A beneficial relationship: associated trees facilitate termite colonies (Macrotermes michaelseni) in Namibia

Abstract

In many tropical ecosystems, termites are important ecosystem engineers. Mound-building termite species often contribute to topographical landscape heterogeneity, and the associated accumulation of soil moisture and nutrients affects the spatial distribution of plant communities. Plants that grow next to the termite mound are known to benefit from the provided nutrients and water storage. The other way around, protection against erosion and cooling effects on the microclimate of the mounds imposed by associated trees have been described. However, little is known regarding the influence of tree parameters and the long-term effects on colony development. In an interval of 12 yr, we studied all Macrotermes michaelseni mounds occurring within one square kilometer of Namibian thornbush savanna in terms of morphology, life history, and associated plants, with a focus on the role of associated trees which grow directly adjacent to the termite mounds. Here, we present the results of a combined analysis of the character states recorded at the termite mound and the associated trees. More than 50% of all 345 recorded termite mounds were associated with a tree. Mounds with associated trees showed increased activity, growth, and height. Mound growth within the study period was correlated positively with the canopy coverage by the associated tree, while total mound heights were correlated non-linear with medium canopy coverage of about 40%. Mounds with a tree covering the sun's average zenith (north) were taller, grew most within 12 yr, and were more likely to be active than those with no northern canopy coverage. Mounds associated with the evergreen Boscia albitrunca attained highest average height (195.06 AE 11.76 cm). We conclude that vital parameters like growth, size, and activity of the termite colonies are facilitated by shading associated trees. These benefits were shown to take significant effect within the span of 12 yr. This indicates a proper longterm mutualism, not a loose alliance and not an obligative eu-symbiosis. Therefore, we propose to use the term "partner tree" for trees associated with M. michaelseni mounds.

Spatial patterns and life histories of Macrotermes michaelseni termite mounds reflect intraspecific competition: insights of a temporal comparison spanning 12 years

“…This illustrates the long‐term advantages of tree‐associated colonies. Wildermuth et al (2021) suspect a self‐reinforcing cycle of both profiting ‘partners'. We conclude that the advantages of tree‐associated M. michaelseni mounds reflect within the intraspecific self‐thinning process, as Wildermuth et al (2021) showed that mound‐tree associations are regularly distributed.…”

“…If the hole was repaired the next day, we assumed the mound to be active. Mounds with open ventilation systems or no signs of activity were recorded as inactive and outwash pediment remains without any elevated mound structure were recorded as remnant (Grohmann et al 2010, Wildermuth et al 2021).…”

“…Furthermore, we identified and recorded every tree above 1.50 m total height that was located within the outwash pediment of a termite mound. These trees were classified as ‘partner trees' (Wildermuth et al 2021).…”

“…All termite mounds and eroded mound remnants (see example in Fig. 1b) in the observatory were mapped in 2007 and 2019 (Wildermuth et al 2021) by two different observer teams, following the approach of Grohmann et al (2010). We chose ground inspection as the most appropriate method since more than 30% of all termite mounds were concealed by the canopy of associated trees on aerial or satellite images.…”

“…Another possible factor influencing termite mound distributions are mound-associated trees, which were shown to have facilitative effects on growth and activity of M. michaelseni mounds (Wildermuth et al 2021). However, termite mounds are known to benefit associated trees themselves (Traoré et al 2008b, Joseph et al 2011, Støen et al 2013) and typical termite mound-associated woody plants are mostly found on large termite mounds (Joseph et al 2013, Wildermuth et al 2021). Thus, it remains unclear whether this mutualism is initiated by the trees or the termite mounds.…”