A thorny issue: Woody plant defense and growth rates in an East African savanna

Authors: WIGLEY, BENJAMIN - The Tata Institute Of Fundamental Research; COETSEE, CORLI - South African National Parks; Augustine, David; RATNAM, JOYASHREE - The Tata Institute Of Fundamental Research; HATTAS, DAWOOD - University Of Cape Town; SANKARAN, MEHESH - The Tata Institute Of Fundamental Research

Submitted to: Journal of Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/18/2019
Publication Date: 1/25/2019
Citation: Wigley, B., Coetsee, C., Augustine, D.J., Ratnam, J., Hattas, D., Sankaran, M. 2019. A thorny issue: Woody plant defense and growth rates in an East African savanna. Journal of Ecology. 107:1839-1851. https://doi.org/10.1111/1365-2745.13140.
DOI: https://doi.org/10.1111/1365-2745.13140

Interpretive Summary: Trees and shrubs that grow in savanna ecosystems use varying strategies to reduce the impacts of browsing herbivores on their growth and survival. One general strategy is to grow leaves with low nutrient content, which makes them less attractive to browsers. The other is to grow structural defences (e.g. thorns), or dense thickets of branches that physically prevent browsers from eating the leaves. We studied the strategies that are used by 6 tree species growing in an East African savanna, and how those strategies change when browsers are experimentally prevented from consuming them. Specifically, we quantified how nine plant defence traits (including structural and chemical traits) respond to the presence versus absence of browsers (using a long-term herbivore exclusion experiment), and how effective the different defence strategies are at protecting against intense herbivory (by measuring growth rates with and without herbivores present). The dominant savanna tree species at our sites were found to all employ a nutrition and defence strategy, but separated into three clusters as defences were variable (cluster 1: high nitrogen in leaves, short spines, high carbon-based secondary metabolites (CBSMs); cluster 2: high nitrogen in leaves, long spines, low CBSMs; cluster 3: high nitrogen in leaves, no spines, low CBSMs). Structural defences (spine length, branching) were generally found to be induced by herbivory, while leaf N increased and CBSMs (condensed tannins, total polyphenols) decreased or did not respond to herbivory. The species with a combination of long spines and increased ‘caginess’ of branches maintained the highest growth rates in the presence of intense browsing, while the cluster with short spines and high CBSMs performed the worst under intense browsing. We propose that high levels of variability in the way that nutrient and defence traits combine may contribute to the coexistence and resilience of the often closely related species comprising savanna trees and shrubs.

Technical Abstract: Savanna woody plant species typically separate into two strategies based on their defences; a low nutrient strategy and a nutrition and defence strategy with the latter including structural or architectural traits. At three paired long-term exclosure sites exposed to intense levels of mammal herbivory at Mpala Ranch, Kenya we investigate: 1) whether defence strategies employed by the dominant fine- and broad-leaved woody savanna species align with previously proposed strategies, 2) how nine plant defence traits (including structural and chemical traits) respond to herbivory and 3) how effective the different defence strategies are at protecting against intense herbivory (by measuring growth rates with and without herbivores present). The dominant savanna tree species at our sites were found to all employ a nutrition and defence strategy, but separated into three clusters as defences were variable (cluster 1: high N, short spines, high carbon based secondary metabolites (CBSMs); cluster 2: high N, long spines, low CBSMs; cluster 3: high N, no spines, low CBSMs). Structural defences (spine length, branching) were generally found to be induced by herbivory, while leaf N increased and CBSMs (condensed tannins, total polyphenols) decreased or did not respond to herbivory. The cluster with long spines combined with increased ‘caginess’ of branches maintained the highest growth rates, while the cluster with short spines and high CBSMs performed the worst under intense browsing. We propose that high levels of variability in the way that nutrient and defence traits combine may contribute to the coexistence and resilience of the often closely related species comprising savanna woody communities, especially under conditions where resources are not particularly limiting.