Root growth of different oak provenances in two soils under drought stress and air warming conditions

Background and aims

Oaks are considered to be drought- and thermo-tolerant tree species. Nevertheless, species and provenances may differ in their ecological requirements. We hypothesised that (i) provenances from xeric sites are better adapted to drought than those from more humid sites, (ii) oaks direct root growth towards resource-rich layers, and (iii) air-warming promotes root growth.

Methods

To test different provenances of Quercus robur, Q. petraea and Q. pubescens, we conducted a model ecosystem experiment with young trees, grown on acidic and calcareous soil, subjected to drought, air warming, the combination of warming and drought, and a control.

Results

The results were only in partial agreement with the first hypothesis. As expected the provenances originating from drier sites produced more biomass than those from more humid sites under drought conditions. Surprisingly, however, they reacted more sensitive to water limiting conditions, as they produced also substantially more biomass under well-watered conditions. The drought treatment reduced root mass substantially in the upper soil. In agreement with the second hypothesis this led to a shift in the centre of root mass to lower depth, where water was still more available than closer to the soil surface. In contrast to the third hypothesis, the air-warming treatment, which was very mild however compared to climate change scenarios, had no significant effects on root growth.

Conclusions

Given that the provenances from drier sites showed more biomass loss at water limiting conditions than provenances from more humid sites, it remains questionable whether provenances from drier sites are better suited for a future climate.     

Which seed origin provides better tolerance to flooding and drought when restoring to face climate change?

Our goal was to establish the tolerance to flooding and drought of seedlings from a hydric gradient of different seed sources to provide recommendations for forest restoration in the face of climate change. We used Drimys winteri var. chilensis, a tree species that grows from extreme arid zones to wetlands along Chile, as the study subject. We expected that seedlings of xeric origin would perform better in drought conditions than populations from moist environments, and vice versa for flooding tolerance. We collected D. winteri seeds from xeric, mesic and wet environments. Seedlings at two development stages were submitted to an extreme flooding and drought treatment during 2 or 4 months in a common garden. After the flooding and drought assays finished, the number of surviving and damaged seedlings, lenticels and adventitious root presence, height, new leaves and specific leaf area, shoot/root ratio, water potential and/or chlorophyll fluorescence (Fv/Fm), were recorded. We found that flooding and drought affected almost all the parameters studied negatively. The xeric population seedlings, at both development stages studied, were the most tolerant to the drought and, unexpectedly, also to the flooding treatment. We recommend restoring with seedlings of xeric origin especially in arid areas where sudden flooding is frequent, as occurs in the Andes Mountains. In the face of climate change, we recommend carrying out common garden and field studies before advising which population origin should be used for restoration, since they do not always respond in accordance with expected patterns of local adaptation.     

台湾桤木和四川桤木种源苗木对水分胁迫的生理响应

采用PEG6000,研究了4种台湾桤木种源和2种四川桤木种源苗木对水分胁迫的生理响应。结果表明：（1）随着水分胁迫强度增加,叶片质膜相对透性增大,四川桤木种源质膜透性比台湾桤木种源增加幅度大。（2）在轻度的水分胁迫下,6个种源SOD活性均呈上升趋势,POD活性的增加只发生在PEG10％的水分胁迫强度以内,轻度水分胁迫下,MDA含量上升幅度小于重度胁迫强度;在PEG30％胁迫处理时,台湾桤木叶片MDA含量平均上升30．22％,而四川桤木上升了40．44％,表明台湾桤木种源膜脂过氧化程度较四川桤木种源轻,受害程度相对要小一些。（3）水分胁迫下,叶片脯氨酸含垃均有不同程度的增加．在PEG20％水分胁迫处理下,各种源脯氨酸积量增加的幅度为：台湾桤木苗栗种源（AF２）（68．9％）〉台湾桤木福建南平种源（AF４）（60．2％）〉台湾桤木台东种源（AF２）（54．7％）〉台湾桤木台中种源（AF1（47．6％）〉四川桤木广元种源（AC2）（39.7%）＞金堂种源（AC１）（39.1%）。水分胁迫条件下,叶片可溶性糖含量呈下降趋势。６种种源苗木在水分胁迫条件下,由于脯氨酸的积累,有一定的渗透调节能力,但这种渗透调节能力又是十分有限的。（４）采用隶属函数法,对各种源的抗旱生理进行综合评价,其耐旱牛理强弱的顺序是：AF2、AF1、AF3、AF4、AC2、AC1,台湾桤木比四川桤木具有较强的耐旱生理适应性。从耐旱生理角度讲,凡是适合四川桤木生长的区域,基本上亦适合台湾桤木,在引种载培中,应选择抗旱性稍强一些的台湾桤木苗栗种源和台中种源。     

