater. SOILS All the soils are azonal, - immature and poorly developed. (i) Soils under the Metrosideros / Leptospermum and Leptospermum- 5uttonia associations. Top: l"-2" - Littst 6" - Fibrous organic or humic layer 12" - A mat of roots with an increasing inorganic component of weathered scoria and scoriaceous particles with depth (ii) Soils under Leptospermum and Olearia-Leptospermum communities. Top: 1" - Litter 2-3" - Fibrous layer &42" - Root mat with large proportions of organic matter.

2 -46- (iii) Soils of rock faces. Little or no soil formation except where humus collects in crevices creating small pockets of soil. BOITIC INFLUENCES It is difficult to assess the damage done to the vegetation by wallabies and other animals. The evidence of their activities is revealed by the browsed condition of the Astelia, (terrestrial in this locality), by the lack of seedling trees and by the numerous well-worn wallaby tracks. It is seen that nearly half the Leptospermum scrub cover is senescent or deadwood; much of which is suffering from fire blight and inhabited by wetas. VEGETATION The vegetation of the crater is essentially a scrub cover of various Leptospermum communities which may be separated as follows: (i) Metrosideros/Leptospermum stands of the crater rim. Canopy: Height ft Totally Metrosideros excelsa. This is the most distinctive species in this stand which borders the crater above the height of 750 ft. Density of canopy varies with aspect; it is very dense on the west-facing slope. (Transect la and 3A). Sub-Canopy: field Layer: Height 6-8 ft Height 3ft Leptospermum - Suttonia - Olearia scrub layer. Height and composition varies with aspect. On the west-facing slope this layer is absent (Compare Transects la and 3A). Species common to canopy and sub-canopy with Leucopogon, Cyathodes and Hebe. (Transect 3A).

3 -47- Ground layer: Fern and lichen (Transects la and 3A). (ii) Leptospermum - scrub hardwood stands of the crater floor and foot-slopes Transects 4 and 2C). Canopy: Height 3-6 ft Leptospermum scoparium stunted and spreading, associated with Olearia. (50 per cent Leptospermum senescent and deadwood. ) Sub-Canopy: Height 1 ft Gaultheria antipoda, Cyathodes acerosa, Leucopogon fasciculatus. Ground Layer: Mosses and lichens. (iii) Olearia-Leptospermum la and 3B). stands of the upper wall of the crater (Transects Canopy: Height 5-8 ft Leptospermum scoparium and Olearia furfuracea. The latter becomes more prominent up to an altitude of 750 ft. Sub-Canopy: Height 2 ft Shrubs of Cyathodes acerosa, Hebe salicifolium, Olearia furfuracea and Leptospermum Ground Layer: Fern, lichens and mosses. (iv) Leptospermum-Suttonia stands af local occurrence on the upper crater Wall and crater rim. Canopy: Height 7-8 Healthy Leptospermum scoparium and Suttonia australis. Sub-Canopy: Height 1-2 ft Cyathodes acerosa with Olearia furfuracea and Leptospermum scoparium Ground Layer: Moss and lichens.

4 -48- (y) Vegetation of rock faces of the upper crater wall (Transects la, 3C, 2B). Field Layer: 1-2 ft Ferns, Scirpus nodosus and stunted shrubs. ENVIRONMENTAL FACTORS Three important physical factors are considered to be affecting plant growth within the crater through their influence on soil moisture conditions. These are: 1. Aspect 1. Aspect 2. Gradient 3. Elevation within the crater The vegetation of the steep, shaded, south-facing slope shows a response to the moister conditions in being dense and vigorous (Transect 1). On the north-facing slope, drier conditions have presumably been responsible for the lower statured shrub population (Transect 2). Drier conditions have allowed the continued existence of scoria slips wich are normally insignificant on such gradients. These slips adversely affect the rate of colonization and reduce the density of the plant cover. 2. Gradient As gradient increases there is a noticeable decrease in shrub height. This is demonstrated by the vegetation profile on the north-facing slope, (Transect 2A and 2C), where a lowering of gradient from 40 to 30 is reflected by an increase in canopy height from 3 ft to 7 ft (Transects 3B and 3C show a comparable pattern). Combined with the change in shrub height is a change in floristic composition. Suttonia australis appears only on the gentler slopes. Pomaderris phylicaefolia is absent here, but is present on the steeper slopes above and below. Its distribution appears to be affected not so much

