Understory Plant Species Richness Research Articles

Effects of canopy density on understory plant diversity in Robinia pseudoacacia plantations on the Loess Plateau of China

To solve the problems of understory plant shortage and biodiversity reduction caused by high tree density of Robinia pseudoacacia plantations on the Loess Plateau in northwest China, we analyzed the data collected by field survey and from literatures. We used the upper boundary line method to examine the impacts of canopy density on understory plant diversity. Based on field survey at the Guanshan Forest Farm of Jingchuan County of Gansu Pro-vince, the species number of understory plants in R. pseudoacacia plantations was higher than that in natural grassland (91 vs. 78). The dominant species varied with canopy density, which was completely different from that of natural grassland. A comprehensive analysis of both literature data and field survey data showed that, when mean annual precipitation (MAP) <550 mm or >550 mm, the increases in canopy density first led to a stable understory plant coverage and then a sharp or slight decrease, and understory plant biomass showed a sharp and continuous decrease or a slight increase and then decrease. Understory plant species richness and other diversity indices (Shannon, Simpson, and Pielou indices) first increase and then decrease, with bigger variation range under lower MAP. In all, the characteristics (coverage, biomass and species diversity, etc.) of understory plant community in R. pseudoacacia plantations were strongly affected by canopy density, with higher sensitivity under lower MAP. There was a general threshold range of canopy density of 0.45-0.6. When canopy density was above or below this thre-shold range, it would lead to a rapid decrease of the most characteristics of understory plant community. Therefore, keeping canopy density within the rational range of 0.45-0.6 in the management of R. pseudoacacia plantations is the key to make all the above-mentioned understory plant characteristics at relatively high level.

Response of understory plant species richness and tree regeneration to thinning in Pinus tabuliformis plantations in northern China

BackgroundChinese pine (Pinus tabuliformis Carr.) is one of the major afforestation species in northern China and plays a key role in restoring forest ecosystems and preserving soil and water. However, most Chinese pine plantations are experiencing ecological problems such as the low diversity of understory plants and difficulty in natural regeneration. Thinning has been widely used to maintain and improve a variety of forest ecosystem services from plantations. To date, however, few studies have been conducted to systematically determine the effects of thinning on understory plant diversity and the regeneration of Chinese pine in plantations. MethodsWe conducted a literature search, and selected 22 publications covering a total of 83 treatments related to thinning effects on the species richness of understory plants and 15 publications covering a total of 43 treatments related to thinning effects on the regeneration of Chinese pine, in tree plantations of northern China. The data from the literature were synthesized and evaluated with meta-analysis approach to determine the treatment effects. ResultsCompared with the control stands, thinning increased the species richness of shrubs and herbs by an average of 25.3% and 26.5%, respectively. While the varying thinning intensities all had significantly positive effects on the species richness of understory plants, only moderate thinning (30%–50%) had a positive effect on the density of regenerating seedlings and saplings of Chinese pine (60.2%). The species richness of understory plants was greatest after 14 years of thinning with an increase of 36.3%, whereas the density of regenerating Chinese pine seedlings and saplings reached a maximum after ≥11 years of thinning with an increase of 76.5%, compared to that of the unthinned stands. Thinning in the half-mature plantations had the greatest effects on the understory shrub richness (44.1%) and the density of regenerating Chinese pine seedlings and saplings (86.5%). Both single and multiple thinning were found to significantly promote the species richness of understory plants and the density of regenerating Chinese pine seedlings and saplings, and the positive effects of thinning were greater in areas with a humidity index (HI) ​< ​30 than in areas with an HI ​≥ ​30. In general, age group, planting density and recovery time were prominent factors affecting the species richness of understory plants, whereas the slope, HI and recovery time were the dominant controls of the density of regenerating Chinese pine seedlings and saplings, indicating differential effects of thinning on the species richness of understory plants and the regeneration capacity of Chinese pine in plantations. ConclusionThinning appears to be a feasible management measure to improve the understory plant diversity and regeneration capacity of Chinese pine in plantations. We postulate that moderate thinning in half-mature forest stands with an HI ​< ​30 can help effectively promote the species diversity of understory plants and the natural regeneration of Chinese pine, thereby maintaining a more resilient stand structure and the development of Chinese pine plantations.

