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.     

Recovery of Populus tremuloides seedlings following severe drought causing total leaf mortality and extreme stem embolism

In contrast with other native Populus species in North America, Populus tremuloides (aspen) can successfully establish itself in drought‐prone areas, yet no comprehensive analysis has been performed on the ability of seedlings to withstand and recover from a severe drought resulting in complete leaf mortality. Here, we subjected 4‐month‐old aspen seedlings grown in two contrasting soil media to a progressive drought until total leaf mortality, followed by a rewatering cycle. Stomatal conductance (g_s), photosynthesis and transpiration followed a sigmoid decline with declining fraction of extractable soil water values. Cessation of leaf expansion occurred close to the end of the linear‐decrease phase, when g_s was reduced by 95%. Leaf mortality started after g_s reached the lowest values, which corresponded to a stem–xylem pressure potential (Ψ_xp) of ?2.0 MPa and a percent loss of stem hydraulic conductivity (PLC) of 50%. In plants with 50% leaf mortality, PLC values remained around 50%. Complete leaf mortality occurred at an average stem PLC of 90%, but all seedlings were able to resprout after 6–10 days of being rewatered. Plants decapitated at soil level before rewatering developed root suckers, while those left with a 4‐cm stump or with their stems intact resprouted exclusively from axillary buds. Resprouting was accompanied by recovery of stem hydraulic conductivity, with PLC values around 30%. The percentage of resprouted buds was negatively correlated with the stem %PLC. Thus, the recovery of stem hydraulic conductivity appears as an important factor in the resprouting capacity of aspen seedlings following a severe drought.     

Defoliation increases carbon limitation in ectomycorrhizal symbiosis of <Emphasis Type="Italic">Betula pubescens</Emphasis>

Boreal forest trees are highly dependent on root-colonizing mycorrhizal fungi. Since the maintenance of mycorrhizal symbiosis implies a significant carbon cost for the host plant, the loss of photosynthetic leaf area due to herbivory is expected to reduce the host investment in mycorrhizae. We tested this hypothesis in a common garden experiment by exposing ectomycorrhizal white birch (Betula pubescens Ehrh.) seedlings to simulated insect defoliation of 50 or 100% intensity during either the previous or the current summer or repeatedly during both seasons before harvest. The shoot and root growth of the seedlings were distinctly reduced by both 100% defoliation and repeated 50% defoliation, and they were more strongly affected by previous-year than current-year defoliation. The root to shoot ratio significantly decreased after 100% defoliation, indicating reduced proportional allocation to the roots. Ergosterol concentration (i.e. fungal biomass) in the fine roots decreased by 100% defoliation conducted either in the year of harvest or in both years. No such decrease occurred following the 100% defoliation conducted in the previous year, indicating the importance of current photosynthates for fungal symbionts. The trend was similar in the colonization percentage of thick-mantled mycorrhizae in the roots, the most marked decline occurring in the repeatedly defoliated seedlings. The present results thus support the prediction that the plant investment in ectomycorrhizae may decline as a response to foliage loss. Moreover, the colonization percentage of thick-mantled mycorrhizae correlated positively with the ratio of leaf to heterotrophic plant biomass in the defoliated birch seedlings, but not in the control ones. This tends to indicate a stronger carbon limitation of ectomycorrhizal colonization in defoliated seedlings.     

Interactive effects of defoliation and an AM fungus on plants and soil organisms in experimental legume–grass communities

