Mean height increase in saplings of a keystone woody savanna species over 15 years similar to that over a single season

Woody structure, particularly that created by large trees, is crucial to savanna biodiversity pattern and process. We assessed changes in vertical and horizontal canopy over 15 years of saplings of Vachellia (Acacia) erioloba (camelthorn), a keystone species and one of the only trees to grow large in arid savanna on low rainfall areas of Kalahari sands, southern Africa. We also assessed whether the fastest growing saplings over the 2003-2004 growing season were also the fastest growing over the following 15 years. We measured height, canopy extent, and diameter of largest stem at ground level of 120 saplings of V. erioloba in November 2003, remeasuring these in March 2004. In November 2019, we were able to find 67 of these saplings, and we measured them again to assess growth between 2004 and 2019. We modeled interindividual explanatory variables for growth (i.e., measures taken in 2004 of height, diameter, number of stems, evidence of fire, and largest stem diameter). We also compared residuals of growth between 2003 and 2004 with those between 2004 and 2019, to assess whether individuals showed similar progress relative to their peers over the two periods considered. Height increase was extremely variable, but mean increase over 15 years was only 45.4 mm, similar to that measured for the 2003-2004 growing season (43.1 mm). Mean horizontal canopy showed far greater growth (mean 120.2 mm over 15 years, compared with 7.12 mm over 2003-2004) than vertical growth. Individuals that had shown the fastest increase in horizontal canopy in 2003-2004 tended to be the same individuals that grew fastest over the following 15 years, but even the highest "achievers" showed increases of only 47.2 mm and 37.5 mm year(-1) in average horizontal canopy extent and height, respectively. This study demonstrates how crucial long-term studies are, particularly in arid systems. Many individuals may be trapped in the "sapling" class and could take decades to become large trees. Woody sapling growth was not cumulative, and modeling studies in savannas should be cautious in extrapolating growth from one season to several years.