Tree line dynamics in the tropical <scp>A</scp>frican highlands – identifying drivers and dynamics

Jacob, Miró; Annys, Sofie; Frankl, Amaury; Ridder, M. De; Beeckman, Hans; Guyassa, Etefa; Nyssen, Jan

doi:10.1111/jvs.12215

Cited by 35 publications

(32 citation statements)

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“…The land management hypothesis would be in agreement with the study of Jacob et al . () about tree line dynamics in the tropical highlands of Africa. This study indicates that tree lines have not risen to higher altitudes in the tropical African highlands because of high anthropozoogenic pressure, which caused stabilization and even recession of the tree lines below their natural climatic limit.…”

Section: Discussionmentioning

confidence: 97%

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North Ethiopian Afro‐Alpine Tree Line Dynamics and Forest‐Cover Change Since the Early 20th Century

et al. 2014

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High‐altitude forests are very important for local livelihood in the vulnerable environment of the densely populated tropical highlands. Humans need the ecosystem services of the forest and directly impact the forest through livestock herding, fire, and wood harvesting. Nevertheless, temperature‐sensitive tree lines in the tropics are scarcely investigated in comparison with higher northern latitudes. In this study, the Erica arborea L. tree line is studied in a tropical mountain in the North Ethiopian highlands: Lib Amba of the Abune Yosef Mountain range (12°04′N, 39°22′E, 3993 m asl). The present tree line and forest cover was recorded by high‐resolution satellite imagery from Google Maps and field data (2010–2013), while historical forest cover was studied from aerial photographs (1965–1982) and repeat photography (1917–2013). The aerial and satellite images were orthorectified and classified in forest/non‐forest binary maps. The binary forest layers were used to detect forest‐cover change and tree line dynamics by image differencing between the three time layers (1965–1982–2010). These maps and a terrestrial photograph indicate two periods of deforestation (1917–1965 and 1982–2013), whereas the forest cover was stable between 1965 and 1982. Deforestation was especially severe (with 63%) between 1982 and 2010, associated with a population increase from 77 to 153 inhabitants per square km. There is significant evidence that the elevation of the E. arborea L. tree line increased from 7 to 15 vertical meters between 1965 and 2010, in an area with decreasing anthropozoogenic pressure. Copyright © 2014 John Wiley & Sons, Ltd.

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Section: Discussionmentioning

confidence: 97%

“…Improved understanding about tree line dynamics and deforestation patterns and their driving processes in tropical highlands is vital for sustainable land management strategies in these vulnerable high‐altitude areas and will help us to understand the effects of climate change on these environments (Jacob et al ., ).…”

Section: Introductionmentioning

confidence: 97%

North Ethiopian Afro‐Alpine Tree Line Dynamics and Forest‐Cover Change Since the Early 20th Century

et al. 2014

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“…The low location of the tree line—relative to the estimated climatic tree line—is typical for Mt. Cameroon and is presumably a result of volcanic activity and rainfall regimes (Jacob et al, ). Note that Mt.…”

Section: Discussionmentioning

confidence: 99%

Forest structure determines spatial changes in avian communities along an elevational gradient in tropical Africa