We established a 13‐week greenhouse experiment based on replicated microcosms to test whether the effects of defoliation on grassland plants and soil organisms depend on plant species composition and the presence of arbuscular mycorrhizal (AM) fungi. The experiment constituted of three treatment factors – plant species composition, inoculation of an AM fungus and defoliation – in a fully factorial design. Plant species composition had three levels: (1) Trifolium repens monoculture (T), (2) Phleum pratense monoculture (P) and (3) mixture of T. repens and P. pratense (T+P), while the AM inoculation and the defoliation treatment had two levels: (1) no inoculation of AM fungi and (2) inoculation of the AM fungus Glomus claroideum BEG31, and (1) no trimming, and (2) trimming of all plant material to 6 cm above the soil surface three times during the experiment, respectively. At the final harvest, AM colonization rate of plant roots differed between the plant species compositions, being on average 45% in T, 33% in T+P and 4% in P. Defoliation did not affect the colonization rate in T but raised the rate from 1% to 7% in P and from 20% to 45% in T+P. Shoot production and standing shoot and root biomass were 48%, 85% and 68% lower, respectively, in defoliated than in non‐defoliated systems, while the AM fungus did not affect shoot production and root mass but reduced harvested shoot mass by 8% in non‐defoliated systems. Of the plant quality attributes, defoliation enhanced the N concentration of harvested shoot biomass by 129% and 96% in P and T+P, respectively, but had no effect in T, while the C concentration of shoot biomass was on average 2.7% lower in defoliated than in non‐defoliated systems. Moreover, defoliation reduced shoot C yield (the combined C content of defoliated and harvested shoot biomass) on average by 47% across all plant species compositions and shoot N yield by 37% in T only. In contrast to defoliation, the AM fungus did not affect shoot N and C concentrations or shoot N yield, but induced 10% lower C yield in non‐defoliated systems and 17% higher C yield in defoliated T. In roots, defoliation led to 56% and 21% higher N concentration in P and T+P, respectively, and 28% higher C concentration in P, while the mycorrhizal fungus lowered root N concentration by 9.7% in defoliated systems and had no effect on root C concentrations. In the soil, the nematode community was dominated by bacterivores and the other trophic groups were found in a few microcosms only. Bacterivores were 45% more abundant in defoliated than in non‐defoliated systems, but were not affected by plant species composition or the AM fungus. Soil inorganic N concentration was significantly increased by defoliation in T+P, while the mycorrhizal fungus reduced NH₄–N concentration by 40% in T. The results show that defoliation had widespread effects in our experimental systems, and while the effects on plant growth were invariably negative and those on bacterivorous nematodes invariably positive, most effects on plant C and N content and soil inorganic N concentration varied depending on the plant species present. In contrast, the effects of defoliation did not depend on the presence of the AM fungus, which suggests that while the relative abundance of legumes and grasses is likely to have a significant role in the response of legume–grass communities to defoliation, the role of AM fungi may be less important. In line with this, the AM fungus had only a few significant effects on plant and soil attributes in our systems and each of them was modified by defoliation and/or plant species composition. This suggests that the effects of AM fungi in legume–grass communities may largely depend on the plant species present and whether the plants are grazed or not.     

热浪频次及间隔时间对闽楠和木荷苗木水力结构和非结构性碳水化合物的影响

近年来亚热带地区极端气候事件热浪发生频率增加,热浪频次及间隔时间的变化使热浪发生的模式及其对植物的胁迫方式更加多样化。高频热浪不仅通过热胁迫影响植物的碳固持速率,还会间接形成水分胁迫造成植物水力结构发生障碍,影响碳水化合物的运输。然而,目前亚热带树木水力结构和非结构性碳水化合物(NSC)对复杂热浪的模式的响应仍不明确。以亚热带主要阔叶树种闽楠(Phoebe bournei)和木荷(Schima superba)苗木为研究对象进行了热浪模拟试验,关注不同热浪频次(单次,两次)及重复热浪间隔时间(短间隔、中间隔、长间隔)对苗木茎部水力结构特征及NSC的影响,使用冲洗法测定水力结构中的导水率(K_h)、最大导水率(K_max)、比导率(K_s)、木质部栓塞百分数(PLC),使用蒽酮-硫酸比色法测定茎段非结构性碳水化合物含量。结果表明,(1)闽楠和木荷的水力结构和非结构性碳水化合物在树种间存在显著差异;(2)不同热浪频次对闽楠和木荷的K_max和PLC影响存在显著差异;(3)重复热浪间隔时间变长,木荷茎栓塞减轻,而闽楠茎栓塞增加,且植株栓塞越严重,茎NSC含量越少。总体上,闽楠的水力传输系统对热浪抗性较弱,在热浪后栓塞严重,导水率下降且无法完全恢复,且NSC含量与栓塞程度相关性较弱;而木荷水力传输系统抗性较强,在热浪后导水能力可能恢复至未受干扰水平,且其恢复程度与NSC含量紧密相关。该研究结果表明,高频热浪的发生会显著影响闽楠和木荷苗木茎部的导水能力,且不同间隔时间的重复热浪事件对植物水力结构的影响存在差异性,并且两个亚热带阔叶树种对热浪伴随的高温和水分胁迫的耐受性和耐受机制存在差异。     

