Assessing carbon storage and sequestration by Canada's urban forests using high resolution earth observation data

Urban trees are important components of the landscape and offer numerous benefits; both socio-economical and biophysical. Urban trees act as a sink for CO₂, helping to offset carbon emissions from urban areas by removing the greenhouse gas from the atmosphere through photosynthesis. Environment Canada develops estimates of Canada's greenhouse gas emissions and removals which are submitted annually to the United Nations as part of ongoing commitments under the United Nations Framework Convention for Climate Change. As part of these reporting commitments countries are required to develop estimates of emissions and removals of Greenhouse Gas that are the result of direct impact of human activities in the Land-Use, Land-Use Change and Forestry Sector. Here, we present an approach which involves sampling high resolution aerial photographs to determine urban tree coverage across Canada's major urban areas. Our results suggest Canadian urban areas have an estimated tree canopy cover of 27%. This tree cover is estimated to store approximately 34,000 kt C and annually sequester approximately 2500 kt of CO₂. These estimates show significant improvement over previous methods used to provide Canadian estimates. The methods developed here are easily repeatable which allow for temporal changes to be analyzed and assessed over time.     

Long-term impact of road salt (NaCl) on soil and urban trees in Edmonton,Canada

The application of de-icing salts for winter road maintenance is recognized as a major contributor to the decline of urban trees. We conducted a long-term monitoring program across several locations in the City of Edmonton (Alberta, Canada) to evaluate the impact of roadway salt application on tree species widely planted in boulevards and right-of-ways: Ulmus americana, Fraxinus pennsylvanica, Pinus contorta, and Picea glauca. Soil and leaf samples were collected from a total of 16 sites over six years. There were four sites selected for each tree species: three mid- to high- traffic roadside sites that received regular winter maintenance and one non-serviced site (control). Sampling was performed three times per year from late spring to late summer. Airborne salinity was assessed in four locations at different distances from the road. In 50% of the roadside sites, soil electrical conductivity (EC) values exceeded 2 dS m⁻¹. Soil pH in all of the roadside sites was also significantly higher than in the control sites, with values ranging from 7.6 to 8.5. In all four species, trees growing in sites with high soil EC had increased leaf Na concentrations and reduced leaf chlorophyll concentrations. Among the airborne monitoring sites, Na deposition in high traffic locations was over four-fold higher than those measured in the control location. Furthermore, Na levels remained relatively high at 20–50 m from the main road. Our data suggest that while soil salinity is among the main stressors affecting roadside trees in Edmonton, salt spray deposition may also have a significant impact on trees located close to high vehicle traffic areas and dense road networks. Our study highlights the importance of collecting data over several years and from multiple locations to account for the spatial and temporal heterogeneity of the urban environments in order to better evaluate the impact of road salt application on urban trees.     

Assessing the performance of urban forest carbon sequestration models using direct measurements of tree growth

Across cities worldwide, people are recognizing the value of greenspace in ameliorating the health and well-being of those living there, and are investing significant resources to improve their greenspace. Although models have been developed to allow the quantification of ecosystem services provided by urban trees, refinement and calibration of these models with more accurate site- and species-specific data can increase confidence in their outcomes. We used data from two street tree surveys in Cambridge, MA, to estimate annual tree mortality for 592 trees and diameter growth rates for 498 trees. Overall tree turnover between 2012 and 2015 was relatively low (annualized 3.6% y⁻¹), and mortality rate varied by species. Tree growth rates also varied by species and size. We used stem diameter (DBH) and species identity to estimate CO₂ sequestration rates for each of 463 trees using three different model variations: (1) i-Tree Streets, (2) Urban Tree Database (UTD) species-specific biomass allometries and growth rates, and (3) empirically measured growth rates combined with UTD biomass allometries (Empirical + UTD). For most species, the rate of CO₂ sequestration varied significantly with the model used. CO₂ sequestration estimates calculated using i-Tree Streets were often higher than estimates calculated with the UTD equations. CO₂ sequestration estimates were often the lowest when calculated using empirical tree growth estimates and the UTD equations (Empirical + UTD). The differences among CO₂ sequestration estimates were highest for large trees. When scaled up to the entire city, CO₂ sequestration estimates for the Empirical + UTD model were 49.2% and 56.5% of the i-Tree Streets and UTD estimates, respectively. We suggest future derivations of ecosystem service provision models allow localities to input their own species-specific growth values. By adding capacity to easy-to-use tools, such as i-Tree Streets, we can increase confidence in the model output.     

