Interactive effects of fertilization and disturbance on community structure and resource availability in an old-field plant community

Summary The interactive effects of fertilization and disturbance on plant community structure and resource availability were studied by supplying four levels of nitrogen and applying four intensities of tilling to a 30 year old field in a factorial design for 2 year. Live above-ground biomass, root biomass, and litter generally increased with nitrogen supply and decreased with disturbance. Species composition varied significantly, with annuals increasing with both nitrogen and disturbance, but with perennials unaffected by nitrogen and decreased by disturbance. Species diversity decreased with disturbance, but decreased with nitrogen only in undisturbed vegetation. Root: shoot ratios decreased with added nitrogen, leaf allocation decreased with disturbance, and flowering allocation increased. Surprisingly, stem allocation was unaffected by disturbance. This result reflected a shift from vertical stems to horizontal stems as disturbance increased. Resource measurements suggested that the vegetation responded to interactions between the treatments as well as to direct treatment effects. Variation in light penetration was reduced by fertilization in undisturbed vegetation but not in tilled plots; variability was not directly affected by disturbance. The availability of nitrogen, the limiting soil nutrient, increased with fertilization but was not significantly affected by disturbance. In contrast, the ratio of ammonium to nitrate was significantly reduced by disturbance but unaffected by supply rates, suggesting that nitrogen may have had different effects under different disturbance regimes, even though its total availability was constant. While many community responses to fertilization and disturbance conformed to those reported earlier, resource and allocation measurements indicated that their interactions are not always predictable from their separate effects.     

Nutrient Addition Dramatically Accelerates Microbial Community Succession

The ecological mechanisms driving community succession are widely debated, particularly for microorganisms. While successional soil microbial communities are known to undergo predictable changes in structure concomitant with shifts in a variety of edaphic properties, the causal mechanisms underlying these patterns are poorly understood. Thus, to specifically isolate how nutrients – important drivers of plant succession – affect soil microbial succession, we established a full factorial nitrogen (N) and phosphorus (P) fertilization plot experiment in recently deglaciated (∼3 years since exposure), unvegetated soils of the Puca Glacier forefield in Southeastern Peru. We evaluated soil properties and examined bacterial community composition in plots before and one year after fertilization. Fertilized soils were then compared to samples from three reference successional transects representing advancing stages of soil development ranging from 5 years to 85 years since exposure. We found that a single application of +NP fertilizer caused the soil bacterial community structure of the three-year old soils to most resemble the 85-year old soils after one year. Despite differences in a variety of soil edaphic properties between fertilizer plots and late successional soils, bacterial community composition of +NP plots converged with late successional communities. Thus, our work suggests a mechanism for microbial succession whereby changes in resource availability drive shifts in community composition, supporting a role for nutrient colimitation in primary succession. These results suggest that nutrients alone, independent of other edaphic factors that change with succession, act as an important control over soil microbial community development, greatly accelerating the rate of succession.     

Temporal variability in soil microbial communities across land-use types

Although numerous studies have investigated changes in soil microbial communities across space, questions about the temporal variability in these communities and how this variability compares across soils have received far less attention. We collected soils on a monthly basis (May to November) from replicated plots representing three land-use types (conventional and reduced-input row crop agricultural plots and early successional grasslands) maintained at a research site in Michigan, USA. Using barcoded pyrosequencing of the 16S rRNA gene, we found that the agricultural and early successional land uses harbored unique soil bacterial communities that exhibited distinct temporal patterns. α-Diversity, the numbers of taxa or lineages, was significantly influenced by the sampling month with the temporal variability in α-diversity exceeding the variability between land-use types. In contrast, differences in community composition across land-use types were reasonably constant across the 7-month period, suggesting that the time of sampling is less important when assessing β-diversity patterns. Communities in the agricultural soils were most variable over time and the changes were significantly correlated with soil moisture and temperature. Temporal shifts in bacterial community composition within the successional grassland plots were less predictable and are likely a product of complex interactions between the soil environment and the more diverse plant community. Temporal variability needs to be carefully assessed when comparing microbial diversity across soil types and the temporal patterns in microbial community structure can not necessarily be generalized across land uses, even if those soils are exposed to the same climatic conditions.     