Direct and indirect interactions between shrubs and the flowering annual community in an arid ecosystem

Shrubs commonly shape the plant community in arid ecosystems such as deserts. Plant interactions between shrubs and the associated species can be both negative and positive, and direct and indirect. The importance of shrub effects on reproduction is also a key component of facilitation studies in deserts. Here, we tested the hypothesis that shrubs facilitate understory plant community composition and reproduction through both direct and indirect interactions mediated by the associated plant neighbourhood. Using the shrub species Larrea tridentata, we tested the effects of facilitation on understory plant density, floral density, and species richness in the Mojave Desert, California. We found that shrubs decreased the net community-wide plant assembly measures relative to open microsites including total density and species richness at the site tested. There was also evidence that shrubs directly decreased plant fitness by reducing floral density, but this effect depended on the species. The net density and diversity of the annual plant community also influenced plant and floral density under shrubs suggesting significant indirect effects between shrubs and all abundant annual plant species. While both direct and indirect interactions between shrubs and the annual plant community were species-specific, the effects of these interactions had greater impact on plant density than floral density measures. Annual plant density and to a lesser extent floral density are thus driven by both direct and indirect interactions with shrubs.

Co‐occurring bunchgrasses are associated with different plant species in dry pine savannas

Grass species are often included during restoration to reinstate feedbacks with fire. In biodiverse pine savannas of the southeastern United States, wiregrass (Aristida beyrichiana) is the preferred candidate for restoring low‐intensity fires through the understory. Less common but locally co‐dominant in dry pine savannas, pineywoods dropseed (Sporobolus junceus) is another bunchgrass candidate for restoration. If two bunchgrass species associate with different suites of plants in the understory, restoring both could support a twofold goal of restoring fire regimes and plant biodiversity. To guide restoration goals, we tested the hypothesis that in natural (i.e. not recently restored) dry pine savannas in North Florida where wiregrass and dropseed are co‐dominant, they are associated with different understory plant species richness, occurrence, and abundance under or adjacent to their canopies. Species richness of associated plants did not differ between bunchgrass species or with distance from bunchgrass (i.e. under vs. adjacent). Some associated plant species, however, were more likely to occur and had greater abundance with one or the other bunchgrass species irrespective of distance. Our results suggest that including more than one bunchgrass species in restoration plantings could increase dry pine savanna biodiversity while also reinstating plant‐fire feedbacks.

Effects of Pine Plantation on Native Ecuadorian Páramo Vegetation

Exotic pine plantations, installed mainly for timber production and carbon sequestration purposes, cover an ever-expanding area of equatorial and tropical regions; however, their economic and environmental benefits are under debate due to their potential negative effects on native plant species conservation. The native understory plant species richness and cover under pine (Pinus radiata D. Don) plantations were compared with natural grasslands in the Ecuadorian Páramo. We analyzed the vegetation in four zones: Antisana (8-year-old pines), Cajas (16-year-old pines), Tisaleo (41-year-old pines) and Cotopaxi (53-year-old pines). The total understory plant cover decreased between 29% and 90% under pine plantations in all zones. The mean species richness in the pine plantations decreased by 44% in Antisana, Cajas and Tisaleo but not in Cotopaxi. Pine plantations strongly reduced the abundance of herbaceous light-demanding species (59%), except small herbs whose cover increased under pine (17.6%). Shrub cover was also negatively affected in Tisaleo and Cotopaxi (7.4%). Pine afforestation effects on Páramo vegetation depend mainly on canopy cover which changes with age and growing conditions (altitude) and secondarily on the vegetation state at the time of planting, resulting from land use history and ecological conditions. The pine effect was therefore more pronounced in the young and dense plantations of Antisana and Cajas than in the old and open stand of Cotopaxi, whose understory vegetation was more similar to that of grasslands.

