Islands Of Fertility Research Articles

Effects of Caragana microphylla patch and its canopy size on “islands of fertility” in a Mongolian grassland ecosystem

In arid and semiarid regions, variations in “islands of fertility” accompanied by discontinuous vegetation is frequently observed. However, the effects of vegetation patches on soil, including the influence of canopy size, are not fully understood, particularly under conditions of severe grazing. We examined the effects of patches of mound-forming shrub, Caragana microphylla, and the plant’s canopy size on these islands of fertility in a heavily grazed Mongolian grassland. In 11 patches with various canopy sizes (32.5–180 cm in diameter), we compared the chemical properties of soils among three microsites: Mound, Below, and Around, which were inside, below, and outside of C. microphylla mounds, respectively. Total carbon (C) and most essential elements for the plants were more concentrated in Mound, but total nitrogen (N) and nonlimiting elements, such as exchangeable sodium (Na), did not significantly differ among microsites. Larger canopies more strongly affected the enrichment of total C and most essential elements, including total N, in Mound. These results suggest that C. microphylla patches substantially enrich total C and most essential elements and that the extent of enrichment was intensified with canopy size. However, under severe grazing, total N may be relatively more affected by the redistribution of resources through grazing, particularly when the canopy size is small.

Grazing ungulates select for grasses growing beneath trees in African savannas

In savannas, isolated large trees can form ‘islands of fertility’, referring to their elevated soil nutrients and their effect on light and water availability in their direct surroundings. Consequently, a quality difference between understorey grasses and open grassland can develop, creating patches of highly nutritious forage for grazing ungulates. Grass species composition beneath and outside of tree canopies was determined in a savanna system of Kruger National Park, South Africa. Direct observations were used to test whether grazing ungulates, i.e., impala, Burchell's zebra and blue wildebeest graze relatively more often beneath than outside of large tree canopies. Additionally, it was investigated whether they selected feeding locations according to allometric scaling and to their sex, and if feeding behaviour was influenced by weather conditions. Instantaneous scan sampling showed that ungulates preferred beneath-canopy grasses, independently of weather conditions. Grass species composition differed beneath and outside tree canopies, and beneath-canopy grasses exhibited more bite marks than outside canopy grasses. Blue wildebeest grazed least often beneath canopies compared to the other species. Females of impala were found to feed on beneath-canopy forage more often than males. Thus, large isolated trees strongly influenced the feeding behaviour of grazing ungulates. The current decline in large savanna trees should therefore be retarded and protective measures should be taken.

Spatial variation in biodiversity and trophic structure of soil nematode communities in a subtropical savanna parkland: Responses to woody plant encroachment

Woody plant encroachment into grasslands generally increases spatial heterogeneity of soil properties and processes and creates “islands of fertility” beneath woody canopies. However, little is known regarding the potential for these changes to influence spatial variation in soil fauna. We quantified population sizes, biodiversity, and trophic structure of soil nematode communities in a savanna parkland where woody plant clusters developed in grassland during the past century. Total nematode density was constant across transects from centers of woody clusters into grasslands. Family richness and Simpson's Dominance Index indicated nematode communities in grasslands and grassland/cluster edges were significantly more diverse than positions within the woody clusters. Relative densities of all nematode trophic groups changed significantly along the transect. Bacterivores nearly doubled in relative density from grassland (35%) to cluster centers (60%), apparently in response to higher concentrations of soil microbial biomass in wooded areas. Relative densities of plant parasitic nematodes decreased along transects from grasslands (35%) to centers of woody clusters (10%), implying decreased nematode herbivory in woody clusters despite much higher root biomass there. The structure index indicated nematode communities within woody clusters were more simplified than those in grassland and edge communities due to reductions in densities of omnivores and predators. Although nematode densities were lower at 10–20 than 0–10 cm, nematode community characteristics were generally similar between soil depths. Changes in nematode community and trophic structure described here could influence biogeochemical processes, species interactions, and successional processes in regions where woody plants are encroaching into grasslands.

