Showing papers on "Disturbance (geology) published in 2009"
TL;DR: This technical note deals with the attitude tracking and disturbance rejection problem of spacecraft for a class of persistent disturbances with unbounded energy which include the sinusoidal disturbance as a special case.
Abstract: In this technical note, we deal with the attitude tracking and disturbance rejection problem of spacecraft for a class of persistent disturbances with unbounded energy which include the sinusoidal disturbance as a special case. The approach involves the integration of techniques from robust control, adaptive control, and robust output regulation theory.
272 citations
TL;DR: In this paper, a sliding mode control (SMC) scheme is proposed for a class of nonlinear systems based on disturbance observers, where the disturbance that cannot be directly measured is estimated using a nonlinear disturbance observer.
Abstract: In this paper, a sliding mode control (SMC) scheme is proposed for a class of nonlinear systems based on disturbance observers. For a nonlinear system, the disturbance that cannot be directly measured is estimated using a nonlinear disturbance observer. By choosing an appropriate nonlinear gain function, the disturbance observer can well approximate the unknown disturbance. Based on the output of the disturbance observer, an SMC scheme is presented for the nonlinear system, and the stability of the closed-loop system is established using Lyapunov method. Finally, two simulation examples are presented to illustrate the features and the effectiveness of the proposed disturbance-observer-based SMC scheme.
213 citations
10 Jun 2009
TL;DR: The capabilities of extended state observer (ESO) for estimating uncertainties is discussed and it is demonstrated that ESO can deal with a large class of disturbances.
Abstract: The capabilities of extended state observer (ESO) for estimating uncertainties is discussed in this paper. The scope of the disturbances that can be observed by LESO with bounded observing errors is given. The observing errors for several typical disturbances-constant disturbance, sine disturbance, ramp disturbance, and square wave disturbance are further analyzed. It is demonstrated that ESO can deal with a large class of disturbances. Finally, the results are tested by simulations.
165 citations
TL;DR: In this article, the authors used time series stacks of Landsat images (LTSS) to evaluate the dynamics of seven national forests (NFs) in eastern United States, including the De Soto NF, the TalladegaNF, the Francis Marion NF, and the Uwharrie NF in southeastern US.
156 citations
TL;DR: In this paper, the authors discuss anthropogenic changes that are particularly likely to compromise resilience of Pacific salmon and management actions that could help bring the current patterns of disturbance regimes more in line with the historical template.
Abstract: Because resilience of a biological system is a product of its evolutionary history, the historical template that describes the relationships between species and their dynamic habitats is an important point of reference. Habitats used by Pacific salmon have been quite variable throughout their evolutionary history, and these habitats can be characterized by four key attributes of disturbance regimes: frequency, magnitude, duration, and predictability. Over the past two centuries, major anthropogenic changes to salmon ecosystems have dramatically altered disturbance regimes that the species experience. To the extent that these disturbance regimes assume characteristics outside the range of the historical template, resilience of salmon populations might be compromised. We discuss anthropogenic changes that are particularly likely to compromise resilience of Pacific salmon and management actions that could help bring the current patterns of disturbance regimes more in line with the historical template.
145 citations
01 May 2009
TL;DR: In this article, the authors proposed a method to solve the problem of "uniformity" in the literature.and.and, and, respectively, the authors' work.
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125 citations
TL;DR: Using the HUMPOL simulation computer model, this paper explored the effects of various factors characteristic of mosaic cultural landscapes on the Relevant Source Area of Pollen (RSAP sensu Sugita) of small communities.
99 citations
TL;DR: It was found that both propagule pressure and disturbance were required for invasion to occur, and only recruits that had settled onto bare space survived beyond three months, precluding invader persistence in undisturbed communities.
