Forest Understorey Research Articles

Large mammalian herbivores modulate plant growth form diversity in a tropical rainforest

Abstract The world’s terrestrial biomes are broadly classified according to the dominant plant growth forms that define ecosystem structure and processes. Although the abundance and distribution of different plant growth forms can be strongly determined by factors such as climate and soil composition, large mammalian herbivores have a strong impact on plant communities, thus defaunation (the local or functional extinction of large animals) has the potential to alter the compositional structure of plant growth forms in natural ecosystems. Tropical rainforests sustain a high diversity of growth forms, including trees, palms, lianas, shrubs, herbs and bamboos, all of which play important ecosystem functions. Here, we experimentally evaluate how large mammalian herbivores affect the dominance, diversity and coexistence of these major tropical forest plant growth forms, by monitoring communities of saplings on the understorey in 43 paired exclusion plots in a long‐term replicated exclusion experiment in the understorey of the Atlantic forest of Brazil. Over the course of 10 years large herbivore exclusion decreased diversity among growth forms, increased the absolute abundance of palms and trees (22% and 38% respectively) and increased the diversity of species within these two groups, to the detriment of other growth forms. Furthermore, all pairwise relationships between growth forms were positive on plots where herbivores had access, whereas several strong negative relationships emerged in plots where herbivores were excluded. This occurred despite strong background directional temporal trends affecting plant communities in both experimental treatments across the region. Synthesis. Our work indicates that the defaunation alters growth form dominance by favouring palms and trees while eroding diversity among growth forms and coexistence on a temporal scale. Large herbivore mammals promote diversity among growth forms, preventing the hyper‐dominance of trees and palms, yet without supressing the diversity of species within growth forms. We argue that large herbivore mammals affect growth forms through several non‐mutually exclusive mechanisms, including herbivory, seed dispersal and physical disturbance, as well as differential effects linked to the morphological and physiological adaptations of growth forms. We conclude that defaunation might lead to profound impacts on important ecosystem functions underpinned by growth form diversity, and result in vertical and horizontal structural simplification of tropical rainforests.

Regeneration from seed in herbaceous understorey of ancient woodlands of temperate Europe.

European ancient woodlands are subject to land use change, and the distribution of herbaceous understorey species may be threatened because of their poor ability to colonize isolated forest patches. The regeneration niche can determine the species assembly of a community, and seed germination traits may be important descriptors of this niche. We analysed ecological records for 208 herbaceous species regarded as indicators of ancient woodlands in Europe and, where possible, collated data on seed germination traits, reviewed plant regeneration strategies and measured seed internal morphology traits. The relationship between plant regeneration strategies and ecological requirements was explored for 57 species using ordination and classification analysis. Three regeneration strategies were identified. Species growing in closed-canopy areas tend to have morphological seed dormancy, often requiring darkness and low temperatures for germination, and their shoots emerge in early spring, thus avoiding the competition for light from canopy species. These species are separated into two groups: autumn and late winter germinators. The third strategy is defined by open-forest plants with a preference for gaps, forest edges and riparian forests. They tend to have physiological seed dormancy and germinate in light and at higher temperatures, so their seedlings emerge in spring or summer. Seed germination traits are fundamental to which species are good or poor colonizers of the temperate forest understorey and could provide a finer explanation than adult plant traits of species distribution patterns. Seed dormancy type, temperature stratification and light requirements for seed germination are important drivers of forest floor colonization patterns and should be taken in account when planning successful ecological recovery of temperate woodland understories.

