Urbanization drives unique latitudinal patterns of insect herbivory and tree condition

Urban landscapes are characterized by high proportions of impervious surface resulting in higher temperatures than adjacent natural landscapes. In some cities, like those at cooler latitudes, trees may benefit from warmer urban temperatures, but trees in many cities are beset with problems like drought stress and increased herbivory. What drives patterns of urban tree health across urbanization and latitudinal temperature gradients? In natural systems, latitude–herbivory relationships are well‐studied, and recent temperate studies have shown that herbivory generally increases with decreasing latitudes (warmer temperatures). However, the applicability of this latitude–herbivory theory in already‐warmed urban systems is unknown. In this study, we investigated how the interaction of urbanization, latitudinal warming and scale insect abundance affected urban tree health. We predicted that trees in warmer, lower latitude cities would be in poorer health at lower levels of urbanization than trees at cooler, higher latitudes due to the interaction of urbanization, latitudinal temperature and herbivory. To evaluate our predictions, we surveyed the abundance of scale insect herbivores on a single, common tree species Acer rubrum in eight US cities spanning 10° of latitude. We estimated urbanization at two extents, a local one that accounted for the direct effects on an individual tree, and a larger one that captured the surrounding urban landscape. We found that urban tree health did not vary with latitudinal temperature but was best predicted by local urbanization and herbivore abundance. We did not observe increased herbivore abundance in warmer, lower latitudes cities, but instead herbivore abundance peaked in the mid latitudes of our study. This study demonstrates that urban landscapes may deviate from classical theory developed in natural systems and reinforces the need for research reconciling ecological patterns in urban landscapes.     

Do cities simulate climate change? A comparison of herbivore response to urban and global warming

Cities experience elevated temperature, CO₂, and nitrogen deposition decades ahead of the global average, such that biological response to urbanization may predict response to future climate change. This hypothesis remains untested due to a lack of complementary urban and long‐term observations. Here, we examine the response of an herbivore, the scale insect Melanaspis tenebricosa, to temperature in the context of an urban heat island, a series of historical temperature fluctuations, and recent climate warming. We survey M. tenebricosa on 55 urban street trees in Raleigh, NC, 342 herbarium specimens collected in the rural southeastern United States from 1895 to 2011, and at 20 rural forest sites represented by both modern (2013) and historical samples. We relate scale insect abundance to August temperatures and find that M. tenebricosa is most common in the hottest parts of the city, on historical specimens collected during warm time periods, and in present‐day rural forests compared to the same sites when they were cooler. Scale insects reached their highest densities in the city, but abundance peaked at similar temperatures in urban and historical datasets and tracked temperature on a decadal scale. Although urban habitats are highly modified, species response to a key abiotic factor, temperature, was consistent across urban and rural‐forest ecosystems. Cities may be an appropriate but underused system for developing and testing hypotheses about biological effects of climate change. Future work should test the applicability of this model to other groups of organisms.     

Species of street tree is important for southern hemisphere bird trophic guilds

Abstract Urban environments are highly modified with unique assemblages of bird species. Much of the research on urban bird assemblages comes from the northern hemisphere. Southern hemisphere bird assemblages differ from northern hemisphere assemblages in that they contain a large proportion of nectarivores. In this study we focus on bird use of street trees in Australia. We investigate the relative influence of tree species (plane tree, Platanus x acerifolia; red gums, Eucalyptus camaldulensis; jacaranda, Jacaranda mimosifolia; bottlebrush, Callistemon citrinus), season and the environment surrounding street trees, on the abundance and species richness of birds in Adelaide, South Australia. Our study considers birds in terms of granivore, nectarivore and insectivore trophic guilds. Nectarivores accounted for the greatest proportion of observations, in terms of abundance, in each of the tree species investigated. Species of street tree was a significant influence on bird use of the trees for all birds and each dietary guild. Red gums were used more than the other tree species by nectarivores, while plane trees were used more than the other trees by insectivores. Use of the tree species by granivores varied with the season. The measures of the environment surrounding street trees were largely unimportant with the exception of traffic for nectarivores in some cases. Urban avifauna use street trees and the species of tree will strongly influence its use by birds. However, the pattern of use of street tree species varied at different times of year and differed between different trophic guilds of birds. The dominance of nectarivores in the southern avifauna will likely produce different patterns of urban environment use to northern hemisphere avifaunas.     