Mycorrhizal type of woody plants influences understory species richness in British broadleaved woodlands.

Summary Mature temperate woodlands are commonly dominated by ectomycorrhizal trees, whereas understory plants predominantly form arbuscular mycorrhizal associations. Due to differences in plant–fungus compatibility between canopy and ground layer vegetation the ‘mycorrhizal mediation hypothesis’ predicts that herbaceous plant establishment may be limited by a lack of suitable mycorrhizal fungal inoculum.We examined plant species data for 103 woodlands across Great Britain recorded in 1971 and in 2000 to test whether herbaceous plant species richness was related to the proportion of arbuscular mycorrhizal woody plants. We compared the effect of mycorrhizal type with other important drivers of woodland plant species richness.We found a positive effect of the relative abundance of arbuscular mycorrhizal woody plants on herbaceous plant species richness. The size of the observed effect was smaller than that of pH. Moreover, the effect persisted over time, despite many woodlands undergoing marked successional change and increased understorey shading.This work supports the mycorrhizal mediation hypothesis in British woodlands and suggests that increased abundance of arbuscular mycorrhizal woody plants is associated with greater understory plant species richness.

Light heterogeneity affects understory plant species richness in temperate forests supporting the heterogeneity-diversity hypothesis.

One of the most important drivers for the coexistence of plant species is the resource heterogeneity of a certain environment, and several studies in different ecosystems have supported this resource heterogeneity–diversity hypothesis. However, to date, only a few studies have measured heterogeneity of light and soil resources below forest canopies to investigate their influence on understory plant species richness. Here, we aim to determine (1) the influence of forest stand structural complexity on the heterogeneity of light and soil resources below the forest canopy and (2) whether heterogeneity of resources increases understory plant species richness. Measures of stand structural complexity were obtained through inventories and remote sensing techniques in 135 1‐ha study plots of temperate forests, established along a gradient of forest structural complexity. We measured light intensity and soil chemical properties on six 25 m² subplots on each of these 135 plots and surveyed understory vegetation. We calculated the coefficient of variation of light and soil parameters to obtain measures of resource heterogeneity and determined understory plant species richness at plot level. Spatial heterogeneity of light and of soil pH increased with higher stand structural complexity, although heterogeneity of soil pH did not increase in conditions of generally high levels of light availability. Increasing light heterogeneity was also associated with increasing understory plant species richness. However, light heterogeneity had no such effects in conditions where soil resource heterogeneity (variation in soil C:N ratios) was low. Our results support the resource heterogeneity–diversity hypothesis for temperate forest understory at the stand scale. Our results also highlight the importance of interaction effects between the heterogeneity of both light and soil resources in determining plant species richness.

Negative effects of long-term phosphorus additions on understory plants in a primary tropical forest

Human activities have disturbed global phosphorus (P) cycling by introducing substantial amounts of P to natural ecosystems. Although natural P gradients and fertilization studies have found that plant community traits are closely related to P availability, it remains unclear how increased P supply affects plant growth and diversity in P-deficient tropical forests. We used a decadal P-addition experiment (2007–2017) to study the effects of increased P input on plant growth and diversity in understory layer in tropical forests. We monitored the dynamics of seedling growth, survival rate, and diversity of understory plants throughout the fertilization period under control and P addition at 15 g P m−2 yr−1. To identify the drivers of responses, P concentration, photosynthesis rate and nonstructural carbon were analyzed. Results showed that long-term P addition significantly increased P concentrations both in soil pools and plant tissues. However, P addition did not increase the light-saturated photosynthesis rate or growth rate of the understory plants. Furthermore, P addition significantly decreased the survival rate of seedlings and reduced the species richness and density of understory plants. The negative effects of P addition may be attributed to an increased carbon cost due to the tissue maintenance of plants with higher P concentrations. These findings indicate that increased P supply alone is not necessary to benefit the growth of plants in ecosystems with low P availability, and P inputs can inhibit understory plants and may alter community composition. Therefore, we appeal to a need for caution when inputting P to tropical forests ecosystems.

