Canopy Effects Research Articles

Leaf chemical defences and insect herbivory in oak: accounting for canopy position unravels marked genetic relatedness effects.

Highly controlled experiments document that plant genetic diversity and relatedness can shape herbivore communities and patterns of herbivory. Evidence from the field is, however, scarce and inconsistent. We assessed whether a genetic signal underlying herbivory can be detected in oak woodlands when accounting for variation at smaller (within-tree) and larger (among-stand) scales. We tested relationships between tree genetic relatedness, leaf chemical defences and insect herbivory for different canopy layers in 240 trees from 15 pedunculate oak (Quercus robur) forest stands. We partitioned sources of variability in herbivory and defences among stands, individuals and branches. Leaf defences, insect herbivory and their relationship differed systematically between the upper and the lower tree canopy. When accounting for this canopy effect, the variation explained by tree genetic relatedness rose from 2.8 to 34.1 % for herbivory and from 7.1 to 13.8 % for leaf defences. The effect was driven by markedly stronger relationships in the upper canopy. Our findings illustrate that considerable effects of the host plant genotype on levels of leaf chemical defences and associated insect herbivory can be detected in natural tree populations when within-individual variation is properly accounted for.

Varying Effects of Urban Tree Canopies on Residential Property Values across Neighborhoods

As more land area than ever is covered with impermeable surfaces causing problems in the environment, urban trees are effective not only in mitigating environmental problems in the built environment and reducing buildings’ energy use, but also in increasing social and economic benefits. However, the benefits urban trees provide are not evenly distributed but rather disproportionately distributed in high-income neighborhoods. The purpose of this study is to estimate the varying effects of urban trees based on a variety of factors that have influence on tree canopy coverage, including land constraints, new developments, financial capacity to maintain trees, and neighborhood characteristics. Using a unique dataset that includes 24,203 single-family residential sales from 2007 to 2015 merged with Urban Tree Canopy (UTC), this study utilizes spatial models to empirically evaluate the impact of UTC on residential property values in the housing market. Multi-Level Mixed (MLM) models are used to capture the varying effects of tree cover, based on land constraints, new development, financial capacity, and neighborhood characteristics. The findings show the effect of trees is positive and varies across neighborhoods, and implication of the results to best achieve specific desired outcomes in tree-related policies and urban development are apparent.

Evaluation of the Effect of Dolomitic Lime Application on Pastures—Case Study in the Montado Mediterranean Ecosystem

The Montado ecosystem, predominant in the Mediterranean region, consists of poor soils, a sparse cover of cork and holm with an understory of natural biodiverse pastures, grazed by animals in extensive regime. The recommended procedure for increasing productivity of these pastures is based on the application of phosphate fertilizer. One of the main productivity-limiting factors is, however, associated with soil acidity. The objective of this work was to evaluate the simultaneous effect of the holm oak canopy and the application of dolomitic lime on the productivity and quality of a permanent biodiverse pasture, grazed by sheep, in an acid soil (pH = 5.4 ± 0.3). Pasture was monitored at the end of autumn 2018 and winter and spring 2019. The results show that amendment of soil acidity is a slow and gradual process that improves soil Mg/Mn ratio and has a positive impact on pasture productivity and quality. Pasture crude protein availability (CP, kg·ha−1), which is based on both pasture dry matter yield (kg·ha−1) and quality (CP, %), proved to be a very practical indicator of the contributions of tree canopy and soil acidity correction to the holistic management of the Montado ecosystem.

Impacts of urban roadside forest patches on NO2 concentrations

Abstract Although it is commonly believed that trees can improve air quality, recent studies have shown that such pollution mitigation can be negligible – or that tree canopies can even increase pollutant concentrations near their sources compared to adjacent treeless areas. We explored the impacts of urban roadside forest patches on the concentrations of nitrogen dioxide in summer and winter in the Helsinki Metropolitan Area, Finland, and especially investigated if canopy cover can result in increased concentrations of NO2 below the canopy. Our results, however, did not show significantly higher – or lower – NO2 concentrations underneath tree canopies compared to levels above canopies. Neither did NO2 levels at the below-canopy sampling height differ significantly between forest patches and adjacent open, treeless areas. The lack of a canopy effect may derive from the rather small size of the forest patches, and – compared to previous studies with similar design – divergent tree species composition forming a dense canopy structure. Our results corroborate previous studies that the potential ecosystem services offered by urban near-road forests are more likely due to benefits other than those related to the removal of air pollutants.