The Reinvigoration of Apple Trees by the Inarching of Vigorous Rootstocks

Summary

The purpose of this research was to determine if established, orchard-grown apple trees that were con®ned in root-restricting membranes received adequate water for growth when irrigated. Previous data had shown positive effects of root restriction on reducing shoot extension growth in apple. Soil matric potential (Ψ_sm), leaf stomatal conductance (g) and leaf water potentials (Ψ_l) were measured over daily cycles during the period of maximum tree water use (July and August). Measurements were also made of the Ψ_sm and fruit size throughout the growing season. Daily and seasonal Ψ_sm showed that the soil within the root restricting membranes (+R) when irrigated (+I) remained closed to field capacity (<100 HPa), which was not the case for unirrigated soil within the membranes (-I +R). Ψ_l measured before dawn, showed that similar levels of drought stress were evident between irrigated and restricted (+I +R) and unrestricted (+I -R and -I -R) trees. The Ψ_sm, and Ψ_l for trees with roots within restricting membranes were significantly more negative, in the absence of irrigation (-I +R). Measurement of g showed that root restricted trees were transpiring at similar rates in the presence of irrigation (+I +R) as unrestricted trees (+I -R) with or without irrigation. Stomatal closure could not explain the increased Ψ_l observed for the restricted irrigated (+I +R) treatment compared with unrestricted (-R) trees. A reduced stomatal aperture was the most likely explanation for the reduction in growth previously observed with the restricted unirrigated trees. Fruit size was also affected by root restriction and the effect became greater as the roots became more restricted with tree age. Similarly, there was also a negative effect, in one year, of root restriction on fruit size at harvest, even in the presence of irrigation. Data show that reductions in soil water availability, Ψ_l and g, for the root-restricted trees (+I +R), were unlikely to be the causes for the previously observed reductions in shoot growth (tree size). These results imply that other factors were in operation, among which root-synthesized chemical regulators of shoot growth are the most likely candidates.     

Porous-permeable pavements promote growth and establishment and modify root depth distribution of Platanus × acerifolia (Aiton) Willd. in simulated urban tree pits

In dense urban areas with heavy pedestrian traffic, current trends favor covering tree pits with porous-permeable pavement over installing grates or leaving the soil exposed. However, pavement cover potentially modifies soil moisture and temperature, altering tree growth and overall resilience, especially when coupled with heat stress and drought in a changing climate. This study evaluated the response of newly planted London plane (Platanus × acerifolia ‘Bloodgood’) trees to porous-permeable resin-bound gravel pavement and associated alterations in soil water distribution and temperature, in two distinct physiographic regions in Virginia, USA. Simulated urban tree pits were either covered with porous-permeable pavement or left unpaved, and root growth and depth, soil water content and temperature, and tree stem diameter measured over two growing seasons. At both sites, trees in paved tree pits grew larger than trees without pavement. Stem diameters were 29% greater at the Mountain site and 51% greater at the Coastal Plain site, as were tree heights (19% and 38% greater), and above ground dry biomass (67% and 185% greater). Roots under pavement developed faster and shallower, with many visible surface roots. In contrast, unpaved tree pits had almost no visible surface roots, and at the Mountain site only occupied an average area of 7 cm² within the 1-m² tree pits, compared with 366 cm² in paved tree pits. Pavement may have extended the root growing season by as much as 14 days, as the average soil temperature for the month of October was 1.1 °C and 1.2 °C higher under pavement than in unpaved pits. Porous-permeable pavement installations in tree pits accelerated establishment and increased growth of transplanted trees, but may result in shallower root systems that can damage pavement and other infrastructure. In addition, shallow root systems may prevent water extraction from deeper soils, compromising drought resilience.     

