Tree species identity and forest composition affect the number of oak processionary moth captured in pheromone traps and the intensity of larval defoliation

1. Pure forests are often seen as being more prone to damage by specialist pest insects than mixed forests, and particularly mixed forests associating host and nonhost species. We addressed the effect of tree diversity on oak colonization and defoliation by a major specialist pest, the oak processionary moth (OPM)
2. We quantified the number of male OPM moths captured and larval defoliation in pure stands of two oak host species (Quercus robur and Quercus petraea) and in mixed stands associating the two oak species or each oak species with another nonhost broadleaved species. We conducted two complementary studies to test the effect of host species and stand composition: (i) we used pheromone trapping to compare the number of males OPM captured throughout the distribution of oak hosts in France and (ii) we noted the presence of OPM nests and estimated defoliation in mature forests of north‐eastern France.
3. Oak species and stand composition significantly influenced the number of male OPM captured and defoliation by OPM larvae. Quercus petraea was consistently more attractive to and more defoliated by OPM than Q. robur. Both oak trees were attacked more in pure stands than in mixed stands, in particular mixed stands associating oaks with another (nonhost) broadleaved species.
4. The results of the present study support the view that mixed forests are more resistant to specialist pest insects than pure stands, and also indicate that this trend depends on forest composition. Our study provides new insights into OPM ecology and has potential implications for forest management, including the management of urban forests where OPM causes serious human health issues.

     

Induced plant defenses breached? Phytochemical induction protects an herbivore from disease

Although wound-induced responses in plants are widespread, neither the ecological nor the evolutionary significance of phytochemical induction is clear. Several studies have shown, for example, that induced responses can act against both plant pathogens and herbivores simultaneously. We present the first evidence that phytochemical induction can inhibit a pathogen of the herbivore responsible for the defoliation. In 1990, we generated leaf damage by enclosing gypsy moth larvae on branches of red oak trees. We then inoculated a second cohort of larvae with a nuclear polyhedrosis virus (LdNPV) on foliage from the damaged branches. Larvae were less susceptible to virus consumed on foliage from branches with increasing levels of defoliation, and with higher concentrations of gallotannin. Defoliation itself was not related to any of our chemistry measures. Field sampling supported the results of our experiments: death from virus among feral larvae collected from unmanipulated trees was also negatively correlated with defoliation. In 1991, defoliation and gallotannin were again found to inhibit the virus. In addition, gallotannin concentrations were found to be positively correlated with defoliation the previous year. Compared with previous results that demonstrated a delecterious effect of induction on gypsy moth pupal weight and fecundity, the inhibition of the virus should confer an advantage to the gypsy moth. Since leaf damage levels increase as gypsy moth density increases, and since leaf damage inhibits the gypsy moth virus, there is the potential for positive feedback in the system. If phytochemical induction in red oak can inhibit an animal pathogen such as LdNPV, it suggests to us that induction in red oak is a generalized response to tissue damage rather than an adaptive defense against herbivores.     

Refoliation of deciduous canopy trees following severe insect defoliation: comparison of <Emphasis Type="Italic">Fagus crenata</Emphasis> and <Emphasis Type="Italic">Quercus crispula</Emphasis>

Deciduous trees can survive severe defoliation by herbivores and often refoliate in the same season. Refoliation following severe defoliation represents compensatory regrowth to recover foliage biomass. Although the relationship between defoliation intensity and degree of refoliation at the individual level has been quantified following artificial defoliation for saplings and small trees, no study has examined the relationship for canopy trees and interspecific differences in this relationship. In this study, defoliation by gypsy moths in an outbreak year and subsequent refoliation were visually surveyed for canopy trees of Fagus crenata (n?=?80) and Quercus crispula (n?=?113) in central Japan. Defoliation and refoliation estimates were scored in 10% classes as the ratio to foliage present before defoliation. The degree of refoliation and the proportion of refoliated trees were high in severely defoliated trees. For 60 and 100% defoliated trees, respective refoliations were 2 and 66% for F. crenata, and 37 and 88% for Q. crispula. All of the 90 and 100% defoliated trees refoliated. These results indicate that severely defoliated trees show an increased need for refoliation to maintain metabolism. Beta regression analysis showed that Q. crispula possessed higher refoliation capability than F. crenata. This is likely associated with the relatively large storage reserves and recurrent growth flush pattern of oak species, which are strong characteristics of oaks and adaptive for response to herbivory and catastrophic disturbances. Interspecific differences in refoliation capability may exert differential effects on forest ecosystem processes, such as influencing the growth of understory species.     

