How reliable is the monitoring of permanent vegetation plots? A test with multiple observers

Questions: A multiple plot design was developed for permanent vegetation plots. How reliable are the different methods used in this design and which changes can we measure? Location: Alpine meadows (2430 m a.s.l.) in the Swiss Alps. Methods: Four inventories were obtained from 40 m² plots: four subplots (0.4 m²) with a list of species, two 10m transects with the point method (50 points on each), one subplot (4m²) with a list of species and visual cover estimates as a percentage and the complete plot (40 m²) with a list of species and visual estimates in classes. This design was tested by five to seven experienced botanists in three plots. Results: Whatever the sampling size, only 45‐63% of the species were seen by all the observers. However, the majority of the overlooked species had cover < 0.1%. Pairs of observers overlooked 10‐20% less species than single observers. The point method was the best method for cover estimate, but it took much longer than visual cover estimates, and 100 points allowed for the monitoring of only a very limited number of species. The visual estimate as a percentage was more precise than classes. Working in pairs did not improve the estimates, but one botanist repeating the survey is more reliable than a succession of different observers. Conclusion: Lists of species are insufficient for monitoring. It is necessary to add cover estimates to allow for subsequent interpretations in spite of the overlooked species. The choice of the method depends on the available resources: the point method is time consuming but gives precise data for a limited number of species, while visual estimates are quick but allow for recording only large changes in cover. Constant pairs of observers improve the reliability of the records.     

Can digital image classification be used as a standardised method for surveying peatland vegetation cover?

The ability to carry out systematic, accurate and repeatable vegetation surveys is an essential part of long-term scientific studies into ecosystem biodiversity and functioning. However, current widely used traditional survey techniques such as destructive harvests, pin frame quadrats and visual cover estimates can be very time consuming and are prone to subjective variations. We investigated the use of digital image techniques as an alternative way of recording vegetation cover to plant functional type level on a peatland ecosystem. Using an established plant manipulation experimental site at Moor House NNR (an Environmental Change Network site), we compared visual cover estimates of peatland vegetation with cover estimates using digital image classification methods, from 0.5 m × 0.5 m field plots. Our results show that digital image classification of photographs taken with a standard digital camera can be used successfully to estimate dwarf-shrub and graminoid vegetation cover at a comparable level to field visual cover estimates, although the methods were less effective for lower plants such as mosses and lichens. Our study illustrates the novel application of digital image techniques to provide a new way of measuring and monitoring peatland vegetation to the plant functional group level, which is less vulnerable to surveyor bias than are visual field surveys. Furthermore, as such digital techniques are highly repeatable, we suggest that they have potential for use in long-term monitoring studies, at both plot and landscape scales.     

Phenological shifts of the alien colonizer Bunias orientalis: Image-based analysis of temporal niche separation

Abstract. Phenological development of the colonizer Bunias orientalis L. was investigated in permanent plots in herbaceous vegetation, using photography and image analysis. In all habitats but one, B. orientalis showed a two-phased rosette growth behaviour, with a first peak cover reached in spring, roughly coinciding with an early peak cover of the matrix vegetation, and a second autumn growth flush occurring in a phase of reduced matrix vegetation cover. This seasonally bimodal growth pattern of B. orientalis appears to partly compensate for its competitive inferiority in crowded herbaceous vegetation. Small or overshadowed individuals of this species particularly profit from higher above-ground performance or release from overlap in autumn. The significance of temporal niche separation for survival and growth of B. orientalis individuals varies with habitat conditions, being most apparent in occasionally disturbed habitats with a relatively low frequency of gap formation. Despite some limitations, image analysis proved to be useful for phenological investigations within herbaceous plant stands.     

