Responses of nonstructural carbohydrates to shoot removal and soil moisture treatments in <Emphasis Type="Italic">Salix nigra</Emphasis>

Aboveground disturbances are common in dynamic riparian environments, and Salix nigra is well adapted with a vigorous resprouting response. Soil moisture stresses are also common, and S. nigra is flood tolerant and drought sensitive. The objective of this study was to quantify nonstructural carbohydrate (NSC) reserves in S. nigra following shoot removal and soil moisture treatments. NSC reserves provide energy for regeneration of shoot tissue until new functional leaves are developed. Three soil moisture treatments: well-watered (W), periodic flooding (F) and drought (D); and three shoot removal treatments: no shoots removed (R0), partial shoot removal (R1), and complete shoot removal (R2) were applied. Plants were harvested when new shoot development was observed (day 13). Statistical significance in the 3 × 3-factorial design was determined in two-factor ANOVA at P < 0.05. Both roots and cuttings were important reservoirs for NSC during resprouting response, with decreases in root (31%) and cutting (14%) biomass in R2 compared to R0. Rapid recovery of photosynthetic surface area (from 15 to 37% of R0) was found in R1. A clear pattern of starch mobilization was found in roots in R0, R1 and R2, with lowest root starch concentration in W, F higher than W, and D higher than F. Shoot starch concentration was lower in F and D compared to W in R0, however, in R1 shoot starch was reduced in W compared to F and D, possibly indicating reduced rates of translocation during soil moisture stress. Evidence of osmotic adjustment was found in roots and shoots with higher total ethanol-soluble carbohydrates (TESC) during soil moisture stress in F and D treatments. Total plant NSC pool was greater in F and D treatments compared to W, and progressively reduced from R0 to R1 to R2. Results indicated negative effects of drought, and to a lesser extent periodic flooding on resprouting response in S. nigra, with implications for reduced survival when exposed to combined stresses of aboveground disturbance and soil moisture.     

干旱-高钙对麻栎幼苗非结构性碳水化合物含量和分配的影响

土壤高钙胁迫是干旱-半干旱区影响树木生长的重要环境因子,为阐明干旱-高钙对树木非结构性碳水化合物(Non-structural carbohydrate,NSC)的含量和分配的影响,以麻栎幼苗为研究对象,阐明干旱和干旱-高钙条件对其生长、光合特征及非结构碳水化合物含量与分配的影响。结果表明:干旱显著降低麻栎幼苗生物量,而干旱-高钙处理较干旱进一步降低了麻栎生物量;干旱-高钙在处理初期就能显著抑制麻栎幼苗净光合速率,处理3个月后干旱和干旱-高钙处理的麻栎幼苗光合速率均显著低于对照;干旱处理麻栎幼苗平均非结构性碳水化合物含量增加19.90%,干旱-高钙处理麻栎幼苗整株的平均NSC含量则显著降低25.62%;干旱和干旱-高钙对麻栎幼苗NSC在不同器官间分配也产生不同影响,干旱条件下麻栎幼苗茎中NSC含量增加最多,较对照增加了52.34%,且淀粉的增高幅度(61.94%)高于可溶性糖(25.53%),干旱、高钙共同作用下麻栎幼苗全株平均NSC含量显著减少的同时,NSC积累在叶中,叶NSC含量显著提高32.31%,根、茎中NSC含量则分别显著降低了49.38%和35.31%。干旱-高钙胁迫降低麻栎幼苗NSC含量,且会减少NSC向枝干和根系分配。     

Post-fire mortality and water relations of three congeneric shrub species under extreme water stress — a trade-off with fecundity?