Intra- and inter-specific leaf trait responses of understorey species to changes in forest maturity

Coppice forest management impacts understorey vegetation. This is particularly true for habitat specialists (priority elements in conservation) who prefer closed canopy conditions associated with mature forests. Most trait-based studies of forest understorey focus on interspecific differences, which neglect the role of intraspecific variability. Here, we tackled this issue by examining how leaf traits respond (intra- and inter-specifically) to different stages of stand maturity for habitat specialist and generalist species. We measured the leaf area (LA), leaf dry matter content (LDMC) and specific leaf area (SLA) for 1,932 individuals belonging to 71 understorey species in 18 beech coppice forests grouped in three stages of stand maturity (i.e., post-logging, n = 5; recovering, n = 5; old stands, n = 8). We assessed leaf trait variation across stages of forest maturity for the entire community, and for specialists and generalists separately using monotonic trend tests. Then, we disentangled the proportion of variation attributable to intraspecific variability and interspecific differences of specialists and generalists using variance decomposition. We found significant positive trends in community-level SLA and negative ones in LDMC with increasing stand maturity. Beech forest understorey specialists (but not generalists) explained these patterns. The contribution of intra- and inter-specific components was dependent upon the trait under consideration and habitat specialization. For specialists, intraspecific variability was significant for all leaf traits and could explain 15.2% of model variance for SLA, 4.2% for LA and 3.1% for LDMC along the stand maturity gradient. Interspecific differences were significant for LDMC (29.8% of variance explained) and SLA (7.6%). For generalists, intraspecific variability was not significant; interspecific differences were significant for LA and explained 26.2 % of variability along the stand maturity gradient. We revealed that the maturity of coppice stands have different effects on generalist and specialist species. Habitat specialists are responsive to subtle changes in forest stand age both intra- and interspecifically, with their prevalence being function- and trait-dependent. Interspecific differences explained most of the LDMC (resource conservation) variance, while intraspecific variability accounted most of the SLA (light capture) variance. Generalists contribute to community-level trait variation, yet these species were less responsive to changes in forest stand age. To assess the impact of forest management on understories and refine the effectiveness of monitoring plans, we recommend focusing on habitat specialists, ideally including both trait variability within- and among-species.

Interests of Livestock Activities within Agroforestry Systems in Guadeloupe: Stakeholders’ Opinions and Farmers’ Perceptions

The Valab project (Integrated Ecosystemic value-enhancement of the Guadeloupean Forest Agrobiodiversity) is a participatory action research that aims to explore the feasibility of diversification activities within the forest understorey. In the exploratory phase of the project, we studied stakeholders’ opinions on the positive and negative impacts of introducing livestock activities within a forest environment, as well as breeders’ constraints or motivations for setting up livestock units within agroforestry systems in the private forests of Guadeloupe. To establish a diagnosis of the current situation, 51 stakeholders (St) and 49 farmers (Fr) were interviewed on their perceptions, opinions, motivations and preferences. Forty-one percent of the St has had a generally positive appreciation of livestock activities, while on the opposite 31% have had a resolutely negative posture depending on their area of expertise. Their views on the possible positive effects or negative interactions between livestock and agroforestry activities were analysed in order to suggest possible approaches for the authorities or associations. Unanimously, the farmers replied that their motivations followed economic strategies: for self-consumption or sale. But their decision-making processes were also non-economic. Many of them (36% Fr) gave importance to their activity in the construction of their identity, whether it was through a desire to perpetuate traditions or to maintain family activities. The main constraints identified were natural constraints (61% Fr), including topography, climate and predators. While 28% of Fr notified their motivations for environmental protection. Another constraint was of a technical nature (44% Fr) since farmers asked for more technical references or support services. Further researches are required to study the interactions between animals and the undergrowth by describing their respective ecosystem services in order to enhance their positive interactions.

Simulating climate change in situ in a tropical rainforest understorey using active air warming and CO2 addition.