5 -40- by slope itself but its ability to grow wherever competition is low. A similar change in floristic composition is seen on the 60 gradient of the north-facing slope (Transect 2B). Gradient appears to be a critical factor affecting plant distribution. The steeper the slope, the rougher the ground surface becomes due to instability of the substratum. This factor, coupled with the low moisture retention presents an inhospitable environment for plant establishment. On the steepest slopes, only plants tolerating a low moisture supply and minimum depth of soil have established themselves and then only in crevices and small soil pockets. 3. Elevation within the Crater There is an overall decrease in the shrub height from the rim to the floor of the crater. It is seen that, for each individual aspect, the shrub height towards the bottom of the crater is not as tall as that of shrubs on a similar gradient near the top (cf. Transects 2A and C and la and B). More marked is the change in floristic composition according to elevation within the crater. Below an altitude of 750 ft Metrosideros excelsa is absent. The constant height of these trees (30-40 ft) accentuates the abrupt change at this level. Olearia furfuracea (7-8 ft tall) is more prominent in scrub on the mid-slopes than in the Leptospermum stands on the footslopes and crater floor. Transects la and B illustrate this, as does Transect 3D. Tables 1 and 2 show that a marked increase in numbers of Olearia furfuracea occurs below 750 ft. Also related to elevation within the crater is a grading of scoria size. On the crater rim the scoria is fine textured. It grades into a coarse scoria on the lower crater walls with scoria boulders on the crater floor. This transition in substrate may play a part in producing the observed vegetation pattern and variation in floristic composition. The structural form of the crater may, too, produce a temperature

6 -50- gradient affecting the vegetation pattern. OONclUSION The character of the vegetation of the crater bears some correlation with three environmental factors considered namely, aspect, gradient and elevation within the crater. These three factors doubtless have a profound influence on microclimates and edaphic factors, such as soil moisture anddrainageconditions. The heterogeneous character of the plant cover, its structure, stature and floristic composition and local dominance of some species must also result from many other circumstances, e. g. differences in seed source etc. New colonisers such as Nothopanax arboreum and Knightia excelsa are making themselves evident on the crater rim. It will be interesting to observe their future development and the fate of the dying Leptospermum scoparium cover. The marked absence of the many species such as Senecio kirkii, Griselinia lucida. Clematis, Edwardsia microphylla, Cardiomanes reniforme etc. found plentifully on the lava fields of the island serves to accentuate the distinctiveness and difference of character of the crater vegetation and its development. More information is required concerning the history of the crater and its vegetation before a full picture can be presented. The outer slopes of the cone and adjacent areas are also covered by Leptospermum communities which all require detailed study and mapping. This is made easier because Leptospermum has only established on the central scoria mass of which the cones are composed. It is significant that Leptospermum communities have not established on surrounding lava fields. This fact can only be evaluated if more intensive study is made on the vegetation and environment of both habitats.

7 -51- TABLE 1 Relative Frequencies of Major Species on Transect up North facing Slopes. Metrosideros Above 750 ft Association Below 750 ft Stratum 1 and Species % % % % 1. Metrosideros excelsa 9 2. Leptospermum scoparium L. ericoides Olearia furfuracea Suttonia australis Cyathodes acerosa Leucopogon fasciculatus Hebe salicifolia Meuhlenbeckia complexa Pomaderris phylicaefolia 23. Pteridium esculentum Comparison of these two tables shows (a) greater density of species on the south-facing slope, (b) greater height of species on the southfacing slope and a gradual decrease in height with descent into the bottom of crater, (c) a definite line at about 750 ft below which Metrosideros excelsa is not present. TABLE 2 Relative Frequencies of Major Species on the South-Facing Transect - Metrosideros Above 750 ft Stratum 1 and 2 Association Below 750 ft Species 1. Metrosideros excelsa 2. Leptospermum scoparium 4. Olearia furfuracea 5. Suttonia australis 6. Cyathodes acerosa 9. Hebe salicifolia 11. Pomaderris phylicaefolia 23.. Pteridium esculentum % %

8 -52- TABLE 3 Relative Frequencies of Major Species on Strip Transects up the East and West-Facing Slopes. West-Facing East-facing Length 63 ft 57 ft Width 1 yd 1 yd Stratum 5-6 ft 1-2 ft 6-8 ft 4 ft Ground Species % % % % % Leptospermum scoparium Olearia furfuracea Hebe salicifolia Cyathodes acerosa Leucopogon fasciculatus Pomaderris phylicaefolia Pteridium esculentum Gaultheria antipoda Suttonia australis Total No. of Plants KEY TO SPECIES IN PROFILE DIAGRAMS Me Metrosideros excelsa Di Dianella intermedia Ls Leptospermum scoparium Tl Thelymitra longifolia Le L. ericoides Sn Scirpus nodosus Of Olearia furfuracea Pe Pteridium esculentum Sa Suttonia australis Al Asplenium lucidum Ca Cyathodes acerosa Af A, flabellifolium Lf Leucopogon fasciculatus Ps Pyrrosia serpens Hs Hebe salicifolia CI Cladonia and moss Mc Muehlenbeckia complexa Us Usnea sp.?p Pomaderris phylicaefolia Ga Gaultheria antipoda Cr Coprosma robusta Sm Stellaria media Nc Nothopanax arboreum Cls Cheilanthes sieberi As Acaena sanguisorbe EC Erythaea centaurum Cg Ctenopteris grammitidis

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10 -54- irahstct 4. WeST FiKCma