Water quality and wetland vegetation responses to water level variations in a university stormwater reuse reservoir: Nature-based approaches to campus water sustainability.

In response to climate-driven water shortages, Duke University in 2014 constructed a water reuse reservoir and wetland complex (Pond) to capture urban stormwater and recycle water to provide campus cooling and reduce downstream loading of nutrients and sediment into Jordan Lake, a regional water supply. We postulated that even with significant water level changes due to withdrawals, the Pond would function to reduce downstream nutrients and sediment once wetland plants became established in the littoral zone. Throughout the project (2015-2021), baseflow nutrient concentrations downstream decreased, with Unfiltered Total Nitrogen (UTN) falling by 44 % and Unfiltered Total Phosphorus (UTP) by 50 %. Storm mean concentrations decreased by 31 % for UTN, 54 % for UTP, and 72 % for Total Suspended Solids (TSS). The annual reductions in mass fluxes (UTN, UTP, and TSS) were between 58 and 85 % across a range of storm intensities. Regardless of water level, temperature, pH, and oxygen concentrations downstream were not significantly changed. Between 2015 and 2020, a littoral survey of planted and naturally introduced species showed that wetter years resulted in a greater number of species across a gradient of three inundation zones (i.e., moist, wet, and aquatic). Conversely, dryer years resulted in fewer species across overlapping zones. The dominant plants that successfully colonized the Pond are all obligate wetland species despite the Pond's highly variable water depths and periods of inundation. The final plant populations were dominated by invasive native species supporting the self-design theory of plant succession as nearly half of the original planted species died. The reuse Pond design (pond-wetland complex) showed the capability of using stormwater runoff for campus cooling while improving water quality services and providing habitat for wetland plants. Thus, campuses with watershed runoff capture capability should consider a nature-based recycling approach as part of their water sustainability program.

Litterfall and nutrient transfer dynamics in a successional gradient of tropical dry forest in Colombia / Dinámica de la hojarasca y transferencia de nutrientes en un gradiente sucesional de bosque seco tropical en Colombia

Introduction: The litterfall production, foliar nutrient dynamics and decomposition are essential to maintain nutrient cycling, soil fertility, and carbon regulation in terrestrial ecosystems. With several studies addressing the variation of these processes, their dynamics in tropical dry forests (TDFs) remain unclear, due to its complex interaction of biotic and abiotic factors. Objective: To evaluate litterfall, nutrient potential return and use efficiency, and decomposition variation in a TDF successional gradient in Tolima, Colombia. Methods: We quantified litterfall from November 2017 to October 2019 in 12 plots distributed in four successional stages: initial, early, intermediate, and late forests. We identified key tree species in foliar litter production and characterized the foliar decomposition of these species. At the community level, we quantified the C, N and P potential return, the N and P use efficiency, and the C:N and N:P ratio. Subsequently, we analyze relationships between vegetation characteristics and some soil chemical properties with these ecological processes. Results: We found that total litterfall in late forests (8.46 Mg ha-1 y-1) was double that found in initial forests (4.45 Mg ha-1 y-1). Decomposition was higher in initial (k = 1.28) compared to intermediate (k = 0.97) and late forests (k = 0.87). The nutrient potential return didn't change along succession, but it did show differences between study sites. The structural development and species richness favored litterfall, while soil chemical conditions influenced nutrient returns and decomposition. Conclusions: TDFs could recover key ecosystem function related to litterfall and nutrient dynamics after disturbances cessation; however, the soil quality is fundamental in return and release of nutrients.