Initial conditions influence effects of prescribed burns and deer exclosure fences on tree regeneration and understory diversity in Appalachian oak-dominated forests

Sustaining oak-dominated forests that support regionally important biodiversity in the Appalachian Mountains is a substantial challenge for land managers due to mesophication – a region-wide phenomenon in which forest composition shifts from oaks to shade tolerant, mesic species such as maples. To inform oak forest management that includes biodiversity preservation as a management objective, we established a two treatment factorial design experiment at two sites to determine the effects of prescribed fire and deer herbivory on tree regeneration and understory plant diversity in dry oak-dominated forests. Data were collected before and after treatments were applied. Prescribed burns altered species composition to favor fire-adapted species and deer exclosure fences protected woody and herbaceous species sensitive to deer browse at both sites, however the initial conditions in the forests profoundly influenced the effects of these treatments. Prescribed fires promoted oak seedlings over maple seedling where oak seedlings were well stocked. Conversely, burning favored maple seedlings to the detriment of oak seedlings in areas where chronic over-browsing resulted in a depauperate understory. Although fire reduced understory plant cover at both sites, overall understory species richness was not affected by burning. This suggests that prescribed fire did not have detrimental effects on forest understory diversity, although herbaceous species diversity was significantly reduced by burning the site with an initially depauperate understory. Fencing resulted in large increases in species richness and cover of understory plants, including more than doubling the cover of tree seedlings and the abundance of tall seedlings in fenced plots, as well as shifts in understory composition towards species sensitive to deer browse. The herbaceous layer in this study contained nearly 6 species for every one species found in the tree canopy, reaffirming the importance of the herbaceous layer in the biodiversity of Appalachian oak-dominated forests. These results provide guidance to managers of protected areas seeking to enhance oak regeneration while maintaining or promoting native plant diversity in the forest understory.

Stoichiometric traits (N:P) of understory plants contribute to reductions in plant diversity following long-term nitrogen addition in subtropical forest.

Nitrogen enrichment is pervasive in forest ecosystems, but its influence on understory plant communities and their stoichiometric characteristics is poorly understood. We hypothesize that when forest is enriched with nitrogen (N), the stoichiometric characteristics of plant species explain changes in understory plant diversity. A 13‐year field experiment was conducted to explore the effects of N addition on foliar carbon (C): N: phosphorus (P) stoichiometry, understory plant species richness, and intrinsic water use efficiency (iWUE) in a subtropical Chinese fir forest. Four levels of N addition were applied: 0, 6, 12, and 24 g m−2 year−1. Individual plant species were categorized into resistant plants, intermediate resistant plants, and sensitive plants based on their response to nitrogen addition. Results showed that N addition significantly decreased the number of species, genera, and families of herbaceous plants. Foliar N:P ratios were greater in sensitive plants than resistant or intermediate resistant plants, while iWUE showed an opposite trend. However, no relationship was detected between soil available N and foliar N, and soil N:P and foliar N:P ratios. Our results indicated that long‐term N addition decreased the diversity of understory plants in a subtropical forest. Through regulating water use efficiency with N addition, sensitive plants change their N:P stoichiometry and have a higher risk of mortality, while resistant plants maintain a stable N:P stoichiometry, which contributes to their survival. These findings suggest that plant N:P stoichiometry plays an important role in understory plant performance in response to environmental change of N.