Spatial Heterogeneity of Soil Nutrients and Respiration in the Desertified Grasslands of Inner Mongolia, China

There is a limited knowledge of spatial heterogeneity in soil nutrients and soil respiration in the semi-arid and arid grasslands of China. This study investigated the spatial differences in soil nutrients and soil respiration among three desertified grasslands and within two shrub-dominated communities on the Ordos Plateau of Inner Mongolia, China in 2006. Both soil organic carbon (SOC) and total nitrogen (TN) were significantly different ( P < 0.01) among the three desertified grasslands along a degradation gradient. Within the two shrub-dominated communities, the SOC and TN contents decreased with increasing distance from the main stems of the shrub, and this “fertile island” effect was most pronounced in the surface soil. The total soil respirations during the growing season were 131.26, 95.95, and 118.66 g C m −2, respectively, for the steppe, shrub, and shrub-perennial grass communities. The coefficient of variability of soil respiration was the highest in the shrub community and lowest in the steppe community. CO 2 effluxes from the soil under the canopy of shrub were significantly higher than those from the soil covered with biological crusts and the bare soil in the interplant spaces in the shrub community. However, soil respiration beneath the shrubs was not different from that of the soil in the inter-shrub of the shrub-perennial grass community. This is probably due to the smaller shrub size. In the two shrub-dominated communities, spatial variability in soil respiration was found to depend on soil water content and C:N ratio.

Termite-induced heterogeneity in African savanna vegetation: mechanisms and patterns

Objectives: To (1) assess the strength of evidence for the role of termites in vegetation heterogeneity in African savannas, and (2) identify the mechanisms by which termites induce such heterogeneity. Location: African savannas. Methods: We conducted a review of the literature, a meta-analysis and qualitative systems analysis to identify mechanisms to explain the observed patterns. Results: The review provided evidence for termite-induced heterogeneity in floristic composition and vegetation patterning in savannas across Africa. Termites induced vegetation heterogeneity directly or indirectly through their nest-building and foraging activities, associated nutrient cycling and their interaction with mammalian herbivores and fire. The literature reviewed indicated that termite mounds essentially act as islands of fertility, which are responsible for ecosystem-level spatial heterogeneity in savannas. This was supported by the meta-analysis, which demonstrated that mounds of Ancistrotermes, Macrotermes, Odontotermes (family Macrotermitinae), Cubitermes (family Termitinae) and Trinervitermes (Nasutitermitinae) are significantly enriched in clay (75%), carbon (16%), total nitrogen (42%), calcium (232%), potassium (306%) and magnesium (154%) compared to the surrounding savanna soil. Conclusions: Termite activity is one of the major factors that induce vegetation patterning in African savannas. The implications of this are discussed and research questions for future studies and modelling efforts are indicated.

Under-canopy microclimate within sand dunes in the Negev Desert

Summary Scattered shrubs are a common phenomenon in many arid landscapes. Once established, shrubs are known to create “islands of fertility”, i.e., preferential habitats for annuals and animals. In an attempt to characterize the physical conditions prevailing under the shrub, radiation, temperatures and soil moisture (0–40 cm) following rain were measured during 1993–1995 at the shaded under-canopy (UC) and at the exposed inter-shrub habitat (EXP) of two pairs of shrubs located at the north- and south-facing slopes of dunes in the Nizzana research site, western Negev Desert, Israel. In addition, the soil organic matter (SOM) and the fine (silt and clay) content (FC) were also measured. Whereas the differences in the amounts of SOM and FC were small, daylight temperatures at UC were substantially lower (6–15 °C), subsequently resulting in extended time during which the UC habitat remained wet. Moisture was retained for up to 10.5–42.6% longer at UC in comparison to EXP, mainly explained by the shading effect. SOM was found to explain only 8.6–19.6% of the results. By shading, shrubs in the Negev Desert may thus provide relatively wetter conditions for annuals, rendering them an advantage over inter-shrub habitats at this harsh arid environment.

Islands of fertility: Soil improvement under indigenous homegardens in the savannas of Roraima, Brazil

Homegardens are a common feature of indigenous dwellings in the savannas of Roraima, northern Brazil. In order to evaluate the effect of homegardens on soils, samples were taken in 5 sites each in the categories new homegardens (0–10-years old), established homegardens (15–35-years old) and old homegardens (more than 40-years old) and in adjacent savanna in Araca Indigenous Land, Roraima, Brazil. For comparison, samples were also taken in forest islands located nearby, on a different soil type, under 10-year-old forest fallows and high forest. P and K showed the greatest increases over time in homegarden soils, in comparison with levels found in adjacent savanna and under forest. Ca and Mg also increased in comparison to adjacent savanna, but levels were less than found in forest soils, most likely due to the different parent material. Zn and Fe also showed increases in homegarden soils over time. Cu and Mn levels showed little relation to homegarden age, suggesting greater effects of factors of soil formation than anthropogenic influences. Values for pH were slightly higher in homegardens than in adjacent savanna, while Al was lower, although these changes were poorly fit by regression models. Soil organic matter increased over time under homegardens, but still was lower than levels found under fallows and forest. Soil fertility improvement under indigenous homegardens can be attributed mainly to deposition of residues around dwellings, although further investigation is needed on the role of trees in accessing nutrient pools at greater depths in savanna soils.

