Variations in Wood Density, Annual Ring Width and Vessel Properties of Quercus brantii Affected by Crown Dieback

Tree decline due to climate change results in physiological weaknesses, attacks by harmful pests and pathogens and threats to forest ecosystem stability. In the work described here, the effects of drought on wood density, tree ring width and variations in vessel morphology are investigated in Persian oak (Quercus brantii) in the forest of the Zagros Mountains, Ilam Province, western Iran. Discs are cut from trunks of declined and healthy trees and woodblocks are cut radially from the sapwood near the bark, at a mid-point between the vascular cambium and the pith (middle) and from wood near the pith. Observations are made on transverse sections from the blocks using microscopy. In trees with decline symptoms, wood density is greater than in healthy trees. Furthermore, declining trees have the narrowest ring width, reduced vessel diameter and area and the highest numbers of vessels and tylose in pith towards the bark. It is concluded that changes in anatomical features are associated with the weakening of trees and are components of declining tree health.

Trirachys holosericeus (apple stem borer).

Trirachys holosericeus, a highly polyphagous longhorned beetle, is native to southern Asia from Pakistan to the Philippines. It is a stem-boring pest in natural and planted forests and fruit trees. It attacks primarily hardwoods, and at least one conifer. Both healthy and stressed trees are attacked, often leading to crown dieback and possibly tree death after one or more years of infestation. Cut logs can remain attractive to egg-laying adults for nearly a year after felling. T. holosericeus has not become established outside its native range of Asia, but it could be moved inadvertently in live plants, logs, and solid wood packaging. Chemical treatments, including injecting insecticides into active galleries, are often used on live trees, especially fruit trees. In forest stands, heavily infested trees are often cut and destroyed. For recently cut logs, rapid transport to sawmills and quick utilization, or at least debarking, are recommended.

TREE DAMAGE AND MISTLETOE IMPACT ON URBAN GREEN AREAS

ABSTRACT Green areas in cities provide several benefits to people, however, several biotic and abiotic agents affects the tree’s health. The main goals of this research were to identify tree damages and to assess the impact of mistletoe on dendrometric variables and the crown condition. Four thousand, seven hundred seventy-five trees were evaluated in 16 green areas of eight Mayoralties in Mexico City; each tree was labeled, and total tree height (Th), diameter at breast height (DBH), damage agents (up to three types), and the crown variables: Live crown ratio (Lcr), Crown dieback (Cdie), Mistletoe severity (Ms) and Crown position (Cpo) were recorded. Of the assessed trees, 3318 presented damage. For the type of damage 1, the mistletoes Cladocolea and Struthanthus showed an average incidence of 42%. For type 2, 20 agents and type 3, 11 were identified. Mechanical wounds, sucking insects, vandalism, and cankers were the most common damages. Based on the Principal Component Analysis (PCA), trees of all diameters and heights were infected, however, Struthanthus was associated with taller and larger diameter trees. Lcr was lower, and Cdie and Ms were higher in infected trees. About Cpo, dominant and codominant trees showed the highest levels of severity by mistletoe. The most affected tree genera were Ulmus and Populus (low Lcr and high Cdie and Ms). Cladocolea was found toward southern Mayoralties, and Struthanthus at the north. It is recommended to assess at least two types of damage on each tree, as this will provide a more complete picture of its health. This is the first study in Mexico on the impact of mistletoe on tree crown variables

Amplifying feedback loop between growth and wood anatomical characteristics of Fraxinus excelsior explains size-related susceptibility to ash dieback

Since the 1990s the invasive fungus Hymenoscyphus fraxineus has caused severe crown dieback and high mortality rates in Fraxinus excelsior in Europe. In addition to a strong genetic control of tolerance to the fungus, previous studies have found landscape heterogeneity to be an additional driver of variability in the severity of dieback symptoms. However, apart from climatic conditions related to heat and humidity influencing fungal infection success, the mechanistic understanding of why smaller or slower-growing trees are more susceptible to dieback remains less well understood. Here, we analyzed three stands in Switzerland with a unique setting of 8 years of data availability of intra-annual diameter growth and annual crown health assessments. We complemented this by ring width and quantitative wood anatomical measurements extending back before the monitoring started to investigate if wood anatomical adjustments can help better explain the size-related dieback phenomenon. We found that slower-growing trees or trees with smaller crowns already before the arrival of the fungus were more susceptible to dieback and mortality. Defoliation directly reduced growth as well as maximum earlywood vessel size, and the positive relationship between vessel size and growth rate caused a positive feedback amplifying and accelerating crown dieback. Measured non-structural carbohydrate (NSC) concentrations in the outermost five rings did not significantly vary between healthy and weakened trees, which translate into large differences in absolute available amount of NSCs. Thus, we hypothesize that a lack of NSCs (mainly sugars) leads to lower turgor pressure and smaller earlywood vessels in the following year. This might impede efficient water transport and photosynthesis, and be responsible for stronger symptoms of dieback and higher mortality rates in smaller and slower-growing trees.