Xylosandrus germanus in Central Europe: Spread into and within the Czech Republic

Invasive organisms represent great threats to ecosystems and great challenges to forest management. In Europe, the black timber bark beetle (Xylosandrus germanus) is an invasive secondary pest that mostly attacks the logs of felled trees. We showed the invasion history for Europe and using many local surveys, we summarize the current distribution and other available information on X. germanus in the Czech Republic. We report that this species is distributed from the lowlands to the mountains in the Czech Republic; it is widespread in the eastern half of the country, where it is more abundant in the warmer south and southeast areas than in the cooler areas. Most (78%) of the known localities are at elevation below 400 m a.s.l. Although an ice storm greatly increased X. germanus abundance near the border with Austria, its high abundance did not result in damage to standing trees. Presence of X. germanus in the Czech Republic for over 10 years has not led to heavy tree infestation.     

北京不同类型人工林中蛾类群落特征

近年来,绿化造林工程在北京市大规模展开,涉及林地生境类型多样.林地昆虫群落组成和多样性特征受林分树种组成、立地条件、管护方式的影响.通过对北京市5种人工林样地中蛾类群落结构、多样性和相似性及其时空动态特征的调查,探讨了蛾类多样性变化对生境类型的生态响应.结果 表明:共获得标本7046头,隶属于25科269种,以夜蛾科(...     

Individual tree traits shape insect and disease damage on oak in a climate‐matching tree diversity experiment

Diversifying planted forests by increasing genetic and species diversity is often promoted as a method to improve forest resilience to climate change and reduce pest and pathogen damage. In this study, we used a young tree diversity experiment replicated at two sites in the UK to study the impacts of tree diversity and tree provenance (geographic origin) on the oak (Quercus robur) insect herbivore community and a specialist biotrophic pathogen, oak powdery mildew. Local UK, French, and Italian provenances were planted in monocultures, provenance mixtures, and species mixes, allowing us to test whether: (a) local and nonlocal provenances differ in their insect herbivore and pathogen communities, and (b) admixing trees leads to associational effects on insect herbivore and pathogen damage. Tree diversity had variable impacts on foliar organisms across sites and years, suggesting that diversity effects can be highly dependent on environmental context. Provenance identity impacted upon both herbivores and powdery mildew, but we did not find consistent support for the local adaptation hypothesis for any group of organisms studied. Independent of provenance, we found tree vigor traits (shoot length, tree height) and tree apparency (the height of focal trees relative to their surroundings) were consistent positive predictors of powdery mildew and insect herbivory. Synthesis. Our results have implications for understanding the complex interplay between tree identity and diversity in determining pest damage, and show that tree traits, partially influenced by tree genotype, can be important drivers of tree pest and pathogen loads.     

Tree recovery and seed dispersal by birds: Comparing forest, agroforestry and abandoned agroforestry in coastal Ecuador

We used a highly replicated study to examine vegetation characteristics between patches of intervened forest, abandoned agroforestry systems with coffee and actively managed agroforestry systems with coffee in a tropical landscape. In all habitats, plant structural characteristics, individual abundance, species richness and composition were recorded for the three plant size classes: adult trees, saplings and seedlings. Furthermore, bird species richness and composition, and seeds dispersed by birds were recorded. Tree abundance was higher in forest habitats while saplings and seedlings were more abundant in abandoned coffee sites. Although species richness of adult trees was similar in the three habitats, species richness of saplings and seedlings was much higher in forest and abandoned coffee than in managed coffee sites. However, in spite of their relatively low species richness, managed coffee sites are an important refuge for tree species common to the almost disappeared mature forest in the area. Floristic similarity for adult trees was relatively low between land use types, but clearly higher for seedlings, indicating homogenizing processes at the landscape level. More than half of the saplings and seedling were not represented by adults in the canopy layer, suggesting the importance of seed dispersal by birds between habitats. Our results show that each of the studied ecosystems plays a unique and complementary role as seed source and as habitat for tree recovery and tree diversity.     