Delayed induced responses of birch glandular trichomes and leaf surface lipophilic compounds to mechanical defoliation and simulated winter browsing

Changes in morphology and chemistry of leaf surface in response to herbivore damage may increase plant resistance to subsequent herbivore attack; however, there is lack of studies on induced responses of glandular trichomes and their exudates in woody plants and on effects of these changes on herbivores. We studied delayed induced responses in leaf surface traits of five clones of silver birch (Betula pendula Roth) subjected to various types of mechanical defoliation and simulated winter browsing. Glandular trichome density and concentrations of the majority of surface lipophilic compounds increased in trees defoliated during the previous summer. This induced response was systemic, since control branches in branch defoliated trees responded to the treatments similarly to defoliated branches, but differently from control trees. In contrast to defoliation treatments, simulated winter browsing reduced glandular trichome density on the following summer and had fewer effects on individual surface lipophilic compounds. Moreover, constitutive density of glandular trichomes was negatively correlated with induced total amount of lipophilic compounds per trichome, indicating a trade-off between constitutive and induced resistance in silver birch. Induced changes in leaf surface traits had no significant effect on leaf damage by chewers, miners and gall mites, but increased susceptibility of birch trees to aphids. However, leaf damage by chewers, miners and gall mites in defoliated (but not in control) trees was correlated with concentrations of some fatty acids and triterpenoids, although the direction of relationships varied among herbivore species. This indicates that induction of surface lipophilic compounds may influence birch resistance to herbivores. Our study thus demonstrated both specificity of elicitation of induced responses of birch leaf surface traits by different types of damage and specificity of the effects of these responses on different types of herbivores.Electronic Supplementary Material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

Density- and growth stage-dependent responses to defoliation in two rhizomatous grasses

Summary Responses to defoliation were studied in two tallgrass prairie perennials (Andropogon gerardii and Panicum virgatum) established from seed at three densities. P. virgatum was also grown from transplanted rhizomes of established clones. Plants of both species displayed a continuum of responses to defoliation, from large reductions in biomass, tillering and seed production to significant increases in one or more performance measures. In crowded populations, defoliation shifted plants into subordinate positions within the competitive hierarchy. Plants competing intraspecifically and those that were initially small suffered more from defoliation than either plants grown at low density or those that were larger than their neighbors. At the highest plant density, the effects of defoliation or initial plant size were overshadowed by the effects of crowding. When defoliated and grown at similar densities, P. virgatum and A. gerardii grown from seed showed large reductions in biomass, seed production, and new rhizome production, but established P. virgatum ramets grown from rhizomes showed increases in these performance measures. Thus, herbivory may be particularly detrimental to P. virgatum during juvenile stages before perennating organs have developed. Overcompensation of P. virgatum clones in response to defoliation only occurred if all ramets within the clone were defoliated. In clones containing both defoliated and undamaged ramets, there were no differences in their performance, suggesting that genets are capable of integrating the effects of differential defoliation among shoots. Defoliated P. virgatum clones allocated a smaller fraction of their total biomass to new rhizomes, indicating that the short-term regrowth response following defoliation may incur a longer-term cost associated with gradual reduction in biomass of the perennating organs and reduced genet success.     