Effects on soil, microclimate and vegetation of the native-invasive Retama monosperma (L.) in coastal dunes

The aim of this study was to analyse and quantify the effects of the canopy of the native-invasive N-fixer woody shrub Retama monosperma in the dune ecosystem, affecting the structure and function of the dune environment as well as plant community, in the context of the facilitation mechanism. Air temperature and relative humidity; soil pH, electric conductivity, organic matter (OM) and nutrient content; above and below-ground vegetation biomass, litter mass, species richness and Shannon diversity were determined and compared from sampling plots below the R. monosperma canopy and in canopy gaps within a coastal dune system in SW Spain. The relationships between soil OM and nutrient contents and above and below-ground vegetation biomass, litter mass, species richness and Shannon diversity were also assessed. A predominance of positive interactions was confirmed. The canopy of R. monosperma ameliorated temperature extremes beneath, and soil OM and nutrient concentrations were increased by 188–466%, compared to those found in gaps. Plant biomass increased by 442% beneath the canopy and was composed almost exclusively of herbaceous annuals. Plant diversity was not affected. Plant communities were clearly structured as fertility islands, distributed in an environmentally stressful dune matrix characterized by scarce vegetation cover and low biomass.

The spatial extent of termite influences on herbivore browsing in an African savanna

Termite mounds form islands of fertility in savanna landscapes and create foraging hotspots for herbivores, but the magnitude and spatial extent of these influences is unknown. We mapped terrain, termite mound and woody vegetation three-dimensional (3-D) structure at 56 cm resolution across a large-scale (254 ha), long-term (34 years) herbivore exclusion experiment in the Kruger National Park, with the Carnegie Airborne Observatory (CAO). We compared vegetation 3-D structure in areas protected from herbivores with those accessible to herbivores, both on termite mounds and in the landscape matrix between termite mounds. Termite mound density was 1.1 ha −1 across the study area and mound size did not differ between protected and accessible areas. Woody vegetation canopy cover was ∼100% greater on protected than accessible mounds, but was only ∼20% greater in the protected inter-mound matrix when compared to the accessible matrix. Woody canopy height class distributions differed significantly between protected and accessible areas, with the tallest vegetation (>10 m) occurring on protected termite mounds. The impacts of herbivore browsing were evident at distances of up to 20 m from termite mound centres. Spatial analysis of mound distribution revealed that the sphere of termite mound influence constitutes ∼20% of the total landscape. Termite influences on herbivore browsing operate at scales much larger than the spatial extent of their mound building activities.

Forestry insularity effect of four Mimosa L. species (Leguminosae-Mimosoideae) on soil nutrients of a Mexican semiarid ecosystem

Information about forestry insularity of plants on soil nutrients will be critical for selecting plant species for agrosilvopastoral or fertility reclamation programs in dry ecosystems. We explored the effects of four Mimosa species (M. lacerata, M. luisana, M. polyantha and M. texana var. filipes) and of rainfall seasonal variation on soil nutrients in a semiarid ecosystem located at the Tehuacan-Cuicatlan Valley, Mexico. Soil samples were taken from outside and under the canopy at three positions (trunk, middle, edge) in all four Mimosa species; ten plants per species. The soil pH, organic matter (SOM), organic carbon (SOC), total nitrogen (Nt), available phosphorus (Pi), and major cations (Ca, Mg, K and Na) were determined. Our results showed that Mimosa species improve the soil under their canopies creating fertile islands with higher SOM, SOC, total N and Pi cycling than the soil in open areas (OA). The insularity effect was significantly species-dependent, where SOM, SOC, Nt and Pi decreased consistently from trunk to OA in all four Mimosa species; however, magnitude varied among species. Likewise, differences in the quantity of soil cations were observed among Mimosa species; though, an insularity gradient trunk-open area was not observed. All these effects were consistent across the species studied and showed little seasonal variability, suggesting a strong forestry insularity of Mimosa species on soil fertility. Of all the four Mimosa species studied, M. lacerata was the most effective in accumulating SOM and nutrients in the soil, for which it would be a good option to implement in agrosilvopastoral or fertility reclamation programs in this semiarid ecosystem.

