Understory Richness Research Articles

Additive effects of N addition and changing precipitation on soil respiration in a climate transitional forest

Global carbon (C) cycling can be greatly affected by the increasing nitrogen (N) deposition and changing precipitation regimes in terrestrial ecosystems. Nevertheless, whether N enrichment and precipitation variability interactively or additively influence soil C release during a long-term period remain largely elusive. Here, a seven-year (2016–2022) field experiment was conducted to explore the responses of soil respiration to N addition, decreased and increased precipitation in a subtropical-warm temperate forest in Central China. The results showed soil respiration was elevated by 11.0%, 12.2%, and 12.3% under N addition, decreased precipitation, and increased precipitation, respectively, primarily originating from the enhancements in plant root respiration. Increased precipitation led to a temporal increase in soil respiration over the experimental period, mostly due to the elevating understory richness and reducing soil C/N ratio. There were additive effects between N addition and changing precipitation on soil respiration observed in our study, suggesting that the impacts of concurrent global change drivers can be predicted from their respective influences. These findings indicated that global change may intensify soil C release from climate transition forests, thus helping to accurately assess the climate change–C feedback in natural ecosystems.

How big is the footprint? Quantifying offsite effects of mines on boreal plant communities

Threats from mining to the biodiversity and ecological services of boreal forests are increasing as demand for minerals increases globally. However, much less is known about how offsite effects of mines affect understory communities as they occur outside the immediate location of mining and are often overlooked during ecological evaluations. We conducted an extensive field survey along 1-km transects surrounding six mine sites of different mining stages (operating vs non-operating), that crossed four ecosystem types (deciduous, coniferous, mixed forests and open canopy) in Canada’s boreal forest. The offsite effects of mines on the understory were quantified using vascular plants (woody and herbaceous), bryophytes and lichens. Mine offsite effects impacted understory diversity and composition. Understory richness and cover was more reduced near operating than non-operating mines. Mining stage mainly significantly altered understory diversity and community structure in deciduous and mixed forests while understory communities were more resistant to the offsite effects in coniferous forest (P > 0.05). The footprint was quantified using the influenced distance and the strongest effects were generally within 0.2 km from mines. Given the predicted changes in boreal forest ecosystems with encroachment of deciduous species into coniferous forests and the increased sensitivity of mixed and deciduous forests, the area affected by offsite effects of mines could grow in the future. We suggest that offsite effects should be included in ecological evaluations.

Chronic browsing by an introduced mammalian herbivore in a tropical island alters species composition and functional traits of forest understory plant communities

AbstractMammalian herbivores have large‐scale impacts on vegetation, altering structure, and species composition, especially in tropical grasslands and savannas. However, there is limited understanding of the potential impacts of mammalian herbivores in tropical wet forests, where they are typically less abundant. We investigated the effects of an introduced mammalian herbivore chital (Axis axis) on vegetation structure, composition and leaf functional traits of tropical evergreen forests of the Andaman Islands, India. Across seven islands, representing a gradient of herbivore densities, increasing chital presence was associated with decreased understory richness, understory density and adult tree richness, but was not related to adult tree density or size class distributions. We also found a significant decrease in community level leaf palatability traits (specific leaf area decreased and leaf thickness increased) with increasing chital habitat use. This community level shift in leaf trait values was better explained by intraspecific variation in leaf traits across islands rather than changes in species composition. In summary, we show persistent long‐term impacts of an introduced mammalian herbivore on understory tropical tree communities although there is little impact on adult tree communities. Our results also show that functional traits of species can be altered in response to novel herbivory, even at herbivore densities where there are no detectable impacts on adult forest structure or composition. Such altered functional traits may potentially alter ecosystem functioning in these forests even without changes in vegetation structure.Abstract in Hindi is available with online material.

