Experimental Approaches to Clarify Phenomena of Competition

It is one thing to expect competition to be an important process in plant ecology, but quite another to actually demonstrate its effects experimentally. Three different lines of experiments have developed over the years: (1) laboratory experiments on species pairs, (2) neighbour- removal experiments in the field and (3) phytometer studies. Additionally, observational, non-manipulative studies attempt to attribute patterns in occurrence, abundance or growth to competition, but since they are purely correlative, this line of evidence is much weaker. To differentiate competition from the indirect effects of trophic interactions (Sect. 19.2), experiments also manipulate grazing pressure, fungal infections and mycorrhization. These experiments may be additionally necessary to understand plant-plant interactions comprehensively, but they do not quantify competition itself.

Laboratory studies allow standardised conditions, controlled densities of plants and detailed assessments of root growth, nutrient leaching and so forth. Additionally, only lab experiments allow the implementation of sophisticated competition designs (Box 19.1). Lab experiments have consistently revealed strong below- and above-ground competition in a wide range of species. This line of research has been largely discontinued, as the evidence is clear. Moreover, lab experiments primarily confirmed that competition can occur, not that they actually matter in the field.

Accordingly, neighbour removal studies have been the main approach to investigating effects of competition in the field. In this approach, a target plant individual is cleared of all neighbours, while in the control it remains untouched. Better growth in isolation is then taken as evidence of suppression of growth when the plant is grown with neighbours. And indeed, reviews and meta-analyses of neighbour-removal experiments unequivocally confirm the importance of competition for plant growth in virtually any system (Gurevitch et al. 1992). The main shortcoming of this approach is that above-ground neighbour removal leads to dead below-ground biomass, which releases nutrients as it decomposes, confounding neighbour competition with fertilisation. Tying shoots aside reduces light competition, but split-root experiments are only possible in the lab. It seems that neighbour- removal experiments are falling out of favour, too, and mesocosm experiments, linking lab and field setups, are rarely used to address plant competition.

The third experimental approach to plant competition focuses on measuring the intensity of competition in the entire plant community. The focus is on the intensity of competition, not the exact effect of one species on another. Accordingly, the idea of using a so-called competition-o-meter was born, building on using organisms as sensors for soil moisture or nutrient status (“phytometer”). Effectively, identical individuals of a “suitable” plant species are transplanted into existing plant communities and neighbour-free controls. The difference in growth quantifies competition. The results of this approach are equivocal with respect to competition intensity. This is largely due to the choice of the phytometer, which affects the results as well as the confounding effects of grazing and mycorrhization. This line of experimentation has thus not been able to deliver as reliable results as the two previously mentioned approaches.

All three approaches have been used to address the question of competition intensity along productivity gradients. One might expect competition to be more severe at high productivity, when growth is fast and plant individuals tall (Grime 2001). At low productivity, Grime argued, the environment is so challenging that competition plays only a minor role. In contrast, Oksanen (1990) argued that under stress any additional adversity imposed by competition will have more severe consequences. Eventually, this debate was resolved: biomass was much less reduced by competition under stressful conditions, but since plants also grow less there, the relative effect of competition was as pronounced under nutrient limitation as under fertile conditions.