Tree age and soil phosphorus conditions influence N2-fixation rates and soil N dynamics in natural populations of Acacia senegal

Acacia senegal, an important leguminous tree in arid and semi-arid environments, has shown promise as a multipurpose species, including gum production and soil fertility improvement, linked with N₂-fixation capabilities. Of particular interest are ontogenetic and edaphic effects on A. senegal performance in natural populations. Our research objectives were to investigate the effect of tree age and site phosphorus conditions on (1) tree N₂-fixation and (2) soil N and C dynamics in natural stands of A. senegal var. senegal, Baringo District, in the Rift Valley, Kenya. Sites consisted of A. senegal saplings (9 months) and mature A. senegal trees (7 years) along an edaphic gradient of soil P availability. A single-tree neighborhood approach was employed using a two by two factorial design: site conditions [high and low soil P contents] and tree age class [juvenile and mature]. Soil (N and C pools and fluxes) and plant metrics were quantified. A soil transfer experiment was also employed to confirm age and site effects on soil N mineralization. On the high soil P site, A. senegal had significantly lower foliar (¹⁵N levels than neighboring non-leguminous species (Balanites aegyptiaca), while foliar δ¹⁵N values in A. senegal on the low P site exhibited no significant difference with our reference plant, B. aegyptiaca. Across P sites, B. aegyptiaca had similar foliar δ¹⁵N values. These results indicate that the rate of N₂-fixation of A. senegal trees, as determined with foliar ¹⁵N natural abundance methodology, increased with increasing soil P availability in these natural populations. However, N₂-fixation rates declined with age. Although soil texture and soil CO₂ efflux did not differ between sites or across ages, soils under mature A. senegal at the high P site exhibited significantly greater total N content and total C content in comparison to soils at the low P site and under juvenile plants. Furthermore, under mature A. senegal trees, soil N mineralization rates were significantly greater as compared to under saplings. Soil transplants confirmed that soil microbial activity may be stimulated under mature trees as N mineralization rates were 2-3 fold greater compared to under A. senegal saplings. Our findings suggest that tree age and soil P availability are important factors in the nitrogen budget of natural populations of A. senegal, determining N₂-fixation rates, and potentially influencing soil total N and C pools and soil mineral N. This study provides information regarding the adaptation of A. senegal under differing edaphic conditions thus increasing accuracy of management support for A. senegal populations as productive agroforests.     

Abiotic factors related to the incidence of the Austrocedrus chilensis disease syndrome at a landscape scale

In this paper, the incidence of the A. chilensis disease syndrome in the “16 de Octubre” Valley (Chubut, Argentinean Patagonia) was related to landscape climatic, topographic and edaphic attributes, using remote sensing, geographic information systems and statistical methods. A strong relationship between the occurrence and incidence of the A. chilensis disease syndrome and site variables related to poor soil drainage was found. Non-allophanized soils with fine textures on flat and wavy soil phases, geomorphologies associated to alluvial processes, and low elevations and gentle slopes were positively related to the incidence of the disease. These relationships at a landscape scale agree with previous studies carried out at the stand level. A logistic predictive model of diseased occurrence was developed for the study area considering aspect, elevation, slope, mean annual precipitation and soil phase (classified according to predominant slopes).     

Diversity and structure of woody vegetation across areas with different soils in Gonarezhou National Park,Zimbabwe

Despite the importance of edaphic factors in influencing woody vegetation diversity and structure in savannas, there is still limited research on the topic across most savanna ecosystems. Here we investigate the differences and/or similarities of woody vegetation diversity and structure across areas with different edaphic factors (i.e. soil group) in Gonarezhou National Park, Zimbabwe. We stratified our study area into two strata based on soil group, namely siallitic soil in northern Gonarezhou and regosol soil in central Gonarezhou. Data were collected from 96 sample plots between March and April in 2011. Our results showed significantly higher woody species diversity in siallitic soil stratum compared to regosol soil stratum. In contrast, there were no significant differences in woody vegetation structure between the two study strata. Our results suggest that soil variations play an important role in influencing woody vegetation diversity more than woody vegetation structure in Gonarezhou. Future studies should investigate the role of soils on a specific woodland type's composition and structure, and also determine the interactive effects between soils and other environmental determinants in Gonarezhou and similar protected areas in savanna ecosystems.     