Spatial variation in a grassland on soil rich in heavy metals

Abstract. The vegetation in a grassland area in the prefecture of Kilkis (N. Greece), known for its surface Cu-mineralization, was studied. 43 quadrats were established along a transect through an area where the vegetation formed patches of different size. Cover-abundance and frequency estimates for all species were made. Normal Association Analysis revealed five quadrat groups characterized by Trifolium scabrum, Linaria pelisseriana, Anthoxanthum ovatum, Gypsophila muralis and Minuartia hirsuta ssp. falcata. 43 soil samples were analysed for pH, organic C, CaCO₃, total content of Fe, Zn, Pb, Cu, Mn, Ca, Mg, K, Na and soil texture. From a discriminant analysis performed on soil data five soil groups resulted, which are highly related to the five vegetation groups. Thymus sibthorpii, Minuartia hirsuta ssp. falcata and Rumex acetosella are the most important taxa with respect to physiognomy and patchi-ness of the vegetation. The number of species in each group of quadrats is affected by the relative favourableness or severity of the soil conditions. Metal contents, mainly Zn, Cu, Mg and Na, and soil texture are considered to be among the main factors controlling the structure and physiognomy of the vegetation.     

The relationship between psyllid leaf galls and redbay (Persea borbonia) fitness traits in sun and shade

Spatial autocorrelation in vegetation has been discussed extensively, but little is yet known about how standard plant sampling methods perform when confronted with varying levels of patchiness. Simulated species maps with a range of total abundance and spatial autocorrelation (patchiness) were sampled using four methods: strip transect, randomly located quadrats, the non-nested multiscale modified Whittaker plot and the nested multiscale North Carolina Vegetation Survey (NCVS) plot. Cover and frequency estimates varied widely within and between methods, especially in the presence of high patchiness and for species with moderate abundances. Transect sampling showed the highest variability, returning estimates of 19–94% cover for a species with an actual cover of 50%. Transect and random methods were likely to miss rare species entirely unless large numbers of quadrats were sampled. NCVS plots produced the most accurate cover estimates because they sampled the largest area. Total species richness calculated using semilog species-area curves was overestimated by transect and random sampling. Both multiscale methods, the modified Whittaker and the NCVS plots, overestimated species richness when patchiness was low, and underestimated it when patchiness was high. There was no clear distinction between the nested NCVS or the non-nested modified Whittaker plot for any of the measures assessed. For all sampling methods, cover and especially frequency estimates were highly variable, and depended on both the level of autocorrelation and the sampling method used. The spatial structure of the vegetation must be considered when choosing field sampling protocols or comparing results between studies that used different methods.     

Comparing survey methods for monitoring vegetation change through time in a restored peatland

Monitoring is an essential step to assess vegetation trajectories post-restoration and ultimately evaluate success. In this paper, we compare two monitoring methods, the line-point intercept (LPI) and the permanent plot (PP) methods, for evaluating plant recovery of a restored cut-over peatland (8.5 ha), following a “moss layer transfer technique”. We used the LPI method to estimate covers (from frequency measures) for each plant species using a systematic grid of approximately 5,700 points (every 3 m × 5 m). In parallel, 43 PP (3 m × 8 m) were established and used to evaluate plant covers. The post-restoration recovery of vegetation was assessed against a reference ecosystem encompassing the variation in species cover from natural undisturbed peatlands in the same region. For all plant groups considered, the LPI consistently showed higher cover estimates than the PP method. Discrepancy between the two methods was particularly evident for the Ericaceae group. A complementary sampling method, the line-intercept (LI), showed strong correlations with the visual estimations of Ericaceae covers (akin to PP), suggesting an overestimation from the LPI method. Most life form groups of the restored peatlands are developing a structure similar to the regional reference ecosystem 8 years post-restoration with the herb group being still most dissimilar. Indeed, when analyzing the temporal evolution of the different key peatland plant components, several are within the range of regional abundance values or moving positively towards range of cover abundance of the reference system such as Sphagnum cover, a key peat-accumulating plant group.     

