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Over the last decade, the field of plant ecology has significantly developed and expanded, especially in research concerning the herb layer and ground vegetation of forests. This revised second edition accounts for that growth, presenting research that approaches the ecology of the herb layer of forests from a variety of disciplines and perspectives. The book synthesizes the research of top ecologists and biologists on herbaceous layer structure, composition, and dynamics of a variety of forest ecosystem types in eastern North America. The 2003 first edition of this book was praised for containing the most extensive listing of herb-layer literature in existence. This second edition brings this material up to date, revised to include current research and data. The book incorporates quantitative data to support analyses that was previously unavailable during the publication of the first edition. Also featured are six entirely new chapters, focused on the response of the herbaceous layer to a wide variety of natural and anthropogenic disturbances.

Despite a growing awareness that the herbaceous layer serves a special role in maintaining the structure and function of forests, this stratum remains an underappreciated aspect of forest ecosystems. In this article I review and synthesize information concerning the herb layer's structure, composition, and dynamics to emphasize its role as an integral component of forest ecosystems. Because species diversity is highest in the herb layer among all forest strata, forest biodiversity is largely a function of the herb-layer community. Competitive interactions within the herb layer can determine the initial success of plants occupying higher strata, including the regeneration of dominant overstory tree species. Furthermore, the herb layer and the overstory can become linked through parallel responses to similar environmental gradients. These relationships between strata vary both spatially and temporally. Because the herb layer responds sensitively to disturbance across broad spatial and temporal scales, its dynamics can provide important information regarding the site characteristics of forests, including patterns of past land-use practices. Thus, the herb layer has a significance that belies its diminutive stature.

• Strategies of herbaceous species in deciduous forests are often characterized by the timing of life history phases (e.g. emergence, flowering, leaf senescence) relative to overstory tree canopy closure. Although springtime photosynthesis is assumed to account for the majority of their annual carbon budgets, the 12-month photosynthetic trajectories of forest herbs have not been quantified. • We measured the temporal dynamics of carbon assimilation for seven native herbaceous perennials and the biennial Alliaria petiolata, a widespread invader in eastern North American forests. We assessed the relative importance of spring, summer, and autumn to species-level annual carbon budgets. • Spring-emerging species showed significant variation in carbon assimilation patterns. High spring irradiance before canopy closure accounted for 39–100% of species-level annual carbon assimilation, but summer and autumn accounted for large proportions of some species’ carbon budgets (up to 58% and 19%, respectively). Alliaria was phenologically unique, taking advantage both autumn and spring irradiance. • Although spring-emerging understory species are often expected to rely on early-season irradiance, our results highlight interspecific differences and the importance of mid–late season carbon gain. Phenological strategies of forest herbs are a continuum rather than discrete categories, and invasive species may follow strategies that are underrepresented in the native flora.

Summary Trait‐based models of ecological communities typically assume intraspecific variation in functional traits is not important, although such variation can change species trait rankings along gradients in resources and environmental conditions, and thus influence community structure and function. We examined the degree of intraspecific relative to interspecific variation, and reaction norms of 11 functional traits for 57 forest understorey plant species, including: intrinsic water‐use efficiency (iWUE), Δ15N, five leaf traits, two stem traits and two root traits along gradients in light, nitrogen, moisture and understorey cover. Our results indicate that interspecific trait variation exceeded intraspecific variation by at least 50% for most, but not all traits. Intraspecific variation in Δ15N, iWUE, leaf nitrogen content (LNC) and root traits was high (47–70%) compared with most leaf traits and stem traits (13–38%). Δ15N varied primarily along gradients in abiotic conditions, while light and understorey cover were relatively less important. Intrinsic water‐use efficiency was related primarily to light transmission, reflecting increases in photosynthesis relative to stomatal conductance. Leaf traits varied mainly as a function of light availability, with some reaction norms depending on understorey cover. Plant height increased with understorey cover, while stem‐specific density was related primarily to light. Resources, environmental conditions and understorey cover did not contribute strongly to the observed variation in root traits. Gradients in resources, environmental conditions and competition all appear to control intraspecific variability in most traits to some extent. However, our results suggest that species cross‐over (i.e. trait rank reversals) along the gradients measured here are generally not a concern. Intraspecific variability in understorey plant species traits can be considerable. However, trait data collected under a narrow range of environmental conditions appears sufficient to establish species rankings and scale between community and ecosystem levels using trait‐based models. Investigators may therefore focus on obtaining a sufficient sample size within a single set of conditions rather than characterizing trait variation across entire gradients to optimize sampling efforts. A lay summary is available for this article. Lay Summary

