Tropical Africa high mountain top ecosystems, Afroalpine ecosystems, occur in isolated patches restricted to peaks of the high mountains along the Great Rift Valley and Cameron-Nigeria mountain ranges between Tropic of Capricorn and Tropic of Cancer (Hedberg, 1964; Gehrke and Linder, 2014). These spectacular ecosystems are habitats for unique life forms. Afroalpine plants own distinctive traits and are specially adapted to frost and drought. They exhibit specific morphological and functional adaptations to the predominant diurnal climate extreme, i.e., the high thermal fluctuation between day and night. Moreover, the spatial isolation of mountain that ranges over a long-timescale have supported the evolution of many endemics (Steinbauer et al., 2016; Flantua et al., 2020; Testolin et al., 2021). The uniquely adapted species are sensitive to perturbation, and climate change (Vuilleumier and Monasterio, 1986; Nagy and Grabherr, 2009; Buytaert et al., 2011), which is the most pervasive of the various threats to the planet’s biodiversity (Malcolm et al., 2006).
The recently observed climate change across tropical regions is significantly higher than the global average. For example, there are observed temperature increases for the tropical rainforest regions at a mean rate of 0.26 +/-0.05°C per decade, with an intensification during the El Niño events (Malhi and Wright, 2004). Climate will continue warming up throughout the coming century in response to changes in radiative forcing arising from anthropogenic emissions of greenhouse gases and aerosols (IPCC 2014).
Climate change will induce thermal isotherm shifts, disrupting the stability of Afroalpine ecosystems and affecting the unique plant diversity, distribution, and species richness, leading to unexpected species and functional groups reorganization and massive endemic extinction (Malcom et al., 2006; Kreyling et al., 2010). Our species distribution modeling (SDM) to test altitudinal gradient/range shift is widely applied to assess the impact of climate change on a few selected plant diversity and distribution. It is a powerful method for testing biota's ecological and evolutionary responses to geophysical influences, such as temperature and moisture changes.
In general, there are two categories of altitude related factors of environmental changes: (i) those physically tied to meters above sea level (m asl), such as atmospheric pressure, temperature, moisture, and clear-sky turbidity; and (ii) those that are not generally altitude specific, such as hours of sunshine, wind, geology, fire, and even human land use (Körner, 2007). Here, we tested the impact of change in temperature related bioclimatic variables on the diversity and distribution of two common Erica spp. of the Bale Mountains and their implication for extensive Afroalpine plateau ecosystems.
Mountains show high biodiversity due to elevational gradients, complex surface structures, and disturbance regimes resulting in a broad range of site conditions and ecological niches (Beierkuhnlein, 2007; Körner, 2007; Nagy and Grabherr, 2009). Many mountain plant species have long life cycles reflected in woody structures above- and belowground, longevity, limited dispersal capacity resulting in inertia at the level of ecosystems (Razgour et al., 2020). The rapid spatial shift of ecosystems or local adaptation to novel climatic conditions is unlikely to happen by plants with broad phenotypic plasticity. For these reasons, tropical alpine and montane ecosystems and the immense biodiversity they harbor are particularly sensitive to climate change induced warming (Malcolm et al., 2006; Razgour et al., 2020). Hence, the anticipated thermal isotherm shifts can disrupt the stability of these ecosystems.
The Bale Mountains of south-central Ethiopia form the largest contagious massif of most extensive plateaus above 3000 m asl in Africa, supporting the most extensive Ericaceous vegetation on the continent (Miehe and Miehe, 1994). Erica spp., commonly known as heathers or heaths, belong to the subfamily Ericoidae, comprising acid loving, woody plants (Wesche et al., 2000; McGuire et al., 2005). The ericaceous vegetation is one of the Afroalpine/Afromontane plants with a broader distribution range, with a high thermal tolerance range and adaptation potential that outcompete some Afroalpine plants (Gehrke and Linder, 2014).
In the Bale Mountains, climate change induced warming and thermal anomalies are expected to generate future environmental conditions that could favor the expansion and dominance of plants with a more comprehensive habitat range, such as Ericaceous vegetation. Kreyling et al. (2010), Wana and Beiekuhnlein (2010), and Kidane et al. (2019) speculated altitudinal range shift and range contraction because of climate change at the Gughe mountains (Southern Ethiopia) and the Bale Mountains, respectively.
Recent SDMs focus on extinction risks of species or groups rare and under threat of extinction, keystone species, or functional type (Urban 2015). Little is known about the potential range loss among the widespread species such as Erica, which is critical as even slight declines in such species can significantly affect ecosystem structure, function, and services (Warren et al., 2013).
To date, the impacts of projected climate change, the extent of the current distribution range and suitable habitat, and the main bioclimatic factors that control Erica's expansion and distribution in the Bale Mountains are not well studied. However, the role of fire, land use, and herbivory as central players in Erica dynamics are relatively more researched (e.g., Wesche et al., 2000, 2003, 2008; Gizaw et al., 2013; Johansson et al., 2014,2018; Gil-Romera et al., 2019). In general, the role of climate change in determining Erica distribution and its implication to associated alpine and subalpine flora is lacking.
Considering the severity of the anticipated climate change, we raised the question, whether the future novel environmental conditions or habitats shift will favor further expansion and dominance of plants with a broader habitat range, such as Erica. We hypothesize that climate change will create novel suitable and unsuitable habitats that will be colonized by plant species that are adapted to the new environmental conditions. We further hypothesize that due to its broad phenotypic plasticity, Erica will respond to considerable changes and prevail in the location of its potential suitable habitats. Hence, we applied Species Distribution Models (SDMs) and model ensemble to model Erica's extent in its current and future distribution range. We projected its future distribution under two RCPs (RCP4.5) average temperature increases 1.4oC (0.9 to 2.0) and RCP8.5 (2070s) with an average temperature increase of 3.7oC (2.6 to 4.8).
Understanding Erica’s current and future distribution and ecological range in the face of climate change is essential to the science and knowledge basis of Erica's ecology and biogeography. It also contributes to the planning and development of sound local management strategies. This research aims to model the current Erica distribution range under the current condition and projected climate change. Specifically, 1) to model the current distribution of Erica, 2) to identify the main bioclimatic variables that control Erica's distribution, and 3) to model the future potential distribution of Erica and discuss its implication to the Afroalpine plants.