Future climate‐change effects on plant growth are most effectively studied using microclimate‐manipulation experiments, the design of which has seen much advance in recent years. For tropical forests, however, such experiments are particularly hard to install and have hence not been widely used. We present a system of active heating and CO2 fertilization for use in tropical forest understoreys, where passive heating is not possible. The system was run for 2 years to study climate‐change effects on epiphytic bryophytes, but is also deemed suitable to study other understorey plants. Warm air and CO2 addition were applied in 1.6‐m‐tall, 1.2‐m‐diameter hexagonal open‐top chambers and the microclimate in the chambers compared to outside air. Warming was regulated with a feedback system while CO2 addition was fixed. The setup successfully heated the air by 2.8 K and increased CO2 by 250 ppm on average, with +3 K and +300 ppm as the targets. Variation was high, especially due to technical breakdowns, but not biased to times of the day or year. In the warming treatment, absolute humidity slightly increased but relative humidity dropped by between 6% and 15% (and the vapor pressure deficit increased) compared to ambient, depending on the level of warming achieved in each chamber. Compared to other heating systems, the chambers provide a realistic warming and CO2 treatment, but moistening the incoming air would be needed to avoid drying as a confounding factor. The method is preferable over infrared heating in the radiation‐poor forest understorey, particularly when combined with CO2 fertilization. It is suitable for plant‐level studies, but ecosystem‐level studies in forests may require chamber‐less approaches like infrared heating and free‐air CO2 enrichment. By presenting the advantages and limitations of our approach, we aim to facilitate further climate‐change experiments in tropical forests, which are urgently needed to understand the processes determining future element fluxes and biodiversity changes in these ecosystems.

Complementary allometric model of understory tree biomass in the semi-deciduous rainforest of Cameroon

Forest understorey contains remarkable plant diversity, contributing to the heterogeneous environments for other biotic gatherings and soil supplement aggregations. Nonetheless, the biomass of the understorey vegetation is neglected because of the lack of appropriate allometric equations without which there are uncertainties in biomass estimation. This study was aimed at developing multispecies allometric equations that will be used to estimate the aboveground biomass of different compartments (trunk, crown, and leaves) of understorey trees in the semi-deciduous rainforest of Cameroon. Understorey tree diameter (1.0-10.0 cm), height, and crown diameter were measured on 1023 trees as biomass predictors. The results showed that the fit of the model improves with more predictive variables, four of which were considered in all studied compartments (trunk, crown, and leaves). Existing specific and pantropic allometries based on diameter tend to overestimate the aboveground biomass of understorey trees when compared to the allometry developed in this study. This study highlighted the importance of a specific aboveground biomass allometric equation for understorey trees. Furthermore, the multispecies allometric equation developed for understorey trees complements those recently developed for overstorey trees, thereby contributing to the total aboveground woody biomass estimation in semi-deciduous forest in the Congo Basin.

Changes in forest structure drive temperature preferences of boreal understorey plant communities

Abstract The local climate in forest understories can deviate substantially from ambient conditions. Moreover, forest microclimates are often characterized by cyclic changes driven by management activities such as clear‐cutting and subsequent planting. To understand how and why understorey plant communities change, both ambient climate change and temporal variation in forest structure have to be considered. We used inventories from 11,436 productive forest sites in Sweden repeated every 10th year 1993–2017 to examine how variation in forest structure influences changes in the average value of minimum and maximum temperature preferences of all species in a community, that is, community temperature indices (CTIs). We then evaluated to what extent these changes were driven by local extinctions and colonizations, respectively, and to what extent the difference in CTI value between two inventories was related to changes in forest density and in macroclimate. Lastly, we tested whether effects on CTI change by these two drivers were modified by topography, soil moisture and tree species composition. CTI values of the understorey plant communities increased after clear‐cutting, and decreased during periods when the forest grew denser. During the period immediately after clear‐cutting, changes were predominately driven by colonizations of species with a preference for higher temperatures. During the forest regeneration phase, both colonizations by species preferring lower temperatures and local extinctions of species preferring higher temperatures increased. The change in understorey CTI over 10‐year periods was explained more by changes in forest density, than by changes in macroclimate. Soil moisture, topography and forest tree species composition modified to some extent the effects of changes in forest density and in macroclimate on understorey CTI values. Synthesis. Via stand manipulation, forest management impacts the effects of regional climate on understorey plant communities. This implies that forest management by creating denser stands locally even can counterbalance the effects of regional changes in climate. Consequently, interpretations of changes in the mean temperature preference of species in forest understorey communities should take forest management regimes into account.

Forest understorey communities respond strongly to light in interaction with forest structure, but not to microclimate warming.