What happens when stems are embolized in a centrifuge? Testing the cavitron theory

Vulnerability curves (VCs) measure the ability of vessels to retain metastable water without embolisms that lower the hydraulic conductivity of stems. The fastest method of measuring VCs is the centrifuge technique and the Cochard cavitron is a method that allows measurement of hydraulic conductivity of stems while they are spinning. This paper describes the pattern of embolism that results after spinning the stems of hybrid aspen (Populus tremula×P. tremuloides) and two hybrid cottonwoods (P38P38 P. balsamifera×P. simonii and Northwest, which is a hybrid of P. deltoides×P. balsamifera). It is recognized that the pattern of embolism induced in a centrifuge ought to differ from the pattern during natural dehydration of plants because the profiles of tension vs distance greatly differ under the two modes of inducing stress. The pattern of embolism was visualized by a staining technique and quantified by traditional measurements of percentage loss conductivity (PLC) performed on subsample segments excised from spun stems. We found a pattern of embolism approximating that expected from theory: (1) PLC near the axis of rotation exceeded the average; (2) PLC was quite high near the ends of the stems, even though tension ought to be zero; (3) large vessels cavitated before small vessels; (4) more embolism occurred near the base than near the apex of the stems. However, we could not always scale up from subsample conductivity and PLC to whole‐stem conductivity. This pattern of embolism is interpreted in terms of vessel diameter and vessel length.     

Defoliation and woody plant (shape Prosopis glandulosa) seedling regeneration: potential vs realized herbivory tolerance

Herbivory by rodents, lagomorphs and insects may locally constrain woody plant seedling establishment and stand development. Recruitment may therefore depend either upon plant tolerance of herbivory, or low herbivore abundance, during seedling establishment. We tested potential herbivory tolerance by quantifying growth, biomass allocation, and survival of defoliated Prosopis glandulosa seedlings under optimal abiotic conditions in the absence of competition. Realized tolerance was assessed by clipping seedlings of known age grown in the field with and without herbaceous competition.At 18-d (= 'young') or 33-d (= 'old') of age, seedlings in the growth chamber were clipped just above the first (cotyledonary) node, above the fourth node, or were retained as non-clipped controls. Potential tolerance to defoliation was high and neither cohort showed evidence of meristematic limitations to regeneration. Clipping markedly reduced biomass production relative to controls, especially belowground, but survival of seedlings defoliated 5 times was still 75%. Contrary to expectations, survival of seedlings defoliated above the cotyledonary node 10 times was greater (P<0.10) for 'young' (75%) than 'old' (38%) seedlings. Under field conditions, survival of defoliated 11-month-old P. glandulosa seedlings was 59% after one defoliation and only 13% after six defoliations.Results indicate P. glandulosa is potentially tolerant of repeated shoot removal early in its life cycle. Seedling tolerance to defoliation under field conditions therefore appears dependent upon abiotic stresses or resource limitations rather than a lack of intrinsic adaptations for shoot replacement or a depletion of the seedlings' bud bank. Curtailment of root growth, a consequence of top removal observed in the growth chamber experiment, may reduce the capacity of P. glandulosa seedlings to acquire soil resources needed for meristem activation and shoot growth under field conditions. The importance of resource availability to post-defoliation regeneration was implicated in the field experiment, where survival, shoot elongation, and aboveground productivity of clipped seedlings was greatest in plots without herbaceous interference.In light of the marked increases in P. glandulosa abundance in grasslands in recent history, our results suggest that (1) utilization of Prosopis seedlings by herbivores may be infrequent or sporadic, (2) periodic episodes of seed production and germination may satiate herbivore populations, and/or (3) Prosopis seedling establishment occurs during periods of low herbivore density.     

Axial and radial profiles in conductivities, water storage and native embolism in trunks of young and old-growth ponderosa pine trees

Ponderosa pine has very wide sapwood, and yet the spatial and temporal use of that sapwood for water transport is poorly understood. Moreover, there have been few comparisons of function in tips of old-growth trees in comparison with young trees. In the present study, axial and radial specific conductivity (k_s), leaf specific conductivity (LSC), leaf specific conductance (k_l), native embolism and the compartmentalization of sapwood water storage were characterized in trunks of young and old-growth trees. Trunks of young trees had lower k_s, lower LSC and lower native embolism [corresponding to 5% loss of conductivity (PLC)] than trunks of old-growth trees. However, k_l in young trees was 3.5 times higher than in old-growth trees, supporting the hypothesis that tall trees have a reduced ability to transport water to their leaves. Water storage (capacitance) of young trees was not significantly different than at the base of old-growth trees. Although the top of the old-growth trees had similar k_s, LSC and k_l to the young trees for a given cambial age, they had higher native embolism and lower capacitance. There was no trade-off between k_s and native embolism at any height. In the tree crown, outer sapwood had 35–50% higher k_s than the inner sapwood and 17–25 PLC lower native embolism. At the base of the old trees, there was no significant difference in native embolism between the outer, middle and inner sapwood, showing that refilling of embolisms was complete despite the 130-year difference in wood age among these radial positions. Although during the dry season the inner sapwood tended to be more saturated than the outer sapwood, the outer part of the sapwood contributed up to 60% of the overall stored water. Safer xylem, higher capacitance and higher k_l would appear adaptive in the young trees for regulating their water resource, which is likely to be less reliable than the water availability of older trees with their more developed root system.     

