Margot Vanhellemont

Impact of avian frugivores on dispersal and recruitment of the invasive Prunus serotina in an agricultural landscape

Although seed dispersal is considered to be a key process determining the spatial structure and spread of non-native plant populations, few studies have explicitly addressed the link between dispersal vector behaviour, seed distribution and seedling recruitment to gain insight into the process of exotic species invasion within a fragmented landscape context. The present study analyses the relationship between avian frugivory and spatial patterns of seed deposition and seedling recruitment for an expanding population of the invasive Prunus serotina in a hedgerow network landscape in Flanders, Belgium. We quantified fruit production, observed frugivores, and determined the spatial distribution of bird droppings and P. serotina seedlings. A relatively diverse assemblage of frugivores visited P. serotina seed trees, with Columba palumbus and Turdus merula being by far the most important dispersers. Landscape structure strongly affected dispersal vector behaviour and the spatial distribution of perching birds, droppings and seedlings. Frugivorous birds non-randomly dispersed seeds to perching sites and an association between perching birds, seed deposition and seedling recruitment was found. Results indicate that landscape structure contributes to non-random seed deposition of P. serotina by common local frugivores. Cutting the larger seed trees is proposed as the most feasible measure to slow down the invasion rate.

Plasticity in response to phosphorus and light availability in four forest herbs.

The differential ability of forest herbs to colonize secondary forests on former agricultural land is generally attributed to different rates of dispersal. After propagule arrival, however, establishing individuals still have to cope with abiotic soil legacies from former agricultural land use. We focused on the plastic responses of forest herbs to increased phosphorus availability, as phosphorus is commonly found to be persistently bioavailable in post-agricultural forest soils. In a pot experiment performed under field conditions, we applied three P levels to four forest herbs with contrasting colonization capacities: Anemone nemorosa, Primula elatior, Circaea lutetiana and Geum urbanum. To test interactions with light availability, half of the replicas were covered with shade cloths. After two growing seasons, we measured aboveground P uptake as well as vegetative and regenerative performance. We hypothesized that fast-colonizing species respond the most opportunistically to increased P availability, and that a low light availability can mask the effects of P on performance. All species showed a significant increase in P uptake in the aboveground biomass. The addition of P had a positive effect on the vegetative performances of two of the species, although this was unrelated to their colonization capacities. The regenerative performance was affected by light availability (not by P addition) and was related to the species’ phenology. Forest herbs can obviously benefit from the increased availability of P in post-agricultural forests, but not all species respond in the same way. Such differential patterns of plasticity may be important in community dynamics, as they affect the interactions among species.

The effects of sampling method and vegetation type on the estimated abundance of Ixodes ricinus ticks in forests

Estimating the spatial and temporal variation in tick abundance is of great economical and ecological importance. Entire-blanket dragging is the most widely used method to sample free-living ixodid ticks. However, this technique is not equally efficient in different vegetation types. The height and structure of the vegetation under study will not only determine the likelihood of a tick-blanket contact, but will also determine the rate of dislodgement. The purpose of this study was therefore to determine whether the alternative strip-blanket is more effectively in picking up ticks than the standard entire-blanket. Sampling was carried out in four forest understory vegetation types that differed in height and structure on five collection dates between April and September 2008. A total of 8,068 Ixodes ricinus ticks was collected (778 adults, 1,920 nymphs, and 5,370 larvae). The highest numbers of ticks were collected along the forest trails, where the dominant vegetation consisted of short grasses. The lowest numbers of ticks were collected in bracken-fern-dominated sites, where the vegetation seriously hampered tick sampling. Surprisingly, in each vegetation type, significantly more nymphs and adults were collected using the entire-blanket. However, the strip-blanket was more effectively in collecting larvae, especially in dense and tall vegetation.

Former land use affects the nitrogen and phosphorus concentrations and biomass of forest herbs

The colonization rates of understorey plants into forests growing on former agricultural land differ remarkably among species. Different dispersal and recruitment largely account for the contrasting colonization rates, but different effects of the soil legacies of former agricultural land use on plant performance may also play a role. Seven herbaceous forest species were sampled in paired post-agricultural and ancient forest stands to study whether land-use history has an effect on the aboveground nutrient concentrations (N, P and N:P ratios) and biomass of forest herbs and, if so, whether slow and fast colonizing species respond differently. Results showed that P concentrations were significantly affected by former land use with higher concentrations in the post-agricultural stands. N concentrations were unaffected and N:P ratios were significantly higher in the ancient stands. Nutrient concentrations varied considerably among species, but the variation was unrelated to their colonization capacity. Six out of the seven species had higher biomass in the post-agricultural stands relative to the ancient stands, and the degree to which the species increased biomass was positively related to their colonization capacity, i.e., the fast colonizing species showed the strongest increase. Such differential responses to past land use may contribute to the contrasting colonization capacity of forest plants. Land-use history thus affected both the nutrient concentrations and biomass of forest herbs, and only the biomass response was related to colonization capacity.

