Early response of herbaceous vegetation to Rhododendron ponticum subsp. baeticum invasion in European Atlantic forests

Questions

Rhododendron ponticum subsp. baeticum is an invasive shrub of growing concern in continental Europe, but little is known about its impact on native plant communities. Here we ask: do environmental conditions differ between forest stands invaded by it and uninvaded stands? Do these differences correlate with R. ponticum's cover? Are these differences associated with differences in taxonomic and functional diversity of vascular plant species of the herb layer? Can these vegetation changes be explained by the sorting of certain life-history traits by R. ponticum-induced environmental changes?

Location

Several forests invaded by R. ponticum in the French Atlantic domain.

Methods

We recorded vegetation composition and a number of environmental variables in 400-m² plots that were established in 64 paired forest stands (32 invaded vs 32 uninvaded). We compiled traits from existing databases. We computed several metrics of taxonomic and functional diversity. We compared environmental variables and diversity metrics between invaded and uninvaded stands. We used correlation and regression analyses to relate them with R. ponticum's cover. We ran RLQ and fourth-corner analyses to explore the relationships between R. ponticum invasion, environmental variables, species traits, and vegetation composition.

Results

Independent of its abundance, R. ponticum invasion was associated with lower light arrival at the forest floor and increased litter thickness. Concomitantly, species richness and diversity and trait diversity were reduced. The major driver of species assemblages was soil pH, which strongly interacted with the invasion gradient. R. ponticum did not sort species according to traits associated with shade tolerance and thick-litter tolerance. However, tree and shrub saplings were more abundant in invaded than uninvaded stands, at the expense of graminoid and fern species.

Conclusions

As R. ponticum becomes the dominant shrub, it exerts new selection forces on life-history traits of extant species, mostly via reduced light availability, increased litter thickness, and physical competition, thereby reducing taxonomic and functional diversity of the herb layer, without impeding tree and shrub self-regeneration, at least in the short term.     

A functional assessment of the response of grassland vegetation to reduced grazing and abandonment

Question: Predicting the impact of land‐use change on vegetation is vital to understanding how biodiversity and ecosystem function may respond. Is it correct to assume that abandonment is an extreme form of grazing reduction? Location: Borders and central Scotland. Methods: The analysis used data sets from two identical experiments where the impacts of two unfertilized, extensively grazed treatments and one unfertilized abandoned treatment were compared against the species dynamics of a pasture subject to normal, productive grazing management over a 16‐year period. Initial multivariate analysis using Principal Response Curves was used to assess if particular traits were associated with either extensive or abandoned treatments, and was checked using univariate tests of individual traits. RLQ analysis followed by clustering into response groups was used to assess if species behaved in a similar manner between sites. Results: For many traits/attributes the shift in value or proportion was approximately linear across the extensification treatments as grazing was removed. However, certain traits showed step changes and quadratic responses. Leaf dry matter content, an important effect trait, was in the latter group. Most traits/attributes and species behaved similarly at the two sites. However, traits such as regenerative strategy, seed length, longevity and mass and seed bank type behaved differently, indicating that they are not predictable response traits. Conclusion: The results indicate that responses to grazing removal during extensification are largely straightforward and largely independent of species pool. However, there are discrepancies that suggest that simple analyses of the impacts of land‐use changes such as grazing reduction may hide more complex responses.     

Matching data sets from two different spatial samples

Abstract. Methods for coupling two data sets (species composition and environmental variables for example) are well known and often used in ecology. All these methods require that variables of the two data sets have been recorded at the same sample stations. But if the two data sets arise from different sample schemes, sample locations can be different. In this case, scientists usually transform one data set to conform with the other one that is chosen as a reference. This inevitably leads to some loss of information. We propose a new ordination method, named spatial‐RLQ analysis, for coupling two data sets with different spatial sample techniques. Spatial‐RLQ analysis is an extension of co‐inertia analysis and is based on neighbourhood graph theory and classical RLQ analysis. This analysis finds linear combinations of variables of the two data sets which maximize the spatial cross‐covariance. This provides a co‐ordination of the two data sets according to their spatial relationships. A vegetation study concerning the forest of Chizé (western France) is presented to illustrate the method.     