Abstract: Propagule pressure is fundamental to invasion success, yet our understanding of its role in the marine domain is limited. Few studies have manipulated or controlled for propagule supply in the field, and consequently there is little empirical data to test for non-linearities or interactions with other processes. Supply of non-indigenous propagules is most likely to be elevated in urban estuaries, where vessels congregate and bring exotic species on fouled hulls and in ballast water. These same environments are also typically subject to elevated levels of disturbance from human activities, creating the potential for propagule pressure and disturbance to interact. By applying a controlled dose of free-swimming larvae to replicate assemblages, we were able to quantify a dose-response relationship at much finer spatial and temporal scales than previously achieved in the marine environment. We experimentally crossed controlled levels of propagule pressure and disturbance in the field, and found that both were required for invasion to occur. Only recruits that had settled onto bare space survived beyond three months, precluding invader persistence in undisturbed communities. In disturbed communities initial survival on bare space appeared stochastic, such that a critical density was required before the probability of at least one colony surviving reached a sufficient level. Those that persisted showed 75% survival over the following three months, signifying a threshold past which invaders were resilient to chance mortality. Urban estuaries subject to anthropogenic disturbance are common throughout the world, and similar interactions may be integral to invasion dynamics in these ecosystems.
96 citations
TL;DR: It is demonstrated that spatial contrasts of burrow counts are broadly useful to indicate the existence of a human-induced disturbance effect on urban beaches and highlighted a number of critical, hitherto unknown, issues in the application of this monitoring technique.
Abstract: Sandy beaches comprise one of the most important coastal resources worldwide, providing habitats to threatened vertebrates, supporting underappreciated invertebrate biodiversity, and delivering crucial ecosystem services and economic benefits to mankind. Monitoring of the natural resource condition of sandy beaches and assessments of the ecological impacts of human disturbance are, however, rare on sandy shores. Because a crucial step in developing beach monitoring is to identify and test biological indicators, we evaluated the utility of using population densities of ghost crabs (genus Ocypode) to measure how beach biota respond to human pressures. Densities of crabs—estimated via burrow counts—were quantified at two sites exposed to high and low levels of human disturbance on an urban beach in eastern Australia. Human disturbance consisted of pedestrian trampling and shoreline armouring which led to the loss of dune habitat. Overall, crab numbers were halved in disturbed areas, but contrasts between impact and control sites were not necessarily consistent over time and varied between different levels of the shore: stronger and more consistent effect sizes were recorded on the upper shore than further seawards. In addition to lowering crab densities, human disturbance also caused shifts in intertidal distributions, with a greater proportion of individuals occurring lower on the shore in the impacted beach sections. The number of visible burrow openings also changed in response to weather conditions (temperature and wind). We demonstrate that spatial contrasts of burrow counts are broadly useful to indicate the existence of a human-induced disturbance effect on urban beaches; we also highlight a number of critical, hitherto unknown, issues in the application of this monitoring technique; these encompass three broad dimensions: (1) a need for standardised protocols; (2) unresolved causal links between observed patterns and putative pressures; and (3) uncertainties of how organisms responds specifically to both natural and human changes of environmental conditions on sandy shores.
84 citations
TL;DR: This analysis demonstrates that when species have distinct fire response strategies, disturbance allows for stable species coexistence, and shows how the size of the coexistence region depends on fire frequency and dispersal ability.
Abstract: We investigate the effects of disturbance on species coexistence using a general mathematical model. The model can be applied to a variety of communities, and we show how it applies in particular to communities of shrubs in Mediterranean heathlands. Our analysis demonstrates that when species have distinct fire response strategies, disturbance allows for stable species coexistence. Furthermore, we show how the size of the coexistence region depends on fire frequency and dispersal ability. The stabilizing mechanism is classified as the spatial storage effect, which is identified by the covariance between environmental and competitive responses. This is the first time that disturbance, defined as a fluctuating mortality factor, has been definitively shown to promote coexistence via the storage effect. Moreover, we show that the biological driver is a trade-off between resistance and resilience to disturbance. The resistance-resilience trade-off is a biological mechanism of coexistence under patchy disturbance. However, the resistance-resilience trade-off has not previously featured in mathematical models of species coexistence. Although the storage effect depends on fluctuations in life-history parameters presumed to result from environmental variation, rarely are life-history parameters explicitly linked to environmental phenomena. Here the link is clear and concrete, allowing better definition of the intended application.