不同种源栓皮栎幼苗叶片水分关系和保护酶活性对干旱胁迫的响应

 为了鉴别不同种源的栓皮栎（Quercus variabilis）耐旱性,对4个种源的3年生盆栽幼苗进行了控制条件下的土壤干旱胁迫实验,系统测定了超氧化物歧化酶(SOD)和过氧化氢酶(CAT)活性、MDA（丙二醛）含量、膜透性,以及叶片P-V曲线导出的水分关系参数（Ψπsat、Ψπtlp、WSDtlp、εmax）。结果表明：在土壤干旱胁迫下,种源4（黄龙）和种源3 (秦岭北坡) 抗氧化能力较强, 种源2 (伏牛山)居中, 种源1（巴山）的表现最差。干旱胁迫使各种源 MDA的含量及膜透性均有不同程度的提高,其中种源4的MDA含量及膜透性比较平稳, 种源1变化剧烈。在干旱胁迫下,各种源渗透调节和保持膨压的能力均有提高,但存在差异,以种源4和种源3表现较好,种源2居中,种源1较差。应用Fuzzy方法,对不同种源的叶片水分关系参数、保护酶活性和膜透性在干旱胁迫中的动态进行综合评判,认为不同种源的耐旱性强度次序为：种源4>种源3>种源2>种源1。其中种源4与种源1和种源2差异显著(p<0.05)。     

Ammonia volatilization during drought in perennial C4 grasses of tallgrass prairie

We measured foliar NH₃ volatilization as part of our study of the decrease (up to 40%) in shoot N concentration during drought in three perennial C₄ grasses of tallgrass prairie. Volatilization of recently expanded leaves was quantified using cuvettes and acid traps for Spartina pectinata, Andropogon gerardii, and Schizachyrium scoparium, a mesic, intermediate, and xeric species, respectively. In general, volatilization decreased during drought, approaching zero as stomates closed, and increased with plant N status and drought tolerance. Prior to drought, NH₃ volatilization was greater in xeric than mesic species (179 and 131 vs. 115 ng m^-2 s^-1 for individual leaves of S. scoparium and A. gerardii vs. Sp. pectinata). During a 2–3 week drought, whole-shoot volatile N losses can exceed 5% of total plant N in these species, accounting for 2–10% of the decrease in shoot percent N (again, xeric > mesic). Drought-induced N retranslocation of shoot N to roots and rhizomes is responsible for c. 63% of the decrease in percent N in Sp. pectinata, 28% in A. gerardii, and 8% in S. scoparium. The remainder of the decrease in percent N is attributable to growth dilution of existing shoot N, accounting for 34, 65, and 87% of the decrease in shoot percent N during drought in Sp. pectinata, A. gerardii, and S. scoparium, respectively. Thus, the relative importance of volatilization, retranslocation, and dilution in decreasing foliar percent N during drought in prairie grasses is species dependent and related to drought tolerance.     

The photoprotective role of epidermal anthocyanins and surface pubescence in young leaves of grapevine (Vitis vinifera)

