Annual diameter growth of Pterocarpus angolensis (Kiaat) and other woodland species in Namibia

Pterocarpus angolensis DC. (Kiaat) is one of the most preferred timber species that occurs in most southern African countries, including Namibia. In Namibia, the species mainly occurs across the five regions in the north-eastern part of the country where commercial timber harvesting has taken place since the 1940s. Information about the growth rates of the species is currently lacking, yet this information is required to determine a sustainable harvesting regime in Namibia. Therefore, more data about diameter increment is needed. This study aims to (1) determine annual diameter increment of P. angolensis in order to assist in planning for sustainable harvesting levels in the northern regions of Namibia, (2) compare growth of P. angolensis with other woodland species (Burkea africana, Baikiaea plurijuga, Terminalia sericea and Schinziophyton rautanenii) and (3) compare P. angolensis growth rates found in northern Namibia with those found in other countries within its distribution range. A total of 111 increment cores and 73 stem discs were collected for tree-ring analysis. The results showed that the average diameter increment of P. angolensis was higher in the Kavango regions (0.71-0.80 cm yr−1) than in neighbouring regions (0.35-0.41 cm yr−1). The northern Namibian growth rate is estimated as 0.50 cm yr−1 and lies within the growth range reported for southern Africa. Growth rates of P. angolensis were more than 0.10 cm yr−1 lower than rates of T. sericea and S. rautanenii, but 0.10 cm yr−1 higher than B. africana and B. plurijuga. Age-diameter relationships of P. angolensis may be a good indicator of tree age if a sufficient number of samples are obtained. Our results revealed a rotation cycle of 95-100 years to reach the minimum harvesting diameter of 45 cm in the Kavango regions. This rotation cycle should be a good guideline for sustainable management of this species. This study suggests that simple ring counting on increment cores and stem discs with additional diameter data can provide valuable information on growth rates and rotation cycles. Additional data and work on older trees (>100 years) is required to complete the age-diameter framework.