Photosynthetically active radiation transmittance of forest plantation canopies in the Ethiopian highlands

Hemispherical photographs and a quantum sensor were used to study the canopy photosynthetic photon flux density (PPFD) transmittance of five forest plantation species in Ethiopia. Red/far-red (R/FR) ratios and temperatures beneath the forest plantations were also measured. The species studied were: Eucalyptus globulus, Pinus patula, Cupressus lusitanica, Grevillea robusta, and Juniperus procera. The canopies of E. globulus, P. patula and G. robusta transmitted about three times as much PPFD as J. procera or C. lusitanica plantations. Furthermore, plantations of G. robusta and E. globulus had the highest, while J. procera had the lowest R/FR ratio. In contrast to J. procera and C. lusitanica, E. globulus and G. robusta had relatively open crowns, higher crown-bases and lower leaf area indices, and as a result, their canopies had higher PPFD transmittance percentages as well as higher below-canopy R/FR ratios and temperatures. From the perspective of understory light regime, plantations of E. globulus and G. robusta are best suited for promoting the natural regeneration of native species. In general, opening of gaps in plantations of heavy-shading tree species may increase the herbaceous layer ground cover, enhance the colonisation and growth of native woody species and consequently may also increase the floristic diversity of mono-specific plantations.