Dominant Woody Species Research Articles

Water relations and mineral nutrition of closely related woody plant species on desert dunes and interdunes

Vegetation on dunes and interdunes in hot, subtropical deserts is profoundly influenced by the temporal and spatial variation in availability of water and nutrients in the landscape. We hypothesised that water is more available to plants on the dunes but that nutrients are in greater concentrations on the interdunes in the Great Sandy Desert, Western Australia. During the course of 2 years, we examined water relations and photosynthesis of six dominant woody species throughout each season, in addition to foliar δ13C, δ15N and nutrient composition. In general, stomatal conductance (gs) was greater and leaf water potential (ΨL) less negative for dune species than for closely related species on the interdunes. The largest tree species in the landscape, Corymbia chippendalei ((D.J.Carr & S.G.M.Carr) K.D.Hill & L.A.S.Johnson), occurred only on the dunes, and maintained moderate gs values year round, ranging between 240 mmol H2O m–2 s–1 in the wet season and 160 mmol H2O m–2 s–1 in the middle of the dry season. It also displayed a relatively stable ΨL, between –0.3 and –0.5 MPa at predawn, and between –1.3 and –1.6 MPa at midday throughout the year, unlike the closely related species on the interdunes, Eucalyptus victrix (L.A.S.Johnson & K.D.Hill), which always displayed significantly lower ΨL values (0.2–1.1 MPa more negative). The two Grevillea species displayed ΨL values within a similar range as for C. chippendalei, while the Acacia species exhibited consistently more negative values, especially late in the dry season. Considerable reductions in gs occurred at this time for all species, except C. chippendalei. Rates of photosynthesis (A) followed the trends in gs, yet δ13C values varied little between related species in the dune and interdune habitats. Mineral nutrient concentrations in soil and foliage tended to be greater in the interdunes. Average N : P ratio in foliage was 28 : 1, indicating P was more limiting than N. Soil depth and texture, in conjunction with their effects on water availability and root growth, were considered to be the most influential factors affecting plant distribution in the Great Sandy Desert. It is concluded that dunes hold relatively more water than adjacent interdunes, sustaining more favourable water status in deep-rooted species from this habitat, further into the dry season. Conversely, species on the interdunes must be more desiccation tolerant and develop root systems with greater ability to penetrate conglomerated lateritic gravel layers in order to access water where and when it is available.

Does plant-derived smoke affect seed germination in dominant woody species of the Mediterranean matorral of central Chile?

Studies performed in the fire-prone Mediterranean-type climate shrublands of Australia, California, and South Africa have shown that plant-derived smoke enhances seed germination in many species. Unlike other areas with similar climate, central Chile stands out for the absence of natural fires, suggesting that smoke may not be expected to promote germination. However, anthropogenic fires have been frequent since several millennia, and the role of fire on shaping fire functional traits is not clear at this point. The aim of this study was to evaluate the effects of plant-derived smoke on seed germination of some predominant native woody species from the Mediterranean matorral of central Chile. We exposed seeds of 18 woody species to plant-derived smoke for 30 min and assessed their germination. Five species failed to germinate under either the experimental and control conditions. Smoke significantly stimulated germination in three species, while decreasing it in eight. Species showing smoke-inhibited germination tend to be major dominants in the Chilean matorral vegetation. The three smoke-stimulated species are known colonizers. This suggests that current human-caused fires could drastically change the structure of Chilean matorral. Although our results suggest fire have not played a major role in shaping adaptations for seed germination of woody species in the Chilean matorral, more investigation about ephemeral species is needed. This study provides the first results about smoke-related germination in the Mediterranean-type zone of central Chile, generating the opportunity to investigate the evolutionary context and distribution of smoke-stimulated germination in all the Mediterranean-type ecosystems.

