Fine root dynamics of oak saplings in response to Phytophthora cinnamomi infection under different temperatures and durations

The belowground effects of Phytophthora cinnamomi on 1‐year‐old saplings of two common oak species in mid‐Atlantic US forests, white (Quercus alba) and red oak (Q. rubra), were examined after incubation in pathogen‐infested soilless potting mix. Fine root lengths (0–1.5 mm in diameter) of both oak species were quantified after incubation at successive 30‐day intervals up to 300 days, for a total of 10 incubation periods. In addition, colony‐forming units (CFU) of P. cinnamomi were quantified after white oak saplings were incubated in infested soilless potting mix at different temperature/duration combinations that reflect soil conditions present in the mid‐Atlantic United States. Impact of P. cinnamomi on fine root lengths of red and white oak saplings varied considerably over time. Significant periods of fine root loss occurred primarily during spring, when bud break and leaf flush began for both oak species. Red oaks had 17% fine root loss on average, while white oaks appeared more resistant to P. cinnamomi infection with a 2% decrease in fine roots over the course of the experiment. Phytophthora cinnamomi CFU declined significantly with exposure to all incubation temperatures except 8°C. This was in contrast to in vitro experiments, where the optimum temperature for mycelial growth was determined to be 21°C and above. Significant fine root loss caused by P. cinnamomi depended on plant phenology and the oak species tested. Extreme soil temperatures have a significant adverse impact on temporal changes of P. cinnamomi population.     

Responses of Acer saccharum canopy trees and saplings to P, K and lime additions under high N deposition

Heavy atmospheric nitrogen (N) deposition has been associated with altered nutrient cycling, and even N saturation, in forest ecosystems previously thought to be N-limited. This observation has prompted application to such forests of non-N mineral nutrients as a mitigation measure. We examined leaf gas-exchange, leaf chemistry and leaf and shoot morphological responses of Acer saccharum Marsh. saplings and mature trees to experimental additions of non-nitrogenous mineral nutrients (dolomitic lime, phosphorus + potassium (P + K) and lime plus P + K) over 2 years in the Haliburton region of central Ontario, which receives some of the largest annual N inputs in North America. Nutrients were adsorbed in the mineral soil and taken up by A. saccharum trees within 1 year of fertilizer application; however, contrary to expectation, liming had no effect on soil P availability. Saplings and canopy trees showed significant responses to both P + K fertilization and liming, including increased foliar nutrient concentration, leaf size and shoot extension growth; however, no treatment effects on leaf gas-exchange parameters were detected. Increases in shoot extension preceded increases in diameter growth in saplings and canopy trees. Vector analysis of shoot extension growth and nutrient content was consistent with sufficiency of N but marked limitation of P, with co-limitation by calcium (Ca) in saplings and by Ca, Mg and K in canopy trees.     

Different sensitivity of isoprene emission, respiration and photosynthesis to high growth temperature coupled with drought stress in black poplar (Populus nigra) saplings

The effects of the interaction between high growth temperatures and water stress on gas-exchange properties of Populus nigra saplings were investigated. Water stress was expressed as a function of soil water content (SWC) or fraction of transpirable soil water (FTSW). Isoprene emission and photosynthesis (A) did not acclimate in response to elevated temperature, whereas dark (R(n)) and light (R(d)) respiration underwent thermal acclimation. R(d) was ~30% lower than R(n) irrespective of growth temperature and water stress level. Water stress induced a sharp decline, but not a complete inhibition, of both R(n) and R(d). There was no significant effect of high growth temperature on the responses of A, stomatal conductance (g(s)), isoprene emission, R(n) or R(d) to FTSW. High growth temperature resulted in a significant increase in the SWC endpoint. Photosynthesis was limited mainly by CO(2) acquisition in water-stressed plants. Impaired carbon metabolism became apparent only at the FTSW endpoint. Photosynthesis was restored in about a week following rewatering, indicating transient biochemical limitations. The kinetics of isoprene emission in response to FTSW confirmed that water stress uncouples the emission of isoprene from A, isoprene emission being unaffected by decreasing g(s). The different kinetics of A, respiration and isoprene emission in response to the interaction between high temperature and water stress led to rising R(d)/A ratio and amount of carbon lost as isoprene. Since respiration and isoprene sensitivity are much lower than A sensitivity to water stress, temperature interactions with water stress may dominate poplar acclimatory capability and maintenance of carbon homeostasis under climate change scenarios. Furthermore, predicted temperature increases in arid environments may reduce the amount of soil water that can be extracted before plant gas exchange decreases, exacerbating the effects of water stress even if soil water availability is not directly affected.     