Growth,nutrition and gas exchange of Pinus resinosa following artificial defoliation

Summary In three experiments, red pine (Pinus resinosa Ait.) seedlings and trees were subjected to artificial defoliations of varying intensities and subsequent growth, gas exchange and nutritional responses were monitored. In Experiment 1, 2-year-old seedlings received 0, 1 or 2 50% defoliations during a single growing season and were maintained in 1 of 3 low nutrient supply treatments. In Experiment 2, response of 4-year-old seedlings was monitored in the year following 0, 25, 50 or 75% defoliation, while in Experiment 3, response of 11-year-old trees was measured 1 year after being defoliated by 0, 33 or 66%. Regardless of intensity of defoliation, or plant size, clipped plants made qualitatively similar allocational and metabolic adjustments over time. First, leaf diffusive conductance and rates of net photosynthesis were stimulated, especially by light to intermediate defoliation. However, there was no effect of defoliation on foliar nitrogen concentration, and elevated gas exchange rates apparently resulted from altered root-shoot dynamics. Second, allocation of new biomass was preferentially shifted towards foliage at the expense of roots, gradually restoring (but undershooting or overshooting) the ratio of foliage: roots of control plants. During the period when foliage: root balance was being restored, the stimulation of needle gas exchange rates disappeared. Plants defoliated by 25% overcompensated in terms of whole plant growth (were larger at harvest than controls), due to shifts in allocation and enhanced photosynthesis. Defoliated plants also stored a proportionally greater share of their carbohydrate reserves in roots than did control plants, even 1 year after clipping.     

Tradeoffs between hydraulic efficiency and mechanical strength in the stems of four co-occurring species of chaparral shrubs

Possible tradeoffs between efficiency of water transport and mechanical strength were examined in stems of two congeneric pairs of co-occurring chaparral shrubs. First, since previously published results indicated that Adenostoma sparsifolium (Rosaceae) had greater specific conductivity (k _s or hydraulic conductivity per xylem transverse area) than A. fasciculatum, it was hypothesized that A. sparsifolium would have greater vessel lumen area per square millimeter of xylem area, and less mechanical strength, than A. fasciculatum. Secondly, since Ceanothus megacarpus (Rhamnaceae) is a non-sprouter (unable to sprout from the root crown following fire or other major disturbance) whereas C. spinosus is a sprouter and thus able to form new stems following disturbance, it was hypothesized that C. megacarpus would have greater mechanical strength, but lower k _s, than C. spinosus. Both hypotheses were supported. Based upon computer-aided image analyses, A. sparsifolum had significantly higher mean and maximum vessel diameters (16.4, 40.5 vs. 14.6, 35.7 μm), a 34% greater percent vessel lumen area, and a two-fold greater measured and theoretical k _s than A. fasciculatum. This corresponded to 14% lower stem density (wet weight/volume) and less mechanical strength, with a 37% lower modulus of elasticity (MOE) and a 30% lower modulus of rupture (MOR) than A. fasciculatum. Similarly, C.␣spinosus had a significantly higher maximum vessel diameter (52.7 vs. 41.8 μm) and a 92% higher theoretical k _s (and 43% higher measured k _s) than C. megacarpus. This corresponded to a 9% lower stem density and 20% lower MOR than for C. megacarpus. Thus, at least within these two congeneric pairs of chaparral shrubs growing together in the same habitat, there may be tradeoffs between mechanical strength and conductive efficiency of the stem xylem which correspond to differences in transport physiology and life history traits of sprouter versus non-sprouter species.     