Understory plant community responses to widespread spruce mortality in a subalpine forest

AbstractAimsSpruce beetles (Dendroctonus rufipennis) are causing widespread spruce (Picea spp.) mortality in subalpine forests in western North America. Spruce beetles are changing forest structure and composition by killing a dominant overstory species, but we know little about how the understory community responds to the increase in resource availability brought about by spruce mortality, what mechanisms drive its response, or how its response affects other forest properties and processes.LocationGlacier Lakes Ecosystem Experiments Site, Medicine Bow Mountains, Wyoming, USA.MethodsWe measured understory community cover and richness in 75 permanent plots during and 10 years after an epidemic spruce beetle outbreak, and measured trait values for 46 common understory species. We used linear regression to determine how the understory community has changed over time and along a gradient of spruce mortality, and to evaluate the relative support for two mechanisms contributing to species responses.ResultsUnderstory cover nearly doubled between sampling periods and increased the most where spruce mortality was most severe. Understory richness doubled and showed a weak positive trend with spruce mortality. Understory species with the largest increases in cover were the most frequent across the landscape before the disturbance, were the tallest at maturity and had the lowest leaf turgor loss points. Fir seedling density decreased over time, with decreases lessening with increases in understory cover. Changes in spruce seedling density were not predicted by changes in understory cover.ConclusionsOur findings highlight some of the diverse ways in which understory communities can be altered by spruce beetle outbreaks, and how the direction and magnitude of change can depend on the amount of spruce mortality as well as on priority effects and traits of pre‐disturbance species. Our findings also highlight how understory community changes can have implications for other forest properties and processes, such as tree regeneration and forest recovery.

Understory diversity are driven by resource availability rather than resource heterogeneity in subtropical forests

Understory vegetation accounts for most of the plant diversity, particularly in plantation forests, and contributes significantly to energy flows and nutrient cycling. However, we lack a mechanistic understanding of understory diversity associated with resource availability and heterogeneity across forest types over a wide range of stand development stages. We sampled the understory vegetation of Chinese fir plantations and natural secondary forests (4–40 years old) and used primary forests as a reference in subtropical China. The percent covers of all understory plant species were estimated at the shrub layer (height >1.3 m) and herb layer (≤1.3 m). We quantified understory light availability and heterogeneity, substrate diversity and soil nutrients and employed principal component analysis to reduce multiple physical and chemical soil properties to PC1 and PC2, with PC1 positively correlating with soil carbon and total nitrogen, and PC2 with available soil nitrogen and phosphorus. Structural equation modelling (SEM) was employed to test the direct and indirect influences of forest type, stand age, light availability and heterogeneity, substrate diversity and soil nutrients on the diversity of understory species. We found that shrub layer richness was higher in the secondary natural and primary forests than in the plantations, but plantations had higher bryophyte richness. Shrub cover was positively correlated with shrub richness and had a positive indirect effect on shrub richness but a negative indirect effect on herb richness via its effect on soil carbon and nitrogen. Stand age has a positive direct effect on bryophyte richness. There were strong positive correlations among the richness of understory layers and life forms. Our results suggest that physical and chemical soil properties, specifically soil nutrient availability, rather than light availability and heterogeneity or substrate diversity, were the main drivers that affected subtropical forests' understory richness.

Effects of variable-density thinning on non-native understory plants in coniferous forests of the Pacific Northwest

Old-growth forests serve as critical habitat for many sensitive species, but management practices have diminished their prevalence, and former regions of old-growth are now dominated by second-growth stands lacking the structural heterogeneity, diversity, and species richness that these older forests possess.In western Washington state in the Pacific Northwest of the United States, the Olympic Habitat Development Study was designed to address this issue and hasten the development of specific old-growth features in second-growth stands using variable-density thinning. One concern with such methods, however, is the potential to introduce non-native plants, which can have negative ecological and economic impacts. Here, we examine how variable-density thinning influences desirable forest characteristics, such as increased plant species diversity, versus the less desirable effects of non-native plant species introduction. We test two hypotheses regarding plant invasions. First, thinning would promote establishment of non-native, shade-intolerant species, but their abundance would gradually decline over time. Second, thinning disturbance and increased heterogeneity of canopy cover would initially promote understory richness of all species, although richness would decline over time with canopy closure and increased cover of shrubs and regenerating trees. We found that the number and cover of non-native species initially increased after thinning, peaking at 16 species present in variable-density thinned treatments in year three. By year 17, 11 species remained throughout the seven 6.5 ha treatment plots sampled, and cover was negligible. As predicted, species richness increased following thinning, however, native species richness remained elevated through year 17, contrary to our hypothesis. Furthermore, native species diversity also increased following thinning and remained higher in thinned treatments than controls through year 17. Our results show that variable-density thinning in temperate coniferous forests can enhance native, but not exotic, plant richness and diversity in the long term.