Above‐ground biomass distribution among species during early old‐field succession

Abstract. The interdependence of species richness and plant biomass has widely been accepted as a general biodiversity rule. However, there is no information about how relationships are established during colonization and how total biomass is distributed among plants. The main objective of this study was to determine the role of several factors which we have hypothetized as affecting biomass distribution among species in an early old‐field community. To test this hypothesis, we conducted an experiment in a deeply ploughed Agropyro‐Rumicion crispi community in the Basque Country (Spain). Light, water and soil nitrogen content were factorially manipulated resulting in eight experimental treatments. We also examined several site features, which could potentially influence final biomass distribution: seed bank composition, soil physico‐chemical heterogeneity and cover and density of the dominant plant species in the community. A partition hypothesis testing approach using Redundancy Analysis was conducted to determine the fraction of biomass distribution variability assigned to each treatment and site feature. The most important species, in terms of biomass, were Agropyron repens, Sinapis arvensis, Arrhenatherum bulbosum and Picris echioides. As a general conclusion, biomass distribution among species during early secondary succession primarily depends on light availability and nitrogen supply. Several soil variables, such as magnesium, calcium and clay contents, also explain a relevant fraction of the biomass distribution among plant species. On the contrary, we found no effect of seed bank composition on biomass distribution. Finally, the total species number and cover of dominant species such as Sinapis arvensis, may determine final biomass distribution.     

Effects of soil erosion on the floristic composition of plant communities on marl in northeast Spain

Abstract. This study explored the validity of three responses of vegetation to increased soil erosion: reduction of vegetation cover, number of species and reduced substitution of species. 201 relevés, including edaphic and geomorphological data, were surveyed in the intensely eroded Eocene marls of the Prepyrenees (NE Spain). Changes in plant species’ presence in relevés from different degradation stages were compared. The level of vegetation degradation was defined as the total phanerogam cover which, in the studied area, was correlated to the degree of soil erosion. The considered trends were validated. Reduction of phanerogam cover and species number were gradual from low to high‐eroded areas. Vegetation degradation explained 48% of the species number variance. In the later stages of degradation a significant substitution of species was not observed, only a lower frequency of occurrence of several species that appeared in the whole set of relevés. Through the process of degradation, 47% of species displayed significantly reduced frequencies as degradation increased, none showed a significant increase in frequency. It is concluded that there are no characteristic species in these plant communities that survive in the severely eroded marls. Among the few species that had increased in frequency, most only colonised favourable micro‐environments.     

Spatial analysis of oblique photo‐point images for quantifying spatio‐temporal changes in plant communities

Question: Can spatial analytical techniques be used to extract quantitative measurements of vegetation communities from ground‐based permanent photo‐point images? Location: Mount Aspiring National Park, south‐western South Island, New Zealand. Methods: Sets of ground‐based photographs representing two contrasting vegetation types were selected to test two spatial analytical techniques. In the grid technique, a grid was superimposed onto the photographs and the frequency of species presence in each grid‐square was calculated to estimate species abundance/cover over the defined area. In the object‐oriented technique, the photographs were segmented into meaningful objects, based on the colour of the pixels and the textural patterns of the images, and the area occupied by an object in the image was used to derive species abundance/cover over the area. Results: Both techniques allow quick and easy classification of digital elements into ecologically relevant categories of vegetation components. The grid technique appeared more robust, being quick and efficient, accommodating all image types and providing presence/absence matrices for multivariate analysis. Fewer classes were identified using the object‐oriented technique, in particular for the forest interior site and for small individual plants such as Astelia spp. Conclusions: Both techniques showed potential for the objective quantitative analysis of long‐term vegetation monitoring of cover and changes of several component species, using repeat ground‐based photographs more specifically for grassland habitats. However, both rely to various degrees on manual classification. Corrective factors and strict protocols for taking the photographs are necessary to account for variation in view angles and to compute values more representative of absolute species abundance.     