The effects of intraspecific competition and light transmission within the canopy on wheat yield in a wide-precision planting pattern

The wide-precision planting pattern has become widely used in the North China Plain as a practice for increasing wheat yield. However, the effects of tillering development and light transmission within canopy on wheat yield under different sowing widths have not been clearly described. Therefore, a two-year experiment was conducted, including four different seeding widths (6 cm, W6; 8 cm, W8; 10 cm, W10; 12 cm, W12) and the traditional planting pattern with seeding width of 4 cm (W4). The results indicated mainly positive effects by the reduced intraspecific competition, specifically all three yield components of W6 and W8 were higher than those for W4. The configurations with more than 10-cm seeding width were mainly affected by the negative effect of a relative homogeneous canopy, leading to the weakened light transmission, leaf senescence, and reduced grain number per spike. Finally, the yields of W6 and W8 were significantly higher than that of W4, whereas the yield in W12 was lower (though not significantly) than W4. In wheat production, therefore, the appropriate seeding width of 6–8 cm is recommended for farmers, whereas the too wide seeding width, with more than 10 cm, should be avoided.

Modeling the impact of 2D/3D urban indicators on the urban heat island over different seasons: A boosted regression tree approach

Understanding how complex urban factors affect the Urban Heat Island (UHI) is crucial for assessing the impacts of urban planning and environmental management on the thermal environment. This paper investigates the relationships between two-dimensional (2D) and three-dimensional (3D) factors and land surface temperatures (LST) within the Olympic Area of Beijing in different seasons, using the boosted regression tree (BRT) model. The BRT model captures the specific contributions of each urban factor to LST in each season and across a continuum of magnitudes for this factor. The results show that these relationships are complex and highly nonlinear. The four most common dominant factors are the Normalized Difference Built-up Index (NDBI), the Normalized Difference Vegetation Index (NDVI), a gravity index for parks (GPI), and average building height (BH). The most important factor in spring is NDBI, with a 45.5% contribution rate. In the other seasons, NDVI is the dominant factor, with contributions of 40% in summer, 21% in autumn, and 19% in winter. NDVI has an overall negative impact on LST in spring and summer, with a quadratic nonlinear decreasing curve, but a positive one in autumn and winter. The 2D land-use variables are most strongly related to LST in summer and spring, but 3D building-related variables have stronger impacts in colder weather. The Sky View Factor (SVF), a 3D measure of urban morphology, has also strong impacts in summer and winter. Both a building-based and a DSM-based SVFs are computed. The latter accounts for buildings, bridges, and trees. In contrast to a building-based SVF, the DSM-based SVF reduces LST when it varies between 0 and 0.75, reflecting the effects of high-density tree canopies that increase shades and evapotranspiration while blocking sky view. The marginal effect curves produced by the BRT are often characterized by thresholds. For instance, the maximal NDVI effect in summer takes place when NDVI = 0.7, suggesting that a very intense green coverage is not necessary to achieve maximal thermal results. Implications for urban planning and environmental management are outlined, including the increased use of evergreen trees that provide thermal benefits in both summer and winter.

Repeated fire shifts carbon and nitrogen cycling by changing plant inputs and soil decomposition across ecosystems

