Bark thickness determines fire resistance of selected tree species from fire-prone tropical savanna in north Australia

We investigated the fire resistance conferred by bark of seven common tree species in north Australian tropical savannas. We estimated bark thermal conductance and examined the relative importance of bark thickness, density and moisture content for protecting the cambium from lethal fire temperatures. Eucalypt and non-eucalypt species were contrasted, including the fire-sensitive conifer Callitris intratropica. Cambial temperature responses to bark surface heating were measured using a modified wick-fire technique, which simulated a heat pulse comparable to surface fires of moderate intensity. Bark thickness was a better predictor of resistance to cambial injury from fires than either bark moisture or density, accounting for 68% of the deviance in maximum temperature of the cambium. The duration of heating required to kill the cambium of a tree (τ_c) was directly proportional to bark thickness squared. Although species did not differ significantly in their bark thermal conductance (k), the thinner barked eucalypts nevertheless achieved similar or only slightly lower levels of fire resistance than much thicker barked non-eucalypts. Bark thickness alone cannot account for the latter and we suggest that lower bark moisture content among the eucalypts also contributes to their apparent fire resistance. Unique eucalypt meristem anatomy and epicormic structures, combined with their bark traits, probably facilitate resprouting after fire and ensure the dominance of eucalypts in fire-prone savannas. This study emphasises the need to take into account both the thermal properties of bark and the mechanism of bud protection in characterising the resprouting ability of savanna trees.     

Ants,fire, and bark traits affect how African savanna trees recover following damage

Bark damage resulting from elephant feeding is common in African savanna trees with subsequent interactions with fire, insects, and other pathogens often resulting in tree mortality. Yet, surprisingly little is known about how savanna trees respond to bark damage. We addressed this by investigating how the inner bark of marula (Sclerocarya birrea), a widespread tree species favoured by elephants, recovers after bark damage. We used a long‐term fire experiment in the Kruger National Park to measure bark recovery with and without fire. At 24 months post‐damage, mean wound closure was 98, 92, and 72%, respectively, in annual and biennial burns and fire‐exclusion treatments. Fire exclusion resulted in higher rates of ant colonization of bark wounds, and such ant colonization resulted in significantly lower bark recovery. We also investigated how ten common savanna tree species respond to bark damage and tested for relationships between bark damage, bark recovery, and bark traits while accounting for phylogeny. We found phylogenetic signal in bark dry matter content, bark N and bark P, but not in bark thickness. Bark recovery and damage was highest in species which had thick moist inner bark and low wood densities (Anacardiaceae), intermediate in species which had moderate inner bark thickness and wood densities (Fabaceae) and lowest in species which had thin inner bark and high wood densities (Combretaceae). Elephants prefer species with thick, moist inner bark, traits that also appear to result in faster recovery rates.     

Anatomical inferences on aerial bud protection of three Eugenia shrub species from the Cerrado

• Location and degree of protection of aerial buds are important functional traits in disturbance- or stress-prone environments since aerial buds ensure the development of new organs under favourable growing conditions. This study was carried out in a Brazilian Cerrado area under regeneration after long-term Pinus cultivation, where the trees were clear-cut in 2012 and the remaining material was burned in 2014.
• After the fire treatment, several species resprouted from belowground organs and their aboveground organs were directly exposed to full sunlight. We collected 15 terminal branches with fully expanded leaves from three individuals of each of three Eugenia species to investigate if those with well-developed belowground organs invest in bark for aboveground bud protection. The samples were analysed using light and electron microscopy.
• In addition to terminal and axillary buds, all species presented accessory buds, and the number varied according to the node analysed. None of the aerial buds were protected by bark, but all were well protected by cataphylls and densely pubescent leaf primordia. There were also inter- and intra-petiolar colleters that released a mucilaginous protein exudate. The distance between the shoot apical meristem and the outer surface was longer in the terminal bud than in axillary buds. The bud leaf primordia covering the shoot apical meristem had a thick cuticle, unicellular non-glandular trichomes that accumulate phenolic and lipophilic compounds, and secretory cavities.
• Our study shows that all three Eugenia species studied here had highly protected aerial buds allocated from belowground organs. These morphological traits may improve the chances of the species' persistence in areas subjected to frost events, low relative humidity, high irradiance and harmful UV levels.