79 citations
TL;DR: Ant communities in the southeastern United States displayed a hump-backed species richness curve against an index of landscape disturbance as mentioned in this paper, showing that ant abundance was greatest with a relative disturbance of 85%.
TL;DR: In implementing an EAFM, managers will need to consider the possibility that, even in areas with high chronic fishing disturbance, further increases in fishing activity may still cause addi- tional damage to benthic invertebrate communities.
Abstract: Demersal fishing alters seabed habitats and affects the structure and functioning of ben- thic invertebrate communities At a critical level of disturbance, such communities may approach an equilibrium disturbed state in which a further increase in disturbance has little additional impact Such arguments have been used to suggest that an ecosystem approach to fisheries management (EAFM) should protect lightly fished areas and deflect fishing activity into areas that are already intensively fished In this study, the effects of variation in fishing disturbance on the secondary pro- duction, species diversity, abundance, biomass, and community structure of benthic infauna were examined in a region of the German Bight (North Sea) that has been intensively trawled for decades Variation in fishing disturbance across the study area was determined using automated position reg- istration and vessel monitoring through satellite Even in such a heavily fished area, linear regression analyses revealed that biomass, species richness, and production decreased significantly with increasing fishing intensity Although redundancy analyses (RDA) showed that sediment character- istics were influential in determining the structure of the infauna community, partial RDA revealed that fishing continued to have an impact on community structure in terms of biomass These results suggest that, in implementing an EAFM, managers will need to consider the possibility that, even in areas with high chronic fishing disturbance, further increases in fishing activity may still cause addi- tional damage to benthic invertebrate communities
TL;DR: In this article, the authors considered the disturbance analysis of a second-order system with unknown but bounded disturbance, and showed that the states of the secondorder system will be stabilized to a region containing the origin, where the radius of this region is determined by the control parameters and can be rendered as small as desired.
Abstract: In this paper, using finite-time control method, we consider the disturbance analysis of a second-order system with unknown but bounded disturbance. We show that the states of the second-order system will be stabilized to a region containing the origin. The radius of this region is determined by the control parameters and can be rendered as small as desired. The rigorous stability analysis is also given. Compared with the conventional PD control law, the finite-time control law yields a better disturbance rejection performance. Numerical simulation results show the effectiveness of the method.
TL;DR: The most important impacts in the past 50 years are derived from the following activities, in decreasing importance: (1) mining and quarrying; (2) deforestation, agriculture and grazing; (3) building (widespread urbanisation, isolated homes, etc.) and related infrastructures (roads, sewer systems, aqueducts, waste dumps, etc.).
Abstract: Evaluating the human disturbance on karst areas is a difficult task because of the complexity of these peculiar and unique environments. The human impact on karstic geo-ecosystems is increasingly important and there is an increasing need for multidisciplinary tools to assess the environmental changes in karst areas. Many disciplines, such as biology, geomorphology, hydrology and social-economical sciences are to be considered to sufficiently evaluate the impact on these intrinsically vulnerable areas. This article gives an overview of the evolution of environmental impact on karst areas of the island Sardinia (Italy). For this particular case, the most important impacts in the past 50 years are derived from the following activities, in decreasing importance: (1) mining and quarrying; (2) deforestation, agriculture and grazing; (3) building (widespread urbanisation, isolated homes, etc.) and related infrastructures (roads, sewer systems, aqueducts, waste dumps, etc.); (4) tourism; (5) military activities. To evaluate the present environmental state of these areas the Disturbance Index for Karst environments [Van Beynen and Townsend (Environ Manage 36:101–116)] is applied in a slightly modified version. Instead of considering the indicators of environmental disturbances used in the original method, this slightly modified index evaluates the disturbances causing the deterioration of the environmental attributes. In the Sardinian case study, 27 disturbances have been evaluated, giving rise to the definition of a Disturbance Index ranging between 0 (Pristine) and 1 (highly disturbed). This Disturbance Index simplifies the original KDI method, appears to adequately measure disturbance on Mediterranean karst areas and could be applied with success to other similar regions.