Persistent effects of fire severity on ponderosa pine regeneration niches and seedling growth

Several recent studies have documented how fire severity affects the density and spatial patterns of tree regeneration in western North American ponderosa pine forests. However, less is known about the effects of fire severity on fine-scale tree regeneration niche attributes such as understory plant composition and cover, surface fuel abundance, and soil properties, or how these attributes in turn affect regenerating ponderosa pine growth. Using 1-m2 plots centered on 360 ponderosa pine seedlings that regenerated naturally after the Pumpkin Fire in 2000 in Arizona, we quantified regeneration niche attributes 13 years post-fire and measured their associations with seedling growth 11–16 years post-fire. Plots were established in a) unburned, b) moderate-severity, and two types of high-severity (100% tree mortality) burns, either c) adjacent to residual live forest edges (high-severity edge plots), or d) >200 m from any residual live trees (high-severity interior plots). We found that all burned plots had greater understory plant species richness, percent cover of forbs, exotic plants and coarse wood, as well as higher soil pH, sand and gravel content, and lower soil clay content compared to unburned plots. High-severity burn plots had the greatest total understory plant and shrub cover, the most herbaceous fine fuel biomass, and tended to have the highest soil nitrogen content compared to other burn severity classes. Ponderosa pine growth (i.e., stem diameter at root collar (DRC) and length of terminal leader) was lowest in the unburned compared to burned plots, and ponderosa pine terminal leader growth was consistently greater in the high-severity edge plots compared to other severities. Finally, niche characteristics such as overstory tree canopy cover (−), soil phosphate (+), and cover of coarse wood (+), forbs (+), and the native grasses, mountain muhly (+) and squirreltail (−), were important explanatory variables of ponderosa pine growth. Exotic plant cover did not have a negative association with ponderosa pine growth. These results suggest that if ponderosa pine seeds can disperse and germinate, and if seedlings can survive the first few critical years after germination, then low overstory canopy cover and abundant forbs or coarse wood may be associated with increased growth rates. Alternatively, forbs may be responding to the same site benefits as the seedling; and abundant forbs, coarse wood, and fine fuels might also put seedlings at increased risk of mortality from subsequent fire, at least until they are taller and more fire resistant.

Long-term effects of vegetation and soil on the microbial communities following afforestation of farmland with Robinia pseudoacacia plantations

Soil microorganisms play an important role in the functional connection of nutrient cycling between plants and soil ecosystems, but the exact responses of microbial communities to change in plant characteristics and soil properties remain uncertain. We analyzed the soil microbial communities of afforested tree stands of varying ages to assess the effects of vegetation and soil quality on soil microbial communities. We studied Robinia pseudoacacia plantations in the Loess Plateau for three age classes (RP13, RP29, and RP44), and compared results from these classes to one farmland (FL) control. The responses of soil microbial communities to changes in understory flora and soil physicochemical properties, including the Shannon index, richness, coverage, and biomass were analyzed. Compared with FL, afforestation significantly increased the Shannon index, species richness, coverage, and biomass of understory plants, which were highest in RP29. The concentrations of soil organic carbon (SOC), total nitrogen (TN), nitrate nitrogen (NO3−-N), and ammoniacal nitrogen (NH4+-N) were highest in RP44. The soil microbial community diversity index was significantly affected by afforestation. The relative abundances of bacterial populations were also affected, with Proteobacteria being dominant in RP29 and Actinobacteria being dominant in FL. Similarly, in fungi, the relative abundance of Basidiomycota was highest in FL, while the relative abundances of Zygomycota were highest in RP29. Pearson correlation analysis revealed that the Shannon and Chao indices of soil fungi were significantly positively correlated with SOC and NH4+-N. RDA showed that SOC, NH4+-N, and soil water content (SWC) were significantly associated with soil bacterial community composition. Soil fungal community composition were significantly affected by the Shannon and Richness indexes of understory flora. The results showed that characteristics of understory flora, soil properties, and soil microbial community composition were all affected by afforestation. The composition of the fungal community was correlated with changes in the diversity of understory flora. The changes in nutrients such as organic carbon and inorganic nitrogen was the key to the changes of soil microorganisms as plant colonization.

