Resistance in oak (Quercus spp.) to defoliation by Tortrix viridana L. in Roudsea Wood National Nature Reserve

Resistance in oak ( Quercus spp.) to defoliation by caterpillars of Tortrix viridana L. was studied in an oak wood and a mixed wood in Roudsea Wood National Nature Reserve. When defoliation is severe in the oak wood a few trees there and most of the oaks in the mixed wood retain relatively undamaged canopies. Their resistance was found to be phenological. The resistant trees in the oak wood flush either later or earlier than those that become defoliated; those in the mixed wood flush later. On late-flushing trees the buds open too late for newly hatched larvae to penetrate them, the resultant mortality reducing the larval population below the level at which damage is severe. The early-flushing trees carry as many larvae per leaf cluster as non-resistant trees but do not show severe defoliation because they produce more foliage and 'grow away' from the attack.     

河南林州植物物候变化特征及其原因分析

根据河南省林州市1987年至2004年的物候和气象资料,运用一元线性回归法和相关分析法分析了林州市近20年来植物物候的变化特征及其对气候变化的响应。结果表明,近20年来,林州地区的毛白杨(Populus tomentosa Carr.)、刺槐(Robinia hispida Linn.)、梧桐(Firmiana simplex W.F.Wight)和白梨(Pyrus bretschneideri Rehd.)等木本植物的春季物候期提前,秋季物候期变化不一致,生长期延长;车前(Plantago asiatica Linn.)、藜(Chenopodium album Linn.)和苍耳(Xanthium sibiricum Patr.)等草本植物的春季物候期变化不一致,秋季物候期均提前,生长期缩短。木本植物春季物候变化受冬末春初气温变化的影响最大、日照次之、降水最小,秋季物候期对气候变化基本没有响应;草本植物的物候期主要受气温影响,降水能促进草本植物开花。木本植物的春季物候变化可作为反映气候变化的代用指标。     

Low temperature tolerance and starvation ability of the oak processionary moth: implications in a context of increasing epidemics

• 1 We investigated how modifications in winter and spring temperature conditions may affect the survival of a spring‐hatching Lepidoptera, the oak processionary moth Thaumetopoea processionea.
• 2 Supercooling and chilling injury experiments indicate that eggs are especially cold hardy at the start of the winter period, although this ability is reduced later in the season. In the spring, young larvae are sufficiently cold hardy to ensure no direct mortality as a result of late frosts.
• 3 A comparison of phenological models shows that neonate larvae may await the unfolding of new oak leaves for relatively long periods (e.g. 1–30 days). Under both low (4°C after 5 days at 16°C) and high temperature experimental scenarios (constant 16°C), the majority of neonate larvae can survive starvation for more than 2 weeks.
• 4 Larvae may also suffer from food depletion once their development has been initiated (e.g. during cold springs) if the threshold temperature for feeding is not reached for several consecutive days, or in the case of late frosts affecting foliage availability. When temperature is reduced to 4°C, developing larvae become inactive and do not feed anymore; their starvation survival capability is reduced to approximately 2 weeks (cold spring hypothesis). At 16°C, developing larvae that are deprived of food can only survive for 10 days (late frost hypothesis).
• 5 We conclude that, in the oak processionary moth, neonate larvae are relatively well adapted to early hatching relative to budburst, ensuring them the highest foliage quality for development. In some years, however, phenological asynchrony or cold spring conditions may affect the persistence of populations at the limits of the species' range.

     

The climate to growth relationships of pedunculate oak in steppe

Pedunculate oak (Quercus robur L.) is a long-lived species that dominates the extra–zonal natural forests in the steppe landscape of southeastern Ukraine. Although Q. robur is considered to be one of the most important species in European dendrochronology, it has received little attention in the steppe zone because of its scarcity in the often-degraded steppe forests. Nevertheless, a small and unique patch of old-growth oak exists within the boundary of Donetsk, a large industrial center in Eastern Europe. This forest is a remnant of an ancient wood and includes several dozen old-age trees that can contribute to filling some of the spatial gaps in pedunculate oak dendrochronology in Eastern Europe. In this study, we aim to determine the effect of climatic variables on pedunculate oak growth in the steppe zone, and to estimate the longevity of this species in the heterogeneous conditions of an urban forest. A total of 20 trees were cored for this study, varying in age from 55 to 254. The resulting tree-ring chronology correlates strongly with local precipitation in spring and summer, and with local temperature in April, June and July. Moving correlation analysis indicates a shift over the last 80 years in the relationship between oak growth and late winter and early spring temperatures, as well as between oak growth and precipitation in February and August. These findings imply that warming has caused both an advance in oak phenology and changes in the climatic conditions in early spring.     

