Comparison of Growth Rings of Dichrostachys Cinera And Senegalia Mellifera Along a Rainfall Gradient: Implications For Bush Encroachment in a Changing Climate

ABSTRACT

Tree ring studies, known as dendrochronology, have been an important tool for studying climate variations since the beginning of the 1900s. This is because the information preserved in tree rings of most tree species for many years serves as environmental archives for both abiotic and biotic factors, tools that can be used to monitor climate changes. This study was the first to compare the growth rings of Dichrostachys cinerea and Senegalia mellifera along a rainfall gradient to infer implications for bush encroachment in a changing climate. Rainfall gradient in this study was defined by three study sites namely; 1) John Alphons Pandani Research Station (600 mm); 2) Farm Onyoka (450 mm) and 3) Farm Ebenhaezer (250 mm). Stem discs of different sizes of the two species were sampled from each site and polished for dendrochronological study. The growth rings of D. cinerea trees were substantially influenced by the amounts of site rainfall, but the growth rings of S. mellifera trees were not influenced by site rainfall. Therefore, D. cinerea is a better sensor and indicator of rainfall variability and climate shifts than S. mellifera, which tolerates dry climates as much as the wet seasons. However, the growth patterns of D. cinerea trees did not differ significantly along a rainfall gradient (H = 4.256, df= 2, p= 0.119 > 0. 05). On the contrary, S. mellifera tree growth patterns differed significantly (H = 42.366, df= 2, p= 0.000 < 0. 05). Senegalia mellifera trees from John Pandeni grew faster compared to the ones from Ebenhaezer and Onyoka. The study concluded that species response to climate change vary by geographical location, through the influence of complex interactions of various factors including rainfall. For further dendroclimatology studies in Namibia, site and species should be carefully selected to include species that are more sensitive to soil moisture ii (rainfall) and/or other climatic factors. Steeper rainfall gradient should be investigated in order to fully understand site climate variability of the recent past and to enable future projection of the results using climate models.