Recruitment Patterns in a Mediterranean Oak Forest: A Case Study Showing the Importance of the Spatial Component

Abstract The spatial patterns of recruits in a Mediterranean forest in Southwest Portugal were examined. We aimed to investigate how several environmental and canopy variables contribute to the observed patterns, and how relevant the spatial component is in explaining the variance in the density of recruits. We took a census of every recruit of four dominant woody species (Arbutus unedo, Quercus faginea, Q. suber, and Viburnum tinus), mapping their position inside two forest plots. For a total of 309 recruits, we measured the following variables: soil moisture, slope, canopy density, herbaceous and litter ground cover, overlying species and height, and distance to the nearest adult of the same and different species. Spatial pattern analysis, principal coordinate analysis of neighbor matrices, canonical correspondence analysis, and variance partitioning were performed to detect significant deviations from complete spatial randomness and identify conspecific and interspecific patterns, characterize regeneration niches, and evaluate the importance of the spatial component. Results showed the existence of significant community structure at the recruitment stage. The distance between recruits and the nearest conspecific adult was the main explanatory variable. Light availability, soil moisture, litter accumulation, and the overlying species were also significant factors in explaining the variance in the density of recruits. The best model accounted for 37% of the overall variance in the density of recruits, and most (80%) of the explained variance corresponded to spatially structured variance. This case study shows the importance of the spatial component in understanding the forest regeneration patterns under Mediterranean conditions and will contribute to the implementation of ecologically based management actions to preserve the remaining forest fragments.

Nutrient resorption and patterns of litter production and decomposition in a Neotropical Savanna

Summary Deposition of nutrients due to anthropogenic activities has the potential to change nutrient availability in nutrient‐limited ecosystems with consequences for plant and ecosystem processes. Species‐specific and ecosystem responses to the addition of nutrients were studied in a field experiment conducted in a Savanna (Cerrado sensu stricto) on dystrophic soil in central Brazil. Three fertilization treatments (N, P and N plus P additions) and unfertilized control were replicated in four 15 × 15 m plots per treatment. Five of the dominant woody species were studied. Specific leaf area (SLA) was not affected by the treatments, but species responded differently in terms of foliar nutrient concentration, resorption efficiency and proficiency to the increased soil N and P availability. Responses of N and P resorption suggested different levels of regulation for these two limiting elements, with a decrease in N and P resorption proficiency depending on the treatment. In general, under N fertilization, senesced leaves presented higher N concentration, whereas combined fertilization with N and P resulted in senesced leaves richer in P. The concomitant variation in P resorption efficiency and proficiency indicates a stronger regulation between them compared to N. Results indicated a highly efficient and complete P resorption, while N concentrations in senesced leaves indicated intermediate or incomplete resorption efficiencies, supporting the idea that Cerrado plants might be more limited by P than by N. At the ecosystem level, higher N concentration in the leaf litter was measured at the end of the dry season in the +NP‐fertilized plots, resulting in a significantly faster decomposition rate in relation to the control treatment. Responses of litterfall and decomposition rates indicated that the system was generally more responsive to the combined addition of N and P than to fertilization with N or P alone over the relatively short time span of this study.

Factors affecting soil P dynamics in temperate volcanic soils of southern Argentina

During volcanic soil development, factors controlling plant-available P shift from hydrolysis of P bound to primary Ca compounds to mineralization of organic P, and an increasing role of amorphous materials in P retention. Monospecific forests of the Andean–Patagonian region are distributed in a narrow strip characterized by young volcanic soils and decreasing west–east precipitation. We analyzed sequential P fractionation, P retention, active Al and Fe, and acid phosphatase activity in the superficial soil (0–15 cm) of ten dominant woody species, and related these data to previous results on soil chemical and biological fertility, and green leaf P concentration. Higher plant-labile P (water–Pi + NaHCO 3–Pi) corresponded to soils with higher total P, and this was directly related to the increase of organic P (NaHCO 3–Po + NaOH–Po). Phosphatase activity was mainly explained by the increase of organic C and the decrease of primary minerals (P–HCl). The strongest determinant of P retention was the formation of active Al, which increased with acidification and losses of cations and primary minerals. In the drier extreme of the precipitation gradient, we found the lowest values of active Al and P retention coupled with high exchangeable cations and neutral pH. The effect of plant species increased in wetter areas, and was reflected in differential patterns of SOM accumulation, base cation cycling and active Al formation. When comparing plant functional groups, potential net N mineralization and nitrification, phosphatase activity, organic C, organic P (NaHCO 3–Po) and green leaf P were significantly higher in broad-leaf deciduous species than in conifers.