Mortality and water relations of three cooccurringHakea species were studied over the first year following a wildfire in scrub-health in southwestern Australia.Hakea ruscifolia regenerated from both resprouting adults and seedlings, whereasH. smilacifolia andH. polyanthema regenerated only from seedlings. We monitored seedling and resprout survival relative to pre-fire numbers and water relations (shoot water potential, transpiration, pressure-volume parameters) in order to determine the relationship between plant mortality and water relations over the critical first summer drought. Seedlings ofH. polyanthema (few initial seedlings per parent) showed little mortality (8%), achieved through both drought avoidance (low transpiration, high predawn water potential and relative water content (RWC), substantial osmotic adjustment) and drought tolerance later in the season. Seedlings ofH. smilacifolia andH. ruscifolia (high seedling/parent ratios) showed little drought avoidance and high mortality (54–68%). The remaining seedlings spent 3–4 months in a wilted condition (up to 3.6 MPa and 45% RWC below the turgor loss point inH. ruscifolia) indicating marked drought tolerance of the survivors. In contrast to its seedlings,H. ruscifolia resprouts were successful drought avoiders and experienced no mortality. The high level of survival and drought resistance ofH. polyanthema was consistent with its large seedlings (via large seeds) and low initial fecundity. The study highlights the importance of the interaction between ecophysiology-morphology and demography in determining the recruitment strategies of plants.     

Differential effect of drought regimes on the seedling performance of six floodplain grassland species

• The performance of seedlings is crucial for the survival and persistence of plant populations. Although drought frequently occurs in floodplains and can cause seedling mortality, studies on the effects of drought on seedlings of floodplain grasslands are scarce. We tested the hypotheses that drought reduces aboveground biomass, total biomass, plant height, number of leaves, leaf area and specific leaf area (SLA), and increases root biomass and root‐mass fraction (RMF) and that seedlings from species of wet floodplain grasslands are more affected by drought than species of dry grasslands.
• In a greenhouse study, we exposed seedlings of three confamilial pairs of species (Pimpinella saxifraga, Selinum carvifolia, Veronica teucrium, Veronica maritima, Sanguisorba minor, Sanguisorba officinalis) to increasing drought treatments. Within each plant family, one species is characteristic of wet and one of dry floodplain grasslands, confamilial in order to avoid phylogenetic bias of the results.
• In accordance with our hypotheses, drought conditions reduced aboveground biomass, total biomass, plant height, number of leaves and leaf area. Contrary to our hypotheses, drought conditions increased SLA and decreased root biomass and RMF of seedlings. Beyond the effects of the families, the results were species‐specific (V. maritima being the most sensitive species) and habitat‐specific. Species indicative of wet floodplain grasslands appear to be more sensitive to drought than species indicative of dry grasslands.
• Because of species‐ and habitat‐specific responses to reduced water availability, future drought periods due to climate change may severely affect some species from dry and wet habitats, while others may be unaffected.

     

Drought and shade deplete nonstructural carbohydrate reserves in seedlings of five temperate tree species

Plants that store nonstructural carbohydrates (NSC) may rely on carbon reserves to survive carbon‐limiting stress, assuming that reserves can be mobilized. We asked whether carbon reserves decrease in resource stressed seedlings, and if NSC allocation is related to species' relative stress tolerances. We tested the effects of stress (shade, drought, and defoliation) on NSC in seedlings of five temperate tree species (Acer rubrum Marsh., Betula papyrifera Marsh., Fraxinus americana L., Quercus rubra L., and Quercus velutina Lam.). In a greenhouse experiment, seedlings were subjected to combinations of shade, drought, and defoliation. We harvested seedlings over 32–97 days and measured biomass and NSC concentrations in stems and roots to estimate depletion rates. For all species and treatments, except for defoliation, seedling growth and NSC accumulation ceased. Shade and drought combined caused total NSC decreases in all species. For shade or drought alone, only some species experienced decreases. Starch followed similar patterns as total NSC, but soluble sugars increased under drought for drought‐tolerant species. These results provide evidence that species deplete stored carbon in response to carbon limiting stress and that species differences in NSC response may be important for understanding carbon depletion as a buffer against shade‐ and drought‐induced mortality.     