Premise Determining which traits characterize strategies of coexisting species is important to developing trait‐based models of plant communities. First, global dimensions may not exist locally. Second, the degree to which traits and trait spectra constitute independent dimensions of functional variation at various scales continues to be refined. Finally, traits may be associated with existing categorical groupings. Methods We assessed trait integration and differentiation across 57 forest understory plant species in Douglas‐fir forests of western Oregon, United States. We combined measurements for a range of traits with literature‐based estimates of seed mass and species groupings. We used network analysis and nonmetric multidimensional scaling ordination (NMS) to determine the degree of integration. Results We observed a strong leaf economics spectrum (LES) integrated with stem but not root traits. However, stem traits and intrinsic water‐use efficiency integrated LES and root traits. Network analyses indicated a modest grouping of a priori trait dimensions. NMS indicated that multivariate differences among species were related primarily to (1) rooting depth and plant height vs. specific root length, (2) the LES, and (3) leaf size vs. seed mass. These differences were related to species groupings associated with growth and life form, leaf lifespan and seed dispersal mechanisms. Conclusions The strategies of coexisting understory plant species could not be reduced to a single dimension. Yet, species can be characterized efficiently and effectively for trait‐based studies of plant communities by measuring four common traits: plant height, specific leaf area, leaf size, and seed mass.

The alpine semi-arid river valleys of the eastern Qinghai-Xizang Plateau are characterized by strong environmental heterogeneity and ecological fragility. Herbaceous plants dominate these ecosystems; however, integrative studies linking niche differentiation, interspecific associations, and community stability remain limited, particularly under extreme alpine conditions. Based on consecutive field surveys conducted from 2021 to 2025 in Qamdo City, we investigated 109 sampling plots comprising 327 herbaceous quadrats. We quantified niche breadth and niche overlap of dominant species and analyzed interspecific associations and community stability using classical ecological indices and statistical approaches. Dominant species showed clear differentiation between broad-niche generalists and narrow-niche specialists (Levins: 1.03-19.18; Shannon: 0.67-3.15). Overall niche overlap was low, with 70% of values within 0-0.5, indicating strong resource-use differentiation. The herb layer exhibited a weak and non-significant positive overall interspecific association. Significant positive associations mainly occurred among broad-niche species, whereas negative associations were concentrated among species with similar resource requirements. Community stability was insufficient, with a Euclidean distance of 17.22 from the theoretical stable point, suggesting limited structural and functional resilience. Pronounced niche differentiation and generally weak interspecific associations represent key adaptive strategies of herbaceous communities in this alpine semi-arid valley system. These patterns reduce competitive exclusion but may also constrain overall community stability. The findings provide important ecological insights and a scientific basis for biodiversity conservation and ecosystem restoration on the Qinghai-Xizang Plateau.

Increasing rates of atmospheric deposition of nitrogen (N) present a novel threat to the biodiversity of terrestrial ecosystems. Many forests are particularly susceptible to excess N given their proximity to sources of anthropogenic N emissions. This study summarizes results of a 25‐yr treatment of an entire central Appalachian hardwood forest watershed via aerial applications of N with a focus on effects of added N on the cover, species richness, and composition of the herbaceous layer. Research was carried out on two watersheds of the Fernow Experimental Forest (FEF), West Virginia. The long‐term reference watershed at FEF (WS4) was used as a reference; WS3 was experimentally treated, receiving three aerial applications of N per year as (NH4)2SO4 totaling 35 kg N ha−1 yr−1, beginning in 1989. Cover of the herbaceous layer (vascular plants ≤1 m in height) was estimated visually in five circular 1‐m2 subplots within each of seven circular 400‐m2 sample plots spanning all aspects and elevations of each watershed. Sampling was carried out in early July of each of the following years: 1991, 1992, 1994, 2003, and 2009—2014, yielding 10 yr of data collected over a 23‐yr period. It was anticipated that the N treatment on WS3 would decrease species richness and alter herb layer composition by enhancing cover of a few nitrophilic species at the expense of numerous N‐efficient species. Following a period of minimal response from 1991 to 1994, cover of the herb layer increased substantially on N‐treated WS3, and remained high thereafter. There was also a coincidental decrease in herb layer diversity during this period, along with a sharp divergence in community composition between WS4 and WS3. Most changes appear to have arisen from unprecedented, N‐mediated increases of Rubus spp., which are normally associated with the high‐light environment of openings, rather than beneath intact forest canopies. These findings support the prediction that N‐mediated changes in the herbaceous layer of impacted forests are driven primarily by increases in nitrophilic species.

