Current Plant Community Research Articles

The impact of deicer and anti-icer use on plant communities in stormwater detention basins: Characterizing salt stress and phytoremediation potential.

We present the results of a 1-year study that quantified salt levels in stormwater, soils, and plant tissues from 14 stormwater detention basins across Northern VA in an above-average snow year. We characterize (1) the level of salt stress plants experience, (2) the extent to which current plant communities feature salt tolerant species, and (3) the capacity of these species to phytoremediate soils and reduce the impacts of deicer and anti-icer use. Our results suggest that detention basin vegetation experience a range of salt stress levels that depend on drainage area type (roads: moderate to high > parking lots: low to moderate > pervious areas: none). Established thresholds for salt sensitive vegetation (Na+, Cl+, electrical conductivity, sodium adsorption ratio, exchangeable sodium percentage) were exceeded at least twice in stormwater or soils from all systems draining roads and half of systems draining parking lots. Winter exceedances were most common, but saline conditions did persist into the growing season, particularly at sites draining roads. Two hundred fifty-five plant species were identified across all detention basins, including 48 natives capable of tolerating elevated salt levels (electrical conductivity ≥2 dS/m). Within-tissue concentrations of sodium and chloride ions were highest in Typha (latifolia and angustifolia) (11.1mg Na+/g; 30mg Cl-/g), making it our top phytoremediation candidate. Scaling these concentrations up, we estimate that a standard-size highway detention basin (2000-3000m2) with 100% cattail cover can phytoremediate up to 100kg of Na+ and 200kg of Cl- per year. Uptake at this level is not sufficient to offset winter salt application, constituting only 5-6% of basin inputs. This suggests that phytoremediation should not be considered a standalone solution to basin salinization, although it could be one approach of many in a broader salt management strategy.

No evidence of a northward biome shift of treeline in the Mackay Lake region, north-central Canada

ABSTRACT Major changes in boreal and subarctic climates have occurred in recent decades, but uncertainty exists as to how biota are responding, e.g., whether the subarctic forest-tundra (‘treeline’) will advance northward. Ground-based vegetation data from the Mackay Lake region of the Northwest Territories were used to determine if there is evidence of a northward biome shift. Near the treeline, current plant communities are similar to those documented 40–67 years ago. There was little or no evidence of recent tree regeneration by seed, expansion of tree stands, or colonization by trees of adjacent tundra. Black spruce stems require ~75 years to reach 2.5 m in height and be discernible as trees on photography and satellite imagery. Branches 20–30 years old are the preferred age for initiation of an upright stem from layering. Upright stems of clonal spruce within the forest-tundra may be far younger than their near-ground or belowground stems. Whether northward biome migration will occur will depend on the interplay between climatic, landscape, and soil factors, species migration capacities, wildfires, and whether afforestation outpaces deforestation. In the forest-tundra, climatic change may not result in significant biome shifts, but rather in changes in the relative abundance of species already present.

Restoration of Vegetation Greenness and Possible Changes in Mature Forest Communities in Two Forests Damaged by the Vaia Storm in Northern Italy.

Windstorms are rare in the Southern Alps, but their frequency is increasing due to climate change. This research analyzed the vegetation of two spruce forests in Camonica Valley (Northern Italy) destroyed by the Vaia storm to evaluate the vegetation responses to blowdown damage. In each study area, the normalized difference vegetation index (NDVI) was used to evaluate the change in plant cover and greenness from 2018 (before the Vaia storm) to 2021. Furthermore, floristic-vegetation data were analyzed to identify current plant communities and develop models of plant succession. The results showed that the two areas, although located in different altitudinal vegetation belts, are undergoing the same ecological processes. NDVI is increasing in both areas, and pre-disturbance values (~0.8) should be reached in less than ten years. Nevertheless, the spontaneous restoration of pre-disturbance forest communities (Calamagrostio arundinaceae-Piceetum) should not occur in both study areas. In fact, the two plant succession trends are characterized by pioneer and intermediate stages with young trees of Quercus petraea and Abies alba, typical of more thermophilic mature forest communities compared to pre-disturbance ones. These results could reinforce the trend of the upward shift in forest plant species and plant communities in response to environmental changes in mountain areas.