AbstractFires shape the biogeochemistry and functioning of many ecosystems, and fire frequencies are changing across much of the globe. Frequent fires can change soil carbon (C) and nitrogen (N) storage by altering the quantity and chemistry of plant inputs through changes in plant biomass and composition as well as the decomposition of soil organic matter. How decomposition rates change with shifting inputs remains uncertain because most studies focus on the effects of single fires, where transient responses may not reflect responses to decadal changes in burning frequencies. Here, we sampled seven sites exposed to different fire frequencies. In four of the sites, we intensively sampled both soils and plant communities across four ecosystems in North America and Africa spanning tropical savanna, temperate coniferous savanna, temperate broadleaf savanna, and temperate coniferous forest ecosystems. Each site contained multiple plots burned frequently for 33–61 years and nearby plots that had remained unburned over the same period replicated at the landscape scale. Across all sites, repeatedly burned plots had 25–185% lower bulk soil C and N concentrations but also 2–10‐fold lower potential decomposition of organic matter compared to unburned sites. Soil C and N concentrations and extracellular enzyme activities declined with frequent fire because fire reduced both plant biomass inputs into soils and dampened the localized enrichment effect of tree canopies. Examination of soil extracellular enzyme activities revealed that fire decreased the potential turnover of organic matter in the forms of cellulose, starch, and chitin (P < 0.0001) but not polyphenol and lignin (P = 0.09), suggesting a shift in soil C and N cycling. Inclusion of δ13C data from three additional savanna sites (19–60 years of altered fire frequencies) showed that soil C losses were largest in sites where estimated tree inputs into soils declined the most (r2 = 0.91, P < 0.01). In conclusion, repeated burning reduced C and N storage, consistent with previous studies, but fire also reduced potential decomposition, likely contributing to slower C and N cycling. Trees were important in shaping soil C and N responses across sites, but the magnitude of tree effects differed and depended on how tree biomass inputs into soil responded to fire.

Impact of seagrass establishment, industrialization and coastal infrastructure on seagrass biogeochemical sinks

The study of a Posidonia sinuosa sedimentary archive has delivered a millenary record of environmental change in Cockburn Sound (Western Australia). Ecosystem change is a major environmental problem challenging sustainable coastal development worldwide, and this study shows baseline trends and shifts in ecological processes in coastal ecosystems under environmental stress. The concentrations and fluxes of biogeochemical elements over the last 3,500 years indicate that important changes in ecosystem dynamics occurred over the last 1,000 years, in particular after ~1900's, probably related to establishment of seagrass meadows in the area and to local and regional human activities (industry and coastal development), respectively. The establishment of seagrasses ~1,000 years ago in the area of study is supported by the appearance of Posidonia fibres from ~40 cm soil depth until the core top, higher δ13C values indicating a larger contribution of seagrass-matter to the soil organic carbon pool, and increased concentration of fine sediments driven by the effect of seagrass canopy in enhancing sedimentation. The comparison of organic carbon, nutrients and metal concentrations and fluxes between pre- and post-establishment of seagrasses shows that seagrass establishment resulted in up to 9-fold increase in the soil biogeochemical sink. In ~1900's, shifts in the concentrations of metals, carbonates, organic carbon, sediment grain size, and δ13C and δ15N values of the organic matter were detected, demonstrating an alteration in seagrass ecosystem functioning following the onset of European settlement. Anthropogenic activities, and in particular the construction of a causeway in 1970's, enhanced seagrass soil organic carbon and metal accumulation rates by 36- and 39-fold, respectively, showing that human-made structures can enhance the biogeochemical sink capacity of seagrasses. Here we reconstruct the impact of human activities on seagrass ecosystem dynamics and blue carbon, which can inform local management of Cockburn Sound and seagrass conservation for climate change mitigation and adaptation.

Complementarity in nurse plant systems: soil drives community composition while microclimate enhances productivity and diversity

Nurse plants facilitate the establishment of other species under their canopies through improvement of microclimate and soil conditions beneath their canopies. We aimed to test whether the effect of nurse soil on understorey communities was independent of the canopy effect, and to what extent soil properties (i.e., nutrients, pH, moisture, temperature, aggregates) can predict understorey biomass and diversity. In a field experiment, we extracted three soil blocks (20 × 20 × 15 cm) under the canopy of eighteen Retama sphaerocarpa shrubs --grouped into small, medium, and large canopy sizes (n = 6). Soil blocks were distributed in a stratified-random design, so that each shrub received back three soil blocks (one from under each Retama canopy size). At the end of the growing season plant biomass, abundance, diversity and soil properties were recorded. Canopy and soil independently affected understorey community structure. Biomass and diversity increased under large Retama canopies irrespective of soil origin, whereas biomass was higher -and plant diversity lower- in soils from large Retama shrubs irrespective of canopy size. Biomass and species abundance were influenced more by soil than by canopy. There was, however, complementarity between these factors, as biomass increased in soils with high moisture and low temperature, while diversity was highly influenced by canopy size. Soil, rather than microclimate amelioration by the nurse, was the main driver of understorey community structure, soil moisture playing a pivotal role. This is highly relevant as drought is expected to increase in drylands worldwide.

