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Acacia Trees (acacia + tree)
Selected AbstractsBelow-ground competition between trees and grasses may overwhelm the facilitative effects of hydraulic liftECOLOGY LETTERS, Issue 8 2004F. Ludwig Abstract Under large East African Acacia trees, which were known to show hydraulic lift, we experimentally tested whether tree roots facilitate grass production or compete with grasses for below-ground resources. Prevention of tree,grass interactions through root trenching led to increased soil water content indicating that trees took up more water from the topsoil than they exuded via hydraulic lift. Biomass was higher in trenched plots compared to controls probably because of reduced competition for water. Stable isotope analyses of plant and source water showed that grasses which competed with trees used a greater proportion of deep water compared with grasses in trenched plots. Grasses therefore used hydraulically lifted water provided by trees, or took up deep soil water directly by growing deeper roots when competition with trees occurred. We conclude that any facilitative effect of hydraulic lift for neighbouring species may easily be overwhelmed by water competition in (semi-) arid regions. [source] The population of Acacia tree species producing gum arabic in the Karamoja region, UgandaAFRICAN JOURNAL OF ECOLOGY, Issue 3 2007Simon P. Egadu Abstract The study aimed at determining the population status of the different Acacia tree species producing gum arabic in the undisturbed, grazed and cultivated habitats in the Karamoja region, Uganda. A total of 135 sample plots each measuring 20 × 20 m2 (0.04 ha) with each habitat having 45 plots were selected and established in the seven counties using a simple random sampling technique. The tree species present, their abundances and sizes were recorded. Twelve Acacia species were identified and a total of 5535 recorded in the sampled area. Out of these, five were gum-producing acacias. Acacia senegal dominated the acacias in all the seven counties and in all habitats of Karamoja with Acacia nilotica (72.3%), Acacia seyal (13.4%), Acacia sieberiana (4%) and Acacia gerrardii (2.6%). Non-gum-producing acacias constituted 7.19% of the total abundance. The tree densities increased with increase in tree size in the undisturbed and grazed habitats but decreased in the cultivated habitat. Most Acacia trees were of large size, an indication of old age and poor regeneration that could affect their future population status. It is recommended that further investigations be carried out into the causes of poor regeneration of Acacia species. Résumé Cette étude visait à déterminer le statut de la population de différentes espèces d'Acacia qui produisent de la gomme arabique dans des habitats non perturbés, pâturés et cultivés de la région de Karamoja, en Ouganda. On a sélectionné un total de 135 plots échantillons mesurant chacun 20 × 20 mètres (0,4 ha), chaque habitat en comptant 45, dans les sept comtés, en utilisant une technique simple d'échantillonnage au hasard. On a rapporté les espèces d'arbres présentes, leur abondance et leur taille. On a identifié douze espèces d'Acacia, pour un total de 5535 dans la zone d'échantillon. Parmi elles, cinq produisent de la gomme. Acacia senegal dominait dans les sept comtés et dans tous les habitats avec Acacia nilotica (72.3%), Acacia seyal (13.4%), Acacia sieberiana (4%) et Acacia gerrardii (2.6%). Les acacias ne produisant pas de gomme représentaient 7.19% de l'abondance totale. La densité des arbres augmentait avec leur taille dans les habitats non perturbés et pâturés, mais diminuait dans les habitats cultivés. La plupart des acacias étaient des arbres de grande taille, signe d'un grand âge et d'une régénération médiocre qui pourraient affecter le statut de la population dans le futur. On recommande de mener des investigations plus poussées pour connaître les causes de cette faible régénération des espèces d'Acacia. [source] Structure and composition of Acacia xanthophloea woodland in Lake Nakuru National Park, KenyaAFRICAN JOURNAL OF ECOLOGY, Issue 4 2006N. Dharani Abstract The woody vegetation of Lake Nakuru National Park occurs along rivers, lakeshores and flood plains. Four different sites within the Acacia xanthophloea woodlands were selected for the study. Vegetation structure was not significantly different in the four woodlands used for the study, but these sites differed in the relative density of Acacia trees. Regeneration of A. xanthophloea differed in each site, with the highest regeneration rates