Introducción: La producción de hojarasca, la dinámica de nutrientes foliares y la descomposición son esenciales para mantener el ciclo de nutrientes, la fertilidad del suelo y la regulación del carbono en ecosistemas terrestres. Con diversos estudios que abordan estos procesos, su variación en los bosques secos tropicales (BSTs) permanece incierta, por su compleja interacción de factores bióticos y abióticos. Objetivo: Evaluar la caída de hojarasca, el retorno potencial de nutrientes y eficiencia de uso, y la variación en descomposición en un gradiente sucesional de un BST en Tolima, Colombia. Métodos: Cuantificamos la caída de hojarasca entre noviembre 2017 y octubre 2019 en 12 parcelas distribuidas en cuatro estados sucesionales: bosque inicial, temprano, intermedio y tardío. Identificamos las especies arbóreas clave en la producción de hojarasca y caracterizamos la descomposición foliar de estas especies. A nivel comunitario, cuantificamos el retorno potencial de C, N y P, la eficiencia de uso de N y P y la relación C:N y N:P. Posteriormente, analizamos las relaciones entre las características de la vegetación y algunas propiedades químicas del suelo con estos procesos ecológicos. Resultados: Encontramos que la caída total de hojarasca en los bosques tardíos (8.46 Mg ha-1 año-1) fue el doble de la hallada en bosques iniciales (4.45 Mg ha-1 año-1). La descomposición fue mayor en bosques iniciales (k = 1.28) en comparación con bosques intermedios (k = 0.97) y tardíos (k = 0.87). El retorno potencial de nutrientes no cambió con el avance de la sucesión vegetal, pero exhibió diferencias entre los sitios de estudio. El desarrollo estructural y la riqueza de especies favorecieron la caída de hojarasca, mientras que las condiciones químicas del suelo influyeron en el retorno de nutrientes y descomposición. Conclusiones: Los BSTs tienen la capacidad de recuperar la función ecosistémica de aporte de hojarasca fina, retorno y liberación de nutrientes después del cese de alteraciones antrópicas; sin embargo, la calidad del suelo es fundamental en el retorno y liberación de nutrientes.

Echoes of the past: Agricultural legacies shape the successional dynamics of protected semi-natural dry grasslands.

European semi-natural dry grasslands are among the most endangered terrestrial ecosystems, being recognised as habitats of community interest by the EU Habitats Directive. The occurrence and preservation of these habitats depend on a combination of anthropogenic and natural factors, although little is known regarding the role of past land-use changes. Here, we investigated the role of time since cultivation abandonment as a major driver of grassland successional dynamics in the Mediterranean agro-pastoral system of Alta Murgia, southern Italy. By integrating cartographic information on the past agricultural land-use with the main abiotic constraints (patch area, slope and aspect), we used generalised additive mixed models to test for the probability of occurrence of current grassland habitat types along time since cultivation abandonment (10 to 200 years). Our results disclosed the successional sequence of grassland plant communities since crop abandonment in the study area, highlighting that the distribution of semi-natural grassland communities largely depends on land use history besides current environmental patterns. Among the habitat types protected under the EU Habitats Directive, we highlighted that xero-thermic communities may represent an intermediate step of grassland succession after cultivation abandonment, while more mesic perennial communities indicate a late successional stage. These successional dynamics are further modulated by mesoclimatic conditions associated with slope and aspect, especially in case of long-standing pastures that were not historically affected by agricultural transformations. Our findings can contribute to a deeper understanding of dynamics relevant to spontaneous vegetation recovery in open environments, which is a prerequisite for setting up effective grassland conservation and restoration actions. Furthermore, our results underline the value of integrating historical maps and current information for the assessment of habitat conservation status.

Restoration of Vegetation Greenness and Possible Changes in Mature Forest Communities in Two Forests Damaged by the Vaia Storm in Northern Italy.

Windstorms are rare in the Southern Alps, but their frequency is increasing due to climate change. This research analyzed the vegetation of two spruce forests in Camonica Valley (Northern Italy) destroyed by the Vaia storm to evaluate the vegetation responses to blowdown damage. In each study area, the normalized difference vegetation index (NDVI) was used to evaluate the change in plant cover and greenness from 2018 (before the Vaia storm) to 2021. Furthermore, floristic-vegetation data were analyzed to identify current plant communities and develop models of plant succession. The results showed that the two areas, although located in different altitudinal vegetation belts, are undergoing the same ecological processes. NDVI is increasing in both areas, and pre-disturbance values (~0.8) should be reached in less than ten years. Nevertheless, the spontaneous restoration of pre-disturbance forest communities (Calamagrostio arundinaceae-Piceetum) should not occur in both study areas. In fact, the two plant succession trends are characterized by pioneer and intermediate stages with young trees of Quercus petraea and Abies alba, typical of more thermophilic mature forest communities compared to pre-disturbance ones. These results could reinforce the trend of the upward shift in forest plant species and plant communities in response to environmental changes in mountain areas.