TL;DR: In this article, a streamwise favorable-to-adverse pressure gradient laminar boundary layer is studied by means of direct numerical simulation, a water-tunnel experiment and an adjoint-based parabolic theory suited to study spatial optimal growth.
Abstract: Steady linear three-dimensional disturbances are investigated in a two-dimensional laminar boundary layer. The boundary layer is subject to a streamwise favourable-to-adverse pressure gradient and eventually undergoes separation. The separating flow corresponds to the first part of a pressure-induced laminar-separation bubble on a flat plate. Streamwise disturbance development in such a flow is studied by means of direct numerical simulation, a water-tunnel experiment and an adjoint-based parabolic theory suited to study spatial optimal growth. A complete overview of the disturbance evolution in various areas of the favourable-to-adverse pressure gradient laminar boundary layer is given. Results from all investigation methods show overall good agreement with respect to disturbance growth and shape within the entire domain. In the favourable pressure-gradient region and, again, slightly downstream of separation, transient growth caused by the lift-up effect dominates disturbance behaviour. In the adverse pressure-gradient region, a modal instability is observed. Evidence is presented that this instability is of Gortler type.
TL;DR: The present study suggests a disturbance estimator design method for application to a recently published, two-degree-of-freedom, control scheme for open-loop, unstable processes with time delay, and a new analytical method on the basis of the IMC design principle, featuring only one user-defined tuning parameter, is developed.
Abstract: The present study suggests a disturbance estimator design method for application to a recently published, two-degree-of-freedom, control scheme for open-loop, unstable processes with time delay. A simple PID controller cascaded with a lead-lag filter replaces the high-order disturbance estimator for enhanced performance. A new analytical method on the basis of the IMC design principle, featuring only one user-defined tuning parameter, is developed for the design of the disturbance estimator. Several illustrative examples taken from previous works are included to demonstrate the superiority of the proposed disturbance estimator. The results confirm the superior performance of the proposed disturbance estimator in both nominal and robust cases. The proposed method also offers several important advantages for industrial process engineers: it covers several classes of unstable process with time delay in a unified manner, and is simple and easy to design and tune.
TL;DR: In this paper, the authors show that the choice of the disturbance model does not affect the closed-loop performance if appropriate covariances are used in specifying the state estimator, regardless of the plant's true unknown disturbance source.
Abstract: Integrated white noise disturbance models are included in advanced control strategies, such as Model Predictive Control, to remove offset when there are unmodeled disturbances or plant/model mismatch. These integrating disturbances are usually modeled to enter either through the plant inputs or the plant outputs or partially through both. There is currently a lack of consensus in the literature on the best choice for the structure of this disturbance model to obtain good feedback control. We show that the choice of the disturbance model does not affect the closed- loop performance if appropriate covariances are used in specifying the state estimator. We also present a data based autocovariance technique to estimate the appropriate covariances regardless of the plant's true unknown disturbance source. The covariances estimated using the autocovariance technique and the resulting estimator gain are shown to compensate for an incorrect choice of the source of the disturbance in the disturbance model. © 2009 American Institute of Chemical Engineers AIChE J, 2009
TL;DR: A new type of composite control scheme of disturbance-observer-based control and terminal sliding mode control for uncertain structural systems by integrating the disturbance observer with TSMC law so that the disturbance with the exogenous system can be estimated and compensated, and external excitation can be attenuated in finite time.
Abstract: A new type of composite control scheme of disturbance-observer-based control and terminal sliding mode control (TSMC) is proposed for uncertain structural systems. The disturbance are supposed to include two parts. One is generated by an exogenous system, which can represent the harmonic signals with modelling perturbations in structural system. The other part is external excitation in H2-norm context. The disturbance observers based on regional pole placement and D-stability theory are presented, which can be designed separately from the controller design. By integrating the disturbance observer with TSMC law, the disturbance with the exogenous system can be estimated and compensated, and external excitation can be attenuated in finite time. Finally, simulations for a four-degree-of-freedom building model excited by 1940 EI Centro earthquake excitation are given to demonstrate the effectiveness of the approach and compare the proposed results with the previous schemes with accuracy.