Forests harbour large spatiotemporal heterogeneity in canopy structure. This variation drives the microclimate and light availability at the forest floor. So far, we do not know how light availability and sub-canopy temperature interactively mediate the impact of macroclimate warming on understorey communities. We therefore assessed the functional response of understorey plant communities to warming and light addition in a full factorial experiment installed in temperate deciduous forests across Europe along natural microclimate, light and macroclimate gradients. Furthermore, we related these functional responses to the species' life-history syndromes and thermal niches. We found no significant community responses to the warming treatment. The light treatment, however, had a stronger impact on communities, mainly due to responses by fast-colonizing generalists and not by slow-colonizing forest specialists. The forest structure strongly mediated the response to light addition and also had a clear impact on functional traits and total plant cover. The effects of short-term experimental warming were small and suggest a time-lag in the response of understorey species to climate change. Canopy disturbance, for instance due to drought, pests or logging, has a strong and immediate impact and particularly favours generalists in the understorey in structurally complex forests.

Arbuscular mycorrhizal fungi promote small-scale vegetation recovery in the forest understorey.

Root-associating arbuscular mycorrhizal (AM) fungi foster vegetation recovery in degraded habitats. AM fungi increase nutrient availability for host plants; therefore, their importance is expected to be higher when nutrient availability is low. However, little is known about how small-scale variation in nutrient availability influences plant and AM fungal communities in a stable ecosystem. We conducted a 2-year field study in the understorey of a boreonemoral forest where we examined plant and AM fungal communities at microsites (15cm diameter) with intact vegetation cover and at disturbed microsites where vegetation was cleared away and soil was sterilized to remove soil biota. We manipulated soil nutrient content (increased with fertilizer, unchanged, or decreased with sucrose addition) and fungal activity (natural or suppressed by fungicide addition) at these microsites. After two vegetation seasons, manipulations with nutrient content resulted in significant, although moderate, differences in the content of soil nutrients (e.g. in soil phosphorus). Suppression of fungal activity resulted in lower richness, abundance and phylogenetic diversity of AM fungal community, independently of microsite type and soil fertility level. Plant species richness and diversity decreased when fungal activity was suppressed at disturbed but not in intact microsites. The correlation between plant and AM fungal communities was not influenced by microsite type or soil fertility. We conclude that small-scale variation in soil fertility and habitat integrity does not influence the interactions between plants and AM fungi. The richness, but not composition, of AM fungal communities recovered fast after small-scale disturbance and supported the recovery of species-rich vegetation.

Germination of small tropical seeds has distinct light quality and temperature requirements, depending on microhabitat.

The coexistence of plant species in tropical rainforests is related to specific abiotic resources, varying according to the occurrence microhabitat of each species. Light quality is the main abiotic factor influencing germination of small seeds; however, studies often do not discriminate its effect from that of light irradiance. This study compared specific requirements for seed germination of ten small-seeded species, with restricted occurrence in only one of three contrasting microhabitats: forest understorey, edge of clearings and open areas. Laboratory experiments were carried out to test temperature regime (constant or fluctuating), light quality (R:FR) and light irradiance (PAR), which reproduce high and low conditions commonly found in the microhabitats. Seed germination of all species occurred between 20 and 30°C, only seeds of open area species were able to germinate at 35°C and no species required alternating temperatures to germinate. Irrespective of species and microhabitat, a decrease in the R:FR reduced the germination percentage; however, there were differences in the capacity to germinate at low R:FR. The values of R:FR50% were higher for open area and edge species (0.441-0.345) than for understorey species (0.181-0.109), with few exceptions. For all species and most of the tests, germination was not influenced by PAR. Light quality is the most important light signal for germination of small seeds; irradiance has little effect. Our results suggest two distinct patterns of germination for small-seeded species: open area and edge species are light-demanding and require high R:FR to germinate, while understorey species are shade-tolerant and germinate at low R:FR. These differences are responsible for distinct microhabitat occurrence and help to explain the coexistence of species in tropical forests.

Forest understorey plant responses to long-term experimental warming, light and nitrogen addition.