Ecosystem Coupling and Ecosystem Multifunctionality May Evaluate the Plant Succession Induced by Grazing in Alpine Meadow.

Most alpine meadow on the Tibetan Plateau are at different stages of community succession induced by grazing practices. Quantifying the succession sequence and assessing the dynamics of plant composition, ecosystem coupling, and multifunctionality across successional stages are essential for reasonable restoration of degraded alpine meadow. Here, we selected areas with different grazing disturbance histories and used them as a space series (i.e., space-for-time substitution) to study the community succession. Our work quantified the plant succession sequence of alpine meadow induced by grazing with plant functional group approach. The plant succession sequence is from the tall sedge community with erect growth to the short undesirable toxic forbs community with prostrate growth. Ecosystem coupling, ecosystem multifunctionality and their relationships were all the lowest in Stage 4. Compared to Stage 4, the ecosystem multifunctionality index increased in Stages 1, 2, and 3 by 102.6, 89.8, and 207.6%, respectively; the extent of ecosystem coupling increased by 20.0, 16.8, and 21.2%, respectively. Our results indicated that the driving factors of ecosystem coupling and ecosystem multifunctionality were soil factor individual in early successional stage to plant-soil simultaneously in late successional stage. Our results also highlighted the importance of toxic weeds during the late stage of degraded succession and suggest that the expansion of toxic plants is a consequence of their greater suitability from a successional perspective. The findings of this study would provide valuable guidance for optimizing the management and restoration practice of alpine meadow.

Short-Term Temporal Patterns in Herbivore Beetle Assemblages in Polyculture Neotropical Forest Plantations.

Although insect herbivorous communities in tropical forests are known to exhibit strong seasonality, few studies have systematically assessed temporal patterns of variation in community structure and plant-herbivore interactions in early successional arboreal communities. We assessed seasonal and interannual variation of the diversity and composition of herbivorous beetles and the tree-herbivore network in a recently established polyculture forest plantation, during the dry and the rainy seasons of 2012 and of 2013. Species richness was similar between years, while the ecological diversity was higher in 2012. Comparing seasons, no differences were found in 2012, whereas in 2013, the species richness and ecological diversity were higher during the dry season. The species composition differed radically across years and seasons. Moreover, a quantitative nested pattern was consistently found across both temporal scales, more influenced by species densities. We found temporal changes in the species strength, whereas connectance and interaction evenness remained stable. Rapid temporal changes in the structural complexity of recently established polyculture plantations and the availability and quality of the trophic resources they offer may act as drivers of beetle diversity patterns, promoting rapid variation in herbivore composition and some interacting attributes. Nonetheless, network structure, connectance, and interaction evenness remained similar, suggesting that reorganizations in the distribution of species may determine the maintenance of the patterns of interaction. Further work assessing long-term temporal dynamics of herbivore beetle assemblages are needed to more robustly relate diversity and interaction patterns to biotic and abiotic factors and their implications in management programs.

Temporal Dynamics of Soil Virus and Bacterial Populations in Agricultural and Early Plant Successional Soils.

As reported in many aquatic environments, recent studies in terrestrial ecosystems implicate a role for viruses in shaping the structure, function, and evolution of prokaryotic soil communities. However, given the heterogeneity of soil and the physical constraints (i.e., pore-scale hydrology and solid-phase adsorption of phage and host cells) on the mobility of viruses and bacteria, phage-host interactions likely differ from those in aquatic systems. In this study, temporal changes in the population dynamics of viruses and bacteria in soils under different land management practices were examined. The results showed that bacterial abundance was significantly and positively correlated to both virus and inducible prophage abundance. Bacterial and viral abundance were also correlated with soil organic carbon and nitrogen content as well as with C:N ratio. The seasonal variability in viral abundance increased with soil organic carbon content. The prokaryotic community structure was influenced more by land use than by seasonal variation though considerable variation was evident in the early plant successional and grassland sites. The free extracellular viral communities were also separated by land use, and the forest soil viral assemblage exhibiting the most seasonal variability was more distinct from the other sites. Viral assemblages from the agricultural soils exhibited the least seasonal variability. Similar patterns were observed for inducible prophage viral assemblages. Seasonal variability of viral assemblages was greater in mitomycin-C (mitC) induced prophages than in extracellular viruses irrespective of land use and management. Taken together, the data suggest that soil viral production and decay are likely balanced but there was clear evidence that the structure of viral assemblages is influenced by land use and by season.

