Introduction

Some interactions between plants are uniquely conspicuous elements of certain tropical forests; the giant lianas that wend through the canopy and the epiphyte-laden branches of cloud forests are striking examples. Nevertheless, the fundamental processes involved are no different from those in extra-tropical communities, even though diverse, sometimes uniquely tropical, mechanisms may be involved. An individual of one plant species interacting with an individual of a second plant species can lead to any of the same five outcomes at any latitude, and these consist of all combinations of negative, positive and neutral effects (except the non-interaction described by the mutually neutral interaction, 0/0). But interactions among plants in forests seldom involve such simple one-on-one relationships. More commonly, multiple players are involved and the interactions change with time: the liana binds crowns of several trees, the fallen palm frond damages multiple seedlings, and the solum is shared by roots of many species. Furthermore, positive and negative interactions occur simultaneously, so the observer sees only an integrated net effect of multiple interactions (Holmgren et al. 1997).

Most symbiotic (mutually positive) interactions in tropical forests involve relationships between plants and animals or between plants and microbes – fungi, bacteria, algae – described elsewhere in this volume. What, if anything, distinguishes plant–plant interactions from plant–microbe, plant–animal or animal–animal interactions? Even though they employ different biotic services (pollination, dispersal; see Ghazoul, Chapter 10, this volume; Muller-Landau & Hardesty, Chapter 11, this volume), higher plants (with a handful of exceptions) all use the same abiotic resources: water, carbon dioxide, photosynthetically active solar radiation and the same suite of 13 mineral elements.