Observations on T hemeda australis- Eucalyptus Savannah 10 Papua P. C. H EYLIGERSI SAVANNAHS, vegetation types with a ground Fires are common throughout the dry season, cover dominated by grasses and an open tree at the end of which most of the ground cover storey, are extensive in the coastal lowlands of has been burnt. Regrowth starts after some the Central District, Territory of Papua (Fig. rain has fallen. Areas burnt early in the dry 1) . These lowlands were included in a regional season can have a fair cover at the end of it survey carried out in 1962 by a team of the and are liable to burn off again. At the time of Division of Land Research and Regional Sur­ our investigation, notwithstanding high rainfall vey, CSIRO. in June, the dry season was already well ad­ This paper describes the vegetation of savan­ vanced and fire had destroyed the ground nah near jackson's Airport, about 12 km east vegetation of a part of the area under in­ of Port Moresby. Emphasis is laid on correla­ vestigation. tion with edaphic conditions. The field work, Wild life in the area is very scarce and its in cooperation with the geomorphologist and influence on the vegetation is negligible. Graz­ the pedologist of the team, was done in July ing by cattle is restricted to a few fenced prop­ and August 1963. ·erties. Th e following data pertinent to this area are Two transects were selected for observations: extracted from the survey report (Mabbutt et transect 1 was located 3 km south of jackson's Airport near the Riga Road ; transect 2 about aI., 1965). 1.5 km northeast of the airport. Levels were The climate is monsoonal: strong south­ taken along each transect and at selected situa­ easterly winds prevail during the dry season tions pits were dug, varying in depth between from May till October, in which monthly rain­ 1.0 and 2.5 m. fall averages 30 mm, and light variable winds during the rest of the year with monthly rain­ falls averaging 160 mm. Table 1 gives more TOPOGRAPHIC AND SOILS DESCRI PTION OF detailed information about rainfall, together THE TRANSECTS with data on temperature and evaporation. Each transect comprised a fairly straight, The landscape, made up of strike ridges and smooth hill slope attaining 22° to 30 ° and with vales, is underlain by fairly steeply dipping minor rock outcrop, passing into a shallowly rocks of Tertiary age. Cherty shale, marl, and dissected foot slope mainly between 0° 30' and limestone form the rounded ridges; the vales 5° and up to 1200 m long, and ended at a small have been cut in less resistant tuff. Relief is in strike stream, which on transect 1 has a narrow the order of 100 m. Lithosols and regosols are bordering flood-plain. found on the ridges, brown clay soils and tex­ Much runoff is as sheet flow: slope wash on ture-contrast soils on the higher parts of the hill slopes results in small terracettes, and on flatter land, and dark clay soils and alluvial foot slopes leaves a fairly abundant lag gravel soils near and along drainage lines. The vege­ (Mabbutt and Scott, 1966) . tation is savannah, predominantly of the Each transect had a similar sequence of soils Th emeda amtralis-EttcalyptllS type, with tall (Scott, unpub lished data) . Th e hills are occu­ grass vegetation and forest along permanent pied by red regosols, which are deeply devel­ streams. oped at the lower slope of site 2. On the adjacent foot slopes texture-contrast soils are 1 D ivision of Land Research and Regional Survey, CSIRO, Canberra, Australia. Manus cript received found, whilst black clay soils occupy the September 21, 1965. remainder of the flatter country, forming a 477 478 PACIFIC SCIENCE, Vol. XX, October 1966 FIG. 1. View from the crest of a str ike ridge over the savannah country just east of, but similar to, that in which the transects were laid out. The foothill in the foreground rises fairly abruptly from the foot slopes, which end against a forested narro w flood-plain. In the farther distance is anoth er strik e ridge shadowed by clouds and backed by Bootless Inlet. belt of 50 m width parallel with the creek at E. papltana, and some scattered A lbizia procera; transect 1, but extending over 300 m from the and a grass cover dominated by T bemeda gully at transect 2. The soil of the flood-plain australis, with Het eropogon contortus and is an organic black clay. Sebima neruosum commonly associated, the latter especially on stonier soils. A shrub layer is only locally developed; A lbizia procera and T H E VEGETATION Cycas media are the more important species. The T bem eda amtralis-Eltcalyptlts savannah This type of savannah occurs on both transects, has a tree stratum of one or more of the except for a fringe of forest and tall grass on Eucalyptlts species E. alba, E. confertiflora, and the narrow flood-plain. Tbemeda australis-Eucalyprsa Savannah In PapUa-HEYLIGERS 479 ...:l Th e Trees -< '" 0 00 ::> ci r...: c-nO\ N""lN N ('(') DISTRIBUTION PATTERNS (Fi g. 2) : Ellcalyp­ z .