Interactions Between Soil Erosion Processes and Fires: Implications for the Dynamics of Fertility Islands

Shrub encroachment in arid and semiarid rangelands, a worldwide phenomenon, results in a heterogeneous landscape characterized by a mosaic of nutrient-depleted barren soil bordered by nutrient-enriched shrubby areas known as “fertile islands.” Even though shrub encroachment is considered as a major contributor to rangeland degradation, little is known about mechanisms favoring the reversibility of the early stages of this process. Here we synthesize the interactions between fires and soil erosion processes, and the implications of these interactions for management of rangelands. The burning of shrub vegetation develops relatively high levels of soil hydrophobicity. This fire-induced water repellency was shown to enhance the soil erodibility in and around burned shrub patches. The fire-induced enhancement of local-scale soil erosion results from changes in the interparticle bonding forces between the soil grains, thus altering the way moisture is retained in the soil. It has been shown—with a number of wind-tunnel studies, field-scale manipulative experiments, microtopographic measurements, and isotopic tracer studies—how the fire-erosion interactions affect the dynamics of fertility islands. Further we propose a new conceptual model of resource “island” dynamics that explains some of the findings previously reported in the literature on the interactions between aeolian processes and arid-land vegetation. In particular, we highlight the ability of fires to enhance the erodibility of nutrient-rich soils accumulated under the shrubs favoring the redistribution of soil resources, thereby contributing to the reversibility of the early stages of shrub encroachment. La invasión de arbustos en pastizales áridos y semiáridos, un fenómeno mundial, genera un paisaje heterogéneo caracterizado por un mosaico de suelo estéril sin nutrientes, rodeado por un área de arbustivas enriquecida en nutrientes que se conocen como “islas de fertilidad”. Aunque el aumento de los arbustos se considera como el mayor factor de la degradación de los pastizales, poco se sabe acerca de los mecanismos que favorecen el regreso a las etapas iniciales de este proceso. Aquí resumimos la interacción entre fuego y el proceso de erosión del suelo y las implicaciones de estas interacciones relacionadas al manejo de pastizales. La quema de la vegetación arbustiva genera relativamente altos niveles de hidrofobicidad en el suelo. Este fuego induce repelencia al agua y ha mostrado erosión del suelo cerca y alrededor de los espacios entre los arbustos. El fuego erosiona, a nivel local, dando como resultado cambios de las fuerzas de enlace entre partículas entre los granos de suelo y por tanto alterando la forma de retención de la humedad. Se ha demostrado con un número de estudios en túneles de viento, experimentos manipulados a nivel escala-campo, con quemas controladas, y mediciones micro topográficas; y estudios con trazadores de isotopos—cómo la interacción fuego-erosión afecta la dinámica de la invasión arbustiva. Además proponemos un nuevo modelo conceptual del recurso “isla” la dinámica que explica algunos de los resultados previamente reportados en la literatura sobre las interacciones entre procesos eólicos y vegetación de zonas áridas. En particular, subrayamos la habilidad del fuego para incrementar la erosión de los suelos ricos en nutrientes acumulados bajo los arbustos favoreciendo la redistribución de los recursos del suelo, por lo tanto contribuyendo al regreso de las etapas iniciales de la invasión de arbustivas.

Termites, vertebrate herbivores, and the fruiting success of Acacia drepanolobium