Feral pigs in a temperate rainforest ecosystem: disturbance and ecological impacts

Feral pigs (Sus scrofa) are a widespread invasive species, and cause biotic disturbance. This study evaluated the impacts associated with ground disturbance by feral pigs in the North Island of New Zealand. Exclosure cages were erected over feral pig-disturbed ground and visually undisturbed ground (the latter as controls). Buried resin bags and litter bags were located in these plots to examine differences in soil nutrients and decomposition rates and seedling/sapling recruitment (abundance, species composition and richness) was monitored over 21 months. No difference was found in the litter decomposition between the disturbed and visually undisturbed plots. Significantly more nitrate (NO₃-N/NO₂-N) was found in the disturbed exclosures. Seedling density was not significantly affected by feral pig disturbance. However, seedling/sapling species richness was lower in disturbed areas. Species composition changes occurred at disturbed sites with species increasing and decreasing in density after feral pig disturbance. However, no pattern was observed between species that were negatively affected by feral pig disturbance. This study shows that feral pig disturbance affects vegetation through direct removal, but also indirectly through increased nitrate, potentially leading to seedling and sapling species composition changes. Feral pigs are known to return to previously disturbed areas to re-disturb. These areas may remain in a re-disturbed state if not protected, and through continued disturbance and increased nitrate, ecosystem changes may occur, especially in characteristically nutrient poor environments.     

Functional Traits Help Predict Post-Disturbance Demography of Tropical Trees

How tropical tree species respond to disturbance is a central issue of forest ecology, conservation and resource management. We define a hierarchical model to investigate how functional traits measured in control plots relate to the population change rate and to demographic rates for recruitment and mortality after disturbance by logging operations. Population change and demographic rates were quantified on a 12-year period after disturbance and related to seven functional traits measured in control plots. The model was calibrated using a Bayesian Network approach on 53 species surveyed in permanent forest plots (37.5 ha) at Paracou in French Guiana. The network analysis allowed us to highlight both direct and indirect relationships among predictive variables. Overall, 89% of interspecific variability in the population change rate after disturbance were explained by the two demographic rates, the recruitment rate being the most explicative variable. Three direct drivers explained 45% of the variability in recruitment rates, including leaf phosphorus concentration, with a positive effect, and seed size and wood density with negative effects. Mortality rates were explained by interspecific variability in maximum diameter only (25%). Wood density, leaf nitrogen concentration, maximum diameter and seed size were not explained by variables in the analysis and thus appear as independent drivers of post-disturbance demography. Relationships between functional traits and demographic parameters were consistent with results found in undisturbed forests. Functional traits measured in control conditions can thus help predict the fate of tropical tree species after disturbance. Indirect relationships also suggest how different processes interact to mediate species demographic response.     

Spatio-Temporal Variation in Contrasting Effects of Resident Vegetation on Establishment,Growth and Reproduction of Dry Grassland Plants: Implications for Seed Addition Experiments

