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Ecologists often invoke interspecific facilitation to help explain positive biodiversity–ecosystem function relationships in plant communities, but seldom test how it occurs. One mechanism through which one species may facilitate another is by ameliorating abiotic stress. Physiological experiments show that a chronic excess of light can cause stress that depresses carbon assimilation. If shading by a plant's neighbours reduces light stress enough, it may facilitate that plant's growth. If light is instead most often a limiting factor for photosynthesis, shading may have an adverse, competitive effect.
In a temperate tree diversity experiment, we measured stem growth rates and photosynthetic physiology in broadleaf trees across a gradient of light availability imposed by their neighbours. At the extremes, trees experienced nearly full sun (monoculture), or were shaded by nearby fast‐growing conifers (shaded biculture).
Most species had slower growth rates with larger neighbours, implying a net competitive effect. On the other hand, the two most shade‐tolerant species (Tilia americana and Acer negundo) and the most shade‐intolerant one (Betula papyrifera) had faster stem growth rates with larger neighbours. The two shade‐tolerant species had the greatest increases in photoinhibition (reduced dark‐acclimated Fv/Fm) across the gradient of increasing light availability, which suggests they are more vulnerable to chronic light stress. While most species had lower carbon assimilation rates in the shaded biculture treatment, T. americana had rates up to 25% higher. T. americana also dropped its leaves 3–4 weeks earlier in monocultures, curtailing its growing season.
We conclude that although large neighbours can cause light limitation in shade‐intolerant species, they can also increase growth through abiotic stress amelioration in shade‐tolerant species. Finally, in shade‐intolerant B. papyrifera, we find a pattern of stem elongation in trees with larger neighbours, which suggests that a shade avoidance response may account for the apparent positive trend in stem volume.
Synthesis. Both positive and negative species interactions in our experiment can be explained in large part by the photosynthetic responses of trees to the light environment created by their neighbours. We show that photosynthetic physiology can help explain the species interactions that underlie biodiversity–ecosystem function relationships. The insights that ecologists gain by searching for such physiological mechanisms may help us forecast species interactions under environmental change.
Survival and growth of forest tree seedlings are influenced by many abiotic and biotic forces that may vary across space and time. The simultaneous influences of habitat heterogeneity, temporal environmental variability (including disturbance regimes), and biotic interactions are difficult to disentangle, yet understanding the relative importance of these factors for tree seedling dynamics is critical for conservation of forest biodiversity. Most long term, spatially explicit studies of tree seedling ecology have been set in the tropics; much less attention has been given to tree seedlings in temperate forests. We monitored the survival of over 3000 individual seedlings over an eight-year period in a temperate forest in northeast Wisconsin, USA. Results from four seedling censuses demonstrated that both conspecific density and environmental variables significantly affected seedling survival. Higher densities of conspecific trees consistently reduced the probability of seedling survival over time. At the community level, relatively common species were negatively influenced by neighboring conspecific trees and exhibited higher per capita and per basal area mortality. The negative impacts of high conspecific tree density were most pronounced in areas of higher light and moisture, but these interactions varied over time, as did the importance of other abiotic variables. Patterns of less common species were more clearly explained by abiotic variables, with shifting relevance of specific variables according to species abundance. Conspecific negative density dependence, which interacts with resource gradients, and habitat conditions that shift over time are influencing community composition in this northern temperate forest.
Debris flows are one of the most common geomorphic processes in steep mountainous areas. The control of their propagation on alluvial fans is fundamental; valley bottoms are usually characterised by high damage potential because they contain concentrations of inhabitants and infrastructure. It is well known that forests have a protective function in that they reduce the triggering of debris flows, as well as hinder their motion and promote deposition, but a quantitative assessment of these effects is still lacking. Using laboratory experiments that simulate debris-flow depositional processes, this research investigated the ability of forests to reduce debris-flow runout and depositional area. The experiments considered two different forest types, high forests and coppice forests, and four volumetric concentrations of sediment (0.50, 0.55, 0.60, and 0.65). The results confirmed that the sediment concentration of the flow is a key factor in determining the geometry of the deposits. On the other hand, forests can reduce debris-flow runout distance and, in general terms, affect the characteristics of their deposits. The results showed that vegetation appear to reduce debris-flow motion especially when the debris-flow kinematic load at the fan apex is low. About the sediment concentration of the mixture, high forest did not exhibit a clear behaviour while coppice forest appears to promote significant deposition at all of the tested concentrations, and this effect increases with the solid concentration (reductions in runout between approximately 20% and 30% at CV=0.50 and CV=0.65, respectively, were observed). Due to their higher tree density, in fact, coppice forests seem to have a better protective effect than the rigid trunks of high forest trees. For this last type of forest, a relationship between the H/L ratio, which represents energy dissipation, have been found and compared with the scenario without forest.
Landslides influence the capacity for safe and sustainable development of mountainous environments. This study explores the spatial distribution of and the interactions between landslides that are mapped using global positioning system (GPS) and extensive field surveys in Mazandaran Province, Iran. Point-pattern assessment is undertaken using several univariate summary statistical functions, including pair correlation, spherical-contact distribution, nearest-neighbor analysis, and O-ring analysis, as well as bivariate summary statistics, and a mark-correlation function. The maximum entropy method was applied to prioritize the factors controlling the incidence of landslides and the landslides susceptibility map. The validation processes were considered for separated 30% data applying the ROC curves, fourfold plot, and Cohen's kappa index. The results show that pair correlation and O-ring analyses satisfactorily predicted landslides at scales from 1 to 150 m. At smaller scales, from 150 to 400 m, landslides were randomly distributed. The nearest-neighbor distribution function show that the highest distance to the nearest landslide occurred in the 355 m. The spherical-contact distribution revealed that the patterns were random up to a spatial scale of 80 m. The bivariate correlation functions revealed that landslides were positively linked to several linear features (including faults, roads, and rivers) at all spatial scales. The mark-correlation function showed that aggregated fields of landslides were positively correlated with measures of land use, lithology, drainage density, plan curvature, and aspect, when the numbers of landslides in the groups were greater than the overall average aggregation. The results of analysis of factor importance have showed that elevation (topography map scale: 1:25,000), distance to roads, and distance to rivers are the most important factors in the occurrence of landslides. The susceptibility model of landslides indicates an excellent accuracy, i.e., the AUC value of landslides was 0.860. The susceptibility map of landslides analyzed has shown that 35% of the area is low susceptible to landslides.