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.     

Tree and impervious cover change in U.S. cities

Paired aerial photographs were interpreted to assess recent changes in tree, impervious and other cover types in 20 U.S. cities as well as urban land within the conterminous United States. National results indicate that tree cover in urban areas of the United States is on the decline at a rate of about 7900 ha/yr or 4.0 million trees per year. Tree cover in 17 of the 20 analyzed cities had statistically significant declines in tree cover, while 16 cities had statistically significant increases in impervious cover. Only one city (Syracuse, NY) had a statistically significant increase in tree cover. City tree cover was reduced, on average, by about 0.27 percent/yr, while impervious surfaces increased at an average rate of about 0.31 percent/yr. As tree cover provides a simple means to assess the magnitude of the overall urban forest resource, monitoring of tree cover changes is important to understand how tree cover and various environmental benefits derived from the trees may be changing. Photo-interpretation of digital aerial images can provide a simple and timely means to assess urban tree cover change to help cities monitor progress in sustaining desired urban tree cover levels.     

The influence of increasing tree cover on mean radiant temperature across a mixed development suburb in Adelaide,Australia

The implementation of trees in urban environments can mitigate outdoor thermal stress. Growing global urban population and the risk of heatwaves, compounded by development driven urban warmth (the urban heat island), means more people are at risk of heat stress in our cities. Effective planning of urban environments must minimise heat-health risks through a variety of active and passive design measures at an affordable cost. Using the Solar and Longwave Environmental Irradiance Geometry (SOLWEIG) model and working within the bounds of current urban design, this study aimed to quantify changes in mean radiant temperature (T_mrt) from increased tree cover at five different 200 × 200 m urban forms (including compact mid-rise development, residential and open grassy areas) within a suburb of Adelaide, Australia during summer. Following a successful validation of SOLWEIG, street trees were strategically distributed throughout each of the five urban forms and the model run over five warm sunny days (13–17 February 2011). Results showed spatially averaged daytime (7:30–20:00) T_mrt reduced by between 1.7 °C and 5.1 °C at each site, while under peak heating conditions (16 February, 14:00) T_mrt reduced by between 2.0 °C and 7.1 °C. The largest reduction in T_mrt under peak warming conditions was at the residential site, despite having the fewest number of trees added. Directly below clusters of trees, T_mrt could be reduced by between 14.1 °C and 18.7 °C. SOLWEIG also highlighted that more built-up sites showed higher T_mrt under peak warming conditions due to increased radiation loading from 3D urban surfaces, but over the course of the day, open sites were exposed to greater and more uniform T_mrt. This study clearly demonstrates the capacity of street trees to mitigate outdoor thermal stress and provides guidance for urban planners on strategic street tree implementation.     

The consequences of cultural practices on growth and cropping of peach trees subjected to localized irrigation: Effects of soil management,nitrogen fertilization and tree density

SUMMARY

Cultural practices were studied in peach tree orchards irrigated with microsprinklers, herbicide strips being maintained along the tree rows. Differences in fruit cropping between treatments varied in the same way as tree vigour, the most vigorous trees having the highest production capacities. Concerning soil management, ground cover vegetation (permanent or temporary) improved fruit production compared with soil cultivation. According to leaf analysis, ground cover vegetation appeared to ameliorate the conditions of trees nutrition. This effect was probably mainly due to increasing soil resistance to compaction; applying organic matter was not effective in improving the performance of trees grown with soil cultivation compared with those grown with ground cover vegetation. Increasing fruit tree density also contributed to increased fruit production. Nevertheless, more than 600 trees per hectare did not allow further increase in fruit yield for the experimental cultivar. Lastly, nitrogen fertilization seemed particularly important for improving the fruiting of peach trees.     

