The effectiveness of post-fire soil disturbance in restoring fynbos after alien clearance

Abstract

The control of invasive alien plants is one of the major tasks facing conservation managers of South African fynbos vegetation. Extensive clearance work currently is underway, but the ultimate success of these operations depends upon good indigenous vegetation recovery. Fynbos species potentially may recruit from persistent seed banks in the soil, but surface seed banks may become depleted under dense aliens and deeper seed banks may fail to germinate. In this study, the effect on fynbos recovery of disturbing the soil mechanically, to a depth of 100–150mm, following alien clearance, was investigated. It was hypothesised that soil disturbance would promote germination from persistent, soil-stored seed banks. It was further hypothesised that soil disturbance would have a greater effect following high compared to low fire intensities. Soil disturbance had no effect on total plant recruitment. There was a significant negative relationship between fire intensity and plant density two years post-fire. Recruitment levels at the study site were low compared to those at sites invaded for two to three decades. This indicates that fynbos seed persistence may decline rapidly after four decades of burial. Soil disturbance resulted in higher indigenous species richness and projected canopy cover at the final census, two years post-fire. Changes to guild structure following soil disturbance included an increase in the cover of shrubs relative to graminoids and forbs, and small-seeded species relative to larger-seeded myrmecochorous species. Soil disturbance caused similar changes to guild structure in both high and low fire intensity plots. Soil disturbance is not recommended as a universal treatment following the clearance of dense stands of aliens. Used in conjunction with indigenous sowing, it may improve restoration at long-invaded, deep-soil sites, following fires of unnaturally high intensity. Post-fire soil disturbance should promote species richness in seedling recruits, plant growth conditions and the development of vegetation cover, while sowing would increase plant density and re-introduce guilds not present in relict soil seed banks.