Intervention of Nitrogen Evolution on Species Diversity of Broad-leaved Forest Plants in Xianweng Mountain

Nitrogen evolution is one of the important factors driving biodiversity change. It is generally believed that the species diversity structure will be changed, and under the conditions of exogenous nitrogen addition, grasses and deciduous shrubs have a competitive advantage over hybrid grasses and evergreen shrubs. For the competition between plants in the same life type, the interaction between different life-type plants is not involved. Therefore, from the changes in forest ecosystem structure and nitrogen elements, the response of different life-type plants to nitrogen evolution can be further explored. The lower plant species of the deciduous broad-leaved forest-Mongolian carp are studied, and different gradients of nitrogen (0, 40, 80, 120 kg N·ha−1·yr−1) are added to the four gradients of extraneous nitrogen effects of nitrogen evolution on forest biodiversity in the north temperate zone. The results of five-year continuous nitrogen addition experiments showed that nitrogen addition significantly reduced the species richness and diversity of understory plants, and changed the species composition of the community. Nitrogen addition increased the species richness of shrub plants and diversity, reduced the richness of herbaceous plants, and reduced the important value of grasses and increased the important value of hybrid grasses. The study showed that long-term nitrogen addition significantly changed the species composition of understory plants, and the response of different life-type plants to nitrogen addition was also different.

Understory Vegetation Richness and Diversity of Eucalyptus benthamii and Pinus elliottii Plantations in the Midsouth US

AbstractIn the Western Gulf region of the United States cold-tolerant eucalyptus have been explored as pulpwood feedstock. However, non-native plantations may alter understory species diversity, modifying environmental conditions and soil characteristics. Few studies have compared eucalyptus plantations with native ecosystems to understand the impact on understory vegetation in the United States. In this study, we compared understory plant species richness and diversity during 2014–2016 in (1) slash pine (Pinus elliottii) established in 2008, (2) slash pine established in 2013, and (3) and Camden white gum (Eucalyptus benthamii) established in 2013. Overstory characteristics, soil pH, and soil nutrient concentrations were measured to understand factors that affected understory species richness and diversity. Results indicated a decline in understory species richness over time, with Camden white gum in an intermediate condition between same-age slash pine (highest richness) and older slash pine (lowest richness). Leaf area index, soil pH and K, and tree height were the most important factors influencing understory species richness and diversity. The adoption of fast-growing eucalyptus on these sites will probably accelerate the deterioration of natural habitats and reduce open-condition species in favor of shade-tolerant species, overturning the conservation efforts already put in place by governmental agencies and conservation groups.

Concealed by darkness: How stand density can override the biodiversity benefits of mixed forests

AbstractHigher tree species richness often increases the diversity of other taxonomic groups and is promoted in many managed forest systems. The reason for the higher diversity is due to, for example, inter‐specific differences in resource filtering of the trees. Resource filtering also depends on the total density of the tree layer, but the interaction between tree species composition and density is, so far, unexplored. Likewise, few studies have addressed whether mixtures of tree species host higher diversity, or support higher productivity, of other taxa than monocultures of the same tree species. We use a gradient in overstory tree species composition, ranging from pure Norway spruce (Picea abies) to pure Birch (Betula sp.), combined with a gradient from open to closed forest, to assess the joint effects of tree species mixture and forest density on the understory vegetation. The cover and species richness of understory vascular plants increased with an increasing proportion of birch and decreased with increasing forest density, while the cover of bryophytes decreased with an increasing proportion of birch and increasing forest density. There were clear interactions between tree species composition and forest density; the decrease in vascular plants with forest density was stronger in forest dominated by spruce than in forests dominated by birch, and the positive effect of an increasing proportion of birch was smaller in dense than in open forests. There were no indications of mixed forests supporting a higher species richness or plant cover than any of the two monocultures. Additionally, few species (10%) showed tendencies toward a higher probability of occurrence in mixed forests, and most of these also occurred in open pure stands. Our results indicate a potential conflict between goals for diversified forests and increased biomass production, as the positive effects for understory vegetation from mixtures may be lost to concomitant increases in forest density. Furthermore, the limited number of species benefiting from mixtures per se indicates that similar biodiversity benefits for understory vegetation may be obtained at landscape levels from the increased use of broadleaf and lower‐density production stands, as from mixtures.

