Plant productivity and nitrogen gas fluxes in a tallgrass prairie landscape

We explored relationships between plant productivity and annual fluxes of nitrogen (N₂) and nitrous oxide (N₂O) in a tallgrass prairie landscape in central Kansas. Our objective was to develop predictive relationships between these variables that could be used in conjunction with remote sensing information on plant productivity to produce large-area estimates of N gas fluxes. Our hypothesis was that there are inherent relationships between plant productivity and N gas fluxes in tallgrass prairie because both are controlled by water and N availability. The research was carried out as part of a multi-investigator project, the First ISLSCP Field Experiment (FIFE, ISLSCP = International Satellite Land Surface Climatology Program), directed toward the use of remote sensing to characterize land-atmosphere interactions. Fluxes of N₂ (denitrification) and N₂O were measured using soil core techniques. Estimates of annual flux were produced by temporal extrapolation of measured rates. Annual aboveground net primary productivity (ANPP) was estimated from measurements of the maximum standing crop of plant biomass. There were strong relationships between ANPP and N gas fluxes, and between a satellite remote sensing-based index of plant productivity (normalized difference vegetation index, NDVI) and gas fluxes. We used these relationships to convert images of NDVI into images of N gas fluxes for one 83 ha watershed and for the entire 15 by 15 km FIFE site. These images were used to compute mean landscape gas fluxes (0.62 g N m^-2 y^-1 for N₂, 0.66 g N m^-2 y^-1 for N₂O) and total N gas production for the two areas. Our flux and production values are useful for comparison with values produced by simulation models and site-specific studies, and for assessing the significance of N gas production to ecosystem and landscape scale processes related to nutrient cycling, water quality and atmospheric chemistry.     

Modeling seasonal dynamics of small fish cohorts in fluctuating freshwater marsh landscapes

Small-bodied fishes constitute an important assemblage in many wetlands. In wetlands that dry periodically except for small permanent waterbodies, these fishes are quick to respond to change and can undergo large fluctuations in numbers and biomasses. An important aspect of landscapes that are mixtures of marsh and permanent waterbodies is that high rates of biomass production occur in the marshes during flooding phases, while the permanent waterbodies serve as refuges for many biotic components during the dry phases. The temporal and spatial dynamics of the small fishes are ecologically important, as these fishes provide a crucial food base for higher trophic levels, such as wading birds. We develop a simple model that is analytically tractable, describing the main processes of the spatio-temporal dynamics of a population of small-bodied fish in a seasonal wetland environment, consisting of marsh and permanent waterbodies. The population expands into newly flooded areas during the wet season and contracts during declining water levels in the dry season. If the marsh dries completely during these times (a drydown), the fish need refuge in permanent waterbodies. At least three new and general conclusions arise from the model: (1) there is an optimal rate at which fish should expand into a newly flooding area to maximize population production; (2) there is also a fluctuation amplitude of water level that maximizes fish production, and (3) there is an upper limit on the number of fish that can reach a permanent waterbody during a drydown, no matter how large the marsh surface area is that drains into the waterbody. Because water levels can be manipulated in many wetlands, it is useful to have an understanding of the role of these fluctuations.     

Simulated productivity of heterogeneous patches in Southern African savanna landscapes using a canopy productivity model

A daily model of terrestrial productivity is used to simulate the annual productivity of heterogeneous vegetation structure at three savanna/woodland sites along a large moisture gradient in southern Africa. The horizontal distributions of vegetation structural parameters are derived from the three-dimensional canopy structure generated from detailed field observations of the vegetation at each site. Rainfall and daily climatic data are used to drive the model, resulting in a spatially explicit estimate of vegetation productivity in 100 m² patches over an area 810,000 m² (8,100 patches per site). Production is resolved into tree and grass components for each subplot. The model simulates the relative contribution of trees and grasses to net primary productivity (NPP) along the rainfall gradient. These simulated production estimates agree with previously published estimates of productivity in southern African savannas. Water-use efficiency of each site is directly related to the structural composition of the site and the differing water-use efficiencies for tree and grass functional types. To assess the role of spatial scale in governing estimates of vegetation productivity in heterogeneous landscapes, spatial aggregation is performed on the canopy mosaic at the northern-most (wettest) site for 625 m², 2500 m² and 5625 m² resolutions. These simulations result in similar overall patterns of average NPP for both trees and grasses, but drastically reduced distributions of productivity due to reduced structural heterogeneity. In particular, the aggregation of the detailed spatial mosaic to coarser resolutions is seen to eliminate information regarding demographic processes such as regeneration and mortality, and the dependence of grass productivity on over-story density. These results indicate that models of system productivity in savanna/woodland ecosystems must retain high spatial resolution to adequately characterize multi-year structural responses and to accurately represent the contribution of grass biomass to overall ecosystem production.This revised version was published online in May 2005 with corrections to the Cover Date.     