Plant community response to thinning and repeated fire in Sierra Nevada mixed-conifer forest understories

Fire suppression in the western United States has significantly altered forest composition and structure, resulting in a higher risk of stand-replacing fire, large-scale drought mortality, and bark beetle outbreaks. Mechanical thinning and prescribed fire are common treatments designed to reduce high-severity fire risk, but few studies have tracked long-term understory plant community response to these treatments with repeated fire application that emulates historic fire regimes. We evaluate changes in understory plant diversity and composition, as well as light availability, soil moisture, and litter depth over two decades following a factorial field experiment crossing thinning and two applications of prescribed fire at the Teakettle Experimental Forest (TEF) in the southern Sierra Nevada. We compare responses in experimental fuels treatments against those in nearby old-growth, mixed-conifer forests with restored low-severity fire regimes in Yosemite and Kings Canyon National Parks. We hypothesized that 1) understory plant richness, evenness, and beta diversity would increase with each burn event; 2) repeated fire after initial thinning would produce the highest understory plant diversity; 3) the second burn would reduce shrub cover, especially in the initially thinned treatments that demonstrated vigorous Ceanothus cordulatus growth after initial treatments; 4) the second burn would increase light availability and bare ground while reducing litter depth; and 5) treatments with initial thinning followed by multiple prescribed burns would show the greatest similarities to reference forests in diversity metrics, shrub cover, and environmental conditions. Although initially local (10 m2-scale) understory plant richness and diversity increased most following thinning combined with prescribed fire, this treatment did not generate understory conditions similar to those in nearby reference forests over the long term. Vigorous shrub growth resulted in low understory evenness and beta diversity over time, which a secondary burn treatment did not alter. Burning without thinning retained a more heterogeneous understory following initial treatment. Two years after the second burn treatment, this burning without thinning resulted in high understory richness and evenness similar to reference forest understories. Our results suggest management treatments may need to focus on creating heterogeneity in burn effects to limit shrub cover and foster more diverse understory communities.

Understory plant diversity under variable overstory cover in Mediterranean forests at different spatial scales

Mediterranean forest ecosystems are of high conservation value and therefore increasingly managed to restore and promote their natural biodiversity and complexity. The present study aimed to assess plant species diversity of East-Mediterranean conifer forests as affected by overstory cover manipulation at multiple spatial scales. Understanding this interaction in water-limited forest ecosystems is particularly challenging as overstory-understory relations involve both water and light constrains. The experiment was conducted in a mature (40y) planted Pinus halepensis forest in the Jerusalem Mountains of Israel. Understory plant community structure was monitored for five years in sixteen experimental plots (0.5 ha) representing a range of overstory cover levels (leaf area index-LAI = 0–4 m2m−2) created by tree thinning. Light and water availability in the forest understory were monitored using fisheye camera (irradiance), predawn leaf water potential (PLWP) of dominant species and soil water content (SWC) measurements. Plant diversity was measured at increasing spatial scales (grain sizes, 0.01–400 m2), using hierarchical nested sampling. Irradiance and SWC increased with decreasing LAI while PLWP was not affected. Stand-level understory richness (α diversity) increased linearly with decreasing LAI, indicating a consistent, positive resource availability-diversity relationship. The results pointed towards strong dependency of species richness on light availability as compared to its dependency on water availability that was less definite. Understory species composition varied across LAI levels (β diversity at the forest level) indicating species turnover, mainly between annual herbs benefiting from reduced overstory cover vs. woody species benefiting from higher cover. The relationships between overstory cover and understory richness and composition were rank-invariant across grain sizes. These relationships were stronger at the larger (10, 100, 400 m2) compared to the smaller (0.01, 0.1, 1 m2) grain sizes. The 10-m2 grain-size, corresponding to the crown projection area of individual trees, showed the strongest relationship. Observed patterns of overstory cover–understory richness suggest that resource availability, rather than spatial resource heterogeneity, was the main factor shaping understory diversity. Furthermore, grain-size effects on this relationship support “environmental filtering” rather than “resource density” as the main mechanism through which reduced overstory cover promoted understory diversity.'Synthesis and applications’: Our findings demonstrate the importance of overstory cover as a key factor determining plant diversity in water-limited East-Mediterranean forests. We highlight the important role of spatial heterogeneity of overstory cover at both the stand (intra-plot) and forest (inter-plot) scales and support dynamic variable thinning as a management strategy to enhance forest biodiversity and complexity.