BACKGROUND AND AIMS: Depending on cultivar, surfaces of young leaves of Vitis vinifera may be glabrous-green ('Soultanina') or transiently have anthocyanins ('Siriki') or pubescence ('Athiri'). A test is made of the hypothesis that anthocyanins and pubescence act as light screens affording a photoprotective advantage to the corresponding leaves, and an assessment is made of the magnitude of their effect. METHODS: Measurements were made on young leaves of the three cultivars in spring under field conditions. Photosynthetic gas-exchange and in vivo chlorophyll fluorescence were measured. Photosynthetic and photoprotective pigments were analysed by HPLC. KEY RESULTS: Compared with glabrous-green leaves, both anthocyanic and pubescent leaves had greater dark-adapted PSII photochemical efficiency and net photosynthesis. In leaves possessing either anthocyanins or pubescence, the ratio of xanthophyll cycle components to total chlorophyll, and mid-day de-epoxidation state of the xanthophyll cycle were considerably smaller, than in glabrous-green leaves. These differences were more evident in pubescent leaves, probably indicating that trichomes were more effective in decreasing light stress than anthocyanins in the epidermis. CONCLUSIONS: Light screens, especially in the form of pubescence, decrease the risk of photoinhibition whilst allowing leaves to maintain a smaller content of xanthophyll cycle components and depend less on xanthophyll cycle energy dissipation. This combination of photoprotective features, i.e. decreased photon flux to the photosynthetic apparatus and lower xanthophyll cycle utilization rates may be particularly advantageous under stressful conditions.     

蒙古莸叶片解剖结构的地理种源变异及其对环境变化响应的意义

长期受到生长环境影响而形成的遗传变异对植物生长发育有着显著的影响。叶片是植物对环境变化最敏感的器官, 了解叶片解剖结构在不同环境中产生的适应性变异是探索植物对环境适应的基础。同质园试验是研究遗传与环境因素对植物生长代谢等影响的一种有效方法, 该研究利用同质园试验排除了环境梯度的影响, 通过常规石蜡切片、多重比较、相关性分析、一般线性模型分析等方法, 对7个不同种源地的蒙古莸(Caryopteris mongholica)叶片解剖结构及其影响因素进行了定量比较。结果表明, 7个种源地的蒙古莸叶片均为等面叶, 无海绵组织分化, 其上表皮细胞较下表皮细胞厚, 上栅栏组织较下栅栏组织厚; 叶片各解剖结构参数间存在显著的自相关性, 不同种源叶片解剖结构存在显著差异: 随种源地年平均气温升高, 叶厚度、栅栏组织厚度呈增大趋势, 其中, 最西南部的阿左旗种源蒙古莸叶片的上下栅栏组织、叶厚度及叶片结构紧密度值均最大, 表现出明显的抗旱特征。种源地经纬度、气温、降水等对解剖结构指标有显著的影响, 其解释程度为34.09%-81.43%。同质园试验说明, 种源地气候差异驱动的遗传变异是引起不同种源叶片解剖结构差异的重要因素。     

Developmental instability as an index of adaptation to drought stress in a Mediterranean oak

An increase in temperature and water deficits caused by the ongoing climate change might lead to a decline growth rates and threaten the persistence of tree species in drought-prone areas within the Mediterranean Basin. Developmental instability (the error in development caused by stress) may provide an index of the adaptability of woody plants to withstand climatic stressors such as water shortage. This study evaluated the effects of drought stress on growth variables in three stands of a Mediterranean oak (Quercus faginea) exposed to differing climatic conditions (xeric, mesic and cooler) along an altitudinal gradient in northeastern Spain, in two climatically contrasting years (wet and dry years). Two indices of developmental instability, fluctuating and translational asymmetries, which reflect environmental stress, were measured in leaves and current-year shoots, respectively. We also measured branch biomass and fractal complexity of branches as indicators of the species’ performance. After a period of drought the individuals’ at the most xeric site presented lower developmental instability and less branch biomass than did the individuals from the mesic and cooler sites. We interpret that difference as an adaptive response to drought which reflects a trade-off between maintenance of homeostasis and growth when water is scarce. The study demonstrated that developmental instability constitutes a useful index to assess the degree of adaptation to stressful environmental conditions. The assessment of developmental instability in sites and years with contrasting climatic conditions provides a means of quantifying the capacity of plants to develop plastic adaptive responses to climatic stress.     