梭梭幼苗的存活与地上地下生长的关系

近几十年来,全球气候变化以及人类活动的加剧,导致古尔班通古特南缘荒漠地区的降水与地下水位发生显著的改变,这些改变必然导致荒漠植物用水策略的适应性变化。以古尔班通古特沙漠南缘原始沙漠中建群种梭梭的1年生幼苗为研究对象,对气象因子、土壤含水率、梭梭幼苗死亡率、地上地下高度(深度)、面积、生物量状况进行了全生长期的连续监测,以期探明1年生梭梭的生长、生存规律。结果表明:梭梭幼苗在土壤水分较为充沛的4—6月生长迅速,但随着土壤水分的消耗,7月地上地下部分都出现了不同程度的萎缩,在8月又恢复生长。在全年生长期中,幼苗死亡率基本呈现逐月下降的趋势。这说明在干旱来临前,梭梭幼苗快速生长以抢占更多资源;当遭遇干旱时,幼苗通过同化枝凋落有效的维持了根系供水与地上部分需水之间的平衡,保证了存活同化器官的光合能力,从而降低了死亡率。同时,幼苗以牺牲地上部分生长为代价将更多的光合产物转向根系,使得幼苗能够获得更多的水分保证其生长、生存。     

The fate of recently fixed carbon after drought release: towards unravelling C storage regulation in Tilia platyphyllos and Pinus sylvestris

Carbon reserves are important for maintaining tree function during and after stress. Increasing tree mortality driven by drought globally has renewed the interest in how plants regulate allocation of recently fixed C to reserve formation. Three‐year‐old seedlings of two species (Tilia platyphyllos and Pinus sylvestris) were exposed to two intensities of experimental drought during ~10 weeks, and ¹³C pulse labelling was subsequently applied with rewetting. Tracking the ¹³C label across different organs and C compounds (soluble sugars, starch, myo‐inositol, lipids and cellulose), together with the monitoring of gas exchange and C mass balances over time, allowed for the identification of variations in C allocation priorities and tree C balances that are associated with drought effects and subsequent drought release. The results demonstrate that soluble sugars accumulated in P. sylvestris under drought conditions independently of growth trends; thus, non‐structural carbohydrates (NSC) formation cannot be simply considered a passive overflow process in this species. Once drought ceased, C allocation to storage was still prioritized at the expense of growth, which suggested the presence of ‘drought memory effects’, possibly to ensure future growth and survival. On the contrary, NSC and growth dynamics in T. platyphyllos were consistent with a passive (overflow) view of NSC formation.     

多效唑和干旱胁迫对毛竹实生苗活力、光合能力及非结构性碳水化合物的影响

以1年生毛竹实生苗为研究对象,研究多效唑对不同水分条件下毛竹实生苗的叶绿素含量、光合参数、非结构性碳水化合物(NSC)含量、碳氮比、根系活力的影响。设置3个水分梯度:W1(75%相对田间持水量,CK)、W2(50%相对田间持水量,中度干旱)和W3(35%相对田间持水量,重度干旱),以及2个多效唑浓度:P1(0mg/L)、P2(40mg/L)。结果表明:随干旱强度增加,P1W1、P1W2、P1W3处理叶色逐渐变淡。与对照P1W1相比,P1W2和P1W3处理下叶绿素a、叶绿素b、类胡萝卜素、叶绿素a/b和叶绿素总含量显著下降(P0.05),Pn、Tr、WUE显著下降(P0.05),Ls显著上升(P0.05),毛竹叶片及根系中非结构性碳水化合物(NSC)含量显著上升(P0.05),毛竹根系活力显著下降。多效唑处理后,P2W2和P2W3的叶片色素含量相对于P2W1显著提高,但P2W2与P2W3无显著差异。同时,施加多效唑使Pn显著提高,P2W3较P1W3增加了146.9%。此外,P2W3处理使可溶性糖大量积累,达最大值3.41mg/g;毛竹叶片及根系淀粉含量显著上升,根系活力显著提高。试验揭示了多效唑通过提高干旱水平下毛竹实生苗的根系活力、光合速率,增加光合色素、非结构性碳水化合物含量,并将养分从地上转移到地下部分,进而抵御干旱胁迫带来的伤害。     