Successful establishment of plants is limited by both biotic and abiotic conditions and their interactions. Seedling establishment is also used as a direct measure of habitat suitability, but transient changes in vegetation might provide windows of opportunity allowing plant species to colonize sites which otherwise appear unsuitable. We aimed to study spatio-temporal variability in the effects of resident vegetation on establishment, growth and reproduction of dry grassland species in abandoned arable fields representing potentially suitable habitats. Seeds were sown in disturbed (bare of vegetation and roots) and undisturbed plots in three fields abandoned in the last 20 years. To assess the effects of temporal variation on plant establishment, we initiated our experiments in two years (2007 and 2008). Seventeen out of the 35 sown species flowered within two years after sowing, while three species completely failed to become established. The vegetation in the undisturbed plots facilitated seedling establishment only in the year with low spring precipitation, and the effect did not hold for all species. In contrast, growth and flowering rate were consistently much greater in the disturbed plots, but the effect size differed between the fields and years of sowing. We show that colonization is more successful when site opening by disturbance coincide with other suitable conditions such as weather or soil characteristics. Seasonal variability involved in our study emphasizes the necessity of temporal replication of sowing experiments. Studies assessing habitat suitability by seed sowing should either involve both vegetation removal treatments and untreated plots or follow the gradient of vegetation cover. We strongly recommend following the numbers of established individuals, their sizes and reproductive success when assessing habitat suitability by seed sowing since one can gain completely different results in different phases of plant life cycle.     

Disturbance-level-dependent post-disturbance succession in a Eurasian steppe

Anthropogenic disturbances may decrease as we take measures to control them. However, the patterns and mechanisms of post-disturbance ecosystem succession have rarely been studied. Here we reported that disturbance level determined the importance of stochastic relative to deterministic changes in ecosystem components (plant community composition, soil microbial community composition, and soil physicochemical indices), and thus predefined the pattern of post-disturbance ecosystem succession. We proposed a theoretical framework with five disturbance levels corresponding to distinct succession patterns. We conducted a nitrogen addition experiment in a temperate steppe, monitored these ecosystem components during “disturbance” treatment (2010–2014) and post-treatment “succession” (2014–2018). The disturbance level experienced by each component in each treatment was inferred by fitting the observed succession patterns into the theoretical framework. The mean disturbance level of these components was found to increase quadratically with nitrogen addition rate. This was because increasing nitrogen addition reduced the importance of stochastic relative to deterministic changes in these components, and these changes had a quadratic relationship with disturbance level. Overall, our results suggested that by monitoring the importance of stochastic relative to deterministic changes in an ecosystem, we can estimate disturbance levels and predict succession patterns, as well as propose disturbance-level-dependent strategies for post-disturbance restoration.

     

Fertilization of boreal forest reduces both autotrophic and heterotrophic soil respiration

The boreal forest is expected to experience the greatest warming of all forest biomes, raising concerns that some of the large quantities of soil carbon in these systems may be added to the atmosphere as CO₂. However, nitrogen deposition or fertilization has the potential to increase boreal forest production and retard the decomposition of soil organic matter, hence increasing both tree stand and soil C storage. The major contributors to soil‐surface CO₂ effluxes are autotrophic and heterotrophic respiration. To evaluate the effect of nutrient additions on the relative contributions from autotrophic and heterotrophic respiration, a large‐scale girdling experiment was performed in a long‐term nutrient optimization experiment in a 40‐year‐old stand of Norway spruce in northern Sweden. Trees on three nonfertilized plots and three fertilized plots were girdled in early summer 2002, and three nonfertilized and three fertilized plots were used as control plots. Each plot was 0.1 ha and contained around 230 trees. Soil‐surface CO₂ fluxes, soil moisture, and soil temperature were monitored in both girdled and nongirdled plots. In late July, the time of the seasonal maximum in soil‐surface CO₂ efflux, the total soil‐CO₂ efflux in nongirdled plots was 40% lower in the fertilized than in the nonfertilized plots, while the efflux in girdled fertilized and nonfertilized plots was 50% and 60% lower, respectively, than in the corresponding nongirdled controls. We attribute these reductions to losses of the autotrophic component of the total soil‐surface CO₂ efflux. The estimates of autotrophic respiration are conservative as root starch reserves were depleted more rapidly in roots of girdled than in nongirdled trees. Thus, heterotrophic activity was overestimated. Calculated on a unit area basis, both the heterotrophic and autotrophic soil respiration was significantly lower in fertilized plots, which is especially noteworthy given that aboveground production was around three times higher in fertilized than in nonfertilized plots.     

