RWANDA TREE DESIGN

Rwanda TREE has three large existing multi-species plantations in Rwanda (20 species, 5400 trees) with large variation in climate and with water and nutrient manipulation treatments at each site (Figure 1). The sites are within 170 km distance but exhibit large variation in elevation (1300-2400 m.a.s.l.), mean daytime (18-24 °C) and mean maximum daytime (24-31 °C) temperatures and precipitation (1050-2100 mm). The sites are located in three different vegetation zones: montane rainforest (Sigira), transitional rainforest (Rubona), and evergreen and semi-evergreen bushland and thicket (Makera). The relative distribution of precipitation over the year is similar at all sites, with highest rainfall in March–May and a dry period in June–August, extending into September at the warmest site. The montane rainforest site at Sigira is at the edge of the protected Nyungwe forest, which forms the largest block of lower montane rainforest remaining in Africa. All sites are located at research stations owned by Rwanda Agriculture and Animal Resources Development Board (RAB) and have basic field work facilities and staff for maintenance.

The plant material and plantations were established during 2016-2017. After seed collection and propagation at a nursery at the mid-elevation site Rubona, seedlings were transplanted at the three experimental sites in January 2018. After successful establishment of seedlings that were irrigated when needed, experimental treatments began in 2019. Each site has 18 plots (6 treatments x 3 replicate plots), including three water levels (low, mid, high) in factorial combinations with two nutrient levels (unfertilized, fertilized + limed) (Figure 2 and 3). The three water levels mimic the natural precipitation at the three sites. Increased and decreased water supply compared to natural precipitation is achieved by irrigation and rain exclusion shelters, respectively (Figure 4). The fertilization treatment is applied to compensate for differences in fertility and among sites. Each plot is 15 x 15 m with plant spacing of 1.5 x 1.5 m, allowing 100 plants per plot; 5 replicates of 20 species (Figure 2, top-left). Trees from all species are randomly distributed within each plot.

Each plantation and plot includes 20 species with different growth strategies and climate of origin (Table 1): half early-successional and half late-successional species equally representing characteristic species from both montane rainforest (at high elevation site) and transitional rainforest (at mid elevation site). This design allows us to determine climate responses of trees and ecosystem process in relation to tree successional strategy and climate of origin.

Figure 1.

Schematic overview of the experimental design at the experimental sites. The information about each site include name, geographic location, potential vegetation type, elevation, mean daytime temperature, mean maximum daytime temperature, and annual precipitation.

Figure 2.

Plot design at each site along the elevation gradients. Two nutrient treatments (low=unfertilized, high) and three water treatments (low, mid, high) will be applied. Each treatment is replicated three times in a full factorial design. Each plot has 100 plants (20 species x 5 replicates) with 1.5 m x 1.5 m spacing.

Figure 3.

Drone picture of the Rubona research site (intermediate elevation site). Photo: Charis Unmanned Aerial Solutions Ltd.

Figure 4.

Photos of irrigation system and rainout shelters at Rubona research site.

Table 1.

Tree species and their taxonomic families and successional identities.

*These are not distinct representatives of their successional groups and have been recorded as both early- and late-successional species in previous studies.