中国35个城市行道树树种组成特征研究

明确城市森林树种组成是城市森林建设与管理的基础。以往研究多局限在特定城市或区域，难以准确揭示其普遍规律。本研究基于百度街景选取全国35个主要城市，共设置88 632个样点。并根据地理区域将这些城市划分为北方、南方、青藏和西北城市，比较不同区域城市行道树树种组成差异。结果表明：①基于街景调查我国常用行道树共99种，最常见的是杨树（Populus spp.）、柳树（Salix spp.）、香樟（Cinnamomum camphora）、雪松（Cedrus deodara）和国槐（Sophora japonica）；②我国南方城市行道树种多选用阔叶树如香樟，北方和西北城市行道树则以杨树、柳树和槐树为主，青藏地区行道树以杨树和针叶树为主；③依据10/20/30经验法则，仅有昆明和桂林城市行道树在种的水平上配置较为合理，其他城市植物配置均不合理，尤其克拉玛依市的杨树相对多度高达61.2%，长沙市和杭州市的樟树比例超过树种总数的50%，单个树种优势明显，4个区域行道树树种水平上配置均不合理。今后在行道树种选择时北方、西北和青藏区域城市应减少杨树、柳树、松树和槐树等的利用，南方地区应增加适宜本土生长的其他树木种类。本研究为我国合理规划城市行道树，系统开展多城市对比和综合提升生态服务功能提供了重要参数。     

Decreased losses of woody plant foliage to insects in large urban areas are explained by bird predation

Despite the increasing rate of urbanization, the consequences of this process on biotic interactions remain insufficiently studied. Our aims were to identify the general pattern of urbanization impact on background insect herbivory, to explore variations in this impact related to characteristics of both urban areas and insect–plant systems, and to uncover the factors governing urbanization impacts on insect herbivory. We compared the foliar damage inflicted on the most common trees by defoliating, leafmining and gall‐forming insects in rural and urban habitats associated with 16 European cities. In two of these cities, we explored quality of birch foliage for herbivorous insects, mortality of leafmining insects due to predators and parasitoids and bird predation on artificial plasticine larvae. On average, the foliage losses to insects were 16.5% lower in urban than in rural habitats. The magnitude of the overall adverse effect of urbanization on herbivory was independent of the latitude of the locality and was similar in all 11 studied tree species, but increased with an increase in the size of the urban area: it was significant in large cities (city population 1–5 million) but not significant in medium‐sized and small towns. Quality of birch foliage for herbivorous insects was slightly higher in urban habitats than in rural habitats. At the same time, leafminer mortality due to ants and birds and the bird attack intensity on dummy larvae were higher in large cities than in rural habitats, which at least partially explained the decline in insect herbivory observed in response to urbanization. Our findings underscore the importance of top‐down forces in mediating impacts of urbanization on plant‐feeding insects: factors favouring predators may override the positive effects of temperature elevation on insects and thus reduce plant damage.     

Variation in ecosystem services of street tree assemblages can guide sustainable urban development

城市行道树集合生态服务功能变化研究及对城市可持续发展管理的启示     

Tree colonization by the Asian longhorn beetle,Anoplophora glabripennis (Coleoptera: Cerambycidae): effect of habitat and tree suitability

Tree colonization and feeding activity of the invasive wood‐borer Asian longhorn beetle (Anoplophora glabripennis), an Asian pest introduced into North America and Europe, was studied in a newly invaded area in Italy. The hypothesis being tested was that the reproductive success of the insect depend on habitat type and tree suitability. Adult beetles were caged on branches of host and nonhost species, in both urban and forest habitats. Two months later, number and size of feeding patches on plant tissues, eggs laid, and surviving larvae were assessed. Bark concentration of C and N was also measured from the same trees. Results indicated that the mean area of plant tissues consumed by adult feeding was significantly larger on trees growing in forest than in urban habitat, although within the same habitat there were no differences between susceptible and nonsusceptible trees. ALB tree colonization, in terms of number of eggs laid and young larvae survival, was not affected by habitat while it was higher on susceptible trees. Although trees growing in forests had a lower nitrogen concentration, they allowed colonization rates similar to those of trees growing in the urban habitat. Hence, the amount of carbon and nitrogen did not fully explain tree suitability or habitat selection. We suggest compensatory feeding as a potential mechanism that might explain this peculiar situation, as supported by a more intensive feeding activity recorded on trees in the forest. Suitability of different trees may be due to other factors, such as secondary chemical compounds.     