TL;DR: It is found that cleared ski runs retained many ecological similarities to reference forests and might even offer some added benefits by possessing greater plant species and functional diversity than either forests or graded runs.
Abstract: In mountain regions around the world, downhill ski areas represent a significant source of anthropogenic disturbance while also providing recreation and revenue. Ski-run creation always results in some level of disturbance, but disturbance intensity varies greatly with construction method. Ski runs may be established either by clearing (cutting and removing tall vegetation) or by clearing and then machine-grading (leveling the soil surface with heavy equipment). To quantify how these different intensities of initial disturbance affect ecosystem properties, we extensively surveyed vegetation, soils, and environmental characteristics on cleared ski runs, graded ski runs, and adjacent reference forests across seven large downhill ski resorts in the northern Sierra Nevada, USA. We found that the greater disturbance intensity associated with grading resulted in greater impacts on all ecosystem properties considered, including plant community composition and diversity, soil characteristics relating to processes of nutrient cycling and retention, and measures of erosion potential. We also found that cleared ski runs retained many ecological similarities to reference forests and might even offer some added benefits by possessing greater plant species and functional diversity than either forests or graded runs. Because grading is more damaging to multiple indicators of ecosystem function, clearing rather than grading should be used to create ski slopes wherever practical.
TL;DR: A recent Ecology Letters paper of Fisher et al. (2008) utilized a modelling framework to investigate disturbance effects on forest biomass dynamics but it contains serious methodological and conceptual errors.
Abstract: A recent Ecology Letters paper of Fisher et al. (2008) utilized a modelling framework to investigate disturbance effects on forest biomass dynamics. But it contains serious methodological and conceptual errors. Associated conclusions are unlikely to be correct.
10 Jun 2009
TL;DR: A brief survey of a little known field of disturbance estimation and subsequent cancellation, a field with a long history and still rather disorganized, which provides a powerful alternative to the modern control paradigm in how real world control problems are viewed and solved.
Abstract: This is a brief survey of a little known field of disturbance estimation and subsequent cancellation, a field with a long history and is still rather disorganized. Researchers and results are scattered over almost two centuries, across East and West: from Jean-Victor Poncelet's Principle of Invariance in 1829, to Jingqing Han's conception of Active Disturbance Rejection in 1995 and beyond. But the field in recent years is maturing and coming into a focus with significant practical and theoretical implications abound. It provides a powerful alternative to the modern control paradigm in how real world control problems, of which disturbance rejection is a central theme, are viewed and solved. In this paper a reader will find a brief history of ideas, a new, unifying, problem formulation, and a summary of recent stability analysis results.
TL;DR: A new algorithm is proposed to the design of an adaptive observer for an unknown sinusoidal disturbance that affects the output of a non-minimum phase linear control plant.
Abstract: A new algorithm is proposed to the design of an adaptive observer for an unknown sinusoidal disturbance that affects the output of a non-minimum phase linear control plant.
TL;DR: Rapid assessments of the richness and diversity of fruit‐feeding butterflies and dung beetles in Ba Be National Park, Vietnam are carried out with baited traps and buffalo dung‐baited pitfall traps.
Abstract: . 1. We carried out rapid assessments of the richness and diversity of fruit-feeding butterflies (sampled with baited traps) and dung beetles (sampled with buffalo dung-baited pitfall traps) at 20 sites across an anthropogenic forest disturbance gradient in Ba Be National Park, Vietnam.
2. We investigated measures of diversity, richness, and functional composition for individual taxa in relation to the degree of disturbance, and verified whether dung beetles and butterflies showed congruent trends.
3. For butterflies, overall species richness increased with forest disturbance, but the richness of rare species decreased. Species diversity was uncorrelated with disturbance.
4. In dung beetles, species richness was unrelated to forest disturbance, but species diversity increased with forest disturbance. The richness of dung beetles in the telecoprid (roller) guild declined with forest disturbance.
5. There was no significant correlation between dung beetles and butterflies across sites for either species richness or species diversity.
