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

Abstract

Background and aims: Species in plant communities segregate along fine-scale hydrological gradients. Although this phenomenon is not unique to fynbos, this community regenerates after fire and therefore provides an opportunity to study the ecological genesis of hydrological niche segregation.

Methods: Following wildfires at two field sites where we had previously mapped the vegetation and monitored the hydrology, seeds were moved experimentally in >2500 intact soil cores up and down soil-moisture gradients to test the hypothesis that hydrological niche segregation is established during the seedling phase of the life cycle. Seedling numbers and growth were then monitored and they were identified using DNA bar-coding, the first use of this technology for an experiment of this kind.

Key results: At the site where niche segregation among Restionaceae had previously been found, 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. Seedling weight was also significantly greater in a competition treatment where the roots of other species were excluded. No such effects were detected at the control site where niche segregation among Restionaceae was previously found to be absent.

Conclusions: The finding that seedling growth on hydrological gradients in the field is affected by soil moisture status and by root competition shows that hydrological niche segregation could potentially originate in the seedling stage. The methodology, applied at a larger scale and followed-through for a longer period, could be used to determine whether species are differently affected by soil moisture.

Fig. 1.

Results of Araya et al. (2011) showing the distribution, along gradients of water-table depth, of representative species of Restionaceae in the vegetation before it was burned within our plots at (A) Jonkershoek and (B) Steenbras, Western Cape, South Africa. Significant niche segregation was present among Restionaceae at Steenbras, but not at the Jonkershoek control site (data not shown).

Fig. 2.

The locations of experimental quadrats superimposed on hydrological maps of SEVa (sum exceedance value for aeration stress) and SEVd (sum exceedance value for drought stress) at (A) Jonkershoek and (B) Steenbras. The maps were created from a hydrological model parameterized with field data.

Fig. 3.

Mean seedling weight (± s.e.) in the Steenbras experiment, as affected by (A) the soil moisture stratum into which a core was translocated, and (B) the sleeve-competition treatment. No significant effects were detected in the version of this experiment conducted at Jonkershoek.