黄土高原小流域土壤养分的空间异质性

利用地理信息系统的空间分析功能,通过地统计学的半变异函数定量研究了黄土高原典型小流域土壤养分的空间异质性特征。结果表明;土壤有机质,全氮,有效氮,全磷和有效磷的理论模型均为球状模型,由随机因素引起的空间变异占空间总变异的比例小,其值分别为13.333%,10.938%,22.000%,9.091%和27.536%,反映5种养分具有较强的空间自相关格局,但它们的空间自相关范围具有明显的差异。土壤全氮和有效氮变程小,分别是90m和110m,有机质次之,变程是120m,而土壤全磷和有效磷的变程最大为160m,研究成果将有效地指导土壤的取样设计,以及进行土壤养分的空间内插和制图。     

Disturbance of forest by trampling: Effects on mycorrhizal roots of seedlings and mature trees of Fagus sylvatica

The effects of disturbance by recreational activities (trampling) on changes in soil organic matter (SOM) and on mycorrhizal roots of seedlings and mature trees were studied in four stands of a beech (Fagus sylvatica L.) forest near Basel, Switzerland. At each site, comparable disturbed and undisturbed plots were selected. Disturbance reduced ground cover vegetation and leaf litter. Beech seedlings had lower biomass after disturbance. Ergosterol concentration in seedling roots, an indicator of mycorrhizal fungi, was lower in two of the four disturbed plots compared to undisturbed plots; these two disturbed sites had especially low litter levels. Based on ergosterol measurements, mycorrhizas of mature trees did not appear to be negatively affected by trampling. Total fine roots and SOM were higher in the disturbed than in the undisturbed plots at three sites. At the fourth site, fine roots and SOM in the disturbed areas were lower than in the undisturbed areas most probably due to nutrient input following picnic activities. Principal component analysis revealed a close correlation between SOM and fine roots of mature trees as well as litter and seedling biomass. Trampling due to recreational activities caused considerable damage to the vegetation layer and in particular to the beech seedlings and their mycorrhizal fine roots, whereas, roots of mature trees were apparently resilient to trampling.     

Differential controls by climate and substrate over the heterotrophic and rhizospheric components of soil respiration

The partitioning of soil respiration rates into the component processes of rhizospheric respiration (because of live roots and those microorganisms that subsist on root exudations) and heterotrophic respiration (because of decomposer microorganisms that subsist on the oxidation of soil organic matter) is difficult to accomplish through experimental observation. In order to minimize disturbance to the soil and maximize preservation of the natural relationships among roots, rhizospheric microorganisms, and decomposers, we conducted a girdling experiment in a subalpine forest dominated by lodgepole pine trees. In two separate years, we girdled trees in small forest plots (5–7 m in diameter) and trenched around the plots to sever invading roots in order to experimentally stop the transport of photosynthate from needles to roots, and eliminate rhizospheric respiration. Soil respiration rates in plots with trees girdled over 1 year prior to measurement were higher than those in plots with trees girdled 2–3 months prior to measurement. These results suggest that any stimulation of respiration because of the experimental artifact of fine root death and addition of labile carbon to the pool of decomposer substrates is slow, and occurs beyond the first growing season after girdling. Compared with control plots with nongirdled trees, soil respiration rates in plots with girdled trees were reduced by 31–44% at the mid‐summer respiratory maximum. An extreme drought during one of the 2 years used for observations caused greater reductions in the heterotrophic component of soil respiration compared with the rhizospheric component. In control plots, we observed a pulse in K₂SO₄‐extractable carbon during the spring snowmelt period, which was absent in plots with girdled trees. In control plots, soil microbial biomass increased from spring to summer, coincident with a seasonal increase in the rhizospheric component of soil respiration. In plots with girdled trees, the seasonal increase in microbial biomass was lower than in control plots. These results suggest that the observed seasonal increase in rhizospheric respiration rate in control plots was because of an increase in rhizospheric microbial biomass following ‘soil priming’ by a spring‐time pulse in dissolved organic carbon. Winter‐time, beneath‐snow microbial biomass was relatively high in control plots. Soil sucrose concentrations were approximately eight times higher during winter than during spring or summer, possibly being derived from the mechanical damage of shallow roots that use sucrose as protection against low‐temperature extremes. The winter‐time sucrose pulse was not observed in plots with girdled trees. The results of this study demonstrate that (1) the rhizospheric component of soil respiration rate at this site is significant in magnitude, (2) the heterotrophic component of soil respiration rate is more susceptible to seasonal drought than the rhizospheric component, and (3) the trees in this ecosystem exert a major control over soil carbon dynamics by ‘priming’ the soil with sugar exudates during the late‐spring snowmelt period and releasing high concentrations of sucrose to the soil during winter.     