Development and feeding efficiency of Malacosoma neustrium larvae reared with Quercus spp. leaves

Observations on larval development of Malacosoma neustrium were conducted both in a cork oak stand and in the laboratory by using leaves of different host trees (cork oak, holm oak and downy oak) as food source. Instars were determined using head capsule and frass measurements. In the field the larvae progressed up to the fifth instar before pupating, and the increase in head capsule width followed Dyars Rule with a rate of increase (R.I.) value of 1.74. The same number of instars was determined for the larvae reared with cork oak (R.I. = 1.73) and holm oak (R.I. = 1.70) leaves. The caterpillars reared with downy oak foliage completed larval development in five, six and seven instars and the R.I. values obtained were 1.60, 1.52 and 1.44 respectively. A lower mortality was recorded for the larvae reared on holm oak. Growth and feeding indices were determined for the larvae from the third up to the last instar. The highest leaf consumption was detected for the fifth instar larvae reared on holm oak. For the caterpillars which completed five instars before pupating, the relative consumption rate (RCR) decreased from the third up to the fifth instar: from 4.8 to 1.7 (cork oak), from 7.4 to 3.3 (holm oak) and from 14.3 to 2.1 (downy oak). The relative growth rate (RGR) was highest during the fourth stadium (0.24, 0.27 and 0.33 for larvae reared with cork oak, holm oak and downy oak leaves respectively) and decreased in the fifth instar (0.09, 0.14 and 0.14 for larvae reared with cork oak, holm oak and downy oak leaves respectively), probably because of greater expense of energy due to the approach of maturity. Feeding and growth indices could be useful to define a defoliation prediction model.     

Genetic determination of tannins and herbivore resistance in <Emphasis Type="Italic">Quercus ilex</Emphasis>

Genetic variability of trees influences the chemical composition of tissues. This determines herbivore impact and, consequently, herbivore performance. We evaluated the independent effects of plant genotype and provenance on the tannin content of holm oak (Quercus ilex) and their consequences for herbivory and performance of gypsy moth (Lymantria dispar) larvae. Oak seedlings of 48 open-pollinated families from six populations were grown in a common garden in central Spain. Half the plants were subjected to defoliation by gypsy moth larvae and the other half were destructively sampled for chemical analysis. Tannin content of leaves did not differ significantly among populations but differed significantly among families. Estimates of heritability (h ²) and quantitative genetic differentiation among populations for tannin content (Q _ST) were 0.83 and 0.12, respectively. Defoliation was not related to the tannin content of plants nor to spine and trichome densities of leaves, although positive family–mean associations were observed between defoliation and both seed weight and plant height (P < 0.003). Among the oak populations, differential increase in larval weight gain with defoliation was observed. Leaf tannin content in Q. ilex is genetically controlled but does not influence defoliation or predict performance of the larvae. Different efficiencies of food utilisation depending on the oak genotypes indicate that other plant traits are influencing the feeding patterns and fitness of L. dispar and consequent population dynamics.     

