Competition by herbs as a limiting factor in shrub invasion in grassland: a test with different growth forms

We tested the hypothesis that seedling establishment, the critical stage in the invasion of grassland by shrubs, is limited by competition with perenn ial grasses in seasonally wet/dry savannas. We placed seeds of two invasive exotic shrubs - Cryptostegia grandiflora, a woody vine, and Acacia nilotic a, an arborescent legume - into pots with a wide range of existing above- a nd below-ground herbaceous biomass provided by either a tussock or a stolon iferous perennial grass. We also imposed different levels of watering frequ ency (5, 10 and 21 d), nutrient addition (+ and -) and grass clipping inten sity (no clipping, clipped to 5 cm and clipped to 25 cm). There was no effe ct of any treatment on shrub seedling emergence or survival and all of the seedlings that emerged survived the 90-d growing period. Herbaceous competi tion also failed to have an effect on biomass accumulation in shrub seedlin gs. More frequent watering significantly increased above- and below-ground biomass accumulation for both shrub species and nutrient addition significa ntly increased Cryptostegia biomass accumulation. Based on these results, w e question the proposition that reduction in competition by herbs via Lives tock grazing has been a significant factor in determining the rate or patte rn of exotic shrub increase in the seasonally wet/dry tropics. We also ques tion the suitability of the two-layer soil moisture hypothesis as a basis f or management practices to control the ingress of woody species into grassl ands and open savannas.