光强对杉木幼苗形态特征和叶片非结构性碳含量的影响

选取南方重要的造林树种杉木(Cunninghamia lanceolata(Lamb.)Hook)幼苗为研究对象,通过搭建遮荫棚设置5个光照强度(分别为自然光照的100%、60%、40%、15%和5%),研究了幼苗在不同光照强度下的生长形态、生物量积累及分配、叶片的非结构性碳含量(NSC)特征。结果显示:(1)叶长、叶宽和叶面积在40%光照强度下最大,而比叶面积和叶片相对含水量随着光照强度的降低呈递增趋势;(2)随着光照强度的降低,杉木幼苗各器官生物量下降,根生物量比和根冠比降低,茎和叶生物量比增加;(3)杉木幼苗在60%光照强度下叶片非结构性碳含量最高,5%光照强度下含量最低;(4)杉木幼苗比叶面积与叶生物量以及与非结构性碳含量之间存在极显著的负相关关系(P0.01),叶生物量与非结构性碳含量之间存在极显著的正相关关系(P0.01)。杉木幼苗能够通过形态学上的可塑性来适应不同的光强环境,提高光竞争能力和生存适合度,但在5%光照强度下,由于较难维持碳收支平衡而不利于其生长和存活。     

Comparative physiology of burned and unburned Rhus laurina after chaparral wildfire

Summary Laurel Sumac (Rhus laurina) is a dominant member of the coastal chaparral community of southern California that survives periodic burning by wildfires by resprouting from a lignotuber (root crown). We investigated the physiological basis for resprouting by comparing shoot elongation, leaf nitrogen content, tissue water status, leaf conductance to water vapor diffusion, and photosynthetic rates of post-fire R. laurina to those of adjacent unburned shrubs. Resprouts had higher rates of shoot elongation, leaf conductance, and photosynthesis than mature, unburned shrubs. Leaf nitrogen contents were elevated in burned shrubs even though their leaves developed interveinal chlorosis. A comparison of soil water potential to predawn water potential indicated that roots of R. laurina remain active below 2 m during the first summer drought after wildfire. Our results support the hypothesis that lignotubers not only contain dormant buds that develop into aerial shoots after wildfire but they also supply nutrient resources that enhance shoot elongation. Because R. laurina is relatively sensitive to drought, yet very successful in its rapid recovery after fire, maintaining an active root system after shoot removal may be the primary function of the massive lignotuber formed by this species.     

Comparative ecophysiology ofLarix kaempferi (Lamb.) Carr. andAbies veitchii Lindl. II. Mechanisms of higher drought resistance of seedlings ofL. kaempferi as compared withA. veitchii

The underlying mechanisms of drought resistance in seedlings ofLarix kaempferi as compared with those inAbies veitchii were investigated. First, seedlings of the two species were desiccated in air of differing water potential, and their survival rates were compared to detect any difference in desiccation tolerance between them. Contrary to differences in their drought resistance,A. veitchii showed higher survival. Next, the shoot heights and root depths of the two species were compared to detect any difference in desiccation avoidance. Although the shoot height differed little between the two species, the roots ofL. kaempferi seedlings were much deeper than those ofA. veitchii seedlings, and the death rate under progressing drought was well correlated with the root extensibility of each species. It was thus concluded thatL. kaempferi seedlings are able to avoid desiccation by extending their roots in deep soil faster thanA. veitchii seedlings, thereby resisting to drought. This paper is dedicated to Prof. Toshiro Saeki to mark his fruitful career in plant ecology.     

