Australasian Plant Conservation
Originally published in Australasian Plant Conservation 19(1) June - August 2010, p 13-14
Regrowth and conservation in New Guinea
University of Durham and Qatar University. Email: firstname.lastname@example.org
(top) Early stage regrowth at Tomb (sweet potato mounds still visible under grass).
Cane grassland with pandan in foreground at Maip.
(bottom) Patch of secondary woodland at Ganonkiyba with primary forest on ridge. Photos: Paul Sillitoe
There are extensive areas of secondary regrowth in the Highlands of Papua New Guinea, ranging from coarse grass to cane grass to woodland. Regardless of their familiarity with, and detailed knowledge of regrowth vegetation, people of the Southern Highlands are strangely casual about the plants that colonise an abandoned garden site. They make little effort to manage such successions, in the sense of planting and/or encouraging certain species over others. According to them they have little control over the process, e.g. whether it is grasses or tree seedlings that colonise a site, or what species of grass or tree. Rather, they allow nature to take its course. Succession is largely influenced by seed supplies from surrounding vegetation, e.g. trees are more likely to colonise sites adjacent to forest and grasses are more likely in grassland.
The actions of gardeners may nonetheless influence the sequence of natural regeneration to some extent. The length of time for which they cultivate sites and the thoroughness with which they clear them of natural vegetation may affect the species composition of subsequent regeneration. If they coppice and pollard some trees, these may shoot again, ensuring that some of the site’s original species recolonise it. Trees that commonly send up shoots again include Lithocarpus, Ficus, Artocarpus, among others. They may circumvent otherwise gradual succession sequences, resulting in a cover of mature trees far sooner than is usual. People may also influence the species composition of some abandoned sites by what they cultivate on them. They may plant some trees and shrubs that will remain when they abandon a garden. Among those which yield edible and usable products are the Naep Casuarina (Casuarina oligodon), Aenk Pandans (Pandanus julianettii) and Aegop Cordylines (Cordyline fruticosa), together with the occasional Poiz Fig (Ficus wassa) and Shuwat Fig tree (Ficus dammaropsis).
People are well aware of succession, as evident in their classification of various stages. Abandoned gardens go through a series of overlapping plant sequences that are botanically varied, with initially quickly changing communities. When initially abandoned, the species composition and abundance varies depending on environmental conditions, but in broad terms the same pattern repeats itself. At the time of abandonment certain rapidly growing pioneer grasses such as Arthaxon, Paspalum, Ischaemum, Setaria and Isachne and herbs, which include Bidens pilosus, Adenostemma lavenia, Cynoglossum javanicum, Crassocephalum, Polygonum, Viola, and Rubus, flourish at the expense of the few remaining crop plants. Some crops, such as Sweet Potato, Highland Pitpit (Setaria), bananas and Sugar Cane, compete successfully against invading weeds and remain for some time. This herbaceous regrowth stage may be followed by a succession of coarse grasses such as Ischaemum and Imperata, with Leersia swamp grass in wet depressions. The occasional tree seedling or juvenile cane grass clump may also occur. A site under Ischaemum may be brought back under cultivation after a period of grassy fallow, whereas Imperata marks a heavily disturbed site with fertility problems (e.g., thin topsoil, repeated burning, etc.). If natural regeneration proceeds, either Cane Grass or tree saplings, or a mixture of both, begin to grow. These first stages of regrowth support scant wildlife populations, largely small rodents and a few grass seed feeding bird visitors.
The Cane or Sword Grass community is dominated by Miscanthus, which comprises robust two to three metre high culms and grows in dense clumps that produce a thick cover frequently impenetrable without a bush knife. However, the grass community is more open where species are less mature and more widely spaced, particularly near homesteads where the rooting activities of foraging pigs create patches of exposed earth. Sword grass regrowth predominates in the majority of valleys where people live and cultivate between l600 m and 2000 m, interspersed with current cultivations and grassy regrowth of recently abandoned gardens. It is second in extent only to montane forest, which it has gradually replaced as human population and anthropogenic activities have expanded. Repeated cultivation, occasional grass fires, and other disturbances contribute to the maintenance of such fallows following forest clearance, together with environmental factors responsible for seedling death, such as soil-plant nutrient imbalances and sub-optimal drainage conditions. Thick brakes of Saezuwp Fern (Dicranopteris) may occur among the cane and occasional clumps of Low Fern (Thelypteridaceae). Other cane grasses sometimes occur, including Job’s Tears (Coix), brakes of Wild Sugar (Saccharum) and patches of Tall Grass (Pennisetum) on wooded margins. Scattered trees of cultivated Screw Pines (Pandanus), and the Graceful Tree Fern (Cyathaceae) are also common as succession progresses. Forests may eventually succeed cane grassland where left undisturbed for long enough. Cane grassland supports relatively meagre wildlife populations consisting primarily of small rodents and some birds, particularly in comparison with montane forest, which supports diverse wildlife including marsupials, rodents and birds, some of them eye-catching and large.
Secondary forest is the alternative long-term floristic succession to Cane Grass. When cleared garden plots in rainforest areas are abandoned they rapidly regenerate into patches of secondary forest. Tree regrowth also occurs in pockets throughout the Cane Grassland zone but has a markedly different floristic composition to montane forest and a considerably lower canopy at ten to twenty metres. Dominant species include fast growing soft-wooded trees primarily, such as various Spurges (Euphorbiaceae), Pipers (Piperaceae), Nettles (Pipturus), Figs (Ficus), Umbrella Trees (Schefflera), Dillenias (Saurauia), Silkwoods (Cryptocarya), Woolly Cedars (Trema) and Switchsorrels (Dodonaea), several of them attractive to feeding birds when fruiting. The Henk Tree Fern (Cyatheaceae) is also a common understory tree. Where Cane Grasses (Miscanthus, Coix) exceed a certain indeterminate number, the regrowth becomes more akin to Cane Grassland; there is no sharp distinction between these two vegetation communities. The ground cover is on the whole considerably less dense than primary forest, consisting of various coarse and creeping grasses and shrubs, sometimes growing to waist height. Ferns are also common, notably in sprays across the forest floor (Thelypteridaceae) and sometimes in tangles (Pteridium and Dicranopteris), together with tall leafy herbs, notably gingers (Zingiberaceae).
Regenerating forest patches in grasslands rarely develop into mature wooded stands because of clearing by land holders for gardens and repeated collection of firewood and raw materials. Nonetheless, if left undisturbed it is likely that montane forest would eventually establish itself. Humans also influence succession through the actions of domesticated animals, e.g. foraging pigs may damage garden cultivations so extensively that they are abandoned by landholders.
The location of secondary forest influences the diversity of wildlife it comprises. For example, when surrounded by montane forest supporting abundant wildlife, secondary forest fauna is are likewise plentiful, and when located in cane grassland where wildlife is limited, fauna is sparse; although tree fruits can attract birds in considerable numbers when ripe. Hunters frequently visit areas of secondary woodland within the forest that support fruiting vegetation, including Saurauia, Trema, Maesa, Schefflera and Cryptocarya, which attract animals such as cuscus, ringtails and giant rats. The influence that areas of disturbed vegetation have on animal populations, particularly ecotones (transition zones between different plant communities) is noteworthy. Human interference may lead to an increase in some animal populations, by creating vegetation communities and ecotones that yield shoots and fruits popular as food with some game. Some archaeologists suggest that humans in the past may have deliberately interfered with the vegetation in places, to promote mixed plant communities, whereas today this occurs as a by-product of swidden farming.