Long-term vegetation response following post-fire straw mulching

BackgroundStraw mulching is one of the most common treatments applied immediately post fire to reduce soil erosion potential and mitigate post-fire effects on water quality, downstream property, and infrastructure, but little is known about the long-term effects on vegetation response. We sampled six fires that were mulched between 9 and 13 years ago in western US dry conifer forests. We compared understory plant species diversity and abundance, tree seedling density and height by species, and fractional ground cover on mulched and unmulched paired plots.ResultsMulch did not influence understory plant diversity, species richness, or fractional ground cover. However, on mulched plots, tree seedlings grew taller faster, especially on north-facing aspects, and there was slightly more graminoid cover. Mulch did not affect overall tree seedling density, but there were fewer ponderosa pine (Pinus ponderosa Lawson & C. Lawson) and more Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) in mulched areas, especially on south-facing slopes.ConclusionsManagers will be able to weigh the long-term implications of mulching against the short-term reductions in soil erosion potential. While there are many concerns about vegetation suppression and exotic species introduction from using straw mulch, our study suggests that the long-term effects are subtle 9 to 13 years after post-fire mulching.

Plant richness and composition in hardwood forest understories vary along an acidic deposition and soil-chemical gradient in the northeastern United States

AimsA century of atmospheric deposition of sulfur and nitrogen has acidified soils and undermined the health and recruitment of foundational tree species in the northeastern US. However, effects of acidic deposition on the forest understory plant communities of this region are poorly documented. We investigated how forest understory plant species composition and richness varied across gradients of acidic deposition and soil acidity in the Adirondack Mountains of New York State.MethodsWe surveyed understory vegetation and soils in hardwood forests on 20 small watersheds and built models of community composition and richness as functions of soil chemistry, nitrogen and sulfur deposition, and other environmental variables.ResultsCommunity composition varied significantly with gradients of acidic deposition, soil acidity, and base cation availability (63% variance explained). Several species increased with soil acidity while others decreased. Understory plant richness decreased significantly with increasing soil acidity (r = 0.60). The best multivariate regression model to predict richness (p < 0.001, adjusted-R2 = 0.60) reflected positive effects of pH and carbon-to-nitrogen ratio (C:N).ConclusionsThe relationship we found between understory plant communities and a soil-chemical gradient, suggests that soil acidification can reduce diversity and alter the composition of these communities in northern hardwood forests exposed to acidic deposition.

Fire legacies in eastern ponderosa pine forests.

