Understanding how plant–plant interactions influence the development of plant populations has been a central theme in plant ecology over the last several decades. Shading by neighbouring plants was quickly identified as a factor that could have dramatic impacts on plant populations, leading to a highly skewed population size-structure with a concentration of individuals in the smallest size classes (Weiner & Thomas 1986). Under high densities, these small individuals may die, resulting in fairly predictable relationships between average plant size and density (White & Harper 1970), through a process commonly referred to as self-thinning. The idea that intense competition can lead to highly skewed size distributions and size-dependent mortality are fundamental concepts in plant population ecology.
However, this foundation has developed cracks. In the 1990s, it was established that competition above ground and competition below ground have fundamentally different consequences for plant populations, due to differences in the size-asymmetry of these forms of competition (Schwinning & Weiner 1998). The traditional model of competition leading to increased population skewness and self-thinning was based on the assumption that competition is size-asymmetric, as it is when plants compete for light. However, when competition is size-symmetric, as it is for soil-based resources, population size-structures are less affected by competition.
In their paper Positive interactions can increase size inequality in plant populations Cheng-Jin Chu and colleagues present an elegant model which further chips away at this fundamental concept by showing that highly size-skewed populations can develop not only through competitive interactions, but also as a result of strong facilitative interactions in stressful environments. The idea that facilitation could be an important process in natural communities gained considerable traction following the publication of an influential paper 15 years ago (Bertness & Callaway 1994). Since then, there has been a proliferation of studies highlighting the frequency of positive interactions in plant communities, and the need to better integrate positive interactions into ecological theory and understanding (Callaway 2007, and see Brooker & Callaway's Special Feature on Facilitation in Plant Communities, published in this issue). Chu et al. (2009) make a significant step in this direction, describing how positive interactions can alter the development of population size-structure and the process of self-thinning.
Chu et al. (2009) use an individual-based 'zone-of-influence' model to explore the interactive effects of competition, facilitation and abiotic stress on the development of plant size-hierarchies. A key outcome of the model is the prediction that facilitation can increase size inequality, particularly at low plant densities. At high densities, competitive interactions overwhelm facilitative interactions, also leading to increased size inequality. As a result, the overall prediction is that size inequality should be high at both low and high densities, and low at intermediate densities. The authors find support for this prediction using a simple field experiment in an alpine meadow, with the lowest levels of size inequality (measured as the coefficient of variation) found at intermediate planting densities.
The implications of this work are significant. At a minimum, this study suggests that simply observing levels of size inequality in different populations will not provide clear insight into the biological processes that are occurring. Instead, high levels of size inequality can emerge from different forms of interaction, i.e. competition and facilitation. The paper serves as a good example of the need to continually review and revise fundamental concepts in response to new information, allowing the march of science to continue moving forward.
Associate Editor, Journal of Ecology
- Bertness, M.D. & Callaway, R. (1994) Positive interactions in communities. Trends in Ecology and Evolution, 9, 191–193.
- Brooker, R.W. & Callaway, R.M. (2009) Facilitation in the conceptual melting pot. Journal of Ecology, 97, 1117–1120.
- Callaway, R.M. (2007) Positive interactions and interdependence in plant communities. Springer, Dordrecht.
- Chu, C.-J., Weiner, J., Maestre, F.T, Xiao, S., Wang, Y.-S., Li, Q., Yuan, J.-L., Zhao, L.-Q., Ren, Z.-W. and Wang, G. (2009) Positive interactions can increase size inequality in plant populations. Journal of Ecology, 97, 1401–1407.
- Schwinning, S. & Weiner, J. (1998) Mechanisms determining the degree of size asymmetry in competition among plants. Oecologia, 113, 447–455.
- Weiner, J. & Thomas, S.C. (1986) Size variability and competition in plant monocultures. Oikos, 47, 211–222.
- White, J. & Harper, J.L. (1970) Correlated changes in plant size and number in plant populations. Journal of Ecology, 58, 467–&.
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