ГАЗОНАСИЧЕНІСТЬ ПОВЕРХНЕВОГО ГРУНТОВОГО ГОРИЗОНТУ В МЕЖАХ ТОРФОМАСИВУ «БУЧМАНИ»

Purpose. Determining the gas saturation of the soil over the Buchmani peat massif. Methods. The study was conducted using a comprehensive methodology of structural-thermo-atmo- geochemical research (STAHGR), including thermometric, emanation and gas-geochemical studies (radon, thoron, carbon dioxide, helium, hydrogen), thermometric studies and gas-geochemical surveys for free hydrocarbons (methane, ethane, propane, butane, isobutane, ethylene, propylene, isopentane, pentane, hexane). The author personally described the current plant communities in the peatlands and determined the depth of peat at the observation points. Results. For the first time, a complex of STAHGR tests was performed within the Buchmany peat bog to assess the gas saturation of the surface soil horizon. The research was carried out in the chromatographic laboratory of the Department of Geoecology and Exploration Research of the Institute of Geological Sciences of the National Academy of Sciences of Ukraine. During the surveys, hydrocarbon gases, helium, hydrogen and carbon dioxide were detected over the peat bog, which gives reason to hope that more in-depth studies will be conducted in the future on peatlands in Ukraine. Different types of peatlands (lowland, upland and transitional) have different vegetation, but the plants: three-parted bentgrass (Bidens tripartita L.), peppercorn (Persicaria hydropiper (L.) germinated on all three types. The maximum depth in the study area was 1.25 m, the minimum depth was 0.25 m. No correlation between peat depth and gas concentration was observed during the study. Scientific novelty. For the first time, a study using the STAHGR method was carried out and the gas saturation of the surface soil horizon within the Buchmanу peat massif was assessed. Practical value. The results obtained using the STAHGR methodology can be used to assess the amount of gases studied above the peatland and the danger of their accumulation within peat massifs.

Current and legacy effects of neighborhood communities on plant growth and aboveground herbivory

Current and legacy effects can greatly affect the growth of a focal plant and its interactions with herbivores and such effects can be mediated by above- and belowground effects. However, determining the relative importance of current and legacy above- and belowground effects in natural conditions is a major challenge. In a long-term grassland experiment, we examined the relative importance of the current and legacy above- and belowground effects of plant communities on the growth and aboveground herbivore damage on a focal plant, Leucanthemum vulgare. Focal plants were planted into tubes with soil collected from different plant communities and placed back into the plant communities. Weekly, plant growth and damage were recorded and after 12 weeks plant biomass was measured. We analyzed how well aboveground and belowground characteristics of the current and legacy plots explained plant growth and herbivory. We found both current plant communities and legacy plant communities significantly affected plant growth (shoot biomass and the number of leaves) and herbivory. Root biomass of the focal plants was influenced by current plant communities only. Current and legacy above- and belowground characteristics explained 12% and 11% of the variation in shoot biomass. Root biomass was mainly explained by current above- and belowground characteristics with a total explained variation of 10%, while legacy effects explained 3%. Legacy effects explained most variation in the number of leaves during the first two weeks of measurements, and the effect remained present during the growth season. In contrast, characteristics of the current community explained most of the variation in herbivory throughout the growth period, with on average 6% explained variance aboveground vs. 5% belowground. Our grassland field study highlights that both current and legacy effects influence plant growth, but herbivory on focal plants is caused by current neighborhood effects only and not by legacy effects.