Quantifying the nitrogen retention capacity of natural wetlands in the large-scale drainage basin of the Baltic Sea

We estimate the nitrogen retention capacity of natural wetlands in the 1.7 million km² Baltic Sea drainage basin, using a wetland GIS data base. There are approximately 138,000 km² of wetlands (bogs and fens) in the Baltic Sea drainage basin, corresponding to 8% of the area. The input of nitrogen to natural wetlands from atmospheric deposition was estimated to 55,000–161,000 ton y¹. A map of the deposition of both wet and dry nitrogen is presented. The input from the human population was estimated to 255,000 ton y¹ in terms of excretory release in processed sewage water. There may also be leakage from forests and agricultural land into the wetlands. Due to lack of data on hydrology and topography, such potential nitrogen sources are not accounted for here. The capacity of the wetlands to retain the atmospheric deposition of nitrogen was estimated to 34,000–99,000 ton y¹. The potential retention by wetlands was estimated to 57,000–145,000 ton y¹ when the nitrogen input from the human population was added. If drained wetlands were to be restored and their area added to the present wetland area, the nitrogen retention capacity was estimated to increase to 196,000–261,000 ton y¹. Our results indicate that existing natural wetlands in the Baltic Sea drainage basin annually can retain an amount of nitrogen which corresponds to about 5–13% of annual total (natural and anthropogenic) nitrogen emissions entering the Baltic Sea. The ecosystem retention service performed by wetlands accounts for a substantial nitrogen removal, thereby reducing the eutrophication of the Baltic Sea.     

Reponses of mamey sapote (Pouteria sapota) trees to continuous and cyclical flooding in calcareous soil

Physiological and growth responses of ‘Pantin’ and ‘Magana’ mamey sapote (Pouteria sapota) trees to continuous and cyclical flooding were studied in a series of experiments. Trees were grown in containers in a very gravelly loam soil and were subjected to continuous flooding of the root zone for 30–66 days (Experiments 1 and 2) or alternating flooding–unflooding cycles for 50 days (Experiments 3–5). For all experiments, the control treatment consisted of nonflooded trees. Net CO₂ assimilation (A) and stomatal conductance (g_s) decreased within 3 days of continuous flooding and internal CO₂ concentration was significantly higher in leaves of flooded than nonflooded plants. In the cyclic flooding experiments, trees were flooded in 3- to 6-day cycles and then unflooded for the same time periods. Stomatal conductance and A decreased within 3 days of flooding, leaf epinasty occurred between days 5 and 10, leaf senescence and abscission occurred between days 15 and 30, and branch dieback and tree death occurred between days 30 and 60. Three cycles of 3-day flooding and 3-day recovery of trees had little effect on leaf gas exchange of ‘Magaña’ trees. Similarly, ‘Pantin’ trees survived 3 cycles of 6 days of flooding interspersed with 3–6 days of recovery despite consistent decreases in g_s and A during flooding. Stomatal conductance and A of both mamey sapote cultivars decreased within a few days of flooding and this species appears to have intermediate flooding tolerance compared with other tropical fruit crops based on tree survival.     

