Suitability of native tree species across regions of Rwanda in relation to climate sensitivity and ecosystem services

Abstract

Our ability to select native tree species for forest restoration and ecosystems services in tropical areas is limited by insufficient knowledge of tree growth, survival and climate sensitivity. This thesis aims to enhance the understanding of the suitability of tree species from highland tropical forest in current and future climates across Rwanda's agro-ecological regions through three experiments and one literature study. Two experiments with young trees were established at three sites along an elevation gradient (2400 to 1300 m a.s.l., temperature difference of 5.4 °C): one involving multispecies plantations of 20 species and another with potted trees of two species planted in the same soil at all sites. A third experiment, conducted in the Ruhande Arboretum (∼1700 m a.s.l.) studied shade tolerance of six species. All experiments included a mix of early (ES) and late (LS) successional species, with the multispecies plantations containing species with dominant distribution in different vegetation types and elevations (transitional rainforest at 1600 – 2000 and montane rainforest at > 2000 m a.s.l). The literature study assessed the suitability of 81 native tree species to different potential natural vegetation systems of east Africa and their contributions to different ecosystem services. The results from the multispecies plantation experiment showed that warming stimulates early tree growth in most early-successional species, particularly those originating from transitional rainforests. In contrast, several late-successional species, especially from higher elevations, did not respond or grew slower and had higher mortality at warmer sites. Findings from the potted tree experiment, although only limited to two species, aligned with results from the multispecies plantations, indicating that warming and not soil differences was the primary explanation of the observed site differences in tree growth in the larger study. In warmer climates, total biomass increased in the ES species without altering biomass allocation. In contrast, in the LS species, only root mass increased at warmer sites. The shading experiment revealed that dense canopy conditions significantly reduced the total tree biomass differently between species. However, under open sky, late-successional (shade-tolerant) species grew equally well as early-successional (shade-intolerant) species, suggesting tree growth differences in the elevation gradient were not due to species differences in light tolerance. The literature study of East African tree species showed clear relationships between elevation, climate, and species traits. Wood density increases as precipitation decreases, which it does with decreasing elevation. Among selected species, most provide multiple ecosystem services: 83% medicinal, 79% construction, 68% fuel, 58% edible, 56% cultural and 53% supporting and 31% regulating. The experimental studies suggest that in a warmer climate, higher-elevation and late-successional species may face increased competition from lower-elevation and early-successional species. When combined with findings from the literature, these results indicate that climate change will likely decrease the provisioning of ecosystem services specific to some late-successional species, as well as the biodiversity and carbon storage of Afromontane forests. Key words: Native trees, Elevation gradient, Tree growth and mortality, Afromontane rainforest, Transitional rainforest, Successional group.