Relating tree vigour to the soil and landscape characteristics of an olive orchard in a marly area of southern Spain

The relationships between tree vigour and soil and landscape properties of a rainfed olive orchard located on a marly slope of southern Spain were studied. The different properties and thickness of three soil layers (viz. the secondary carbonate-free layer, the solum, and the layer penetrated by roots), and the properties of the 0–30 cm layer, were used as soil variables. Olive tree vigour and soil properties were found to be strongly correlated. Much of the variability in tree vigour was related to soil morphology. The thickness of the root zone and the average CaCO₃ content in the secondary carbonate-free layer and the upper 30 cm of soil accounted for ∼58 and ∼68%, respectively, of the variance in canopy volume. Tree vigour was related to some properties of the root zone, such as clay content and cation exchange capacity, expressed on a surface basis. Topographic properties such as distance to summit, slope gradient and profile curvature were also found to be significantly correlated with tree vigour, but had less influence than intrinsic soil properties.     

Micro-scale heterogeneity in urban forest soils affects fine root foraging by ornamental seedlings of Buddhist pine and Northeast yew

Urban soils are frequently characterized by a strong heterogeneity caused by intense anthropogenic activity and land use changes. Soil heterogeneity is commonly known to affect tree root development, but little has been detected concerning root foraging by ornamental trees in heterogeneous urban soils at micro-scale. In this study, Buddhist pine [Podocarpus macrophyllus (Thunb.) D. Don] and Northeast yew (Taxus cuspidata S. et Z.) were selected as ornamental tree species for a two-year study. In the first-year, seedlings were cultured under contrasting photoperiods to generate different morphologies. In the second year, seedlings were transplanted to pots filled with soils collected from an urban forest. Controlled-release fertilizers (N-P₂O₅-K₂O, 14-13-13) were evenly broadcasted to a half patch of the pot (heterogeneity) or to both halves (homogeneity) on the surface 5 cm beneath the pot-top at the rate of 0.135 g N seedling⁻¹. In the fertilized heterogeneous patch, larger Buddhist pine seedlings had greater dry weight, length, surface area, volume, number of tips, and morphological foraging-precision in fine roots. Compared to Northeast yew seedlings under natural photoperiod in the first year, those under the extended photoperiod had larger size, greater fine root biomass, and length but lower foraging-precision in the second year. N and P concentrations in second-year fine roots mainly increased with the availability of patches generated by fertilization for both species. In conclusion, the ability to forage for nutrients by ornamental tree seedlings in heterogeneous urban forest soils was species-specific. Buddhist pine seedlings had higher foraging precision in heterogeneous urban soils than Northeast yew seedlings due to their response to the extended photoperiod during culture.     

Factors influencing urban tree planting program growth and survival in Florida,United States

High levels of mortality after installation can limit the long-term benefits associated with urban tree planting initiatives. Past planting projects funded by the Florida Forest Service were revisited two to five years after installation to document tree survival and growth and assess program success. Additionally, various site (e.g., soil compaction, installed irrigation) and tree-related (e.g., species, nursery production method, initial size at planting) factors were noted to assess their impact on tree growth. Results show that the overall establishment rate for the 26 sites (n = 2354 trees) was high, with 93.6% of trees alive at the time of final inspection. On-site irrigation played a significant role in tree survival and growth, especially for Magnolia grandiflora (97.7% survival on irrigated sites; 73.8% survival on non-irrigated sites). Findings from this work validate the effectiveness of current program policies which include maintenance of tree quality within the first year after planting, and offer further insights regarding the impacts of season of planting and initial size of nursery stock on plant growth and development.     