Assessing the lysis of diverse pollen from bulk environmental samples for DNA metabarcoding

Pollen is ubiquitous year-round in bulk environmental samples and can provide useful information on previous and current plant communities. Characterization of pollen has traditionally been completed based on morphology, requiring significant time and expertise. DNA metabarcoding is a promising approach for characterizing pollen from bulk environmental samples, but accuracy hinges on successful lysis of pollen grains to free template DNA. In this study, we assessed the lysis of morphologically and taxonomically diverse pollen from one of the most common bulk environmental sample types for DNA metabarcoding, surface soil. To achieve this, a four species artificial pollen mixture was spiked into surface soils collected from Colorado, North Carolina, and Pennsylvania, and subsequently subjected to DNA extraction using both the PowerSoil and PowerSoil Pro Kits (Qiagen) with a heated incubation (either 65 °C or 90 °C). Amplification and Illumina sequencing of the internal transcribed spacer subunit 2 (ITS2) was completed in duplicate for each sample (total n, 76), and the resulting sequencing reads taxonomically identified using GenBank. The PowerSoil Pro Kit statistically outperformed the PowerSoil Kit for total DNA yield. When using either kit, incubation temperature (65 °C or 90 °C) used had no impact on the recovery of DNA, plant amplicon sequence variants (ASVs), or total plant ITS2 reads. This study highlighted that lysis of pollen in bulk environmental samples is feasible using commercially available kits, and downstream DNA metabarcoding can be used to accurately characterize pollen DNA from such sample types.

Assessing soil and land health across two landscapes in eastern Rwanda to inform restoration activities

Abstract. Land degradation negatively impacts water, food, and nutrition security and is leading to increased competition for resources. While landscape restoration has the potential to restore ecosystem function, understanding the drivers of degradation is critical for prioritizing and tracking interventions. We sampled 300–1000 m2 plots using the Land Degradation Surveillance Framework across Nyagatare and Kayonza districts in Rwanda to assess key soil and land health indicators, including soil organic carbon (SOC), erosion prevalence, vegetation structure and infiltration capacity, and their interactions. SOC content decreased with increasing sand content across both sites and sampling depths and was lowest in croplands and grasslands compared to shrublands and woodlands. Stable carbon isotope values (δ13C) ranged from −15.35 ‰ to −21.34 ‰, indicating a wide range of historic and current plant communities with both C3 and C4 photosynthetic pathways. Field-saturated hydraulic conductivity (Kfs) was modeled, with a median of 76 mm h−1 in Kayonza and 62 mm h−1 in Nyagatare, respectively. Topsoil OC had a positive effect on Kfs, whereas pH, sand, and erosion had negative effects. Soil erosion was highest in plots classified as woodland and shrubland. Maps of soil erosion and SOC at 30 m resolution were produced with high accuracy and showed strong variability across the study landscapes. These data demonstrate the importance of assessing multiple biophysical properties in order to assess land degradation, including the spatial patterns of soil and land health indicators across the landscape. By understanding the dynamics of land degradation and interactions between biophysical indicators, we can better prioritize interventions that result in multiple benefits as well as assess the impacts of restoration options.

Historical and current fire regimes in ponderosa pine forests at Zion National Park, Utah: Restoration of pattern and process after a century of fire exclusion

Plant community resilience in frequent-fire forests of western North America has been compromised by over a century of fire exclusion. However, there are several western National Parks where fires have been reintroduced over the past several decades, including Zion National Park in southwest Utah. Here we reconstruct historical components of fire regimes in ponderosa pine dominated forests at Zion and compare these to recent fire frequency, current plant community and fuels structure, and potential fire behavior. Historical fires burned every 9–10 years on average up until 1879, when fires ceased contemporaneous with introduction of Euro-American livestock grazing and timber harvest in upland forests. Abundant tree regeneration occurred after fire exclusion, with tree density averaging 45 trees ha−1 in reconstructed 1880 forests versus 106 trees ha−1 today. Intervals between recent (since 1988) wildfires and prescribed fires in these same stands ranged from 7 to 13 years, similar to historical fire timing. Depending on whether plots had burned from zero to three times in recent fires, we found significant differences in canopy base heights (increased), duff and litter depths (decreased), and percent cover of grass and forbs (increased), but not tree density, tree basal area, shrub height, shrub cover, or woody fuels. Combined effects of recent fires on overstory and understory structure resulted in a significant difference in likelihood of crown fire occurrence, declining from a mean of 58% in plots with no fire since 1879 to 13% in plots with three fires since 1988. Significant effects were generally seen after two or three fires, suggesting it is the reintroduction of the fire regime and not just individual fire events that restore resiliency. Overall, effects of recent fires are building on the latent resiliency of ponderosa pine forests at Zion National Park, although questions remain about extent and future dynamics of oak and manzanita shrubfields that occupy similar environmental settings, along with a general lack of ponderosa pine regeneration across all plots.

