A Private Tree By-Law’s Contribution to Maintaining a Diverse Urban Forest: Exploring Homeowners’ Replanting Compliance and the Role of Construction Activities in Toronto, Canada

Many municipalities are working to protect and grow their urban forest, including adopting private tree regulations. Such regulations typically require property-owners to apply for a permit to remove trees and, if the permit is granted, plant replacement trees. Even with such regulations, many private trees are removed each year, particularly on residential property. Property-level construction activity, including expanding building footprints, replacing an older home with a new one, and increasing hardscaping, is emerging as a key driver of residential tree loss. This study addresses whether homeowners who receive a permit to remove one or more trees comply with the requirement to plant replacement trees to better understand the effect of private tree regulation. We explore this question through a written survey of homeowners who received a tree removal permit and site visits in Toronto (Ontario, Canada). While 70% of all survey participants planted the required replacement trees 2 to 3 years after receiving the permit, only 54% of homeowners whose permit was associated with construction planted. Additionally, most replacement trees were in good health but were dominated by a few genera. We also found significant differences in replacement planting and tree survival across the city’s 4 management districts. This study highlights that if resources supporting private tree regulations are limited, tree permits associated with construction should be prioritized for follow-up. Additionally, guidance about diverse species to plant should be communicated to ensure that private tree regulations are supporting the long-term protection of the urban forest.

Tracheid and Pit Dimensions Hardly Vary in the Xylem of Pinus sylvestris Under Contrasting Growing Conditions

Maintaining sufficient water transport via the xylem is crucial for tree survival under variable environmental conditions. Both efficiency and safety of the water transport are based on the anatomical structure of conduits and their connections, the pits. Yet, the plasticity of the xylem anatomy, particularly that of the pit structures, remains unclear. Also, trees adjust conduit dimensions to the water transport distance (i.e., tree size), but knowledge on respective adjustments in pit dimensions is scarce. We compared tracheid traits [mean tracheid diameter d, mean hydraulic diameter dh, cell wall reinforcement (t/b)2], pit dimensions (diameters of pit aperture Da, torus Dt, margo Dm, and pit border Dp), and pit functional properties (margo flexibility F, absolute overlap Oa, torus overlap O, and valve effect Vef) of two Scots pine (Pinus sylvestris L.) stands of similar tree heights but contrasting growth rates. Furthermore, we analyzed the trends of these xylem anatomical parameters across tree rings. Tracheid traits and pit dimensions were similar on both sites, whereas Oa, O, and F were higher at the site with a lower growth rate. On the lower growth rate site, dh and pit dimensions increased across tree rings from pith to bark, and in trees from both sites, dh scaled with pit dimensions. Adjusted pit functional properties indicate slightly higher hydraulic safety in trees with a lower growth rate, although a lack of major differences in measured traits indicated overall low plasticity of the tracheid and pit architecture. Mean hydraulic diameter and pit dimension are well coordinated to increase the hydraulic efficiency toward the outer tree rings and thus with increasing tree height. Our results contribute to a better understanding of tree hydraulics under variable environmental conditions.

The effect of nitrogen source and quantity on disease expression of Neonectria ditissima in apple

The effects of nitrogen on the interaction between apple trees and European canker caused by Neonectria ditissima are not well understood. Previous field and laboratory studies have shown that nitrogen affects N. ditissima disease development, germination and germ-tube growth in vitro but the type of nitrogen source has not been examined in vivo. Therefore, the aim of this study was to determine the effects of root-applied nitrogen from different sources on the development of European canker on inoculated potted trees. One-year-old ‘Royal Gala’ trees were planted in a low-nitrogen growth substrate and treated with a range of concentrations of calcium ammonium nitrate (CAN) or other nitrogen sources (Ca(NO3)2, KNO3, (NH4)2SO4, NH4NO3, urea, YaraMila™) at equivalent molar rates of nitrogen as the highest CAN treatment. Treatments were applied during the growing season (Nov to May). The control treatment received no applied nitrogen. Bud and leaf scar wounds were inoculated at leaf fall with N. ditissima conidia. Tree growth and health, disease progression and leaf nitrogen content were monitored. The rate of nitrogen application affected tree diameter and leaf nitrogen content while the nitrogen source mainly affected tree survival, powdery mildew incidence, leaf weights, leaf nitrogen and European canker symptom expression. Trees treated with (NH4)2SO4 had the lowest survival rates and highest leaf nitrogen content. Disease expression was highest with NH4NO3 and lowest with KNO3 applications. The control plants (which received no additional nitrogen), showed the least amount of both growth and disease expression. Applications of CAN, even at the lowest rate (20 g), increased disease susceptibility. Increasing rates of CAN applications did not significantly increase disease incidence. Nitrogen concentration is an important factor in the disease development of European canker of apple. Field evaluation is recommended to further validate these results.