Drought tolerance and growth in populations of a wide‐ranging tree species indicate climate change risks for the boreal north

Choosing drought‐tolerant planting stock in reforestation programs may help adapt forests to climate change. To inform such reforestation strategies, we test lodgepole pine (Pinus contorta Doug. ex Loud. var latifolia Englm.) population response to drought and infer potential benefits of a northward transfer of seeds from drier, southern environments. The objective is addressed by combining dendroecological growth analysis with long‐term genetic field trials. Over 500 trees originating from 23 populations across western North America were destructively sampled in three experimental sites in southern British Columbia, representing a climate warming scenario. Growth after 32 years from provenances transferred southward or northward over long distances was significantly lower than growth of local populations. All populations were affected by a severe natural drought event in 2002. The provenances from the most southern locations showed the highest drought tolerance but low productivity. Local provenances were productive and drought tolerant. Provenances from the boreal north showed lower productivity and less drought tolerance on southern test sites than all other sources, implying that maladaptation to drought may prevent boreal populations from taking full advantage of more favorable growing conditions under projected climate change.     

Cyclitols and carbohydrates in leaves and roots of 13 Eucalyptus species suggest contrasting physiological responses to water deficit

In many tree species, physiological adaptations to drought include the accumulation of osmotically active substances and/or the presence of particular compatible solutes, among them cyclitols. Recently, the cyclitol quercitol was identified in species of Eucalyptus, a diverse genus whose speciation is probably driven by adaptation to water availability. We subjected seedlings of 13 Eucalyptus species from different ecosystems ('mesic' and 'xeric') and different sub-generic taxonomic groups to 10 weeks of water deficit (WD) treatment. Pre-dawn water potentials (psi(pdwn)) and relative water content (RWC) were determined in shoots, and total osmolality, soluble low-molecular-weight carbohydrates and cyclitols were measured in leaves and roots. Responses to water deficit followed two distinct patterns: Eucalyptus species from 'mesic' environments adjusted concentrations of sucrose (through increased levels of sucrose and decreases in RWC) in response to water deficit, whereas 'xeric' species increased concentrations of quercitol (through reductions in RWC). In root tissues, only species from xeric environments contained high levels of quercitol and mannitol, increasing under WD conditions. We suggest that the former (mesic) strategy may be beneficial to respond to short-lasting drought conditions, because sucrose is easily metabolized, whereas the latter (xeric) strategy may relate to an effective acclimation to longer-lasting drought. These physiological response groups are also related to taxonomic groups within the genus.     

不同种源乌桕幼苗对干旱胁迫的生理响应

以不同种源1年生乌桕幼苗为材料,采用盆栽控水试验研究了持续干旱条件下其外部形态及主要生理指标的变化,探讨各种源乌桕的抗旱性特性与差异。结果显示:(1)随着干旱持续时间的延长,各种源乌桕幼苗生长均受到不同程度抑制,外形表现出明显的缺水特征;其叶片相对含水量逐渐减小,复水后明显回升。(2)在极度干旱条件下(停水后第12天),幼苗叶片净光合作用速率仍维持一定的强度,而且细胞膜透性的变化不显著;在干旱胁迫条件下,幼苗叶片的过氧化物酶和超氧化物歧化酶活性增强,可溶性糖和可溶性蛋白质质量分数增加,丙二醛含量上升,脯氨酸积累,复水后呈现相反的变化趋势。(3)极度干旱条件下,不同种源乌桕幼苗间除丙二醛和相对电导率外,其它生理指标的变化差异显著或极显著,即不同种源具备比较稳定的抗旱性差异。(4)采用隶属函数法对不同种源幼苗的抗旱性分析发现,安徽黄山、河南商城两种源综合评价指数最大,为抗旱性较强的种源。研究表明,各种源乌桕幼苗能够通过调控自身保护酶活性和渗透调节物质含量来有效缓解干旱胁迫的伤害,从而表现出较强的抗旱性和适应性,但种源间存在稳定的差异,并以安徽黄山和河南商城种源抗旱性较强,可应用推广于季节性干旱区造林。     

The adaptation of Dianthus carthusianorum L. (Caryophyllaceae) to growth on a zinc–lead heap in southern Poland

Plants of Dianthus carthusianorum from a calamine (zinc–lead) waste heap and from unpolluted stands were compared in the field and under controlled laboratory conditions. It was found that the waste-heap plants differed significantly from those in the normal population in respect to the following morphological traits: lower weight of aerial parts, shorter and narrower leaves, smaller number of leaves per plant. In combination with shorter, less numerous shoots, these features reduce the transpiration area of the plants. The general habit (predominance of forms with short and trailing shoots) of the waste-heap population points to adaptation to a xeric environment. Under controlled conditions all of the above traits were maintained through three successive generations. They are thus genetically controlled. Root tolerance tests showed that the waste-heap plants had higher tolerance to zinc and lead. These results indicate that strong selection processes have generated a clear ecotypical differentiation for heavy metal tolerance and drought tolerance in a zinc–lead-rich waste-heap population of D. carthusianorum (variety).     