Climate change, eutrophication and intensified forest management are affecting forest understorey plants, a major component of forest biodiversity. The main impacts of these drivers have often been studied, but we lack a good understanding of how key understorey species are affected by potential interactive effects of these drivers and which species drive community changes. Here we assessed the responses of 15 species occurring in the understorey of a deciduous temperate forest to experimental warming, light addition and enhanced nitrogen inputs in permanent plots surveyed for 9years. We analysed vegetation cover and key functional traits (plant height, specific leaf area and reproductive traits) at the species level and identified the species driving community change with principal response curves (PRC). Light addition and warming, and to a lesser extent also nitrogen addition, had profound effects on cover and functional traits. Many species showed directional change over time, and this change can either be strengthened or weakened by treatments, indicating the importance of long-term monitoring. Against expectations, we observed few interactions between treatments. Species responses to treatments were related to ecological strategies (generalists versus forest specialist). Generalists, such as Rubus fruticosus, benefitted from the warming and light treatments and outcompeted forest specialists. This might ultimately lead to biotic homogenization. Since the treatment effects of light and warming were additive, keeping the canopy closed will only mitigate, but not stop, the effects of global warming on the forest understorey plants.

Evolution of Impatiens (Balsaminaceae) in the Albertine Rift – The endemic Impatiens purpureoviolacea complex consists of ten species

AbstractThe Albertine Rift harbours a highly diverse flora with numerous endemic species. An important component of the forest understorey is the herbaceous genus Impatiens. Fieldwork in Burundi, the Democratic Republic of the Congo and Rwanda as well as morphological studies indicated that the Albertine Rift endemic Impatiens purpureoviolacea represents a species complex. We analyzed the hidden diversity of the complex using morphological and molecular data supplemented by herbarium studies. We found that the Impatiens purpureoviolacea complex can be divided into morphologically and phylogenetically well characterized clades containing ten species and a natural hybrid. We describe all of these species, provide a species key and analyze their evolutionary history. Beside Impatiens purpureoviolacea and I. gesneroidea, the already described I. urundiensis is resurrected from synonymy. Two varieties, Impatiens purpureoviolacea var. longicalcarata and I. gesneroidea var. superglabra are raised to species status, and five new species (Impatiens elwiraurzulae, I. lotteri, I. ludewigii, I. lutzmannii, I. versicolor) and a new natural hybrid (I. ×troupinii) are described. Within the mostly insect‐pollinated species of the clade, two bird‐pollinated species (Impatiens gesneroidea, I. superglabra) evolved independently. The clade split from its sister taxon in the Pliocene and started diversifying during the Pliocene/Pleistocene transition in parallel to an increased mountain uplifting and volcanic activity in the Albertine Rift. It further diversified during the Pleistocene, likely due to the changes in forest cover and connectivity induced by climatic fluctuations.

Influence of Environmental Factors on Forest Understorey Species in Northern Mexico

Background: Understorey plants are key to maintaining forest structure and functioning. They protect the soil, improve its structure and fertility, reduce water run-off and sustain the below-ground biota, amongst other ecological services. However, little is known about the environmental conditions that regulate the occurrence of these plants. This study focuses on determining how canopy cover influences the occurrence of understorey species and identifying the most important soil properties that affect these species. The study area was a pine-oak forest in the Sierra Madre Occidental, an important source of ecological services for northwestern Mexico. Methods: To assess the conditions influencing the presence of herbaceous and shrub species, 25 soil variables were examined in relation to the species occurring in forest gaps and under the canopy. Sampling was conducted in five plots, each of 100 × 100 m. In each plot, 4 subplots, each of 20 × 20 m, were each subdivided in a grid of 2 × 2 m units, in which the presence-absence of herbaceous and shrub species was recorded (2000 units in total). Soil samples were extracted for analysis from the central point in each subplot. Data were analyzed using a Binomial Logistic Model (BLM) and Random Forest (RF) classification. Results: Understorey species were more strongly affected by soil variables than by their location in gaps or below canopy. The concentrations of Ca, P, K, Fe, Na, C, Zn, Mn, nitrates, organic matter, sand, silt, and percentage water saturation were statistically significantly associated with the presence of some plant species, whilst no significant differences were found in regard to preference for gaps or canopy, although several species were more frequent in open areas. Conclusions: Given the importance of the understorey cover in forest system functioning, we propose that understorey should be considered in integrated management and conservation practices for the temperate forests of northern Mexico.