Differential effects of reindeer on high Arctic lichens

Abstract. We studied the effects of Svalbard reindeer on the abundance of lichens in Spitsbergen. A survey was carried out in 14 areas with contrasting reindeer densities. Separate cover estimates for crustose, fructose and foliose lichens were taken in each area, and related to the density of reindeer pellet groups, a measure of reindeer density. Dominant macro lichen families were identified in 10 areas, and a full record of macrolichen species was taken in four additional areas. Variation in reindeer density is partially due to past overhunting, and subsequent incomplete recovery, releasing some areas from reindeer grazing for 100–200 yr. The cover of fruticose lichens was negatively related to reindeer pellet group density, indicating suppression by Svalbard reindeer. This makes their impact comparable to other members of the Rangifer genus around the northern hemisphere. The generally recorded low abundance of lichens in the diet of Svalbard reindeer compared to other Rangifer species, therefore, was interpreted as the depletion of fruticose lichens in Spitsbergen, and a subsequent switch to alternative foods. Of all fruticose lichens, Stereocaulon spp. appeared least sensitive to grazing. Crustose and foliose lichen cover was independent of reindeer pellet group density. The cover of crustose lichens was significantly related to latitude, with greater cover in more northern areas. Foliose lichens were more abundant in places where moss cover was high. We conclude that the impact of Svalbard reindeer on lichens is dependent on growth form, with fruticose lichens suffering from grazing, whereas foliose lichens might indirectly benefit from higher densities of reindeer or, like crustose lichens, be controlled by other factors.     

Burrowing activities of kangaroo rats and patterns in plant species dominance at a shortgrass steppe-desert grassland ecotone

Abstract. Our objective was to evaluate the effects of burrowing activities by banner-tail kangaroo rats (Dipodomys spectabilis Merriam) on plant community structure and species dominance for two patch types at the ecotone between shortgrass steppe and desert grassland in New Mexico, USA. 10 mounds produced by kangaroo rats were selected in patches dominated by Bouteloua gracilis (the dominant in shortgrass steppe communities) and 10 mounds were selected in patches dominated by B. eriopoda (the dominant in Chihuahuan desert grasslands). Plant cover and density by species were sampled from three locations associated with each mound: the mound proper, the edge of the mound in the transition area, and the off-mound vegetation. Similar cover of B. eriopoda for the edges of mounds in both patch types indicates the ability of this species to respond to animal disturbances regardless of the amount of cover in the surrounding undisturbed vegetation. By contrast, cover of B. gracilis was low for all mounds and mound edges in patches dominated by this species. Much higher cover of B. eriopoda on mound edges compared to the undisturbed vegetation in B. gracilis-dominated patches indicates that kangaroo rats have important positive effects on this species. Lower cover of perennial grasses and higher cover of forbs, shrubs, and succulents on the edges of mounds in B. eriopoda-dominated patches compared to patches dominated by B. gracilis indicate the importance of surrounding vegetation to plant responses on disturbed areas. Our results show that kangaroo rats have important effects on both species dominance and composition for different patch types, and may provide a mechanism for small-scale dominance patterns at an ecotone; thus providing further support for their role as keystone species in desert grasslands.     

Linear relationships between aboveground biomass and plant cover in low open herbaceous vegetation

Abstract. Four herbaceous plant species of a sand dune area and several herb species of an open early‐successional patch were investigated for the occurrence of a simple relationship between aboveground biomass and plant cover. Without exception linear regressions of aboveground biomass on plant cover were found with slope factors depending on the growth form of the species. These results suggest that (early) growth of herbaceous plants in low and (temporarily) open vegetation is not affected by possible constraints caused by a decreasing ratio of plant cover to aboveground biomass. The obtained linear relationships could be used for rapid non‐destructive determination of aboveground biomass by image‐analysis of cover data.     

A model of alpine species distribution in relation to snowmelt time and altitude

Abstract. The focus of this study is the response of species to time of snowmelt and altitude in alpine areas and an examination of changes in species response to snowmelt as altitude increases and temperature decreases. Transects (n= 43) were placed evenly along an altitudinal gradient at Finse, Hardanger‐vidda, western Norway, from ridges to late snowbeds. These gradients were systematically sampled (‘Repeated Gradient Analysis, RGA’) and an adjusted F‐test was used to determine repeated trends in species distribution along the transects. Of the 41 taxa analysed 22 showed a significant change in expected occurrence in response to time of snowmelt (when a site becomes free of snow) as altitude increased. Three types of response were observed: (1) no change in response: (2) increased occurrence as altitude increases, i.e. the taxon invades snow‐free sites as altitude increases, and (3) decreased occurrence as altitude increases, i.e. the taxon retreats from snow covered areas. It is suggested that the changes in response are due to both environmental factors (temperature related) and biological interactions. Decreases in expected occurrence are probably due to increased environmental severity as altitude increases (temperature related decreases). These species are represented by taxa preferring intermediate cover of snow. The invasion of earlier snow‐free sites is probably due to reduced competition from lee‐side taxa as altitude increases. A predictive model based on the species‐environmental relationships suggests that a 1°K temperature increase changes the limits of occurrence in response to time of snowmelt from 3 to 20 days for the different taxa.     