Questions What is the long‐term impact of deer browsing on the diversity of the herbaceous layer and tree species regeneration? Which parameters of regeneration of the tree species regeneration and the herbaceous layer best indicate browsing impact? Study site Dinaric Mountains, Slovenia. Methods We studied the long‐term impact of red deer on mixed temperate forests by comparing the tree species regeneration and herbaceous layer vegetation under two treatments: deer present, no deer. We analyzed the regeneration of trees older than one year up to a diameter at breast height of 10 cm (categorized into five height classes) and the cover abundance of plant species in the herbaceous layer (<50 cm). Results There were no significant differences between the treatments in the number of tree species regeneration. Browsing impact on the most abundant tree species—European beech, silver fir and sycamore—varied, indicating differences in palatability. When deer were present, the number of silver fir regenerating was significantly lower in all height classes, and regeneration of European beech specimens shorter than 50 cm was more abundant, while for sycamore shorter seedlings (<20 cm) were more abundant, and regeneration of taller specimens (≥50 cm) was significantly less abundant. The reduction of tree species diversity during natural regeneration is evident. There were no significant differences between the treatments in the number and diversity of plant species in the herbaceous layer; however, the abundance of 13 plant species differed significantly between the treatments. Deer reduce the density of tree species saplings due to long‐term browsing and thus indirectly increase tree species diversity in the regeneration up to 20 cm in height and plant species diversity in the herbaceous layer. Conclusions We documented the direct and indirect impact of deer on the diversity of tree species regeneration, but only found an indirect influence on the diversity of the herbaceous layer. The height structure of palatable tree species regeneration appears to be the most appropriate indicator of browsing impact. The observed browsing impact leads to the complete dominance of European beech in the tree species composition of forest stands. Naturally regenerating tree species showed different, size‐dependent responses to the impact of red deer. Deer prevented recruitment of palatable tree species above the browse layer. No plant species in the herbaceous layer went locally extinct because of deer; however, deer influenced the abundance of some plant species. Deer indirectly increased plant species diversity in the herbaceous layer by reducing the density of tree saplings.

Leaf functional traits play critical roles in plant functioning. Although the functional traits of overstory trees have been extensively studied, minimal research has been conducted regarding understory species, despite the understory layer is an important component of temperate forests. Such insufficiency limit the broader understanding of processes and functions in forest ecosystems, particularly when under the increasing atmospheric nitrogen (N) deposition. Here, we investigated the responses of 18 leaf functional traits in six understory herbaceous species within young and mature stands (three species per stand) in larch ( Larix principis-rupprechtii ) plantations that subjected to 12 years of anthropogenic N addition. We found that N addition did not significantly impact the photosynthetic traits of understory herbaceous species in either stand; it only led to increased chlorophyll content in Geum aleppicum Jacq. Similarly, with the exception of decreases in the predawn leaf water potential of Sanguisorba officinalis L., N addition did not significantly affect leaf hydraulic traits. With the exception of changes to adaxial epidermis thickness in Potentilla chinensis Ser. (decreased) and G. aleppicum (increased), N addition had negligible effects on leaf anatomical traits and specific leaf area, however, interspecific variations in the plasticity of leaf anatomical traits were observed. Stable responses to N addition were also observed for nonstructural carbohydrates (NSC) and their components (soluble sugars and starch), with the exception of Polygonum divaricatum L., which exhibited increases in NSC. Overall, our results suggest that the functional traits of understory herbaceous species exhibit stability under conditions of long-term N enrichment in temperate plantations.

ABSTRACT Soil bacteria and understorey plants interact and drive forest ecosystem functioning. Yet, knowledge about biotic and abiotic factors that affect the composition of the bacterial community in the rhizosphere of understorey plants is largely lacking. Here, we assessed the effects of plant species identity (Milium effusum vs. Stachys sylvatica), rhizospheric soil characteristics, large-scale environmental conditions (temperature, precipitation and nitrogen (N) deposition), and land-use history (ancient vs. recent forests) on bacterial community composition in rhizosphere soil in temperate forests along a 1700 km latitudinal gradient in Europe. The dominant bacterial phyla in the rhizosphere soil of both plant species were Acidobacteria, Actinobacteria and Proteobacteria. Bacterial community composition differed significantly between the two plant species. Within plant species, soil chemistry was the most important factor determining soil bacterial community composition. More precisely, soil acidity correlated with the presence of multiple phyla, e.g. Acidobacteria (negatively), Chlamydiae (negatively) and Nitrospirae (positively), in both plant species. Large-scale environmental conditions were only important in S. sylvatica and land-use history was not important in either of the plant species. The observed role of understorey plant species identity and rhizosphere soil characteristics in determining soil bacterial community composition extends our understanding of plant-soil bacteria interactions in forest ecosystem functioning. Rhizosphere soil bacterial community composition shows high habitat dependency. This study reported the influence of plant identity, soil chemistry, climate, nitrogen deposition and land-use history on its dynamics across Europe.