桂西北喀斯特峰丛洼地表层土壤养分时空分异特征

本文运用传统统计方法研究了典型喀斯特峰丛洼地4个植被演替阶段(草地、灌木林、次生林、原生林)表层(0—15 cm)土壤养分的空间分异特征,并分析其成因。结果表明：(1)不同植被演替阶段土壤养分（有机碳、全氮、碳氮比、全磷、全钾、碱解氮、速效磷、速效钾）及pH值有明显差异（P<0.05）;除全钾外,其他养分含量均随植被正向演替而增加。(2)地貌部位对养分含量的影响不尽一致：各植被演替阶段的磷、钾及原生林的碳氮均符合养分的“洼积效应”,只有受人为干扰较强的草地、灌木林、次生林的碳氮养分存在随坡位升高而增加的现象。通过本研究得知,植被演替对养分积累具有积极作用;磷素（速效磷3.55—11.41 mg/kg）、钾素（速效钾64.05—105.75 mg/kg）是该区域的养分限制性因子;该区域养分的“倒置现象”并不普遍存在。     

Development of microtopography in a semi-arid grassland: Effects of disturbance size and soil texture

Our objective was to evaluate effects of disturbance size and soil texture on the development of microtopography for a shortgrass plant community in north central Colorado USA. Disturbances, defined as the death of individual plants, were created in 1984 and 1985 to evaluate development through time of the small-scale pattern of perennial bunchgrasses and bare soil openings that characterize this semiarid grassland. Disturbed plots of three sizes (50, 100, 150 cm-diameter) comparable in size to naturally-occurring disturbances were produced by killing plants at two sites differing in soil texture (sandy loam, clay loam). Disturbed plots were not manipulated after being created. In 1993, a laser surveying instrument was used to measure heights of crowns of individual plants of the dominant species, the perennial bunchgrass Bouteloua gracilis ([H.B.K.] Lag. ex Griffiths), and bare soil openings between plants for two locations: within each disturbance and in the surrounding undisturbed landscape.Differences between crown heights of plants and bare soil openings were comparable for both the undisturbed landscape and inside disturbances indicating that small-scale microtopography had recovered within nine years after disturbance occurred. However, complete recovery to the undisturbed state had not occurred since crown heights of plants relative to bare soil openings were significantly less on disturbed than undisturbed locations. Differences in height between plant crowns and bare soil openings on disturbed plots increased as disturbance size increased, indicating greater soil redistribution with increasing plot size. Larger differences in height were also found on plots on the fine- than the coarse-textured soil, indicating the importance of soil particle size and plant cover type to the development of microtopography. Differences in height between microsites on disturbed plots were positively related to total plant cover and negatively related to cover of B. gracilis indicating the importance of this species to reducing erosion on disturbed areas.In this semiarid grassland, patterns in microtopography were heterogeneous, likely as a result of the small-scale redistribution of soil between bare soil openings and B. gracilis plants through time. Our results indicate that this redistribution of soil is affected by disturbance size, soil texture, and patchy plant cover. The major effect of small-scale disturbances on patterns in microtopography of the shortgrass steppe are causing plant death and exposing soil to erosional and depositional processes.     