Effects of Different Canopy and Tree Shape on Fruit Quality of Huangguogan

In this paper, the fruit of Huangguoganwas used as the test material. The fruit quality was measured according to different canopies and tree shapes. The results showed that the quality of fruits in upper canopy layers was superior to that in lower canopy layers. The difference of intrinsic quality,like soluble solids, VitaminC and sugar content,was significant in different canopies; but the difference of appearance quality was small. And on different tree shapes, the fruit quality of round-headed tree was the best, followed by the open-center tree.

The Duality of Reforestation Impacts on Surface and Air Temperature

AbstractEvidence is mounting that temperate‐zone reforestation cools surface temperature (Tsurf), mitigating deleterious effects of climate warming. While Tsurf drives many biophysical processes, air temperature (Ta) is an equally important target for climate mitigation and adaptation. Whether reductions in Tsurf translate to reductions in Ta remains complex, fraught by several nonlinear and intertwined processes. In particular, forest canopy structure strongly affects near‐surface temperature gradients, complicating cross‐site comparison. Here the influence of reforestation on Ta is assessed by targeting temperature metrics that are less sensitive to local canopy effects. Specifically, we consider the aerodynamic temperature (Taero), estimated using a novel procedure that does not rely on the assumptions of Monin‐Obukhov similarity theory, as well as the extrapolated temperature into the surface layer (Textrap). The approach is tested with flux tower data from a grass field, pine plantation, and mature hardwood stand co‐located in the Duke Forest (North Carolina, USA). During growing season daytime periods, Tsurf is 4–6 °C cooler, and Taero and near‐surface Textrap are 2–3 °C cooler, in the forests relative to the grassland. During the dormant season, daytime differences are smaller but still substantial. At night, differences in Taero are small, and near‐surface Textrap is warmer over forests than grasslands during the growing season (by 0.5 to 1 °C). Finally, the influence of land cover on Textrap at the interface between the surface and mixed layer is small. Overall, reforestation appears to provide a meaningful opportunity for adaption to warmer daytime Ta in the southeastern United States, especially during the growing season.

Effect of crop canopy and herbicide application on kochia (Bassia scoparia) density and seed production

AbstractUnderstanding the effects of crop management practices on weed survival and seed production is imperative in improving long-term weed management strategies, especially for herbicide-resistant weed populations. Kochia [Bassia scoparia (L.) A.J. Scott] is an economically important weed in western North American cropping systems for many reasons, including prolific seed production and evolved resistance to numerous herbicide sites of action. Field studies were conducted in 2014 in a total of four field sites in Wyoming, Montana, and Nebraska to quantify the impact of different crop canopies and herbicide applications on B. scoparia density and seed production. Crops used in this study were spring wheat (Triticum aestivum L.), dry bean (Phaseolus vulgaris L.), sugar beet (Beta vulgaris L.), and corn (Zea mays L.). Herbicide treatments included either acetolactate synthase (ALS) inhibitors effective on non-resistant B. scoparia or a non–ALS inhibiting herbicide effective for both ALS-resistant and ALS-susceptible B. scoparia. Bassia scoparia density midseason was affected more by herbicide choice than by crop canopy, whereas B. scoparia seed production per plant was affected more by crop canopy compared with herbicide treatment. Our results suggest that crop canopy and herbicide treatments were both influential on B. scoparia seed production per unit area, which is likely a key indicator of long-term management success for this annual weed species. The lowest germinable seed production per unit area was observed in spring wheat treated with non–ALS inhibiting herbicides, and the greatest germinable seed production was observed in sugar beet treated with ALS-inhibiting herbicides. The combined effects of crop canopy and herbicide treatment can minimize B. scoparia establishment and seed production.