Response of Beetle Assemblages (Insecta: Coleoptera) to Patch Characteristics and Habitat Complexity in an Ever-Expanding Urban Landscape in the Yucatán Peninsula, Mexico

Abstract Insect diversity levels and change in remnant urban habitats have been poorly examined in Neotropical regions. The aim of this study was to evaluate the diversity and composition of foliage/shrub dwelling beetles (Insecta: Coleoptera) regarding the influence of patch attributes (i.e., patch size, connectivity, and surrounding matrix) and within-patch habitat complexity, in an urban landscape at Chetumal, Yucatán peninsula, Mexico. Despite the potential species-specific responses to urban landscape pattern, our study reveals that there are effects of patch spatial features and habitat complexity on overall beetle diversity. These effects are as follows: 1) relatively larger urban patches contained higher overall beetle richness, 2) beetle species composition varied significantly regarding understory vegetation structure and tree richness, and 3) patches of intermediate vegetation age had highest beetle richness. This research provides a case study of patch and habitat features related to Neotropical insect assemblages in urban settlements, underlying the importance of using ecological information to offer management recommendations in ever-expanding urban tropical settlements.

Woody species richness and turnover in expanding Mediterranean forests: a story of landscape legacies influenced by climatic aridity

The current expansion of forests in Europe is expected to be affected by transient colonization credits (i.e. species deficits compared to long-established forests), especially due to plant species with limited dispersal ability. We aimed to disentangle the role of forest connectivity and climatic aridity in the assembly of woody plant communities present in expanding Mediterranean forests by studying species with different types of dispersal. We used a space-for-time approach in recent (post 1980), middle-aged (1956–1980) and long-established forests (pre-1956) along an aridity and forest connectivity gradient in NE Spain. We recorded woody canopy and understory species and classified them as either vertebrate-, non-vertebrate- or human-dispersed species (i.e., crops). Species turnover between forests of different ages was limited to particular species and did not give rise to colonization credits determined by plant dispersal type. Connectivity and aridity drove understory richness in a similar way in forests of different ages. Within this general pattern, non-vertebrate-dispersed species were especially promoted by benign conditions. Sites with high forest connectivity had either the richest or the most impoverished understories given, respectively, low or high aridity. Thus, the rescue effect was modulated by aridity, which acted as a limiting factor. Mediterranean forest richness and composition are strongly dependent on climatic constraints and the ‘rescue effects’ (connectivity) originating in surrounding forest areas. These two factors shape the composition of expanding forests in different ways depending on the ability of species to disperse.

Influence of Abiotic and Biotic Factors on Soil Microbial Biomass in Robinia pseudoacacia Plantations in the Loess Hilly Region

The ecological productivity of the Robinia pseudoacacia L. (RP) widely cultivated on the Loess hilly region has been widely questioned with its aging. Soil microbial biomass (SMB) plays a key role in soil nutrient dynamics and productivity of the ecosystems. Understanding the main ecological drivers of SMB is supposed to be of importance for ecosystem functioning of RP in the Loess hilly region. In this study, we identified the most influential factors affecting SMB at 2 layers (0–10 cm and 10–30 cm) using forward selection in terms of plant characteristics (forest age, tree height, diameter at breast height, tree canopy, crown base height, herb height, herb number, herb coverage, herb ground diameter and herb diversity), soil physiochemical characteristics (soil bulk density, pH, water content, soil organ carbon, soil total and available nutrient content) and topographical properties (elevation, aspect and slope). We also analyzed individual and interactive effects (plant–soil, plant–topography, soil–topography, plant–soil–topography) using general linear model (GLM) analysis. Among all plant variables, tree canopy and understory richness had the greatest impact on SMB. The soil variables with the greatest impact on SMB were bulk density and available phosphorus. Elevation was the most important topographic factor affecting SMB. When we considered the interactive effects among plant, soil physicochemical and topographical variables on SMB, a significant interaction effect occurred at a depth of 10–30 cm soil layer. We concluded that individual effects of abiotic and biotic factors significantly affect SMB at 0–10 cm, while the interaction of these factors mainly played roles at 10–30 cm. These results provide a basis for maintaining soil health and productivity using efficient SMB by providing suitable abiotic and biotic habitats.