Temporal patterns of soil nutrients in a Panamanian moist forest revealed by ion-exchange resin and experimental irrigation

The effect of seasonal water availability on soil nutrients and soil N transformations was investigated by irrigating two large plots of mature tropical forest on Barro Colorado Island (BCI), Panama, during the dry season for five consecutive years. Methods included (i) nutrient accumulation by ion-exchange resins placed on the surface of the mineral soil for contiguous 21-day periods, (ii) monthly mineral soil (0–10 cm) extractions and incubations for inorganic N and P concentrations, and (iii) leaching loss of nutrients from leaf litter samples. Rates of nutrient accumulation by the resins showed a great deal of variation between sampling dates and among years in control plots; albeit, seasonal patterns were slight, except for the highest Ca values near the end of the wet season and inorganic P (P_i) and SO₄ values that peaked during the dry season. Irrigation had remarkably little effect on nutrient accumulation rates by resins, except for an increase in Mg and Na values, but did affect the timing in the temporal variation in K, Na, N_i and P_i values. In contrast, inorganic N (N_i) and P_i pools and N transformation rates in the mineral soil hardly varied among sampling dates and did not show any response to irrigation. We hypothesize that the timing of leaf litterfall and nutrient leaching from forest floor litter can set up temporal patterns in the levels of soil nutrient at the surface of the mineral soil, but the temporal patterns essentially disappear with depth in the mineral soil.     

Vegetation responses to different spatial patterns of soil disturbance in burned and unburned tallgrass prairie

Pocket gopher (Geomyidae) disturbances are created in spatiallypredictable patterns. This may influence resource heterogeneity and affectgrassland vegetation in a unique manner. We attempt to determine the extent towhich density and spatial pattern of soil disturbances influence tallgrassprairie plant community structure and determine how these disturbances interactwith fire. To investigate the effects of explicit disturbance patterns we createdsimulated pocket gopher burrows and mounds in various spatial patterns.Simulated burrows were drilled into the soil at different densities inreplicated plots of burned and unburned prairie. Separate plots of simulatedmounds were created in burned and unburned prairie at low, medium, or high mounddensities in clumped, uniform, or random spatial dispersions. In both burned and unburned plots, increased burrow density decreasedgraminoid biomass and increased forb biomass. Total-plant and graminoid biomasswere higher in burned than unburned plots while forb biomass was higher inunburned plots. Total-plant species richness was not significantly affected byburrow density or burning treatments, but graminoid species richness increasedin unburned plots and forb species richness increased in burned plots. Plant species richness was temporarily reduced directly on mounddisturbances compared to undisturbed prairie. Over time and at larger samplingscales, the interaction of fire and mound disturbance patterns significantlyaffected total-plant and graminoid species richness. The principal effect inburned and unburned prairie was decreased total-plant and graminoid speciesrichness with increased mound disturbance intensity. Although species richness at small patch scales was not increased by anyintensity of disturbance and species composition was not altered by theestablishment of a unique guild of disturbance colonizing plants, our studyrevealed that interactions between soil disturbances and fire alter the plantcommunity dominance structure of North American tallgrass prairie primarily viachanges to graminoids. Moreover, these effects become increasingly pronouncedover time and at larger spatial sampling scales.     