     

Variations in bark-stripping by red deer Cervus elaphus across Europe

• 1 The literature on bark‐stripping by red deer Cervus elaphus in Europe is reviewed to reveal quantitative variation in this behaviour and relate it to deer density and local characteristics such as dominant tree species, occurrence of artificial feeding, altitude, region and size of the study site. We also review the importance of bark in red deer diets over the seasons and discuss the causes of bark‐stripping, focusing on the significance of bark as food.
• 2 Over the 36 sites examined, the rate of bark‐stripping was highly variable (from 0 to 84% of susceptible trees debarked), with less damage in Scotland than in other European sites for which bark‐stripping rates were higher at high red deer density. Altitude, the size of the study site, the number of dominant tree species and the occurrence of artificial feeding do not significantly relate to the rate of bark‐stripping.
• 3 Bark sometimes made up a large proportion of red deer diet (> 10%), especially in areas with severe winters (high levels of snow), whereas in study sites with mild winters, bark was practically not eaten at all.
• 4 These results suggest that severe bark‐stripping could be related to a reduction in food resource availability. This food availability hypothesis needs to be better documented, dealing particularly with the possible interaction between food availability and red deer density.

     

Fire-tolerance mechanisms of common woody plant species in a semiarid savanna in south-western Zimbabwe

The frequency of fire has increased in savannas yet few studies have assessed how plants persist when subjected to long‐term disturbance by fire. We investigated the contributions of bark thickness and resprouting to the persistence of woody plants in two fire trials that were started in 1948 and 1949. The number of resprouts per individual, bark thickness, basal diameter and height of woody plants were measured in unburnt plots and those burnt annually, triennially and quinquennially during the late dry season. Changes in tree density, number of resprouts and individuals in different height classes between 1963 and 2002 were assessed. Bark thickness varied among species and also increased with increases in basal diameter. Generally, plants with thick bark survived fire more than those with thin bark. Resprouting was the major fire survival strategy for most species. The number of resprouts produced per plant ranged from 4 ± 3 (Acacia rehmanniana) to 14 ± 9 (Pseudolachnostylis maprouneifolia). Fire reduced species richness in plots burnt annually and triennially by 47% and 6% respectively. Species richness increased in unburnt plots (5%) and those burnt quinquennially (16%). Most woody species survived fire through a combination of traits.     

Thermal properties and fauna on the bark of trees in two different African ecosystems

Summary The thermal properties of 26 African tree species in two different ecosystems were studied using thermocouples. In a subtropical moist forest were three bark types of trees: species with thin and smooth bark types with low values of insulation across the bark; species with a more structured bark type and medium insulation values; and species with deep-fissured or scaly bark types and high insulation values. Only these latter trees are able to survive openings in the subtropical forest and stand alone on edges of forest gaps. In the savanna all tree species showed adaptations in the structure of their bark in different forms: many tree species shade their trunks. Some have low bark insulation and these are known to be sensitive to fires. Some tree species show high bark insulation and do not shade their trunks. Tree species with white bark avoid overheating of their surface by reflection of the radiation. The arthropod community living exclusively on the bark was investigated for the first time on South African trees, on ten tree species. In the two different ecosystems this habitat is occupied by different arthropod groups. In the subtropical forest Acari, Araneae, Opiliones, Isopoda, Myriopoda, Blattodea, Psocoptera, Heteroptera, Coleoptera, Formicidae, and Nematocera (Diptera) are the main arthropod groups living exclusively on the bark of trees. In the savanna Pseudoscorpiones, Araneae, Collembola, Blattodea, Psocoptera, Coleoptera, Neuroptera, Termites, Formicidae, Hymenoptera and Brachycera (Diptera) are the main arthropod groups living exclusively on the bark of trees. Within one ecosystem on one bark type the dominant species are similar; richly structured bark types have a richer fauna. In the forest, bark arthropod diversity is related to the bark structure of the constituent trees, and the arthropod communities on the bark would reflect changes in the structure of the forest. Forests comprising tree species with different bark types would have a richer total bark arthropod fauna. Specialists on richly structured bark types would die out if tree species composition were altered by man to give stands consisting only of tree species with smooth bark types. Bark arthropods in a subtropical moist forest have different proportions of herbivorous and fungivorous compared to carnivorous species than those on the bark of trees in a savanna.     