Pruning of small fruit crops can affect habitat suitability for Drosophila suzukii

Insect activity, survival, and development are affected by climatic conditions that elicit effects at multiple scales. Pruning small fruit crop canopies alters the microclimate, which in turn may influence insect pest activity. We investigated the effect of three canopy density treatments (high, medium, low) on Drosophila suzukii (Matsumura) (Diptera: Drosophilidae) fruit infestation in blueberries and caneberries using a two-year, multi-state experiment. We quantified the effect of canopy density on canopy microclimate, fruit quality (total soluble solids, fruit firmness), and yield. To better understand heterogeneity in canopy microclimate, parameters were further separated by canopy location (exterior vs. interior) in Maryland. In both crops, meta-analyses reveal small magnitude effects of the plant canopy on microclimate, whereas analysis of variance did not separate these effects, with mean canopy differences of 0.1–0.7 °C and 0.5–1.3 % relative humidity (RH) between caneberry canopy densities and locations. In caneberry multi-state trials, 0.14 fewer D. suzukii larvae (g fruit)−1 occurred on average in the low canopy density treatment, and 0.2 fewer D. suzukii larvae (g fruit)−1 occurred in exterior raspberries in Maryland compared with the canopy interior. Artificially infested blueberry fruit indicated immature D. suzukii survival within fruit can vary across canopy densities and locations. Although lower total yield was produced in low density canopies, canopy density did not influence berry quality or marketable yield. Microhabitats provide important shelter from extreme environmental conditions; the availability of shelter and ability to locate it affects insect pest populations and distributions. Understanding how crop canopy microclimate affects D. suzukii infestation can inform efforts to develop habitat manipulation tactics and improve the efficiency of fruit production.

Variable effects of forest canopies on snow processes in a valley of the central Spanish Pyrenees

AbstractSnowpacks and forests have complex interactions throughout the large range of altitudes where they co‐occur. However, there are no reliable data on the spatial and temporal interactions of forests with snowpacks, such as those that occur in nearby areas that have different environmental conditions and those that occur during different snow seasons. This study monitored the interactions of forests with snowpacks in four forest stands in a single valley of the central Spanish Pyrenees during three consecutive snow seasons (2015/2016, 2016/2017 and 2017/2018). Daily snow depth data from time‐lapse cameras were compared with snow data from field surveys that were performed every 10–15 days. These data thus provided information on the spatial and temporal changes of snow–water equivalent (SWE). The results indicated that forest had the same general effects on snowpack in each forest stand and during each snow season. On average, forest cover reduced the duration of snowpack by 17 days, reduced the cumulative SWE of the snowpack by about 60% and increased the spatial heterogeneity of snowpack by 190%. Overall, forest cover reduced SWE total accumulation by 40% and the rate of SWE accumulation by 25%. The forest‐mediated reduction of the accumulation rate, in combination with the occasional forest‐mediated enhancement of melting rate, explained the reduced duration of snowpacks beneath forest canopies. However, the magnitude and timing of certain forest effects on snowpack had significant spatial and temporal variations. This variability must be considered when selecting the location of an experimental site in a mountainous area, because the study site should be representative of surrounding areas. The same considerations apply when selecting a time period for study.

Forest mosses sensitively indicate nitrogen deposition in boreal background areas

Mosses take up nitrogen (N) mainly from precipitation through their surfaces, which makes them competent bioindicators of N deposition. We found positive relationships between the total N concentration (mossN%) of common terrestrial moss species (feather mosses Pleurozium schreberi and Hylocomium splendens, and a group of Dicranum species) and different forms of N deposition in 11–16 coniferous forests with low N deposition load in Finland. The mosses were collected either inside (Dicranum group) or both inside and outside (feather mosses) the forests. Deposition was monitored in situ as bulk deposition (BD) and stand throughfall (TF) and detected for ammonium (NH4+-N), nitrate (NO3−-N), dissolved organic N (DON), and total N (Ntot, kg ha−1yr−1). Ntot deposition was lower in TF than BD indicating that tree canopies absorbed N from deposition in N limited boreal stands. However, mossN% was higher inside than outside the forests. In regression equations, inorganic N in BD predicted best the mossN% in openings, while DON in TF explained most variation of mossN% in forests. An asymptotic form of mossN% vs. TF Ntot curves in forests and free NH4+-N accumulation in tissues in the southern plots suggested mosses were near the N saturation state already at the Ntot deposition level of 3–5 kg ha−1yr−1. N leachate from ground litterfall apparently also contributed the N supply of mosses. Our study yielded new information on the sensitivity of boreal mosses to low N deposition and their response to different N forms in canopy TF entering moss layer. The equations predicting the Ntot deposition with mossN% showed a good fit both in forest sites and openings, especially in case of P. schreberi. However, the open site mossN% is a preferable predictor of N deposition in monitoring studies to minimize the effect of tree canopies and N leachate from litterfall on the estimates.

