Longevity and growth of Acacia tortilis; insights from 14C content and anatomy of wood

Background: Acacia tortilis is a keystone species across arid ecosystems in Africa and the Middle East. Yet, its life-history, longevity and growth are poorly known, and consequently ongoing changes in tree populations cannot be managed in an appropriate manner. In other arid areas parenchymatic bands marking growth zones in the wood have made dendrochronological studies possible. The possibilities for using pre- and post-bomb 14C content in wood samples along with the presence of narrow marginal parenchymatic bands in the wood is therefore tested to gain further insight into the age, growth and growth conditions of A. tortilis in the hyper-arid Eastern Desert of Egypt. Results: Based on age scenarios and the Gompertz growth equation, the age of trees studied seems to be from 200 up to 650 years. Annual radial growth estimated from calibrated dates based on the post-bomb 14C content of samples is up to 2.4 mm, but varies both spatially and temporally. Parenchymatic bands are not formed regularly. The correlation in band pattern among trees is poor, both among and within sites. Conclusion: The post-bomb 14C content of A. tortilis wood gives valuable information on tree growth and is required to assess the age scenario approach applied here. This approach indicates high longevities and slow growth of trees. Special management measures should therefore be taken at sites where the trend in tree population size is negative. The possibilities for dendrochronological studies based on A. tortilis from the Eastern Desert are poor. However, marginal parenchymatic bands can give insight into fine scale variation in growth conditions and the past management of trees

Endangered Dracaena ombet Population in the Red Sea Hills, Sudan, Recovers After Abrupt Change

The endangered and endemic Nubian dragon blood tree, Dracaena ombet, has been feared extirpated from core distribution areas in the Red Sea Hills, Sudan, after reported mass death events in the 20th century. Populations of dragon tree species are generally reported to be in decline, with a noticeable lack of recruitment and possible poor resilience. Rare recruitment events are, however, normal for species with remnant population dynamics, and when eventually occurring, such events can restore seemingly degraded populations. In response to recently reported observations of dracaena saplings in a historically core distribution area of the Red Sea Hills, we assess the status of this long-lived arboreal species. We describe a current realized niche, investigate a potential range shift by comparing the spatial distribution of saplings and older individuals, and assess population recovery based on pre-disturbance system identity derived from qualitative, historical observations. We document a beginning recovery of the dracaena population in the study area. Around half of the mapped population are individuals in the sapling stage, and they are in good health. Its current realized niche is described by higher altitudes, steeper slopes, more concave landscape forms and east-facing aspects compared to areas where dracaena individuals are absent. However, for the new generation of dracaena saplings we find signs of a leaning range shift where saplings are shifted towards higher altitudes near the mist-influenced escarpment. A full collapse and eventual extirpation of the endangered Dracaena ombet population may at best be averted, or at least delayed in the study area. Our resilience analysis indicates that a full recovery will be a slow process due to the inherent natural climate variability of arid lands, only allowing sporadic regeneration. Considering this species’ information legacy, saplings seem to be well equipped to survive such variability, but perhaps within a restricted safe operating space. Conservation measures should therefore be taken to secure the survival of the new generation along with broader spatial scale studies to confirm whether our findings reflect a regional phenomenon.publishedVersio