The Janzen‐Connell (JC) hypothesis is a major ecological explanation for high species richness, in particular in tropical forest ecosystems. Central components of the JC hypothesis are noncompetitive effects of distance and density dependence, two drivers that contribute independently to species coexistence, but are ultimately linked in the field. However, although numerous studies provide evidence for either distance‐ or density‐dependent effects based on observational data, experimental testing of simultaneous and interactive effects of distance and density has rarely been conducted, especially in a comprehensive multispecies approach. Here, we make use of the forest Biodiversity‐Ecosystem Functioning project (BEF) –China to estimate distance‐ and density‐dependent effects in a reciprocal tree seedling transplant experiment of 11 tree species. We deployed 13,490 juveniles of all 11 species in their own (home) and in all foreign monocultures (away), as well as at three different levels of planting density, thereby testing for distance and density effects, respectively. In addition, to quantify the amount to which density effects were brought about by potential additional effects of intraspecific competition, we set up a common garden experiment with different levels of planting density, where an additional “shadow” treatment controlled for effects of canopy shading. Although the “away” and “high‐density” treatments significantly impaired the performance and productivity of seedlings, leaf damage and survival was exclusively affected by either the home/away or the density treatment, respectively. Negative density‐dependent effects on leaf damage were less pronounced in the “home” treatment, showing that the effects were not additive. In addition, results obtained in the Common Garden Experiment showed that negative effects of high density may be also brought about by intraspecific competition as an alternative density‐responsive mechanism and less by true JC effects. Overall, our results provide strong support on a multispecies basis for the influence of host‐specific effects already operating in early stages of a forest plantation. However, they also emphasize the need to account appropriately for potential additional density‐responsive mechanisms such as intraspecific competition or microenvironmental conditions when addressing the role of JC effects for species coexistence.
An important factor controlling tree species diversity is conspecific density dependence (CDD). Adult trees associated with arbuscular mycorrhiza (AM) and ectomycorrhiza (ECM) can exhibit negative and positive CDD effects on conspecific recruitment, respectively. However, the extent to which these mycorrhizal associations affect spatial distributions of individual trees and their relative abundances within forests through CDD remains uncertain. We analysed changes in spatial correlations between adults and conspecific juveniles at different growth stages of five hardwood species in a 6-ha plot of an old-growth forest using a point pattern analysis. The clump sizes of large individuals were also evaluated using the Iδ index (a measure of individual dispersion) in 24 species. In two AM-associated species, juveniles were distributed at greater distances with increasing size or were always distributed at a distance from adults, resulting in small clumps of adults. In contrast, juveniles of two ECM-associated species were distributed close to adults during early or late growth stage, resulting in large clumps of adults. Juveniles of an ECM-associated species disappeared with increasing size, probably due to shade intolerance. In 24 tree species with large numbers of individuals within a plot, the relative basal area was related to both mycorrhizal type and maximum diameter, suggesting that the relative abundance of a species is largely related to its mycorrhizal associations and maximum plant size. This study strongly demonstrated that mycorrhizal associations play an important role in determining the spatial distribution patterns and community structure of tree species through CDD.
Maintaining tree diversity
Negative interaction among plant species is known as conspecific negative density dependence (CNDD). This ecological pattern is thought to maintain higher species diversity in the tropics. LaManna et al. tested this hypothesis by comparing how tree species diversity changes with the intensity of local biotic interactions in tropical and temperate latitudes (see the Perspective by Comita). Stronger local specialized biotic interactions seem to prevent erosion of biodiversity in tropical forests, not only by limiting populations of common species, but also by strongly stabilizing populations of rare species, which tend to show higher CNDD in the tropics.
Science , this issue p. 1389 ; see also p. 1328
1. Mixed conifer-hardwood forests can be more productive than pure forests and they are increasingly considered as ecosystems that could provide adaptation strategies in the face of global change. However, the combined effects of tree-to-tree competition, rising atmospheric CO2 concentrations and climate on such mixtures remain poorly characterized and understood.
2. To fill this research gap, we reconstructed 34-year series (1980-2013) of growth (basal area increment, BAI) and intrinsic water-use efficiency (iWUE) of Scots pine (Pinus sylvestris L.) – European beech (Fagus sylvatica L.) mixed stands at two climatically contrasting sites located in the southwestern Pyrenees. We also gathered data on tree-to-tree competition and climate variables in order to test the hypotheses that (i) radial growth will be greater when inter-specific competition exceeds intra-specific competition, i.e. when species complementarity occurs, and (ii) enhanced iWUE could be linked to improved stem radial growth.
3. Growth of both species was reduced when intra-specific competition increased. Species complementarity was linked to improved growth of Scots pine at the continental site, whilst competition overrode any complementarity advantage at the drought-prone Mediterranean site. Beech growth did not show any significant response to pine admixture likely due to shade tolerance and the highly competitive nature of this species. Increasing inter-specific competition drove recent iWUE changes, which increased in Scots pine but decreased in European beech. The iWUE enhancement did not involve any growth improvement in Scots pine. However, the positive BAI-iWUE relationship found for beech suggests an enhanced beech growth in drought-prone sites due to improved water use.
4. Synthesis. Complementarity may enhance growth in mixed forests. However, water scarcity can constrict light-related complementarity for shade intolerant species (Scots pine) in drought-prone sites. BAI-iWUE relationships were negative for Scots pine and positive for European beech. These contrasting behaviours have got implications for coping with the expected increasing drought events in Scots pine-European beech mixtures located near the ecological limit of the two species. Complementarity effects between tree species should be considered to avoid overestimating the degree of future carbon uptake by mixed conifer-broadleaf forests.
Tree roots have long been recognized to increase slope stability by reinforcing the strength of soils. Slope stability models include the effects of roots by adding an apparent cohesion to the soil to simulate root strength. No model includes the combined effects of root distribution heterogeneity, stress-strain behavior of root reinforcement, or root strength in compression. Recent field observations, however, indicate that shallow landslide triggering mechanisms are characterized by differential deformation that indicates localized activation of zones in tension, compression, and shear in the soil. These observations contradict the common assumptions used in present models. Here we describe a new model for slope stability that specifically considers these effects. The model is a strain-step discrete element model that reproduces the self-organized redistribution of forces on a slope during rainfall-triggered shallow landslides. We use a conceptual sigmoidal-shaped hillslope with a clearing in its center to explore the effects of tree size, spacing, weak zones, maximum root-size diameter, and different root strength configurations. The model is driven by root data of Norway spruce obtained from laboratory and field measurements. Simulation results indicate that tree roots can stabilize slopes that would otherwise fail without them and, in general, higher root density with higher root reinforcement results in a more stable slope. Root tension provides more resistance to failure than root compression but roots with both tension and compression offer the best resistance to failure. Lateral (slope-parallel) tension can be important in cases when the magnitude of these forces is comparable to the slope-perpendicular tensile forces. In these cases, lateral forces can bring to failure tree-covered areas with high root reinforcement. Slope failure occurs when downslope soil compression reaches the soil maximum strength. When this occurs depends on the amount of root tension upslope in both the slope-perpendicular and slope-parallel directions. Roots in tension can prevent failure by reducing soil compressive forces downslope. When root reinforcement is limited, hillslopes form a crack parallel to the slope near its top. Simulations with roots that fail across this crack always resulted in a landslide. Slopes that did not form a crack could either fail or remain stable, depending on root reinforcement. Tree spacing is important for the location of weak zones but tree location on the slope (with respect to where a crack opens) is as important. Finally, for the specific cases tested here, large roots, greater than 20 mm, are too few too contribute significantly to root reinforcement. Omitting roots larger than 8 mm predicted a landslide when none should have occurred. Intermediate roots (5 to 20 mm) appear to contribute most to root reinforcement and should be included in calculations.
To fully understand the mechanisms of shallow landslide triggering requires a complete re-evaluation of the traditional apparent-cohesion approach that does not reproduce the incremental loading of roots in tension or in compression. Our model shows that it is important to consider the forces held by roots in a way that is entirely different than done thus far. Our work quantifies the contribution of roots in tension and compression which now finally permits to analyze more realistically the role of root reinforcement during the triggering of shallow landslides.