Large‐scale manipulation of oviposition substrata affects egg supply to populations of some stream‐dwelling caddisflies

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武夷山常绿阔叶林乔木与灌木小枝树皮主要功能性状的趋同

树皮是维管形成层以外的所有组织,其功能性状是反映植物生态适应策略的重要研究对象。为了揭示亚热带山地乔、灌木小枝树皮功能性状特征及其关联性,采用独立样本T检验和标准主轴回归方法对武夷山19种常见木本植物小枝树皮的主要功能性状的基本特征及其相关性进行了分析。结果发现：(1)树皮厚度、树皮密度与树皮含水量在乔、灌木之间无显著差异,其均值分别为(0.78±0.06)mm、(0.58±0.03)mg/cm³和(155.07±14.16)%。树皮碳(C)、氮(N)、磷(P)生态化学计量特征在乔、灌木之间差异也不显著,C、N、P平均值分别为(492.43±4.25)mg/g、(8.90±0.74)mg/g和(0.61±0.06)mg/g,树皮C∶N、C∶P与N∶P平均值分别为60.76±4.17、905.53±67.36与15.19±0.84。(2)树皮性状之间存在关联,其中树皮厚度与密度、密度与含水量均呈异速生长关系,而树皮厚度与含水量、氮含量与磷含量均呈等速生长关系。乔木树皮碳含量和磷含量间、灌木树皮密度和氮含量间均呈异速生长关系,乔木与灌木各性状间关系差异不显著。结果表明...     

Distribution of early‐arriving saproxylic beetles on standing dead Scots pine trees

• 1 The community composition of early‐arriving saproxylic beetles on 80 standing and recently deceased Scots pine trees (Pinus sylvestris L.) was examined.
• 2 In total, 34 species of saproxylic beetles were found, comprising at least four well‐defined groups of species identified by correspondence analysis.
• 3 Bark thickness, trunk/branch diameter and height above ground significantly affected community composition, with bark thickness being the most important factor.
• 4 Overall, 13.7% of the variance in species composition was explained by section of the tree that was sampled, a variable that encompasses the three aforementioned parameters.

     

Tree age and bark thickness as traits linked to frost ring probability on Araucaria araucana trees in northern Patagonia

Frost events may damage the cambium and consequently the newly produced tracheids whose cell walls have not yet completed their lignifications, leading to the formation of frost rings. This study deals with the presence of frost rings in Araucaria araucana trees according to cambial age and bark thickness, under the assumption that these factors may be involved in physical or physiological mechanisms that increase resistance to freezing temperatures that impact the cambial tissue. The study was conducted in northern Patagonia at two sites of contrasting geomorphology, and therefore potentially associated with a differential degree of exposure to extreme cold. Wood plus bark cores were extracted from main stems at two heights from the ground and from each of the four cardinal point directions for 30 individuals per site. A Linear Mixed Model and a Generalized Linear Mixed Model were applied in order to relate the bark thickness and the frequency of frost rings in accordance with the different sampling points on the stem. It was observed that as bark becomes thicker with cambial age, the frequency of frost rings decreases, indicating a possible thermal-induced mechanism of bark protection. Consequently, there is an increase in the presence of frost rings at the younger stages of tree life. Although the mechanisms of cold hardiness in trees can be complex, including aspects of the tree physiology, our data indicated that as tree age increases, the thickness of the bark is higher, resulting in a potential effect of isolation and passive protection against the harmful effects of frosts. This mechanism may be relevant in the ecology, conservation and management of forests faced with extreme variability in future climate and changing scenarios.     

