Growth in three Karoo shrub species under various grazing treatments: the tracking of marked individuals over 25 years

Growth studies of long-lived shrubs are uncommon. In semi-arid rangelands, livestock and wildlife grazing influence growth processes and may interact with changing climate. Tierberg-LTER, excluded from livestock since 1987, and two adjacent ranches allowed for a comparative growth study of three shrub species (<i>Osteospermum sinuatum</i> – palatable, <i>Pteronia empetrifolia</i> – moderately palatable, and <i>P. pallens</i> – unpalatable) across contrasting present ungulate herbivory (none in exclosures, wildlife only, or livestock and wildlife combined) and past livestock herbivory (moderate and overgrazed). Marked individuals were measured at irregular intervals over 25 years. Stem basal diameter (mm), height (m) and two canopy diameters (m) were measured at the onset of the study and in 2014. Mean basal diameter growth (mm y⁻¹) varied between species and treatments. Change in mean attribute values was influenced by interactions with grazing history and treatments. Some historical and grazing treatments had a negative effect on the palatable <i>O. sinuatum</i> and no significant effect on moderately palatable <i>P. empetrifolia</i> or unpalatable <i>P. pallens</i>. The age of the originally marked individuals are unknown but &gt;70% of these individuals survived over the 25-year period. Our results support previous findings that dwarf shrubs in deserts are slow growing and long lived, and that grazing regime affects plant growth.

针对长寿灌木的生长研究较为罕见。在半干旱放牧区中，家畜与野生草食动物的采食会影响植物生长过程，且可能与气候变化产生交互作用。自1987年起禁止家畜放牧的Tierberg长期生态研究样地（Tierberg-LTER）与两个相邻牧场，为三种灌木物种的对比生长研究提供了实验基础：这三种灌木分别为适口性良好的*Osteospermum sinuatum*、适口性中等的*Pteronia empetrifolia*，以及适口性较差的*P. pallens*。研究涵盖当前不同水平的有蹄类采食压力（禁牧围栏样地无采食、仅野生有蹄类采食、家畜与野生有蹄类混合采食）与既往家畜采食历史（分为中度放牧与过度放牧两类）。研究人员在25年间以不规则的监测间隔对标记植株进行了多次测定，在研究启动时与2014年，分别测定了植株的茎基直径（毫米）、株高（米）以及两个冠幅直径（米）。平均茎基直径生长速率（毫米·年⁻¹）因物种与处理组而异，各观测性状平均值的变化受到放牧历史与处理组交互作用的影响。部分既往与当前放牧处理对适口性良好的*O. sinuatum*存在负面影响，但对适口性中等的*P. empetrifolia*以及适口性较差的*P. pallens*无显著影响。初始标记个体的树龄未知，但其中超过70%的个体在25年监测期内存活。本研究结果佐证了既往研究结论：荒漠矮灌木生长缓慢且寿命悠长，且放牧制度会显著影响植物生长。