Junipers enable heavily browsed rowan saplings to escape ungulates in boreal forest

Recent increases in ungulate herbivore populations have intensified browsing pressure in northern forest ecosystems. High browsing pressure affects recruitment into tree populations and saplings are among the most impacted and critical tree stages. This calls for research on factors that enhance sapling survival and promote recovery of herbivore preferred species. One understudied factor in ungulate dominated boreal forests is the ability of young trees to escape large herbivores by utilising “nurse plants”. First, we examined the impact of ungulate browsing on the abundance and size of rowan (Sorbus aucuparia), a functionally important tree species in an old-growth boreal pine forest, western Norway. The responses were compared between exclosures and ungulate-access plots over ten years (2001–2011) combined with investigations along a browsing intensity gradient (Exp. A). In a second experiment we investigated whether rowan saplings escape browsing and enhance growth and survival by growing within nurse plants. Consequently, we planted rowan saplings in ungulate browsing areas, inside and outside coniferous juniper (Juniper communis) bushes, and analysed the treatment effect on browsing, growth, and survival of rowan saplings across five years (2013–2018; Exp B). Excluding large herbivores resulted in a large increase in rowan saplings which were six times more abundant in exclosures (0.44 ind./m2) compared to ungulate-access plots (0.07 ind./m2) after ten years (Exp. A). The abundance of rowan saplings also decreased with higher browsing intensity. Moreover, the rowans in exclosures grew to almost twice the height of the ungulate-access rowans. The protected rowan saplings experienced significantly less browsing damage, and only 25% of the protected plants experienced heavy browsing or died after the first winter, compared to 70% of the unprotected plants (Exp. B). After five years, 50% more unprotected than protected plants experienced heavy browsing or mortality, and growth in protected plants was nearly 50% greater over the period. The negative impact of ungulate browsing on abundance and growth of rowans can be counteracted by positive plant-plant interactions. Rowan saplings growing within nurse plants showed significantly less browsing and better performance indicating high functional importance of the forest understorey. Forest managed with multi-layered and multi-species understoreys may therefore also be beneficial for tree recruitment in browsed boreal forest landscapes.

Inter‐ and intraspecific spatial distributions, spatial segregation by dominants and emergent neutrality in understorey plants

Tradeoffs between intra and interspecific distances of individual plants determine the spatial distributions of local plant communities and small‐scale community assembly. Here we use an individual based approach and null model standardization to compare intra‐ and interspecific nearest neighbour distances of plant ramets as well as mean differences in functional trait expression to nearest heterospecifics within local communities of temperate forest understorey plants in northern Poland. We found a general tendency towards clustered intraspecific and segregated interspecific ramet distributions. Spatial distribution patterns of understorey plants differed considerably between two nearby forest sites that mainly differed in the age structure of trees. Individuals of clonal species tended to have significantly smaller intraspecific and larger interspecific nearest neighbour distances than seed dispersing species. Our results indicate a major influence of dominant plant species on the spatial distributions of subdominants. Trait expressions related to plant growth (specific leaf area and leaf mass) and reproduction (seed mass) were significantly segregated in comparison to the spatial distribution of conspecific individuals and less so in comparison to heterospecific individuals. We argue that spatially explicit analyses of plant community assembly need to consider respective distributions of important traits and that the tradeoffs of contrasting trait distributions trigger small scale plant community composition and species distributions. We present a general framework for the analysis of plant community assembly based on the spatial distributions of individual plants and functional traits.