Overyielding in mixed pine forests with belowground complementarity: impacts on understory

The impact of biodiversity loss on the functioning of forest ecosystems has become a central issue in ecology. Most reports of the positive effects of tree mixture on the biodiversity–productivity relationship focus on mixtures that combine tree species with contrasting traits. Nevertheless, little is known about how coniferous mixtures of the same genus affect forest productivity, what mechanisms are involved, and how the understory is affected. Here, we assessed the effect of mixed versus monospecific stands of Pinus sylvestris L. and P. pinaster Ait. on productivity, its impact on the understory, and its relationship with soil water and fertility, based on research with six triplets (6 triplets × 3 forest stands × 1 plot = 18 plots) in North-Central Spain. Each triplet consisted of two plots dominated either by P. sylvestris or P. pinaster and of one mixed plot that contained both species. Productivity, at the stand and neighborhood levels, and the understory richness, and soil water and fertility at the stand level were analyzed. A positive effect of pine mixture on productivity was observed at the smaller spatial scale, and it had no negative effect on the understory richness. The greater space-use efficiency (higher tree density and basal area) of both Pinus species in the admixtures was related to soil water and fertility niche complementarity. The fundamental role of scale in determining the relationship between species richness and ecosystem functioning in forests is highlighted.

Strong influences of stand age and topography on post-fire understory recovery in a Chinese boreal forest

Soil resource quantity and heterogeneity as well as overstory characteristics change along with stand age after fire disturbance, imposing strong influences on post-fire understory recovery. In this study, we aimed to assess how soil resource (nitrogen quantity and heterogeneity), topographic position (aspect), overstory characteristics (basal area), and stand age affected understory characteristics (cover, richness, evenness, and composition) during boreal forest succession after fire disturbance. We collected plant community and environmental data from 76 stands aged at 4 to 203 years post-fire in the boreal forest of Northeast China. We separately tested the effects of the considered factors on the characteristics of total understory, shrubs, and herbs. Total understory cover was higher at the south-facing slopes and flat locations than at north-facing slopes. Species richness of total understory, shrubs, and herbs was higher at the flat locations and south-facing slopes than at the north-facing slopes. The cover of total understory and shrubs showed increasing trends along with stand age, soil nitrogen quantity, and overstory basal area as well as decreasing trends along with soil nitrogen heterogeneity, while the cover of herbs showed reversed trends across all the three topographic positions. Understory richness showed decreasing trends along with soil nitrogen quantity as well as increasing trends along with soil nitrogen heterogeneity and overstory basal area. Total understory evenness showed flat trends along with soil nitrogen quantity, heterogeneity, and stand age as well as decreasing trends along with overstory basal area. Understory community composition was mainly affected by topographic position, stand age, and overstory basal area. During understory recovery, community composition shifted from shade-intolerant species to shade-tolerant species. Our results highlight the importance of topography and stand age in post-fire understory recovery in boreal forests of China, mediated through availability of light and soil nutrients.

Effectiveness of Restoration Treatments for Reducing Fuels and Increasing Understory Diversity in Shrubby Mixed-Conifer Forests of the Southern Rocky Mountains, USA

Exclusion of natural surface fires in warm/dry mixed-conifer forests of the western U.S. has increased potential for stand-replacing crown fires and reduced resilience of these systems to other disturbances, such as drought and insect attack. Tree thinning and the application of prescribed fire are commonly used to restore more resilient ecological conditions, but currently, there is a lack of long-term data with which to evaluate restoration treatment effectiveness in forest types where resprouting shrubs dominate understory communities. At a mixed-conifer site in southwestern Colorado, we compared forest structure and understory vegetation responses to three restoration treatments (thin/burn, burn, and control) over 10 years in a completely randomized and replicated experiment. Forest density, canopy cover, and crown fuel loads were consistently lower, and crown base height was higher, in thin/burn than burn or controls, but the effects diminished over time. Ten years following treatment, >99% of all plant species within both treatments and the control were native in origin. There were no differences between treatments in understory richness, diversity, cover, or surface fuels, but graminoid cover more than doubled in all treatments over the 15-year monitoring period. Similarly, there was more than a 250% increase post-treatment in shrub density, with the greatest increases in the thin/burn treatment. In addition, we saw an increase in the average shrub height for both treatments and the control, with shrub stems >80 cm becoming the dominant size class in the thin/burn treatment. Conifer seedling density was significantly lower in thin/burn compared with burn and control treatments after 10 years. Taken together, these conditions create challenges for managers aiming to reestablish natural fire patterns and sustain mixed-conifer forests. To limit the dominance of resprouting shrubs and facilitate conifer regeneration after overstory thinning and prescribed fire, managers may need to consider new or more intensive approaches to forest restoration, particularly given current and projected climate change.