Effects of cutting and nitrogen deposition on biodiversity in Cantabrian heathlands

AbstractQuestions: Does the diversity of heathland vegetation change when subjected to experimental disturbances such as cutting and nitrogen fertilization? Do changes in the vegetation structure negatively affect the regeneration of the dominant species Calluna vulgaris? Is cutting an alternative method of conserving the diversity and maintaining the structure of heathlands in the Cantabrian Mountains?Location: Calluna vulgaris heathlands on the southern slopes of the Cantabrian Mountain range, NW Spain.Methods: A total of 60 plots were treated with different combinations of cutting and twice the estimated atmospheric deposition of nitrogen (56 kg‐N.ha−1.yr−1). The changes in the cover values of the species present were monitored over a five year study period. The cover values were used to calculate abundance and species richness.Results: Fertilizing with nitrogen allows biodiversity to increase over time. However, the greatest biodiversity is associated with the cutting plus fertilization treatment, since cutting allows gaps to be opened that are easily colonized by pioneer annual species, while fertilization mainly favours an increase in the mean number of perennial herbs (graminoids and forbs). Increased perennial herb richness also corresponds to a rise in their cover values. The recovery of the dominant woody species in these communities, Calluna vulgaris, is not impeded by the increase in perennial herbs species' cover values.Conclusions: In the Calluna vulgaris heathlands studied, cutting plus fertilization allowed an increase in biodiversity over time. No displacement of the dominant woody species, Calluna vulgaris, is observed due to the presence of the perennial herbs. Cutting patches of heathland is recommended as a mechanism for maintaining high vegetation diversity, when grazing is not possible.

¿Cuánto vale una reserva forestal?: el caso de El Robledal

Se presentan los resultados de valoración económica directa e indirecta de la Reserva Forestal Protectora El Robledal obtenidos a 2005. Esta reserva es un bosque andino seco y se encuentra ubicada en los municipios de Guachetá y Ráquira, departamentos de Cundinamarca y Boyacá, en la jurisdicción de la Corporación Autónoma Regional de Cundinamarca (CAR), con una elevación entre los 2.700 y los 3.200 MSNM y con una un área de 400,2 ha. Con base en una imagen satelital LANDSAT TM de 1994, se estima la cobertura boscosa de la reserva en tan solo 173 ha y se caracteriza el bosque en cuanto a estructura y composición a través del levantamiento aleatorio de 12 cuadrantes de 2 por 50 m. La especie leñosa dominante es el roble (Quercus humboldtii Bonpl.) Finalmente, se procedió a hallar los valores económicos de uso directo de subsistencia y productivo y de uso indirecto de los servicios ambientales establecidos para la investigación: valor económico de la madera en pie, recolecta de madera y producción de carbón vegetal por la población aledaña a la reserva y finalmente del bosque como sumidero de carbono. Para tal fin se utilizaron metodologías de valoración económica tales como precios de mercado, valoración contingente, costes evitados o inducidos, basadas en costos y transferencia de beneficios. Se concluye que el valor más alto esta asociado con la madera en pie y el valor de subsistencia, producto de la tala de bosques para la producción ilegal de carbón vegetal, es insignificante y motivo de preocupación. Una recomendación de política es trasladar el ingreso derivado de los habitantes de la zona por el valor de subsistencia a la protección de la reserva. Por tratarse de una reserva forestal muy antigua y a la luz del desarrollo sostenible y el tiempo ecológico, este recurso debería tratarse como no renovable.

Effects of cutting and nitrogen deposition on biodiversity in Cantabrian heathlands

Questions: Does the diversity of heathland vegetation change when subjected to experimental disturbances such as cutting and nitrogen fertilization? Do changes in the vegetation structure negatively affect the regeneration of the dominant species Calluna vulgaris? Is cutting an alternative method of conserving the diversity and maintaining the structure of heathlands in the Cantabrian Mountains? Location: Calluna vulgaris heathlands on the southern slopes of the Cantabrian Mountain range, NW Spain. Methods: A total of 60 plots were treated with different combinations of cutting and twice the estimated atmospheric deposition of nitrogen (56 kg-N.ha−1.yr−1). The changes in the cover values of the species present were monitored over a five year study period. The cover values were used to calculate abundance and species richness. Results: Fertilizing with nitrogen allows biodiversity to increase over time. However, the greatest biodiversity is associated with the cutting plus fertilization treatment, since cutting allows gaps to be opened that are easily colonized by pioneer annual species, while fertilization mainly favours an increase in the mean number of perennial herbs (graminoids and forbs). Increased perennial herb richness also corresponds to a rise in their cover values. The recovery of the dominant woody species in these communities, Calluna vulgaris, is not impeded by the increase in perennial herbs species' cover values. Conclusions: In the Calluna vulgaris heathlands studied, cutting plus fertilization allowed an increase in biodiversity over time. No displacement of the dominant woody species, Calluna vulgaris, is observed due to the presence of the perennial herbs. Cutting patches of heathland is recommended as a mechanism for maintaining high vegetation diversity, when grazing is not possible.

