Are trees of intermediate density more facilitative? Canopy effects of four East African legume trees

Abstract

Questions Do East African legume trees differ in their canopy effects? Have species with intermediate canopy density more pronounced facilitative effects on understorey primary production, community composition and diversity? How are canopy effects related to species' encroachment status, and modulated by a local aridity gradient? Location Upland and lowland environments of the Borana rangelands, an arid thornbush savanna in southern Ethiopia. Methods We harnessed pastoralists' local ecological knowledge to rank the encroachment status of six legume tree species previously known as Acacia, and correlated this rank to a dendrometric proxy of canopy density. Vegetation relevés (1 m²) were placed in sub‐canopy and adjacent inter‐canopy habitats of four legume tree species that differed in canopy density and encroachment status (Vachellia bussei, V. drepanolobium, V. seyal, V. tortilis). Using mixed‐effects ANOVA, we evaluated effects of tree species and habitat (sub‐canopy or inter‐canopy) on total, forb and grass biomass, and on species diversity, comparing results for lowland and upland sites. Effects on floristic composition were assessed via PERMANOVA and NMDS. Results Species' encroachment status and canopy density were negatively correlated. Most pronounced facilitative effects (more frequent and larger differences between sub‐canopy and inter‐canopy habitats) were found for a species with intermediate canopy density (V. bussei). Diversity in its understorey vegetation increased up to 32% (total biomass: 29%, forb biomass: 40%). In contrast, the species with the most open canopy (V. drepanolobium; also the most encroaching species) never exerted significant facilitative effects. In contrast to expectations, canopy effects were more frequent in (climatically less arid) upland environments, probably due to a redistribution of water within the landscape. Large differences in facilitative effects across tree individuals indicate that a high intraspecific variability of canopy traits and/or small‐scale differences in abiotic site conditions have partly overridden species‐specific differences in canopy architecture. Conclusion Our study provides new insights on why encroaching legume tree species may decrease herbaceous production: they tend to have more open canopies, with small facilitative effects on sub‐canopy herbaceous production. We thus recommend promoting an open savanna with the tree layer dominated by species with a high facilitative potential to maintain forage provision and species diversity.

Forb ecology research in dry African savannas: Knowledge, gaps, and future perspectives

“… Superscripts refer to selected studies only. 1 Buitenwerf et al (), 2 O'Connor (), 3 Gilo and Kelkay (), 4 Jacobs and Naiman () , 5 Odadi et al (), 6 Young et al (), 7 Zerbo et al (), 8 Dreber and Esler (), 9 O'Connor, , 10 Shackleton (), 11 Walker and Knoop (), 12 Belsky et al (), 13 Augustine (); 14 Ludwig, De Kroon, Berendse, & Prins, (), 15 van der Waal et al (), 16 Linstädter et al (), 17 Muvengwi et al (), 18 Masunga et al () , 19 Smit (), 20 Dreber et al (), 21 Rutherford et al (), 22 Hejcmanová et al (), 23 Odadi et al (), 24 Tessema et al (), 25 Angassa and Oba (), 26 Keya (), 27 Metzger, Coughenour, Reich, and Boone (), 28 Hanke et al (), 29 Tessema et al (), 30 Wesuls et al (), 31 Riginos et al (), 32 Riginos and Grace (), 33 Koerner et al (), 34 Belsky et al (), 35 Linstädter et al (), 36 Britz and Ward (), 37 Burkepile et al (), 38 Eby et al (), 39 Mlambo et al (), 40 Chikorowondo et al (), 41 Clegg and O'Connor (), 42 Traill (). …”

“…Herbaceous productivity is strongly colimited by water, nutrients, and sunlight (Walker & Knoop, ; Belsky et al, ; Belsky, Mwonga, & Duxbury, ; Augustine, ; Ludwig, De Kroon, Berendse, & Prins, ; Linstädter, Bora, Tolera, & Angassa, ; Muvengwi, Witkowski, Davies, & Parrini, ; Van der Waal et al, ). Despite their shading effects on the understory vegetation, large savanna trees act as fertility islands by means of continuous nutrient inputs which facilitate standing herbaceous biomass (Belsky et al, ; Ludwig et al, ; Mlambo, Nyathi, & Mapaure, ; Weltzin & Coughenour, ).…”

“…Despite their shading effects on the understory vegetation, large savanna trees act as fertility islands by means of continuous nutrient inputs which facilitate standing herbaceous biomass (Belsky et al, ; Ludwig et al, ; Mlambo, Nyathi, & Mapaure, ; Weltzin & Coughenour, ). Forbs may contribute up to 40%–50% of the total herbaceous biomass under tree canopies (Linstädter et al, ; Ludwig et al, ; Mlambo et al, ), which is significantly higher compared to areas outside the canopy zone (Belsky et al, , ; Linstädter et al, ; Ludwig et al, ; Mlambo et al, ). The facilitative effects of trees on forbs and grasses are often influenced by local livestock grazing pressure (Belsky et al, ) and are known to vary with the tree species (Belsky et al, ; Mlambo et al, ) and its position in the landscape (Linstädter et al, ).…”

“…Forbs may contribute up to 40%–50% of the total herbaceous biomass under tree canopies (Linstädter et al, ; Ludwig et al, ; Mlambo et al, ), which is significantly higher compared to areas outside the canopy zone (Belsky et al, , ; Linstädter et al, ; Ludwig et al, ; Mlambo et al, ). The facilitative effects of trees on forbs and grasses are often influenced by local livestock grazing pressure (Belsky et al, ) and are known to vary with the tree species (Belsky et al, ; Mlambo et al, ) and its position in the landscape (Linstädter et al, ). Differences in canopy architecture (Linstädter et al, ), tree size, and density (Ludwig et al, ; Riginos & Grace, ) have implications for shading, but also on moisture availability (Van der Waal et al, ) and nitrogen enrichment of the subcanopy soil (Ludwig et al, ; Weltzin & Coughenour, ).…”

“…The facilitative effects of trees on forbs and grasses are often influenced by local livestock grazing pressure (Belsky et al, ) and are known to vary with the tree species (Belsky et al, ; Mlambo et al, ) and its position in the landscape (Linstädter et al, ). Differences in canopy architecture (Linstädter et al, ), tree size, and density (Ludwig et al, ; Riginos & Grace, ) have implications for shading, but also on moisture availability (Van der Waal et al, ) and nitrogen enrichment of the subcanopy soil (Ludwig et al, ; Weltzin & Coughenour, ). Nitrogen‐fixing canopy trees, such as Acacia species, were reported to have variable facilitative effects on the herbaceous layer.…”