On the palaeoclimatic potential of a millennium-long oak ring width chronology from Slovakia

Although Slovakia is largely forested and rich in historical buildings, it is one of the few European countries without a millennium-long tree-ring chronology. In this study, we gather all available oak ring width data from Slovakia, establish a new composite chronology and assess its climate sensitivity. The nation-wide oak network includes 276 samples from historical buildings and 1028 modern series from material that was randomly collected at sawmills and wood submission sites across Slovakia. The final composite oak record covers the period from CE 967–2013, reflects a distinct hydroclimatic signal from late spring to early summer, and is highly correlated with other oak chronologies from surrounding countries. Although this study reveals a high degree of growth coherency and climate sensitivity inherent to the new Slovakian oak ring width chronology, changes in sample size at the transition from modern to relict material and further back in time limit any applicability to palaeoclimatic analysis.     

Egg development and diapause: ecophysiological and genetic basis of phenological polymorphism and adaptation to varied hosts in the green oak tortrix,Tortrix viridana L. (Lepidoptera: Tortricidae)

Experiments concerned 8 tortrix populations associated with varied oak species. They showed that the egg development included a phase of diapause. Completion of embryogenesis at 20 degrees C was used as a criterion for whether diapause was completed. Under semi-natural conditions diapause terminated in late autumn or early winter, then eggs developed continuously, without postdiapause winter quiescence, even in severe cold. The eggs from the populations associated with holm or cork oak completed diapause then hatched later than those from the populations associated with sessile or pubescent oak. Reciprocal crossbreedings confirmed that this phenological polymorphism was genetically determined. Under constant temperatures the physiological state of diapausing eggs, assessed by measuring their cold requirements to complete diapause by exposure to 8 degrees C, varied gradually with increasing age. This diapause development was strongly temperature-dependent. Cold requirements of diapausing eggs were much higher in a late-hatching than in an early-hatching population. This explains the phenological polymorphism of the tortrix: the more eggs need cold, the later they complete diapause in autumn, and the later they hatch in spring. Egg cold requirements varied widely within populations too, which resulted in large variations in the date of diapause end among individuals. The date of egg hatch was influenced by the temperatures occurring during diapause and postdiapause, but apparently not by photoperiod.     

Rapid recycling of triose phosphates in oak stem tissue

We report the carbon-13 and oxygen-18 isotope ratios in cellulose from the early and late wood of pedunculate oak (Quercus robur L.). The δ¹³ C value of the early wood correlates best with that of the late wood of the previous year. The δ¹⁸O value of the early wood correlates best with that of the late wood of the same year. We suggest that a biochemical explanation of these data is that there is a rapid cycle between hexose monophosphates and triose phosphates in oak stem tissue during cellulose synthesis. Evidence in support of this explanation is provided by the intramolecular distribution of ¹⁴C in labelled fructose extracted from cores of wood that had been supplied with [1?¹⁴C]- and [6-¹⁴C]glucose.     

秦岭东段栓皮栎枝条非结构性碳水化合物含量的季节动态

枝条是碳供应器官和碳需求器官的连接者, 研究其非结构性碳水化合物(NSC)含量的季节变化对理解树木体内的碳分配至关重要。该研究以秦岭东段栓皮栎(Quercus variabilis)优势群落为研究对象, 于2016年5月至2017年5月, 在其分布的海拔上下限(650 m和970 m), 通过在展叶期采用旬尺度和在非展叶期采用月尺度相结合的周期性取样方法(共计12次), 测定栓皮栎枝条NSC组分及含量, 并观测同期叶片物候变化。结果表明: (1)栓皮栎枝条NSC含量随季节波动较小, 变化差异不显著。但枝条可溶性糖含量(高海拔)或淀粉含量(低海拔)在一定生境条件下, 均存在明显的季节波动, 说明栓皮栎枝条可溶性糖和淀粉之间存在动态转化过程。(2)栓皮栎枝条NSC组成以可溶性糖为主(61%), 这可能是该树种在暖温带季风气候区所采取的生长策略。(3)土壤含水量(正相关)和饱和水汽压差(负相关)分别是在高海拔和低海拔影响栓皮栎枝条NSC含量的主导环境因子, 说明相比高海拔, 低海拔的栓皮栎可能对高温引起的水分胁迫更敏感。(4)结合叶片物候发现, 栓皮栎枝条NSC含量最大值出现在萌芽前(3月中下旬, 11%左右), 最小值出现在展叶后期(4月末, 5%左右), 叶片萌芽展叶后枝条NSC含量下降。总体而言, 枝条NSC含量在高低海拔不存在显著差异, 但春季萌芽前后存在显著差异, 海拔引起的叶片物候时间差极可能是造成这一现象的主要原因。研究结果说明, 栓皮栎叶片物候会直接影响枝条NSC含量的季节变化, 枝条NSC含量对叶片萌芽生长至关重要, 研究结果有助于加深对栓皮栎树体内碳调配机制的理解。     