Optimization of saline water level for sustainable Barnea olive and oil production in desert conditions

The main aim of the present study is to evaluate the effects of long-term irrigation with water at three different levels of salinity on Barnea olive trees with a view to optimize vegetative growth, productivity and oil quality. The study was carried out on trees growing in the Negev desert of Israel. The intermediate salinity (4.2 dS m⁻¹ EC) inhibited growth significantly only in the first year after planting, while from the second year onward retardation of vegetative growth as compared with the control treatment (1.2 dS m⁻¹ EC) was slight and non-significant. However, the high salinity (7.5 dS m⁻¹ EC) retarded tree growth significantly. Furthermore, the intermediate treatment led to significant increase in tree productivity relative to the other treatments, and also to an increase in olive oil yield. In conformity with the greater number of fruits produced, the olives of the 4.2 dS m⁻¹ EC treatment tended to be smaller.No significant differences were found between saline- and control-water irrigated trees in terms of olive oil basic quality parameters, such as free fatty acids, peroxide value, and fatty acid profile. The saline treatments increased the levels of certain antioxidant components (polyphenols and Vitamin E) in the oil extracted from the olives as compared with the control. The data obtained clearly show that, of the three water salinities tried, the moderate level of 4.2 dS m⁻¹ EC is best suited for production of olives and olive oil under the conditions prevailing in the central Negev, at least during the first 9 years from planting. The sustainability of Barnea cultivation under moderately saline water is discussed.     

Natural bee-honey based biostimulants confer salt tolerance in onion via modulation of the antioxidant defence system

Plant biostimulants are an emerging trend of crop management products which can enhance nutrient uptake, plant growth and productivity under various abiotic stresses. The ability of diluted bee-honey extract (DHE) to enhance the tolerance of onion plants to salinity stress has been investigated. Two-season field experiments were consecutively conducted in 2015/16 and 2016/17 to study the effect of 25–50 g/L DHE foliar application on growth, yield, physio-biochemical attributes, and antioxidative defence of two onion cultivars (i.e. Giza^Red and Giza-20) grown on a saline soil (EC = 8.81 dS m^?1). Results exhibited that DHE significantly increased biomass production, bulb yield and water use efficiency, leaf photosynthetic and pigments contents. Additionally, DHE applicati\on also improves osmoprotectants, membrane stability index (MSI) and relative water content (RWC), and enzymatic and non-enzymatic antioxidants of both onion cultivars in comparison with untreated control plants. In general, both cultivars showed a similar response to the DHE. Results of this study highlighted the potential impact of DHE as a promising plant bio-stimulant for overcoming the harmful effects of soil salinity stress by increasing the activity of plant antioxidative defence systems     

Ethylene production as an indicator of stress conditions in hydroponically-grown strawberries

As a soilless system, hydroponics eliminates competing weeds and soil-born pests while conserving water and providing conditions that can be quickly altered to suit specific crops. However, stress-induced physiological conditions may arise within the system from factors such as mechanical injury, pests, or inconsistent nutrient flow rates that result in some plants receiving too much or too little water. Most abiotic stress conditions result in increased production of the plant hormone ethylene. High levels of ethylene inhibit growth, cause premature ripening, and induce the onset of senescence, potentially reducing the productivity of hydroponically-grown crops. In this study, we demonstrate that assessing ethylene levels from leaves of hydroponically-grown strawberry plants can be used as an early indicator of stress conditions. Our results demonstrate that there is no significant correlation between ethylene production and temperatures ranging from 15 to 37 °C or with light intensities ranging from 63 to 1500 μmol m⁻² s⁻¹. However, an increase in ethylene production tended to be positively correlated with sampling time; levels were higher during midday compared to early morning or later afternoon. The daily ethylene fluctuations under greenhouse conditions due to sampling time, light intensity, or temperature changes were not significantly high enough to indicate stress conditions. Overall, three system analyses showed altered ethylene production in plants farthest from the pump supplying the nutrient solution. This effect was interpreted to be caused by excess accumulation of nutrient solution around the plant roots, causing increased ethylene production in the leaves. Our results indicate that different watering patterns, manifested as pump pressure or drainage control, was the more difficult component to control in the design of these hydroponic systems. For example, in one system, an increase in ethylene production was measured for the position farthest from the pump, and resulted in those plants having a lower number of flowers and significantly reduced overall plant radii relative to the system average. In a separate experiment, plants from trays that had been flooded for 24 h showed a significant decrease in the plant radii and number of leaves and flowers 1 month after the flooding treatment. We conclude that system-wide ethylene measurements can be used to identify stressed plants within hydroponic systems. This type of analysis would be especially useful as an indicator of general stress conditions no matter the cause, identifying locations that may result in lower plant productivity.     

