Evaluation of recruitment methodologies for Boscia albitrunca in the southern Kalahari, South Africa

Abstract

Boscia albitrunca is protected under the National Forest Act (Act No. 84 of 1998) as well as under the Northern Cape Nature Conservation Act (Act No. 9 of 2009). The survival of this species is threatened by farming practices such as high browsing pressure and thicket control; development pressures of habitat destruction and tree losses coupled with the limited rehabilitation success recorded to date. Even though B. albitrunca germinates relatively easy from seed and can be regenerated using vegetative cuttings, long-term survival of both cuttings and seedlings has hitherto proved challenging. Hundreds of tree destruction permits are issued by the Department of Forestry, Fisheries and the Environment (DFFE) with the requirement to compensate for its loss through the re-establishment of three to ten trees for every B. albitrunca lost. Even though these requirements are enforced through a monitoring and reporting system, these compensation requirements have not resulted in the expected survival and/or increase of B. albitrunca specimens in the regions they were lost. This study aimed to determine to what degree soil amelioration affect the recruitment rate of B. albitrunca when sowing (freshly harvested or stored seeds) and when planting vegetative cuttings. The soil characteristics and establishment rates of naturally recruited B. albitrunca saplings in areas where this species was legally removed were also evaluated. Nursery trials, field trials and veld assessments were undertaken within a 50 km radius of Kathu town in the Southern Kalahari. A total of 15 soil samples were collected and analysed. To test the germination, seeds were harvested by hand, cleaned, dried and stored from two respective farms in the study area. Seeds of different ages were then planted in both ameliorated and untreated soils whereafter germination was monitored daily for the first thirty days and then bi-weekly for survival until hardening. To assess vegetative propagation, root cuttings and stem cuttings were collected from adult trees. Sealed and unsealed cuttings were planted in planting bags (nursery trials) or directly into the ground (field trials). The same monitoring regime as for seed germination applied. The field trials tested the establishment of the hardened saplings and additional sourced saplings, as well as the survival rate of planted cuttings. The additional saplings were also hardened. Field trails were monitored for survival and growth bi-annually for a period of 18 months. During the same period, naturally recruiting saplings were georeferenced and monitored bi-annually in an area where topsoil clearance took place for construction. This study found that B. albitrunca prefers well-draining sandy soils, although germination and establishment is not significantly affected by soil nutrient or chemical values. Amelioration further had no significant effect on germination. B. albitrunca can be successfully recruited from seeds or sealed stem cuttings. Larger cutting sizes had better establishment, and the youngest seeds had the highest mean germination. The correct cleaning and storage of seeds is however very important, and if done correctly, seeds remain viable for up to 12 months. Hothouses were further proven to have shorter germination periods than outdoor trials. A survival of below 25% were recorded for field trial saplings after 18 months, despite their stunted above ground growth and the coppicing ability of B. albitrunca were confirmed.