Seasonal variations in leaf gas exchange of plantain cv. Hartón (Musa AAB) under different soil water conditions in a humid tropical region

The seasonal effect of soil water availability on leaf gas exchange of plantain plants cv. Hartón growing on two different texture soils (loamy and clayey) were evaluated. Soil water deficits corresponded to 48, 24 and 4 days without precipitation. Daily measurements of leaf gas exchange and microclimatic conditions were carried out at 2 h intervals in a humid tropical environment south of Maracaibo Lake, Venezuela. The results show that cv. Hartón is sensitive to conditions of low water deficit on loamy and to a much greater degree on clayey soils. A marked reduction in leaf conductance (g_s) was observed under severe as well as moderate deficit (below 50 mmol m⁻² s⁻¹) on clayey soils. Under low deficit g_s increases to values between 60 and 100 mmol m⁻² s⁻¹. The same trend was observed in plants on loamy soils but higher g_s for all conditions were obtained compared with plants on clayey soil. Stomatal closure produced a reduction of 85 and 55% of total assimilation (A_tot) for severe and moderate deficit in plants on clayey soils, respectively. While plants on loamy soil exhibited a 65 and 35% reduction, respectively. Water use efficiency (WUE) consistently decreased as available soil water decreased on both soil types. Independently of soil water conditions, higher WUE were always obtained for loamy soils. This suggests that cv. Hartón does not have the ability to adjust the CO₂ assimilation to transpiration ratio in order to optimize gas exchange. This evidences the importance of maintaining high conditions of available soil water in order to avoid lower assimilation rates that probably influence negatively on yield and fruit quality.     

Growth and establishment of container-grown London planetrees in response to mulch,root-ball treatment and fertilization

We conducted two experiments to evaluate the impact of cultural treatments on growth and establishment of container-grown London planetrees (Platanus × acerifolia’ Bloodgood’). In both experiments, 48 trees grown in 100 l (#25) black plastic containers were assigned at random to one of three root-ball treatments prior to planting; no treatment (Control), outer 3 cm of roots removed around entire root-ball (Shave), or outer circling roots disentangled from the root-ball (Tease). In Experiment 1, half of the trees were fertilized with 400 g of controlled release fertilizer (15-9-12; N-P₂O₅-K₂O) at planting and the remainder of the trees were not fertilized. In Experiment 2, half of the trees were mulched with an 8 cm deep × 2 m diameter ring of coarse ground pine bark at planting and the remainder of the trees were not mulched. In Experiment 1, fertilization at planting increased SPAD chlorophyll content on two of four measurement dates but did not affect cumulative height or caliper growth after two years. After two growing seasons, root-ball treatments (shaving or teasing) increased root growth outside the original root-ball compared to control trees. Both root-ball treatments also reduced circling roots. In Experiment 2, mulching at planting increased soil moisture and cumulative tree height and diameter growth. Shaving increased new root growth and both root-ball treatments improved root architecture and reduced circling roots. Overall, the study demonstrates that root-ball manipulations can stimulate new root growth and reduce circling roots. Mulch is a valuable aid to conserve soil moisture and increase tree growth. Fertilization at planting provided little benefit in this experiment, which may have been related to a high level of soil fertility at the site or nutrient loading of the trees from nursery culture prior to transplanting.     

Tree stability in an improved soil to withstand wind loading

As trees are planted in close proximity to properties and humans in a dense city like Singapore, uprooting of trees can pose safety concerns. Previous research studies have shown that soil properties are important factors that govern tree stability. Soil properties can be improved by mixing top soil (TS) with coarse-grained soil. The effects of soil improvement using coarse-grained soil on tree stability were investigated in this study. Granite chip was used as coarse-grained soil to improve the original top soil. Static analysis and finite element modeling were performed to study tree stability in an improved soil. Factors investigated include root geometry and soil shear strength parameter. Finite element results showed good agreement with static analysis in determining the maximum wind force needed to uproot the tree. Parametric study showed that the wind force needed to uproot the tree was influenced by the mode of failure and the magnitude was dependant on the root geometry and the soil properties. A mixture of 20% top soil and 80% granite chip by dry mass was the optimum mixture for withstanding the maximum wind force needed to uproot the tree.     

