Introduction to the Special Feature on Mechanisms of Plant Competition

Abstract

The James Hutton Institute, Aberdeen, UKThis Special Feature brings together some of the latestideas and evidence about how plants compete with oneanother. That there are new ideas about this enduring sub-ject might come as a surprise to many ecologists. Indeed,when invited by the editors of Functional Ecology to guest-edit this Special Feature, our first reaction was to ask our-selves, ‘Surely all that stuff is so well known that there isnothing new to say.’ As is so often the case, more soberreflection revealed a contradictory reality. There is in facta lot still to be said about plants’ competitive mechanisms,far more than can be covered in these few pages. And wehope you will be convinced that in exploring this topic, weare not just tinkering around the edges, but are addressingissues that should lie at the heart of current ecologicalthinking.Most plant ecologists who have given serious thought tothe subject will have asked questions similar to the follow-ing:1. What is competition?2. How does it happen?3. Where is it happening?4. When is it happening?5. How can we quantify it?6. How can we be sure we are really measuring it and notsomething that just looks like competition?7. What is the influence (if any) of competition on a plantcommunity and on the fitness of its members?8. How does competition rate as an ecologically significantprocess compared with other things, such as environ-mental severity and habitat stability?Regrettably, few of us have yet to find satisfying answersto most of these questions. Of course, basic processes ofplant competition have been known for a long time, atleast at a conceptual, macroscopic level. Plants thatachieve greater root growth and faster uptake of waterand nutrients or grow taller to produce extensive canopiesto shade smaller neighbours, and which capitalize on thatbiophysical superiority by producing more offspring ordurable vegetative structures, are likely to have an advan-tage over less well-endowed neighbours, other things beingequal. But characterizing the precise cause-and-effect rela-tionships that allow those and associated processes tooccur, quantifying their interactions with others and, cru-cially, revealing their impacts (if any) on populationdynamics or community structure have proven to befraught with practical difficulties. Theory is of little help:the notion that applying Lotka–Volterra models equatesto truly understanding competition has been debunkedrepeatedly (Simberloff 1982; Peters 1991, pp. 56–8; Shipley2010, pp. 22–25).Nevertheless, it is impossible to imagine plant ecologywithout competition as one of its basic tenets, even plantecology based on neutral models (Hubbell 2005). Fewdoubt that competition remains a useful and powerfulidea, despite its reputation for fostering confusion, and thetendency of some ecologists to give it too much conceptualdominance, a stance lately criticized by Grime & Pierce(2012, p. 22) as ‘the product of some very muddled, non-Darwinian thinking’. In terms of understanding what com-petition is and what it does, it perhaps is not too fancifulto say that ecology is in a similar state to physics beforeRutherford and Bohr characterized atomic structure orgenetics until Watson and Crick discovered the doublehelix. Atoms and genes were useful, and powerful ideaslong before their structures and modes of action wereknown, but only after those breakthroughs were made,could experiments be fully explained and, most impor-tantly, experimental outcomes predicted. Competition hasyet to yield to ecology’s equivalents of Rutherford et al.,whoever they may turn out to be, such that the idea ofcompetition is transformed into a tangible, measurableprocess (or series of processes) that improves ecologicalunderstanding rather than confuses it. One (but not theonly) prerequisite for achieving that goal is to understandmuch more about how, and under what circumstances,competitive processes operate. Which brings us nicely tocompetitive mechanisms.Plants compete for resources: light; nutrients; water.Each resource has its own idiosyncrasies in terms of howits spatial and temporal availability is determined andresponds to local depletion by competing plants (Craine &Dybzinski 2013). The scope of natural selection to producestructures better able to achieve resource capture in anenvironment comprising other individuals that are alsopotentially capturing those resources is limited by multiple