Tree ring isotopic composition, radial increment and height growth reveal provenance-specific reactions of Douglas-fir towards environmental parameters

In the search of timber species being tolerant towards summer droughts, which are expected to be more frequent in future, Douglas-fir is often discussed as a potential alternative for spruce in Central Europe. To assess physiological and growth reactions of Douglas-fir provenances towards climate- and weather-related environmental conditions we took advantage of a provenance trial with three sites in south-western Germany located along an elevation gradient. We examined six different provenances of Douglas-fir from North America for oxygen (δ¹⁸O) and carbon (δ¹³C) stable isotope composition in tree rings as well as for radial increment for a 7 year period and long-term height growth. Our results show that different Douglas-fir provenances clearly vary in their drought sensitivity at the driest and warmest site in the valley as shown by the radial growth decline in the extreme dry and hot year 2003. The growth decline in the provenances Pamelia Creek, Cameron Lake, Duncan Paldi and Conrad Creek could be clearly attributed to a reduction in stomatal conductance as assessed by the relations between δ¹⁸O and δ¹³C in the tree rings. These responses were not related to the long-term average climate at the places of origin of the provenances and the provenances with the lowest long-term (height) growth potential were the ones least affected in radial increment by the extreme drought of 2003. When selecting suitable Douglas-fir provenances, which are adapted to the climatic conditions projected for the future we thus might need to take into account the trade-off between the adaptation to extreme drought periods and the long-term growth performance. Site-specific evaluations of the probability of extreme drought events are thus needed to select the appropriate provenances.     

Warming and drought do not influence the palatability of Quercus pubescens Willd. leaves of four European provenances

In the context of global warming, the impact of extreme drought events on trees and biotic interactions with herbivore insects is widely unknown. A faster range expansion of insects in a changing climate could lead to mass propagations of pests in forests. Therefore, the aim was to investigate the influence of climatic alterations on leaf palatability. We exposed juvenile Quercus pubescens Willd. individuals of four European provenances (Bulgaria, Germany, Hungary, and Italy) to warming and drought. In addition, we conducted a palatability experiment with the pre-exposed Q. pubescens leaves and the caterpillars of the generalist forest pest Lymantria dispar L. (gypsy moth). Consumed leaf dry material, density of trichomes, and specific leaf area were examined. Surprisingly, neither warming nor drought affected the leaf palatability, but palatability was related to the density of trichomes. The Bulgarian provenance of Q. pubescens, which had the lowest density of trichomes, was most palatable. These findings suggest that global warming and drought might not lead to more frequent infestations of the four tested European Q. pubescens provenances by L. dispar caterpillars in the future.     

The ecological significance of annuals on a Mediterranean grassland (Mt Ventoux,France)

A study was carried out on a Mediterranean limestone grassland to search for the ecological significance of the occurrence of annuals. In the studied grassland, most of annuals are concentrated in patches. Authors tested if annuals (1) correspond to the R-strategy (ruderal) according to Grime's theory, i. e. they are adapted to live in disturbed, productive habitats, or (2) if they present an adaptation to a xeric climate in keeping with Raunkiaer.The correspondence analyses and the examination of plants strategies reveal that:(1) annuals are grouped with the more xerophytic perennials in the ordination,(2) the cover of annuals is higher in plots with a high cover of S species (stress-tolerators) and a lower cover of C species (competitors) and CS species (intermediate strategy).These results constitute arguments to integrate annuals of Mediterranean grasslands into stress-tolerators. In the discussion are examined the importance of patches in which annuals are preferentially located. Although patches are generated by disturbances, the presence of annuals is explained by their life cycle fitting the microclimate variations, characterized by an extreme summer drought. Their cycle is not adapted here to the frequency of disturbances (unless the summer drought is considered as annual disturbance - this last point of view is discussed). In the studied grassland, annuals are then better thought of as highly developed stress-tolerators than as ruderals. A generalization is put forward by distinguishing two cases: in xeric habitats, annuals are stress-tolerant, and in productive habitats, they are ruderal.Nomenclature: follows Kerguélen (1993)     