High carbon losses from established growing sites delay the carbon sequestration benefits of street tree plantings – A case study in Helsinki,Finland

We assessed the net carbon (C) sequestration dynamics of street tree plantings based on 10 years of measurements at two case study sites each with different tree species in Helsinki, Finland. We assessed C loss from tree soils and tree C accumulation, tested the applicability of pre-existing growth and biomass equations against observations, and estimated the time point for the beginning of net C sequestration for the studied street tree plantings. The tree woody biomass C accumulation in the first 10 years after planting was 18–32 kg per tree. At the same time the C loss from the growth media was at least 170 kg per growth media volume (25 m³) per tree. If this soil C loss was accounted for, the net C sequestration would begin, at best, approximately 30 years after planting. Biomass equations developed for traditional forests predicted more stem biomass and less leaf and branch biomass than measured for the species examined, but total aboveground biomass was generally well predicted.     

Using indicators of land-use development intensity to assess the condition of coastal wetlands in Hawai‘i

Although wetland condition assessment procedures have been developed, validated, and calibrated in the continental United States, they have not yet been fully developed or field-tested for wetlands in Hawai‘i. In order to address the need for comprehensive assessment methods for Hawaiian coastal wetlands, our research compared three indicators of landscape condition (landscape development intensity, road density, and forest cover) with wetland condition as measured by rapid assessment methods (RAM) and detailed field data collected on soil and water quality. We predicted that wetlands located in the least developed landscapes would have more nutrient rich soils, yet lower nutrient levels in the surface water, and would receive the highest rapid assessment scores. The hypotheses of our study were generally supported. However, while the correlations between landscape variables and δ¹⁵N isotopes and CRAM scores were relatively strong, the correlations between the landscape indicators and the other Level II and III field indicators were not very strong. These results suggest that further calibration and refinement of metrics is needed in order to more accurately assess the condition of Hawaiian coastal wetlands. A more detailed land use map, in addition to more comprehensive assessments of wetland water quality and biotic integrity would likely improve the relationships between indicators of landscape condition and wetland condition. Nonetheless, our research demonstrated that landscape analysis at larger scales (1,000 m buffers and watersheds) could provide managers with valuable information on how regional stressors may be affecting wetland water quality (measured as δ¹⁵N in plant tissue) as well as overall wetland condition (RAM scores).     

Predicting canopy and trunk cross-sectional area of silver linden (Tilia tomentosa) in confined planting cutouts

Urbanized land is characterized by the dominance of paved surfaces. Increasing tree canopy in urbanized areas has been identified as an effective way to reduce stormwater runoff, sequester carbon, improve air and water quality, and otherwise mitigate the environmental impacts and increase the livability of cities. However, attaining sufficient tree canopy in urban areas remains an elusive goal. Site design characteristics such as cutout size may limit urban tree growth and complicate efforts to predict future canopy, especially in highly paved systems such as parking lots. We studied 25 silver lindens (Tilia tomentosa Moench) grown for 14 years at one site, in pavement cutouts of various sizes. Regression analysis, even on these limited data, indicated a strong relationship between tree size and canopy projection area and unpaved soil surface area, but not soil depth. Cutout size explained 70% of the variability in tree canopy projection area and 77% of the variability in trunk cross-sectional area. The addition of other variables, such as soil bulk density, did not improve the model. Trees growing in parking lot cutouts <5.3 m² attained only limited size, regardless of the level of soil compaction. In larger cutouts, however, increases in soil bulk density from 1.1 to 1.5 Mg/m³ were associated with a 70% reduction in trunk cross-sectional area. In order to create urban sites with a sustainable tree canopy, site design must provide large areas of uncompacted soil for trees and protect this soil from compaction during use. Urban tree growth models that incorporate cutout characteristics are needed to predict future canopy area with confidence.     

Estimating regional forest productivity and water yield using an ecosystem model linked to a GIS