Chilling and heat requirements for leaf unfolding in European beech and sessile oak populations at the southern limit of their distribution range

With global warming, an advance in spring leaf phenology has been reported worldwide. However, it is difficult to forecast phenology for a given species, due to a lack of knowledge about chilling requirements. We quantified chilling and heat requirements for leaf unfolding in two European tree species and investigated their relative contributions to phenological variations between and within populations. We used an extensive database containing information about the leaf phenology of 14 oak and 10 beech populations monitored over elevation gradients since 2005. In parallel, we studied the various bud dormancy phases, in controlled conditions, by regularly sampling low- and high-elevation populations during fall and winter. Oak was 2.3 times more sensitive to temperature for leaf unfolding over the elevation gradient and had a lower chilling requirement for dormancy release than beech. We found that chilling is currently insufficient for the full release of dormancy, for both species, at the lowest elevations in the area studied. Genetic variation in leaf unfolding timing between and within oak populations was probably due to differences in heat requirement rather than differences in chilling requirement. Our results demonstrate the importance of chilling for leaf unfolding in forest trees and indicate that the advance in leaf unfolding phenology with increasing temperature will probably be less pronounced than forecasted. This highlights the urgent need to determine experimentally the interactions between chilling and heat requirements in forest tree species, to improve our understanding and modeling of changes in phenological timing under global warming.     

Two sides of a coin: host‐plant synchrony fitness trade‐offs in the population dynamics of the western spruce budworm

Conifer‐feeding budworms emerge from overwintering sites as small larvae in early spring, several days before budburst, and mine old needles. These early‐emerging larvae suffer considerable mortality during this foraging period as they disperse in search of available, current‐year buds. Once buds flush, surviving budworms construct feeding shelters and must complete maturation before fresh host foliage senesces and lignifies later in the summer. Late‐developing larvae suffer greater mortality and survivors have lower fecundity when feeding on older foliage. Thus, there is a seasonal trade‐off in fitness associated with host synchrony: early‐emerging budworms have a greater risk of mortality during spring dispersal but gain better access to the most nutritious foliage, while, on the other hand, late‐emerging larvae incur a lower risk during the initial foraging period but must contend with rapidly diminishing resource quality at the end of the feeding period. We investigate the balance that results from these early‐season and late‐season synchrony fitness trade‐offs using the concept of the phenological window. Parameters associated with the variation in the phenological window are used to estimate generational fitness as a function of host‐plant synchrony. Because defoliation modifies these relationships, it is also included in the analysis. We show that fitness trade‐offs characterizing the phenological window result in a robust synchrony relationship between budworm and host plant over a wide geographic range in southern British Columbia, Canada.     

The Temporal Distribution and Carbon Storage of Large Oak Wood in Streams and Floodplain Deposits

We used tree-ring dating and ¹⁴C dating to document the temporal distribution and carbon storage of oak (Quercus spp.) wood in trees recruited and buried by streams and floodplains in northern Missouri, USA. Frequency distributions indicated that oak wood has been accumulating in Midwest streams continually since at least the late Pleistocene, about 14,000 calibrated radiocarbon years before present (cal. BP). The median residence time of an oak bole in the study streams was 3,515 years (n = 200). More than 30% of sampled oak wood entered the floodplain sediments and stream waters within the last 1,000 years, though very few samples dated to the last 150 years. Temporal variability in the record of oak recruitment to streams suggests a potentially strong influence from shifts in climate and fluvial processes, although other possible influences are addressed. Recent human impacts on streams have altered the dynamics of oak input and sequestered carbon with unknown long-term consequences. The long duration of carbon storage (mean age = 1,960 years) in this waterlogged environment appears to be strongly limited by decreasing wood density resulting from reductions in cell wall thickness. Lack of evidence of biotic degradation may imply that wood loss is largely due to abiotic hydrolyses. These findings document a continuous and long-term form of carbon storage that is sensitive to changes in climate and anthropogenic alteration of fluvial processes.     