干旱胁迫下红砂幼苗非结构性碳水化合物动态变化特征

该试验以荒漠区主要建群种红砂幼苗为研究对象,设置适宜水分(CK)、轻度干旱(MD)、中度干旱(SD)和重度干旱(VSD)4个胁迫处理(即田间持水量的80%、60%、40%和20%),采用盆栽控水试验,分别测定干旱胁迫15、30、45和60 d时红砂幼苗的叶、茎、粗根和细根中非结构碳水化合物(NSC)及其组分的含量,分析不同胁迫强度下不同干旱持续时间红砂幼苗NSC的动态变化及各组分差异,以揭示红砂NSC对干旱胁迫的响应机制。结果表明:(1)干旱胁迫强度和胁迫持续时间对红砂幼苗不同器官NSC及其组分均有显著影响,其中胁迫持续时间对NSC动态变化的影响尤为显著。(2)干旱胁迫初期,红砂叶中的NSC含量呈下降趋势,而茎中的NSC含量呈上升趋势,粗根和细根中NSC含量在各胁迫处理下基本保持稳定。(3)干旱胁迫后期,红砂叶和茎中的可溶性糖、淀粉和NSC含量逐渐增加,而粗根和细根中的淀粉和NSC含量呈下降趋势(中度干旱除外),且这一时期重度干旱处理下各器官可溶性糖和NSC的含量明显高于CK。研究发现,重度干旱胁迫能显著诱导提高红砂幼苗不同器官中的NSC含量,并通过分解根中淀粉和增加叶片中可溶性糖含量的方式来调节细胞渗透势平衡,以维持细胞活力,进而保持红砂在干旱胁迫后期的存活。     

Growth features of Acacia tortilis and Acacia xanthophloea seedlings and their response to cyclic soil drought stress

Seedlings of Acacia tortilis (Forsk) Hyne and Acacia xanthophloea Benth. were raised under controlled glasshouse conditions. Control plants were watered daily while other treatments involved withholding water for 2, 4 and 6 days with 1‐day rehydration to container capacity. Compared to A. tortilis, A. xanthophloea seedlings showed higher leaf area, relative growth rates and total dry weight production under adequate water supply conditions. However, with increased water stress, A. xanthophloea seedlings could not alter their pattern of carbon allocation, retaining their root : shoot (r : s) ratio of about 0.5. By comparison, A. tortilis seedlings shifted carbon allocation to the roots, leading to a r : s ratio of 1.5 in water‐stressed seedlings, compared to 0.5 in the control plants. The ability of A. tortilis to reallocate carbon to the roots away from the shoots and to actually increase root growth compared to A. xanthophloea was a dehydration postponement strategy that may be important in species survival during drought.     

Effects of shoot excision on in situ soil and root respiration of wheat and soybean under drought stress

Little information is available about the variability of root-derived respiration rate in relation to biotic factors such as photosynthesis and substrate availability in roots. Here we examine the role of decreased carbohydrates availability on root-derived respiration through removal of above ground biomass. Spring wheat (Triticum aestivum L. cv. Longchun 8139) and soybean (Glycina max L. cv. Tianchan 2) were grown in the field under a moveable rain shelter, and subjected to three different water regimes: (1) well-watered control; (2) moderate drought stress, and (3) severe drought stress. Root-derived respiration before and after shoot clipping, and the concentration of total nonstructural carbohydrate, malic and citric acid were measured for spring wheat and soybean. Root-derived CO₂ flux and total nonstructural carbohydrate concentration of clipped wheat decreased by 38% and 31%, respectively. However, for soybean the root- derived CO₂ flux and total nonstructural carbohydrate concentrations were only 58% and 62% of control, respectively, indicating the root respiration rate was controlled by the availability of carbon in the root. A significant positive correlation between total nonstructural carbohydrate concentration of the root and soil water content was observed in unclipped plants. Total nonstructural carbohydrate contributed 93% of the variance in root-derived respiration. Our results clearly show, that in the field, the availability of carbon substrate in roots determines root-derived respiration and plays a key link between soil moisture and root-derived respiration. A period of time is needed for root respiration to return to “steady-state” after shoot removal and this period needed is strongly dependent on species and soil water content.     