Temperature-Dominated Driving Mechanisms of the Plant Diversity in Temperate Forests, Northeast China

Climate, topography, and tree structure have different effects on plant diversity that vary with spatial scale. In this study, we assessed the contribution of these drivers and how they affect the vascular plant richness of different functional groups in a temperate forest ecosystem in Northeast China. We investigated about 0.986 million plants from 3160 sites to quantify the impact of annual mean temperature, sunshine duration, annual precipitation, standard deviation of diameter at breast height, and forest type on richness of vascular plants (total species, tree, treelet, shrub, and herb, separately) using the gradient boosting model. The results show that annual mean temperature had the strongest impact on plant richness. The tree richness peaked at intermediate annual mean temperature and sunshine duration and increased with annual precipitation. The Shannon diversity index and Simpson dominance index increased with annual precipitation and standard deviation of diameter at breast height, decreased with sunshine duration, and peaked at intermediate annual mean temperature and forest type. The total richness and understory richness increased with annual mean temperature and standard deviation of diameter at breast height and peaked at intermediate sunshine duration and annual precipitation. A comprehensive mechanism was found to regulate the plant diversity in forest ecosystems. The relationship between tree richness and annual mean temperature with latitudinal effect could be affected by the differences in number and size of tree individuals, indicating that plant diversity varies with the utilization of energy. The force driving plant richness varied with the functional group due to the different environmental resource requirements and the life history strategies of plants layers.

Can mixed pine forests conserve understory richness by improving the establishment of understory species typical of native oak forests?

A positive effect of mixed pine forests ( Pinus sylvestris L. and Pinus pinaster Ait.) on the understory richness and tree regeneration was observed with respect to monospecific stands. Understory species typical of the native Pyrenean oak forests in the Iberian Peninsula contribute to maintaining high understory richness in such mixed pine forests. The influence of stands characteristics on the understory in mixtures that combine coniferous tree species of the same genus deserves more study since they are frequent in Spain. To assess the effect of mixed versus monospecific stands of Pinus sylvestris L. and Pinus pinaster Ait. on the main tree species regeneration and understory species composition. Tree regeneration and understory species composition were inventoried in eighteen forest plots (6 triplets) in North-Central Spain. Each triplet consisted of two plots dominated either by Scots pine or Maritime pine and one mixed plot that contained both species. The basal area (%) of both Pinus species was the only characteristic of the stands that significantly influenced the understory composition and tree regeneration. Characteristic species of humid and temperate zones, including P. sylvestris regeneration, dominated in Scots pine stands, and typical species of well-drained Mediterranean areas, including P. pinaster regeneration, dominated in Maritime pine stands. In mixed stands, the highest regeneration of the native Pyrenean oak with respect to monospecific stands was accompanied by understory species typical of native oak forests that share the same regeneration niche. Mixed pine forests allow the development of understory species better than monospecific forests.

Understory response to overstory and soil gradients in mixed versus monospecific Mediterranean pine forests

Many studies highlight the role of mixed versus monospecific forests to provide numerous ecosystem services. Most reports of the positive effects of tree mixture on biodiversity focus on coniferous–deciduous combinations, but little is known about the effects of mixtures combining two coniferous tree species. We assessed the effects of mixed versus monospecific stands of Pinus sylvestris and P. pinaster on the understory richness and composition and its relationship with the soil status, based on research with six triplets in northern Spain. In ten square meter quadrats randomly located per plot, the cover of every understory vascular plant species was estimated visually and data were codified according to Raunkiaer’s life-forms. One soil pit of 50 cm depth was dug in each plot to determine the soil water (water holding capacity) and fertility (carbon and exchangeable cations stocks) status. A water-stress gradient associated with the overstory composition indicated that P. pinaster tolerates lower soil water content than P. sylvestris. Mixed stands are under greater water stress than monospecific P. sylvestris stands but maintain the same level of understory richness. Also, a soil fertility gradient defined by organic carbon and exchangeable magnesium stocks was identified. Hemicryptophytes, whose abundance is greater in mixed stands, were the only understory life-form positively correlated to soil fertility. We conclude that the mixture of both Pinus species should continue to be favored in the study area because it helps to maintain understory richness under greater water-stress conditions and improves soil fertility.

