Editor’s Choice 101:1

Issue 101:1 of Journal of Ecology will be published online very soon.  The latest Editor’s Choice is Tree effects on grass growth in savannas: competition, facilitation and the stress-gradient hypothesis by Dohn et al. 

Editor’s Choice 101:1

Ecologists have been scratching their heads over how competitive and facilitative interactions vary with ecosystem productivity and abiotic stress for over 20 years (Bertness & Callaway 1994).  One befuddling aspect is that the prediction of outcomes along gradients is much more obscure when stress is produced by a variation in an important resource, such as water, rather than something unrelated to a resource, such as temperature or soil salinity.  Thus, understanding shifts in the importance and intensity of interactions along resource gradients has taken on substantial significance in the literature, and yet there is a great deal of variation in outcomes among relevant studies (Callaway 2007).


Justin Dohn and Niall Hanan at their savanna field site in Mali, West Africa.
Photo credit: Andrew Tredennick

In this context, this issue’s Editor’s Choice, “Tree effects on grass growth in savannas: competition, facilitation and the stress-gradient hypothesis”, by Justin Dohn of Colorado State University and colleagues, takes a major step forward through an explicitly targeted meta-analysis of tree canopy effects on understory productivity in arid and semi-arid savannas in Africa and North America.  They found a clear and marked shift from net competitive effects to net facilitative effects of trees on understory productivity with decreasing annual precipitation across all savanna.  In other words, in wetter savannas the effect of tree canopies on understory productivity was negative relative to that in the open grassland, but in drier savannas trees increased productivity in the understory.  The very large biogeographic scale at which the authors worked swamped the nagging issue of different local community-scale patterns, which can vary wildly, obviating a so called major weakness of traditional community ecology, “it’s overwhelming emphasis on localness” (Lawton 1999).


An arid savanna in northern Mali. Greater grass growth beneath tree canopies than in interstitial areas, is evident, due to the facilitative effects of trees on the microclimate of subcanopy regions.
Photo credit: Justin Dohn

Curiously, the pattern in African savannas operated on a different scale compared with that in North America.  These differences remained even when canopy effects were tested against more complex aridity metrics, suggesting that the timing of rainfall or perhaps some more subtle geographic process or history alters how aridity-related stress affects interactions among plants.  Dohn et al. conducted their meta-analysis with ecosystem productivity as the dependent variable, and it is important to note that this is just one of many variables that might respond to the effects of savanna trees or other facilitators.  It would be interesting to see similar meta-comparisons of community diversity or the growth responses of individual beneficiary species.  Regardless, this study goes a very long way towards helping us understand how the regulation of ecological processes at local spatial and temporal scales affects large-scale and general patterns in nature (Brooker et al. 2009).

Ray Callaway
Associate Editor, Journal of Ecology

Bertness, M.D. & R.M. Callaway (1994) Positive interactions in communities. Trends in Ecology and Evolution 9:191-193.

Brooker, R.W.,  R.M. Callaway, L.A. Cavieres, Z. Kikvidze, C. J. Lortie, R. Michalet, F.I. Pugnaire, A. Valiente-Banuet & T. G. Whitham (2009) Don’t diss integration: a comment on Ricklefs’s disintegrating communities. American Naturalist 174:919-927.

Callaway, R.M. (2007) Positive Interactions and Interdependence in Plant Communities. Springer, Dordrecht.

Lawton, J.H. (1999) Are there general laws in ecology? Oikos 84:177-192.

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