Salt spray and edaphic factors maintain dwarf stature and community composition in coastal sandplain heathlands

North American coastal sandplain heathlands are unique in species composition and vegetation, but the extent to which edaphic factors influence the structure of these communities is currently debated. It was hypothesized that salt spray and edaphic factors maintain the dwarf stature and community composition of heathlands by limiting plant growth and excluding competitively dominant woody species close to the ocean. Field surveys were carried out to investigate the spatial patterns of salt spray accumulation, soil salt and soil moisture. High salt spray correlated significantly with increased leaf necrosis and water stress in Myrica pensylvanica and with decreased plant height. Plant community composition changed across a salt spray and soil gradient, as well. Distinctive sub-communities were identified that separated according to soil salt and soil moisture but salt spray was the main factor affecting sites occupied only by heathland vegetation. Results from this study suggest that salt spray suppresses the growth of heathland plants in close proximity to the ocean, and therefore maintains the low stature in these dwarf shrublands. This research also demonstrates that the physical environment influences the community structure in heathlands, particularly by limiting tree species from growing in high salt spray, low water availability sites.

Extant and potential vegetation of an old-growth maritime Ilex opaca forest

The extant and potential (seed bank) vegetation of a rare maritime holly forest on Fire Island, New York was described to assess whether treefall gaps act as a mechanism for the persistence of the species composition of this plant community over time. The Sunken Forest overstory is dominated by Ilex opaca, Amelanchier canadensis and Sassafras albidum. A survey of canopy gaps indicated canopy openings compose 11.3% of the land within the Sunken Forest (16 ha). The composition and density of the seed bank were described using the emergence method. Germination from soil samples placed in the greenhouse was monitored over 2 years. Sixteen species germinated with an average propagule density of 215±41 germinants per square metre. An early successional species (Rhus copallinum) dominated the seed bank, but the late-successional, shade-tolerant I. opaca was also present. Though only one species in the seed bank did not appear in the current vegetation, species abundance differed between vegetation strata. The mean cover and density of the ground-layer flora were higher beneath treefall gaps than closed canopy. Sapling density did not differ between the two canopy conditions, but the dominant species differed with A. canadensis occupying several closed canopy plots and P. serotina saplings appearing more often in gap plots. Most of the dominant canopy species are present in the seed bank and ground layer but are not present in the shrub and sapling layer, with the exception of A. canadensis. Current (2002) sapling density is lower than three decades ago for all species except P. serotina, which is now the dominant woody species in the Sunken Forest understory. The results of this study indicate that if the cause of the sapling reduction is lessened or removed, the characteristic species of the overstory of this unusual plant community may rebound and redevelop a sapling and shrub layer akin to that present before the increase in Odocoileus virginianus on the island.

Soil microbial activity and litter turnover in native grazed and ungrazed rangelands in a semiarid ecosystem

Long-term overgrazing is known to influence soil microbiological properties and C sequestration in soil organic matter. However, much remains to be known concerning overgrazing impacts on soil microbial activity and litter turnover in heavily grazed rangelands of Central Iran. Aboveground litter decomposition of three dominant species (Agropyron intermedium, Hordeum bulbosum, and Juncus sp.) were studied using a litter bag experiment under field conditions in three range sites of Central Iran, a site with continuous grazing, a site ungrazed for 17 years with dominant woody species (80% cover), and a site ungrazed for 17 years with dominant pasture species (70% cover). Soil samples were taken from 0 to 30 cm depth and analyzed for their chemical and microbiological properties. Results demonstrate that soil organic C and total N contents and C/N ratios were similar for both ungrazed and grazed sites, while available P and K concentrations significantly decreased under grazed conditions. It was also evident that range grazing decreases soil respiration and microbial biomass C, suggesting a lower recent annual input of decomposable organic C. Nevertheless, grazing conditions had no significant effect on litter decomposition indicating soil microclimate is not affected by grazing animals in this ecosystem. It is concluded that overgrazing may presumably depress microbial activity through either reduced input of fresh plant residue into the surface soil or lack of living roots and exudates for stimulating microbial activity. This study also suggests that 17 years of livestock exclusion might be insufficient time for expected C accumulation in soil.