Climate Changes Challenges to the Management of Mediterranean Montado Ecosystem: Perspectives for Use of Precision Agriculture Technologies

Montado is an agro-silvo-pastoral system characterized by a high complexity as a result of the interactions between climate, soil, pasture, trees, and animals. It is in this context that management decisions must be made, for example with respect to soil fertilization, grazing, or animal supplementation. In this work, the effect of the tree canopy on the spatial and temporal variability of the soil and productivity, quality, and floristic composition of the pasture was evaluated. Precision agriculture (PA) technologies for monitoring soil and pasture were also evaluated. The study was carried out between October 2015 and June 2018 in an experimental field of 2.3 ha. The results showed: (i) The positive impact of trees and animal grazing on soil fertility; (ii) the influence of inter-annual variability of precipitation on the pattern of pasture vegetative cycle; (iii) the positive effect of trees in pasture quality; (iv) the negative effect of trees in pasture productivity; (v) the role of pasture floristic composition as an indicator of soil limitations or climatic changes; (vi) the potential of technologies associated with the concept of PA as express tools to decision making support and for the optimization of the herbaceous stratum and the dynamic management of grazing in this ecosystem in a holistic and sustainable form.

The canopy effect in filamentous algae: Improved modeling of Cladophora growth via a mechanistic representation of self-shading

For decades, nuisance algal growth has wreaked havoc in systems across the world. It has been particularly problematic in the Laurentian Great Lakes. Although managing nutrient loads has resulted in some mitigation, ecosystem perturbations in the last two decades have resulted in favorable conditions for a Cladophora resurgence.This paper reports on improvements to the Great Lakes Cladophora Model, which has been used to inform management of Great Lakes nuisance algal growth since it was first developed over 30 years ago (Auer et al., 1982; Auer and Canale, 1982; Canale and Auer, 1982a,b). Like earlier versions, this recent configuration of the model, GLCM v3, simulates algal biomass density (g dry mass m−2) and stored (or cellular) phosphorus content (P as % dry mass) over the spring, summer and fall growth cycle. Two major advances over previous versions of the model are presented: a) an improved characterization of the light and temperature response surfaces driving gross growth and respiration and b) the development and implementation of a segmented-system (canopy) approach for simulating the impact of self-shading (carrying capacity) on growth. Prior versions of the GLCM treated the algal mat as a lumped system, utilizing the logistic model to simulate the carrying capacity effect. In that approach, a carrying capacity coefficient (Xmax, maximum biomass density) placed a ceiling on biomass accrual. However, that approach was not mechanistic (i.e., physiologically supported) and empirical specification of the coefficient is undermined by significant intra- and inter-site variability. This uncertainty places too much emphasis on Xmax as a tuning parameter. The signal contribution of this work to the development of the GLCM v3 is the replacement of the lumped system approach with a vertically segmented, mechanistic treatment of self-shading: the “canopy effect.” Here, biomass accrual is mechanistically governed by light attenuation through the canopy as quantified by kalg, the vertical extinction coefficient for light passing through the mat. This coefficient may be determined by direct, in situ measurement and offers much less freedom for use as a tuning parameter. The advances in Cladophora growth modeling provided here and embodied in the GLCM v3 offer a more mechanistic and robust tool than previous versions, strengthening its credibility for the management of nuisance algal growth such as that mandated by the Great Lakes Water Quality Agreement of 2012. Further, the vertical determination of net growth provides a more mechanistic basis for future modeling of other key processes such as sloughing (detachment) in both the Great Lakes and other natural waterbodies.

Direct and Indirect Effects of Overstory Canopy and Sex-Biased Density Dependence on Reproduction in the Dioecious Shrub Shepherdia canadensis (Elaeagnaceae)