-< z .-< tus alba occurred over the whole transect 1; its -< height ranged from 9 to 16 rn, On the black clay and adjacent textur e-contrast soils it was o 00 accompanied or predominated by E. conjerti­ .-< flora, of about the same height. Relative pro­ portions of E. alba and E. confertiflora varied between 6:4 and 2:8. On the rest of the texture­ contrast soils and the regosols E. alba was joined by E. papuana, varying in height be­ o .-< tween 10 and 20 m and mostly remaining sub­ N ~ ordinate. Relative proportions varied between .g" 9:1 and 5:5 . Somewhere In the central part .~'" of the texture-contrast soils, over a distance of <U about 50 m, an overlap In the area of E. .S ~ rr:r: confertiflo ra and of E. papttana occurred (Fi g. <3 T"-lf"\N«l Ov-\ ::> N N NN ..... \O 3) . On the transition to the organic black clay -< .-< soils some specimens of E. papllana also oc­ curred. E. papttana occurred over the whole transect 2, with heights rangin g from 10 to 18 m on .-< .-< N the flat country and from 17 to 22 m on the o. \O r<\V"\ r<)V"\ ec NN N ..... ..... ..... slopes and along the gully. E. alba, 9-13 m '" .-< high, was concentrated on the texture-contrast 0\ 0 0\ soils, but some trees occurred along the gully o\ r---:~ OO \C":j" N N N('(')c<\t<""l and one single tree was growing on the slope. .-< On the texture -contrast soils the relative pro­ 00V\ 0 portions of E. alba and E. papllana varied be­ O"": ~NV"\O\ ('('"\ N NN r- ..... tween 8 :2 and 5:5. Not a single specimen of N .-< E. confertiflora was seen. ff") ~«: The density is in the order of 150 trees per ...... oo":!'-.::r r- O ('(') N N"'t' 0\""1'" hectare. A more exact determination of density .-< N .-< was abandoned because of signs of wartime dis­ turbance such as local cutting of trees, trees that were pushed over, and patches of rather dense eucalypt regrowth. Moreover, it is un­ <U likely that this would have contributed very ~ 8'" much to an explanation of the described dis­ 8 tribution pattern. Observations of the 1962 a survey show that this rather haphazard pattern is a general feature. More detailed regional investigations may reveal the causal factors. ROOT SYSTEMS : Root systems were studied in 11 pits, which had been dug at the foot of the trunks, by removing as much soil from the roots as was possible without undue time­ consuming effort. They usually appeared to consist of a small taproot and about five main horizontal roots branching from the base of the 480 PACIFIC SCIENCE, Vol. XX, October 1966 TRANSECT 1 SCALE OF ~.:.:~ . TOPOGRAPHY TRANSECT 2 .' 25m I__-< .0 . 00" r-v-v-v-v- 100m o••••••••• 00• 0 .:.:.:.:.::.:.r::::::::::::::::::::::::::/:::::·:··:·:··:::·; ':':" :':':'~'::' .. Eucalyptus Ground cover Soi 1 t ype 0.... .f. alba //// I . ..Joemeda-neteropoqon ~~ ....81ac kca I y SOl'1 group • . .Epapuana \\\ \\\ \\\ ....llJemedo-Copi//ipfdium ~.. .. :.Text ure - contrast soil group ~ . .f. confertijlora III \1, 1// . • ... Ihemeaa-sehima ::::::::: Red regosol group -<.///. Sehima - Heleropoqon series FIG. 2. Diagram of the transects, showing the distribution and mutual proportions of the Eucalyptus species, and the distribution of the ground cover groups and of the soil types. trunk. The horizontal roots of E. papuana are flo ra, where they gradually emerge from the often oval in section with the long axis vertical, base. In the case of E. alba the trunk is usually and divide at or near the base in several hori­ a little swollen, but sometimes is considerably zontal roots above each other (Fig. 4) . so. With a trunk diameter of 24 em, the diam­ Horizontal roots run for distances up to eter of the base may vary between 30 and 45 about 25 m more or less parallel with the em. Just below the surface the base is con­ surface, often not deeper than 0.2 m, only stricted, under which constriction the main very gradually tapering. Near the base vertical roots branch off. roots branch off from horizontal roots, and are The taproot, if present, may reach a depth particularly common in some specimens of E. of 1.3 m but is usually shorter. It branches off alba. In some cases these roots run down verti­ under acute-angle, thick roots, which show the cally more than 1.5 m, but more often they same features as described above for the ver­ gradually bend away obliquely, and may even tical roots.