Stability of landscape trees in engineered and conventional urban soil mixes

Urban trees are frequently exposed to unsuitable soil conditions that can hamper root system development, potentially affecting both tree health and stability. Engineered soil designs have been developed to increase soil volume for trees planted in confined spaces, and past research has shown that these designs improve growing conditions. However, tree stability in these engineered soils has received limited attention from researchers. In this study, we evaluated the stability of two tree species of contrasting soil quality tolerance (Prunus serrulata and Ulmus parvifolia) after 3 years growth in two skeletal soil mixes, in a suspended pavement design (uncompacted soil), and in a conventionally prepared soil pit. Tree stability was evaluated by measuring trunk resistance to a lateral deflecting force applied with a rope winch system under both ambient and near-saturated soil conditions. Although heavily irrigating the experimental soils had no effect on tree stability, species-specific responses to soil mixes were observed. P. serrulata grown in the gravel-based skeletal soil showed greater trunk deflection resistance than trees grown in the other soil treatments, yet the stability of U. parvifolia was unaffected by soil type. These species-specific responses were consistent with earlier observations of root development in which P. serrulata grew up to 60 times greater root length in gravel-based skeletal soil whereas U. parvifolia root growth was similar in all soil treatments. This research provides evidence that certain tree species planted in conventional tree pits may be more prone to uprooting due to poor root development and that root anchorage might be improved for these species by utilizing a skeletal soil mix.     

Multiparametric model of urban park cooling island

This paper presents a study of mitigation of the heat island effect in the built environment with urban (city) parks. The park cooling island (PCI) effect, considering park grass cover and trees’ density and age, is determined for selected extreme summer days at various wind speeds under the optimum soil water conditions in the root zone based on an all-day quasi-stationary thermal response. PCI was determined numerically by coupling a CFD model of an urban park and quasi-steady state, two-zone thermal response boundary condition models of each park element. The boundary models are evaluated in form of multi-parameter approximation polynomials taking into account the sensible and latent heat transfer and the geometrical, optical and thermal properties of park elements. Three-dimensional CFD modelling was used for the determination of temperature, humidity and air velocity fields in an urban park with a size of 140 m × 140 m. Based on the comparison of the measured and numerically determined air temperatures in the tree crowns, we proved that the method of linking the models is adequate for temperature and flow condition modelling in the city park environment.The results are presented in the form of local PCI as the difference between local air temperature in the pedestrian zone and the reference air temperature preceding the park. The study proved that it is possible to normalise the cooling effect using the specific dimensionless coefficient of leaf area (LAI_sp), which includes an approximation of the density of trees planted in the park and their size or age. It was found out that the cooling effect of the park is up to ?4.8 °C at LAI_sp, equal to 3.16, which corresponds to a planting density of 45 trees per hectare, with an age of 50 years. It was also found that with the length of the park cooling effect change decreases. The optimal length of the park with a LAI_sp 3 is 130 m.     

Predicting dry matter production of cauliflower (Brassica oleracea L. botrytis) under unstressed conditions: I. Photosynthetic parameters of cauliflower leaves and their implications for calculations of dry matter production