Indigenous peoples’ habitation history drives present‐day forest biodiversity in British Columbia's coastal temperate rainforest

Abstract Long‐term human habitation has transformed the earth's surface. The combination of time and complex human–environment interactions in remote regions of North America has likely resulted in modified landscapes, though we often consider these regions free of human influence due to the absence of industrial development. We examined long‐term impacts of human resource‐use on British Columbia's coastal rainforest communities. We focused on the region's widespread habitation sites with extensive shell middens to test the legacy of ancient human occupation in present‐day plant communities. Ten habitation sites and 10 control sites in similar locales were selected for floristic surveys and soil sampling. We tested whether plant communities at habitation sites reflected a ‘cultural plant‐use legacy’, with greater presence of culturally significant plant species, and/or a ‘marine nutrient subsidy legacy’ from human use, with increases in species that prefer nutrient‐rich soils. We found that the habitation sites had different plant assemblages than the control sites and were dominated by plants with both higher nutrient requirements and cultural significance. We demonstrate that long‐term occupation has led to strong differences in plant community structure between sites, countering the notion that this is a pristine landscape. We emphasize the value of interdisciplinary approaches and considering past human resource‐use when examining current plant communities. A plain language summary is available for this article.

Nitrogen recycling in coupled green and brown food webs: Weak effects of herbivory and detritivory when nitrogen passes through soil

Abstract The study of coupled green and brown food webs has improved our ability to understand how nutrient cycling and plant communities respond to perturbations. Yet, it is still difficult to predict how rapidly and consistently changes in one food web propagate to the other. An area of particular uncertainty is the response of plants to the changes in leaf litter nitrogen release caused by herbivores and detritivores. We combined a field experiment and theoretical analysis to assess how herbivory, fertilization, and detritivory changed leaf litter nitrogen release and plant growth. We produced leaf litter in greenhouse cages with a factorial combination of grasshopper herbivory and additional nitrogen fertilizer. Our experiment used the resulting 15N‐labelled leaf litter to measure isopod litter consumption, litter nitrogen release, and plant nitrogen uptake in the field. We then used a dynamical systems model to distinguish whether plant growth relied (a) directly on litter nitrogen release or (b) on other nitrogen pathways so that changes in litter nitrogen release have little immediate impact. Our empirical results show that the effect of nitrogen fertilization on isopod processing of leaf litter was stronger than the effect of herbivory. Regardless, the available soil nitrogen and plant growth were independent of litter nitrogen release. Our dynamical model attributes the independence to other nitrogen sources and sinks, which buffer the plant‐available nitrogen pool. Isopods and litter with a history of herbivory reduced above‐ground plant growth when we accounted for initial field mesocosm conditions, especially the abundance of the competitively dominant goldenrods. Consequently, the impact of isopods and litter traits do propagate to influence plant growth, but do not have a large enough effect to overcome initial differences in plant biomass and community composition in one year. Synthesis. Our results indicate animals in green and brown food webs can alter plant litter nitrogen release without any significant nutrient recycling effect on plant growth. Considering the availability of external and soil‐based nitrogen sources, as well as the current plant community and microbial biomass, may be critical to determining when plant growth will respond to animal‐mediated changes in litter decomposition.

Fen restoration: defining a reference ecosystem using paleoecological stratigraphy and present-day inventories

