General.

A plant-community, without destroying its primitive stamp and general ecological and floristic structure, may readily be altered by a change in the relative abundance of its constituents, which may be so marked that a species of but little moment in the primitive community, or one introduced, may become common, or even dominant, and in this way a new community come into being.

The modifications may consist merely of increase or decrease in abundance of certain species already present but, on the other hand, species either page 357indigenous, exotic, or both together, frequently join the community and become definite members.

The causes of modifications are principally grazing or burning, or these combined; also seeds may be purposely sown or plants planted, or land may be drained, forest felled; indeed, there are many causes. In associations containing edible species, the relative palatability¹ of their various members plays the foremost part and it can be seen how grazing, or browsing, may rapidly alter the composition of certain classes of pasture or forest without affecting their general physiognomy.

Fire, of course, functions in proportion to the damage done and in regard to the species destroyed or damaged. In forest, regeneration after fire will depend, to some extent, on the ability of certain species to put forth suckers or the contrary. It also destroys the humus of the forest-floor, so bringing about new conditions for the germination of seeds.

Both grazing &c. and burning lead to increase in light-demanding species, both in number and variety, but such additions as arrive come, almost invariably, from plants very near the scene of damage. On the other hand, stock brought from some distant locality frequently bring in seeds of plants not present in the association (e. g. species of Acaena, Danthonia pilosa, Carex resectans, Poa maniototo).

Some examples of modified communities.

The student of New Zealand vegetation has usually not far to go in order to see examples of modified associations; indeed, much of the apparently primitive vegetation he will encounter in the lowland and montane belts will be more or less modified. This has already been stressed in Part II, where various modified communities are dealt with. As the changes in these partly-modified communities become, by degrees, intensified, according as exotic or indigenous species rule, so will the communities become exotic-induced or indigenous-induced. All the same, so long as the ecological conditions governing a modified community remain constant, it will bear the stamp of fixity. Thus, many modified types of vegetation may well be expected to remain for a long time much as they now are provided no new disturbing factor appears. Virtually all lowland and montane tussock-grassland, swamp and salt-meadow, and a good deal of forest, dune and even rock-vegetation come into this class.

In some of the modified communities certain conspicuous species play page 358such an important part as to affect their physiognomy. In what follows various examples are given.

Where Ammophila arenaria has been purposely planted to arrest dune-movement, self-sown plants may invade the adjacent primitive sand-grass association and grow side by side with Desmoschoenus spiralis or Spinifex hirsutus; so, too, Oenothera odorata may become abundant and make many parts of more stable dunes, and also sandy river-beds, gay with its yellow blossoms. Sambucus nigra has become firmly established in certain lowland forests (its seeds brought by birds) where the light has been greatly increased through destruction of undergrowth; also, in the different environment of shady gullies in semi-arid Central Otago, it is now a common member of certain patches of scrub. Hypericum Androsaemum is frequent both in damaged lowland forest and in montane Pteridium heath (SO.), it having come in after fire. Senecio mikanioides climbs over small trees and shrubs on the outskirts of semi-coastal forest in some parts of North Island and north of South Island. Verbascum Thapsus is now a most important member of the plant-covering of river-terraces and other stony or dry spots in the montane belt of South Island; by the settlers it is well known by the name "tobaco-plant". Plantago Coronopus forms extensive colonies in certain salt-meadows looking exactly as if indigenous; so, too, Lepturus incurvatus is equally at home in the same formation; in places, some of the indigenous species of salt-meadow are the hosts of the parasitic Cuscuta Epithymum. Pinus radiata has extended into the Leptospermum shrubland (VP.) and, in course of time, should make a distinct association. Hypochoeris radicata is a most important constituent of low tussock-grassland.