Soil surface properties in Mediterranean mountain ecosystems: Effects of environmental factors and implications of management

Understanding soil processes is fundamental to the success of forest restoration programs. We compared different types of soils in Mediterranean mountain forests with respect to their edaphic environments and influence of vegetation cover and lithology. We then used this information to determine the suitability of current forest restoration programs in these ecosystems.     

The impact of former agriculture on habitat conditions and distribution patterns of ancient woodland plant species in recent black alder (Alnus glutinosa (L.) Gaertn.) woods in south-western Poland

Little is known about the influence of former agricultural use of soils on the forest recovery process in post-agricultural black alder (Alnus glutinosa) woods – the most fertile and the wettest forest habitats among the European temperate forest types. Thus, studies focusing on edaphic, hydrological and light conditions responsible for colonization mechanisms present in such woodlands adjoining ancient ones were undertaken in south-western Poland. In the 16 m² quadrats of the 33 transects laid out perpendicularly across the ancient-recent forest boundary, data were collected on herb layer composition, chemical soil properties, as well as illumination level. Additionally, groundwater level in the spring months was recorded in piezometers. The number and cover of true woodland herbs were higher in ancient woods, regardless of forest type. Soils in ancient woodlands reached higher levels of Al³⁺, K⁺, cation exchange capacity (CEC), available K, P, and organic C, whereas their pH was lower. The illumination level of the forest floor was greater in recent woods. Linear regression showed that recent wood age had a negative effect on pH and base cations, but positive on Fe²⁺, Ca²⁺, available P and Mg, CEC, and on C and N contents. CCA results showed that woodland age, pH, humus type, groundwater level, available Mg and K were always among those variables having the highest contribution in explaining the distribution pattern of woodland species in recent woods. Total N and available P contents were always higher in ancient woodland soils than in recent, and their content grew with time. Thus, they cannot be treated as indicators of former agricultural use of recent alder wood soils. Urtica dioica, Poa trivialis and Galium aparine, the three competitive herbs, avoided sites with a high level of groundwater, combined with poor illumination level. In order to create the best possible conditions allowing for effective forest recovery in habitats of such high fertility, it is essential to maintain a good water regime and shade in the forest floor. This in turn reduces the competitive exclusion of woodland flora by the aggressive herbs and facilitates the immigration of typical woodland herbs.     

The decline of Austrocedrus chilensis forests in Patagonia, Argentina: soil features as predisposing factors

The forests of Austrocedrus chilensis in southern Argentina suffer mortality from “mal del ciprés”, whose causes remain unknown. The purpose of this work was to establish the relation of soil features with the occurrence of the disease. In Río Grande Valley, Chubut Province, Argentina, 14 areas with “mal del ciprés” were selected for study. The spatial pattern of the decline varied among the different areas and was classified as aggregated and disaggregated. In each area, symptomatic and asymptomatic plots were established and characterized by 11 edaphic and topographic variables. Three forest areas where the disease was totally absent were also included. Site features were related to the occurrence of the decline using principal component analysis and cluster analysis. Results indicated that soil properties related to poor internal drainage, such as the proximity to water streams, non-allophanized soils of fine textures, and redoximorphic features, act as predisposing factors to the development of “mal del ciprés”. Poor soil drainage was strongly associated not only with the occurrence of the disease, but also with its spatial pattern. Symptomatic and asymptomatic plots presented similar edaphic features in areas with a disaggregated distribution of the decline and were grouped together in the multivariate analysis. This result suggests that large areas with such a pattern are prone to develop the decline.     