Koncept ekologických fenoménů v interpretaci středoevropské vegetace [Ecological phenomena concept: the interpretation of the Central-European vegetation

A review of literature referring to floristic, vegetation and habitat diversity in Bohemian Massif has resulted to a revision and detailed classification of ecological phenomena. Each phenomenon represents an open non-hierarchical system of lithological, hydrological, topoclimatic and biotic aberrations, that differ from common features of surrounded “zonal“ environment. Two main groups of ecological phenomena have been distinguished: (i) large-scale ecological geo-phenomena originated by prominent lithological factors, and (ii) meso-scale ecological topo-phenomena produced by local action of hydrology, topoclimate, and air currents. With regard to the first group, the following geo-phenomena play a particular role in the Central European vegetation: (a) sandstone, (b) limestone and dolomite (karst phenomenon), (c) marl (flysch and loess), (d) neo-volcanic, and (e) serpentinite phenomena. With regard to the latter group, the following meso-scale phenomena contribute to the diversity of habitats in landscape scenery: (f) river (or valley), (g) summit, (h) frost hollow, and (i) scree (and glacial cirque) phenomena. The “insular“ feature of the ecological phenomena in vegetation cover is expressed mainly by the occurrence of treeless ecosystems, such as swamps, bogs, marshes, steppes, rock faces and scree ecosystems, etc. Relatively higher species richness can indicate most of ecological phenomena (they should be detected by the distance from the regional species-area curve), and by the presence of rare plant species, especially by the relic and/or endemic species, which survive on extreme habitats due to the lower competition of prevailing dominant woody or graminoid species.

Koncept ekologických fenoménů v interpretaci středoevropské vegetace [Ecological phenomena concept: the interpretation of the Central-European vegetation

A review of literature referring to floristic, vegetation and habitat diversity in Bohemian Massif has resulted to a revision and detailed classification of ecological phenomena. Each phenomenon represents an open non-hierarchical system of lithological, hydrological, topoclimatic and biotic aberrations, that differ from common features of surrounded “zonal“ environment. Two main groups of ecological phenomena have been distinguished: (i) large-scale ecological geo-phenomena originated by prominent lithological factors, and (ii) meso-scale ecological topo-phenomena produced by local action of hydrology, topoclimate, and air currents. With regard to the first group, the following geo-phenomena play a particular role in the Central European vegetation: (a) sandstone, (b) limestone and dolomite (karst phenomenon), (c) marl (flysch and loess), (d) neo-volcanic, and (e) serpentinite phenomena. With regard to the latter group, the following meso-scale phenomena contribute to the diversity of habitats in landscape scenery: (f) river (or valley), (g) summit, (h) frost hollow, and (i) scree (and glacial cirque) phenomena. The “insular“ feature of the ecological phenomena in vegetation cover is expressed mainly by the occurrence of treeless ecosystems, such as swamps, bogs, marshes, steppes, rock faces and scree ecosystems, etc. Relatively higher species richness can indicate most of ecological phenomena (they should be detected by the distance from the regional species-area curve), and by the presence of rare plant species, especially by the relic and/or endemic species, which survive on extreme habitats due to the lower competition of prevailing dominant woody or graminoid species.

Foliar 15N natural abundance indicates phosphorus limitation of bog species

Foliar delta15N, %N and %P in the dominant woody and herbaceous species across nutrient gradients in New Zealand restiad (family Restionaceae) raised bogs revealed marked differences in plant delta15N correlations with P. The two heath shrubs, Leptospermum scoparium (Myrtaceae) and Dracophyllum scoparium (Epacridaceae), showed considerable isotopic variation (-2.03 to -15.55 per thousand, and -0.39 to -12.06 per thousand, respectively) across the bogs, with foliar delta15N strongly and positively correlated with P concentrations in foliage and peat, and negatively correlated with foliar N:P ratios. For L. scoparium, the isotopic gradient was not linked to ectomycorrhizal (ECM) fractionation as ECMs occurred only on higher nutrient marginal peats where 15N depletion was least. In strong contrast, restiad species (Empodisma minus Sporadanthus ferrugineus, S. traversii) showed little isotopic variation across the same nutrient gradients. Empodisma minus and S. traversii had delta15N levels consistently around 0 per thousand (means of -0.12 per thousand and +0.15 per thousand respectively), and S. ferrugineus, which co-habited with E. minus, was more depleted (mean -4.97 per thousand). The isotopic differences between heath shrubs and restiads were similar in floristically dissimilar bogs and may be linked to contrasting nutrient demands, acquisition mechanisms, and root morphology. Leptospermum scoparium shrubs on low nutrient peats were stunted, with low tissue P concentrations, and high N:P ratios, suggesting they were P-limited, which was probably exacerbated by markedly reduced mycorrhizal colonizations. The coupling of delta(15)N depletion and %P in heath shrubs suggests that N fractionation is promoted by P limitation. In contrast, the constancy in delta15N of the restiad species through the N and P gradients suggests that these are not suffering from P limitation.