古田山亚热带常绿阔叶林树皮厚度的变异特征

树皮是木本植物茎干最外层结构,具有保护茎干、养分储存与运输等重要作用。因此,树皮厚度是一项非常重要的功能性状,其变异不仅影响树皮的各种生态功能,还能影响群落构建与物种共存。然而,以往对树皮厚度的研究集中于火灾易发生态系统,对火灾不易发生的亚热带常绿阔叶林的研究仍较缺乏。测量了古田山国家级自然保护区亚热带常绿阔叶林内树种的树皮厚度,并检验了总树皮厚度、内树皮厚度与外树皮厚度在各分类群间以及功能群间的差异。结果发现：1）39个树种807个个体的总树皮厚度、内树皮厚度与外树皮厚度均值分别为1.90 mm、1.38 mm和0.54 mm。漆树科（Anacardiaceae）、杨梅科（Myricaceae）以及亚热带常绿阔叶林代表类群壳斗科（Fagaceae）、山茶科（Theaceae）的树皮厚度较大。短柄枹（Quercus serrata）、木荷（Schima superba）、小叶青冈（Cyclobalanopsis myrsinifolia）等树种的树皮厚度较大。2）种间、科间的各树皮厚度差异均显著。不同功能类群间,乔木类群的各树皮厚度均较灌木类群大,常绿类群的各树皮厚度均较落叶类群大（内树皮厚度除外）。本次研究结果表明,相对于火灾易发生态系统中的树皮厚度,古田山亚热带常绿阔叶林群落内的树皮厚度相对较薄,表明这些森林树种对当地湿润气候的适应性。同时,树皮厚度在各种分类水平与功能群水平间的显著变异,反映了群落内不同生态策略的共存。     

Reactions of the fauna on the bark of trees to the frequency of fires in a North American savanna

Summary The arthropod communities living on the bark of the oak species Quercus macrocarpa and Q. ellipsoidalis were investigated in a North American oak savanna. Differences were found in the community structure of the arthropods living on the bark of these two tree species, although they have the same fissured bark type. In the North American oak savanna ecosystem the most important disturbance factor is fire, which maintains species richness. Highest numbers of species and specimens were found at moderately disturbed sites. Three main ecological groups of arthropods living on the bark of trees can be distinguished in relation to the degree of disturbance: (1) Inhabitants of bark of trees restricted to undisturbed sites: they do not occur in fire-disturbed areas; (2) Inhabitants of bark of trees adapted to a moderate degree of disturbance: many species occur in high numbers only in moderately disturbed areas; and (3) Specialist inhabitants of bark of trees in heavily disturbed areas. The number of specimens of these species increases per trunk with the frequency of disturbance.     

Invasion windows for a global legume invader are revealed after joint examination of abiotic and biotic filters

• Successful alien plant invasion is influenced by both climate change and plant–plant interactions. We estimate the single and interactive effects of competition and extreme weather events on the performance of the global legume invader Lupinus polyphyllus (Lindl.).
• In three experimental studies we assessed (i) the stress tolerance of seedling and adult L. polyphyllus plants against extreme weather events (drought, fluctuating precipitation, late frost), (ii) the competitive effects of L. polyphyllus on native grassland species and vice versa, and (iii) the interactive effects of extreme weather events and competition on the performance of L. polyphyllus.
• Drought reduced growth and led to early senescence of L. polyphyllus but did not reduce adult survival. Fluctuating precipitation events and late frost reduced the length of inflorescences. Under control conditions, interspecific competition reduced photosynthetic activity and growth of L. polyphyllus. When subjected to competition during drought, L. polyphyllus conserved water while simultaneously maintaining high assimilation rates, demonstrating increased water use efficiency. Meanwhile, native species had reduced performance under drought.
• In summary, the invader gained an advantage under drought conditions through a smaller reduction in performance relative to its native competitors but was competitively inferior under control conditions. This provides evidence for a possible invasion window for this species. While regions of high elevation or latitude with regular severe late frost events might remain inaccessible for L. polyphyllus, further spread across Europe seems probable as the predicted increase in drought events may favour this non‐native legume over native species.