The effect of drought on C and N stable isotopes in different fractions of leaves, stems and roots of sensitive and tolerant beech ecotypes

Beech seedlings from 11 German climatic provenances were exposed to a realistically timed drought treatment in a greenhouse experiment. The stable isotope composition of carbon (C) and nitrogen (N) was analysed in pooled bulk material of roots, stems and leaves, as well as in the aqueous extracts and starch fractions. The delta 13C values increased in bulk samples (BS) of roots, stems and leaves by drought, although no leaf growth occurred during the experimental period. A clear drought effect on delta 13C in aqueous extracts was detected in leaves. In aqueous extracts of stems and roots as well as in starch fractions of all organs, abundance of delta 13C also tended to be increased by drought, but this effect was not statistically significant. For both delta 13C and delta 15N, enrichment was observed from the site of uptake/ source to the site of use/sink. A gradient for delta 13C in all fractions from leaves (-29.49, -28.89 and -27.85 per thousand) to stems (-28.81, -27.48 and -26.98 per thousand) and to roots (-27.60, -26.37 and -26.48 per thousand) was detected in BS, aqueous extracts and starch, respectively. An opposite gradient for delta 15N was found in BS: 1.59 per thousand, 1.84 per thousand and 3.05 per thousand in roots, stems and leaves, respectively. delta 15N was neither affected by drought in the BS nor in aqueous extracts, but an effect of provenance was observed. Particularly in roots and stems, drought-sensitive provenances showed the strongest shifts in delta 13C induced by drought and the lowest delta 15N values. In the present experiment, delta 13C values were more affected by the environmental factor drought, while delta 15N values were more affected by the genetic factor provenance.     

Responses to drought preconditioning in<Emphasis Type="Italic"> Eucalyptus globulus</Emphasis> Labill. provenances

Shoot water relations and morphological responses to drought preconditioning were studied by subjecting 5-month-old seedlings of three provenances of Eucalyptus globulus to different water regimes for 36 days in a greenhouse pot study. Moderately stressed plants were watered every 6 days and severely stressed plants were watered every 9 days. Control plants were watered daily. Drought cycles induced significant changes in morphological and physiological characteristics. Preconditioned seedlings were smaller in size, root collar diameter, height, and leaf area than control seedlings. Shoot/root ratio was not affected by drought. Osmotic potential at full turgor (ψπ_FT) and osmotic potential at turgor loss point (ψπ_TLP) were significantly lower and the magnitude of osmotic adjustment was significantly higher under the severe than under the moderate stress treatment. In severely stressed plants a decrease of turgid mass/dry mass contributed to osmotic adjustment. In a subsequent acclimation test, preconditioned seedlings showed higher values of stomatal conductance, predawn relative water content and water potential and lower mortality than control plants. These variables were significantly related to ψπ_FT. We assume that the reduced leaf area and osmotic adjustment observed in preconditioned seedlings contributed to drought acclimation in the selected E. globulus provenances leading to better rates of gas exchange and improved water status than non-conditioned plants. Provenances exhibited differences in their responses to drought, albeit mainly morphological differences. E. globulus subsp. bicostata from Tumbarumba grew more quickly (larger diameter and height relative growth rate) than the other provenances, implying a greater ability to tolerate water stress. It can be expected that preconditioned seedlings will display greater tolerance of water stress than non-conditioned plants and perform better during early establishment (higher survival and early growth).     