Choosing past and present-day indicators could strengthen the reference ecosystem used for ecological restoration projects. Based on the paleoecological analysis of four peat cores and the characterization of 13 contemporary natural sites, the reference ecosystem for minerotrophic peatlands in southeastern Canada is composed of two broad categories of plant assemblages described as tall-sedge and Sphagnum–Thuja/brown moss. In paleoecological peat profiles, tall-sedge communities were found at the transition between aquatic and terrestrial, and were associated with high graminoid production, riverine peatlands, and elevated water table in the present-day analyses. Sphagnum–Thuja communities resemble the present-day vegetation found in natural basin type peatlands. Except for Sphagnum warnstorfii Russ., these communities, with high taxonomical diversity, contain more generalist species from boreal peatland vegetation, such as Rhododendron groenlandicum (Oeder) Kron & Judd, Thuja occidentalis L., Linnaea borealis L., and Maianthemum trifolium L. They grow in dryer or shady habitats such as hummocks or forest understory. The importance of brown mosses was revealed by paleoecological analysis. Overall, findings from both approaches are complementary: paleoecological stratigraphy informs us about past ecosystem dynamics, while present-day inventories allow us to define current plant communities and their major environmental characteristics. The range of variability of vegetation and environmental variables found in these studies are essential tools for fen restoration projects.

Rapid spread of a fleshy-fruited species in abandoned subalpine meadows - formation of an unusual forest belt in the eastern Carpathians

In recent decades, most subalpine hay meadows and pastures have been abandoned, and trees have been recolonizing these sites where forest existed before agricultural activity. This study examined how woody vegetation, dominated by the deciduous fleshy-fruited tree Sorbus aucuparia (rowan), expanded on subalpine meadows in the Western Bieszczady Mountains (eastern Carpathians, Poland) after the cessation of agricultural use. The aims were to determine the abundance of rowan in the woody vegetation, to estimate the rate of rowan expansion in the studied area, and to characterize the variability of rowan stands and growth forms. Rowan dominated the current plant community of abandoned subalpine meadows, though this species is not considered a rapid colonizer of open areas and was not frequent in the uppermost forest belt before the colonization. The whole area was encroached by rowans in a very short period of time 60-70 years ago. Rowan tree density was similar throughout the elevational gradient but the growth form changed, becoming more shrub-like with increasing elevation. Rowan stands will likely be the main element of the subalpine belt in this region in the upcoming decades. At present, no tree species can be considered a rapid successor to rowan in the area.

A Survey of the New River Aquatic Plant Community in Response to Recent Triploid Grass Carp Introductions into Claytor Lake, Virginia

Abstract Aquatic plant communities play critical roles in the form and function of stream ecosystems. In this study, we surveyed the aquatic-plant community along a 39-km reach of the New River, VA, in response to triploid Ctenopharyngodon idella (Grass Carp) stockings to control Hydrilla verticillata (Hydrilla) in Claytor Lake. We utilized drift-net sampling methods and visual observations to document the current plant community in this reach. Nine of 12 aquatic plant species identified in our survey have been documented as preferred forage for Grass Carp. These findings may indicate that migrating Grass Carp could alter the plant community in this reach. We recommend continued monitoring of this system to characterize any future effects of Grass Carp herbivory.

Are Cichorieae an indicator of open habitats and pastoralism in current and past vegetation studies?

Cichorieae, one of the six tribes of the sub-family Cichorioideae (Asteraceae), produces a well-recognisable fenestrate pollen type. In the Mediterranean area, the significance of high percentages of Cichorieae pollen from archaeological layers is still questioned. We assessed the presence of Cichorieae as indicators of open habitats and pasturelands in current plant communities by comparing data on vegetation composition with pollen spectra from two Hellenistic sites of Basilicata (southern Italy): Difesa San Biagio in the low valley of the river Bradano and Torre di Satriano in the Lucanian Apennines. We also analysed the pollen morphology bringing to the discrimination of size classes within the fenestrate type of Cichorieae. Pollen spectra from the considered archaeological sites have low forest cover (7% on average); Asteraceae and Poaceae are prevalent; Cichorieae account to ca. 23%; coprophilous fungal spores are varied and present high concentrations. In surface soil samples collected near the sites, Cichorieae pollen is about 12%. In current vegetation types, an increasing abundance of Cichorieae was observed from salt marshes, forests and shrublands to open habitats and grasslands. This is coherent with the actual land cover around the study sites and the findings of the archaeological sample that point to an open landscape dominated by pastures and cultivated fields. Our integrated approach confirmed that today Cichorieae are common in secondary pastures and in some types of primary open habitats of southern Italy: hence, high percentages of this pollen can be considered a good indicator of these habitats even in past environment reconstructions.