Habitat of Osmoderma eremita Scop. (Coleoptera: Scarabaeidae), a beetle living in hollow trees

The habitat of Osmoderma eremita, a vulnerable species in Europe restricted to tree cavities, was examined in southeastern Sweden. The occurrence of O. eremita larvae and fragments, larval frass and imagines were investigated in 135, 72 and 21 living oak trees with wood mould cavities, respectively. Living individuals and fragments were only found in hollows with frass. The correlation between different characteristics of the oaks and the occurrence of the beetle were examined by building multi-variate models with logistic regression. The frequency of O. eremita is higher in hollows with openings directed towards the sun (S or W) and in cavities with large amounts of wood mould. In one area the frequency was higher in trees which stand in an open or half open surrounding. The tendency to prefer sun exposed sites implies that the forestation of oak meadows, caused by cessation of traditional management, might be detrimental to the species.     

Plant functional traits explain interspecific differences in immediate cyclone damage to trees of an endangered rainforest community in north Queensland

Abstract Cyclones cause profound immediate impacts on tropical rainforest trees, including defoliation, limb loss, snapping of stems and uprooting. Some studies have shown that plant functional traits such as tree size, buttress roots and wood density are correlated with these forms of cyclone damage. On 20 March 2006, Severe Tropical Cyclone Larry crossed the north Queensland coast and proceeded inland across the Atherton Tablelands, impacting the critically endangered Mabi Type 5b rainforest. We investigated the effects of Cyclone Larry on common tree species by categorizing damage to trees as uprooted, snapped, limbs damaged (light, moderate, severe) or upright and estimating levels of defoliation. Damage was then related to functional traits including tree size, presence of buttress roots, wood density, and leaf size and strength. Levels of damage differed between species. Tree size (diameter at breast height) and the presence of buttress roots were not related to damage levels. Wood density was significantly negatively correlated to proportion of trees with snapped stems and significantly positively correlated with the proportion of trees upright with no or light limb damage. Levels of defoliation were significantly related to leaf strength (specific leaf area – SLA) and to leaf width, but not other components of leaf size (area or length) or petiole length. Species with high wood density and low SLA (e.g. Argyrodendron spp.) were found to have high cyclone resistance, the ability to resist damage, while species with low wood density and high SLA (e.g. Dendrocnide photinophylla) exhibited low resistance. However, traits related to low resistance are also those linked to rapid growth and high cyclone resilience, the ability to recover from damage, so it is unlikely that the Mabi forest will experience long‐term changes in floristic composition following Cyclone Larry.     

Improved insect performance from host-plant defoliation: winter moth on oak and apple

ABSTRACT. 1. We test the hypothesis that defoliation has short-term and long-term negative effects on performance of winter moth Operophtera brumata L. on two species of food plants: Garry oak ( Quercus garryana Dougl.) and apple ( Malus domesticus L.).
2. Pupal weight (and hence potential fecundity of females) decreased with increased defoliation in the current year on both tree species.
3. Pupal weight increased, however, with increased level of defoliation in the previous year.
4. Increased weight of pupae from larvae which had fed on previously defoliated trees may buffer population decline by increasing fecundity of moths in the next generation.     

Growth responses of an African savanna tree, <Emphasis Type="Italic">Bauhinia thonningii</Emphasis> Schumacher,to defoliation,fire and climate

This study investigated the growth responses to defoliation, fire-exposure and climate factors of a widespread Africana savanna tree, Bauhinia thonningii Schumacher, at a site in central Zambia. Experimental trees (n = 47) were either exposed to fire (n = 12) in the first half of the dry season or protected from fire (n = 35). Some of the fire-protected trees (n = 12) were subjected to artificial defoliation in two consecutive years. Phenological responses (bud break, leaf flush and leaf production) to fire-exposure and defoliation were monitored on permanently marked sample shoots over a 2-year period. Radial tree growth (diameter at 1.3 m above ground) was measured annually over a 7-year period from 1998 to 2005. During the first two years, defoliation and fire-exposure advanced the onset of bud break and leaf flush but fire-exposed trees produced significantly less leaves than did trees protected from fire, probably because scorching caused more severe shoot die-back than is normal. Leaf production was also significantly affected by experimental treatments and their interaction with year. Although treatments had significant short-term effects on radial growth, previous-year growth significantly influenced current-year growth, thereby confirming the existence of autocorrelation in the time-series growth data of B. thonningii. The interaction between previous-year growth and climate factors explained a significant proportion (25–40%, P < 0.001) of the variance in annual tree growth. Variogram models predicted that a 2-year manual defoliation treatment would shorten the longer-term growth cycle while continuous fire-exposure extended the cycle by one year. The results are useful for the management of savanna trees.     

