Trait divergence of woody species in relation to affinity for termite mounds in Upper Katanga (DR Congo)

Prepared by Fidèle Cuma Mushagalusa, David Bauman, Mylor Ngoy Shutcha & Pierre Meerts

The dry tropical woodlands and savannahs of Africa are characterised by patterns of dramatic soil heterogeneity that result from the activity of termites. The resulting termite mounds typically present higher concentrations of a series of chemical elements, higher clay content and available-water storage capacity. These termite mounds are not only ‘soil condition islands’, but they are also home of an entirely different tree community when compared to the surrounding flora. The level of specialisation to these habitats is such that most tree species occurring on termite mounds are not found off mounds, in the surrounding woodland, and vice versa. However, the affinity of individual tree species to termite mounds has not been assessed so far, based on large-scale inventories of whole-regional floras. Moreover, the mechanisms that underlie the strong habitat specialisation observed at such a fine spatial scale are still poorly understood. In a recent study, we found a first exciting hint regarding these mechanisms: when transplanting seedlings of termite mound species to the surrounding woodland, the seedlings presented higher mortality during the dry season, suggesting that they are more susceptible to water stress.

In this study, we compared morphological and chemical leaf functional traits of tree species naturally occurring on and off termite mounds. These functional traits are morphological, physiological or phenological features of individuals that affect their performances. Here, we focused on leaf traits related to the strategy of resource acquisition, that is, leaf traits correlated with the fast or slow acquisition of resources. A fast acquisitive species, for instance, tend to present higher leaf areas, specific leaf area (i.e. ratio of fresh leaf area and leaf dry mass), leaf nitrogen and phosphorus contents. Because of evolutionary and physiological constraints, species must allocate their limited resources to their different vital functions (i.e. growth, survival and reproduction), so that trade-offs among these functions emerged. As a result, fast acquisitive species tend to resist environmental stresses less, while conservative species invest more in long-lived tissues but acquire resources more slowly. We hypothesised that the termite mound tree species would present traits related to a strategy of fast resource acquisition, given the more fertile and water-rich soils where they occur. We also explored the trait variability expressed within one species that presented a broad distribution, both on and off mounds, to see whether our expectations when comparing different species would also hold among the individuals of the same species.

From the 70 species analysed, 22 were termitophilous (i.e. presented a strong preference for termite mounds), while 43 occurred mostly or exclusively off mounds and five occurred both on and off mounds. The scarcity of broad-niched species was particularly striking. Regarding species traits, we found that termitophilous species presented typical traits of fast resource acquisition, in contrast with tree species of the surrounding woodland. The same variation pattern was found among the individuals of one species occurring both on and off the mounds (i.e. at the intraspecific level).

Our study showed that not only do termite mounds harbour original plant communities contrasting with the surrounding woodland, but these communities are also functionally contrasted, presenting morphological and chemical leaf properties known to be related to a fast resource acquisition syndrome. We also showed that the tree species occurring on and off termite mounds in the tropical dry woodland present very little overlap, and are functionally different both at the interspecific and intraspecific level.

This is a plain language summary for the paper of Cuma et al. published in the Journal of Vegetation Science (https://doi.org/10.1111/jvs.12827).