Spatio‐temporal colonization patterns of forest plant species in a mixed deciduous forest

Abstract. An integrated analysis of the colonization patterns of forest plant species was carried out in a 34‐ha, mixed deciduous forest in northern Belgium. First, we sought to describe the relationships between land use history and environmental conditions. Land use history and soil type were related and negative correlations between pH and secondary forest age were found. The density of the shrub layer increases with secondary forest age. Litter quantity and cover of Urtica dioica were mainly indirectly influenced by land use history. Litter starts accumulating at low pH values and high shrub density and Urtica dioica grows vigorously on nutrient enriched soils where much light can reach the ground. Next, the importance of these human‐altered environmental conditions for the colonization of forest plant species was assessed relative to the importance of dispersal limitation. Therefore, the distribution of 16 forest species was mapped and species‐specific spatio‐temporal isolation measures were calculated. The analysis revealed that the colonization patterns of the slowly colonizing species (i.e. ‘ancient forest plant species’) are best explained by a combination of spatio‐temporal isolation, soil type, pH and the (non‐)cover of Urtica dioica. By contrast, spatio‐temporal isolation was never a limiting factor for good colonizing forest species. Our results suggest that colonization of ‘ancient forest plant species’ is hampered by a combination of dispersal‐ and recruitment limitation and that the relative importance of both factors is species‐specific.     

Relationships between vegetation zonation and altitude in a salt-marsh system in northwest Spain

Abstract. Vegetation zonation in salt marshes has traditionally been attributed largely to altitudinal differences, since altitude determines the temporal pattern of tidal flooding and is thus closely related to proximate determinants of the distribution of species and plant communities. We investigated the distribution of vascular plants and plant communities along a series of altitudinal transects in two salt marshes in the northwestern Iberian Peninsula. Our results indicate that altitudinal range varies significantly both among species and among communities, and confirm that salt-marsh vegetation characteristics (species cover and composition) can be predicted on the basis of altitude, particularly at the lower levels of the profile.     

Patterns of ecological,chorological and taxonomic diversity at both sides of the Strait of Gibraltar

Abstract. Diversity relations in Mediterranean heathlands and the understorey of oak woodlands on sandstone-derived substrates were studied at both sides of the Strait of Gibraltar. Trends in species composition and cover were analysed by Detrended Correspondence Analysis; the first axis, assumed to reflect a main environmental gradient, was used to analyse the patterns of three aspects of community diversity. Species richness, i.e. number of species along a 100-m transect, shows a humpbacked trend along the gradient, with the highest values in the understorey of evergreen Quercus suber woodlands, associated with soils of intermediate fertility and moisture status. The number of endemic species is highest in open heathlands, associated with more extreme conditions of acid, infertile soils on exposed ridges. The taxonomic singularity, as measured by the inverse of the average number of species per genus at each site, is highest at the most fertile and moist sites occupied by semideciduous Q. canariensis woodlands. A comparison between northern (Spanish) and southern (Moroccan) sides of the Strait of Gibraltar shows a general concordance of the trends of woody plant communities along the main environmental gradient. However, significant differences of the southern samples are: (1) lack of some differential, habitat-specific species and greater abundance of widespread generalists; and (2) a general reduction in species diversity, number of endemics and taxonomic singularity. We interpret these differences as affected partly by the smaller extent and fragmentation of sandstone areas in the south, and partly by the higher impact of slashing and grazing there.     

