Anomalously tall vegetation, on anomalously nutrient-rich soils, in an arid climate in southern Namibia

Near Keetmanshoop in southern Namibia (https://en.wikipedia.org/wiki/Keetmanshoop and https://en.wikipedia.org/wiki/Gr%C3%BCnau,_Namibia), there is a limited but considerable incidence of trees that is unexplained by deep groundwater in seasonal drainage lines.

Open savanna, about 5 m high, of Vachellia erioloba (https://www.inaturalist.org/observations/133078127), occurs under mean annual rainfall of about 125 mm (https://maps-namibia.com/namibia-rainfall-map).

At Gellap Ost (http://the-eis.com/elibrary/search/6553 and https://www.nbri.org.na/gellap-ost-research-station and https://www.mindat.org/feature-3357334.html), the substrate on the broadscale plain is a rather sandy-looking dark loam, free of stones, derived from dolerite bedrock.

The upper stratum consists of trees of V. erioloba, up to 6 m high and 6 m wide (https://www.inaturalist.org/observations/133078122 and https://www.inaturalist.org/observations/133078123). The individual trees are, on average, about 50 m apart.

Some of the trees bear the nest-complexes of Philetairus socius (https://www.inaturalist.org/observations/32789834).

Karoid shrubs are scarce, and consist mainly of Rhigozum trichotomum (https://www.inaturalist.org/taxa/355852-Rhigozum-trichotomum).

The main stratum here consists mainly of a short tussock-grass, Stipagrostis obtusa (https://www.inaturalist.org/taxa/594826-Stipagrostis-obtusa). This is grassier than on the shale-derived substrate nearby, which supports Catophractes (https://www.inaturalist.org/taxa/70059-Catophractes-alexandri) and other shrubs, rather than grasses and trees.

In the Keetmanshoop area, V. erioloba is not associated with Kalahari sand, which does not extend this far to the west (https://www.researchgate.net/figure/Geographic-location-black-dot-and-vegetation-structure-of-each-study-site-along-the_fig1_241520991). Instead, it is associated with dolerite (https://www.inaturalist.org/observations/133078121).

In the same general area, Aloidendron dichotomum (https://www.inaturalist.org/taxa/527446-Aloidendron-dichotomum) is unusually tall for an aloe. It occurs on outcrops of dolerite (https://www.inaturalist.org/observations/142126920 and https://www.inaturalist.org/observations/148037408 and https://www.inaturalist.org/observations/109356346 and https://www.inaturalist.org/observations/122831811 and https://www.inaturalist.org/observations/37025903 and https://www.inaturalist.org/observations/160059615 and https://www.inaturalist.org/observations/11046247 and https://www.inaturalist.org/observations/41560387).

Aloidendron dichotomum is arborescent enough to be used for nesting by P. socius (https://www.inaturalist.org/observations/25798893).

DISCUSSION

In general, vegetation can be expected to be tallest in rainy climates, and on moderately nutrient-poor soils.

The climate near Keetmanshoop is arid, and dolerite is a relatively nutrient-rich parent material. Therefore, the incidence of trees hereabouts seems anomalous.

The nutrient-richness of vegetation dominated by V. erioloba and S. obtusa is evident in the great palatability of both species to ungulates.

It is well-known that V. erioloba occurs in Kgalagadi Transfrontier Park (https://en.wikipedia.org/wiki/Kgalagadi_Transfrontier_Park), where mean annual rainfall is about 200 mm. However, the landforms here consist of seasonal drainage lines among dunes of deep sand (https://www.inaturalist.org/observations/40959605), allowing roots to draw water tens of metres deep during drought.

The habitats of V. erioloba and A. dichotomum near Keetmanshoop and Grunau lie immediately adjacent to the Nama Karoo biome (https://en.wikipedia.org/wiki/Nama_Karoo). It is understandable that the vegetation is grassier than in the Nama Karoo, because the proportion of rainfall in summer increases as one proceeds northwards from the southern border of Namibia. However, the incidence of trees on plains and rocky outcrops of dolerite seems incongruous relative to the Nama Karoo, and cannot be explained by the seasonal shift in the rainfall.

Posted on 24 May, 2023 15:10 by milewski milewski

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Posted by milewski 11 months ago

PHENOLOGY OF VACHELLIA ERIOLOBA

In May 2006, I traversed Namibia, including the main road from the southern border through Keetmanshoop.

I found that, throughout Namibia at the time, the trees of Vachellia erioloba, from Keetmanshoop to the eastern Caprivi, were bearing ripe fruits (in the form of pods palatable to ungulates and other herbivores, https://www.inaturalist.org/observations/150312907 and https://www.inaturalist.org/observations/160059068 and https://www.inaturalist.org/observations/154847885 and https://www.inaturalist.org/observations/149217186).

No flowers were present on V. erioloba. However, all individuals bore full foliage, of the usual dull-green hue (https://www.inaturalist.org/observations/162438092 and https://www.inaturalist.org/observations/148763510).

This phenological pattern seemed to be clear and consistent, at this time.

Posted by milewski 11 months ago

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