     

Synchronous rise and fall of bark beetle and parasitoid populations in windthrow areas

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Shifts in functional traits elevate risk of fire‐driven tree dieback in tropical savanna and forest biomes

Numerous predictions indicate rising CO₂ will accelerate the expansion of forests into savannas. Although encroaching forests can sequester carbon over the short term, increased fires and drought‐fire interactions could offset carbon gains, which may be amplified by the shift toward forest plant communities more susceptible to fire‐driven dieback. We quantify how bark thickness determines the ability of individual tree species to tolerate fire and subsequently determine the fire sensitivity of ecosystem carbon across 180 plots in savannas and forests throughout the 2.2‐million km² Cerrado region in Brazil. We find that not accounting for variation in bark thickness across tree species underestimated carbon losses in forests by ~50%, totaling 0.22 PgC across the Cerrado region. The lower bark thicknesses of plant species in forests decreased fire tolerance to such an extent that a third of carbon gains during forest encroachment may be at risk of dieback if burned. These results illustrate that consideration of trait‐based differences in fire tolerance is critical for determining the climate‐carbon‐fire feedback in tropical savanna and forest biomes.     

小蠹虫信息素的研究与应用现状

小蠹虫是世界性林木害虫,猖獗时常可造成林木成片枯死。利用小蠹虫信息素防治该虫,不仅可以有效降低虫口密度,减轻危害程度,还可以监测小蠹虫的发生为害情况。本文从小蠹虫信息素的分离鉴定、信息素的生物合成、小蠹虫对信息素的行为反应等方面综述小蠹虫信息素的研究与应用现状。     

Fire and drought: Shifts in bark investment across a broad geographical scale for Neotropical savanna trees

Savanna tree communities occurring in confluence zones with other biomes likely experience different environmental pressures, resulting in shifts in the selection of individual traits, the combinations of such traits, and species composition. In seasonally dry fire-prone environments, plant survival is presumably associated with adaptive changes in bark properties related to fire protection and water storage. Here, we integrated the multiple functions of the bark to investigate whether different selective pressures could influence patterns of variation in bark structure and allocation across species in a broad geographical range. We measured thickness, density, and water content of the inner and outer bark in branches and the main stem of the 51 most abundant species in three savanna communities differing in climatic aridity, one located at the core region of Cerrado in Central Brazil and the other two at its periphery, in the transition zones with Amazonia and Atlantic forest biomes. We found no difference in outer bark thickness but markedly difference in inner bark thickness between the three plant communities. In the central region, where dry season is long and fire is frequent, branches and main stem showed thicker inner bark. Contrastingly, in the south periphery region, where dry season is short, species showed thinner inner bark in both branches and main stem. Species from the north periphery region, where mean annual precipitation is higher, but fire is frequent and the dry season is also long, showed similar main stem inner bark thickness, but thinner branch inner bark compared to core region species. Our findings support the idea that investing in inner bark thickness and bark moisture may be the most advantageous strategy in plant communities that suffer from high evaporative demand during a long period and are at a high risk of fire.     