Naturally revegetated forest governed by mudflow induced sediment heterogeneity

We investigated an upstream area of the 1926 Taisho Mudflow that occurred at Mount Tokachi, a volcano in central Hokkaido, Japan, to clarify the relationship between natural forest succession and mudflow-induced sediment characteristics. The study area was divided into three parts, i.e., undisturbed, deposition, and scoured areas, based on disturbance regimes. The deposition area was further divided into three different forest stands for a final total of five forest stand types. We assumed that the mudflow regimes created sediment edaphic heterogeneity and undisturbed and island forests supplied seeds for natural revegetation. The undisturbed forest stand comprised pioneer species, whereas a mosaic forest consisting of almost pure stands characterized by Betula ermanii and Picea glehnii developed in the mudflow. This indicates that each plant species has a characteristic ability to establish and adapt initially and later develop into a mosaic forest according to sediment edaphic conditions, particularly depth, grain size distribution, and water and nutrient gradients. The differences in forest species composition and a 30-year time lag between the development of forest stands at the distal edges and the center explain how the cross-sectional sediment edaphic heterogeneity created by the mudflow regimes affected succession and forest development. Furthermore, plants are specific to sediment depth and texture, as well as moisture and nutrient availability, which play important roles in their growth and development; thus, forest stands with contrasting species and age structures developed in the mudflow.     

Effects of serpentine soil factors on Virginia pine (Pinus virginiana) seedlings

Effects of simulated serpentine soil conditions (elevated Mg:Ca ratio and Ni concentration) on seedlings from populations of Virginia pine (Pinus virginiana Mill.) from serpentine and non-serpentine sites were evaluated in sand culture. We determined (1) how seedlings are affected by elevated Mg:Ca ratio and Ni concentrations, (2) if there are interactive effects between Mg:Ca ratio and Ni concentrations on seedling growth, needle pigment concentrations, and nutrition, and (3) if Virginia pine populations from serpentine areas are edaphic ecotypes. A Mg:Ca ratio of 5 and 50 microM Ni both reduced seedling growth compared with control seedlings grown in the presence of the standard Mg:Ca ratio of 0.5 and no Ni. Interactive effects between Mg:Ca ratio and Ni concentrations were highly significant for growth, foliar pigments, and needle and root elemental concentrations. Nickel-mediated reductions in growth and foliar pigment concentrations were less at the serpentine Mg:Ca ratio of 5 than at the standard (non-serpentine) Mg:Ca ratio of 0.5. Foliar N was reduced by Ni concentrations as low as 10 microM, and foliar and root K, Ca and P concentrations were significantly reduced by Ni concentrations above 25 microM, with greater reductions at a Mg:Ca ratio of 0.5 than at a Mg:Ca ratio of 5. There were no population x serpentine soil factor interactions for seedling growth, foliar pigment concentrations, or nutrition, suggesting that seedlings from trees growing on serpentine soils are not edaphic ecotypes. We conclude that serpentine conditions present at the site of seed collection have not resulted in the selection of edaphic ecotypes of Virginia pine with respect to Mg:Ca ratio and Ni concentration.     

Distribution of Selected Timber Species of a Central African Rain Forest in Relation to Topography and Soil Heterogeneity: Implications for Forest Management

Several studies have looked at the relative importance of habitat association of tree species in maintaining tropical tree species diversity at a variety of scales including the importance of edaphic factors. The objective of this study was to test hypotheses on the distribution of important timber species in relation to topographic position and soil chemical parameters. Tree species from a Central African rain forest were selected within a 100-ha plot in southwestern Central Africa Republic. Individuals of Lovoa trichilioides, Nesogordonia papaverifera, Pterocarpus soyauxii, Terminalia superba ≥ 10 cm diameter at breast height (dbh) were inventoried then mapped to topography using a survey laser. A torus translation was performed to test for associations between tree species distribution and topographic position as well as soil mineral nutrients for three of the four species. Trees ≥ 30 cm dbh of all three species showed no association with topographic position. However, the distribution of T. superba was significantly associated with high concentrations of soil mineral nutrients. Evidence from mineral nutrient levels indicates that extrapolating topography to define habitat and soil type can be very misleading. Because many of Central Africa's most important timber species have distributions linked to edaphic factors, it is clear that soils must be considered equally as much as topographic position for management plans of logging concessions within Central African tropical forests.     