Tropical forest understorey riparian and upland composition, structure, and function in areas with different past land use

AbstractQuestionsAlthough past land‐use effects on composition and structure are described for many forest types, forested riparian areas have been overlooked. Riparian areas may contain unique species and high species richness, as well as contribute basal leaf‐litter resources for aquatic fauna. We asked: (a) does past land use alter composition in riparian vs upland areas in wet tropical forest; (b) which vegetation life forms characterize riparian zones; and (c) what is the composition and quantity of leaf litter in riparian zones compared to uplands?LocationRiparian and upland locations in tropical wet forest, Luquillo Forest Dynamics Plot (LFDP), Luquillo Experimental Forest, Puerto Rico.MethodsStratified random sampling was conducted in riparian and upland areas of LFDP with high and low past land use. Understorey vegetation life form composition and cover were sampled in plots. Leaf‐litter species composition and components were estimated with collecting baskets placed throughout riparian zones and uplands.ResultsFerns and lianas were more abundant in riparian areas than uplands. Ordination of species composition groups was distinct for riparian and upland areas with different past land use when based on leaf litter, but this ordination pattern was not as clear in plot vegetation.ConclusionCharacterized by greater presence of ferns and lianas, riparian zones had higher species richness values and greater leaf litter biomass. Distinct leaf‐litter species composition within riparian and upland areas may result from a combination of different land‐use histories, various plant types, and environmental conditions. Although riparian areas are proportionally a small component of the forested landscape, they are significant contributors to ecosystem process, terrestrial and aquatic linkages, and plant community composition.

Multiple soil factors explain eutrophication signals in the understorey vegetation of temperate forests

AbstractQuestionsVegetation in forest understories is determined by a complex interplay of many different environmental factors, including light, water and nutrient availability. Changes in these conditions, especially changes in nitrogen (N) availability, have been widely discussed to cause shifts in species composition and loss of oligotrophic species. We examined whether nitrogen is the most important factor for the decline of nitrogen‐sensitive species in understorey communities, as measured by average Ellenberg N values and the share and number of nitrogen‐sensitive character species.LocationNine different oligo–mesotrophic types of temperate forest in southwest Germany.MethodsWe combined understorey vegetation and soil data from 135 forest stands. To study the importance of variables in explaining the vegetation gradients, we used random forest analyses and non‐metric multidimensional scaling (NMDS) ordinations combined with fitting of environmental vectors.ResultsWe detected strong differences between three examined forest type groups: eutrophication in the understorey vegetation of broad‐leaved forests on carbonate soils was strongly related to high canopy closure and low soil carbon/phosphorus (C/P) ratio. In broad‐leaved forests on acidic soil, base saturation played a major role, but the vegetation was also influenced by the C/N and C/P ratios. In coniferous forests on acidic soil, soil pH and organic layer depth were important, as were the C/N, C/P and N/P ratios.ConclusionOur results show that eutrophication in understorey forest vegetation is not monocausal. Rather, oligotrophic species of forest communities are reduced at sites with higher N or P availability and/or higher base saturation and soil pH. These conditions favor fast‐growing ubiquitous species in the competition for light and other limiting factors. Since these factors were of varying importance between the forest types examined, differences shaping the forest communities need to be regarded when evaluating the impact of environmental changes, for example through atmospheric nitrogen deposition.

Trait‐based responses to land use and canopy dynamics modify long‐term diversity changes in forest understories

AbstractAimLand use is the foremost cause of global biodiversity decline, but species do not respond equally to land‐use practices. Instead, it is suggested that responses vary with species traits, but long‐term data on the trait‐mediated effects of land use on communities are scarce. Here we study how forest understorey communities have been affected by two land‐use practices during 4–5 decades, and whether changes in plant diversity are related to changes in functional composition.LocationFinland.Time period1968–2019.Major taxa studiedVascular plants.MethodsWe resurveyed 245 vegetation plots in boreal herb‐rich forest understories, and used hierarchical Bayesian linear models to relate changes in diversity, species composition, average plant size, and leaf economic traits to reindeer abundance, forest management intensity, and changes in climate, canopy cover and composition. We also studied the relationship between species evenness and plant size across both space and time.ResultsIntensively managed forests decreased in species richness and had increased turnover, but management did not affect functional composition. Increased reindeer densities corresponded with increased leaf dry matter content, evenness and diversity, and decreased height and specific leaf area. Successional development in the canopy was associated with increased specific leaf area and decreased leaf dry matter content and height in the understorey over the study period. Effects of reindeer abundance and canopy density on diversity were partially mediated by vegetation height, which had a negative relationship with evenness across both space and time. Observed changes in climate had no discernible effect on any variable.Main conclusionsFunctional traits are useful in connecting vegetation changes to the mechanisms that drive them, and provide unique information compared to turnover and diversity metrics. These trait‐dependent selection effects could inform which species benefit and which suffer from land‐use changes and explain observed biodiversity changes under global change.

