Andrew John Scott (PhD) - Vegetation recovery and recruitment processes in semi-arid old fields

I'm still figuring out how to get theses online, so I thought I'd start with the Executive Summary of Andrew's excellent PhD thesis on Vegetation recovery and recruitment processes in south-eastern Australian semi-arid old fields. Email me if you want a PDF version (J.Morgan@latrobe.edu.au)

Abstract

Old field succession is still poorly understood in arid and semi-arid regions, and theory predicts that, due to the harshness of the abiotic environment, recovery of native vegetation communities will be slow and less likely to follow a trajectory back toward pre-disturbance condition. The purpose of this study was to explore vegetation recovery after abandonment from cultivation in semi-arid native grasslands in northwestern Victoria, Australia, and to gain a mechanistic understanding of the causes of and constraints on reassembly in this system by investigating recruitment processes at the key life-history stages (seeds, seedlings, adults).

Species richness, composition, abundance and evenness of the standing vegetation were investigated along a chronosequence of time-since-abandonment spanning ~100 years, together with soil properties and cover of the biological soil crust. Most vegetation and soil parameters were found to be converging toward those of adjacent uncultivated grasslands, and some had recovered by ~50 years after abandonment. However, vegetation in old fields remained significantly different from uncultivated grasslands over the span of the study and several native species did not recolonise old fields; cover of lichen also remained significantly reduced. Native annual forbs were found to be important in the assessment of recovery, and it was concluded that an

abiotic threshold has not been crossed but biotic cultivation legacies may be limiting recovery in these grasslands.

The germinable soil seed bank was sampled over two seasons (spring and autumn) and also showed some capacity for recovery, but most native species had low persistence in the soil and many were apparently unable to form a seed bank, even in uncultivated grasslands. Persistent seeds contributed little to post-abandonment recovery in the standing vegetation, and there was low similarity between above- and below-ground species composition in spring. Similarity increased in autumn, indicating the importance of the annual seed rain, and the results suggest that the seed bank follows, rather than causes, patterns in the standing vegetation. An examination of the germination biology of several native annual forbs showed that an overall lack of complex germination requirements apparently explains the generally low persistence of these species in the soil, but the ability to retain a small pool of dormant seeds is consistent with the finding of a small persistent fraction in an otherwise transient seed bank.

Field seedling density and species richness were greatly reduced in the 20 years after abandonment, and did not significantly increase with time. Seed availability rather than environmental conditions (biological soil crust, soil fertility) appeared to be the major driver of seedling recruitment with time-since-abandonment. High compositional similarity between the seedling and adult life-history stages suggested that patterns in the standing vegetation were largely determined at the seed and seedling establishment stages, rather than by post-establishment mortality.

The relative importance of seed and microsite limitation was further investigated by examining the dispersal of seeds from uncultivated grasslands into adjoining old fields, and experimentally tested by adding the seeds of two native annual forbs into 20 year old fields. The results confirmed the major importance of dispersal limitation in driving post-abandonment patterns of occurrence, but also demonstrated some influence of microsite, and emphasize that the two are not mutually exclusive but operate simultaneously to determine post-abandonment vegetation recovery.

These findings have important implications for the management of semi-arid old fields in south-eastern Australia, and suggest that seed introductions are likely to have the largest effect on restoration success.