Acacia trees on the cultural landscapes of the Red Sea Hills

This paper examines interactions between five pastoral nomadic culture groups of the Egyptian and Sudanese Red Sea Hills and the acacia trees Acacia tortilis (Forssk.) Hayne subsp. tortilis and subsp. raddiana growing in their arid environments. A. tortilis is described as a keystone species both ecologically and culturally: the trees play such critical roles in ecosystems and social groups that their removal would greatly impact both systems. Interviews in the field with the Semitic, Arabic-speaking Ma‘aza and Ababda, and the Cushitic, Beja, Bidhaawyeet-speaking Bishaari, Amar Ar and Hadandawa nomads probed the cultural and ecological contexts of acacias in pastoral nomadism, revealing deep insight into traditional ecological knowledge and traditional perceptions and uses of the trees. The paper describes how this knowledge guides pastoral decision-making, with acacias as a particularly critical component of the pastoral livelihood in both normal and stressful circumstances. A. tortilis is the most important reliable vegetation resource for nomads while also providing fuel and other useful products, ecosystem services for people and animals, and increased biodiversity by providing diverse microhabitats and resources for other species. We describe aspects of kinship, territorial organization, spiritual beliefs and tribal law underlying the significance of trees on the cultural landscape. We discuss environmental and economic challenges to human/tree relationships and to pastoral livelihoods. We challenge views of nomads as agents of ecological destruction, and propose maintenance and restoration of traditional pastoralism as viable alternatives in dryland development.publishedVersio

Mortality, Recruitment and Change of Desert Tree Populations in a Hyper-Arid Environment

BACKGROUND: Long-term vegetation changes in hyper-arid areas have long been neglected. Mortality, recruitment and change in populations of the ecologically and culturally important and drought persistent Acacia tortilis and Balanites aegyptiaca are therefore estimated in the Eastern Desert of Egypt, and are related to the primary agents of change, water conditions and human intervention. METHODOLOGY: A change analysis using high-resolution Corona images (1965) in combination with field data (2003) is the basis for recruitment, mortality and change estimates. For assessing the influence of water conditions on patterns in recruitment and survival, different types of generalized linear models are tested. CONCLUSIONS: The overall trend in population size in that part of the Eastern Desert studied here is negative. At some sites this negative trend is alarming, because the reduction in mature trees is substantial (>50%) at the same time as recruitment is nearly absent. At a few sites there is a positive trend and better recruitment. Frequent observations of sprouting in saplings indicate that this is an important mechanism to increase their persistence. It is the establishment itself that seems to be the main challenge in the recruitment process. There are indications that hydrological variables and surface water in particular can explain some of the observed pattern in mortality, but our results indicate that direct human intervention, i.e., charcoal production, is the main cause of tree mortality in the Eastern Desert

Acacia trees on the cultural landscapes of the Red Sea Hills

This paper examines interactions between five pastoral nomadic culture groups of the Egyptian and Sudanese Red Sea Hills and the acacia trees Acacia tortilis (Forssk.) Hayne subsp. tortilis and subsp. raddiana growing in their arid environments. A. tortilis is described as a keystone species both ecologically and culturally: the trees play such critical roles in ecosystems and social groups that their removal would greatly impact both systems. Interviews in the field with the Semitic, Arabic-speaking Ma‘aza and Ababda, and the Cushitic, Beja, Bidhaawyeet-speaking Bishaari, Amar Ar and Hadandawa nomads probed the cultural and ecological contexts of acacias in pastoral nomadism, revealing deep insight into traditional ecological knowledge and traditional perceptions and uses of the trees. The paper describes how this knowledge guides pastoral decision-making, with acacias as a particularly critical component of the pastoral livelihood in both normal and stressful circumstances. A. tortilis is the most important reliable vegetation resource for nomads while also providing fuel and other useful products, ecosystem services for people and animals, and increased biodiversity by providing diverse microhabitats and resources for other species. We describe aspects of kinship, territorial organization, spiritual beliefs and tribal law underlying the significance of trees on the cultural landscape. We discuss environmental and economic challenges to human/tree relationships and to pastoral livelihoods. We challenge views of nomads as agents of ecological destruction, and propose maintenance and restoration of traditional pastoralism as viable alternatives in dryland development

Figure 5

<p>Changes in Wadi Gaetri between A 1996 and B 2003. I: In 2003 a hole in the ground is all that testifies to the presence of the stump (1996). II: A green tree has already lost half of its canopy. In 2003 this tree is completely gone, suggesting live felling for charcoal production.</p