Forests can decrease the risk of shallow landslides by mechanically reinforcing the soil and positively influencing its water balance. However, little is known about the effect of different forest structures on slope stability. In the study area in St Antönien, Switzerland, we applied statistical prediction models and a physically-based model for spatial distribution of root reinforcement in order to quantify the influence of forest structure on slope stability. We designed a generalized linear regression model
and a random forest model including variables describing forest structure along with terrain parameters for a set of landslide and control points facing similar slope angle and tree coverage. The root distribution measured at regular distances from seven trees in the same study area was used to calibrate a root distribution model. The root reinforcement was calculated as a function of tree dimension and distance from tree with the root bundle model (RBMw). Based on the modelled values of root reinforcement, we introduced
a proxy-variable for root reinforcement of the nearest tree using a gamma distribution. The results of the statistical analysis show that variables related to forest structure significantly influence landslide susceptibility along with terrain parameters. Significant effects were found for gap length, the distance to the nearest trees and the proxy-variable for root reinforcement of the nearest tree. Gaps longer than 20 m critically increased the susceptibility to landslides. Root reinforcement decreased with increasing distance from trees and is smaller in landslide plots compared to control plots. Furthermore, the influence of forest structure strongly depends on geomorphological and hydrological conditions. Our results enhance the quantitative knowledge about the influence of forest structure on root reinforcement and landslide susceptibility and support existing management recommendations for protection against gravitational natural hazards.
Reforestation after a landslide facilitates competition between herbaceous plants and arborous plants. Tangible variations in grassland areas in regions susceptible to landslides can only be found within collections of trees. A landslide area in the Sule Watershed was investigated. Relative illuminance results reveal that the Rhodes grass (Chloris gayana Kunth) biomass in this landslide area increases with relative illuminance. A comparison of regions with tree islands indicates that the size of the grassland areas decreased and the number of tree islands increased during 2005-2010. Furthermore, a germination experiment in a soil-seed bank indicates that more woody plant species exist around the tree island than in other areas in the landslide region. Trees in a tree island change the micro-climate of the landslide region, and they gather as many nutrients and as much moisture as possible, enabling vegetation to expand around the tree island. Additionally, the area with Rhodes grass and its biomass declined annually in the tree island region. Investigation results show that tree islands and soil-seed banks are suited to reforestation in landslide regions. The pioneering research will assist regional landslide management in Taiwan.
Aims
Plant roots play an important role in the stability of the slope. The efficiency of the vegetation in the slope stability relies on the planting layout on the slope. Vegetation located at the upper, middle, and lower slopes is investigated to determine the influence of the spatial layout of planting on the stability of slopes. Additionally, the role of plant roots in the mechanical mechanism of the vegetated slope is studied.
Methods
A simple root system architecture with one tap root and four lateral roots is selected, and 3D root structure is integrated with the soil. The 3D finite element analysis is used to model the stability of vegetated slopes.
Results
Soil arching takes place in the soil between root systems. As the spacing between root system increases, the effect of vegetation on the slope stability decreases. The effect of vegetation at the upslope and mid-slope on the safety factor of the slope is better than that at the downslope if the plant root system penetrates into the firm ground.
Conclusions
Assessment of the stability of vegetated slopes may be affected by the root structure pattern, the relationship between the root system and the firm ground, and the modeling method of the root system in the ground.
The influence of root reinforcement on shallow landsliding has been well established through mechanistic and empirical studies, yet few studies have examined how local vegetative patterns influence slope stability. Because root networks spread outward from trees, the species, size, and spacing of trees should influence the spatial distribution of root strength. We documented the distribution and characteristics of trees adjacent to 32 shallow landslides that oc- curred during 1996 in the Oregon Coast Range. Although broadly classified as a conifer-dominated forest, we observed sparse coniferous and abundant hardwood trees near landslide scars in an industrial forest (Mapleton) that experienced widespread burning in the 19th century. In industrial forests that were burned, selectively harvested, and not replanted (Elliott State Forest), swordfern was ubiquitous near landslides, and we observed similar numbers of live conifer and hardwood trees proximal to landslide scarps. We demonstrate that root strength quantified in landslide scarps and soil pits correlates with a geometry-based index of root network contribution derived from mapping the size, species, condi- tion, and spacing of local trees, indicating that root strength can be predicted by mapping the distribution and charac- teristics of trees on potentially unstable slopes. In our study sites, landslides tend to occur in areas of reduced root strength, suggesting that to make site-specific predictions of landslide occurrence slope stability analyses must account for the diversity and distribution of vegetation in potentially unstable terrain.
For over a century, the negative exponential relationship of tree density to diameter has been observed and considered an indicator of equilibrium forest structure, especially at the stand level. More recent studies have suggested that other forms of diameter distributions should be considered “balanced”. Some of these seemingly contradictory findings can be resolved by understanding the impacts of scale and sampling area on the possibility of observing a negative exponential distribution. We calculated the minimum area required to sample the smallest number of trees necessary to produce hypothetical negative exponential distributions. The minimum area is influenced greatly by the maximum diameter and the exponential decrease rate of the distribution. It is also influenced by the width of diameter classes used to summarize the data, and the basal area per hectare of the sample. These differences in minimum required sampling area are important considerations because some recent studies that suggest a rotated sigmoid distribution, rather than negative exponential, is more characteristic of old-growth forests may have used sampling schemes that were insufficient to observe a negative exponential distribution even if one existed. Others have used criteria in selecting sampling locations that ensure a high density of large trees, thereby precluding them from observing a negative exponential distribution even if one existed. Our review suggests that within-stand sampling areas for testing diameter distribution in temperate forests typically need to exceed 1ha, especially in mature to old-growth stages, though mortality rate, diameter class width, and total stand basal area all affect the area or number of prism points needed. Diameter distributions have rarely been assessed at the landscape scale. Nonetheless, it is reasonable to expect that landscape distributions may generally be negative exponential and the slope of the line (i.e. the mortality rate) may be an important measure of landscape dynamics. Old-growth landscapes where natural disturbances are typically small patches will have a lower mortality rate than those where disturbances are more frequent and widespread.
The influence of plant diversity on slope stability was investigated at early phases of succession in a mixed forest in Sichuan, China. The first phase comprised big node bamboo (Phyllostachys nidularia Munro) only. In the second phase, bamboo co-existed with deciduous tree species and in the third phase, deciduous species existed alone. Root density at different depths and root tensile strength were determined for each species. The factor of safety (FOS) was calculated for slopes with and without vegetation for each succession phase. For phase 2, FOS was determined for different species mixtures and positions. In phase 3, simulations were performed with a single tree at the top, middle or toe of the slope. Due to its shallow root system, bamboo contributed little to slope stability. In simulations with the tree at the top or middle of the slope, FOS decreased because tree weight added a surcharge to the slope. FOS increased with the tree at the bottom of the slope. Different mixtures of species along the slope had no influence on FOS. Differences in root tensile strength between species played a small role in FOS calculations, and tree size and density were the most important factors affecting slope stability, excluding hydrological factors.