Measurements of CO₂ exchange of cauliflower leaves were carried out in a field experiment which included two nitrogen fertilisation rates. Irradiance and CO₂ concentration were varied at the leaf level within a leaf cuvette and additionally a temperature treatment was applied to field grown plants moved into climate chambers. These measurements were used to estimate parameter values of a rectangular hyperbola describing cauliflower leaf CO₂ exchange as a function of irradiance and CO₂ concentration. The obtained parameter estimates were used to derive empirical regression equations with temperature and nitrogen content of the leaves as independent variables. The resulting relationships were applied within a simple photosynthesis–respiration based dry matter production model in order to derive functional relationships between light use efficiency and irradiance, leaf area index and temperature.The rectangular hyperbola was able to describe the gas exchange data as varied by irradiance and CO₂ concentration on the single leaf level with sufficient accuracy, but estimates of initial light use efficiency (about 25 μg J⁻¹) were too high because of the bias emanating from the limited flexibility of this model. Light saturated photosynthesis rate (P_max) showed an optimum response to temperature and an increase with increasing nitrogen content of leaves. The initial slope α of the rectangular hyperbola showed no consistent responses to ambient temperature and nitrogen content of leaves. The respiration per unit leaf area β increased exponentially with increasing temperature, resulting in a Q₁₀ value of 1.86. Because only a limited number of plants was evaluated in this study, additional work is needed to further substantiate the results of the gas exchange measurements.The model analysis demonstrated that LUE is independent of the light integral over a range 5–10 MJ m⁻² per day photosynthetically active radiation if one assumes an adaptation of P_max within the canopy and over time according to the incident irradiance. Acclimatisation within the canopy and higher leaf area indices, LAI, reduce the decrease of LUE with irradiance but a substantial decline remains even for LAI values of 4.     

Street trees in Bangalore: Density,diversity, composition and distribution

Once renowned as India’s “garden city”, the fast growing southern Indian city of Bangalore is rapidly losing tree cover in public spaces including on roads. This study aims to study the distribution of street trees in Bangalore, to assess differences in tree density, size and species composition across roads of different widths, and to investigate changes in planting practices over time. A spatially stratified approach was used for sampling with 152 transects of 200 m length distributed across wide roads (with a width of 24 m or greater), medium sized roads (12–24 m) and narrow roads (less than 12 m). We find the density of street trees in Bangalore to be lower than many other Asian cities. Species diversity is high, with the most dominant species accounting for less than 10% of the overall population. Narrow roads, usually in congested residential neighborhoods, have fewer trees, smaller sized tree species, and a lower species diversity compared to wide roads. Since wide roads are being felled of trees across the city for road widening, this implies that Bangalore’s street tree population is being selectively denuded of its largest trees. Older trees have a more diverse distribution with several large sized species, while young trees come from a less diverse species set, largely dominated by small statured species with narrow canopies, which have a lower capacity to absorb atmospheric pollutants, mitigate urban heat island effects, stabilize soil, prevent ground water runoff, and sequester carbon. This has serious implications for the city’s environmental and ecological health. These results highlight the need to protect large street trees on wide roads from tree felling, and to select an appropriate and diverse mix of large and small sized tree species for new planting.     

Contrasting natural regeneration and tree planting in fourteen North American cities

Field data from randomly located plots in 12 cities in the United States and Canada were used to estimate the proportion of the existing tree population that was planted or occurred via natural regeneration. In addition, two cities (Baltimore and Syracuse) were recently re-sampled to estimate the proportion of newly established trees that were planted. Results for the existing tree populations reveal that, on average, about 1 in 3 trees are planted in cities. Land uses and tree species with the highest proportion of trees planted were residential (74.8 percent of trees planted) and commercial/industrial (61.2 percent) lands, and Gleditsia triacanthos (95.1 percent) and Pinus nigra (91.8 percent). The percentage of the tree population planted is greater in cities developed in grassland areas as compared to cities developed in forests and tends to increase with increased population density and percent impervious cover in cities. New tree influx rates ranged from 4.0 trees/ha/yr in Baltimore to 8.6 trees/ha/yr in Syracuse. About 1 in 20 trees (Baltimore) and 1 in 12 trees (Syracuse) were planted in newly established tree populations. In Syracuse, the recent tree influx has been dominated by Rhamnus cathartica, an exotic invasive species. Without tree planting and management, the urban forest composition in some cities will likely shift to more pioneer or invasive tree species in the near term. As these species typically are smaller and have shorter life-spans, the ability of city systems to sustain more large, long-lived tree species may require human intervention through tree planting and maintenance. Data on tree regeneration and planting proportions and rates can be used to help determine tree planting rates necessary to attain desired tree cover and species composition goals.