This study assessed the reproductive success of a temperate dioecious shrub, Canada buffaloberry, Shepherdia canadensis (L.) Nutt., in central Alberta, Canada, by examining the effects of spatial patterns and overstory canopy on flower and fruit production. S. canadensis is more abundant and productive (more fruit) at forest edges and in forest gaps, suggesting a dependence on higher light conditions than is typical of late-seral forests. We used path analysis to demonstrate that flower and fruit production exhibited density-dependent effects at a scale of 50 m2 around focal female plants. Fruit production was positively affected by male intraspecific density (pollen supply) and negatively affected by female intraspecific density (pollen competition), but not correlated with overall intraspecific density. The effects of sex-differentiated density are partly due to pollinator responses to male plant density. Flower production was positively affected by overall intraspecific density. A pollen supplementation trial doubled fruit production relative to a control, demonstrating that local male density (pollen availability) and pollinator activity can limit fruit production in S. canadensis. Canopy cover was negatively related to both flower and total fruit production, with approximately one-third (34%) of the total effect of canopy on fruit production due to the effect of canopy on flower production. The commonly observed negative association between canopy cover and fruit production in buffaloberry, therefore, is partly a result of the reduction first in flower number and second in fruit set. This study clarifies the mechanisms associated with the often-noted observation, but not previously assessed at the level of individuals, that reproductive output in S. canadensis is density dependent, limited by canopy cover through reductions in both flowering and fruit set, and pollinator limited. These findings hold implications for managing animal species that depend on the fruit of S. canadensis and suggest future directions for research on dioecious and actinorhizal species.

A study of the canopy effect on fire regime transition using an objectively defined Byram convective number

Forest fires can be classified into two regimes, depending on Byram convective number NC: a plume-dominated and wind-driven regime. This classification is often based on two-dimensional (2D) studies without considering flame front structures, and is associated with the change of governing heat transfer mechanism. Studies based on three-dimensional (3D) considerations suggest the dependency of the fire regime transition on canopy characteristics CdLAI (CdαsσsHF/2) through which the atmosphere boundary layer (ABL) flow changes to mixing layer (ML) flow. Hence, the primary objective of the present study is to investigate the fire regime transition and its associated heat transfer mechanisms with the aid of drawing a dimensionless configuration space {NC, CdLAI}. This requires an objective definition of the Byram convective number; the 2 m or 10 m high open wind speed (U∞=U2 or U10) often used to define it being arbitrary. Therefore, the present study introduces a scaling method for the reference height (Yref) at which the wind speed Uref is chosen to define a new Byram convective number NC’. A mathematical expression for Yref as a function of canopy/vegetation height is proposed for a small-scale, stationary fire. The new configuration space {NC’, CdLAI} is considered more suitable for investigating the fire regime transition.

Ozone exchange within and above an irrigated Californian orchard

In this study, the canopy effects on the vertical ozone exchange within and above Californian orchard are investigated. We examined the comprehensive dataset obtained from the Canopy Horizontal Array Turbulence Study (CHATS). CHATS typifies a rural central Californian site, with O3 mixing ratios of less than 60 ppb and moderate NO x mixing ratios. The CHATS campaign covered a complete irrigation cycle, with our analysis including periods before and after irrigation. Lower O3 mixing ratios were found following irrigation, together with increased wind speeds, decreased air temperatures and increased specific humidity. Friction velocity, sensible heat and gas fluxes above the canopy were estimated using variations on the flux-gradient method, including a method which accounts for the roughness sublayer (RSL). These methods were compared to fluxes derived from observed eddy diffusivities of heat and friction velocity. We found that the use of the RSL parameterization, which accounts for the canopy-induced turbulent mixing above the canopy, resulted in a stronger momentum, heat, and ozone exchange fluxes above this orchard, compared to the method which omits the RSL. This was quantified by the increased friction velocity, heat flux and ozone deposition flux of up to 12, 29, and 35% at 2.5 m above the canopy, respectively. Within the canopy, vertical fluxes, as derived from local gradients and eddy diffusivity of heat, were compared to fluxes calculated using the Lagrangian inverse theory. Both methods showed a presence of vertical flux divergence of friction velocity, heat and ozone, suggesting that turbulent mixing was inefficient in homogenizing the effects driven by local sources and sinks on vertical exchange of those quantities. This weak mixing within the canopy was also corroborated in the eddy diffusivities of friction velocity and heat, which were calculated directly from the observations. Finally, the influence of water stress on the O3 budget was examined by comparing the results prior and after the irrigation. Although the analysis is limited to the local conditions, our in situ measurements indicated differences in the O3 mixing ratio prior and after irrigation during CHATS. We attribute these O3 mixing ratio changes to enhanced biological emission of volatile organic compounds (VOCs), driven by water stress.