Vegetation-environment relationships in the forests of Chitral district Hindukush range of Pakistan

We investigated the composition of plant communities to quantify their relationships with environmental parameters in the Chitral Hindukush range of Pakistan. We sampled tree vegetation using the Point Centered Quarter (PCQ) method while understory vegetation was sampled in 1.5-m circular quadrats. Cedrus deodara is the national symbol of Pakistan and was dominant in the sampled communities. Because environmental variables determine vegetation types, we analyzed and evaluated edaphic and topographic factors. DCA-Ordination showed the major gradient as an amalgam of elevation (p<0.05) and slope (p<0.01) as the topographic factors correlated with species distribution. Soil variables were the factors of environmental significance along DCA axes. However, among these factors, Mg2+ , K + and N2+ contributed not more than 0.054% 0.20% and 0.073%, respectively, to variation along the first ordination axis. We conclude that the principal reason for weak or no correlation with many edaphic variables was the anthropogenic disturbance of vegetation. The understory vegetation was composed of perennial herbs in most communities and was most dense under the tree canopy. The understory vegetation strongly regulates tree seedling growth and regeneration patterns. We recommend further study of the understory vegetation using permanent plots to aid development of forest regeneration strategies.     

沱江上游深丘地区不同立地土壤抗蚀性、渗透性及其影响因素

通过对沱江上游深丘地区不同立地土壤抗蚀性、渗透性与其各影响因素进行了研究,分析了土壤物理性状各因素及草本盖度、林分类型、郁闭度单独及综合对土壤抗蚀性、渗透性产生的影响,找出了不同立地条件下影响土壤抗蚀性、渗透性的主导因素,并结合当地情况初步探讨了成因,以期为土壤改良措施的选择提供依据。研究发现:1)该地区人为干扰严重,整体上土壤砂性较重,土壤物理结构较差,不利于保水保肥;2)该区影响土壤抗蚀性和渗透性的主导因素为林分类型和林下植被盖度,其中混交林提高土壤抗蚀性和渗透性,改良土壤的效果最好。     

Models for the estimation of single tree volume in four Miombo woodland types

Single tree volume estimation models are produced for four Julbernardia-Brachystegia woodlands in Malawi. The estimation models are based on the regression of volume on one or a number of independent variables derived from diameter and height. In total, 1282 trees from 46 species were felled and individually measured. Site-specific models were produced for the dominant species and important species groups, with separate models for canopy and under-storey species groups from three sites. Compared to diameter, height was a poor predictor of volume and only derivations of diameter were used wherever possible. The models may have wider applicability, particularly in woodlands with similar structure, climatic and edaphic conditions, but should be validated since their applicability outside of the four sample sites has not yet been tested.     

Land‐use changes influence the sporulation and survival of Phytophthora agathidicida,a lethal pathogen of New Zealand kauri (Agathis australis)

Phytophthora agathidicida is the accepted causal agent of dieback in remnant stands of long‐lived indigenous New Zealand kauri (Agathis australis) and poses a significant threat to the long‐term survival of this species. Little is known about the effect of key soil physicochemical characteristics on the growth of P. agathidicida. In this study, we investigated the growth of P. agathidicida in soils collected from adjacent areas under original kauri forest, short rotation pine (Pinus radiata) plantation forest and grazed pastures. A growth response assay was used to quantify asexual (sporangia) and sexual (oospore) spore counts over 8 days in soils sampled from each land‐use. Significantly higher numbers of sporangia (p < 0.001) and oospores (p < 0.01) were found in pasture and pine forest soil within 2 days of the growth assay trials, suggesting these soils may favour asexual/sexual reproduction in the early stages of P. agathidicida establishment compared to kauri forest soils. Additionally, oospore production significantly increased over 8 days in pine forest soil, suggesting that with an increase in inoculum loads, these soils potentially act as pathogen reservoirs. The soil physicochemical properties (e.g., pH, C and N, phosphorus content and electrical conductivity) investigated in this study did not significantly correspond to spore count data between land‐uses, suggesting that differences in growth response are driven by other edaphic factors not explored in the present study.     

The role of graveyards in species conservation and beta diversity:a vegetation appraisal of sacred habitats from Bannu,Pakistan

