Competition between Lemna minuta, Lemna minor, and Azolla filiculoides. Growing fast or being steadfast?

A substantial number of Lemnaceae are invasive outside their natural distribution area. Lemna minuta is considered invasive in several European countries, where it can occur in the same habitat as invasive Azolla filiculoides and native Lemna minor. In this study the presence, abundance and growth rates of all three species were monitored across 24 natural ponds and in a series of mesocosms in order to explore the importance of species invasiveness and habitat invisibility. Field monitoring showed that the distribution of the three species of macrophytes is heterogeneous in space and time. However, the data show no association of nutrient or light levels with plant distribution. Indeed, using reciprocal transplanting experiments it was demonstrated that all species are able to grow in all ponds, even ponds where the species do not naturally occur. It is concluded that distribution of L. minor, L. minuta, and A. filiculoides is not limited by the prevailing physicochemical characteristics of the ponds during the summer period. Remarkably, in these experiments A. filiculoides displayed the highest RGR, and exerted a negative influence on growth rates and surface cover of L. minor and L. minuta. Despite such apparent invasiveness, A. filiculoides was relatively rare in the study area. Rather, the species most abundant was L. minor which has the lowest RGR under field conditions in summer. Therefore, this study shows that the invasiveness of the species during the summer months is not necessarily reflected in the actual distribution pattern in natural ponds. In fact, alien L. minuta and A. filiculoides are under-represented in the monitored area. It is concluded that the interaction of several factors, including growth under winter-conditions and/or dispersal after disturbances, is the major determinant of the abundance and heterogeneous distribution of L. minor, L. minuta, and A. filiculoides in the study area

UV4Plants workshop 2019: UV-B and climate change; impacts on plants and vegetation

Organizer’s report. Editorial-board-reviewed article

From the Editors' Desk

Editorial for journal issueNon peer reviewe

“Step by step”: high frequency short-distance epizoochorous dispersal of aquatic macrophytes

Aquatic macrophytes can successfully colonise and re-colonise areas separated by space and time. The mechanisms underlying such “mobility” are not well understood, but it has often been hypothesised that epizoochory (external dispersal) plays an important role. Yet, there is only limited, and mostly anecdotal, evidence concerning successful epizoochorous dispersal of aquatic macrophytes, particularly in the case of short-distance dispersal. Here we examine in situ and ex situ dispersal of aquatic macrophytes, including three invasive alien species. A high frequency of Lemna minor Linnaeus dispersal was observed in situ, and this was linked to bird-mediated epizoochory. We concluded that wind had no effect on dispersal. Similarly, in an ex situ examination Lemna minuta Kunth and Azolla filiculoides Lamarck, were found to be dispersed with a high frequency by mallard ducks (Anas platyrhynchos). No dispersal was measured for Elodea nuttalli (Planchon) H. St. John. It is concluded that short-distance or “stepping-stone” dispersal via bird-mediated epizoochory can occur with high frequencies, and therefore can play an important role in facilitating colonisation, range expansion and biological invasion of macrophytes

Effects of natural solar UV-B radiation on three Arabidopsis accessions are strongly affected by seasonal weather conditions

Large numbers of studies have reported on the responses of plants that are exposed to a specific dose of ultraviolet-B (UV-B) radiation. However, in the natural environment UV-B is a highly dynamic variable with UV-B intensities depending on, amongst others, geographic, temporal, weather and climatic factors. Furthermore, UV-B effects on plants can potentially be modulated by other environmental variables, and vice versa. This study aimed to characterize UV-B effects on plant morphology and accumulation of UV-screening pigments within the context of an oceanic climate and to assess the potential seasonality of plant UV-B responses. Arabidopsis thaliana was grown outdoors under UV-blocking or transmitting filters. Genotypic differences in the adaptive response to UV-B were assessed at seven time-points over a 12 month period and involved the Arabidopsis accessions Ler, Col-0, and Bur-0. Strong seasonal effects were found on rosette morphology and total UV-screening pigment concentrations across the three accessions. Low temperatures were the main determinant of accumulation of UV-absorbing pigments, with no clear UV-B effect observed at any time throughout the year. There was a significant UV effect on morphology during the summer months, and this was most likely associated with stress. This study shows that UV-effects need to be analysed in the context of weather, and other co-occurring natural factors, and emphasizes the importance of a holistic, multifactorial approach for the investigation of environmentally relevant UV-effects