Hořák

Ferenc

Sedláček

et al. 2019

Journal of Biogeography

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Aim To test if tree species richness and forest structure drive spatial variation in avian communities along a tropical elevation gradient and to present information about the role of detailed forest parameters. Location A 2,000‐m long elevational gradient of tropical forest on Mt. Cameroon, west‐central Africa. Taxon Birds and trees. Methods We performed bird censuses and vegetation mapping at the same plots across six forested sites at elevations of 350, 650, 1,100, 1,500, 1,850, and 2,200 m a.s.l., with 16 plots per elevation. We tested the effects of elevation, forest structure and tree diversity on the species richness, functional diversity and β‐diversity of birds (Bray–Curtis dissimilarity). We used conditional inference trees based on random forests (RF) to investigate these relationships across all elevation sites as well as within elevations. Results Both tree and bird species richness declined monotonically with elevation. Vegetation structure correlated with elevation, and all vegetation attributes significantly differed among elevations. The RF explained 70% of the variance in avian species richness, with the most important predictors being elevation, proportion of dead trees, tree species richness and herb layer coverage. We found that elevation (and shrub layer) was a particularly important predictor of avian functional diversity. We identified no important predictor of bird species richness after standardization within elevations, and the proportion of dead trees was the sole important predictor of functional diversity. Within‐elevation β‐diversity in avian community composition was determined by the dissimilarity of the tree community and differences in leaf area index, solar radiation and spatial distance. The functional dissimilarity was best explained by leaf area index. Main conclusions Apart from elevation itself, spatial distance even within elevations correlated with compositional and functional variation among avian assemblages. Forest structural traits can have a significant influence on distribution of birds. Thus, gaps in the spatial distribution of species such as along elevations might be caused by fine‐scale recognition of suitable habitats.

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“…However, few studies have investigated the importance of traditional land‐use for maintaining these open habitats. In the Bale Mountains, it is likely that most of the heathland would grow into closed‐canopy subalpine forests without fire (Jacob et al., ). In these forests, most of the shade‐intolerant heathland and Afro‐alpine species are absent (Miehe & Miehe, ) and the oldest heathland stands in our study had fewer alpine species and more forest species (Figure ).…”

Section: Discussionmentioning

confidence: 99%

Disturbance from traditional fire management in subalpine heathlands increases Afro‐alpine plant resilience to climate change

Johansson

Frisk

Nemomissa

et al. 2018

Global Change Biology

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Species are often controlled by biotic factors such as competition at the warm edge of their distribution range. Disturbances at the treeline, disrupting competitive dominance, may thus enable alpine species to utilize lower altitudes. We searched for evidence for range expansion in grazed, fire-managed Ethiopian subalpine Erica heathlands across a 25-year chronosequence. We examined vascular plant composition in 48 plots (5 × 5 m) across an altitudinal range of 3,465-3,711 m.a.s.l. and analyzed how community composition changed in relation to increasing competition over time (using a Shade index based on Erica shrub height and cover) and altitude. Species' habitats and altitudinal ranges were derived from literature. Time since fire explained more variation (r = .41) in species composition than altitude did (r = .32) in an NMDS analysis. Community-weighted altitudinal optima for species in a plot decreased strongly with increasing shade (GLM, Standardized Regression Coefficient SRC = -.41, p = .003), but increased only weakly with altitude (SRC = .26, p = .054). In other words, young stands were dominated by species with higher altitudinal optima than old stands. Forest species richness increased with Log Shade index (SRC = .12, p = .008), but was unaffected by altitude (SRC = -.07, p = .13). However, richness of alpine and heathland species was not highest in plots with lowest Shade index, but displayed a unimodal pattern with an initial increase, followed by a decrease when shading increased (altitude was not significant). Our results indicate that disturbance from the traditional patch burning increases the available habitat for less competitive high-altitude plants and prevents tree line ascent. Therefore, maintaining, but regulating, the traditional land use increases the Afro-alpine flora's resilience to global warming. However, this system is threatened by a new REDD+ program attempting to increase carbon storage via fire suppression. This study highlights the importance of understanding traditional management regimes for biodiversity conservation in cultural landscapes in an era of global change.

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Tree line dynamics in the tropical African highlands – identifying drivers and dynamics

Abstract: 38This is an Open Access version of the article: Jacob, M., Annys, S., Frankl, A., De Ridder, M., Beeckman, H., Guyassa, E., Nyssen, J., 2015. Tree line dynamics in the tropical African highlands -identifying drivers and dynamics.

Cited by 35 publications

References 49 publications

North Ethiopian Afro‐Alpine Tree Line Dynamics and Forest‐Cover Change Since the Early 20th Century

North Ethiopian Afro‐Alpine Tree Line Dynamics and Forest‐Cover Change Since the Early 20th Century

Forest structure determines spatial changes in avian communities along an elevational gradient in tropical Africa

Disturbance from traditional fire management in subalpine heathlands increases Afro‐alpine plant resilience to climate change

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