Graveyards or sacred groves are often places of natural vegetation protected by spiritual believers because of their sacred beliefs and indigenous culture.A study of graveyards was conducted to determine their role in species conservation,community formation,and associated indicators and species composition using multivariate statistical approaches.It was hypothesized that variations in the age of graveyards would give rise to diverse plant communities under the impact of various edaphic and climatic factors.Quantitative ecological techniques were applied to determine various phytosociological attributes.All the data were put in MS Excel for analysis in PCORD and CANOCO softwares for cluster analysis(CA),two-way cluster analysis(TWCA),indicator species analysis and canonical correspondence analysis.CA and TWCA through Sorenson distance measurements identified five major graveyard plant communities:(1) FicusBougainvillea-Chenopodium;(2) Acacia-Datura-Convolvulus;(3) Ziziphus-Vitex-Abutilon;(4) Acacia-Lantana-Salsola;and(5) Melia-Rhazya-Peganum.Species such as Capparis decidua,Herniaria hirsuta,Salvadora oliedes and Populus euphratica were only present inside graveyards rather than outside and advocate the role of graveyards in species conservation.The impact of different environmental and climatic variables plus the age of the graveyards were also assessed for comparison of plant communities and their respective indicator species.The results indicate that higher chlorine concentration,age of graveyards,low soil electrical conductivity,lower anthropogenic activities,higher nitrogen,calcium and magnesium concentrations in the soil,and sandy soils were the strong environmental variables playing a significant role in the formation of graveyard plant communities,their associated indicators and species distribution patterns.These results could further be utilized to evaluate the role of edaphic and climatic factors,indicator species and conservation management practices at a greater scale.     

Coarse woody debris in late-successional Picea abies forests in northern Europe: Variability in quantities and models of decay class dynamics

Coarse woody debris pools and their decay class dynamics were studied in three areas of unmanaged boreal forest in northern Finland, and in the Murmansk and Arkhangelsk provinces in northwestern Russia. The study areas had varying climatic and edaphic conditions, and disturbance histories. Living and dead trees (diameter at 1.3 m height ≥10 cm) were measured in five late-successional Picea abies-dominated stands in each of the three areas. Wood density and time since death were determined from randomly sampled dead P. abies, and their decay class dynamics were modeled using stage-based matrix models.     

Understory plant communities of boreal mixedwood forests in western Canada: Natural patterns and response to variable-retention harvesting

We examined patterns of variation in richness, diversity, and composition of understory vascular plant communities in mixedwood boreal forests of varying composition (broadleaf, mixedwood, conifer) in Alberta, Canada, before and for 2 years following variable-retention harvesting (clearcut, 20 and 75% dispersed green tree retention, control). Broadleaf-dominated forests differed from mixedwood or conifer-dominated forests in that they had greater canopy cover, litter depth, soil nitrogen, warmer soils, as well as greater shrub cover, herb and shrub richness and diversity (plot scale). In contrast, conifer, and to a lesser extent mixedwood, forest had greater β diversity than broadleaf forest. Overall, mixedwood and conifer forests were similar to one another, both differed from broadleaf forest. Several species were found to be significant indicators of broadleaf forest but most of these also occurred in the other forest types. Understory composition was related to canopy composition and edaphic conditions. Variable-retention harvesting had little effect on understory cover, richness, or diversity but resulted in reduced richness and β diversity at a larger scale. The clearcut and 20% treatments affected composition in all forest types. Early successional species and those common in disturbed sites were indicators of harvesting while evergreen, shade-tolerant understory herbs were indicators of the control forest and 75% retention harvest. We conclude that it is important to maintain a range of variation in canopy composition of mixedwood forests in order to conserve the associated understory communities. The presence of conifers in these forests has a particularly important influence on understory communities. The threshold for a lifeboat effect of variable-retention harvesting is between 20 and 75% retention. Examination of richness and β diversity at a variety of scales can provide interesting information on effects of harvesting on spatial reorganization and homogenization of understory plant communities.     

Application of aluminium toxicity indices to soils under various forest species

Several aluminium toxicity indices were applied to acid soils developed from various parent materials (granodiorite, slate and limestone) and sustaining various tree species (Quercus robur, Pinus radiata and Eucalyptus nitens), in Galicia, NW Spain. Some indices are based on the composition of the soil solid or liquid phases and some on the composition of the leaf or root tissues. The base cation saturation of the exchange complex was used as an index based on the composition of the soil solid phase. The indices based on the soil solution composition were various cation species concentrations, including phytotoxic aluminium species, or concentration ratios. The concentrations of Al and Ca and the Ca/Al molar ratio were used as indices based on the leaf and root tissue compositions. The application of various indices led to different results. According to indices based on the soil solution or plant tissue compositions, the greatest problems are expected in soils developed from limestone or granodiorite and sustaining P. radiata; fewer problems are expected in soils developed from slate and sustaining Q. robur. On the contrary, the saturation of the exchange complex predicts the highest aluminium toxicity risk in soils developed from slate and sustaining Q. robur, and the lowest risk in soils sustaining E. nitens.     