Mitigation of the hazards posed by debris flows requires an understanding of the mechanisms leading to their initiation. The objectives of this study were to evaluate and document the hydrologic response of a potential debris-flow source area to major rainstorms and to evaluate whether traditional models of hillslope hydrology can account for the observed response. A field site in an area of previous debris-flow activity was instrumented and monitored for two winter seasons. Hydrologic responses for a wide variety of antecedent conditions were recorded, including two storm events that produced well-defined positive pore-pressure pulses at the site and initiated numerous debris flows in the immediate vicinity of the site. The observed hydrologic response was highly dependent on antecedent moisture conditions which can be characterized by soil matric suction measurements. The pressure-head pulses observed had a magnitude of approximately 50 cm of water, were transient, traveled downslope, and exhibited some spatial variability. Traditional models of hillslope hydrology do not fully account for the positive pore-pressure pulses observed high on the hillslope. Key words: debris flow, hillslope hydrology, pore pressure, antecedent moisture, tensiometer, piezometer, field investigation.
To understand landslide regeneration and provide information necessary for restoration, we sampled seed rain, seed pool, and plant cover on two Ecuadorian landslides. We trapped 1304 seeds and found that, while most seeds were in the family Asteraceae, there was substantial variation in seed rain among plant families. Four hundred and seventy-five seedlings emerged from soil samples, including nonvascular and vascular families; again, species in Asteraceae dominated, with species in Piperaceae also very common. Plant cover, consisting of members of four fern families and 20 vascular plant families—with species in Asteraceae, Melastomataceae and Poaceae most common—was scored as a percentage of the total plot area. Principal components analysis (PCA) showed that, for all three of these plant life stages (seed rain, seed-propagule pool, plant cover), spatial variation was dominated by differences between the two landslides rather than within-landslide plot differences. PCA also showed that plots separated best on axes defined by the families Cecropiaceae, Urticaceae, Melastomataceae, Papilionaceae, Asteraceae, and Araceae with clumping of families in PCA space suggesting common successional strategies. Another multivariate technique, canonical correspondence analysis (CCA), showed that the combined seed rain and seed pool data could predict the percent cover of the family Verbenaceae and that the current plant cover families could predict Asteraceae seeds and seedlings. Finally, we use our past and present landslide data, along with multivariate modeling results, to suggest strategies for successful landslide restoration.
Slope stability models traditionally use simple indicators of root system structure and strength when vegetation is included
as a factor. However, additional root system traits should be considered when managing vegetated slopes to avoid shallow substrate
mass movement. Traits including root distribution, length, orientation and diameter are recognized as influencing soil fixation,
but do not consider the spatial and temporal dimensions of roots within a system. Thick roots act like soil nails on slopes
and the spatial position of these thick roots determines the arrangement of the associated thin roots. Thin roots act in tension
during failure on slopes and if they traverse the potential shear zone, provide a major contribution in protecting against
landslides. We discuss how root traits change depending on ontogeny and climate, how traits are affected by the local soil
environment and the types of plastic responses expressed by the plant. How a landslide engineer can use this information when
considering slope stability and management strategies is discussed, along with perspectives for future research. This review
encompasses many ideas, data and concepts presented at the Second International Conference ‘Ground Bio- and Eco-engineering:
The Use of Vegetation to Improve Slope Stability—ICGBE2’ held at Beijing, China, 14–18 July 2008. Several papers from this
conference are published in this edition of Plant and Soil.
The role of vegetation in preventing shallow soil mass movement is now fairly well understood, particularly at the individual plant level. However, how soil is reinforced on a larger scale and the influence of changes in vegetation over time has rarely been investigated. Therefore we carried out a study on the temporal and spatial changes within stands of Cryptomeria japonica D. Don, growing in the Sichuan province of China, an area where shallow landslides are frequent.Soil cores were taken from three neighbouring stands of C. japonica aged 9, 20 and 30 years old and growing on steep slopes. Cores were taken from around trees and the root (<10 mm in diameter) biomass density (root density (RD)) present in each core was measured at different depths. The spatial position of trees at each site was noted and soil shear strength was measured. The tensile strength of a sample of roots from each stand was measured. Using the RD data, the root area ratio (RAR) could be estimated. RAR and root tensile strength were used as input to a model of root reinforcement which determines the additional cohesion, cr, or contribution of vegetation to soil. Data were then incorporated into a two-dimensional model of slope stability developed in the finite element (FE) code, Plaxis, which calculates the safety factor (FOS), or likelihood of a slope to fail under certain circumstances. We calculated the FOS of slopes with and without C. japonica, taking into account the spatial position of trees at each stand.Results showed that RD was highest in the 9-year-old stand, but that root tensile strength was lowest. In the 30-year-old stand, RD was low but a higher root tensile strength compensated for the decrease in RAR. The FOS increased by only 15–27% when vegetation was present, with the greatest augmentation in the 9-year-old stand. The older stands had been thinned over the years, resulting in large gaps between trees, which would be prone to local soil slippage. This spatial effect was reflected in the FE analysis, which showed a significant relationship between the number of trees and distance between groups of trees in the 20- and 30-year-old stands only. Therefore, when managing fragile slopes, care should be taken when thinning, so that large gaps do not exist between trees, the influence of which is accrued over time.
Natural root grafts occur in many tree species, but this widespread phenomenon has received only little attention. For many years, physiological aspects such as transfer of organic materials, water and minerals were considered their major significance. Better anchorage in flooded areas or in windy habitats was also proposed to select for this character. I propose that root grafts provide several additional types of benefit to intact grafted trees, to neighbors of trees that have lost their crowns, and sometimes even to the ones that have lost their crowns. These include: being a mate (female, male or both) for one's own gametes; taking a chance that the grafted neighbor will lose its crown, leaving it with its neighbor's grafted root system; for trees that reproduce vegetatively, there is a good chance that the neighbor is a ramet of the same genet, and root grafting thus supports the same genotype, and since most seeds are dispersed near the parent tree there is a good chance that the grafted neighbor is genetically close. For a grafted root system that has lost its original crown, the genotype continues to live and in certain taxa it still has a chance to resume canopy growth and reproduction. While root grafts may enable acquisition of beneficial fungi or microorganisms from the grafted neighbor, there is a risk of pathogen transmission. Since roots produce various toxins that defend the canopy, root grafts with other genotypes that provide additional types of defensive molecule may increase the tree's resistance to various herbivores and pathogens. In spite of the potential benefits, pathogen transmission and increased neighbor competition may select against the characteristic of root grafting.
The mechanistic basis underpinning forest succession is the gap-phase paradigm in which overstory disturbance interacts with seedling and sapling shade tolerance to determine successional trajectories. The theory, and ensuing simulation models, typically assume that understory plants have little impact on the advance regeneration layer's composition. We challenge that assumption by reviewing over 125 papers on 38 species worldwide that form dense and persistent understory canopies. Once established, this layer strongly diminishes tree regeneration, thus altering the rate and direction of forest succession. We term these dense strata recalcitrant understory layers. Over half of the cases reviewed were linked to increases in canopy disturbance and either altered herbivory or fire regimes. Nearly 75% of the studies declared that competition and allelopathy were the likely interference mechanisms decreasing tree regeneration, yet only 25% of the studies used manipulative field experiments to test these putative mechanisms. We present a conceptual model that links the factors predisposing the formation of recalcitrant understory layers with their interference mechanisms and subsequent impacts on succession. We propose that their presence constricts floristic diversity and argue for their explicit inclusion in forest dynamics theory and models. Finally, we offer management suggestions to limit their establishment and mitigate their impacts.
