A vegetation switch as the cause of a forest/mire ecotone in New Zealand

Abstract

Abstract. Vegetation switches are those processes in which there is positive‐feedback between vegetation and environment, i.e. a vegetation state modifies its environment producing conditions more favourable to itself (Wilson & Agnew 1992). Switches can produce and maintain abrupt ecotones between plant communities. Such a sharp ecotone exists between beech‐podocarp forest and mire vegetation, both on deep peat, in southwest New Zealand. One such site was examined. There was no apparent explanation for the ecotone in the present topography nor in the substrate. Levelled transects through the forest demonstrated that most seedling establishment occurred on dead fallen tree boles. These microsites were significantly richer in N, P and K than the wet sump microsites. We argue that this is a mechanism whereby trees can become established in the forest, but not in the open mire. In the forest, the presence of trees ensures the presence of dead‐log microsites on the ground, permitting tree seedlings to grow. In the mire, there are no such micro‐sites, and trees cannot establish. The ecotone may be sharpened because of the presence of an ecotonal band of the small tree Leptospermum sco‐parium between forest and mire. This species can reproduce vegetatively by root suckers in the mire. Its boles are light, and even if they fall to the mire surface they are not thick enough to form a substrate for tree‐seedling establishment. The larger tree species may be prevented from falling onto the mire by the wind‐sheltering of the forest, and by the zone of Leptospermum. The postulated process would represent a new kind of water‐/nutrient‐mediated switch, of Type 1 (‘One‐sided’). It may occur in many waterlogged forests worldwide.

Linking habitat modification to catastrophic shifts and vegetation patterns in bogs

“…Furthermore, vascular plants may attract and accumulate nutrients from the surrounding environment through advective transport by groundwater, which is driven by the active transpiration of vascular plants (Marschner 1995;Rietkerk et al 2004a;Wetzel et al 2005). Moreover, when trees die and fall down on the bog surface, the logs provide a nutrient-rich environment suitable for successful colonization by vascular plants (Agnew et al 1993). It can thus be concluded that nutrients in the substrate only affect vascular plant growth.…”

The functional ecology of Empodisma minus (Hook, f.) Johnson & Cutler in New Zealand ombrotrophic mires

“…Now the data in Table 2 are evidence that it is the Manuka swamp site which most differs from the rest in its higher concentration of all elements except Nitrogen at depth and higher surface plant manganese content. Since Manuka swamp is a valley through which water fiows (Agnew, Sykes & Wilson, 1993) and not domed, we conclude that it is minerotrophic and look for evidence of ombrotrophism in the other sites by comparison.…”

“…The main, possibly ombrotrophic, part of the mire had been burnt 5-6 yr before, but nearby, isolated by forest, there are a few intact valley swamp openings. One of these we studied and named the Manuka swamp (Agnew, Sykes & Wilson, 1993), because of the conspicuous edge of Leptospermum scoparium Forster and Forster, known as the manuka or Ti Tree.…”

Human-mediated vegetation switches as processes in landscape ecology

“…A second approach is to see hard edges as artificial, but to welcome this, to see the formality of, e.g., the Miller House garden as the victory of mankind and its machine over nature (Gutkind 1952). The third viewpoint is that whilst gradual change is common in nature, hard edges are also present, caused by sharp environmental changes or by natural switches (Wilson and Agnew 1992;Agnew et al 1993); human switches are an example of these edge-sharpening processes, and the hard edges they produce are therefore not to be eschewed.…”

“…It can also sharpen an existing gradient, creating an abrupt ecotone. We are concerned here mainly with the latter process, which clearly has the potential to modify the landscape (Armand 1992;Wilson and Agnew 1992;Agnew et aL 1993). Wilson and Agnew (1992) gave examples of switches mediated by a range of factors, and able to create landscape features.…”