Artemisia halodendron Litters Have Strong Negative Allelopathic Effects on Earlier Successional Plants in a Semi-Arid Sandy Dune Region in China.

Artemisia halodendron Turcz. ex Besser occurs following the appearance of a pioneer species, Agriophyllum squarrosum (L.) Moq., with the former replacing the latter during the naturally vegetation succession in sandy dune regions in China. A previous study revealed that the foliage litter of A. halodendron had strong negative allelopathic effects on germination of the soil seed bank and on the seedling growth. However, whether this allelopathic effect varies with litter types and with the identity of plant species has not yet been studied. We, therefore, carried out a seed germination experiment to determine the allelopathic effects of three ltter types of A. halodendron (roots, foliage, and stems) on seed germination of six plant species that progressively occur along a successional gradient in the semi-arid grasslands in the Horqin Sandy Land of northeastern China. In line with our expectation, we found that the early-successional species rather than the late-successional species were negatively affected by A. halodendron and that the allelopathic effects on seed germination increase with increasing concentration of litter extracts, irrespective of litter types. Our study evidenced the negative allelopathic effects of A. halodendron on the species replacement and on the community composition during dune stabilization in the Horqin Sandy Land. Further studies are needed to better understand the successional process and thus to promote the vegetation restoration in that sandy dune region as A. halodendron itself disappeared also during the process.

Plant-topsoil relationships underlying subalpine grassland patchiness.

Approximately half of the area in the Spanish Central Pyrenees is dedicated to pastures. A decrease in stocking rate coupled with changes in livestock management in recent decades have favoured the expansion of Nardus grasslands, which are considered undesirable for grazing use and for diversity conservation. The objective of this study was to analyse how topsoil properties are related to grassland plant composition occurring in erosion-disturbed (chalk grasslands) and undisturbed (Nardus mat-grasslands) soils in a subalpine area of the Spanish Central Pyrenees. We selected six paired sampling points for a side-by-side comparison of both communities. At each point, we 1) estimated the plant cover of each species through inventories and 2) analysed a set of physical-chemical topsoil properties (0-5 and 5-10 cm depth). Data were analysed through multivariate analysis. We found typical species of Nardus mat-grasslands in the undisturbed sites growing on non-eroded and well-structured soils that were low in calcium and acidic, with high contents of organic matter. In turn, we found earlier-successional grassland communities growing on slopes recently affected by soil erosion processes. The species composition was mainly species from stony slope grasslands and, to a lesser extent, from the long-term snow-covered environments of the high mountains. These soils were shallower and stonier and had a less-stable structure, higher pH, and lower organic matter and calcium content than undisturbed soils. Our results suggest that the differences between both communities emerge and are maintained by soil-plant feedback mechanisms mediated in Nardus mat-grasslands through soil stabilization and acidification and in chalk grasslands through soil erosion and basification. These findings suggest that the subalpine grassland mosaic results from a model of non-equilibrium plant coexistence due to soil disturbance and inexorable succession. Management should be focused on maintaining a disturbance regime, through grazing, sufficient to prevent the spreading of Nardus mat-grasslands.

Long-term succession of aquatic plants reconstructed from palynological records in a shallow freshwater lake.