北京市麻雀体质水平沿城市化梯度的变化

麻雀是城市和乡村均有分布的鸟类物种.快速的城市化正在使城市麻雀的栖息地和食物资源大量减少.以北京市为例,研究了麻雀的体质水平沿城市化梯度的变化,以期为我国城市化过程中的鸟类保护提供参考.研究结果表明城市高层楼房居民区、低层楼房居民区以及大学校园中麻雀的体质指数显著低于郊区环境,而城市公园和平房居民区麻雀的体质水平则与郊区环境的差异较小.随采样点城市化水平的增加,麻雀的体质水平呈下降趋势,高度城市化的环境导致麻雀的体质水平下降.在城市化过程中,增加城市公园的数量及居民区的植被覆盖量可为麻雀等鸟类提供必要的生活资源,从而实现城市化过程中的生物多样性保护.     

Thermal biology,population fluctuations and implications of temperature extremes for the management of two globally significant insect pests

The link between environmental temperature, physiological processes and population fluctuations is a significant aspect of insect pest management. Here, we explore how thermal biology affects the population abundance of two globally significant pest fruit fly species, Ceratitis capitata (medfly) and C. rosa (Natal fruit fly), including irradiated individuals and those expressing a temperature sensitive lethal (tsl) mutation that are used in the sterile insect technique. Results show that upper and lower lethal temperatures are seldom encountered at the field sites, while critical minimum temperatures for activity and lower developmental thresholds are crossed more frequently. Estimates of abundance revealed that C. capitata are active year-round, but abundance declines markedly during winter. Temporal autocorrelation of average fortnightly trap captures and of development time, estimated from an integrated model to calculate available degree days, show similar seasonal lags suggesting that population increases in early spring occur after sufficient degree-days have accumulated. By contrast, population collapses coincide tightly with increasing frequency of low temperature events that fall below critical minimum temperatures for activity. Individuals of C. capitata expressing the tsl mutation show greater critical thermal maxima and greater longevity under field conditions than reference individuals. Taken together, this evidence suggests that low temperatures limit populations in the Western Cape, South Africa and likely do so elsewhere. Increasing temperature extremes and warming climates generally may extend the season over which these species are active, and could increase abundance. The sterile insect technique may prove profitable as climates change given that laboratory-reared tsl flies have an advantage under warmer conditions.     

Conservation value of degraded habitats for forest birds in southern Peninsular Malaysia

Clearance of tropical forest for agricultural purposes is generally assumed to seriously threaten the survival of forest species. In this study, we quantified the conservation value, for forest bird species, of three degraded habitat types in Peninsular Malaysia, namely rubber tree plantations, oil palm plantations, and open areas. We surveyed these degraded habitats using point counts to estimate their forest bird species richness and abundance. We assessed whether richness, abundance, and activities of different avian dietary groups (i.e. insectivores and frugivores) varied among the habitats. We identified the critical habitat elements that accounted for the distribution of forest avifauna in these degraded habitats. Our results showed that these habitats harboured a moderate fraction of forest avifauna (approximately 46–76 species) and their functions were complementary (i.e. rubber tree plantations for moving; open habitats for perching; shrubs in oil palm plantations for foraging). In terms of species richness and abundance, rubber tree plantations were more important than oil palm plantations and open habitats. The relatively high species richness of this agricultural landscape was partly due to the contiguity of our study areas with extensive forest areas. Forecasts of forest-species presence under various canopy cover scenarios suggest that leaving isolated trees among non-arboreal crops could greatly attract relatively tolerant species that require tree canopy. The conservation value of degraded habitats in agricultural landscapes seems to depend on factors such as the type of crops planted and distance to primary forest remnants.     