That tree species mixing may strongly affect tree structure and tree growth is so far hardly considered in tree and stand models. Hence, for a better understanding and design of mixed species stands, the inter-specific facilitation and competition needs better representation in individual tree models. Here, we show for the five most common tree species combinations in Central Europe that mixing causes facilitation and competition reduction in mixed stands and how such effects can be implemented in individual tree stem diameter growth models.
This study was based on 62 long-term experimental plots belonging to 10 chronosequences in Germany with repeated spatially explicit stand inventories from 1991 to 2016. They covered monospecific and mixed species stands of Norway spruce (Picea abies [L.] Karst.), Scots pine (Pinus sylvestris L.), European beech (Fagus sylvatica L.), sessile oak (Quercus petraea [Matt.] Liebl.), European ash (Fraxinus excelsior L.), and sycomore maple (Acer pseudoplatanus L.). The study represented medium and high qualitative site conditions, fully stocked, or only moderately thinned stands of 22–238 years of age, and the mixing patterns ranged from individual-tree to cluster-mixtures.
Based on spatially explicit measurements, we quantified for each tree the intra- or interspecific neighborhood, local stand density, and growth. We applied mixed models to analyze how inter-specific neighborhood modified tree growth.
First, we showed that the inter-specific neighborhood can increase tree growth significantly beyond the level of open-grown trees in intra-specific neighborhoods (net faciltation). The potential growth rates of the fastest growing trees in mixed stands were 14–78% higher compared to monospecific stands matched for size, crown, and site index. The mixing effect differed between species combinations. Second, we provided evidence that mixing on average reduced competition by 16%, and that this effect increased with the mixing proportion. In four out of the five species combinations we observed a competition reduction of both associated tree species (mutualistic relationship). Third, for the mixtures of Norway spruce/European beech and sessile oak/European beech that covered a broader range of site conditions, we found a mainly positive modulation of facilitation and competition depending on site conditions.
We discussed the potential causes for the observed facilitation and competition reduction, their implementation in tree growth models, and the relevance of the findings for the design and management of mixed-species stands.
Allelopathy (i.e., chemical interaction among species) was originally conceived as inclusive of positive and negative effects of plants on other plants, and we adopt this view. Most studies of allelopathy have been phenomenological, but we focus on studies that have explored the ecological significance of this interaction. The literature suggests that studies of allelopathy have been particularly important for three foci in ecology: species distribution, conditionality of interactions, and maintenance of species diversity. There is evidence that allelopathy influences local distributions of plant species around the world. Allelopathic conditionality appears to arise through coevolution, and this is a mechanism for plant invasions. Finally, allelopathy promotes species coexistence via intransitive competition, modifications of direct interactions, and (co)evolution. Recent advances additionally suggest that coexistence might be favored through biochemical recognition. The preponderance of phenomenological studies notwithstanding, allelopathy has broad ecological consequences.
Expected final online publication date for the Annual Review of Ecology, Evolution, and Systematics, Volume 52 is November 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
Root reinforcement is a mechanism through which forests contribute to the prevention and mitigation of shallow instabilities in soils, one of the main hazards in mountain areas. This study aims to assess how spatial tree distribution and whether thinning operations affect root reinforcement in the most common forest types of the European Southern Alps. We measured size and position of the trees of 119 stands, belonging to sweet chestnut, European beech, Norway spruce, and spruce-beech-silver fir forest types. We developed, calibrated and validated a model for estimating root reinforcement at the stand level, using the spatial distribution of tree diameter as the input variable. Finally, we simulated a thinning of 18% stand basal area, selecting removal trees from smaller diameter classes (low thinning) or from random clusters (random cluster thinning), and assessed its effect on root reinforcement. Root reinforcement statistically differed among forest types and the lowest values were found in the sweet chestnut forest type. Irrespective of the forest type, low thinning did not significantly modify root reinforcement, while random cluster thinning reduced it five-fold.
Model-driven individual-based forest ecology has emerged in the 1990s and has given rise to a wealth of publications. At the same time, individual-based methods in forest management have been refined in a number of different countries and steadily grow in importance. For the first time this book integrates three main fields of forest ecology and management, i.e. tree/plant interactions, biometry of plant growth and human behaviour in forests. Individual-based forest ecology and management is an interdisciplinary research field with a focus on how the individual behaviour of plants contributes to the formation of spatial patterns that evolve through time. Key to this research is a strict bottom-up approach where the shaping and characteristics of plant communities are understood to be mostly the result of interactions between plants and between plants and humans. Written in a highly accessible style, the book provides essential information on theories and concepts of individual-based forest ecology and management and introduces point process statistics for analysing plant interactions. This is followed by methods of spatial modelling with a focus on individual-based models. The text is complemented by key concepts of modern plant growth science. Finally new methods of measuring, analysing and modelling human interaction with trees in forest ecosystems are introduced and discussed. For better access and understanding, all methods introduced in this book are accompanied by example code ready to use in the statistical software R and by worked examples. Additional technical details are given in three appendices.
Tree height is essential for assessing carbon budgets and biodiversity. One of the most commonly used assessment methods is a field survey. However, this approach is extremely challenging for obtaining highly accurate estimates in forests with tall and dense canopies. In this study, we utilized airborne remotely sensed mean canopy height (MCH) spatial coverage acquired by high-cost airborne light detection and ranging (lidar) and low-cost unmanned aerial vehicle (UAV) sensors to quantify the tree height for a 590-ha complex tropical forest in the mountainous region of central Taiwan. The performances of the acquisition techniques were evaluated by comparing the statistical relationships of MCH from lidar and MCH from UAV with the concurrently obtained field mean tree height measurement (MTH) at the plot (25 × 20 m) scale. In addition, we further analyzed the forest structural variables that may influence lidar and UAV MCHs by using a general linear model. The results showed that both MCHs derived from lidar and UAV accurately estimated MTH. MCH from UAV had a superior performance to that of a small model offset, and the slope of the model fit line was close to one, which was possibly due to the finer spatial resolution of the UAV imagery. MCH from lidar may be utilized to delineate the entire vertical profile of a forest stand, but MCH from UAV can only detect the upper half of the canopies. This is a result of instrument and data differences. General linear model statistics revealed that the maximum stand height and mean tree age may be the major forest stand structure determinants affecting MCH estimates, which might indicate that the airborne estimations of mean canopy height are mainly governed by large trees within a forest stand.