Dynamics of twolined chestnut borer Agrilus bilineatus as influenced by defoliation and selection thinning

1 The twolined chestnut borer, Agrilus bilineatus (Coleoptera: Buprestidae), is a major mortality agent of stressed oak trees. However, patterns of abundance and population change are not well understood. 2 We studied the spatial and temporal variation in abundance of twolined chestnut borer adults during a gypsy moth, Lymantria dispar (Lepidoptera: Lymnatriidae), outbreak and examined the influence of both defoliation and thinning on twolined chestnut borer abundance. 3 In stands that were defoliated by gypsy moth, extensive defoliation occurred in one year, and major overstory tree mortality followed in the next. Most mortality occurred in the year preceding the peak year of twolined chestnut borer abundance and abundance of twolined chestnut borer was positively associated with defoliation and mortality in the previous year. 4 Twolined chestnut borers were more frequently associated with poor or fair crown condition trees than trees with good crown condition and were more abundant on members of the red oak group than the white oak group.     

Branch growth and leaf numbers of red maple (Acer rubrum L.) and red oak (Quercus rubra L.): response to defoliation

Summary Branch growth and leaf formation from terminal and from lateral buds of red maple (Acer rubrum L.) and red oak (Quercus rubra L.) were measured in response to simulated insect defoliation. A single large branch representative of the crown of each tree was used for enumeration of growth and of bud numbers throughout three successive years of 0, 50, 75, and 100% leaf removal for the entire tree. Leaf number per tree for both species after the last year of defoliation was reduced in direct proportion to the severity of defoliation, in comparison to the predefoliation status of the trees. Bud number per tree for red maple, but not for red oak, was also reduced in proportion to severity of defoliation.Averaged over all defoliation treatments, defoliation reduced branch growth more than leaf production. Furthermore, the reduction in branch growth and leaf production was greater in red oak than in red maple. Three years of successive defoliation reduced the mean lateral plus terminal branch growth by 40% in red oak and by 23% in red maple, while leaf number was reduced 22% in red oak and remained unchanged in red maple. In red maple, 100% defoliation caused greater branch death than the 50 or 75% defoliation treatments, and the amount of death was greater after each successive year of defoliation. In contrast to red maple, undefoliated red oak incurred a substantial amount of branch death throughout the study which was little affected by defoliation treatment.     

The potential for interspecific competition between two abundant defoliators on oak: leaf damage and habitat quality

Abstract. 1. The pedunculate oak, Quercus robur L., suffers high annual levels of spring defoliation in Wytham Woods, Oxon. The two major defoliators, Tortrix viridana L. and Operophtera brumata L., sometimes reach high enough densities to completely defoliate trees.
2. Experiments with larvae of both species enclosed on oak trees in Wytham Woods suggest considerable potential for intra- and interspecific competition between T. viridana and O. brumata.
3. T. viridana and O. brumata consume equivalent amounts of leaf tissue per unit caterpillar density. Despite this, O. brumata is the superior competitor, measured by mortality during the larval feeding period.
4. Evidence is presented that T. viridana larvae depend upon the protected microclimate of leaf rolls to maintain water balance, and that leaf damage may disrupt the construction of, and microclimate within, leaf rolls. O. brumata may therefore be competitively superior because it indirectly interferes with the water balance of T. viridana.
5. The importance of interspecific competition in the dynamics of T. viridana and O. brumata populations is discussed. Stochastic mortality factors acting on first instar larvae of both species probably make intense competition a rare event.     

