Ants, fire and bark traits affect how African savanna trees recover following damage

Bark damage resulting from elephant feeding is common in African savanna trees with subsequent interactions with fire, insects and other pathogens often resulting in tree mortality. Yet, surprisingly little is known about how savanna trees respond to bark damage. We addressed this by investigating how the inner bark of marula (Sclerocarya birrea), a widespread tree species favoured by elephants, recovers after bark damage. We used a long-term fire experiment in the Kruger National Park to measure bark recovery with and without fire. At 24 months post-damage, mean wound closure was 98, 92, and 72% respectively in annual and biennial burns and fire exclusion treatments. Fire exclusion resulted in higher rates of ant colonisation of bark wounds, and such ant colonisation resulted in significantly lower bark recovery. We also investigated how ten common savanna tree species respond to bark damage and tested for relationships between bark damage, bark recovery and bark traits while accounting for phylogeny. We found phylogenetic signal in bark dry matter content, bark N and bark P, but not in bark thickness. Bark recovery and damage was highest in species which had thick moist inner bark and low wood densities (Anacardiaceae), intermediate in species which had moderate inner bark thickness and wood densities (Fabaceae) and lowest in species which had thin inner bark and high wood densities (Combretaceae). Elephants prefer species with thick, moist inner bark, traits that also appear to result in faster recovery rates.

大象取食造成的树皮损伤在非洲稀树草原树木中十分常见，后续与火、昆虫及其他病原体的相互作用往往会导致树木死亡。然而令人意外的是，目前学界对稀树草原树木如何应对树皮损伤的了解仍十分有限。针对这一研究空白，我们以深受大象喜爱的广布树种马鲁拉树（Sclerocarya birrea）为研究对象，探究其树皮损伤后的内树皮修复过程。我们借助克鲁格国家公园的长期火烧控制实验，测量了有火烧与无火烧条件下的树皮修复情况。损伤后24个月时，年度火烧、两年火烧与禁火处理组的平均伤口闭合率分别为98%、92%与72%。禁火处理下树皮伤口的蚂蚁定植率更高，而这类蚂蚁定植会显著降低树皮修复效率。我们还探究了10种常见稀树草原树木对树皮损伤的响应模式，并在控制系统发育效应的前提下，检验了树皮损伤程度、树皮修复能力与树皮性状之间的关联。我们发现，树皮干物质含量、树皮氮（N）含量与树皮磷（P）含量存在系统发育信号，而树皮厚度则无此信号。内树皮厚实湿润且木材密度较低的树种（漆树科）的树皮损伤程度与修复率均最高；内树皮厚度与木材密度中等的树种（豆科）次之；内树皮较薄且木材密度较高的树种（使君子科）则最低。大象更偏好内树皮厚实湿润的树种，而这类性状同时也能提升树木的树皮修复速率。