Multi‐scale patterns and bush encroachment in an arid savanna with a shallow soil layer

Abstract.Question:Bush encroachment (i.e. an increase in density of woody plants often unpalatable to domestic livestock) is a serious problem in many savannas and threatens the livelihood of many pastoralists. Can we derive a better understanding of the factors causing bush encroachment by investigating the scale dependency of patterns and processes in savannas?Location:An arid savanna in the Khomas Hochland, Namibia.Methods:Patterns of bush, grass, and soil nutrient distribution were surveyed on several scales along a rainfall gradient, with emphasis on intraspecific interactions within the dominant woody species,Acacia reficiens.Results:Savannas can be interpreted as patch‐dynamic systems where landscapes are composed of many patches (a few ha in size) in different states of transition between grassy and woody dominance.Conclusions:In arid savannas, this patchiness is driven both by rainfall that is highly variable in space and time and by inter‐tree competition. Within the paradigm of patch‐dynamic savannas, bush encroachment is part of a cyclical succession between open savanna and woody dominance. The conversion from a patch of open savanna to a bush‐encroached area is initiated by the spatial and temporal overlap of several (localized) rainfall events sufficient forAcaciagermination and establishment. With time, growth and self‐thinning will transform the bush‐encroached area into a matureAcaciastand and eventually into open savanna again. Patchiness is sustained due to the local rarity (and patchiness) of rainfall sufficient for germination of woody plants as well as by plant‐soil interactions.

Comparative ecophysiology of Tamarix ramosissima and native trees in western U.S. riparian zones

Over the past century, the natural flow regimes of the major western U.S. rivers have been altered by dams, flow regulation and diversion of water for human use. As a result, the floodplains of many rivers have become drier and more saline than in the pre-dam era, and riparian water tables have declined. These conditions have favored the replacement of native mesic trees such as Populus spp. (cottonwood) and Salix spp. (willow) by saltcedar, ( Tamarix ramosissima), an introduced, stress-tolerant shrub from Eurasia. Saltcedar is now the dominant woody species on many perennial rivers systems in the arid southwestern U.S. and northwest Mexico A review of the research literature shows that saltcedar has greater salt tolerance, drought tolerance, resistance to water stress, and fire tolerance than mesic native trees. On the other hand, under a natural flow regime, native trees are competitive with saltcedar in germination and establishment during a flood year and they have equal or faster growth rates. On rivers that still experience a pulse flood regime or where floods have been reestablished, cottonwood and willow have shown the ability to establish despite the presence of saltcedar. Contrary to previous reviews, the current evidence does not support the conclusion that saltcedar has unusually high evapotranspiration rates or leaf area index that would allow it to dessicate water courses. According to most researchers, an effective management strategy for saltcedar must include the return of a more dynamic hydrological regime to regulated rivers, allowing saltcedar and native trees to coexist to maximize the habitat value of the riparian zone.

Multi-scale patterns and bush encroachment in an arid savanna with a shallow soil layer

Abstract Question: Bush encroachment (i.e. an increase in density of woody plants often unpalatable to domestic livestock) is a serious problem in many savannas and threatens the livelihood of many pastoralists. Can we derive a better understanding of the factors causing bush encroachment by investigating the scale dependency of patterns and processes in savannas? Location: An arid savanna in the Khomas Hochland, Namibia. Methods: Patterns of bush, grass, and soil nutrient distribution were surveyed on several scales along a rainfall gradient, with emphasis on intraspecific interactions within the dominant woody species, Acacia reficiens. Results: Savannas can be interpreted as patch-dynamic systems where landscapes are composed of many patches (a few ha in size) in different states of transition between grassy and woody dominance. Conclusions: In arid savannas, this patchiness is driven both by rainfall that is highly variable in space and time and by inter-tree competition. Within the paradigm of patch-...