栗山天牛幼虫龄数和龄期的测定

栗山天牛Massicus raddei (Blessig)是危害我国东北柞树天然林的重要蛀干害虫, 长期营隐蔽性生活, 3年发生1代, 防治困难。幼虫的龄数和龄期测定是害虫预测预报以及制订其科学治理策略的重要依据。本研究于2008-2011年通过林间定期解剖受害树进行取样, 在辽宁省宽甸县采集不同发育阶段的栗山天牛幼虫, 分别测量幼虫上颚长、 主单眼间距、 前胸背板宽、 中胸气门长和体长等5项形态指标, 利用频次分析的统计方法, 测定了栗山天牛幼虫的龄数。结果表明: 栗山天牛幼虫有6龄, 雌雄性幼虫龄数相同。上颚长、 主单眼间距和前胸背板宽3项指标均可用于分龄, 中胸气门长和体长变异较大, 不宜用作幼虫龄数划分。利用种群众数龄期法计算1-6龄幼虫的平均龄期分别为9.25, 266.85, 48.09, 51.29, 260.33和385.71 d, 幼虫期共1 021.52 d。在我国东北地区, 自然条件下栗山天牛世代发生非常整齐而且高度同步, 完成1代发育需跨越4个年份, 幼虫经历3次越冬, 第1年以2-3龄幼虫越冬, 第2年主要以4-5龄幼虫越冬, 第3年全部以末龄幼虫越冬。研究结果进一步明确了栗山天牛幼虫期的生物学特性, 为生产上合理防治该害虫提供了参考依据。     

Are climatic factors responsible for the process of oak decline in Poland?

Oak decline, a complex process leading to increased mortality of this species, has been observed in Europe for many years. Previous studies suggest that climate conditions, especially drought, may be one of the most important factors that trigger this phenomenon. The paper investigates the radial growth and wood anatomy features of pedunculate oak (Quercus robur) trees of various health status as well as their response to climate conditions. Wood samples including all annual increments were taken at two sites (western and central Poland, 15 trees each). Based on the crown defoliation level, three health groups (healthy, weakened and dead oaks) were distinguished. Cross-sections were prepared with sliding microtome and Cell P image analysis software was used for the measurements. Tree-ring width (TRW), earlywood vessels density (VDen) and non-weighted vessels diameter (VD) were determined and correlated with mean monthly values of temperature, precipitation, vapour pressure, and Palmer Drought Severity Index (PDSI). Radial increment and anatomical parameters were significantly higher for the healthy oaks than for the weakened and the dead trees. TRW showed smaller dependence on climate than analysed anatomical attributes. No obvious pattern of relationship was found between oak radial growth and climate regarding tree health status. Our results revealed that the drought has a weak impact on the process of oak decline on investigated sites in Poland.     

The effect of fodder plant genotype on the variation of larval fitness traits in genotype classes of green oak leafroller moth

The effect of genetic variation of oak (Quercus pubescens L. and Q. petraea L.) on the genotype fitness components in green oak leafroller moth larvae (Tortrix viridana L.) at esterase (Est-4) and protease (Pts-4) loci was studied. The samples of larvae were collected from nine oak trees, whose genetic variation was assayed by RAPD-PCR using primer OPA14. The contributions of the factors of Yoak species/genotype and green oak leafroller moth genotype and their interaction to the variation of important size-related traits of the larvae were evaluated by two-way ANOVA. It was shown that the same larval genotype can display maximum fitness on the trees of one species or genotype and minimal, on the trees of other species or genotype. The interactions between the oak genotype and green oak leafroller moth genotype factors lead to the relationships that appear in statistically significant associations between genotype classes of green oak leafroller moth and oak. These results are discussed from the standpoint of a recently developed new field, community or ecosystem genetics.     