Resorption protection. Anthocyanins facilitate nutrient recovery in autumn by shielding leaves from potentially damaging light levels

The resorption protection hypothesis, which states that anthocyanins protect foliar nutrient resorption during senescence by shielding photosynthetic tissues from excess light, was tested using wild-type (WT) and anthocyanin-deficient mutants of three deciduous woody species, Cornus sericea, Vaccinium elliottii (Chapmn.), and Viburnum sargentii (Koehne). WT Betula papyrifera (Marsh) was included to compare the senescence performance of a species that does not produce anthocyanins in autumn. Plants were subjected to three environmental regimes during senescence: an outdoor treatment; a 5-d high-stress (high light and low temperature) treatment followed by transfer to a low-stress environment and a low-stress treatment that served as control. In the outdoor treatment, the appearance of anthocyanins in senescing leaves of WT plants was concomitant with the development of photo-inhibition in mutant plants of all three anthocyanin-producing species. In the high-stress environment, WT plants maintained higher photochemical efficiencies than mutants and were able to recover when transferred to the low-stress environment, whereas mutant leaves dropped while still green and displayed signs of irreversible photooxidative damage. Nitrogen resorption efficiencies and proficiencies of all mutants in both stressful treatments were significantly lower than the WT counterparts. B. papyrifera displayed photochemical efficiencies and nitrogen resorption performance comparable with the highest of the anthocyanin-producing species in all three senescing environments, indicating a photoprotective strategy divergent from the other species studied. These results strongly support the resorption protection hypothesis of anthocyanins in senescing leaves.     

Carbon,nitrogen, phosphorus,and sulphur concentration and partitioning in beech ecotypes (Fagus sylvatica L.): phosphorus most affected by drought

Beech seedlings of different drought sensitivity originating from 11 German provenances were grown in pots and cultivated in a greenhouse. The present paper aims to give insights on uptake, transport and use of macronutrients, since the knowledge of drought effects on the nutrition of trees is low compared to water relations. Therefore, the elemental composition, the ratio of inorganic to total content, and the partitioning between roots and shoots of carbon, nitrogen, phosphorus, and sulphur were investigated as affected by provenance and drought treatment. Phosphorus and phosphate concentrations decreased in all tissues after a 3 week drought treatment simulating a summer drought period. In roots carbon increased and nitrate decreased, in stems nitrogen decreased but nitrate increased following drought. The observed effects on phosphorus and phosphate are discussed in terms of lower phosphate mobility in the substrate due to lower water availability. The decrease in the ratio of phosphate to phosphorus in the tissues suggests the use of vacuolar phosphate pools for maintaining organic phosphorus homeostasis. The partitioning of all macronutrients was not affected by drought, although phosphorus and phosphate were significantly lowered in tissues. In most of the parameters studied significant differences between provenances were found. The recently observed drought sensitivity of provenances was not reflected in the strength of concentration changes or partitioning of macronutrients by drought over provenances.     

Interannual changes in folivory and bird insectivory along a natural productivity gradient in northern Patagonian forests

Trophic regulation models suggest that the magnitude of herbivory and predation (top-down forces) should vary predictably with habitat productivity. Theory also indicates that temporal abiotic variation and within-trophic level heterogeneity both affect trophic dynamics, but few studies addressed how these factors interact over broad-scale environmental gradients. Here we document herbivory from leaf-feeding insects along a natural rainfall/productivity gradient in Nothofagus pumilio forests of northern Patagonia, Argentina, and evaluate the impact of insectivorous birds on foliar damage experienced by tree saplings at each end of the gradient. The study ran over three years (1997–2000) comprising a severe drought (1998–1999), which allowed us to test how climatic events alter top-down forces. Foliar damage tended to increase towards the xeric, least productive forests. However, we found a predictable change of insect guild prevalence across the forest gradient. Leaf miners accounted for the greater damage recorded in xeric sites, whereas leaf chewers dominated in the more humid and productive forests. Interannual folivory patterns depended strongly on the feeding guild and forest site. Whereas leaf-miner damage decreased during the drought in xeric sites, chewer damage increased after the drought in the wettest site. Excluding birds did not affect leaf damage from miners, but generally increased chewer herbivory on hydric and xeric forest saplings. Indirect effects elicited by bird exclusion became most significant after the drought, when total folivory levels were higher. Thus, interannual abiotic heterogeneity markedly influenced the amount of folivory and strength of top-down control observed across the forest gradient. Moreover, our results suggest that spatial turnovers between major feeding guilds may need be considered to predict the dynamics of insect herbivory along environmental gradients.