Carbohydrate dynamics and mortality in a piñon‐juniper woodland under three future precipitation scenarios

Drought‐induced forest mortality is an increasing global problem with wide‐ranging consequences, yet mortality mechanisms remain poorly understood. Depletion of non‐structural carbohydrate (NSC) stores has been implicated as an important mechanism in drought‐induced mortality, but experimental field tests are rare. We used an ecosystem‐scale precipitation manipulation experiment to evaluate leaf and twig NSC dynamics of two co‐occurring conifers that differ in patterns of stomatal regulation of water loss and recent mortality: the relatively desiccation‐avoiding piñon pine (Pinus edulis) and the relatively desiccation‐tolerant one‐seed juniper (Juniperus monosperma). Piñon pine experienced 72% mortality after 13–25 months of experimental drought and juniper experienced 20% mortality after 32–47 months. Juniper maintained three times more NSC in the foliage than twigs, and converted NSC to glucose and fructose under drought, consistent with osmoregulation requirements to maintain higher stomatal conductance during drought than piñon. Despite these species differences, experimental drought caused decreased leaf starch content in dying trees of both species (P < 0.001). Average dry‐season leaf starch content was also a good predictor of drought‐survival time for both species (R² = 0.93). These results, along with observations of drought‐induced reductions to photosynthesis and growth, support carbon limitation as an important process during mortality of these two conifer species.     

Water relations,gas exchange,and growth of resprouts and mature plant shoots of Arbutus unedo L. and Quercus ilex L.

Resprout and mature plant shoot growth, leaf water status and gas exchange behavior, tissue nutrient content, flowering, and production were studied for co-occurring shallow-rooted (Arbutus unedo L.) and deeprooted (Quercus ilex L.) Mediterranean tree species at the Collserola Natural Park in Northeast Spain Resprouts showed higher growth rates than mature plant shoots. During fall, no differences in eco-physiological performance of leaves were found, but mobilization of carbohydrates from burls strongly stimulated growth of fall resprouts compared to spring resprouts, despite low exposed leaf area of the fall shoots. During summer drought, resprouts exhibited improved water status and carbon fixation compared to mature plant shoots. Shoot growth of Q. ilex was apparently extended due to deep rooting so that initial slower growth during spring and early summer as compared to A. unedo was compensated. Tissue nutrient contents varied only slightly and are postulated to be of minor importance in controlling rate of shoot growth, perhaps due to the relatively fertile soil of the site. Fall flowering appeared to inhibit fall shoot growth in A. unedo, but did not occur in Q. ilex. The results demonstrate that comparative examinations utilizing vegetation elements with differing morphological and physiological adaptations can be used to analyze relatively complex phenomena related to resprouting behavior. The studies provide an important multi-dimensional background framework for further studies of resprouting in the European Mediterranean region.     

Water relations and drought tolerance of young African tamarind (Tamarindus indica L.) trees

Tamarindus indica L. is an important multipurpose tree, indigenous to Africa, now introduced worldwide and known for its drought tolerance. The effects of drought on tamarinds, especially at seedling stage, are hardly investigated. However, this information is important for its conservation and domestication. In a growth chamber experiment we investigated the water relations of African tamarind seedlings under short-term soil drought stress. Initially tamarind seedlings can be considered as drought-tolerant at the expense of internal water storage reserves as they keep on transpiring (sap flow) and growing (diameter fluctuations). They finally spent 20% of their stem and root water storage reserves and experienced stem water potentials near − 3 MPa. Therefore, they can be classified as anisohydric. Their risk-taking behavior led to a high rate of seedling mortality (50%) because of whole plant hydraulic failure. They were not hydraulically efficient and they possessed low water storage capacity in stem and root (45%) due to high tissue density. When re-irrigated, remaining seedlings recovered slowly as a consequence of non-stomatal limitations and partial shoot dieback. Although tamarind seedlings show traits related to drought tolerance, we suggest that the species contains some water saving mechanisms. Contrasts with the co-occurring water-conserving tree species baobab (Adansonia digitata L.) are also discussed.     