Aquatic plants in shallow freshwater lakes play a key role in stabilizing ecological function and providing valuable ecosystem services, yet they are severely degraded worldwide. An improved understanding of long-term aquatic plant succession is critical to investigate the potential driving mechanisms and to facilitate ecological restoration. In this paper, we reconstructed changes in the aquatic plant community over the past century based on palynological records from Changdang Lake, Middle and Lower Yangtze River Basin (MLYB), China. Our results reveal that aquatic plants in Changdang Lake have undergone three clear phases: emergent macrophytes dominated the aquatic vegetation in the 1900s-1970s, submerged macrophytes in the 1970s-1990s, and floating macrophytes increasingly after the 1990s. Significant changes in the aquatic plant communities were caused by increasing anthropogenic pressures, such as damming and nutrient loading from agriculture, aquaculture, and urbanization after the Chinese economic reform. We argue that Changdang Lake is currently in a transition phase between a macrophyte-dominated state and an algae-dominated state. Our palynological record is different from many contemporary studies, which suggest submerged plants dominated most lakes in this region before the 1950s. We suggest that the return of the aquatic plants to their 1970s-1980s state would be a realistic target for lake restoration. Our results show that palynological records can reveal long-term dynamics of macrophytes in shallow lakes for sustainable lake restoration and management.

Community divergence and convergence along experimental gradients of stress and disturbance.

We created 24 mesocosms containing mixtures of herbaceous species arranged along experimentally maintained gradients of stress (external abiotic constraints limiting biomass production) and disturbance (events causing partial or total live biomass destruction) in order to determine the degree to which community assembly is deterministic or historically contingent during succession. In this 7-yr experiment, we found taxonomic divergence and functional convergence during the last 3 yr. Although communities became more functionally dissimilar as the difference in the level of stress increased, they were equally taxonomically different irrespective of the amount of difference between them in terms of stress and disturbance. In addition, comparing communities experiencing the same conditions, taxonomic community structure was more dissimilar as the levels of stress and disturbance decreased. Therefore, community assembly was largely deterministic from a functional perspective but more historically contingent from a taxonomic perspective, and the relative importance of taxonomic historical contingency decreased as the levels of stress and disturbance frequency increased.

Plant Communities Rather than Soil Properties Structure Arbuscular Mycorrhizal Fungal Communities along Primary Succession on a Mine Spoil.

Arbuscular mycorrhizal fungal (AMF) community assembly during primary succession has so far received little attention. It remains therefore unclear, which of the factors, driving AMF community composition, are important during ecosystem development. We addressed this question on a large spoil heap, which provides a mosaic of sites in different successional stages under different managements. We selected 24 sites of c. 12, 20, 30, or 50 years in age, including sites with spontaneously developing vegetation and sites reclaimed by alder plantations. On each site, we sampled twice a year roots of the perennial rhizomatous grass Calamagrostis epigejos (Poaceae) to determine AMF root colonization and diversity (using 454-sequencing), determined the soil chemical properties and composition of plant communities. AMF taxa richness was unaffected by site age, but AMF composition variation increased along the chronosequences. AMF communities were unaffected by soil chemistry, but related to the composition of neighboring plant communities of the sampled C. epigejos plants. In contrast, the plant communities of the sites were more distinctively structured than the AMF communities along the four successional stages. We conclude that AMF and plant community successions respond to different factors. AMF communities seem to be influenced by biotic rather than by abiotic factors and to diverge with successional age.

Los bosques de Abies guatemalensis Rehder de San Marcos, Guatemala: una oportunidad para su restauración ecológica / The forests of Abies guatemalensis Rehder of San Marcos, Guatemala: an opportunity for ecological restoration

El pinabete, Abies guatemalensis Rehder, es una especie distribuida en las montañas altas de Guatemala. Según elConsejo Nacional de Áreas Protegidas (Conap), sus poblaciones se encuentran severamente fragmentadas. Porsu rareza y fragmentación, esta especie se encuentra protegida por el Convenio sobre el Comercio Internacional deEspecies Amenazadas de Fauna y Flora Silvestre (Cites), lo que pone en relieve la importancia de la recuperaciónde sus bosques para su debida conservación. El éxito de su establecimiento puede mejorase al sembrarla junto a plantas nodriza, que le ayudarían a sobrevivir a las condiciones secas del año y las heladas que se presentan en las zonas de mayor altitud del país durante los meses de noviembre a febrero. Este trabajo muestra las características físicas, químicas y de vegetación del área de tres bosques de pinabete (A. guatemalensis) de la parte alta de San Marcos. Además, presenta la propuesta de restauración ecológica estimulada simulando la sucesión vegetal del área. Se elaboraron mapas, y describieron tres bosques de pinabete y sus alrededores; se presenta el estudio de la composición química del suelo y la vegetación y su representación en diferentes etapas de sucesión vegetal. Además, se propone la recuperación de áreas alrededor de parches de bosque, consistente en establecer plantas arbustivas en lugares abiertos donde aún no hay; por su parte en aquellas áreas donde ya se encuentren arbustos, se propone utilizarlos como plantas nodriza para establecer árboles de varias especies buscando mantener biodiversidad deespecies arbóreas, en donde la teoría de la sucesión indica que el pinabete va a dominar al final. Esta propuesta hademostrado viabilidad porque es una práctica conocida fuera de bosques naturales en áreas de San Marcos paraestablecer el pinabete con fines comerciales, que aprovecha la sucesión vegetal natural del área.