Temporal patterns of ground flora response to fire in thinned Pinus–Quercus stands

The ground flora stratum affects stand structure, resource acquisition, nutrient cycling, and taxonomic richness in forest ecosystems. Disturbances such as thinning and prescribed fire alter forest understory growing conditions that generally increase ground flora cover and richness. We studied annual changes in ground flora assemblages over three growing seasons after fire in thinned and frequently burned (3-year rotation) Pinus–Quercus stands. Our results corroborated trends from other studies that indicated greater ground flora richness and cover after thinning and burning compared with thin-only treatments. We also found that the stratum experienced relatively rapid succession between growing seasons that complimented the tolerance succession model. Forbs had reduced cover and richness from increasingly difficult growing conditions over time and were replaced by woody plants, shrubs, and seedlings. This likely occurred from changing competition dynamics that favored quick growth in the first growing season and long-term investment in vertical growth in the third growing season. The successful regeneration pathways also fit ground flora regeneration models and added a unique pathway to strengthen the predictive power of these models. As many stand management goals are focused towards improving biodiversity, prescribed fire and thinning may be used to increase understory richness in Pinus–Quercus stands.

Rhododendron maximum impacts seed bank composition and richness following Tsuga canadensis loss in riparian forests

AbstractSouthern Appalachian riparian forests have undergone changes in composition and function from invasive pathogens and pests. Castanea dentata mortality in the 1930s from chestnut blight (Cryphonectria parasitica) and Tsuga canadensis mortality in the 2000s from the hemlock woolly adelgid (Adelges tsugae) have led to the expansion and increased growth of Rhododendron maximum, an evergreen subcanopy shrub. A better understanding of seed bank characteristics and the various abiotic and biotic factors that affect the seed bank may be useful in determining the restoration potential of forest communities following invasion‐related disturbances. We compared the seed bank of two deciduous forest types: hardwood forests with a dense R. maximum subcanopy (hereafter, RR) and hardwood forests without R. maximum (hereafter, HWD). We evaluated numerous microenvironmental variables through principal component analysis (PCA) and correlated the derived PCA axes scores to seed bank density and richness across forest types. We found that seed bank density was comparable between the forests types; however, seed bank richness was much lower in RR than HWD and the species composition was dissimilar between forest types. Twenty‐eight of 64 (44%) species in the seed bank of HWD were not found in the seed bank of RR. Species that were represented in both forest types were often found in contrasting densities. Most notably, seed bank densities of several woody species were considerably higher in RR (85%) than HWD (45%), while herbaceous seed bank density was lower in RR (11%) than HWD (50%). Mineral soil pH, soil nutrient availability, and soil moisture were lower, and organic soil (Oi + Oe + Oa) depth and mass were greater in the RR than HWD forest type. PCA correlations revealed that PCA4 (represented by understory density and Oe + Oa phosphorus and carbon/nitrogen ratio) was negatively correlated with total seed bank density. PCA1 (represented by Oe + Oa cations and phosphorus, understory richness, ground‐layer cover, and mineral soil pH) and PCA4 were positively correlated with total seed bank richness. These results suggest that the soil seed bank will not be the primary mode of recruitment to establish a diverse and herbaceous‐rich community if a RR is present.

Not just about the trees: Key role of mosaic-meadows in restoration of ponderosa pine ecosystems

Abstract Historical pre-settlement conditions in ponderosa pine ecosystems ranged from savannas ( 6 m from overstory trees) on understory vegetation in five recently thinned ponderosa pine stands and one long-undisturbed stand in Colorado. We also compared historical spatial patterns in mosaic-meadows to current conditions in ponderosa pine stands and determined whether thinning treatments are recreating historical conditions. Mosaic-meadows in ponderosa pine ecosystems declined substantially from a mean of 55% of stand area in 1860–1875 to 7% in 2010–2013 prior to tree thinning. The loss of mosaic-meadows has ecological consequences for the function of ponderosa pine ecosystems because spatial patterns of mosaic-meadows strongly influenced understory vegetation. At our sites, understory cover increased by 3%/m with distance from overstory trees, and understory richness increased from 6 species/m2 within 1 m of overstory trees to 9 species/m2 at 10 m away from overstory trees. Cover and richness of understory vegetation (especially native forbs) responded rapidly to creation of mosaic-meadows within 1 to 4 years after treatment. Thinning treatments at two stands brought the coverage of mosaic-meadows within the historical range of variation, but there was a noticeable lack of mosaic-meadows >12 m from overstory trees at all treated stands. Restoration of the fundamental ecological characteristics of ponderosa pine ecosystems should intentionally include variably sized mosaic-meadows.