We used the PnET-II model of forest carbon and water balances to estimate regional forest productivity and runoff for the northeastern United States. The model was run at 30 arc sec resolution (approximately 1 km) in conjunction with a Geographic Information System that contained monthly climate data and a satellite-derived land cover map. Predicted net primary production (NPP) ranged from 700 to 1450 g m² yr¹ with a regional mean of 1084 g m² yr¹. Validation at a number of locations within the region showed close agreement between predicted and observed values. Disagreement at two sites was proportional to differences between measured foliar N concentrations and values used in the model. Predicted runoff ranged from 24 to 150 cm yr¹with a regional mean of 63 cm yr¹. Predictions agreed well with observed values from U.S. Geologic Survey watersheds across the region although there was a slight bias towards overprediction at high elevations and underprediction at lower elevations.Spatial patterns in NPP followed patterns of precipitation and growing degree days, depending on the degree of predicted water versus energy limitation within each forest type. Randomized sensitivity analyses indicated that NPP within hardwood and pine forests was limited by variables controlling water availability (precipitation and soil water holding capacity) to a greater extent than foliar nitrogen, suggesting greater limitations by water than nitrogen for these forest types. In contrast, spruce-fir NPP was not sensitive to water availability and was highly sensitivity to foliar N, indicating greater limitation by available nitrogen. Although more work is needed to fully understand the relative importance of water versus nitrogen limitation in northeastern forests, these results suggests that spatial patterns of NPP for hardwoods and pines can be largely captured using currently available data sets, while substantial uncertainties exist for spruce-fir.     

Private residential urban forest structure and carbon storage in a moderate-sized urban area in the Midwest,United States

In conjunction with urbanization and its importance as a major driver of land-use change, increased efforts have been placed on understanding urban forests and the provisioning of ecosystem services. However, very little research has been conducted on private property and little is known about the structure and function of privately owned urban forests. This research examines the structure of and carbon storage services provided by private residential urban forests in a moderate-sized Midwestern city. The primary research questions are as follows: What is the structure of private urban forests, and how does it vary across parcels? How much carbon is stored in tree and soil pools of private urban forests, and how does carbon vary across parcels? Ecological inventories were conducted on 100 residential parcels within 14 Neighborhood and Homeowners Associations of varying size and development age. Tree species richness, diversity, density, and diameter distribution were determined on a per parcel basis and for the entire tree population sampled. Further, tree and soil carbon storage were determined for each parcel. Results of this research demonstrated large variability in per-parcel tree metrics. Twelve of the parcels sampled had two or fewer trees, while eleven had greater than 50 trees. Further, tree carbon storage ranged from no carbon to 11.22 kg C m^?2. Alternatively, soil carbon storage was less variable and averaged 4.7 kg C m^?2, approximately 1.9 times higher than the average carbon stored in trees (2.5 kg C m^?2). Management efforts aimed at maintaining or enhancing carbon storage and other ecosystem services should focus on both soil protection and maximizing services in living biomass. Our results demonstrate that sustaining tree-produced ecosystem services requires maintenance of large old trees and species diversity, not only in terms of relative abundance, but also relative dominance, and in combination, species–specific size distributions.     

Irrigation of Myrtus communis plants with reclaimed water: morphological and physiological responses to different levels of salinity

Summary

The influence of irrigation with different sources of reclaimed water on physiological and morphological changes in Myrtus communis plants was investigated to evaluate their adaptability to such conditions. M. communis plants, growing in a growth chamber, were subjected to four irrigation treatments over 4 months (120 d): a control [tap water (0.8 dS m^–1), leaching 10% (v/v) of the applied water] and three reclaimed water irrigation treatments, namely 1.5 dS m^–1 leaching 25% (v/v) of the applied water (RW1), 4.0 dS m^–1 leaching 40% (v/v) of the applied water (RW2), and 8.0 dS m^–1 leaching 55% (v/v) of the applied water (RW3). After treatment, all plants were irrigated with tap water, as for the control plants, for a further 2 months (60 d). At the end of the first period (4 months), none of the myrtle plants showed any adverse change in biomass and the average total dry weight (DW) increased by 53% in treatment RW2. However, at the end of the treatment and recovery period (180 d), accumulations of Cl^– ions, and especially Na⁺ ions, negatively affected the growth of all RW3 plants. Plants irrigated with all three reclaimed water samples had increased difficulty in taking-up water from the substrate (i.e., they had lower leaf water potential and relative water content values). RW2 plants showed a better response in their gas exchange parameters. The use of reclaimed water decreased leaf K⁺/Na⁺ and Ca²⁺/Na⁺ ratios, but no chlorosis or necrosis were observed. The three reclaimed water samples had different effects on the myrtle plants depending on the specific chemical properties of the water. Leaching was found to be important to minimise the negative effects of salinity in the irrigation water.     