Lightning Damage Facilitates Beetle Colonization of Tropical Trees

Lightning is a common agent of disturbance in many forest ecosystems. Lightning-damaged trees are a potentially important resource for beetles, but most evidence for this association is limited to temperate pine forests. Here, we evaluated the relationship between lightning damage and beetle colonization of tropical trees. We recorded the number of beetle holes on the trunks of trees from 10 strike sites (n = 173 lightning-damaged trees) and 10 matching control sites (n = 137 control trees) in Panama. The trunks of lightning-struck trees had 370% more beetle holes than control trees. The abundance of beetle holes increased with increasing total crown dieback among both control and lightning-damaged trees, and with larger tree diameter among lightning-struck trees. Beetle holes also were more abundant in trunk sections of lightning-damaged trees located directly below a damaged section of the crown. The results of this study suggest that lightning damage facilitates beetle colonization in tropical forest trees and provide a basis for investigations of the effects of lightning-caused disturbance on beetle population dynamics and assemblage structure.

Growth and wood anatomical adjustments of Fraxinus excelsior to the infestation of the invasive fungus Hymenoscyphus fraxineus

<p>Since the 1990s the invasive fungus Hymenoscyphus fraxineus has led to severe crown dieback and high mortality rates in Fraxinus excelsior in Europe. In addition to a strong genetic control of tolerance to the fungus, previous studies have found high landscape variability in the severity of dieback symptoms. However, apart from heat and humidity-related climate conditions favoring fungal development the mechanistic understanding of why smaller or slower growing trees are more susceptible to dieback remains less well understood.</p><p>Here, we analyzed three stands in Switzerland with a unique setting of eight years of intra-annual diameter growth and annual crown health assessments, together with ring-width and quantitative wood anatomical measurements preceding the monitoring, to investigate if wood anatomical adjustments can help better explaining the size-related dieback phenomenon.</p><p>We found that slower growing trees or trees with smaller crowns already before the arrival of the fungus were more susceptible to dieback and mortality. We show that defoliation directly reduces growth as well as maximum earlywood vessel size, and that the positive relationship between vessel size and growth rate causes a positive feedback amplifying crown dieback. Because leaf necrosis happens during late summer when ring formation has already finished, photosynthesis is heavily reduced during a time when non-structural carbohydrates (NSCs, sugars and starch) are stored. Thus, we hypothesize that a lack of NSCs (mainly sugars) leads to lower turgor pressure and smaller earlywood vessels in the next year impeding efficient water transport and photosynthesis, and is responsible why smaller and slower growing trees show stronger symptoms of dieback and higher mortality rates.</p>

Regeneration in the Understory of Declining Overstory Trees Contributes to Soil Respiration Homeostasis along Succession in a Sub-Mediterranean Beech Forest

Research Highlights: Tree decline can alter soil carbon cycling, given the close relationship between primary production and the activity of roots and soil microbes. Background and Objectives: We studied how tree decline associated to old age and accelerated by land-use change and increased drought in the last decades, affects soil properties and soil respiration (Rs). Materials and Methods: We measured Rs over two years around centennial European beech (Fagus sylvatica L.) trees representing a gradient of decline in a sub-Mediterranean forest stand, where the number of centennial beech trees has decreased by 54% in the last century. Four replicate plots were established around trees (i) with no apparent crown dieback, (ii) less than 40% crown dieback, (iii) more than 50% crown dieback, and (iv) dead. Results: Temporal variations in Rs were controlled by soil temperature (Ts) and soil water content (SWC). The increase in Rs with Ts depended on SWC. The temperature-normalized Rs exhibited a parabolic relationship with SWC, suggesting a reduced root and microbial respiration associated to drought and waterlogging. The response of Rs to SWC did not vary among tree-decline classes. However, the sensitivity of Rs to Ts was higher around vigorous trees than around those with early symptoms of decline. Spatial variations in Rs were governed by soil carbon to nitrogen ratio, which had a negative effect on Rs, and SWC during summer, when drier plots had lower Rs than wetter plots. These variations were independent of the tree vigor. The basal area of recruits, which was three times (although non-significantly) higher under declining and dead trees than under vigorous trees, had a positive effect on Rs. However, the mean Rs did not change among tree-decline classes. These results indicate that Rs and related soil physico-chemical variables are resilient to the decline and death of dominant centennial trees. Conclusions: The development of advanced regeneration as overstory beech trees decline and die contribute to the Rs homeostasis along forest succession.

Effect of pruning, fertilization and pesticide injection on crown dieback in urban trees in Colombia: Analysis of factors involved

This research evaluated the effect of pruning, fertilization and pesticide injection on crown dieback in urban trees in Colombia and analyzed the factors involved. Systemic insecticides and/or fungicides were applied through injections in the trunk of 15 tree species affected by the progressive deterioration of the crown in the urban forests of the Metropolitan Area of the Aburrá Valley. The presence of progressive deterioration was evaluated qualitatively on a scale from zero to three in an average sample of 12 individuals for each species. Two treatments were used: i) application of insecticide + fungicide, and ii) application of insecticide, both treatments were applied three times, plus a general treatment of pruning and fertilization. Also, two controls were evaluated: healthy trees and diseased trees without treatment. The probability of individual progressive deterioration (PD) during the study period according to the species, treatment applied, and the initial state of affectation was estimated using a generalized linear mixed model. The analysis of factors involved also included planting site, traffic flow of the site, the wood density of the species, and time. The results suggest that the deterioration is a dynamic phenomenon associated with environmental stresses caused mainly by the climatic variability. From the evaluated variables, the species seems to be the most determinant factor for the affectation, since intrinsic variables of the species, like wood density, can favor its appearance. On the contrary, the treatments evaluated did not affect the recovery of the species. The action of insects and pathogens seems to be opportunistic once trees are affected.