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Rainfall partitioning is a key process in forest ecosystems, affecting evaporation, transpiration and water storage in the canopy, litter and soil. Although rainfall partitioning has been well studied in forest ecosystems, less is known about how rainfall partitioning is modified by tree species diversity. To disentangle the effects of tree species identity and diversity on rainfall partitioning, we installed 128 rain gauges in an experimental plantation, located in Zedelgem (Belgium), part of a large, multi-site forest diversity experiment in Belgium (FORBIO). With these rain gauges, we monitored throughfall for a full year, with a temporal resolution of two weeks. Based on these data, we analysed identity (i.e. whether throughfall is species-specific), non-additive (i.e. whether throughfall in mixtures is different from the expected amounts based on monocultures) and richness effects (i.e. how species richness affects throughfall). First of all, we found that rainfall partitioning highly differed between tree species, due to pronounced species identity effects. The presence of Pinus sylvestris [pine] was always associated with lower throughfall. Year-round throughfall was, for example, the lowest in mixtures of pine and Tilia cordata [lime] (43.8 +/- 14.5% of the open-field rainfall) and monocultures of pine (48.6 +/- 11.8%). Addi-tionally, non-additive effects were significant, leading to a 10% decrease of rainfall reaching the forest floor in mixtures compared to the expected throughfall based on measurements in monocultures. Tree species richness effects on rainfall partitioning acted indirectly, mostly via an increased basal area in mixed stands. Our findings suggest that mixing tree species leads to a higher than expected reduction in throughfall, which lowered soil moisture in mixed plots. Whether this reduced throughfall and soil moisture affects ecosystem functioning re-mains to be quantified.

Global environmental changes are expected to alter the functional characteristics of understorey herb-layer communities, potentially affecting forest ecosystem functioning. However, little is known about what drives the variability of functional traits in forest understories. Here, we assessed the role of different environmental drivers in shaping the functional trait distribution of understorey herbs in fragmented forests across three spatial scales. We focused on 708 small, deciduous forest patches located in 16 agricultural landscape windows, spanning a 2500-km macroclimatic gradient across the temperate forest biome in Europe. We estimated the relative effect of patch-scale, landscape-scale and macroclimatic variables on the community mean and variation of plant height, specific leaf area and seed mass. Macroclimatic variables (monthly temperature and precipitation extremes) explained the largest proportion of variation in community trait means (on average 77% of the explained variation). In contrast, patch-scale factors dominated in explaining community trait variation (on average 68% of the explained variation). Notably, patch age, size and internal heterogeneity had a positive effect on the community-level variability. Landscape-scale variables explained only a minor part of the variation in both trait distribution properties. The variation explained by shared combinations of the variable groups was generally negligible. These findings highlight the importance of considering multiple spatial scales in predictions of environmental-change effects on the functionality of forest understories. We propose that forest management sustainability could benefit from conserving larger, historically continuous and internally heterogeneous forest patches to maximise ecosystem service diversity in rural landscapes. (C) 2018 Gesellschaft fur Okologie. Published by Elsevier GmbH. All rights reserved.

Understorey communities can dominate forest plant diversity and strongly affect forest ecosystem structure and function. Understoreys often respond sensitively but inconsistently to drivers of ecological change, including nitrogen (N) deposition. Nitrogen deposition effects, reflected in the concept of critical loads, vary greatly not only among species and guilds, but also among forest types. Here, we characterize such context dependency as driven by differences in the amounts and forms of deposited N, cumulative deposition, the filtering of N by overstoreys, and available plant species pools. Nitrogen effects on understorey trajectories can also vary due to differences in surrounding landscape conditions; ambient browsing pressure; soils and geology; other environmental factors controlling plant growth; and, historical and current disturbance/management regimes. The number of these factors and their potentially complex interactions complicate our efforts to make simple predictions about how N deposition affects forest understoreys. We review the literature to examine evidence for context dependency in N deposition effects on forest understoreys. We also use data from 1814 European temperate forest plots to test the ability of multi-level models to characterize context-dependent understorey responses across sites that differ in levels of N deposition, community composition, local conditions and management history. This analysis demonstrated that historical management, and plot location on light and pH-fertility gradients, significantly affect how understorey communities respond to N deposition. We conclude that species' and communities' responses to N deposition, and thus the determination of critical loads, vary greatly depending on environmental contexts. This complicates our efforts to predict how N deposition will affect forest understoreys and thus how best to conserve and restore understorey biodiversity. To reduce uncertainty and incorporate context dependency in critical load setting, we should assemble data on underlying environmental conditions, conduct globally distributed field experiments, and analyse a wider range of habitat types.

The collection of high-quality calibration data is essential for the estimation of parameter values and reliability of crop models. However, few tools are available to quantify the minimum number of observations needed for parameter estimation. We therefore here applied practical identifiability analysis, based on global sensitivity analysis, to design measurement campaigns on farmers’ fields. We applied the method for parameterization of the AquaCrop model for mid-early potatoes in Belgium. We generated several virtual observational datasets, considering multiple weather and soil conditions, and measurement frequencies and variables. This analysis resulted in experimental designs where measurement campaigns should be conducted over at least two growing seasons and in different soil types, using soil moisture sensors combined with field observations every two weeks. This method showed to be a useful planning tool for the collection of sufficient data for the calibration of process-based crop models.

