What limits recovery of semi-arid oldfields? Seed addition experiments reveal the importance of seed versus microsite limitation

Many native grasslands in southern Australia have been utilised for agriculture. Grazing and cropping are two widespread landuses that have pushed these ecosystems to the brink of extinction. But what happens if you remove agricultural disturbances? Can native ecosystems recover? I've been leading a research team for some time that has been asking: does the function, structure and composition of oldfields return after the cessation of cultivation?

The recovery of native communities after cultivation may be constrained by two key ecological factors: (a) the failure of species to reach a site due to poor dispersal or (b) their failure to survive once there. Seed addition is a common method to test for seed versus microsite limitation. Most studies, however, do not follow populations beyond seedling establishment (to see if long-term persistence occurs), nor do they measure seed dispersal (to see if seed movements really are limiting recovery).

Recently, my grad-student Andrew Scott and I set out to determine the constraints on the recovery of semi-arid grasslands in northern Victoria after cultivation. We examined dispersal across native grassland / oldfield boundaries (using Astroturf to catch seeds at increasing distances from boundaries) and also investigated the relative importance of seed and microsite limitation across multiple life-history stages and generations by adding seed of two grassland forbs that are absent from the early successional recovery after cultivation.

Perhaps unsurprisingly, seed trapping over two seasons showed little movement of native seeds into old fields; most species had extremely localized dispersal of only a few metres. Consequently, similarity between the seed rain and standing vegetation was moderate to high. Hence, seed dispersal is a key constraint to the recovery of oldfields. This might be overcome by the deliberate introduction of propagules.

Goodenia - one of the species we used in seed addition
experiments in northern plains grasslands to test for
seed versus microsite limitation.
(Photo: Andrew Scott)

Seed addition into oldfields showed that two annual species (Goodenia, Rhodanthe) were able to establish in all, and flower in most, oldfields in the first year. Seedling establishment increased with sowing density, consistent with seed limitation. However, the relative importance of microsite limitation increased over the life-spans of the species. Density-dependence reduced the number of flowering plants, resulting in a large decline in seedling density in the following generation. This decline continued so that the initial positive effects of sowing density on seedling numbers disappeared by the fourth generation and hence, the long-term persistence of populations is uncertain. This highlights that seed additions might overcome dispersal barriers, but it does not guarantee that sustainable populations will then develop.

Thus, by monitoring seed dispersal and following experimental populations beyond seedling establishment, a rare achievement in the seed addition research field, we showed that dispersal limits species distributions, but microsite plays an important role in limiting population growth and persistence. Perhaps what this points to is that multiple seed re-introductions (i.e. seeds added across years) will be necessary to recover oldfields.

This work will appear in Oecologia soon.