The influence of woody plants and livestock grazing on grass species composition, yield and soil nutrients in the Middle Awash Valley of Ethiopia

The objectives of the study, conducted during the 2000/01 growing season in the Middle Awash Valley of Ethiopia, were to quantify the effects of two dominant woody species (Acacia tortilis and Balanites aegyptica) on grass species composition, grass dry matter (DM) yield and soil under light, medium and heavy grazing. At each study site two sub-habitats were distinguished, viz. under tree canopies and open grassland. The nutrient status of soil under the canopies of both woody species was found to be higher than the open grassland, especially with regard to soil pH, total N and organic carbon. This was more pronounced in the top 50mm of soil and on the lightly grazed site. Due to the observed association of Panicum maximum with tree canopies, the grass DM yield was higher under tree canopies, but again only at the lightly grazed site. Heavy grazing proved to be a strong overriding effect of the positive influences of the woody plants. The grass species found at the heavily grazed site were mostly annuals and less desirable. This emphasizes the importance of conservative stocking rates and proper pasture management.

Comparative seasonal gas exchange and chlorophyll fluorescence of two dominant woody species in a Holm Oak Forest

Summary Diurnal courses of net CO 2 uptake rate, stomatal conductance (g S ), maximum photochemical efficiency of PSII (Fv/Fm) and apparent photosynthetic electron transport rate (ETR) were measured in Quercus ilex and Phillyrea latifolia in a holm oak forest throughout the seasons of the year. These measurements were complemented with response curves of photosynthetic rates to PPFD and CO 2 concentrations. P. latifolia was better adapted to drought and warm conditions and showed higher net CO 2 uptake, g S and Fv/Fm values than Q. ilex in summer. But in autumn and specially in winter P. latifolia was more sensitive to low temperatures and experienced lower net CO 2 uptake, g S , ETR and Fv/Fm values than Q. ilex . The maximum net CO 2 uptake values for P. latifolia occurred under summer high temperatures whereas maximum net CO 2 uptake values for Q. ilex occurred under winter low temperatures. However, in summer during midday, both species presented null or slightly negative net CO 2 uptake rates. Since in the summer season both species experienced similar ETR values, the lower net CO 2 uptake values of Q. ilex suggest that Q. ilex presented greater photorespiration rates. During winter, very low Fv/Fm values were found especially for P. latifolia , indicating that maximal photochemical efficiency of PSII is very sensitive to low temperatures. However, they were not accompanied by low net CO 2 uptake rates showing that cold photoinhibition determined a potential but not an actual decrease in photosynthetic performance. Under well watered conditions and with high CO 2 concentration and saturated PPFD, Q. ilex was able to increase its photosynthetic rates whereas P. latifolia had lower plasticity to make a profit of optimal environmental conditions. These results show different strategies between these two dominant co-occurring species. They also indicate that the warmer and drier conditions expected for the Mediterranean region in the near decades as a result of climate change will favour drought resistant species with lower photosynthetic rates such as P. latifolia in detriment of more mesic species such as Q. ilex .

An analysis of interspecific associations of Pinus squamata with other dominant woody species in community succession

Pinus squamata is an extremely endangered pine from northeast Yunnan Province of China, which was reported from only one locality where there are 32 individuals in the field. In order to analyze its ecological relationships with other trees and shrubs, a series of techniques, including analysis of variance, χ2 test and percentage of co-occurrence, were used to analyze interspecific associations of dominant woody species in communities with P. squamata. The results indicated significantly positive correlation of the association of overall woody species in the two communities with P. squamata. There is also significantly positive correlation in the community on the north facing slope, but only a positive correlation in the community on the south-west facing slope. Considering that the community on the south-west facing slope is mixed coniferous and broad-leaved forest and that on the north facing slope is evergreen broad-leaved forest, the results imply that the communities with P. squamata may be in succession from mixed coniferous and broad-leaved forest to evergreen broad-leaved forest. Combined with the data of χ2 test and percentage of co-occurrence, we conclude that P. squamata has no significant correlation with other woody species, and its co-occurrence with other trees may be by chance. Finally, its requirements for survival may be threatened by evergreen broad-leaved woody species.