Tree roots provide surface erosion protection and improve slope stability through highly complex interactions with the soil due to the nature of root systems. Root reinforcement estimation is usually performed by in‐situ pullout tests, in which roots are pulled out of the soil to reliably estimate the root strength of compact soils. However, this test is not suitable for the scenario where a soil progressively fails in a series of slump blocks, for example, in unsupported soils near streambanks and road cuts where the soil has no compressive resistance at the base of the hillslope. The scenario where a soil is unsupported on its downslope extent and progressively deforms at a slow strain rate has received little attention, and we are unaware of any study on root reinforcement that estimates the additional strength provided by roots in this situation. Thus, we designed two complementary laboratory experiments to compare the force required to pull the root out. The results indicate that the force required to pullout roots is reduced by up to 50% when the soil fails as slump blocks compared to pullout tests. We also found that, for slump block failure, roots had a higher tendency to slip than to break, showing the importance of active earth pressure on root reinforcement behaviour, which contributes to reduced friction between soil and roots. These results were then scaled up to a full tree and tree stand using the root bundle and field‐measured spatial distributions of root density. Although effects on the force mobilized in small roots can be relevant, small roots have virtually no effect on root reinforcement at the tree or stand scale on hillslopes. When root distribution has a wide range of diameters, the root reinforcement results are controlled by large roots that hold much more force than small roots. This article is protected by copyright. All rights reserved.
Typhoons are the common natural disturbances in Taiwan. In order to study species regeneration, coexistence and maintenance of biodiversity of forests, immediate field surveys and monitoring should be done. In 2008, we surveyed a forest after typhoon season, and monitored species regeneration in the Lienhuachih forest dynamics plot in central Taiwan. Our results showed 11 landslide areas caused by typhoons totalled 9159.47 m², and most landslide areas were located in the valley and on lower slopes. Strong winds caused trees to fall down and branches or the canopy to be damaged; heavy rainfall caused tree death or disappearance. The typhoon caused greater damages to trees with a small diameter at breast hight than to larger ones. The typhoon also caused greater damage to the number of individuals and basal area of pioneer and rare species than it did to dominant species. In addition, we set up 1χ1 m plots and divided landslides into 3 areas, the center of the landslide area, edge of the landslide area, and non-landslide areas, to study differences in regeneration of tree species and understory coverage during 2009-2012. Our results showed that for landslide areas and edges of landslide areas, the density, number of species, and understory coverage increased, but the proportion of pioneer species decreased. The number of seedlings that regenerated changed very dramatically, especially in landslide areas. In contrast, in non-landslide areas, the density, number of species, recruitment, and proportion of pioneer species changed less than in the other 2 areas or were stable. The proportion of pioneer species and average growth per seedling were significantlies higher than in the other 2 areas. Then, we used the Sorenson similarity index to understand similarity between regenerated seedlings and overstory trees. The Sorenson similarity index was the lowest at 26.82% in landslide areas, 37.38% in edges of landslide areas, and 41.65% in non-landslide areas. Overall, our study concluded that when typhoons hit landslide formed, more trees were damaged by heavy rainfall than by strong winds in 2008, and new niches were formed, which benefited tree species regeneration and maintained tree diversity and coexistence in the Lienhuachih broadleaf forest.
Studies aiming at assessment of factors influencing the interdependence among species often rely on assumptions that can significantly change the results. The goal of this study is to evaluate the impact of analytical assumptions on spatial arrangements of trees within mixed species stands. We used the O-ring statistics computed for two species with different shade tolerances (i.e., Fagus sylvatica L. and Quercus petraea L.). The O-ring parameters (i.e., inside radius and width) were evaluated within a replicated design. It was found that inference depends on both radius and width of the ring. We found a sinusoidal dependence between species, with repulsion prevalent at distances less than 1.5 heights of the dominant trees. Same species seems to aggregate at distances ½ height of the dominant trees, and repulse at small and large distances (approximately 8 m and ⩾30 m, respectively). Different species could assist one – another in establishing the spatial arrangement of the stand, but do not help each-other when size is of importance, as competition for common resources is stronger than for the non-common ones. Using reductio ad absurdum we found that width of the ring could change the interpretation. For the mixture European beech – sessile oak narrow rings should be used for research focused on location while wider rings are suited to size related analyses. Employment of inadequate values for analytical tools, such as ring width, can fail to reveal spatial relationships. The existence of parameter values that capture better particular processes indicates that different hypotheses should be investigated considering some intrinsic properties of the analysed processes.
Root reinforcement is a key factor when dealing with slope stability problems and is an important quantitative criterion for the evaluation of the protective function of forests against shallow landslides, as well as for the adoption of appropriate practices in protection forest management. Although many models have been developed to estimate root reinforcement, a reliable quantification that considers both its spatial and temporal variability still remains a challenge. This work aims to extend the understanding of the long term spatial and temporal dynamics of root reinforcement after forest harvest in subalpine spruce forests by supplying new experimental data and applying a state-of-the-art model.We estimated root reinforcement decay 5, 10 and 15 years after timber had been harvested in spruce stands in a small catchment in the Swiss Alps. We collected root distribution data at different distances from the trees and calibrated and validated a root distribution model (RootDis). To estimate root mechanical properties, we tested roots up to 12 mm diameter in the field, and computed root reinforcement for each case study with the Root Bundle Model. Finally, we developed a new model for the estimation of root reinforcement decay, based on the observed change in root distribution after felling and on the decay of the root pullout force. The final result is a model for the spatial-temporal prediction of root reinforcement heterogeneity and dynamics in subalpine spruce forest stands. Five year old harvested spruce forest in the climatic conditions of the study area provides 40% of the root reinforcement of live forest, while 15 years old harvested forest provides no reinforcement at all. Shrub species and natural regeneration could guarantee almost the 30% of the root reinforcement of a live forest after 15 years from cutting. Additional work is now required to further validate the model and implement these results in a slope stability analysis.
Debris flow frequency and magnitude were determined for 33 basins in southwest British Columbia. Basins were first classified as either weathering-limited or transport-limited using a discriminant function based on debris-contributing area, an area-weighted terrain stability number, and drainage density. Multiple regression was used to predict magnitude, peak discharge, frequency and activity (frequency times magnitude) within each group of basins. Model performance was improved by stratifying the total sample of debris flow basins into weathering-and transport-limited groups. Explained variance increased by an average of 15 per cent in the transport-limited sample, indicating that sediment supply conditions in the more active basins are fundamental in predicting debris flow activity. An independent test of the regression models with 11 basins yielded generally good results for debris flow magnitude and peak discharge. Prediction of debris flow frequency proved problematical in weathering-limited basins. The methods developed here provide estimates of debris flow attributes in basins for which few data on past events are available. Copyright (C) 1999 John Wiley & Sons, Ltd.
Understand How to Analyze and Interpret Information in Ecological Point Patterns
Although numerous statistical methods for analyzing spatial point patterns have been available for several decades, they haven’t been extensively applied in an ecological context. Addressing this gap, Handbook of Spatial Point-Pattern Analysis in Ecology shows how the techniques of point-pattern analysis are useful for tackling ecological problems. Within an ecological framework, the book guides readers through a variety of methods for different data types and aids in the interpretation of the results obtained by point-pattern analysis.
Ideal for empirical ecologists who want to avoid advanced theoretical literature, the book covers statistical techniques for analyzing and interpreting the information contained in ecological patterns. It presents methods used to extract information hidden in spatial point-pattern data that may point to the underlying processes. The authors focus on point processes and null models that have proven their immediate utility for broad ecological applications, such as cluster processes.
Along with the techniques, the handbook provides a comprehensive selection of real-world examples. Most of the examples are analyzed using Programita, a continuously updated software package based on the authors’ many years of teaching and collaborative research in ecological point-pattern analysis. Programita is tailored to meet the needs of real-world applications in ecology. The software and a manual are available online.
