Modes of dispersal and sowing in the woody plants of Gorongosa

(writing in progress)


The following refer to woody plants taller than 3 m; by numbers of spp., not individuals.

Miombo canopy consists of about six spp., most being ballistic. Brachystegia is dominant, Julbernardia, Millettia, also ballistic (seeds strike other tree boles)
Mesic Miombo 50% zoochorous (e.g. Rubiaceae), 42% anemochorous, 7% ballistic
Xeric Miombo 18% zoochorous, 78% anemochorous (e.g. Combretum), 3% ballistic

Ken believes that soil moisture is responsible, in miombo

Succession on termite mounds:

  • initially dominance by sun-loving spp., e.g. some spp. of Manilkara or Mimusops (occurring in savanna or forest, wherever there are perch-sites, not necessarily termite mounds)
  • next, in moist areas, bird-dispersed spp. of forest enter under the shady canopy
  • finally, these grow through, and the original sun-loving trees die, leading to fire-free patches of forest with succulent fruits. The spp. entering the upper canopy do not all possess succulent fruits, they tend to be anemochorous (seeds blown from adjacent areas, intercepted by thicket on termite mound, germinate, grow through as replacers). Only some of the canopy trees (e.g. "Chrysophyllum') are zoochorous. Initial heliophytic guild of trees cedes to shade-loving trees with succulent fruits (relatively low plants, in the mid-lower stratum), these grow up while being partly replaced by spp. lacking succulent fruits. Change to shade-loving or thicket understorey birds, i.e. different birds performing the roles of dispersing seeds: Pycnonotus followed by Andropadus and Phyllastrephus. First are clump-adapted, including Treron and frugivorous hornbills, great movers and establishers. Second groupmare more static, multiplying the forest elements, their preferred foods, by their own activities. First, establishment, second, multiplication.

Slopes of Gorongosa mountain:

  • pediment thickets 62% zoochorous, 22% anemochorous, 17% ballistic
  • scrub thicket 55% zoochorous, 37% anemochorous (Vachellia/Senegalia, Albizia, Brachylaena, Dombeya), 7% ballistic

Montane thicket (Buddleia, Philippia, Widdringtonia, Nuxia 70% zoochorous, 4% anemochorous

Montane forest 85% zoochorous, distributed through all strata

Transitional forest (two-thirds of the way down the mountain) 65% zoochorous, 18% ballistic (e.g. Millettia stuhlmannii,

Tropical forest (Newtonia buchananii and 'Teleopsis murch') 75% zoochorous, 25% anemochorous

Riverine forest (not the same as swamp forest, which is oligotrophic) 70% zoochorous, 18% anemochorous (among canopy trees, e.g Albizia, whi hnhas light pods that split to expose the seeds, without throwing them out, similarly to Vachellia seyal)

Swamp forest (black-water system, waterlogged and anaerobic)(not the same as riverine forest):

Breonadia salicina is dominant (mode of dispersal uncertain)
Barrington racemosa is subdominant
Synsepalum brevipes third in abundance zoochorous (birds) fruits succulent but mucilaginous and borne in small quantity
Parkia (pollinated by bats)
Khaya uncommon (elsewhere a big tree on river banks) anemochorous
Syzygium guineense zoochorous
Bridelia micrantha yellow fruits similar to Celtis (one of the few euphorbs with succulent fruits)
Homalium possibly zoochorous
Ficus zoochorous
Erythroxylon red succulent fruit in understorey

Almost all of the thicket and forest types of the Gorongosa area have mainly animal-dispersed woody spp. However, in drier, presumably more fire-prone types, e.g. miombo, <50% of the woody spp. are animal-dispersed. In dry savanna woodland with e.g. Combretum, only 18% of woody spp. are animal-dispersed (and presumably an even smaller percentage have succulent fruits). Here, ballistic and wind-dispersal predominate in the canopy. The incidence of succulent fruits is also small in oligotrophic swamp forest - in sharp contrast to nearby forest on termite mounds. The incidence of succulent fruits is greatest in forest on remote mounds.

Posted on July 11, 2023 02:15 AM by milewski milewski


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