Net photosynthesis,root respiration,and regrowth of Bouteloua gracilis following simulated grazing

Summary Net photosynthesis (P_N), root respiration (R_R), and regrowth of Bouteloua gracilis (H.B.K.) Lag. were examined in the laboratory over a 10-day period following clipping to a 4-cm height to simulate grazing by large herbivores. Net photosynthesis rates of tissue remaining immediately following defoliation were only about 40% as great as preclipping rates. Three days after clipping, P_N rates of defoliated plants had increased to values about 21% greater (per unit leaf area) than those of unclipped controls and remained at that level through Day 10. No statistically significant changes in R_R occurred following defoliation. Biomass of unclipped plants nearly doubled during the 10-day study period, while that of defoliated plants increased 67%. Over half the new growth of defoliated plants was allocated to new leaf blades and only 18% to new roots, while only 33% of the new growth of control plants was allocated to new leaf blades but 29% went to new roots. As a consequence of increased P_N rates and increased carbon allocation to synthesis of additional photosynthetic tissue following defoliation, net CO₂ uptake per plant increased from 9% to 80% of that of the controls from Day 0 through Day 10.     

Defoliation of Brassica napus increases severity of blackleg caused by Leptosphaeria maculans: implications for dual-purpose cropping

Canola (Brassica napus) crops for grazing and grain (dual-purpose) production provide an economic break-crop alternative for dual-purpose cereals in Australian mixed farming systems. Infection by Leptosphaeria maculans is the most prevalent disease in Australian canola crops with airborne inoculum released throughout the autumn and winter when crops are grazed. Glasshouse and field experiments were conducted to investigate the effect of mechanical defoliation (simulated grazing) on disease severity at plant maturity. In glasshouse experiments, stem canker severity increased from 4% to 24% in severely defoliated plants, but light defoliation had no effect compared with undefoliated control plants. Disease severity was increased with defoliation in all field experiments. Defoliation increased crown canker severity from 22.6% to 39.3% at Wagga Wagga and from 3.0% to 7.1% at Canberra and lodging from 9.6% to 11.9% at Naracoorte in the same set of cultivars assessed at each site. The increase in disease severity with defoliation was less in canola lines with moderate to high levels of stem canker resistance. Plants defoliated before stem elongation tended to develop less disease than those defoliated during the reproductive phase of plant growth. These findings suggest that the impact of grazing on L. maculans infection of canola crops can be minimised by sowing cultivars with a high level of stem canker resistance and grazing during the vegetative stage of plant growth prior to stem elongation. Further research is required to determine whether these management strategies are applicable in canola crops defoliated by grazing animals.     

Growth,chemical responses and herbivory after simulated leaf browsing in <Emphasis Type="Italic">Combretum apiculatum</Emphasis>

Vegetative and chemical responses to simulated leaf browsing during the growth season, and their subsequent effect on herbivory, were studied on Combretum apiculatum Sonder (Combretaceae) in Botswana. Treatments (50% and 100% leaf and shoot apex removal) were performed just before the shoot growth curve levelled out, and responses recorded 3 months later, just before leaf fall. Compared to controls, defoliation treatments, removing apical dominance, reduced growth in tree height and increased shoot mortality, although the production of lateral shoots increased. At the end of the trial, there was no difference in total length of annual shoots between treatment groups. Significant refoliation occurred only after 100% defoliation. Refoliated leaves were smaller and the 100% defoliated trees had a lower final leaf biomass. Total leaf biomass production was, however, equal for all treatment groups. Refoliated leaves contained higher levels of N, lower levels of acid-detergent fibre (ADF) and total phenolics, and showed a trend towards lower levels of condensed tannins, compared to leaves on control trees. Such chemical changes may be due to either carbon stress or to younger physiological age of new leaves. In spite of the observed potential increase in food quality, we found no evidence of increased levels of insect or ungulate herbivory on refoliated leaves, which, at least for insect herbivory, may be explained by the reduction in temporal availability of leaves. We conclude that the single severe defoliation was not detrimental to C. apiculatum in the short-term, although the resource loss and induced compensatory growth may produce negative effects during subsequent growth seasons.     