Fungal communities associated with acorn woodpeckers and their excavations

Wood-decay fungi soften wood, putatively providing opportunities for woodpeckers to excavate an otherwise hard substrate, yet the fungal community composition in tree cavities and the specificity of these relationships is largely unknown. We used high-throughput amplicon sequencing of the fungal ITS2 region to examine the fungal communities associated with acorn woodpeckers (Melanerpes formicivorus) and their cavities in mature valley oak (Quercus lobata) and blue oak (Q. douglasii) trees in an oak savannah of central coastal California, USA. Acorn woodpeckers and their excavations harbored over 1500 fungal taxa, including more than 100 putative wood-decay fungi. The fungal communities found on the birds were more similar to those found in excavated cavities than those found in trees without excavated holes. These results suggest that symbiotic associations between acorn woodpeckers and fungi are highly diverse, with low specificity. Symbiotic associations between cavity-excavators and fungi are likely more common and widespread than previously thought.     

The influence of local spring temperature variance on temperature sensitivity of spring phenology

The impact of climate warming on the advancement of plant spring phenology has been heavily investigated over the last decade and there exists great variability among plants in their phenological sensitivity to temperature. However, few studies have explicitly linked phenological sensitivity to local climate variance. Here, we set out to test the hypothesis that the strength of phenological sensitivity declines with increased local spring temperature variance, by synthesizing results across ground observations. We assemble ground‐based long‐term (20–50 years) spring phenology database (PEP725 database) and the corresponding climate dataset. We find a prevalent decline in the strength of phenological sensitivity with increasing local spring temperature variance at the species level from ground observations. It suggests that plants might be less likely to track climatic warming at locations with larger local spring temperature variance. This might be related to the possibility that the frost risk could be higher in a larger local spring temperature variance and plants adapt to avoid this risk by relying more on other cues (e.g., high chill requirements, photoperiod) for spring phenology, thus suppressing phenological responses to spring warming. This study illuminates that local spring temperature variance is an understudied source in the study of phenological sensitivity and highlight the necessity of incorporating this factor to improve the predictability of plant responses to anthropogenic climate change in future studies.     

Diversity of gall wasps (Hymenoptera: Cynipidae) associated with oak trees (Fagaceae: <Emphasis Type="Italic">Quercus</Emphasis>) in a fragmented landscape in Mexico

Habitat fragmentation reduces the available habitat area and increases both the distance between fragments and the amount of fragment edges. Therefore, there are more probabilities of plant population size reduction and species extinction. In the same way, biotic and abiotic changes associated with forest fragmentation can dramatically alter plant growth and phenological patterns. We conducted a 3-year study to analyze effects of habitat fragmentation and seasonal variation on host plant quality (quantity of leaves, diameter at breast height, tree height), gall abundance and species richness in a temperate oak forest. Our results show that host plant quality was significantly higher in isolated oaks and small fragments, increasing the abundance and species richness of oak gall wasp species in most fragmented habitats. Oak canopy cover is altered by forest fragmentation, there being higher production of leaves on trees that are more exposed to fragmentation, and can provide important resources for maintaining gall wasp species diversity in a fragmented landscape. We found higher gall wasp richness and abundance in autumn than in the spring, which matches with the higher quantity of leaves in this season.     

Chilling and forcing temperatures interact to predict the onset of wood formation in Northern Hemisphere conifers