Influences of transect relocation errors on line-point estimates of plant cover

The line-point transect method has been used to estimate plant cover for about nine decades. In particular, the method is often used to determine baseline plant cover and monitor for changes in plant cover over time. In such cases, detection of change requires both the initial transect starting position and angle of orientation are exact in relocation without error. A study was conducted on influences of errors in basal cover estimates that resulted from inexact relocation and orientation of a resample transect. Simulation studies of actual field data showed that variation in plant cover estimates from relocated line-point transects increased with each source of error and combinations of these errors. Relocated transects resulted in unbiased estimates of total-plant cover only when means over all transects are used to detect changes over time. Substantial errors were observed when the mean cover of individually relocated transect was compared to its original transect.     

Conservation of bog plant species assemblages: Assessing the role of natural remnants in mined sites

Abstract. Bogs, economically valuable wetlands, are subjected to exploitation in southern Canada. We addressed plant conservation within bogs mined for peat, in which small undisturbed remnants are left, mostly at the margins of the mined areas. The main goal of the study was to test whether these remnants act as refuges for plants which could recolonize areas that are planned for restoration after mining is completed. Mosses, lichens and vascular plants were sampled in remnants of 24 mined bogs in southeastern Canada during the summer of 1997. The vegetation was also sampled at the margins and centres of 24 nearby natural bogs in plots similar in size to these remnants. Using similarity analysis and ordination techniques, we found that plant species assemblages in remnants of mined bogs differ from those near the margins of natural bogs, and that certain species are associated with the centre of natural bogs, due to the presence of pools. We also showed that water conditions of remnants are affected by drainage due to peat mining. Sphagnum moss showed itself to be a key indicator of mining effects on vegetation. Implications for peat resource management and bog conservation are discussed.     

Relationships between species richness patterns in deciduous forests at the north Estonian limestone escarpment

Abstract. The flora of the deciduous forests at the base of the north Estonian limestone escarpment is species rich, with an exceptionally high number of rare bryophyte species. Relationships between species richness of bryophyte and herb layers and biotic and environmental conditions were studied, using General Linear Mixed Models. Human disturbance (waste deposit, tree damage etc) was significantly negatively correlated with species richness of both plant layers. Soil nitrogen content was negatively and soil water retention positively correlated with bryophyte species richness, while herb richness was unrelated to soil factors. After eliminating the effects of environment, negative correlations in species richness and cover between the bryophyte and herb layers were discovered on finer scales (1 m²), referring to biotic interactions. This relationship was obscured with the simple correlation analysis. On the other hand, the positive correlation in species pools between the bryophyte and herb layers (0.1 ha) was insignificant. The species pools of both bryophyte and herb layers were significantly positively correlated with the species richness of the tree layer. In summary, bryophyte and herb layer richness responded differently to environmental conditions, but human disturbance significantly decreased the richness of both layers. Due to the uniqueness and small area of these forests we recommend protection and restoration of disturbed sites.     

Reproducibility of species lists,visual cover estimates and frequency methods for recording high‐mountain vegetation

Question: When multiple observers record the same spatial units of alpine vegetation, how much variation is there in the records and what are the consequences of this variation for monitoring schemes to detect changes? Location: One test summit in Switzerland (Alps) and one test summit in Scotland (Cairngorm Mountains). Method: Eight observers used the GLORIA protocols for species composition and visual cover estimates in percentages on large summit sections (>100 m²) and species composition and frequency in nested quadrats (1 m²). Results: The multiple records from the same spatial unit for species composition and species cover showed considerable variation in the two countries. Estimates of pseudo‐turnover of composition and coefficients of variation of cover estimates for vascular plant species in 1 m × 1‐m quadrats showed less variation than in previously published reports, whereas our results in larger sections were broadly in line with previous reports. In Scotland, estimates for bryophytes and lichens were more variable than for vascular plants. Conclusions: Statistical power calculations indicated that unless large numbers of plots were used, changes in cover or frequency were only likely to be detected for abundant species (exceeding 10% cover) or if relative changes were large (50% or more). Lower variation could be reached with the point method and with larger numbers of small plots. However, as summits often strongly differ from each other, supplementary summits cannot be considered as a way of increasing statistical power without introducing a supplementary component of variance into the analysis and hence into the power calculations.