On 12 May 2008 the magnitude 8.0 Wenchuan Earthquake erupted on the eastern fringe of the Tibetan Plateau in China, triggering many landslides onto towns and villages, leading to substantial loss of life and damage to property and the infrastructure. Local characteristic cloudy weather posed serious difficulties upon regional remote observation for disaster rescue in the mountainous region. To overcome weather limits to remote sensing, the study advanced one quick methodology to extract primary regional landslides with 8-day Moderate Resolution Imaging Spectroradiometer (MODIS) Normalized Difference Vegetation Index (NDVI) and terrain slope information. The threshold of temporal NDVI jump (value of 0.4) was introduced to discriminate landslide induced sharp NDVI decline from seasonal variations of vegetation cover, while slope information can be used to further confirm land cover change caused by a landslide. The study indicated that the Wenchuan Earthquake induced about 170 km of landslides in the southern half of the Longmenshan Fault Zone, most of which was formerly covered by forest or shrub. An ecological restoration of the areas affected by landslides will therefore be quite difficult. In addition, the result emphasizes that regions with slopes over 15° are exposed to high landslide risk, which should be carefully taken into account for settlement and transportation planning.
The hypothesis that seed or seedling predation by host-specific herbivores can explain the coexistence of the large number of tree species in tropical forests, and produce a low density and uniform dispersion of adults of these tree species, is examined. On theoretical grounds, it is shown that spacing of conspecific adults can account for only a very small fraction of the observed tree species richness in tropical forests unless interadult distances are quite large. Moreover, in growing or declining populations, seed predation will not result in large-scale uniformity in adult dispersion patterns throughout the population. It results in uniformity in equilibrium populations only if the same spacing rule applies to all adult trees. However, adult trees differ enormously in seed production from year to year, and from young (small) to old (large) adults. High variance in seed production from tree to tree and year to year leads to heterogeneity in the intensity of seed predation and in the spacing distance between adults. This explains why the adults of most tropical tree species are clumped or randomly dispersed, and not uniformly spaced, in spite of heavy seed predation in many species. Because tree species are not everywhere as dense as whould be predicted from observed nearest neighbor distances, it is concluded that other factors than seed predation limit the abundance of tropical tree species and prevent single-species dominance. /// Проверяли гипотезу о том, что потребление семян или проростков фитофагами, специализированными к определенному виду кормового растения, может объяснить сосуществование большого числа видов деревьев в тропических лесах, низкую плотность и равномерное распределение зрелых деревьев этих видов. На теоретической основе показано, что пространственное распределение взрослых деревьев одного вида определяется лишь небольной частью всего видового богатства деревьев в тропическом лесу, хотя расстояния между эрелыми деревьями относительно большие. Более того, в развивающихся или угасающих популяциях выедание семян не приводит к крупномасштабной равномерности в характере пространственного распределения взрослых деревьев в пределах распространения популяции. Это приводит равномерности в уровновешенных популяциях лишь в случае, если характер пространственного распределения одинаков у всех взрослыхдеревьев. Однако взрослых деревья сильно различаются по продукции семян в разные годы и в разном возрасте. Большие колебания в продукции семян в разные годы, а таюже индивидуальные колебания приводят к возникновению различий в интенсивности потребления семян и неравномерности распределения взрослых деревьев. Это объясняет, почему распределения взрослых деревьев большинства тропических видов агрегировано или случайно и неравномерно, несмотря на интенсивное отчуждение семян фитофагами у многих видов. Так как отдельные виды деревьев не везде встречаются в той плотности, которую можно предсказать по расстоянию между ближайшими соседними деревьями, сделано заключение, что обилие тропических деревьев лимитируется рядом друтих факторов, помимо выедания семян, которые препетствуют и доминированию отдельных видов.
Sampling of 24-, 34-, and 44-yr-old patch cuts (324-2400 m2) in the Bartlett Experimental Forest, New Hampshire, was undertaken. Shade-tolerant species, especially eastern hemlock Tsuga canadensis and American beech Fagus grandifolia, were relatively more abundant in small gaps and gap edges and generally decreased with increasing gap size. Shade-intolerant species, including paper birch Betula papyrifera and pin cherry Prunus pensylvanica were relatively more abundant in large gaps and gap centers and increased with increasing gap size. Intermediately shade-tolerant species, especially yellow birch Betula alleghaniensis and red maple Acer rubrum, were relatively more abundant in gap centers. Striped maple Acer pensylvanicum was relatively more abundant in gap edges. Many of these relationships were complex due to interactions with gap age and slope. Sugar maple Acer saccharum relative abundance was not associated with gap size or location within a gap. Analyses isolating irradiance as a factor influencing species composition were inconclusive. Other effects of gap disturbance and characteristics associated with different locations in the gap, eg soil conditions and root competition, may play an important role in the gap dynamics. Gap age had a strong effect on species relative abundances; these patterns reflected typical successional sequences in northern hardwood forests. Gap disturbances increased species richness and diversity. Gaps contained species not present in the old-growth forest, and the species compositional variations among different gap sizes suggest that a forest with a range of gap sizes will have high diversity. -from Authors
Within a 44-km2 belt between 530-850 m a.s.l. (subtropical lower montane wet forest) examination of aerial photographs from 1936-1988, and field surveys, revealed 46 landslides; 40% of landslides had map areas (the horizontal projection of their actual areas) between 200-400 m2. Landslides >1800 m2 comprised <10% of the sample, but accounted for c40% of the total map area disturbed. On average, landslides disturb a minimum of 0.3% and 0.08% of the forest map area per century on slopes underlain by intrusive and volcaniclastic rocks, respectively. On 8 landslides <1 yr old, the soil in the lower zone generally had a higher concentration of organic carbon and nutrients than that in the upper zone. Germinable buried seeds of light-demanding species were found only in the lower zone (22 seeds m-2), c5% of the average buried seed density found in adjacent mature forest. Vegetation analyses on a 52-yr chronosequence of 20 landslides showed that regrowth was consistently more vigorous in the lower zone. Species composition is dominated on landslides up to 38 yr old by light-demanding, fast-growing pioneer trees. Revegetation of the upper zone seemed to be retarded by extensive mats of light-demanding ferns. Basal area and floristic composition start to resemble predisturbance conditions on 300-600-m2, 52-yr old landslides. -from Author
The aim of this field investigation was to study the enrichment of biodiversity of the slope at an early phase of succession, initiated by selected pioneers, and to study how this enrichment related to enhancement of the slope stability. Four experimental plots, with differing plant pioneers and number of species (diversity), were designed in order to assess the effects of plant succession on slope stability. Plant growth pattern was assessed by observing the increment in species diversity (number), species frequency and plant biomass. Higher vegetation biomass in a mixed culture situation (LLSS) in the field with Leucaena leucocephala as a pioneer, marked an increase in species diversity after 24 months of observation. In contrast, G (grasses and legume creepers) plot revealed the slowest rate of succession and the lowest above-ground biomass amongst the plots. The mixed-culture plot without L. leucocephala (SS) had also shown a lower biomass, a similar phenomenon observed in a plot grown by L. leucocephala (LL) with low plant diversity. Consequently, these plant growth patterns gave a positive effect on slope stability where the regression study showed that the shear strength was much affected by plant biomass. Meanwhile, throughout the succession process in LLSS plot, root length density reached the highest value amongst the plots, 23 Km m(-3). In relation to this, the saturation level of the slope indicates the unsaturated condition of the soil which resulted in the enhancement of both soil penetrability and soil shear strength of the plot. These attributes reveal a strong positive relationship between the process of natural succession and the stability of slopes. (C) 2010 Elsevier B.V. All rights reserved.
