From the ground up

Peter Grubb wrote a really important paper in 1977 (Biological Reviews 52: 107-145) on the 'regeneration niche' and how it might contribute to the maintenance of local species diversity in plant communities. It's one of those papers I think all new Post-Grads in my Lab should read, and has been central to my thinking about plant community dynamics for a long time.

Few studies in the eucalypt woodlands of south-eastern Australia have examined whether species have fundamentally different regeneration niches. Hence, it remains uncertain whether species coexistence is promoted by differences in regeneration requirements created by substrate heterogeneity.

Recently, Amber Briggs and I published a paper (Briggs & Morgan) examining this question in semi-arid woodlands in Terrick Terrick National Park. Here, biological soil crusts (a mix of lichens, liverworts and mosses) are common and have a very patchy distribution. We wondered whether groundlayer species would germinate differently in areas with different crust components.

We sowed the seeds of five herbaceous species with contrasting seed morphology on top of four patch types(foliose lichen, short-turf moss, tree leaf litter, disturbed crust) and followed their emergence. Germination varied between patch types and, for the largest-seeded species (Maireana excavata), final germination was significantly lower on the biological soil crust and litter patch types because they strongly acted as a physical barrier to seed penetration into the soil. Germination time courses showed that biological soil crusts delayed the timing of germination of these species.

Hence, the patchiness in the environment created by soil crusts might differentially affect the spatial patterning of plant species in semi-arid woodlands by their subtle influence on seedling emergence. Grubb's regeneration niche theory would, therefore, have some support in semi-arid woodlands.

 Figure: The small-scale heterogeneity that biological soil crusts create in semi-arid woodlands. Different plant species respond differently to biological soil crust components, potentially enhancing small-scale species coexistence.