Experimental investigation of the origin of fynbos plant community structure after fire

Background/Question/Methods

Fynbos vegetation in the Cape of South Africa regenerates naturally after fire and one might expect this disturbance to over-ride other influences upon community structure. In fact, we have recently shown (Araya et al. 2011 N.Phyt. 189: 253-258) that plants in fynbos segregate strongly on fine-scale soil moisture gradients. The seedling stage is the most vulnerable in a plant's life history. We therefore advanced the hypothesis that hydrological niche segregation among fynbos plants is initiated after fire by differential survival and growth of seedlings along the soil moisture gradient. Interspecific competition significantly increases the niche differences among meadow species, so we proposed that the same might happen in fynbos. We tested these predictions with an experiment in which soil cores containing seeds were moved up and down soil moisture gradients after fire, but before seed germination had started. We applied a competition treatment in the root zone, reasoning that in post-fire conditions where canopy cover is low and in the highly nutrient-poor soils of fynbos, root competition would be more important than shoot competition. We then monitored seedling numbers and growth and identified them using DNA bar-coding, the first use of this technology for an experiment of this kind.

Results/Conclusions

The size of seedlings was significantly greater, the wetter the location into which they were moved, regardless of the soil moisture status of their location of origin, or of the species. This trend was influenced by an interaction between soil moisture and a root competition treatment, such that competition suppressed growth more in wetter locations than in drier ones. There was also a significant interaction between species and location, such that Restio nudiflorus increased in weight less with increasing soil moisture than did the other Restio species combined. This kind of differential seedling performance could generate hydrological niche segregation,  particularly under the influence of root competition.