Clare J. Trinder

Plant ecology's guilty little secret: understanding the dynamics of plant competition

Summary Plant competition has been studied for decades. Yet, it is still an elusive concept that means different things to different people, is resistant to direct study and is shrouded in semantic and statistical complexity. We still lack basic information about many competitive mechanisms, processes and outcomes and their relationship to other ecological processes, and about how local interactions between individuals are propagated through communities. We suggest here that two critical issues have been overlooked in previous studies. First, there is a need for direct measurements of the process of competition as opposed to indirect mechanisms of competitive outcomes. Biomass has become the ‘industry standard’ for measuring competition, but we suggest that biomass cannot provide unambiguous insights into plant competition because it is the product of too great a range of factors and processes. Second, the use of a single measure of competition at an arbitrarily assigned end point of an experiment misses much of the complexity of dynamic interactions between competing plants and can lead to erroneous interpretations. Here, we suggest approaches to handle these difficulties, using new techniques or the application of well-known methods in a novel way. We also provide examples of systems or questions where the improved understanding these approaches could bring would be of particular benefit. Ultimately, we suggest the need for a major shift in the way in which we consider and measure plant competition to identify broadly agreed rules for variation in its importance, its role in different communities and habitats, and how and whether it influences or drives patterns of species diversity and abundance.

Interactions among fungal community structure, litter decomposition and depth of water table in a cutover peatland

Peatlands are important reservoirs of carbon (C) but our understanding of C cycling on cutover peatlands is limited. We investigated the decomposition over 18 months of five types of plant litter (Calluna vulgaris, Eriophorum angustifolium, Eriophorum vaginatum, Picea sitchensis and Sphagnum auriculatum) at a cutover peatland in Scotland, at three water tables. We measured changes in C, nitrogen (N) and phosphorus (P) in the litter and used denaturing gradient gel electrophoresis to investigate changes in fungal community composition. The C content of S. auriculatum litter did not change throughout the incubation period whereas vascular plant litters lost 30-40% of their initial C. There were no differences in C losses between low and medium water tables, but losses were always significantly less at the high water table. Most litters accumulated N and E. angustifolium accumulated significant quantities of P. C, N and P were significant explanatory variables in determining changes in fungal community composition but explained <25% of the variation. Litter type was always a stronger factor than water table in determining either fungal community composition or turnover of C, N and P in litter. The results have implications for the ways restoration programmes and global climate change may impact upon nutrient cycling in cutover peatlands.

A new hammer to crack an old nut: interspecific competitive resource capture by plants is regulated by nutrient supply, not climate.

Although rarely acknowledged, our understanding of how competition is modulated by environmental drivers is severely hampered by our dependence on indirect measurements of outcomes, rather than the process of competition. To overcome this, we made direct measurements of plant competition for soil nitrogen (N). Using isotope pool-dilution, we examined the interactive effects of soil resource limitation and climatic severity between two common grassland species. Pool-dilution estimates the uptake of total N over a defined time period, rather than simply the uptake of 15N label, as used in most other tracer experiments. Competitive uptake of N was determined by its available form (NO3 − or NH4 +). Soil N availability had a greater effect than the climatic conditions (location) under which plants grew. The results did not entirely support either of the main current theories relating the role of competition to environmental conditions. We found no evidence for Tilmans theory that competition for soil nutrients is stronger at low, compared with high nutrient levels and partial support for Grimes theory that competition for soil nutrients is greater under potentially more productive conditions. These results provide novel insights by demonstrating the dynamic nature of plant resource competition.

Temporal patterns of litter production by vascular plants and its decomposition rate in cut-over peatlands

Peatlands are an important carbon (C) store but many have been drained and damaged by mechanical harvesting. Little is known about ecological processes on abandoned peatlands that have recolonized naturally nor the impact of plants on C balance of these sites. Over the course of 13 months, we measured amounts of litter falling from three different species colonizing an abandoned peat bog in north-eastern Scotland to calculate potential inputs of C and nitrogen (N). In parallel, a litter bag experiment quantified C loss from the litter of these species over 18 months. Calluna vulgaris produced the greatest amounts of litter (276 ± 32.3 g dwt m−2 yr−1); Eriophorum angustifolium produced 10.9 ±2.6 g dwt m−2 yr−1 and Eriophorum vaginatum produced 42.3 ± 3.5 g dwt m−2 yr−1. Carbon loss varied from 30% (E. vaginatum) to 40% (Calluna) over 18 months, but differences among the three species were not significant. Overall, these findings indicate that Calluna can make significant inputs of C and N into degraded cut-over peatlands, but seasonal variation in inputs is considerable.

Introduction to the Special Feature on Mechanisms of Plant Competition

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