Dispersal mode constrains allocation of carbon and mineral nutrients in seeds of forest and savanna trees

• Species vary in seed size and content of stored reserves, which can be related to dispersal strategies and type of habitat in which they are found. We compare seed carbon and nutrient reserves of anemochorous and zoochorous trees from the Cerrado of central Brazil.
• We measured seed dry mass, lipids, non‐structural carbohydrates (starch and total soluble sugars), carbon and mineral nutrients in ten forest and 13 savanna species, each classified as having wind‐ or animal‐dispersed seeds. We used phylogenetically independent contrasts to test for correlations among these traits.
• Seeds of anemochorous species were lighter, with higher concentrations of C, N, P, Ca and Mg. Lipids were the dominant carbon reserve for most anemochorous species, underpinning the importance of allocation to compact carbon reserves. Starch, lipids or soluble sugars were the major carbon reserve in zoochorous seeds. Savanna and forest species did not differ in seed mass or in total carbon reserves. However, seeds of forest species had higher concentrations of starch than seeds of savanna species. Lipid and starch negatively correlated across species, suggesting a trade‐off between starch and lipids as major seed carbon reserves. Calcium was positively correlated with Mn and B, while Mg was positively correlated with C, N, P, K, S, Zn and B. Potassium, S and Cl were positively correlated, while P was positively correlated with Mg and Zn.
• Dispersal mode rather than vegetation type constrained seed mass and seed storage allocation patterns in forest and savanna trees. We provide evidence that similar mechanisms are involved in seed storage of carbon and mineral nutrients across species.

     

Alternative fire resistance strategies in savanna trees

Bark properties (mainly thickness) are usually presented as the main explanation for tree survival in intense fires. Savanna fires are mild, frequent, and supposed to affect tree recruitment rather than adult survival: trunk profile and growth rate of young trees between two successive fires can also affect survival. These factors and fire severity were measured on a sample of 20 trees near the recruitment stage of two savanna species chosen for their contrasted fire resistance strategies (Crossopteryx febrifuga and Piliostigma thonningii). Crossopteryx has a higher intrinsic resistance to fire (bark properties) than Piliostigma: a 20-mm-diameter stem of Crossopteryx survives exposure to 650°C, while Piliostigma needs a diameter of at least 40 mm to survive. Crossopteryx has a thicker trunk than Piliostigma: for two trees of the same height, the basal diameter of Crossopteryx will be 1.6 times greater. Piliostigma grows 2.26 times faster than Crossopteryx between two successive fires. The two species have different fire resistance strategies: one relies on resistance of aboveground structures to fire, while the other relies on its ability to quickly re-build aboveground structures. Crossopteryx is able to recruit in almost any fire conditions while Piliostigma needs locally or temporarily milder fire conditions. In savannas, fire resistance is a complex property which cannot be assessed simply by measuring only one of its components, such as bark thickness. Bark properties, trunk profile and growth rate define strategies of fire resistance. Fire resistance may interact with competition: we suggest that differences in fire resistance strategies have important effects on the structure and dynamics of savanna ecosystems. Received: 16 August 1996 / Accepted: 4 January 1997     

Drought and stand susceptibility to attacks by the European spruce bark beetle: A remote sensing approach

1. Several time-series analyses have demonstrated that after extreme summer drought bark beetle damage increased. However, studies predicting stand susceptibility over large spatial extents are limited by technical constraints in obtaining detailed, spatially-explicit data on infestation spot occurrence.
2. Using a unique dataset of georeferenced bark beetle infestation data, we tested whether the spatial variation of local growing conditions of forest stands, topography, and landscape variables modified the local occurrence of Ips typographus infestations after a severe hot drought in Central Europe.
3. Bark beetle infestation occurrence depended on soil-related aridity intensity, elevation, slope, and soil conditions. We showed that elevation interacted with growing conditions and topography. At low elevations, spruce forests growing on flat areas and wetter soils were more sensitive to the infestations. On the contrary, forests on steep slopes and soils with low water availability were rarely attacked. At the landscape scale, bark beetle damage increased with host tree cover but decreased with compositional diversity.
4. Our findings are generally consistent with the growth-differentiation balance hypothesis that predicts that trees growing under chronic dry conditions tend to be more resistant against biotic disturbances.
5. Spruce stands at low elevations located in homogeneous landscapes dominated by spruce were those more exposed to bark beetles in the initial phase of a drought-induced outbreak.