Dispersal of seeds as a constraint in revegetation of old fields in Renosterveld vegetation in the Western Cape, South Africa

[摘要] ENGLISH ABSTRACT:Species-rich Renosterveld vegetation does not return to old agricultural fields evenafter many years (10-20) of abandonment. In most cases the fields are slowly takenover by non-indigenous species, particularly alien pasture grasses and African lawngrass. While poor survival of indigenous seedlings due to competition, grazing or anyother establishment constraints, might explain the failure of natural vegetation tocolonise old fields, here I am testing the hypothesis that recolonisation by indigenousplant species is limited by seed dispersal. I have quantified changes in seed densitywith distance from the natural vegetation into old fields using seed traps and soil seedbank assessment. I have also looked at seed dispersal in the dung of large herbivores.Shrubs (with the exception of Eytropappus rhinocerotis) were poorly represented inthe seed rain indicating that they might be seed limited. Dicerothamnus rinocerotis (adominant shrub in Renosterveld) and certain geophytes (that were recovered in thesoil bank) have proven that seed dispersal is not the primary constraint to their returnto ploughed fields. Dicerothamnus rinocerotis produces a large number of seeds thatare dispersed long distances into the old field and a number of geophytes were wellrepresented in the soil bank. Large herbivores are dispersing a lot of herbaceous forbsin their dung but make very little contribution to the dispersal of indigenous tussockgrass and shrubs. Seeds of indigenous Renosterveld tussock grasses (e.g Triboliumhispidum) were restricted to the edge of the natural vegetation. My research hasshown that some indigenous seeds, particularly those dispersed in wind and throughlarge mammals, do reach the old ploughed field. I therefore conclude that seeddispersal is not the primary constraint to the natural recovery of populations of someindigenous plant species on old fields, but may limit recovery of certain indigenousgeophytes, grasses and shrubs.

[发布机构] Stellenbosch University