Spatio-temporal litterfall dynamics in a 60-year-old mixed deciduous forest

IntroductionTo increase our understanding of litterfall dynamics in mixed-species forests, seasonal and annual variations in litterfall mass and nutrient concentrations were assessed for a 60-year-old spontaneously developed forest dominated by silver birch (Betula pendula Roth), pedunculate oak (Quercus robur L.), and northern red oak (Quercus rubra L.) in Belgium.ResultsTotal quantities and seasonal patterns of most litterfall fractions were similar over the 29-month study period, but the species differed in start and duration of their leaf shedding period. The spatial distribution of litterfall persisted over the years for leaves, but not for total litterfall because of the varying spatial pattern of fallen twigs and reproductive structures. Consequently, predicting humus build-up based on short-term litterfall measurements may be difficult in mixed forests. Nutrient concentrations in leaf litter differed considerably between the species and throughout the year, but the seasonal pattern did not depend on the species. Betula returned significantly more nutrients to the soil per mass unit than Quercus, except for sodium.ConclusionAs the present stand conditions only allow recruitment of Q. rubra, Betula is being outcompeted, which decreases the nutrient return to the soil and may negatively affect biogeochemical cycling.

Prunus serotina unleashed: invader dominance after 70 years of forest development

Propagule pressure and disturbance have both been found to facilitate invasion. Therefore, knowledge on the history of introduction and disturbance is vital for understanding an invasion process, and research should focus on areas in which the invasive species has not been deliberately introduced or managed to study unconfounded colonization patterns. Comparing the outcome of such spontaneous colonization processes for different ecosystems might provide a useful framework for setting management priorities for invasive species that enter new, uninvaded areas. We focused on the 70-year spontaneous spread of the invasive tree species Prunus serotina in a pine forest in the Netherlands. To reconstruct the invasion pattern, we combined historical maps, tree ring analysis, spatially explicit tree inventory data, seed density data, and regeneration data for both native and non-native species. Prunus serotina was the only species that showed successful regeneration: the species was present throughout the forest in the tree, shrub, and herb layer. Native species were not able to outgrow the seedling stage. Our data demonstrate that P. serotina is a gap-dependent species with high seed production that builds up a seedling bank. We also compared the results of this study with a similar study on P. serotina colonization in a deciduous forest in Belgium, where P. serotina invasion was not successful. The sharp contrast between the outcomes of the two invasion processes shows the importance of studying an invasive species and the recipient ecosystem jointly and made us raise the hypothesis that herbivore pressure may facilitate P. serotina invasion.

The phosphorus legacy of former agricultural land use can affect the production of germinable seeds in forest herbs.

Abstract: Land-use history can have large effects on the different life stages and demography of forest plant species. Here we studied how the legacies of former land use in post-agricultural forests, and increased phosphorus (P) availability in particular, may alter the germinability and seed quantity in populations of the forest herbs Primula elatior and Geum urbanum. We collected seeds in experimental populations of P. elatior and G. urbanum established in post-agricultural and ancient forest stands 10 y ago and determined the number of seeds per fruit and germination percentage. The effect of P availability on the production of germinable seeds was tested in a pot experiment with 3 P levels. Former land use had an impact on the mean germination percentage: seed germinability tended to be higher in post-agricultural compared to ancient forest sites. For G. urbanum, the number of seeds per fruit was also higher in post-agricultural forest. Whereas P availability had no effect on G. urbanum seed quantity and germinability, the germination percentage of P. elatior seeds increased significantly with P supply. Whereas previous studies showed that former agricultural land use can have detrimental effects on particular life stages of forest herbs (e.g., reduced juvenile or adult survival), the production of germinable seeds might rather benefit from it. The environmental legacies of former land use thus affect the various life stages of a plant differently, which results in complex effects of land-use history on the demography of forest plants.

The seedling bank stabilizes the erratic early regeneration stages of the invasive Prunus serotina.

Abstract: We studied the regeneration dynamics of the semi-shade-tolerant invasive tree species Prunus serotina in the understory of 7 pine stands in its introduced range for 4 y, focusing on temporal, spatial, and spatiotemporal patterns. In each 20- × 40-m study plot, we inventoried all trees and shrubs taller than 1 m, counted seedlings in 3 age-height classes (224 subplots), trapped P. serotina seed rain (84 subplots), and identified the P. serotina seed trees. The seed set, seed rain density, and seedling densities of P. serotina all varied between the years and between the study plots, but the temporal stability of the spatial regeneration patterns increased with regeneration stage. There was a clear distinction between (1) seedlings smaller than 20 cm, younger than 6 y, occurring in very high densities and (2) seedlings between 20 cm and 1 m tall, older than 6 y, showing high spatiotemporal stability. Notwithstanding the large year-to-year variation in seed input, P. serotina maintained its regeneration potential in the forest understory by building up a short-lived seedling bank. The seedling bank strategy might represent an efficient way towards site occupancy of shade-tolerant non-native species in forest understories with few shade-tolerant native species. Nomenclature: Kartesz (1994) for the American species and Lambinon et al. (1998) for the European species.

A million and more trees for science

TreeDivNet is the largest network of biodiversity experiments worldwide, but needs to expand. We encourage colleagues to establish new experiments on the relation between tree species diversity and forest ecosystem functioning, and to make use of the platform for collaborative research.

Does neighbourhood tree diversity affect the crown arthropod community in saplings

Mixed forest with multiple tree species is expected to create heterogeneous habitat and diverse niches for the canopy arthropod community. We assessed arthropod abundance, order richness, and community composition in the crowns of saplings of nine temperate tree species in two plantations of a recently established tree diversity experiment in Belgium, and looked for relationships with the diversity and structure of the sapling’s local neighbourhood. The crown arthropod community differed between the two study sites, both in terms of abundances and composition. More arthropods were found in the post-agricultural site; the arthropod community was more complex in the formerly forested site. The tree species identity of a sapling, its apparency, and the phylogenetic diversity of its local neighbourhood all affected the crown arthropod community. Our study suggests that mixing phylogenetically distant tree species creates niches for a complex crown arthropod community.