Execution of the auxin replacement apical dominance experiment in temperate woody species

The classic Thimann-Skoog or auxin replacement apical dominance test of exogenous auxin repression of lateral bud outgrowth was successfully executed in both seedlings and older trees of white ash, green ash, and red oak under the following conditions: (1) decapitation of a twig apex and auxin replacement were carried out during spring flush, (2) the decapitation was in the previous season's overwintered wood, and (3) the point of decapitation was below the upper large irrepressible lateral buds but above the lower repressible lateral buds. Although it has been suggested that neither auxin, the terminal bud, nor apical dominance have control over the outgrowth of the irrepressible buds during spring flush, there is evidence in the present study that indicates that such control over the repressible buds exists. In seedlings, second-order branching, which resulted from decapitation of elongating current shoots, was also inhibited by exogenous auxin in the three species. Hence, the auxin replacement experiments did work on year-old proleptic buds (of branches of older trees) that would have entered the bud bank and also on current buds of seedlings. Cytokinin treatments were ineffectual in promoting bud growth.     

Invasive insects impact forest carbon dynamics

Invasive insects can impact ecosystem functioning by altering carbon, nutrient, and hydrologic cycles. In this study, we used eddy covariance to measure net CO₂ exchange with the atmosphere (NEE), and biometric measurements to characterize net ecosystem productivity (NEP) in oak‐ and pine‐dominated forests that were defoliated by Gypsy moth (Lymantria dispar L.) in the New Jersey Pine Barrens. Three years of data were used to compare C dynamics; 2005 with minimal defoliation, 2006 with partial defoliation of the canopy and understory in a mixed stand, and 2007 with complete defoliation of an oak‐dominated stand, and partial defoliation of the mixed and pine‐dominated stands. Previous to defoliation in 2005, annual net CO₂ exchange (NEE_yr) was estimated at ?187, ?137 and ?204 g C m^?2 yr^?1 at the oak‐, mixed‐, and pine‐dominated stands, respectively. Annual NEP estimated from biometric measurements was 108%, 100%, and 98% of NEE_yr in 2005 for the oak‐, mixed‐, and pine‐dominated stands, respectively. Gypsy moth defoliation strongly reduced fluxes in 2006 and 2007 compared with 2005; NEE_yr was ?122, +103, and ?161 g C m^?2 yr^?1 in 2006, and +293, +129, and ?17 g C m^?2 yr^?1 in 2007 at the oak‐, mixed‐, and pine‐dominated stands, respectively. At the landscape scale, Gypsy moths defoliated 20.2% of upland forests in 2007. We calculated that defoliation in these upland forests reduced NEE_yr by 41%, with a 55% reduction in the heavily impacted oak‐dominated stands. ‘Transient’ disturbances such as insect defoliation, nonstand replacing wildfires, and prescribed burns are major factors controlling NEE across this landscape, and when integrated over time, may explain much of the patterning of aboveground biomass and forest floor mass in these upland forests.     

Early spring defoliation,secondary leaf flush,and leafminer outbreaks on American holly

Summary Adults of Phytomyza ilicicola (Diptera: Agromyzidae), a univoltine specialist leafminer, emerge in close synchrony with leaf flush of American holly and feed on and oviposit in soft, partially expanded leaves. Early spring defoliation, such as commonly results from freezing injury to young shoots, is followed several weeks later by a second flush of young leaves from lateral buds. We simulated this phenomenon by manually defoliating whole small trees and individual shoots of large trees to test the hypothesis that freezing injury can encourage leafminer outbreaks by inducing an abundance of soft, protein rich young leaves late in the adult activity period, when availability of vulnerable leaves becomes limited. Defoliation of small trees one or two weeks after bud break resulted in six- to 13-fold increases in the incidence of feeding punctures and larval mines on second flush leaves as compared with densities on original young leaves of control trees. Similarly, we induced significant increases in feeding punctures and larval mines on second flush leaves of individual defoliated shoots, although leaves that did not open until after the flight period escaped this injury. These observations underscore the capability of adult female P. ilicicola to locate and exploit a small number of phenologically available leaves among many hundreds of older leaves on the same tree. By altering the phenology of leaf flush, certain kinds of environmental stress may predispose perennial plants to outbreaks of early season folivores that restrict their feeding or oviposition to very young leaves.     