The fir, Abies guatemalensis Rehder, is distributed in the highlands of Guatemala. According to the NationalProtected Areas Council (Conap), their populations are severely fragmented. For its rarity and fragmentation,it is protected by the Convention on International Trade in Endangered Species of Wild Fauna and Flora species(Cites), indicating the importance of the recovery of their forests for the proper conservation of the species. Thesuccess of his establishment may be enhanced to sow with nurse plants, which would help to survive the dryconditions of the year and frosts that occur in the higher elevations of the country during the months of Novemberto February. This work shows the features physical and chemical and vegetation in the area of three forests of fir (A. guatemalensis) of the top of San Marcos. In addition, the proposal of ecological restoration simulating plantsuccession in the area. Maps were drawn, and three fir forests and surrounding described; it is shown the study the chemical composition of soil and vegetation and their representation in different ages of plant succession. Further, it is proposed the recovery of areas around patches of forest, consting of which is establish shrubby plants in open spaces where there are not yet; in areas where bushes already are, it is proposed to use it as nurse plants to establish trees of various species seeking to maintain biodiversity of tree species, where the theory of succession indicates that the fir will dominate in the end. This proposal is feasible because as it is known outside natural forests inareas of San Marcos to establish the fir for commercial purposes, which uses the natural plant succession the area.

Effects of root decomposition on plant-soil feedback of early- and mid-successional plant species.

Plant-soil feedback (PSF) is an important driver of plant community dynamics. Many studies have emphasized the role of pathogens and symbiotic mutualists in PSFs; however, less is known about the contribution of decomposing litter, especially that of roots. We conducted a PSF experiment, where soils were conditioned by living early- and mid-successional grasses and forbs with and without decomposing roots of conspecific species (conditioning phase). These soils were used to test growth responses of conspecific and heterospecific plant species (feedback phase). The addition of the roots of conspecifics decreased the biomass of both early- and mid-successional plant species in the conditioning phase. In the feedback phase, root addition had positive effects on the biomass of early-successional species and neutral effects on mid-successional species, except when mid-successional grasses were grown in soils conditioned by conspecifics, where effects were negative. Biomass of early- and mid-successional forbs was generally reduced in soils conditioned by conspecifics. We conclude that root decomposition may increase short-term negative PSF effects, but that the effects can become neutral to positive over time, thereby counteracting negative components of PSF. This implies that root decomposition is a key element of PSF and needs to be included in future studies.

Effect of fire on a monodominant floating mat of Cyperus giganteus Vahl in a neotropical wetland / Efeito do fogo sobre uma comunidade monodominante de Cyperus giganteus Vahl em uma área úmida neotropical

The rhizomatous Cyperus giganteus, abundant in the Pantanal wetland, can dominate extense floodable areas as monodominant communities. The Jacadigo lake has a large area of C. giganteus, where we performed an evaluation on community structure during two months in 2010, before it was hit by a wildfire which top-killed the vegetation, compared to ten months post-fire. We utilized 40 plots of 1m × 1m, along permanent trails, assessing two strata: the upper, near the inflorescence of adult plants, and the lower, close to the water level. Our results show that fire does not affect dominance of C. giganteus, as it maintained the same cover as before fire; species richness is not much altered either - 28 before fire and 34 thereafter. Fire changed the floristic composition, due to the annual variation of species and the ability of some plants to colonize gaps and to regrow after fire from underground organs and seeds. The stratification of the vegetation with characteristic species of upper and lower strata was similar after fire.