Spread of plant pathogens and insect vectors at the northern range margin of cypress in Italy

The Mediterranean cypress (Cupressus sempervirens) is a multi-purpose tree widely used in the Mediterranean region. An anthropogenic range expansion of cypress has taken place at the northern margin of the range in Italy in recent decades, driven by ornamental planting in spite of climatic constraints imposed by low winter temperature. The expansion has created new habitats for pathogens and pests, which strongly limit tree survival in the historical (core) part of the range. Based on the enemy release hypothesis, we predicted that damage should be lower in the expansion area. By comparing tree and seed cone damage by pathogens and pests in core and expansion areas of Trentino, a district in the southern Alps, we showed that tree damage was significantly higher in the core area. Seed cones of C. sempervirens are intensively colonized by an aggressive and specific pathogen (the canker fungus Seiridium cardinale, Coelomycetes), associated with seed insect vectors Megastigmus wachtli (Hymenoptera Torymidae) and Orsillus maculatus (Heteroptera Lygaeidae). In contrast, we observed lower tree damage in the expansion area, where a non-aggressive fungus (Pestalotiopsis funerea, Coelomycetes) was more frequently associated with the same insect vectors. Our results indicate that both insect species have a great potential to reach the range margin, representing a continuous threat of the arrival of fungal pathogens to trees planted at extreme sites. Global warming may accelerate this process since both insects and fungi profit from increased temperature. In the future, cypress planted at the range margin may then face similar pest and pathogen threats as in the historical range.     

Bird predation enhances tree seedling resistance to insect herbivores in contrasting forest habitats

According to the associational resistance hypothesis, neighbouring plants are expected to influence both the insect herbivore communities and their natural enemies. However, this has rarely been tested for the effects of canopy trees on herbivory of seedlings. One possible mechanism responsible for associational resistance is the indirect impact of natural enemies on insect herbivory, such as insectivorous birds. But it remains unclear to what extent such trophic cascades are influenced by the composition of plant associations (i.e. identity of ‘associated’ plants). Here, we compared the effect of bird exclusion on insect leaf damage for seedlings of three broadleaved tree species in three different forest habitats. Exclusion of insectivorous birds affected insect herbivory in a species-specific manner: leaf damage increased on Betula pendula seedlings whereas bird exclusion had no effect for two oaks (Quercus robur and Q. ilex). Forest habitat influenced both the extent of insect herbivory and the effect of bird exclusion. Broadleaved seedlings had lower overall leaf damage within pine plantations than within broadleaved stands, consistent with the resource concentration hypothesis. The indirect effect of bird exclusion on leaf damage was only significant in pine plantations, but not in exotic and native broadleaved woodlands. Our results support the enemies hypothesis, which predicts that the effects of insectivorous birds on insect herbivory on seedlings are greater beneath non-congeneric canopy trees. Although bird species richness and abundance were greater in broadleaved woodlands, birds were unable to regulate insect herbivory on seedlings in forests of more closely related tree species.     

Going undercover: increasing canopy cover around a host tree drives associational resistance to an insect pest

Neighbouring heterospecific plants are often observed to reduce the probability of herbivore attack on a given focal plant. While this pattern of associational resistance is frequently reported, experimental evidence for underlying mechanisms is rare particularly for potential plant species diversity effects on focal host plants and their physical environment. Here, we used an established forest diversity experiment to determine whether tree diversity effects on an important insect pest are driven by concomitant changes in host tree growth or the light environment. We examined the effects of tree species richness, canopy cover and tree growth on the probability of occurrence, the abundance, and volume of galls caused by the pineapple gall adelgid Adelges abietis on Norway spruce. Although tree diversity had no effect on gall abundance, we observed that both the probability of gall presence and gall volume (an indicator of maternal fecundity) decreased with tree species richness and canopy cover around host spruce trees. Structural equation models revealed that effects of tree species richness on gall presence and volume were mediated by concurrent increases in canopy cover rather than changes in tree growth or host tree density. As canopy cover did not influence tree or shoot growth, patterns of associational resistance appear to be driven by improved host tree quality or more favourable microclimatic conditions in monocultures compared to mixed‐stands. Our study therefore demonstrates that changes in forest structure may be critical to understanding the responses of herbivores to plant diversity and may underpin associational effects in forest ecosystems.     