found in the nonfenced plots where browsing took place. Résumé La végétation ligneuse du Parc National de Nakuru se trouve le long des cours d'eau, sur les berges du lac et dans les plaines inondables. Nous avons sélectionné quatre sites dans les zones arborées àAcacia xanthophloea. La structure da la végétation n'était pas significativement différente dans les quatre forêts sélectionnées pour l'étude, mais ces sites différaient quant à la densité relative d'acacias. La régénération des Acacia xanthophloea différait sur chaque site, et le taux de régénération était le plus élevé dans les plots non clôturés fréquentés par les herbivores. [source] Do spatial effects play a role in the spatial distribution of desert-dwelling Acacia raddiana?JOURNAL OF VEGETATION SCIENCE, Issue 4 2000Kerstin Wiegand Abstract. We investigated the spatial pattern of A. raddiana in the Negev desert of Israel in order to gain insights into the factors and processes driving the dynamics of this species. Using a scale-dependent measure, the ring statistic, we analysed both patterns observed in the field and time series of spatial tree distributions produced by a simulation model. In the field, random spacing was the predominant pattern observed. However seedlings were clumped on small scales. We ran the model under two contrasting scenarios representing hypotheses that explain the clumping of seedlings and the random distribution of trees. One hypothesis is that there is spatial heterogeneity in seed distribution, germination and seedling mortality, but that these heterogeneities are not correlated with each other in space. The second hypothesis assumes a correlation between these heterogeneities leading to areas suitable for establishment. However, the suitability of the sites is temporally variable. Furthermore, the second hypothesis assumes density-dependent tree mortality due to competition. Both hypotheses lead to spatial distributions that are in qualitative agreement with the patterns observed in the field. Therefore, the classical view that a clumped seedling distribution and a random pattern of older trees is due to clumped regeneration and density-dependent mortality may not hold for Acacia trees in the Negev. [source] Complex habitat requirements and conservation needs of the only extant Baroniinae swallowtail butterflyANIMAL CONSERVATION, Issue 3 2004Jorge L. León-Cortés The unique butterfly species, Baronia brevicornis, perhaps the oldest taxon in the Papilionidae, is known to have a very restricted distribution in Mexico. Populations are restricted to southern Mexico in deciduous scrub forest where its host-plants, Acacia trees (Leguminosae), are common and widespread. Little is known of the conservation implications of its relationship with its host trees. We recorded fine scale population data for larval and adult B. brevicornis in 22 km2 of fragmented landscape in southern Mexico. Habitat associations determined from over 1319 transect walks reveal that B. brevicornis exhibited an extremely localised distribution, occupying < 1% of the study area and being mostly associated with Acacia woodlots. Detailed analyses of habitat requirements for larva and/or adult B. brevicornis suggest that it lays its eggs on Acacia pennatula and A. macracantha (two newly recorded larval host-plants), that eggs were laid disproportionately on Acacia trees with long branches (,2= 17.7, P < 0.001) and that the probability of finding adult B. brevicornis between occupied and un-occupied Acacia woodlots increased with host-plant density (,2= 18.4, P < 0.001). The results of this study suggest that conservation recommendations for B. brevicornis must consider the condition of the Acacia habitat network, given that Acacia is mostly associated with human-managed grazing systems. Effective conservation will require the establishment of connected networks of patches where natural dynamics can produce new habitats, as well as the creation of new habitats within colonisation distance. This research provides a rare case study of conservation biology of a Neotropical insect, emphasising the importance of using ecological information to provide management recommendations. [source] What helps Opuntia stricta invade Kruger National Park, South Africa: Baboons or elephants?APPLIED VEGETATION SCIENCE, Issue 2 2007L.C. Foxcroft Germishuizen & Meyer (2003) for plant species Abstract Question: Is Opuntia stricta more frequent, and its patches larger, under trees suitable for baboon roosting? If so, does it mean that baboons are major dispersal agents and that plants established