Making Agriculture Carbon Neutral Amid a Changing Climate: The Case of South-Western Australia

Making Australian agriculture carbon neutral by 2050 is a goal espoused by several agricultural organisations in Australia. How costly might it be to attain that goal, especially when adverse climate change projections apply to agriculture in southern Australia? This study uses scenario analysis to examine agricultural emissions and their abatement via reforestation in south-western Australia under projected climate change. Most scenarios include the likelihood of agricultural emissions being reduced in the coming decades. However, the impact of projected adverse climate change on tree growth and tree survival means that the cost of achieving agricultural carbon neutrality via reforestation is forecast to increase in south-western Australia. Agricultural R&D and innovation that enable agricultural emissions to diminish in the coming decades will be crucial to lessen the cost of achieving carbon neutrality. On balance, the more likely scenarios reveal the real cost of achieving carbon neutrality will not greatly increase. The cost of achieving carbon neutrality under the various scenarios is raised by an additional AUD22 million to AUD100 million per annum in constant 2020 dollar terms. This magnitude of cost increase is very small relative to the region’s gross value of agricultural production that is regularly greater than AUD10 billion.

Early Performance of Several Prunus Interspecific Hybrid Rootstocks for Redhaven Peach in Southern Ontario

A multi-year orchard experiment was established to measure the performance of Rootpac®-R, Rootpac®-20, Rootpac®-40, and Rootpac®-70 rootstocks using ‘Redhaven’ peach (Prunus persica var. persica) as the scion, and compared with the Bailey peach seedling rootstock, the current industry standard. Tree survival after five years was 79% on Rootpac-40, whereas the remaining rootstocks showed no tree mortality. Tree vigour and canopy height and width were influenced by rootstock genotype beginning the year of planting in 2016. For the first five years of production, Rootpac-70 consistently produced the largest trees based on TCSA and by year five, all rootstocks produced trees with similar TCSAs, except for Rootpac-70, which was 38% larger than Bailey. By year five, cumulative yields were greatest on Rootpac-70, which were 10% higher than Bailey; cumulative yields of Rootpac-R, Rootpac-20, and Rootpac-30 were 98%, 89%, and 84% that of Bailey, respectively. Cumulative yield efficiency was significantly influenced by rootstock although the magnitude of the differences was small and likely of insignificant commercial importance. Rootpac-40 consistently produced the largest fruit. These results are only reflective of the orchard establishment years and additional data are required before peach producers can make fully informed decisions concerning the rootstocks evaluated in this study for their orchard systems. However, at this juncture, all the Rootpac rootstocks evaluated in this study are likely to impart excessive vigour to be used in a higher density system and offer little advantage over Bailey.

Genotypic variation rather than ploidy level shapes trait expression in a foundation tree species under drought stress and defoliation

With advancing climate change, tree survival increasingly depends on mechanisms that facilitate coping with multiple environmental stressors. At the population level, genetic diversity is a key determinant of a tree species’ capacity to deal with stress. However, little is known about the relative relevance of the different components of genetic diversity for shaping tree stress responses. We compared how two components of genetic diversity, genotypic variation and ploidy level, shape growth, phytochemical, and physiological traits of Populus tremuloides, under environmental stress. In two field experiments we exposed eight diploid and eight triploid aspen genotypes to individual and interactive drought stress and defoliation treatments. We found that: 1) Genotypic differences were critical for explaining variation of most of functional traits and their responses to stress. 2) Ploidy levels generally played a subordinate role for shaping traits, as they were typically obscured by genotypic differences. 3) As an exception to the second finding, we found that triploid trees expressed higher levels of foliar defenses, photosynthesis, and rubisco activity under well-watered conditions, and displayed greater drought resilience than diploids. This research demonstrates that the simultaneous study of multiple sources of genetic diversity is important for understanding how trees will respond to environmental change.

Deriving a Tree Growth Model from Any Existing Stand Growth Model

In this study, a new method was developed to derive a tree survival and diameter growth model from any existing stand-level model, without the need for individual-tree growth data. Predictions from the derived tree model are constrained to match number of trees and basal area per ha as outputted by the stand model. The tree models derived from three different stand models were evaluated against a tree model, in both unadjusted and disaggregated forms. For the same stand-level model, the derived tree model outperformed its counterpart, the disaggregated tree model. Furthermore, except for one stand model with poor performance, the tree models derived from the remaining two stand models delivered comparable results to those obtained from the unadjusted tree model. The tree model derived from one stand model even performed slightly better than the unadjusted tree model. This is significant because the coefficients of the unadjusted and disaggregated tree models had to be estimated from tree-level growth data, whereas the derived tree model required no tree growth data at all. The methodology presented in this study should be applicable when there is no ingrowth or recruitment.