Wetlands, carbon, and climate change

Wetland ecosystems provide an optimum natural environment for the sequestration and long-term storage of carbon dioxide (CO₂) from the atmosphere, yet are natural sources of greenhouse gases emissions, especially methane. We illustrate that most wetlands, when carbon sequestration is compared to methane emissions, do not have 25 times more CO₂ sequestration than methane emissions; therefore, to many landscape managers and non specialists, most wetlands would be considered by some to be sources of climate warming or net radiative forcing. We show by dynamic modeling of carbon flux results from seven detailed studies by us of temperate and tropical wetlands and from 14 other wetland studies by others that methane emissions become unimportant within 300 years compared to carbon sequestration in wetlands. Within that time frame or less, most wetlands become both net carbon and radiative sinks. Furthermore, we estimate that the world’s wetlands, despite being only about 5–8 % of the terrestrial landscape, may currently be net carbon sinks of about 830 Tg/year of carbon with an average of 118 g-C m^?2 year^?1 of net carbon retention. Most of that carbon retention occurs in tropical/subtropical wetlands. We demonstrate that almost all wetlands are net radiative sinks when balancing carbon sequestration and methane emissions and conclude that wetlands can be created and restored to provide C sequestration and other ecosystem services without great concern of creating net radiative sources on the climate due to methane emissions.     

Response of tomato plants to saline water as affected by carbon dioxide supplementation. I. Growth,yield and fruit quality

Summary

Tomato plants (Lycopersicon esculentum (L.) Mill. cv. F144) were irrigated with low concentrations of mixed salts; the highest level (E.C. 7 dS m^–1) simulated conditions used to produce quality tomatoes in the Negev highlands. CO₂ enrichment (to 1200.mmol mol^–1, given during the daytime) increased plant growth at the early stage of development. However, later growth enhancement was maintained only when combined with salt stress. In the absence of CO₂ supplementation, overall growth decreased with salt (7 dS m^–1) to 58% and fresh biomass yields to 53% of the controls. However, under elevated CO₂ concentrations total plant dry biomass was not reduced by salt stress. CO₂ enrichment of plants grown with 7 dS m^–1 salt increased total fresh fruit yields by 48% and maintained fruit quality in terms of total soluble salts, glucose and acidity. Fruit ripening was about 10.d earlier under CO₂ enrichment, regardless of salinity treatment. It is suggested that a combined utilization of brackish water and CO₂ supplementation may enable the production of high-quality fruits without incurring all the inevitable loss in yields associated with salt treatment.     

Unmanned aerial systems for modelling air pollution removal by urban greenery

Urban greenery plays an important role in reducing air pollution, being one of the often-used, nature-based measures in sustainable and climate-resilient urban development. However, when modelling its effect on air pollution removal by dry deposition, coarse and time-limited data on vegetation properties are often included, disregarding the high spatial and temporal heterogeneity in urban forest canopies. Here, we present a detailed, physics-based approach for modelling particulate matter (PM₁₀) and tropospheric ozone (O₃) removal by urban greenery on a small scale that eliminates these constraints. Our procedure combines a dense network of low-cost optical and electrochemical air pollution sensors, and a remote sensing method for greenery structure monitoring derived from Unmanned aerial systems (UAS) imagery processed by the Structure from Motion (SfM) algorithm. This approach enabled the quantification of species- and individual-specific air pollution removal rates by woody plants throughout the growing season, exploring the high spatial and temporal variability of modelled removal rates within an urban forest. The total PM₁₀ and O₃ removal rates ranged from 7.6 g m^-2 (PM₁₀) and 12.6 g m^-2 (O₃) for mature trees of Acer pseudoplatanus to 0.1 g m^-2 and 0.1 g m^-2 for newly planted tree saplings of Salix daphnoides. The present study demonstrates that UAS-SfM can detect differences in structures among and within canopies and by involving these characteristics, they can shift the modelling of air pollution removal towards a level of individual woody plants and beyond, enabling more realistic and accurate quantification of air pollution removal. Moreover, this approach can be similarly applied when modelling other ecosystem services provided by urban greenery.     