Functional response of plant assemblages to management practices in road–field boundaries

Questions

Management practices implemented on road verges are partly established to preserve biodiversity in agricultural landscapes. Their evaluation was primarily based on the analysis of the taxonomic structure and composition of communities. What is the relationship between management practices and the functional characteristics of road–field plants within elements?

Location

West‐central France.

Methods

We sampled the berm, the embankment and the field margin of 40 road–field boundaries located in west‐central France, an area where delayed mowing of some berms has been practised since 2009 for biodiversity reasons. We characterized management practices implemented on the different elements, i.e., the frequency and timing of mowing (early summer or late summer), the frequency of herbicide treatment in field margins and the N input rate. We retrieved from databases seven functional traits and types known to be influenced by management practices. To identify relationships between traits or types and environmental variables we first performed partial RLQ analyses to remove any potential confounding effect of the landscape context studied. We then computed fourth‐corner statistics to quantify relationships between traits or types, environmental variables and partial RLQ axes.

Results

Late mowing of the berm promoted nitrophilous species within berms and competitive rather than ruderal species within arable field margins. The frequency of herbicide treatment in field margins promoted broad‐leaf species within this element and, to a lesser extent, within embankments. Finally, the functional characteristics of communities of the three elements were not influenced by the level of N input in field margins.

Conclusions

In our environmental context, managing road verges affected the functional structure of plant assemblages both within them and within their adjacent arable field margins. We suggest a single early mowing of berms as a valuable practice for both conservation purposes and weed risk control in adjacent field margins.     

Traits reveal how habitat‐quality gradients structure small mammal communities in a fragmented tropical landscape

Understanding the relationships among community structure, vegetation structure and availability of food resources are a key to unravelling the ecological processes that structure biological communities. In this study, we tested (i) whether the composition of small mammal communities changed across gradients in habitat quality in tropical forest fragments, and (ii) whether any observed change could be explained by the functional traits of species. We sampled 24 trapping grids in fragments of semi‐deciduous forest, in each of two 6‐month periods. We considered each trapping grid as a sampling unit, for which we collected three datasets: an environmental matrix (vegetation structure and food resource availability), the abundance of small mammal species (community structure) and a matrix of functional traits (ecological and morphological traits which express tolerance to habitat disturbance and trophic guild). We used an RLQ approach to evaluate the association between traits and environmental gradients. Forest‐specialist and scansorial–arboreal species were associated with more complex habitat that had greater litter and canopy cover and more fallen logs. In relation to trophic guilds, granivore (fruit seeds), insectivorous and omnivorous species were also associated with higher complexity habitat, while frugivores were associated with shrub cover and availability of fruits. We conclude that functional traits (habitat use, use of vertical strata and diet) provide valuable insights into the distribution of small mammals along gradients of habitat quality in tropical forest fragments. We highlight that communities studies in fragmented landscapes should investigate the different components of biodiversity not only in landscape‐scale but also in habitat scale. Abstract in Portuguese is available with online material.     

Substitutes for grazing in semi‐natural grasslands – do mowing or mulching represent valuable alternatives to maintain vegetation structure?

Question: Which management treatments are suitable to replace historically applied grazing regimes? How and why does vegetation structure change following changes in management? Location: Semi‐natural calcareous dry grasslands in southwest Germany. Methods: We analysed changes in floristic and functional composition induced by different management treatments (grazing, mowing, mulching, succession) in long‐term experimental sites. First, floristic and functional distances between the initial conditions and the following years were determined. Second, we used RLQ analyses to include data on abiotic conditions, vegetation composition and functional traits in one common analysis. Finally, we applied cluster analyses on RLQ species scores to deduce functional groups. Results: In contrast to the historical management regime of grazing, all alternative management treatments led to changes in floristic and functional composition, depending on their intensity with respect to biomass removal. The distance analyses showed that mulching twice per year and mowing did not lead to strong changes in floristic or functional composition. However, RLQ analysis clearly provided evidence that only the grazed sites are in equilibrium, indicating that vegetation change still goes ahead. Conclusions: The current study clearly shows that RLQ is a powerful tool to elucidate ongoing processes that may remain hidden when separately analysing floristic and functional data. Alternative management treatments are not appropriate to sustain the typical disturbance dynamics of species‐rich semi‐natural grasslands. The less frequent an alternative management treatment is with respect to biomass removal, the less the floristic and functional structure can be maintained.