In African savannas, vertebrate herbivores are often identified as key determinants of plant growth, survivorship, and reproduction. However, plant reproduction is likely to be the product of responses to a suite of abiotic and biotic factors, including nutrient availability and interactions with antagonists and mutualists. In a relatively simple system, we examined the role of termites (which act as ecosystem engineers--modifying physical habitat and creating islands of high soil fertility), vertebrate herbivores, and symbiotic ants, on the fruiting success of a dominant plant, Acacia drepanolobium, in East African savannas. Using observational data, large-scale experimental manipulations, and analysis of foliar N, we found that Acacia drepanolobium trees growing at the edge of termite mounds were more likely to reproduce than those growing farther away, in off-mound soils. Although vertebrate herbivores preferentially used termite mounds as demonstrated by dung deposits, long-term exclusion of mammalian grazers did not significantly reduce A. drepanolobium fruit production. Leaf N was significantly greater in trees growing next to mounds than in those growing farther away, and this pattern was unaffected by exclusion of vertebrates. Thus, soil enrichment by termites, rather than through dung and urine deposition by large herbivores, is of primary importance to fruit production near mounds. Across all mound-herbivore treatment combinations, trees that harbored Crematogaster sjostedti were more likely to fruit than those that harbored one of the other three ant species. Although C. sjostedti is less aggressive than the other ants, it tends to inhabit large, old trees near termite mounds which are more likely to fruit than smaller ones. Termites play a key role in generating patches of nutrient-rich habitat important to the reproductive success of A. drepanolobium in East African savannas. Enhanced nutrient acquisition from termite mounds appears to allow plants to tolerate herbivory and the reduced defense by a relatively ineffective ant partner. Our results underscore the importance of simultaneously examining top-down and bottom-up effects to understand those factors most important to plant reproductive success.

Carbon‐nitrogen interactions in fertility island soil from a tropical semi‐arid ecosystem

Summary 1. Biological nitrogen (N) fixation by symbiotic and free‐living organisms is considered the main pathway for N soil enrichment in desert and semi‐desert ecosystems. This fact is more noticeable in tropical ecosystems where legume species have a high relative abundance. However, this biological fixation pathway does not guarantee the maintenance of soil N pools, and N conservation pathways are important in understanding controls over soil N cycling. 2. In dryland ecosystems, desert plants can form a ‘fertility island’ (FI) as soils beneath plants show higher concentrations of N and organic matter. 3. Here we assess how carbon (C) and N may interact to conserve soil N within the FI soil of two legume species (Prosopis laevigata and Parkinsonia praecox), one a known N‐fixer and the other believed not to fix N, as well as within adjacent bare ground soil. In a semi‐arid tropical ecosystem in central Mexico, we examined spatial patterns in C and N pools and transformation rates, and we investigated seasonal variations in these relationships. 4. Results show that FI soil C and N could be linked to total N storage through net C and N immobilization in microbial biomass and heterotrophic microbial activity. Soil under P. laevigata canopy had greater total N as well as N accumulated in microbial biomass than soil under P. praecox and bare ground soil. Nevertheless, inorganic N and potential net N mineralization rates were similar under soils of both species, although we expected higher inorganic N and N‐mineralization values in N‐fixer species to explain the greater total N. Higher total N concentrations under P. laevigata probably result from greater inputs of organic C and a higher potential net C mineralization rate in comparison to P. praecox and bare ground soil. 5. Even though N input and output values were not measured, the results highlight the importance of assessing the role of organic C, heterotrophic microbial activity, and N storage in microbial biomass in order to understand controls over N retention in soil N cycling. Thus, soil C‐N interactions could be a control factor of N soil conservation in this tropical semi‐arid ecosystem.

Soil erosion and runoff in different vegetation patches from semiarid Central Mexico

article i nfo Vegetation patches in arid and semiarid areas are important in the regulation of surface hydrological processes. Canopy and ground covers developed in these fertility islands are a natural cushion against the impact energy of rainfall. Also, greater levels of organic matter improve the soil physicochemical properties, promoting infiltration and reducing runoff and soil erosion in comparison with the open spaces between them. During the 2006 rainy season, four USLE-type plots were installed around representative vegetation patches with predominant individual species of Huisache (Acacia sp), Mesquite (Prosopis sp), Prickly Pear or Nopal (Opuntia sp) and Cardon (Opuntia imbricata), to evaluate soil erosion and runoff, in semiarid Central Mexico. A comparative bare surface condition (Control) was also evaluated. Vegetative canopy and ground cover were computed using digital images. Selected soil parameters were determined. Soil erosion was different for the studied vegetation conditions, decreasing as canopy and ground cover increased. There were not significant differences in runoff and soil erosion between the Control and O. imbricata surfaces. Runoff was reduced by 87%, 87% and 98% and soil loss by 97%, 93%, and 99% for Acacia farnesiana, Prosopis laevigata and Opuntia sp, respectively, as compared to the Control. Soil surface physical conditions were different between the low vegetation cover conditions (Control and O. imbricata surfaces) and the greater vegetation cover conditions (A. farnesiana, P. laevigata and Opuntia sp), indicating a positive effect of vegetation patches on the regulation of surface hydrological processes.