The phenology of wood formation is a critical process to consider for predicting how trees from the temperate and boreal zones may react to climate change. Compared to leaf phenology, however, the determinism of wood phenology is still poorly known. Here, we compared for the first time three alternative ecophysiological model classes (threshold models, heat‐sum models and chilling‐influenced heat‐sum models) and an empirical model in their ability to predict the starting date of xylem cell enlargement in spring, for four major Northern Hemisphere conifers (Larix decidua, Pinus sylvestris, Picea abies and Picea mariana). We fitted models with Bayesian inference to wood phenological data collected for 220 site‐years over Europe and Canada. The chilling‐influenced heat‐sum model received most support for all the four studied species, predicting validation data with a 7.7‐day error, which is within one day of the observed data resolution. We conclude that both chilling and forcing temperatures determine the onset of wood formation in Northern Hemisphere conifers. Importantly, the chilling‐influenced heat‐sum model showed virtually no spatial bias whichever the species, despite the large environmental gradients considered. This suggests that the spring onset of wood formation is far less affected by local adaptation than by environmentally driven plasticity. In a context of climate change, we therefore expect rising winter–spring temperature to exert ambivalent effects on the spring onset of wood formation, tending to hasten it through the accumulation of forcing temperature, but imposing a higher forcing temperature requirement through the lower accumulation of chilling.     

Divergent trends in the risk of spring frost damage to trees in Europe with recent warming

Frost events during the active growth period of plants can cause extensive frost damage with tremendous economic losses and dramatic ecological consequences. A common assumption is that climate warming may bring along a reduction in the frequency and severity of frost damage to vegetation. On the other hand, it has been argued that rising temperature in late winter and early spring might trigger the so called “false spring”, that is, early onset of growth that is followed by cold spells, resulting in increased frost damage. By combining daily gridded climate data and 1,489 k in situ phenological observations of 27 tree species from 5,565 phenological observation sites in Europe, we show here that temporal changes in the risk of spring frost damage with recent warming vary largely depending on the species and geographical locations. Species whose phenology was especially sensitive to climate warming tended to have increased risk of frost damage. Geographically, compared with continental areas, maritime and coastal areas in Europe were more exposed to increasing occurrence of frost and these late spring frosts were getting more severe in the maritime and coastal areas. Our results suggest that even though temperatures will be elevated in the future, some phenologically responsive species and many populations of a given species will paradoxically experience more frost damage in the future warming climate. More attention should be paid to the increased frost damage in responsive species and populations in maritime areas when developing strategies to mitigate the potential negative impacts of climate change on ecosystems in the near future.     

Carbon isotope fractionation in tree ring early and late wood in relation to intra-growing season water balance

We determined the stable carbon isotope composition (δ^1.3C) of cellulose extracted from early and late wood in Douglas fir [Pseudotsuga menziexii (Mirb.) Franco] tree rings. Data were obtained for the period 1962 to 1981, at the start of which the trees were 20 years old. A water balance model was used to calculate daily stand transpiration and water deficit. The model incorporates site factors (soil water availability, slope and aspect) and environmental variables (solar radiation, air temperature and rainfall). There was far greater variability in late wood than in early wood δ^1.3C. In wet years, late wood δ^1.3C was significantly lighter (by as much as 2δ) than early wood δ^1.3C but in dry years this difference was reversed. Differences between spring and summer cumulative transpiration accounted for almost 60δ of the variability in differences between early and late wood δ^1.3C. We found excellent correspondence between summer cumulative transpiration and late wood δ^1.3C, with estimates of transpiration accounting for up to 93% of the variability in δ^1.3C. Correlations between early wood δ^1.3C and spring transpiration were generally poor (r2<0.4), but we were able to identify those exceptional years in which there had been a very dry spring. Our results indicate that, while tree ring δ^1.3C correlates reasonably well with basal area increment, it is a far better indicator of inter- and intra-annual variability in water availability than radial growth.     

Phenological plasticity will not help all species adapt to climate change

Concerns are rising about the capacity of species to adapt quickly enough to climate change. In long‐lived organisms such as trees, genetic adaptation is slow, and how much phenotypic plasticity can help them cope with climate change remains largely unknown. Here, we assess whether, where and when phenological plasticity is and will be adaptive in three major European tree species. We use a process‐based species distribution model, parameterized with extensive ecological data, and manipulate plasticity to suppress phenological variations due to interannual, geographical and trend climate variability, under current and projected climatic conditions. We show that phenological plasticity is not always adaptive and mostly affects fitness at the margins of the species' distribution and climatic niche. Under current climatic conditions, phenological plasticity constrains the northern range limit of oak and beech and the southern range limit of pine. Under future climatic conditions, phenological plasticity becomes strongly adaptive towards the trailing edges of beech and oak, but severely constrains the range and niche of pine. Our results call for caution when interpreting geographical variation in trait means as adaptive, and strongly point towards species distribution models explicitly taking phenotypic plasticity into account when forecasting species distribution under climate change scenarios.