Dendrochronological evidence of cockchafer (Melolontha sp.) outbreaks in subfossil tree-trunks from Tovačov (CZ Moravia)

In dendrochronological dating, we encounter the trouble that some samples cannot be dated due to the occurrence of anomalies in the tree-ring series. When dating subfossil wood for the extension of existing master chronologies, this may be an especially unwanted circumstance as subfossil wood is scarce and each sample is valuable. In Moravia (Czech Republic) outbreaks of the cockchafer (Melolontha sp.) used to appear with a disastrous effect on agricultural crops, and, for a dendrochronologist, on samples of wood to be dated. Oak ring width reduction caused by cockchafer induced defoliation can superimpose climate induced growth variations and may complicate dating of historical or subfossil samples. For this study, 33 samples of sub-fossil trunks were assessed; 19 of which were attacked by cockchafer. For comparison, we analysed a total of nine living oak trees from the forest district Strá?nice-Vracov, recently attacked by the cockchafer. Occurrence of the cockchafer was reported there in 1999, 2003 and 2007.We found clear growth patterns with cyclic narrow rings every 4 years. This allowed us to create an artificial curve expressing the recurring cycles of cockchafer outbreaks which assisted us in the detection of the same pattern in subfossil tree growth. The tree-ring series of the attacked living oak stand as well as the 19 individual tree-ring series from subfossil oak trunks could not be dated using standard chronologies and showed a high resemblance to the cockchafer life cycle. Additionally, we found out that the living oak stand was attacked in 1983, 1987, 1991 and 1995 as well. The results support the hypothesis that the periodical reduction observed in tree-rings at regular increments is caused by cockchafer infestation. This evidence is further supported by characteristic anatomical features in early wood. A mean tree-ring series was created from the individual tree-ring series of subfossil oaks even though radiocarbon dating showed completely different ages of samples. This means that the affected tree-ring series cannot be used for dendrochronological dating and dendroclimatological analyses. These series cannot be used without preceding mathematical and statistical modification.     

Correlated responses of root growth and sugar concentrations to various defoliation treatments and rhythmic shoot growth in oak tree seedlings (Quercus pubescens)

Background and Aims

To understand whether root responses to aerial rhythmic growth and contrasted defoliation treatments can be interpreted under the common frame of carbohydrate availability; root growth was studied in parallel with carbohydrate concentrations in different parts of the root system on oak tree seedlings.

Methods

Quercus pubescens seedlings were submitted to selective defoliation (removal of mature leaves, cotyledons or young developing leaves) at appearance of the second flush and collected 1, 5 or 10 d later for morphological and biochemical measurements. Soluble sugar and starch concentrations were measured in cotyledons and apical and basal root parts.

Key Results

Soluble sugar concentration in the root apices diminished during the expansion of the second aerial flush and increased after the end of aerial growth in control seedlings. Starch concentration in cotyledons regularly decreased. Continuous removal of young leaves did not alter either root growth or apical sugar concentration. Starch storage in basal root segments was increased. After removal of mature leaves (and cotyledons), root growth strongly decreased. Soluble sugar concentration in the root apices drastically decreased and starch reserves in the root basal segments were emptied 5 d after defoliation, illustrating a considerable shortage in carbohydrates. Soluble sugar concentrations recovered 10 d after defoliation, after the end of aerial growth, suggesting a recirculation of sugar. No supplementary recourse to starch in cotyledons was observed.

Conclusions

The parallel between apical sugar concentration and root growth patterns, and the correlations between hexose concentration in root apices and their growth rate, support the hypothesis that the response of root growth to aerial periodic growth and defoliation treatments is largely controlled by carbohydrate availability.