Disturbance legacies structure communities and ecological memory, but due to increasing changes in disturbance regimes, it is becoming more difficult to characterize disturbance legacies or determine how long they persist. We sought to quantify the characteristics and persistence of material legacies (e.g., biotic residuals of disturbance) that arise from variation in fire severity in an eastern ponderosa pine forest in North America. We compared forest stand structure and understory woody plant and bird community composition and species richness across unburned, low‐, moderate‐, and high‐severity burn patches in a 27‐year‐old mixed‐severity wildfire that had received minimal post‐fire management. We identified distinct tree densities (high: 14.3 ± 7.4 trees per ha, moderate: 22.3 ± 12.6, low: 135.3 ± 57.1, unburned: 907.9 ± 246.2) and coarse woody debris cover (high: 8.5 ± 1.6% cover per 30 m transect, moderate: 4.3 ± 0.7, low: 2.3 ± 0.6, unburned: 1.0 ± 0.4) among burn severities. Understory woody plant communities differed between high‐severity patches, moderate‐ and low‐severity patches, and unburned patches (all p < 0.05). Bird communities differed between high‐ and moderate‐severity patches, low‐severity patches, and unburned patches (all p < 0.05). Bird species richness varied across burn severities: low‐severity patches had the highest (5.29 ± 1.44) and high‐severity patches had the lowest (2.87 ± 0.72). Understory woody plant richness was highest in unburned (5.93 ± 1.10) and high‐severity (5.07 ± 1.17) patches, and it was lower in moderate‐ (3.43 ± 1.17) and low‐severity (3.43 ± 1.06) patches. We show material fire legacies persisted decades after the mixed‐severity wildfire in eastern ponderosa forest, fostering distinct structures, communities, and species in burned versus unburned patches and across fire severities. At a patch scale, eastern and western ponderosa system responses to mixed‐severity fires were consistent.

Maximizing the monitoring of diversity for management activities: Additive partitioning of plant species diversity across a frequently burned ecosystem

Monitoring understory plant diversity is important, allowing managers to track current diversity status and trends both spatially and temporally at a landscape-scale. Improving precision in quantifying patterns in understory plant diversity improves efficiency in monitoring design and more accurate measures of success of management intervention over time. Patterns of species diversity are dependent upon the scale in which they are examined – an increase in small-scale diversity across a gradient can convert to a decrease in large-scale diversity across that same gradient. Using two extensive datasets including both mined historical data and supplemental experimental data, we performed an additive partitioning of plant diversity to elucidate the hierarchical spatial patterns of understory plant species richness, and independent measures of alpha and beta diversity in the species-rich longleaf pine ecosystem at Eglin Air Force Base in northwestern FL, USA. This analysis allowed us to identify the spatial scale that most effectively captures plant diversity to inform monitoring efforts by using measures of species turnover, specifically beta diversity. We found that while species richness and alpha diversity increased with spatial scale, beta diversity began to reach an asymptote at smaller (1 m2) scales. Furthermore, we found the sampling effort at this 1 m2 scale required as few as 60 plots to effectively estimate plant diversity within management blocks. While our results are attributable to Eglin AFB specifically, these scaling analyses can help to streamline monitoring efforts in other ecosystems that seek to elucidate the individual contributions of diversity components.

Effects of Seasonality, Forest Structure, and Understory Plant Richness on Bee Community Assemblage in a Southern Rocky Mountain Mixed Conifer Forest

Wild bee community assemblages were surveyed in a high-elevation mixed conifer forest in central Colorado at multiple points during the growing season (April–August) and across a range of forest stand densities using blue vane traps. Understory forb communities were also characterized and related to bee species abundance and diversity. Overall γ-diversity was characterized by 19 genera of bees captured representing five families (Andrenidae, Apidae, Colletidae, Halictidae, and Megachilidae) and 39 unique species. Seasonal variation in abundance and community composition was considerable with early-season (April) communities dominated by Osmia spp. and midsummer (June–July) communities dominated by Bombus spp. Bee abundance and α-diversity were on average 88 and 74% higher in midsummer than in early- and late season. Forest basal area was negatively correlated with bee α-diversity and abundance, as well as understory plant species richness. Trapping locations with a stand basal area of ≤7.5 m2/ha exhibited approximately 54% higher bee abundances and 44% higher α-diversity than locations with ≥20.0 m2/ha of basal area. Bee α-diversity and abundance positively correlated with understory species richness suggesting that forested sites with low basal area, likely corresponding to overstory canopy gaps and increased site occupancy by forbaceous species, may drive local abundance and diversity of wild bee assemblages. This study provides basal area threshold values in a mixed conifer forest type that may be useful for resource management practitioners concerned with creating or conserving pollinator habitats.