6. Apparent effects of disturbance were thus sensitive to the particular metric used (species richness or diversity), the taxonomic group studied (butterflies or dung beetles), and the functional group investigated (different guilds of dung beetle).
TL;DR: Tissue losses among the dislodged and fractured sea whips increased throughout the experimental period and were mainly due to predation by the nudibranch Tritonia diomedea, which appeared to react with a strong scavenging response to sea whip lying on the seafloor.
Abstract: The sea whip Halipteris willemoesi occurs in habitats coincident with bottom trawl fisheries in the Gulf of Alaska and Bering Sea and can be damaged by passing trawls. Little is known about the long-term fate of sea whips damaged by trawl gear. Sea whip responses to simulated trawl disturbance were observed in situ over a period of about 1 yr in order to assess delayed mortality from sublethal injuries. Colonies of H. willemoesi were randomly assigned to 3 treatment groups and 1 control group. Treatments were designed to mimic trawl damage including dislodgement, fracture of the axial rod, and soft tissue abrasion. Fifty percent of dislodged colonies demonstrated the ability to rebury their peduncles and recover to an erect position. Most of these colonies eventually became dislodged again without further disturbance and only one was erect at the final observation. None of the fractured colonies were able to repair their axial rods and only one was erect at the experiment’s conclusion. Light tissue abrasion caused only minor tissue losses that lessened over time, and all abraded and control colonies remained erect throughout the experiment. Tissue losses among the dislodged and fractured sea whips increased throughout the experimental period and were mainly due to predation by the nudibranch Tritonia diomedea, which appeared to react with a strong scavenging response to sea whips lying on the seafloor. The presence of predators in areas where sea whips are disturbed may exacerbate trawl effects since damaged or dislodged colonies are more vulnerable to predation.
TL;DR: In this article, the authors quantify the rates and spatial patterns of forest disturbance in private forests, state forests, and a National Park in the Polish Carpathians before and after the collapse of socialism.
Abstract: Forest use can increase substantially during periods of societal change, but it is unclear how harvesting rates differ among different landownership types in such times. Our goal here is to quantify the rates and spatial patterns of forest disturbance in private forests, state forests, and a National Park in the Polish Carpathians before and after the collapse of socialism. We analysed a series of classified Landsat TM images (1988–2000) and a landownership map. Our results showed that disturbance peaked in all ownership types in the immediate transition time. However, disturbance rates in private forests were about five times higher than on public lands. The spatial pattern of disturbances was similar across ownership types, but private forests were more fragmented than state and National Park forests. Our study indicates that institutional strength may determine forest use under different ownership types and highlights the multi-scale, nested control of the drivers of land use change.
TL;DR: In this paper, the ionospheric disturbance dynamo (Ddyn) is attributed to the reduction of the H component amplitude in the equatorial region which constitutes the main characteristic associated with a westward electric current flow, and the localization of the eastward current and consequently the position and the extent of the anti-Sq current vortex changes from one storm to another.
Abstract: . During magnetic storms, the auroral electrojets intensification affects the thermospheric circulation on a global scale. This process which leads to electric field and current disturbance at middle and low latitudes, on the quiet day after the end of a storm, has been attributed to the ionospheric disturbance dynamo (Ddyn). The magnetic field disturbance observed as a result of this process is the reduction of the H component amplitude in the equatorial region which constitutes the main characteristic of the ionospheric disturbance dynamo process, associated with a westward electric current flow. The latitudinal profile of the Ddyn disturbance dynamo magnetic signature exhibits an eastward current at mid latitudes and a westward one at low latitudes with a substantial amplification at the magnetic equator. Such current flow reveals an "anti-Sq" system established between the mid latitudes and the equatorial region and opposes the normal Sq current vortex. However, the localization of the eastward current and consequently the position and the extent of the "anti-Sq" current vortex changes from one storm to another. Indeed, for a strong magnetic storm, the eastward current is well established at mid latitudes about 45° N and for a weak magnetic storm, the eastward current is established toward the high latitudes (about 60° N), near the Joule heating region, resulting in a large "anti-Sq" current cell. The latitudinal profile of the Ddyn disturbance as well as the magnetic disturbance DP2 generated by the mechanism of prompt penetration of the magnetospheric convection electric field in general, show a weak disturbance at the low latitudes with a substantial amplification at the magnetic equator. Due to the intensity of the storm, the magnitude of the DP2 appears higher than the Ddyn over the American and Asian sector contrary to the African sector.