Differential flood stress resistance of two almond cultivar* based on survival,growth and water relations as stress indicators

SUMMARY

Potted almond trees (Amygdalus communis L.) of the two cvs Ramillete and Garrigues were submitted to two treatments: non-flooded (control) and flooded for 7 d in June 1991 under field conditions. After being submerged for one week, the almond trees were removed from the water (recovery period). The effects of flooding on the growth, stomatal behaviour, leaf water potential, osmotic potential and turgor potential were examined through the experimental period. Flooding caused a reduction in root dry weight of 'Ramillete', wilting, chlorosis and necrosis of the leaves, and plant death. Epinasty occurred in treated trees, but it appeared sooner in 'Garrigues' than in 'Ramillete'. Garrigues presented the lower resistance of plant plus soil (R(p+S)) for both treatments. After the flooding period, a progressive reduction of R(p+S) values was noted in 'Garrigues'. The decrease in leaf water potential by flooding in both cultivars can be related to an increase in the resistance to water uptake. Leaf osmotic and turgor potential behaviour confirm the progressive dehydration of leaf tissues. The continous decrease in v|/" \ys and values in 'Ramillete' indicated that the severity of the damage induced by flooding stress was irreversible in this cultivar. The reduction in leaf conductance (g,) can be related to the leaf water deficit by effects of flooding, the recovery of g, for 'Garrigues' occurred 20 d after leaf water potential. The differences between the cultivars suggest that they differ in their ability to withstand flood conditions and their association is not desirable in poorly drained soils.     

The accuracy of land cover-based wetland assessments is influenced by landscape extent

Widespread degradation of wetlands has motivated the development of tools to evaluate wetland condition. The application of field-based tools over large regions can be prohibitively expensive; however, land cover data may provide a surrogate for intensive assessments, enabling rapid and cost-effective evaluation of wetlands throughout whole regions. Our goal was to determine if land cover data could be used to estimate the biotic integrity of wetlands in Alberta??s Beaverhills watershed. Biotic integrity was measured using both plant- and bird-based indices of biotic integrity (IBIs) in 45 wetlands. Land cover data were extracted from seven nested landscape extents (100?C3,000?m radii) and used to model IBI scores. Strong, significant predictions of IBI scores were achieved using land cover data from every spatial extent, even after factoring out the influence of location to address the spatial autocorrelation of land cover classes. Plant-based IBI scores were best predicted using data from 100?m buffers and bird-based IBI scores were best predicted using data extracted from 500?m buffers. Road cover or density and measures of the proportion of disturbed land were consistent predictors of IBI score, suggesting their universal importance to plant and bird communities. Simplified models using the proportion of undisturbed land were less accurate than more detailed models (reductions in r ² of 0.31?C0.32). Regardless of the level of detail in land cover classification, our results emphasize the need to optimize landscape extent for the taxonomic group of interest: an issue that is typically poorly articulated in studies reporting on the development of GIS-based assessment methods. Our results also highlight the need to calibrate models in test areas before scaling up, to ensure predictive accuracy.     

Modeling fire and landform influences on the distribution of old-growth pinyon-juniper woodland

Expansion of Pinus and Juniperus species into shrub steppe in semi-arid regions of the western United States has been widely documented and attributed in part to fire exclusion. If decreased fire frequency has been an important cause of woodland expansion, one would expect to find age structures dominated by younger trees on more fire-prone sites, with old-growth pinyon-juniper woodland limited to sites with lower fire risk. We compared current old-growth distribution with spatial models for fire risk in a 19-km² watershed in central Nevada, USA. Multiple GIS models were developed to represent fire susceptibility, according to abiotic factors representing fuels and topographic barriers to fire spread. We also developed cellular automata models to generate fire susceptibility surfaces that additionally account for neighborhood effects. Rule-based GIS models failed to predict old-growth distribution better than random models. Cellular automata models incorporating spatial heterogeneity of site productivity predicted old-growth distribution better than random models but with low accuracy, ranging from 58% agreement at the single-pixel (0.09-ha) scale to 80% agreement for 20-pixel neighborhoods. The best statistical model for predicting old-growth occurrence included the negative effect of topographic convergence index (local wetness), and the positive effects of solar insolation and proximity to rock outcrops. Results support the hypothesis that old-growth woodlands in the Great Basin are more likely to occur on sites with low fire risk. However, weak relationships suggest that old-growth woodlands have not been confined to fire-safe sites. Conservation efforts should consider the landscape context of old-growth woodlands across a broad landscape, with an emphasis on conserving landscape variability in tree age structure.