Insect abundance in managed forests benefits from multi-layered vegetation

Retention forestry intends to promote biodiversity by retaining deadwood and tree-related microhabitats. Simultaneously, production forests undergo major structural changes by conversion into near-natural forests. As insect biomass is declining, it is important to understand how insect communities respond to management-related changes in forest structure. While some structural elements, such as deadwood, are studied extensively, three-dimensional forest structure is often neglected. Terrestrial laser scanning offers new approaches to quantify three-dimensional structure but their suitability has not been evaluated with field-based insect surveys.To test how insect communities respond to forest structure, we examined insects from window traps from 122 sites in the Black Forest. For total insect abundance and for the seven most abundant taxa, we related deadwood, microhabitats, various conventional stand properties and novel remote sensing-based indices for vegetation structure to total and taxon-specific abundances. Additionally, we assessed the influences of these structural elements on community composition.Total insect abundance and abundances of most taxa were positively related to multi-layered stands, as derived from remote sensing techniques. Furthermore, each taxon responded to some additional forest structural elements. Higher tree diameter, canopy gap fraction and share of deciduous trees increase abundances of the predominantly herbivorous taxa Heteroptera, Sternorrhyncha and Auchenorrhyncha. Community composition was influenced by mean tree diameter and share of deciduous trees. Neither tree-related microhabitats nor deadwood diversity had a detectable effect on insect abundance.We conclude that more elements of forest structure than previously acknowledged are related to insect populations. In particular, multi-layered forest stands have higher insect abundances in the midstorey. The current conversion in continuous-cover forestry in Europe from even-aged, often conifer-dominated forests to uneven-aged, mixed species stands can therefore increase the abundance of a wide range of insect taxa and is possibly one strategy to halt insect decline in forests.     

The comparative growth rates of indigenous street and garden trees in Grahamstown,South Africa

Urban forestry is advocated worldwide as a means of enhancing the liveability of towns and cities, and mitigating some of the anticipated impacts of climate change. Optimisation of the benefits of trees in urban areas is dependent upon knowledge of tree form, growth, and the products and benefits that trees provide. Growth rates are a vital variable for modelling benefits, yet there is a significant gap in knowledge pertaining to growth rates of trees in urban areas, especially indigenous species in developing world countries. Here we report on growth rates of indigenous street and garden trees in Grahamstown, South Africa, using two approaches; tree ring counts on increment cores and mean rates from trees of known planting age. Growth equations for both street and garden trees were derived. There was no significant difference in mean growth rates determined via the two methods. For both methods street trees grew approximately 30% slower than trees in gardens.     

Tree species diversity influences herbivore abundance and damage: meta-analysis of long-term forest experiments

Plant monocultures are commonly believed to be more susceptible to herbivore attacks than stands composed of several plant species. However, few studies have experimentally tested the effects of tree species diversity on herbivory. In this paper, we present a meta-analysis of uniformly collected data on insect herbivore abundance and damage on three tree species (silver birch, black alder and sessile oak) from seven long-term forest diversity experiments in boreal and temperate forest zones. Our aim was to compare the effects of forest diversity on herbivores belonging to different feeding guilds and inhabiting different tree species. At the same time we also examined the variation in herbivore responses due to tree age and sampling period within the season, the effects of experimental design (plot size and planting density) and the stability of herbivore responses over time. Herbivore responses varied significantly both among insect feeding guilds and among host tree species. Among insect feeding guilds, only leaf miner densities were consistently lower and less variable in mixed stands as compared to tree monocultures regardless of the host tree species. The responses of other herbivores to forest diversity depended largely on host tree species. Insect herbivory on birch was significantly lower in mixtures than in birch monocultures, whereas insect herbivory on oak and alder was higher in mixtures than in oak and alder monocultures. The effects of tree species diversity were also more pronounced in older trees, in the earlier part of the season, at larger plots and at lower planting density. Overall our results demonstrate that forest diversity does not generally and uniformly reduce insect herbivory and suggest instead that insect herbivore responses to forest diversity are highly variable and strongly dependent on the host tree species and other stand characteristics as well as on the type of the herbivore.