Understanding the responses of invasive and native populations to environmental change is crucial for reliable predictions of invasions in the face of global change. While comparisons of responses across invasive species with different life histories have been performed before, comparing functional traits of congeneric native and invasive species may help to reveal driving factors associated with invasion. Here we compared morphological functional trait patterns of an invasive species (Impatiens parviflora) with its congeneric native species (I. noli-tangere) along an approximately 1600 km European latitudinal gradient from France (49°34′N) to Norway (63°40′N). Soil nitrogen was recorded during six weeks of the growing season, and light, soil moisture, and nutrient availability were estimated for each sampled population using community weighted means of indicator values for co-occurring species. Temperature data were gathered from nearby weather stations. Both the native and invasive species are taller at higher latitudes and this response is strongest in the invasive species. Seed mass and number of seeds per capsule increase in I. noli-tangere but decrease in I. parviflora towards higher latitudes. Surprisingly, plant height in the invasive I. parviflora decreases with increasing soil nitrogen availability. The latitudinal pattern in seed mass is positively related to temperature in I. noli-tangere and negatively in I. parviflora. Leaf area of both species decreases with increasing Ellenberg indicator values for nitrogen and light but increases with increasing soil moisture. Soil nitrogen concentrations and Ellenberg indicator values for nitrogen have significant positive (I. noli-tangere) and negative (I. parviflora) effects on the number of seeds per capsule. Our results show that the native I. noli-tangere has efficient reproduction at its range edge while the invasive I. parviflora shows a marked decrease in seed size and seed number per capsule. These patterns are unrelated to the growth and obtained size of the plants: even low soil nitrogen availability in the north seemed not to limit plant growth and size. Our results suggest that the invasive I. parviflora tends to become more invasive at lower latitudes by producing heavier seeds and more seeds per capsule.

Forest conversion from native deciduous forests to coniferous stands has been performed in many European regions and resulted in dramatic shifts in understorey plant community composition. However, the drivers for changes in specific understorey plant species remained unclear. Here, we experimentally determine the species-specific effects of light availability and chemical soil characteristics, on the vegetative and regenerative performance of five herbaceous forest understorey plants. Topsoil samples from both spruce and deciduous stands at four locations, with two levels of soil acidity, were collected and used in a common garden experiment. Additionally, three different light levels were applied, i.e., ‘light deciduous’, ‘dark deciduous’ (extra light reduction during summer) and ‘evergreen’ (light reduction during winter). In a second experiment we evaluated the germination of two of these species against the acidity and tree species at the site of origin of the soil samples. The light regime affected both the vegetative and regenerative performance of the understorey species: compared to light deciduous, Anemone nemorosa had a significantly lower performance under the evergreen light regime, Convallaria majalis under dark deciduous and Luzula luzuloides and Galium odoratum under both light regimes. The vegetative performance was lower in soil from acid sites for the acid-sensitive species G. odoratum and Primula elatior . Differences between the soils sampled under deciduous or spruce stands had no effect on the vegetative, or the regenerative performance of these species. By contrast, the germination of L. luzuloides and P. elatior was higher in soils sampled in deciduous stands and in neutral sites. Species-specific responses in vegetative and regenerative performance of adult plants to a changed light regime and soil acidification could be a reason for the changed vegetation composition in converted stands. Also lower germination and establishment of forest understorey species in spruce stands could influence the species distribution after conversion.

The identity and composition of the overstorey trees can have large impacts on the understorey vegetation in temperate forests. Here, we assess the effects of the conversion of mixed deciduous forests to Norway spruce ( Picea abies ) plantations on the understorey vegetation composition and diversity. We also investigated whether the effects of this conversion depend on the soil fertility and if they affect the species pool at the landscape scale. The study area contained ancient deciduous woodlands mainly consisting of Quercus spp., Fagus sylvatica and Carpinus betulus in the Gaume, an area with a mild temperate climate in southern Belgium. Spruce plantations were established 30–50 years ago in this matrix of deciduous woodlands. Differences in overstorey and understorey vegetation, forest floor and soil characteristics between 40 paired plots in spruce plantations and adjacent deciduous forest were compared along a soil fertility gradient. The forest floor mass was higher and the soil pH was lower in spruce plantations. The composition of the understorey vegetation differed between deciduous forest and spruce plantations: the spruce stands contained more light-demanding and acid-tolerant understorey species than the adjacent deciduous stands. The mean understorey species diversity, i.e., the local species diversity in a plot (α-diversity) as well as the total number of species (γ-diversity), did not differ significantly between the deciduous and spruce stands. Hence, the differing species composition of the spruce plantations contributes to a larger overall species pool and higher across site β-diversity. Although the differences in the environmental variables within a plot pair increased along the fertility gradient, the differences in understorey vegetation diversity between deciduous and spruce stands showed no relationship with the gradient. Only the difference in Ellenberg indicator value for soil reaction ( R ) within a pair increased with increasing soil fertility. In line with the abiotic changes, the understorey vegetation composition had shifted towards a more acid-tolerant vegetation type 30–50 years after conversion to spruce plantations. The increase in light-demanding species was probably due to a more intensive thinning management in the spruce stands.