A planta rizomatosa Cyperus giganteus, encontrada no Pantanal, pode dominar extensas áreas inundáveis, formando uma comunidade monodominante chamada localmente de pirizal. A lagoa do Jacadigo, situada na sub-região de Nabileque, possui uma extensa área de pirizal onde foi realizada uma avaliação na estrutura da vegetação durante dois meses no período de 2010, antes de ser atingida por um incêndio que causou perda da cobertura aérea da vegetação e foi comparada com 10 meses de avaliação pós-fogo. Com indicações na literatura de que o rizoma da planta pode favorecer a espécie através de rebrotamento após um incêndio, o objetivo deste trabalho foi avaliar as variações na estrutura da vegetação de pirizal após um evento de fogo. Foram utilizadas 40 parcelas não fixas de 1m × 1m e alocadas sistematicamente através de trilhas avaliando dois estratos: o superior, próximo a inflorescência dos adultos e o inferior, próximo a lâmina d’água. Nos dois meses logo após o fogo, devido ao tamanho reduzido nos indivíduos de C. giganteus foi avaliado apenas um estrato denominado total. Os resultados mostram que o fogo não afeta a dominância de C. giganteus, que se mantém na mesma faixa de cobertura do período anterior ao fogo; a riqueza de espécies não é alterada, 28 no período anterior ao fogo e 34 no período posterior. Após o incêndio, a estrutura da comunidade sofre alteração na composição florística, devido a mudança anual de espécies e a capacidade que certas plantas possuem em colonizar clareiras e rebrotar após o fogo por meio de órgãos subterrâneos e sementes. A estratificação da vegetação com espécies características de estrato superior e inferior se manteve após o fogo, porém com diferença menos acentuada entre as espécies.

Open and disturbed habitats support higher diversity of Syrphidae (Diptera)? A case study during three yr of sampling in a fragment of Araucaria forest in southern Brazil.

The diversity of hoverflies in five different habitats of the Parque Estadual de Vila Velha, Ponta Grossa, Paraná, Brazil, from September 1999 to August 2002 was studied. These five areas had been exposed to various types of anthropogenic disturbance at different times, which resulted in different stages of plant succession. In total, 2,841 specimens of 151 species of Syrphidae were collected using malaise traps. The highest species richness and abundance were found at the edge of the forest (Forest Edge), whereas the lowest was found in the most conserved areas. Evenness values increased along the succession gradient. The accumulation curve of new occurrences of syrphid species nearly approached an asymptote starting on the third month of the third year. Estimates of syrphid species richness using different methods indicated that between 155 and 288 species are found in the study area, according the Michaelis-Menten and the Chao2 estimators, respectively. The richness and abundance of representatives of the subfamily Syrphinae were higher in more open areas, where herbaceous plants predominate. Nineteen hoverfly species can be considered as environmental indicators, as they were collected exclusively or mainly in one of the defined habitat types. The degree of opening of the vegetation seems to be the factor determining the distribution of Syrphidae species, a conclusion based on the finding that grassy clearings in Araucaria Forests had more species. Hence, we conclude that, to become established, Syrphidae communities need conserved mosaic landscapes.

Interpretation of nitrogen isotope signatures using the NIFTE model.