A comparison of the herbicide tolerances of rare and common plants in an agricultural landscape

Declining plant biodiversity in agroecosystems has often been attributed to escalating use of chemical herbicides, but other changes in farming systems, including the clearing of seminatural habitat fragments, confound the influence of herbicides. The present study introduces a new approach to evaluate the impacts of herbicide pollution on plant communities at landscape or regional scales. If herbicides are in fact a key factor shaping agricultural plant diversity, one would expect to see the signal of past herbicide impacts in the current plant community composition of an intensively farmed region, with common, successful species more tolerant to widely used herbicides than rare or declining species. Data from an extensive field survey of plant diversity in Lancaster County, Pennsylvania, USA, were compared with herbicide bioassay experiments in a greenhouse to test the hypothesis that common species possess higher herbicide tolerances than rare species. Five congeneric pairs of rare and common species were treated with 3 commonly used herbicide modes of action in bioassay experiments, and few significant differences were found in the tolerances of rare species relative to common species. These preliminary results suggest that other factors beyond herbicide exposure may be more important in shaping the distribution and abundance of plant species diversity across an agricultural landscape.

Seed supply and the regeneration potential for plantations and shrubland in southern China

Assessing the characteristics of seed supply will be vital to better understand the dynamics of forest regeneration. In this study, we surveyed the aboveground vegetation, the seed rain, the seed bank, and natural seedling emergence in four typical 24-year-old plantations (eucalyptus, mixed-native, mixed-legume, and mixed-conifer) and a naturally successioned shrubland in southern China. The dominant species in the understory were similar among the five plant communities. The seed rain and the seed bank were dominated by shrubs and herbs but indigenous tree species were rare. Species that were common to all five-plant communities represented a great proportion of the seeds in the seed rain and seed bank. The seed rain consisted mostly of seeds derived from the local plant community. Seed abundance was greater in the seed bank than in the seed rain, and species richness was greater in the seed bank and in the corresponding plant community than in the seed rain. Species composition similarity between the seed rain, the seed bank, and the aboveground vegetation was low, because the seed rain contained much fewer species, and the seed bank and aboveground vegetation contained many different species, respectively. These findings indicate that both the seed rain and the seed bank play important roles in providing seeds for plant recruitment in the understory, but the seed bank contributes more than the current seed rain to the diversity of recruited plants. The current plant community has little impact on the qualitative composition of the seed rain and seed bank. Based on these data, it appears that succession to the desired zonal, mature forest community is unlikely to result from seeds in the seed rain or seed bank. Lack of seed availability of desired zonal mature forest species is the main bottleneck currently limiting succession in the plantations. Reintroduction of late-successional species could facilitate the desired succession.

The impact of ecological differentiation and dispersal limitation on species turnover and phylogenetic structure of inselberg's plant communities

Phylogenetic structure analysis is a novel way to address the relative importance of stochastic and deterministic processes governing species assemblages. Here we investigate the phylogenetic structure of the vegetation of inselbergs located in the African rain forest. Inselbergs combine strong ecological gradients at the local scale due to soil depth variation and insular properties at the regional scale. They are therefore ideal models to assess the influence of ecological sorting and dispersal limitation on the phylogenetic structure of plant communities. On 21 inselbergs separated by up to 200 km where five microhabitat‐types were recognized, 311 vegetation plots were inventoried. We found that floristic similarity between plots depended on both microhabitat differentiation and spatial distance, while phylogenetic clustering (i.e. excess of phylogenetic similarity between species from a same plot) only appeared between plots from differentiated microhabitats and increased with ecological distance. Within a microhabitat‐type, the absence of phylogenetic structure between inselbergs indicates that species turnover is probably due to dispersal limitation rather than to regional‐scale variations in environmental factors. Hence, phylogenetic structure analysis can help disentangle the effects of ecological sorting and dispersal limitation on species assemblages. To estimate the time‐scale of the processes generating the phylogenetic structure, we investigated how lineage similarity changes with increasing age in the phylogenetic tree. High lineage similarity levels between ecologically very differentiated plots were only reached at the proximity of the root of the phylogenetic tree. This was observed even when considering only plots sharing no species and indicates that phylogenetic niche conservatism has been important for generating the observed phylogenetic structure. Hence, ancient diversification exerts an impact on the assembly of current plant communities.