Effects of prescribed burning and harvesting on ground-dwelling spiders in the Canadian boreal mixedwood forest

The ‘Natural Disturbance Paradigm’ for forest management seeks to meet conservation goals by modeling industrial harvest in fire-driven forest systems on patterns associated with wildfire. Fire suppression and increased forest harvesting may have detrimental effects on biodiversity, and therefore prescribed burning is suggested to retain legacies of wildfire not emulated under natural disturbance based approaches. The merits of this approach are being tested in the EMEND experiment in the Canadian boreal mixedwood forest. We compared responses of ground-dwelling spiders between sites subjected to prescribed post-harvest burning and retention harvest during three seasons during the first 7 years after disturbance. Overall, 38,661 adult spiders representing 190 species were collected. Estimated species richness was highest in undisturbed sites in all 3 years. Burning had the strongest negative effect on species richness 1–2 years after treatment; however, richness was higher in burns than in harvested sites 5–6 years post-disturbance. Species turnover was highest within controls but tended to increase over time between burned and harvested plots. Lower turnover in burned and harvested sites may reflect habitat homogenization by disturbance, suggesting a management and conservation challenge in relation to naturally disturbed and undisturbed areas. Species were grouped into disturbance-specialists, disturbance-tolerant, disturbance-generalists and generalists; 22 species were significant indicators for untreated sites, 18 for the burn and three for the harvest treatments. No major differences were observed in the spider fauna between harvested and burned areas within the first 6–7 years post-disturbance, and little evidence of recovery toward the pre-harvest fauna was evidenced. However, long term experiments may improve understanding of natural disturbance processes and improve management of boreal forests.     

The effect of environmental heterogeneity on clonal behaviour of Prunella vulgaris L.

Melinis minutiflora Beauv. (Poaceae) is an African grass that is invading mid-elevation Trachypogon savannas in Venezuela. The objective of this study was to investigate the influence of soil fertility, competition and soil disturbance in facilitating Melinis' invasion and growth in these savanna sites. We manipulated soil fertility by adding nitrogen (+N), phosphorus and potassium (+PK), or nitrogen, phosphorus, and potassium (+NPK). We simultaneously manipulated the competitive environment by clipping background vegetation. In a separate experiment, we mechanically disrupted the soil to simulate disturbance. We hypothesized that germination and growth were bottlenecks to early establishment in undisturbed savanna, but that disturbance would alleviate those bottlenecks. We measured Melinis seed germination and subsequent establishment by adding seeds to all plots. We examined Melinis growth by measuring biomass of Melinis seedling transplants, 11 months after they were placed into treatment plots. Germination and establishment of Melinis from seed was extremely low. Of the 80,000 seeds applied in the experiment, only 28 plants survived the first growing season. Mortality of Melinis seedling transplants was lowest in PK fertilized plots, but in the absence of PK mortality increased with N additions and clipping. By contrast, fertilization of the savanna with NPK greatly increased Melinis seedling biomass and this effect was greatly enhanced when competition was reduced (e.g. clipping). Melinis transplant growth responded strongly to soil disturbance- a response not fully explained by removal of competitors (clipping) or changes in soil nutrients and moisture. We suspect that disruption of the soil structure allowed for greater root proliferation and subsequent plant growth. We believe that native savanna is relatively resistant to Melinis invasion, since Melinis seedlings persisted in intact savanna but exhibited little or no growth during the first year. The significant enhancement of Melinis seedling growth with clipping and nutrient additions suggests that low soil nutrients and the presence of native savanna species are important factors in the ability of native savanna to resist Melinis establishment. However, the potential for Melinis growth increases enormously with soil disturbance.     

华北落叶松林细根生物量对土壤水分、氮营养空间异质性改变的响应

在华北落叶松(Larix principis-rupprechtii)林选取采伐干扰样地和未采伐干扰样地进行对比研究, 分析采伐干扰造成林下土壤水分和氮营养空间异质性的改变对细根生物量空间变异的影响。采用空间格局分析的小支撑、多样点的设计原则, 对每个样点的土壤分3层取样(0-10 cm、10-20 cm、20-30 cm)。进行细根(≤1 mm和1-2 mm)生物量与土壤含水量、全氮、硝态氮、铵态氮和土壤pH的偏相关分析, 以及细根生物量变异函数值和土壤各因子变异函数值的线性回归分析。研究结果表明, 在不同样地, 细根生物量与土壤各因子均表现为正相关关系, 不同土层相关性强弱表现各异, 其中土壤含水量与细根生物量的相关性显著。受采伐干扰后, 细根生物量与土壤含水量、全氮、土壤硝态氮空间变异的关联性更趋于明显。多元线性回归分析结果表明, 采伐干扰样地细根生物量的空间变异更多地受到土壤多因子的综合影响, 而未采伐干扰样地的细根生物量受土壤水分、全氮和硝态氮单独效应的影响更大。     

