Shrub demography 1989-1996

This package includes two worksheets “seedlings wild” and “seedlings sown”. The data in “ssedlings wild” are counts of seedlings that emerged in situ from the soil seedbank in 150 mm diameter rings pegged to the soil surface in a variety of microsites sites from 1989 to 1996. Seedling monitoring rings were placed in 55 permanent (5 m x 5 m) plots in indigenous Prince Albert Succulent Karoo vegetation on Tierberg Karoo Research Centre (TKRC) and on the adjacent farms Argentina (ARG) and Tierberg (TBG), as well microsites influenced by insects (ant mounds, termite mounds) or deposition of organic matter (litter trains). These data were originally collected to understand the influence of seed availability, weather, microsite, vegetation clearing and grazing on emergence and survival of seedlings in arid (<200ÿmm p.a.) Karoo shrubland. In undisturbed vegetation 20-800 seedlings emerged per square metre in the austral autumn when conditions were relatively cool and humid. Densities of emerging seedlings were greater in microsites influenced by grazing, insects, litter and shade than in undisturbed or exposed sites. The species composition of seedling assemblages resembled that of the surrounding vegetation. Survival of seedlings varied from 5% in 1989 and 1990 to 25% in 1991 and appeared to be limited by the quantity of rain in winter and spring.

Data in “seedlings sown” are repeated counts of seedlings that emerged from sown seed of the shrubs Pteronia pallens, P. empetrifolia, Tripteris sinuata and the annual Tetragonia echinata. Seeds were sown in May 1990 and monitored until March 1992 on permanent plots in grazed and protected shrubland where neighboring plants were removed or left intact. The objective of the experiment was to understand how competition, grazing and rainfall influenced the emergence and subsequent survival of seeds of perennial and annual plants. Emergence of planted seeds was similar in all treatments, but survival of both annual and perennial species were better on cleared sites than in undisturbed vegetation. Seeds of a common annual Tetragonia echinata (Aizoaceae) maintain a seedbank but those of three perennial shrubs Pteronia empetrifolia, P. pallens and Tripteris sinuata (all Asteraceae) did not.

Acknowledge as follows: Data are the property of SAEON and were collected by SJ Milton, students and assistants, funded by the National Research Foundation, the Percy FitzPatrick Institute, University of Cape Town, and the Department of Science and Technology (DST) through the Biota-DST “Tierberg Reborn project”