Deer exclusion unveils abiotic filtering in forest understorey plant assemblages.

The role of deer (family Cervidae) in ecosystem functioning has traditionally been neglected by forest ecologists due to the animal's scarcity in most parts of the northern hemisphere. However, the dramatic rebound in deer populations throughout the 20th century has brought deer browsing to the forefront of forest ecological questioning. Today there is ample evidence that deer affect tree regeneration, understorey plant and animal diversity, and even litter decomposition. However, the mechanisms underlying the effects of deer on forest ecosystems remain unclear. Among others, the relative role of abiotic factors versus biotic interactions (e.g. herbivory) in shaping plant assemblages remains largely unknown. We used a large-scale experiment with exclosures distributed along abiotic gradients to understand the role of black-tailed deer (Odocoileus hemionus sitchensis) on the forest understorey on the Haida Gwaii archipelago (western Canada), a unique context where most of the key ecological effects of deer presence have already been intensively studied. Our results demonstrate that 20 years of deer exclusion resulted in a clear increase in vascular plant richness, diversity and cover, and caused a decline in bryophyte cover. Exclusion also unveiled abiotic (i.e. soil water availability and fertility) filtering of plant assemblages that would otherwise have been masked by the impact of abundant deer populations. However, deer exclusion did not lead to an increase in beta diversity, probably because some remnant species had a competitive advantage to regrow after decades of over browsing. We demonstrated that long-term herbivory by deer can be a dominant factor structuring understorey plant communities that overwhelms abiotic factors. However, while exclosures prove useful to assess the overall effects of large herbivores, the results from our studies at broader scales on the Haida Gwaii archipelago suggest that exclosure experiments should be used cautiously when inferring the mechanisms at work.

Temperature effects on forest understorey plants in hedgerows: a combined warming and transplant experiment.

Hedgerows have been shown to improve forest connectivity, leading to an increased probability of species tracking the shifting bioclimatic envelopes. However, it is still unknown how species in hedgerows respond to temperature changes, and whether effects differ compared with those in nearby forests. We aimed to elucidate how ongoing changes in the climate system will affect the efficiency of hedgerows in supporting forest plant persistence and migration in agricultural landscapes. Here we report results from the first warming experiment in hedgerows. We combined reciprocal transplantation of plants along an 860-km latitudinal transect with experimental warming to assess the effects of temperature on vegetative growth and reproduction of two common forest herbs (Anemone nemorosa and Geum urbanum) in hedgerows versus forests. Both species grew taller and produced more biomass in forests than in hedgerows, most likely due to higher competition with ruderals and graminoids in hedgerows. Adult plant performance of both species generally benefitted from experimental warming, despite lower survival of A. nemorosa in heated plots. Transplantation affected the species differently: A. nemorosa plants grew taller, produced more biomass and showed higher survival when transplanted at their home site, indicating local adaptation, while individuals of G. urbanum showed greater height, biomass, reproductive output and survival when transplanted northwards, likely owing to the higher light availability associated with increasing photoperiod during the growing season. These findings demonstrate that some forest herbs can show phenotypic plasticity to warming temperatures, potentially increasing their ability to benefit from hedgerows as ecological corridors. Our study thus provides novel insights into the impacts of climate change on understorey plant community dynamics in hedgerows, and how rising temperature can influence the efficiency of these corridors to assist forest species' persistence and colonization within and beyond their current distribution range.