Physical disturbance of an upland grassland influences the impact of elevated UV-B radiation on metabolic profiles of below-ground micro-organisms

This investigation determined the response of soil microbial communities to enhanced UV‐B radiation and disturbance in upland grassland. A factorial field experiment encompassing two levels of UV‐B supplementation (simulating ambient and a 30% increase in stratospheric ozone) and two levels of disturbance (disturbed and undisturbed) was established at Buxton Climate Change Impacts Laboratory, Derbyshire, UK, and maintained for 7 years prior to sampling. Enhanced UV‐B increased microbial utilization of carbohydrates, carboxylic acids, polymers and aromatic compounds present in Biolog® GN plates when inoculated with soils taken from disturbed plots, but did not affect carbon utilization of soil microbial communities associated with undisturbed plots (UV‐B×Disturbance interaction, P<0.05 for each substrate type). UV‐B treatment did not affect numbers of bacteria or fungi. Direct microscopic counts showed fewer bacteria in soil originating from disturbed plots than from undisturbed plots (Disturbance, P<0.001), although a greater number of culturable bacteria and fungi were isolated from disturbed than from undisturbed soils (Disturbance, P<0.001). No UV‐B‐ or disturbance‐related differences in protein, starch or urea hydrolysis were exhibited by bacterial isolates. UV‐B treatment did not affect total plant biomass within undisturbed plots or the biomass of individual groupings of grasses, forbs and mosses. Per cent root length colonized by arbuscular mycorrhizal fungi (AMF) was not affected by enhanced UV‐B radiation in the undisturbed plots. Neither AMF nor plant biomass was measured in disturbed plots. The key findings of this study show that UV‐B‐mediated alterations in carbon utilization occurred in soil microbial communities subjected to disturbance, but such changes were not observed in communities sampled from undisturbed grassland. Differences in the catabolic potential of microbial communities from disturbed grassland subjected to enhanced UV‐B are probably related to plant‐mediated changes in resource availability or quality.     

Seasonal storage of nutrients by perennial herbaceous species in undisturbed and disturbed deciduous hardwood forests

Question: Pollution and eutrophication of surface water is increasingly a problem in agricultural landscapes. Do intact (relatively undisturbed) and degraded forests differ in seasonal nutrient storage and therefore potential to ameliorate nutrient pollution? Location: United States, Midwestern region. Methods: We used three sets of paired plots, where intact plots were located close to disturbed woodlands. Herbaceous perennials located in eight 0.25 m² quadrats in the plots were harvested (in spring and mid‐summer), dried, separated into above‐ and below‐ground plant parts, and weighed to determine biomass. Nitrogen, phosphorus and potassium content of the plant tissues were then determined, and these data combined with biomass to estimate nutrient storage. Results: In spring, intact sites had 62% greater above‐ground biomass than disturbed sites and 75% greater below‐ground biomass. In summer, below‐ground biomass of intact plots was still much greater than that of disturbed plots (73 percent), but above‐ground biomass was similar. Nutrient tissue concentration generally did not differ, nor did soil nutrient levels. The disturbed sites were largely missing one group of species, the spring ephemerals, and this accounted for the difference in biomass and nutrient storage between sites. Conclusions: Relatively undisturbed woodlands in our study had a much greater capacity to store nutrients, and therefore ameliorate nutrient pollution, in early spring. This is significant because spring is also the time of highest potential leaching of nutrients into surface water.