Large areas of agricultural land in Europe have been planted with broadleaved trees during the past decades. The aims of this study were to determine stand-scale colonization rates of European forest herbs in young (14–18 years old) post-agricultural broadleaved forest plantations, to analyze whether colonization rates were related to life-history traits and site characteristics, and to compare the results with studies in older post-agricultural forests. The colonization of herbaceous forest species was mapped in 16 young broadleaved forests and compared with their distribution in adjacent older forests in southern Sweden. Stand characteristics were analyzed in 32 paired sample plots. Colonization rates (m year −1 ) were calculated for 43 forest plant species. For 30 forest plant species we also compared current colonization rates with results from 30 to 75 year old stands. Colonization rates increased with maximum plant height and the potential dispersal distance, but were negatively related to diaspore mass and clonal spread. Soil, canopy and vegetation conditions in young forests were not significantly related to colonization rates. Nevertheless, young forests had higher soil pH and extractable phosphorus concentrations while soil organic matter content, basal area and canopy cover of the trees were higher in the adjacent older forests. Higher resource availability in young forests may favor competitive tall herbs over low-growing ancient forest plants. The comparison with colonization rates into older post-agricultural plantations revealed that while approximately half of the species had similar or lower rates, the other half had on average ca. ten times higher colonization rates in the present study, ranging from 3.0 to 11.5 m year −1 indicating the potential for relatively fast colonization by some forest plants. We conclude that planted post-agricultural forests can start to contribute to restoration of forest plant diversity shortly after canopy closure when located adjacent to older forests. However, many small forest plants, including those species which form extensive flower carpets characteristic of ancient forests, colonize young forests very slowly. Further research should focus on the development of restoration measures which facilitate the colonization process of ancient forest herbs.

Recent anthropogenic climate change is already affecting the range dynamics of many organisms worldwide. Understanding the relative importance of environmental factors and historical processes in determining species’ geographic ranges is crucial for predicting the impacts of climate change. Here, we experimentally assessed the survival and performance of four understorey forest plants ( Ornithogalum pyrenaicum , Scilla bifolia , Iris foetidissima and Symphytum tuberosum ) transplanted beyond their natural range limit over a 7-year period. Since the survival probability of plants transplanted beyond their natural geographic range is expected to decrease with the home-away distance, the species represent a gradient in the distance between the transplant site and their natural geographic range limit (from 50 to 500 km). Three of the four species still survived seven years after introduction. Appreciable survival outside their natural geographic range partly suggests that the ranges of these plant species may be controlled by dispersal limitation. Importantly, an interspecific trend towards decreasing survival with increasing distance from the range limit was apparent. Initial competition from other understorey plants generally limited the survival and performance of the transplanted species. In conclusion, the good survival and performance of the most ‘nearby’ species emphasizes the importance of dispersal limitation in determining range limits, while the poor survival of the ‘farther’ species suggests that their occurrence is (co-)controlled by environmental factors such as the local climate. Finally, we briefly discuss our results within the context of human-mediated assisted migration in the face of climate change.

It is ecologically undesirable to solve forest soil compaction due to mechanized harvesting at large spatial scales using agricultural mechanical soil loosening techniques. We therefore examined whether a stimulation of biological activity through litter manipulation, liming and/or inoculation of the anecic earthworm species Lumbricus terrestris could significantly contribute to the ecological restoration of compacted forest soils by comparing the impact of these treatments on the soil within and beside compacted wheel tracks. The replacement of native litter by litter with a better quality resulted in a faster litter decomposition. However, maximal decay rates were obtained only when litter manipulation, earthworm inoculation and liming were combined. Anecic earthworms were initially absent as soils were probably too acid. Liming as well as litter manipulation had a small positive influence on the numbers of retraced L. terrestris, inducing positive feedback mechanisms on soil pH and litter decomposition rates. None of the treatments, however, had a significant effect on the compaction degrees within or beside tracks within the small study period. L. terrestris realized only a small decrease of bulk density beside the tracks. Within the tracks a similar number of L. terrestris was retrieved, but effects on the compaction degree were negligible. Liming decreased penetration resistance, but only in the absence of anecic earthworms. Endogeic earthworms were overall more abundant than the anecics, especially within tracks where soil water contents and pH values were higher. However, endogeics only had a marginal effect on litter decomposition and although they positively influence soil structure, they could not realize a reduction of the compaction degree, quantified by bulk density and penetration resistance, due to different burrowing habits. Our results indicated that a positive impact of anecic earthworms on the structure of compacted forest soils can be obtained in the long-term, at least in case soil conditions (acidity, nutrient availability and moisture content) are favourable. This can be achieved by conversion of forests towards tree species with high quality litter.