The games plants play: the defensive strategies of Acacia nigrescens and Acacia tortilis with respect to the interaction between elephants and fire

African savannas are shaped by the interaction of two ecological factors: African
elephants (Loxodonta africana) and fires. The response of individual Acacias to
megaherbivores and fire regimes is dependent on their life history strategies. The
resulting architecture of trees is a trade-off between a strategy of growing tall to escape
low-lying fires or of developing thick, insulating bark to protect the internal structure
from heat damage. Allometric studies suggest that there is a strong relationship between
trunk diameter and average bark thickness, as well as trunk diameter and height, in
Acacia nigrescens and Acacia tortilis. When both have similar bark thicknesses, A.
nigrescens tends to grow taller and burn at higher temperatures than A. tortilis. The
combination of bark stripping and field burns show that elephants can have dramatic
effects on the survival abilities of trees during fires and that Acacias have varied life
history strategies in response. Severely debarked (over 50%) A. nigrescens have greater
odds of experiencing interruptions in the water column, as reflected in increased leaf
discoloration and wilting, whereas A. tortilis shows no effect within the same time period.
This suggests that A. nigrescens copes with fire and megafaunal herbivory by increasing
canopy height and A. tortilis copes by investing in bark with greater insular ability to
protect against fire and in mechanical defences to protect against herbivory.

非洲稀树草原的生态格局由两大核心生态因子的相互作用所形塑：非洲象（Loxodonta africana）与野火。金合欢属植物个体对巨型草食动物与火格局的响应，取决于其自身的生活史策略。树木最终形成的构型，是两种生存策略间的权衡：要么通过长高以避开低矮野火，要么演化出厚实的隔热树皮以保护内部组织免受高温损伤。异速生长研究表明，黑荆金合欢（Acacia nigrescens）与弯叶金合欢（Acacia tortilis）的树干直径与平均树皮厚度、树干直径与株高之间均存在显著相关性。当二者树皮厚度相近时，黑荆金合欢通常比弯叶金合欢长得更高，且在火灾中需承受更高的灼烧温度。剥树皮试验与野外火烧试验的结合结果显示，大象可在火灾期间对树木的存活能力产生显著影响，而金合欢属植物则演化出多样的生活史策略以应对此类干扰。剥皮程度超过50%的黑荆金合欢，其木质部水分传导更易出现中断，具体表现为叶片变色率与萎蔫程度升高；而弯叶金合欢在相同时长内未出现此类效应。这表明，黑荆金合欢通过提升冠层高度来应对野火与巨型草食动物取食；而弯叶金合欢则通过投资演化出隔热性能更强的树皮以抵御野火，并强化机械防御结构以抵御草食动物取食。