Subsidy or stress? Tree structure and growth in wetland forests along a hydrological gradient in Southern Europe

In forested wetlands, hydrology exerts complex and sometimes compensatory influences on tree growth. This is particularly true in semi-arid ecosystems, where water can be both a limiting resource and a stressor. To better understand these relationships, we studied hydrologic and edaphic controls on the density, growth, tree architecture and overall productivity of forested wetlands dominated by the tree species Alnus glutinosa and Salix atrocinerea in Southern Europe. We sampled 49 plots set within 21 stands in the Atlantic coastal zone of the Iberian Peninsula, and quantified woody composition, size structure (diameter and height), and radial growth using dendrochronology. Plots were grouped into three saturation classes to compare tree growth characteristics (tree density, degree of sprouting, live basal area and productivity) across levels of saturation. We used Principal Component Analysis (PCA) to create integrated explanatory factors of hydrology, soil nutrient status and soil texture for use in linear mixed models to predict stand characteristics. Increased site saturation favoured a shift in species dominance from Alnus to Salix and resulted in a higher degree of multi-stemmed tree architecture (‘shrubbiness’), particularly for Alnus. Radial growth was negatively correlated with long-term soil saturation; however, annual productivity on a per-tree basis varied by species. Alnus growth and tree density were negatively correlated with waterlogging and fine-textured soils, possibly due to anaerobiosis in the rooting zone. In contrast, Salix growth was more influenced by nutrient limitation. Overall site productivity as measured by annual basal area increment decreased with prolonged saturation. In summary, soil saturation appears to act as a chronic stressor for tree species in this ecosystem. However, these species persist and maintain a dominant canopy position in the most permanently flooded patches through increased sprouting, albeit at a reduced rate of overall biomass accumulation relative to well-drained sites. The diversity in functional responses among wetland forest species has important implications for the conservation and management of these ecosystems. The sustainable management of these ecosystems is directly tied to their vulnerability to changing hydrological conditions. Non-equilibrium modifications to the hydrologic regime from land use and climate change, which are particularly severe in semi-arid regions, may further decrease productivity, integrity and resilience in these stress-adapted communities.     

Agroforestry systems: sources of sinks of greenhouse gases?

The prominent role of forestry and agroforestry systems in the flux and long-term storage of carbon (C) in the terrestrial biosphere has increased global interest in these land-use options to stabilize greenhouse gas (GHG) emissions. Preliminary assessments suggest that some agroforestry systems (e.g., agrosilvicultural) can be CO₂ sinks and temporarily store C, while other systems (e.g., ruminant-based silvopastoral systems) are probably sources of GHG (e.g., CH₄).Agroforestry systems can be significant sources of GHG emissions, especially at low latitudes. Practices such as tillage, burning, manuring, chemical fertilization, and frequent disturbance can lead to emission of CO₂, CH₄, and N₂O from soils and vegetation to the atmosphere. Establishment and management of agroforestry systems incompatible with prevailing edaphic and climatic conditions can accelerate soil GHG emissions. Non-sustainable agroforestry systems are quickly degraded, and woody and herbaceous crops can become significant GHG sources. Silvopastoral systems can result in soil compaction and erosion with significant loss of labile C and N compounds to the atmosphere. Ruminant-based silvopastoral systems and rice paddy agrisilvicultural systems are well documented sources of CH₄ which significantly contribute to the global CH₄ budget.Early assessments of national and global terrestrial CO₂ sinks reveal two primary beneficial attributes of agroforestry systems: 1) direct near-term C storage (decades to centuries) in trees and soils, and, 2) potential to offset immediate GHG emissions associated with deforestation and subsequent shifting agriculture. Within the tropical latitudes, it is estimated that one ha of sustainable agroforestry can provide goods and services which potentially offset 5–20 ha of deforestation. At a global scale, agroforestry systems could potentially be established on 585–1275×10⁶ ha of technically suitable land, and these systems could store 12–228 (median 95) Mg C ha^–1 under current climate and edaphic conditions.The US Government right to retain a non-exclusive, royalty free licence in and to any copyright is acknowledged.