Root Respiration,Photosynthesis and Grain Yield of Two Spring Wheat in Response to Soil Drying

The effects of soil water regime and wheat cultivar, differing in drought tolerance with respect to root respiration and grain yield, were investigated in a greenhouse experiment. Two spring wheat (Triticum aestivum) cultivars, a drought sensitive (Longchun 8139-2) and drought tolerant (Dingxi 24) were grown in PVC tubes (120 cm in length and 10 cm in diameter) under an automatic rain-shelter. Plants were subjected to three soil moisture regimes: (1) well-watered control (85% field water capacity, FWC); (2) moderate drought stress (50% FWC) and (3) severe drought stress (30% FWC). The aim was to study the influence of root respiration on grain yield under soil drying conditions. In the experiment, severe drought stress significantly (p < 0.05) reduced shoot and root biomass, photosynthesis and root respiration rate for both cultivars, but the extent of the decreases was greater for Dingxi 24 compared to that for Longchun 8139-2. Compared with Dingxi 24, 0.04 and 0.07 mg glucose m⁻² s⁻¹ of additional energy, equivalent to 0.78 and 1.43 J m⁻² s⁻¹, was used for water absorption by Longchun 8139-2 under moderate and severe drought stress, respectively. Although the grain yield of both cultivars decreased with declining soil moisture, loss was greater in Longchun 8139-2 than in Dingxi 24, especially under severe drought stress. The drought tolerance cultivar (Dingxi 24), had a higher biomass and metabolic activity under severe drought stress compared to the sensitive cultivar (Longchun 8139-2), which resulted in further limitation of grain yield. Results show that root respiration, carbohydrates allocation (root:shoot ratio) and grain yield were closely related to soil water status and wheat cultivar. Reductions in root respiration and root biomass under severe soil drying can improve drought tolerant wheat growth and physiological activity during soil drying and improve grain yield, and hence should be advantageous over a drought sensitive cultivar in arid regions.     

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.     

民勤绿洲荒漠过渡带人工梭梭种群年龄结构及动态

基于野外样地调查数据,通过分析静态生命表、存活曲线、动态指数等参数,结合时间序列模型,研究了人工梭梭种群年龄结构动态,旨在预测人工梭梭种群演替趋势,为退化人工梭梭种群恢复与重建提供理论依据。结果表明：(1)民勤绿洲荒漠过渡带人工梭梭种群的年龄结构近似于“金字塔”型,幼龄林个体占总物种数的90.1%;(2)人工梭梭种群存活曲线趋近于Deevey-Ⅱ型,为稳定型种群。不同区域人工梭梭种群的第Ⅰ、Ⅱ、Ⅲ龄级个体死亡率和消失率最高;(3)动态量化结果显示人工梭梭种群总体均表现为增长型,对外界干扰敏感性较强;(4)时间序列预测分析表明,在未来2、4个龄级时间后,种群Ⅱ、Ⅳ龄级个体数量大幅度增加,人工梭梭种群发展趋势较好。民勤绿洲荒漠过渡带人工梭梭种群为稳定增长型种群,种群具有一定的自然更新能力。针对该种群幼龄个体死亡率较高的问题,未来建议加强对第Ⅰ、Ⅱ、Ⅲ龄级幼株的精准抚育管理,通过人为辅助措施提高幼苗的保存率,促进人工梭梭林的可持续发展。