Nitrogen cycling in forest soils has been intensively studied for many years because nitrogen is often the limiting nutrient for forest growth. Complex interactions between soil, microbes, and plants and the consequent inability to correlate δ15N changes with biologic processes have limited the use of natural abundances of nitrogen isotopes to study nitrogen (N) dynamics. During an investigation of N dynamics along the 250-year-old successional sequence in Glacier Bay, Alaska, United States, we observed several puzzling isotopic patterns, including a consistent decline in δ15N of the late successional dominant Picea at older sites, a lack of agreement between mineral N δ15N and foliar δ15N, and high isotopic signatures for mycorrhizal fungi. In order to understand the mechanisms creating these patterns, we developed a model of N dynamics and N isotopes (Nitrogen Isotope Fluxes in Terrestrial Ecosystems, NIFTE), which simulated the major transformations of the N cycle and predicted isotopic signatures of different plant species and soil pools. Comparisons with field data from five sites along the successional sequence indicated that NIFTE can duplicate observed patterns in δ15N of soil, foliage, and mineral N over time. Different scenarios that could account for the observed isotopic patterns were tested in model simulations. Possible mechanisms included increased isotopic fractionation on mineralization, fractionation during the transfer of nitrogen from mycorrhizal fungi to plants, variable fractionation on uptake by mycorrhizal fungi compared to plants, no fractionation on mycorrhizal transfer, and elimination of mycorrhizal fungi as a pool in the model. The model results suggest that fractionation during mineralization must be small (˜2‰), and that no fractionation occurs during plant or mycorrhizal uptake. A net fractionation during mycorrhizal transfer of nitrogen to vegetation provided the best fit to isotopic data on mineral N, plants, soils, and mycorrhizal fungi. The model and field results indicate that the importance of mycorrhizal fungi to N uptake is probably less under conditions of high N availability. Use of this model should encourage a more rigorous assessment of isotopic signatures in ecosystem studies and provide insights into the biologic transformations which affect those signatures. This should lead to an enhanced understanding of some of the fundamental controls on nitrogen dynamics.

Rooting depth, water availability, and vegetation cover along an aridity gradient in Patagonia.

Above-and belowground biomass distribution, isotopic composition of soil and xylem water, and carbon isotope ratios were studied along an aridity gradient in Patagonia (44-45°S). Sites, ranging from those with Nothofagus forest with high annual rainfall (770 mm) to Nothofagus scrub (520 mm), Festuca (290 mm) and Stipa (160 mm) grasslands and into desert vegetation (125 mm), were chosen to test whether rooting depth compensates for low rainfall. Along this gradient, both mean above-and belowground biomass and leaf area index decreased, but average carbon isotope ratios of sun leaves remained constant (at-27‰), indicating no major differences in the ratio of assimilation to stomatal conductance at the time of leaf growth. The depth of the soil horizon that contained 90% of the root biomass was similar for forests and grasslands (about 0.80-0.50 m), but was shallower in the desert (0.30 m). In all habitats, roots reached water-saturated soils or ground water at 2-3 m depth. The depth profile of oxygen and hydrogen isotope ratios of soil water corresponded inversely to volumetric soil water contents and showed distinct patterns throughout the soil profile due to evaporation, water uptake and rainfall events of the past year. The isotope ratios of soil water indicated that high soil moisture at 2-3 m soil depth had originated from rainy periods earlier in the season or even from past rainy seasons. Hydrogen and oxygen isotope ratios of xylem water revealed that all plants used water from recent rain events in the topsoil and not from water-saturated soils at greater depth. However, this study cannot explain the vegetation zonation along the transect on the basis of water supply to the existing plant cover. Although water was accessible to roots in deeper soil layers in all habitats, as demonstrated by high soil moisture, earlier rain events were not fully utilized by the current plant cover during summer drought. The role of seedling establishment in determining species composition and vegetation type, and the indirect effect of seedling establishment on the use of water by fully developed plant cover, are discussed in relation to climate change and vegetation modelling.

Relationships between the effects of insect herbivory and sheep grazing on seasonal changes in an early successional plant community.

The effects of spring grazing by sheep and of natural levels of insect herbivory were studied in 1985 on a limestone field abandoned from arable land for four years. A split-plot design was adopted in which paddocks, arranged in Latin squares, were either left ungrazed or heavily grazed by sheep for ten days in April. Within each paddock plots were either sprayed regularly with Malathion-60 or untreated.Natural levels of insect herbivory, compared to the reduced levels in insecticide-treated plots, had effects of similar magnitude to those from the short burst of spring grazing. Many attributes of the grazed/insecticide-treated sward were either increased or decreased by a factor of two within a season. Both types of herbivore caused changes in the direction of plant succession as well as in its rate. Effects on early successional species were large and similar when caused by either type of herbivore. Effects on later successional species were often smaller, but also showed differences in the action of the two herbivore types, as did effects on sward height, species richness and total cover. The effects of sheep and insect herbivory were not always additive or in the same direction.The results suggest that manipulations of both mammal and insect herbivores may be powerful tools for directing changes in plant community composition.