Tree-ring widths and isotopes of artificially defoliated balsam firs: A simulation of spruce budworm outbreaks in Eastern Canada

Defoliation by insects is a major disturbance influencing the forest dynamics in many ecosystems and can affect forest productivity worldwide. The main objective of this research was to further investigate the potential use of tree-ring widths and isotopic compositions to identify different degrees of past spruce budworm defoliation episodes. A secondary objective was to understand the responses of trees to defoliation episodes using carbon isotopes as a proxy to provide insights into subsequent physiological changes. Tree-ring widths, carbon and oxygen isotopic compositions in wood cellulose and gas exchange measurements were compared among 288 balsam fir (Abies balsamea Mill.) seedlings grown in a controlled experiment that involved different intensities of defoliation. Observations were performed over four growing periods. Moderate to heavy-defoliated seedlings showed reduced radial growth and enriched their cellulose carbon isotopic composition probably as a result of mobilized stored carbohydrates enriched in ¹³C. Less severely defoliated seedlings did not show significant reductions in growth and ¹³C enrichments. The gas exchange observations and wood cellulose oxygen isotope compositions do not suggest photosynthetic compensation in the remaining needles although a positive trend in the response of both assimilation rate (A) and stomatal conductance (g_s) to defoliation was observed in the first growing period. Thus it remains open as to which mechanisms were employed to compensate for the reduced carbon source in the mildly defoliated seedlings. While further investigations are advised, the results of this study still help promote the utilization of tree-ring widths in combination with carbon isotopic compositions for reconstructing severe past defoliation events.     

Defoliation effects on carbon and nitrogen substrate import and tissue-bound efflux in leaf growth zones of grasses

Grassland plants suffer regular defoliation, causing loss of photosynthetic activity and internal resources. Consequently, re‐foliation may be substrate‐limited. The present study was undertaken to test the hypothesis that decreased C import in leaf growth zones is (partially) compensated by: (i) mobilization of substrate within growth zones; and (ii) increased efficiency of substrate use in leaf area expansion; but (iii) that these processes depend on the C status of growth zones at defoliation. Mixtures of a C₃ (Lolium perenne L.) and a C₄ grass (Paspalum dilatatum Poir.) were grown at 15 °C (C₃ dominance) and 23 °C (C₄ dominance). Individual plants thus grew in contrasting (light and temperature) environments before being defoliated. Defoliation caused a drastic and immediate decrease in C import, but effects on leaf area expansion were buffered by biomass mobilization in the growth zone and increases in specific leaf area of produced tissue. Thus, over the first 2 d post‐defoliation, the amount of leaf area produced per unit imported C increased by 39 to 102% depending on treatment. The magnitude of these buffering responses was correlated with the concentration of water soluble carbohydrates in the growth zone at defoliation. Similar responses were observed for N, although defoliation effects were smaller and delayed relative to those on C import. This study demonstrates refoliation is sustained by short‐term mobilization of reserves within the growth zone and reduced costs of produced leaf area, but that these mechanisms depend on growth zone C status at defoliation.     

Prolonged experimental drought reduces plant hydraulic conductance and transpiration and increases mortality in a piñon–juniper woodland

Plant hydraulic conductance (k_s) is a critical control on whole‐plant water use and carbon uptake and, during drought, influences whether plants survive or die. To assess long‐term physiological and hydraulic responses of mature trees to water availability, we manipulated ecosystem‐scale water availability from 2007 to 2013 in a piñon pine (Pinus edulis) and juniper (Juniperus monosperma) woodland. We examined the relationship between k_s and subsequent mortality using more than 5 years of physiological observations, and the subsequent impact of reduced hydraulic function and mortality on total woody canopy transpiration (E_C) and conductance (G_C). For both species, we observed significant reductions in plant transpiration (E) and k_s under experimentally imposed drought. Conversely, supplemental water additions increased E and k_s in both species. Interestingly, both species exhibited similar declines in k_s under the imposed drought conditions, despite their differing stomatal responses and mortality patterns during drought. Reduced whole‐plant k_s also reduced carbon assimilation in both species, as leaf‐level stomatal conductance (g_s) and net photosynthesis (A_n) declined strongly with decreasing k_s. Finally, we observed that chronically low whole‐plant k_s was associated with greater canopy dieback and mortality for both piñon and juniper and that subsequent reductions in woody canopy biomass due to mortality had a significant impact on both daily and annual canopy E_C and G_C. Our data indicate that significant reductions in k_s precede drought‐related tree mortality events in this system, and the consequence is a significant reduction in canopy gas exchange and carbon fixation. Our results suggest that reductions in productivity and woody plant cover in piñon–juniper woodlands can be expected due to reduced plant hydraulic conductance and increased mortality of both piñon pine and juniper under anticipated future conditions of more frequent and persistent regional drought in the southwestern United States.     