Are Biological Effects of Desert Shrubs More Important than Physical Effects on Soil Microorganisms?

Vegetation cover plays a major role in providing organic matter and in acting as a physical barrier, with both together contributing to the formation of "fertile islands," which play an active role in prolonging biological activity in desert ecosystems. By undertaking this study, a longterm research, we designed an experiment to separate the two components-the physical and biotic parts of the perennial plants-and to identify the factor that contributes the most to the ecosystem. The study site was located in the northern Negev Desert, Israel, where 50 Hammada scoparia shrubs and 50 artificial plants were randomly marked. Soil samples were collected monthly over 3 years of research at three locations: under the canopy of H. scoparia shrubs, in the vicinity of the artificial plants, and between the shrubs (control). The contribution to microbial activity was measured by evaluation of the microbial community functions in soil. The functional aspects of the microbial community that were measured were CO2 evolution, microbial biomass, microbial functional diversity, and the physiological profile of the community. The results of this study are presented in two ways: (1) according to the three locations/treatments; and (2) according to the phenological situation of the vegetation (annual and perennial plants) in the research field: the growing phase, the drying process, and the absence of annual plants. The only parameters that were found to affect microbial activity were the contribution of the organic matter of perennial shrubs and the growth of vegetation (annual and perennial) during the growing seasons. The physical component was found to have no effect on soil microbial functional diversity, which elucidates the important contribution of the desert shrub in enhancing biological multiplicity and activity.

Did Columbus also open the exploration of the modern diet?

On the 500th anniversary of Columbus's voyage of discovery, one aspect that merits particular attention is its influence on food and nutrition worldwide. Columbus found an unexpected wealth in food resources rather than the gold he sought. The sailors came from a continent where famine was well known and had subsisted for two months on a typical seamen's diet of preserved foods, primarily hardtack and salt pork. They made landfall in green and fertile islands. The foods that constituted the core of the diet of the Americas before 1492--from maize to potatoes, beans to tomatoes, to numerous other fruits and vegetables--became the true patrimony that the inhabitants of the New World bequeathed to humanity. These foods, new to the Old World, provided the basis for the exchange and evolution of networks of food production and consumption which, with many trials and errors and with no preestablished plan, characterize modern nutrition. The diet of Mediterranean countries, now universally recommended for nutritional health, is high in several foods, tomatoes and beans in particular, that originated in the New World.

Temporal patterns of nutrient availability around nests of leaf-cutting ants ( Atta colombica) in secondary moist tropical forest

Leaf-cutting ants consume up to 10% of canopy leaves in the foraging area of their colony and therefore represent a key perturbation in the nutrient cycle of tropical forests. We used a chronosequence of nest sites on Barro Colorado Island, Panama, to assess the influence of leaf-cutting ants ( Atta colombica) on nutrient availability in a neotropical rainforest. Twelve nest sites were sampled, including active nests, recently abandoned nests (<1 year) and long-abandoned nests (>1 year). Waste material discarded by the ants down-slope from the nests contained large concentrations of nitrogen and phosphorus in both total and soluble forms, but decomposed within one year after the nests were abandoned. Despite this, soil under the waste material contained high concentrations of nitrate and ammonium that persisted after the disappearance of the waste, although soluble phosphate returned to background concentrations within one year of nest abandonment. Fine roots were more abundant in soil under waste than control soils up to one year after nest abandonment, but were not significantly different for older sites. In contrast to the waste dumps, soil above the underground nest chambers consistently contained lower nutrient concentrations than control soils, although this was not statistically significant. We conclude that the ‘islands of fertility’ created by leaf-cutting ants provide a nutritional benefit to nearby plants for less than one year after nest abandonment in the moist tropical environment of Barro Colorado Island.

Biogeochemical cycling in forest soils of the eastern Sierra Nevada Mountains, USA