Vegetation significantly affects hillslope hydrological and mechanical properties related to shallow landslide triggering. In view of the complexity of soil plant hydrological interactions, the quantification of root mechanical reinforcement remains a challenge. Herein we present a back analysis of mechanical stability criteria related to a well-characterized vegetated shallow landslide in Italy, focusing on the quantification of lateral and basal root reinforcement. Lateral root reinforcement is included in slope stability estimates by adding a stabilizing force proportional to the scarp surface and root distribution. This stabilizing force is added to the force balance equation for the infinite slope model for different landslide shapes and dimensions. To quantify root reinforcement, we use the Wu model and the fiber bundle model (WM and FBM, respectively). Implementation of the latter model allows the quantification of the stress–strain behaviour of a bundle of roots for different root distributions and mechanical properties. Results of these models are compared highlighting key differences between the two approaches. Calculations using the FBM can explain the overestimation of lateral root reinforcement using WM and the commonly observed overestimation in the factor of safety. The model also quantifies the displacement-dependent behaviour of root reinforcement on vegetated slopes. Lateral root reinforcement can strongly influence the stability of slopes up to a certain area (1000–2000 m2). The magnitude of this stabilizing effect depends on parameters such as inclination, soil mechanical properties, and root distribution.
A large number of methods for the analysis of point pattern data have been developed in a wide range of scientific fields. First-order statistics describe large-scale variation in the intensity of points in a study region, whereas second-order characteristics are summary statistics of all point-to-point distances in a mapped area and offer the potential for detecting both different types and scales of patterns. Second-order analysis based on Ripley's K-function is increasingly used in ecology to characterize spatial patterns and to develop hypothesis on underlying processes; however, the full range of available methods has seldomly been applied by ecologists. The aim of this paper is to provide guidance to ecologists with limited experience in second-order analysis to help in the choice of appropriate methods and to point to practical difficulties and pitfalls. We review (1) methods for analytical and numerical implementation of two complementary second-order statistics, Ripley's K and the O-ring statistic, (2) methods for edge correction, (3) methods to account for first-order effects (i.e. heterogeneity) of univariate patterns, and (4) a variety of useful standard and non-standard null models for univariate and bivariate patterns. For illustrative purpose, we analyze examples that deal with non-homogeneous univariate point patterns. We demonstrate that large-scale heterogeneity of a point-pattern biases Ripley's K-function at smaller scales. This bias is difficult to detect without explicitly testing for homogeneity, but we show that it can be removed when applying methods that account for first-order effects. We synthesize our review in a number of step-by-step recommendations that guide the reader through the selection of appropriate methods and we provide a software program that implements most of the methods reviewed and developed here.
First-order lateral roots originating in the upper part of the taproot of a woody species, usually termed surface roots, grow close beneath the soil surface, even on irregular or sloping ground. In slope condition, in fact, the surface roots can assume upward as well as downward growth. Existing knowledge on the controls over root direction does not fully explain these field observations.Two different soil types and sloping conditions were selected in field condition to explore the behaviour of the surface roots in the woody species Spartium junceum L. The root system 3D architecture was measured with a 3D digitizer and the angle of growth (0° = vertically downwards) and the radial direction (0° = horizontally downslope or northwards) of all root segments measured.Surface roots were more numerous in clay soil than in loam soil, independently from the slope inclination. They had initial angles larger than 90°, i.e. they grew upwards only in clay soil. The subsequent angles of growth maintained this value only in steep-slope condition, showing a clear soil type x slope inclination interaction. The initial angle of all first-order lateral roots decreased linearly with depth of origin on the taproot always in relation to the soil type, with this relationship being stronger in clay soil.These findings showed that the liminal angle (the preferred angle of growth) of surface roots was mainly affected by the soil type rather than the soil surface inclination. Thus, upward growth must stand in the plasticity of the plagiotropic response of these secondary laterals rather than in a strong internal control.
Massive landslides, caused by the catastrophic Chi-Chi earthquake in 1999, occurred at the Jou-Jou Mountain area in the Wu-Chi basin, Taiwan. Multi-temporal satellite images coupled with an NDVI-based index were used to extract landslides and assess the changes. The extracted area soon after the earthquake is 822.97 ha. From 1999 through 2005, the denudation area has declined to 143.22 ha, which indicates that most landslides have been restored. Over six years of natural vegetation succession, vegetation recovery rate indicating vegetation coverage improvement percentage at landslides has reached 89.69%. Based on field surveys, many of native pioneer plants have invaded the denudation sites and formed a vegetated buffer strips to mitigate the impacts on debris-flow hazards and hillslope erosion. Additionally, soil erosion in the denudation sites at different dates was also estimated. The average annual erosion depth in the initial stage of the earthquake reached 6.04 mm, about 2.26 times as high as pre-quake. With vegetation recovery at landslides, soil erosion has significantly reduced after six years. The analyzed results provide very useful information for decision making and policy planning in the landslide area.
To assess the impact of landslides on soil fertility in south Ecuador, we compared the properties of shallow translational debris slides with those of adjacent undisturbed soils. A chronosequence of four small landslides, ca. 0.5, ca. 2–3, ca. 8–10, and ca. 20 years old, was selected on a 30–50°, forest-covered, east-facing slope of the eastern cordillera at 1900–2300 m above sea level (a.s.l.). All soils were Dystric Cambisols. The mean mass of the soil organic layer (sum of all O horizons) increased in the order landslide head (46 t ha−1)
Over 60% of all stems and basal area of tabonuco occurred in unions, clumps of trees interconnected by root grafts. Self and intraspecific grafting were extensive, while interspecific grafting was not common. Grafted trees were tall and had a small crown/DBH ratio. Hurricane damage was significantly higher in isolated individual tabonuco trees than those in unions. A noncompetitive force such as root grafting was probably more important than competitive forces in maintaining the unions of tabonuco, and thus the forest community. -from Authors
Lienhuachih subtropical evergreen broadleaf forest dynamics plot tree species characteristics and distribution patterns
- L-W Chang
- J-L Hwong
- Y-T Chen
- C-Y Yeh
- H-C Lin
- C-C Kuo
- C-C Lin
- I-F Sun
- Yang K-C Chen
- Z-S Wnag
Chang L-W, Hwong J-L, Chen Y-T, Yeh C-Y, Lin H-C, Kuo C-C, Lin C-C, Sun I-F, Yang K-C, Chen Z-S,
Wnag H-H (2012). Lienhuachih subtropical evergreen broadleaf forest dynamics plot tree
species characteristics and distribution patterns. Taiwan Forest Research Institute, Taipei,
Taiwan, pp. 346. [online] URL: http://forest
geo.si.edu/sites/default/files/lienhuachih_stand
_book.pdf