Biomass allocation and leaf chemical defence in defoliated seedlings of Quercus serrata with respect to carbon-nitrogen balance

BACKGROUND AND AIMS: Both nutrient availability and defoliation affect the carbon-nutrient balance in plants, which in turn influences biomass allocation (e.g. shoot-to-root ratio) and leaf chemical composition (concentration of nitrogen and secondary compounds). In this study it is questioned whether defoliation alters biomass allocation and chemical defence in a similar fashion to the response to nutrient deficiency. METHODS: Current-year seedlings of Quercus serrata were grown with or without removal of all leaves at three levels of nutrient availability. KEY RESULTS: Plant nitrogen concentration (PNC), a measure of the carbon-nutrient balance in the plant, significantly decreased immediately after defoliation because leaves had higher nitrogen concentrations than stems and roots. However, PNC recovered to levels similar to or higher than that of control plants in 3 or 6 weeks after the defoliation. Nitrogen concentration of leaves produced after defoliation was significantly higher than leaf nitrogen concentration of control leaves. Leaf mass per plant mass (leaf mass ratio, LMR) was positively correlated with PNC but the relationship was significantly different between defoliated and control plants. When compared at the same PNC, defoliated plants had a lower LMR. However, the ratio of the leaf to root tissues that were newly produced after defoliation as a function of PNC did not differ between defoliated and control plants. Defoliated plants had a significantly lower concentration of total phenolics and condensed tannins. Across defoliated and control plants, the leaf tannin concentration was negatively correlated with the leaf nitrogen concentration, suggesting that the amount of carbon-based defensive compounds was controlled by the carbon-nutrient balance at the leaf level. CONCLUSIONS: Defoliation alters biomass allocation and chemical defence through the carbon-nutrient balance at the plant and at the leaf level, respectively.     

Hydraulic adjustments in aspen (Populus tremuloides) seedlings following defoliation involve root and leaf aquaporins

Main conclusion

Changes in root and leaf hydraulic properties and stimulation of transpiration rates that were initially triggered by defoliation were accompanied by corresponding changes in leaf and root aquaporin expression. Aspen (Populus tremuloides) seedlings were subjected to defoliation treatments by removing 50, 75 % or all of the leaves. Root hydraulic conductivity (L_pr) was sharply reduced in plants defoliated for 1 day and 1 week. The decrease in L _pr could not be prevented by stem girdling and it was accompanied in one-day-defoliated plants by a large decrease in the root expression of PIP1,2 aquaporin and an over twofold decrease in hydraulic conductivity of root cortical cells (L _pc). Contrary to L _pr and L _pc, 50 and 75 % defoliation treatments profoundly increased leaf lamina conductance (K _lam) after 1 day and this increase was similar in magnitude for both defoliation treatments. Transpiration rates (E) rapidly declined after the removal of 75 % of leaves. However, E increased by over twofold in defoliated plants after 1 day and the increases in E and K _lam were accompanied by five- and tenfold increases in the leaf expression of PIP2;4 in 50 and 75 % defoliation treatments, respectively. Defoliation treatments also stimulated net photosynthesis after 1 day and 3 weeks, although the increase was not as high as E. Leaf water potentials remained relatively stable following defoliation with the exception of a small decrease 1 day after defoliation which suggests that root water transport did not initially keep pace with the increased transpirational water loss. The results demonstrate the importance of root and leaf hydraulic properties in plant responses to defoliation and point to the involvement of PIP aquaporins in the early events following the loss of leaves.