under these trees are important foci of Opuntia stricta spread? Location: Skukuza, Kruger National Park, South Africa. Method: We surveyed an area invaded by Opuntia stricta in the Skukuza region of KNP. The survey included plots under potential baboon roosting trees,plots under trees unlikely to support baboons,and paired randomly located open sites. Results: The null hypothesis -tree- Opuntia spatial independence , can be rejected for Acacia nilotica, but not for Spirostachys africana. Opuntia plants are positively associated with Acacia trees suitable for baboon roosting. However, there is no significant difference between frequency of Opuntia under Acacia trees suitable and unsuitable for baboon roosting. It appears that all Acacia trees can serve as nurse trees for Opuntia. Compared to plots under Acacia trees, frequencies of old and robust Opuntia plants are significantly higher in open areas and under dead trees. Conclusions: While baboons may be responsible for long distance Opuntia dispersal (over km),their role is not detectable at a local scale. On the other hand, elephants seem to contribute substantially to the local vegetative propagation of this species. Opuntia establishment and growth are more influenced by micro-habitat than previously thought. [source] Implications of closed ecosystem conservation management: the decline of Rothschild's giraffe (Giraffa camelopardalis rothschildi) in Lake Nakuru National Park, KenyaAFRICAN JOURNAL OF ECOLOGY, Issue 4 2009Rick A. Brenneman Abstract Giraffe were historically free-ranging across most of sub-Saharan Africa but are now most often confined to national parks, conservation areas, or private ranches. Five viable populations of Rothschild's giraffe (Giraffa camelopardalis rothschildi) remain in protected areas in Kenya and Uganda. The viable population in Uganda is Murchison Falls National Park and the four populations in Kenya are Lake Nakuru National Park (LNNP), Ruma National Park, Giraffe Manor, and Mwea Natural Reserve. The Kenya Wildlife Service queried a rapid decline in LNNP giraffe numbers falling from 153 individuals (1995) to 62 individuals (2002) and the failure of that population to recruit young in those years. Significantly reduced browse options, inbreeding depression and preferential lion predation were considered as potential reasons for this trend. Population genetic parameter estimates derived from multilocus genotype analyses suggest that the LNNP population was in good genetic health with respect to the likelihood of inbreeding depression. The population decline coincided with the drought attributed to the 1994 El Niño. Possible dietary complications from highly concentrated tannin levels because of forced over consumption of the park's declining acacia trees may have compromised young giraffe, making them easy and opportunistic prey for the park's lion population. Résumé Dans le passé, des girafes évoluaient librement dans une grande partie de l'Afrique sub-saharienne, mais elles sont désormais confinées le plus souvent dans des parcs nationaux, des aires de conservation ou des ranches privés. Cinq populations viables de girafes de Rothschild (Giraffa camelopardalis rothschildi) subsistent dans des aires protégées au Kenya et en Ouganda. La population viable d'Ouganda est celle du Parc National des Murchison Falls, et les quatre populations du Kenya sont celles du Parc National du Lac Nakuru (LNNP), du Parc National de Ruma, du Giraffe Manor et de la Réserve Naturelle de Mwea. Le Kenya Wildlife Service s'est interrogé au sujet du déclin rapide du nombre de girafes au LNNP qui est passé de 153 individus en 1995 à 62 en 2002, et de l'échec de cette population pour recruter des jeunes ces années-là. Des possibilités alimentaires réduites, une dépression due à l'inbreeding, et une prédation préférentielle par les lions ont été envisagées comme raisons de cette tendance. Des estimations des paramètres génétiques de la population dérivées d'analyses de génotypes multilocus suggèrent que la population du LNNP est en bonne santé génétique en ce qui concerne la possibilité de dépression causée par l'inbreeding. Le déclin de la population a coïncidé avec la sécheresse attribuée à El Niño en 1994. Il est possible qu'elle ait souffert de complications alimentaires à cause de taux de tanins très concentrés, parce qu'elle a été forcée de consommer à l'excès les acacias du parc, qui sont en déclin, et que ces complications aient nui aux jeunes girafes et aient fait d'elles des proies faciles pour la population opportuniste des lions du parc. [source] | |||||||||||||