We review some of the unique features of biogeochemical cycling in forests of the eastern Sierra Nevada Mountains, USA. As is the case for most arid and semi-arid ecosystems, spatial and temporal variability in nutrient contents and fluxes are quite high. “Islands of fertility” are common in these forests, a result of spatial variations in both litterfall and decomposition rates. Dry summer conditions greatly inhibit biological activity in the O horizon, and thus most annual litter decomposition takes place beneath the snowpack when moisture is available. Snowmelt duration is shortened near tree boles because of local warming, resulting in earlier drying of the O horizon, significantly lower decomposition rates, and increased O horizon mass. Water and nutrient fluxes vary spatially because of snowdrift in winter and surface runoff over hydrophobic soils in summer and fall. Moisture variability in the vertical as well as the horizontal dimension has significant consequences for nutrient fluxes. Because of the very dry summers, rooting in the O horizons is absent in these forests, and thus competition between microbes and trees for nutrients in that horizon is non-existent. Nutrients mineralized from the O horizon and not taken up by plants enrich runoff through the O horizons over hydrophobic mineral soils, resulting in very high concentrations of inorganic N and P in runoff waters. Substantial temporal variations in water and nutrient fluxes occur on a seasonal (with snowmelt being the dominant hydrologic event of the year), annual, and decadal basis. The most significant temporal variation is due to periodic fire, which we estimate causes annualized N losses that are two orders of magnitude greater than those associated with leaching and runoff. We hypothesize that fire suppression during the 20th century may have contributed to the deterioration of nearby Lake Tahoe by allowing buildups of N and P in O horizons which could subsequently leach from the terrestrial ecosystem to the Lake in runoff. In general, we conclude that biogeochemical cycling in these forests is characterized by greater spatial and temporal variability than in more mesic forest ecosystems.

Different microhabitats affect soil macroinvertebrate assemblages in a Mediterranean arid ecosystem

Spatio-temporal variability is a key factor in conservation, management and restoration of ecosystems. Spatial heterogeneity is caused in many cases by organisms that are able to modify their environments. This is especially relevant in arid systems, where organisms such as shrubs and ants create patches of high nutrient availability (fertile islands) surrounded by a low-nutrient matrix. Although variations in structure and physiology among shrubs provoke differences in their effects as fertile islands, whether different microhabitats vary in their influence on animal communities is poorly known. The principal aim of our study is to analyse the effects of different shrub species and Messor harvester ant-nest mounds on the structure of soil macroarthropod assemblages in a strongly seasonal desert location of SE Spain. Shrub microhabitats and ant-nest mounds maintained higher species density, abundance and biomass of soil macroinvertebrates than the surrounding soil matrix. The different microhabitats differed in taxonomic and trophic composition, abundance, and biomass of soil macroinvertebrates, at both litter and belowground levels. Also, variations of invertebrate abundance among microhabitats changed throughout sampling periods. Thus the spatio-temporal heterogeneity of the study site affected the distribution and dynamics of the macroinvertebrate community. The above results indicate that the spatio-temporal mosaic created by microhabitat and seasonal variations on macroinvertebrate assemblages is a relevant issue to be considered in conservation, sustainable management and environmental restoration in heterogeneous arid systems to preserve their biodiversity and ecosystem functioning.

Interaction entre érosions hydrique et éolienne sur sols sableux pâturés au Sahel : cas du bassin-versant de Katchari au nord du Burkina Faso

In the Sahel, sandy soils are widespread and support not only most of the pearl millet production, the major staple crop in the region, but also grass production for livestock. Parent sediments of these soils have an aeolian origin and are hence prone to wind erosion. Still, their clay content, even though very low, allows physical crust formation during rainfall leading to runoff and water erosion. Squall lines, major rainfall events of the rainy season, are usually preceded by intense wind. Wind and water erosions are therefore closely associated in both time and space, but they are rarely studied simultaneously. Erosion measurements were performed for two years (2001, 2002) on a small catchment of grazing land (1,4 ha) at Katchari, Burkina Faso, a location typical of the Sahel area with under 500 mm annual rainfall. Wind erosion occurs at the onset of the rainy season, from May to 15th of July, when soil cover is the lowest and before the growth of vegetation. On this non-cultivated area, the same dynamic unfolds as that recorded in millet fields in other sahelian studies. Water erosion occurs throughout the rainy season, but certain intense events produce most of the total annual erosion. Wind causes the largest sediment fluxes leading to both erosion (up to 20 Mg/ha per year) and deposition (up to 30 Mg/ha per year) depending on the area in the catchment. Water erosion is one order of magnitude lower than wind erosion and is more intense where wind erosion is highest. Hence, the same area is eroded by both wind and water. Conversely, areas where aeolian deposition occurs are less affected by water erosion and correspond to fertile islands where vegetation grows. From this study, it comes out that there is on the whole no land degradation at the catchment scale, but an intense aeolian and water dynamic leading to substantial spatial variability typical of sahelian landscapes.