Saltmarshes on peat substrate on the southwest coast of Ireland: edaphic parameters and plant species distribution

Abstract. Saltmarshes on peat substrate are common along the western Atlantic coast of Ireland. The peat which underlies these marshes was formed under freshwater conditions in post glacial times, after which these systems were subjected to a marine transgression. The aim of this study was to determine the relationship between edaphic factors, substrate type and saltmarsh vegetation, specifically investigating the role of edaphic factors in determining the distribution of saltmarsh species Atriplex portulacoides in Ireland. Edaphic parameters measured for each substrate included pH, moisture content, ammonium and nitrate. The peat was found to differ markedly from other substrates. Using canonical correspondence analysis it was found that pH and ammonium were the major drivers in influencing saltmarsh vegetation on peat substrate. Under both in situ and ex situ conditions Atriplex portulacoides showed an affinity for drier substrate and its absence from fringe marshes in Ireland is likely due to a combination of both biotic and abiotic factors, including intolerance to high soil moisture levels

Conspecific negative density dependence in a long-lived conifer, yew Taxus baccata L.

Conspecific negative density dependence in tree species can have major consequences for structuring of communities, yet in temperate forests this phenomenon remains largely unexplored. We investigated density-dependent recruitment in the long-lived conifer yew (Taxus baccata L.), for which low levels of regeneration have been linked with reduced light availability beneath conspecific canopies and speculative autotoxic effects. We combined in situ assessment of yew regeneration in a temperate forest in southwest Ireland with ex situ pot experiments to assess whether light and/or beneath conspecific canopy conditions inhibit the germination and early growth of yew. In field experiments, recruitment was related to distance from adult conspecifics, with higher levels of regeneration found in areas not beneath yew canopies. However, reduced light availability beneath the dense yew canopy was not responsible for this pattern, with abundant seedlings found in areas of equivalent or even lower light availability. Pot experiments showed that seedling survival was highest in deep shade treatments. Experiments also demonstrated that the addition of yew needles negatively affected seedling growth, but not seed germination. Together, our results suggest that under natural forest conditions, the absence of regeneration beneath mature yew canopies, often attributed to low light availability, may at least be partly related to substances in yew canopy or leaf leachates

Light responses of yew (Taxus baccata L.); does size matter?

Key message Photosynthetic capacity of yew needles depends primarily on light environment, with little evidence that developmental stage is a determinant photosynthetic performance. However, needle morphology is mostly determined by plant size. Abstract Populations of yew Taxus baccata are often characterized by a lack of natural regeneration and stands of this species have been afforded priority conservation status within the European Union. Light availability is known to be a key factor influencing recruitment dynamics in yew. To explore potential size-related responses to light, we investigated the distribution, photosynthetic efficiency and needle morphological traits in yew across a gradient of light availability at Atlantic oak woodland in southwest Ireland. Young plants were more common in shaded areas whereas saplings and juvenile yews were distributed in areas of higher light availability. Photosynthetic capacity of yew needles was found to depend primarily on light environment, with little evidence that the developmental stage is a determinant of photosynthetic performance. Photoinhibition was evident in needles sampled from high light environments across all demographic size classes. However, larger plants also showed particularly strong needle morphological responses (needle length, width, thickness, and specific leaf area) to light levels, which were mostly lacking in seedlings. We conclude that there is a dual control of yew light responses; the efficiency of the photosynthetic light reactions is largely linked to the light environment in which the plants grew, while needle morphology is mostly determined by the size of the plants. The findings of this study highlight the importance of considering all life-history stages and multiple traits when evaluating species response to light availability