TL;DR: The results are should be particularly encouraging for managers because in many regions morphological trait data are not readily available, and compiling such data is a very time-intensive task and unlikely to be feasible for most managers to undertake.
TL;DR: It is found that disturbance history was only modestly related to exotic abundance overall, but fire frequency showed the strongest association, and effective CSS conservation will require developing means to curb the negative impacts of exotic plants, which may abound with or without severe or recent disturbance.
Abstract: Many ecosystems of conservation concern require some level of disturbance to sustain their species composition and ecological function. However, inappropriate disturbance regimes could favor invasion or expansion of exotic species. In southern California coastal sage scrub (CSS) fire is a natural disturbance, but because of human influence, frequencies may now be unnaturally high. Other anthropogenic disturbances such as grazing also occur in reserve areas. Managers charged with imposing or tolerating fire or other disturbance within their reserves are concerned that habitat quality may be degraded by an increasing abundance of exotic plants. We used vegetation monitoring data from Camp Pendleton, California, USA, to assess the correlation between past disturbances (frequent fire, agriculture, or grazing and mechanical disturbances) and current exotic species abundance in CSS. We found that disturbance history was only modestly related to exotic abundance overall, but fire frequency showed the strongest association. We also examined whether cover and richness of various native plant life forms (woody species, perennial herbs, and annual herbs) were more strongly influenced by disturbance history or by exotic-plant abundance. Native plant responses varied among life forms, but woody species and annual herbs were generally more strongly and negatively associated with exotic abundance than with disturbance. Effective CSS conservation will require developing means to curb the negative impacts of exotic plants, which may abound with or without severe or recent disturbance. Additionally, more focus should be given to understory herbs showing sensitivity to invasion. Though understudied, native herbs comprise the greatest portion of plant diversity in CSS and are critical to preservation of the community as a whole.
TL;DR: The results suggest that the growth of slow-growing Vaccinium species and soil microbes in boreal forests are not limited by N availability, however, significant changes in the proportion of dwarf shrubs to graminoids and a decrease in the biomass of V. myrtillus demonstrate the susceptibility of understorey vegetation to N enrichment.
Abstract: Question: How do N fertilization and disturbance affect the understorey vegetation, microbial properties and soil nutrient concentration in boreal forests?
Location: Kuusamo (66°22′N; 29°18′E) and Oulu (65°02′N; 25°47′E) in northern Finland.
Methods: We conducted a fully factorial experiment with three factors: site (two levels), N fertilization (four levels) and disturbance (two levels). We measured treatment effects on understorey biomass, vegetation structure, and plant, soil and microbial N and C concentrations.
Results: The understorey biomass was not affected by fertilization either in the control or in the disturbance treatment. Fertilization reduced the biomass of deciduous Vaccinium myrtillus. Disturbance had a negative effect on the biomass of V. myrtillus and evergreen Vaccinium vitis-idaea and decreased the relative proportion of evergreen species. Fertilization and disturbance increased the biomass of grass Deschampsia flexuosa and the relative proportion of graminoids. The amount of NH4+ increased in soil after fertilization, and microbial C decreased after disturbance.
Conclusions: Our results suggest that the growth of slow-growing Vaccinium species and soil microbes in boreal forests are not limited by N availability. However, significant changes in the proportion of dwarf shrubs to graminoids and a decrease in the biomass of V. myrtillus demonstrate the susceptibility of understorey vegetation to N enrichment. N enrichment and disturbance seem to have similar effects on understorey vegetation. Consequently, increasing N does not affect the rate or the direction of recovery after disturbance. Moreover, our study demonstrates the importance of understorey vegetation as a C source for soil microbes in boreal forests.