Historical influences dominate the composition of regenerating plant communities in abandoned citrus groves in north-central Florida

The question of what determines plant community composition is fundamental to the study of plant community ecology. We examined the relative roles of historical land use, landscape context, and the biophysical environment as determinants of plant community composition in regenerating citrus groves in north-central Florida. Results were interpreted in light of plant functional traits. Herbaceous and woody plants responded differently to broad-scale variables; herbs correlated most strongly with surrounding land cover at a scale of 8 km, while the only significant determinant of woody species distributions was local land use history. There were significant correlations between herbaceous species and spatial context, habitat isolation, environmental variables, and historical variables. Partial Mantel tests indicated that each variable provided a unique contribution in explaining some of the variation in the herbaceous dataset. The correlation between woody plants and local historical variables remained significant even with other effects corrected for. In the herbaceous community, species composition was linked to functional traits much as expected from classical theory. While spatial influences in our study system are important for both woody and herbaceous plants, the primary determinant of plant community composition in regenerating citrus groves is historical land use. Our results suggest that the fine-scale mechanisms of local competition, tolerance and facilitation invoked by many classical studies may ultimately be less important than land use history in understanding current plant community composition in regenerating agricultural areas.

Variations in net ecosystem exchange of carbon dioxide in a boreal mire: Modeling mechanisms linked to water table position

In mires, which occupy large areas of the boreal region, net ecosystem CO2 exchange (NEE) rates vary significantly over various timescales. In order to examine the effect of one of the most influencing variables, the water table depth, on NEE the general ecosystem model GUESS‐ROMUL was modified to predict mire daily CO2 exchange rates. A simulation was conducted for a lawn, the most common microtopographical feature of boreal oligotrophic minerotrophic mires. The results were validated against eddy covariance CO2 flux measurements from Degerö Stormyr, northern Sweden, obtained during the period 2001–2003. Both measurements and model simulations revealed that CO2 uptake was clearly controlled by interactions between water table depth and temperature. Maximum uptake occurred when the water table level was between 10 and 20 cm and the air temperature was above 15°C. When the water table was higher, the CO2 uptake rate was lower, owing to reduced rates of photosynthetic carbon fixation. When the water table was lower, NEE decreased owing to the increased rate of decomposition of organic matter. When the water table level was between 10 and 20 cm, the NEE was quite stable and relatively insensitive to both changes within this range and any air temperature changes above +15°C. The optimal water table level range for NEE corresponds to that characteristic of mire lawn plant communities, indicating that the annual NEE will not change dramatically if climatic conditions remain within the optimal range for the current plant community.

A methodology for assessing departure of current plant communities from historical conditions over large landscapes

Fire frequency and severity, and vegetation composition and structure have been altered across much of North America during the past century because of fire exclusion and other land management practices. The cumulative results are now recognized to be partly responsible for dramatic increases in wildland fire severity and declines in ecosystem health. In response, the Departments of Agriculture and Interior are developing a strategy for reducing fire hazard and restoring forest health. A key component of this strategy is the assessment of the departure of current plant communities from historical conditions. Assessing departure is difficult because of limited spatial coverage of data on successional class distribution prior to European settlement (historical) conditions. This article discusses a two-step approach to the problem that first generates data on historical vegetation composition and structure by simulating succession and disturbance processes under historical conditions, and then measures departure by comparing these simulated data to an observation on current conditions. The data are observations on high-dimensional multinomial categorical variables, and pose several problems that limit the usefulness of conventional statistical methods. We propose a method that constructs a linear approximation of the current observation vector from the simulated historic data, and measures the departure of the current observation vector by the length of the residual error vector. A simulation study indicates that this departure measure is substantially more sensitive than conventional outlier detection methods. Our methodology is demonstrated using a pilot region encompassing lands in Utah.