Physiological responses of cork oak and holm oak to infection by fungal pathogens involved in oak decline

The aim of this research was to study the changes in net photosynthesis and stomatal conductance values in 3‐year‐old cork oak and holm oak seedlings growing in natural conditions and inoculated with Apiognomonia quercina, Biscogniauxia mediterranea, Botryosphaeria corticola and Pleurophoma cava. Throughout the 4‐month experimental period, the evolution of visual external symptoms and the values of physiological variables were periodically recorded. All pathogens caused stem lesions around the infection point; however, the lesions caused by B. corticola were longer in both oak species. On cork oak seedlings, all pathogens induced a significant and gradual reduction in net photosynthesis and stomatal conductance values, whereas other physiological disturbances were induced only by B. corticola infections on holm oak seedlings.     

Cold acclimation of Pinus contorta and Pinus sylvestris assessed by chlorophyll fluorescence

Needle samples of six provenances each of lodgepole pine (Pinus contorta Dougl. var. latifolia) and Scots pine (Pinus sylvestris L.), originating from latitudes 55 to 68 degrees N in western Canada and northern Sweden, were collected during the autumn and subjected to freezing temperatures in the range of -8 to -29 degrees C on three occasions in September and October. Needle injury was assessed by two different methods: visual assessment and chlorophyll a fluorescence. Chlorophyll a fluorescence data showed a highly significant correlation with the visual assessments of injury, indicating that the technique can be used as a simple, non-destructive and objective measure for rapid detection of freezing injury and for ranking of needle materials with respect to development of cold acclimation. The analyses showed that, during the autumn, lodgepole pine needles were more hardy and acclimated to low temperatures earlier than Scots pine needles.     

Reduced photosynthesis in old oak (Quercus robur): the impact of crown and hydraulic architecture

We tested the hypothesis that changes in crown architecture of old pedunculate oak trees (Quercus robur L. ssp. robur Kl. et Kr. et Rol.) reduce leaf specific hydraulic conductance of shoots, thereby limiting stomatal conductance and assimilation of affected shoots. At the end of summer 1999, hydraulic conductance and leaf specific hydraulic conductance, measured with a high-pressure flow meter in 0.5- to 1.5-m long shoots, were 27 and 39% lower, respectively, in shoots of low vigor compared with vigorously growing shoots in a 165-year- old stand in southeastern Germany. Two types of bottlenecks to water transport can be identified in shoots of old oak trees, namely nodes and abscission zones. The reduction in hydraulic conductance was especially severe in shoots with diameters of less than 2 mm. Maximum stomatal conductance and maximum net assimilation rate increased significantly with hydraulic conductance and leaf specific hydraulic conductance. Our data support the hypothesis that changes in shoot and consequently crown architecture observed in aging trees can limit photosynthesis by reducing shoot hydraulic conductance. Thus, in addition to increasing pathway length and lower conductivity of xylem in old trees, structural changes in shoot and crown architecture need to be considered when analyzing water relations and photosynthesis in mature and declining trees.     

Effects of partial defoliation on carbon and nitrogen partitioning and photosynthetic carbon uptake by two-year-old cork oak (Quercus suber) saplings

At the end of the growing season in late July, 20-month-old cork oak (Quercus suber L.) saplings were partially defoliated (63% of leaf area) to evaluate their ability to recover leaf area after defoliation. At 18 and 127 days after defoliation, changes in starch and nitrogen pools were determined in leaves and perennial organs, and variations in photosynthetic carbon uptake were investigated. To determine the role of stored nitrogen in regrowth after defoliation, plant nitrogen was labeled in the previous winter by enriching the nutrient solution with 15N. Plants recovered the lost leaf area in 127 days. Although there was remobilization of starch and nitrogen from leaves and perennial organs, the availability of resources for growth in the following spring was not decreased by defoliation. On the contrary, starch concentration in coarse roots was higher in defoliated saplings than in control saplings, presumably as a result of the higher net CO2 exchange rate in newly developed leaves compared with pre-existing leaves.     

Restoring silvopastures with oak saplings: effects of mulch and diameter class on survival,growth, and annual leaf-nutrient patterns

In Southwestern Spain, multifunctional silvopastoral systems consisting of pastureland and open oak woodlands are known as Dehesas. These, and other similar systems of the Mediterranean basin, are currently threatened by increasing intensive land use. As a consequence, oak regeneration is declining and is in need of adequate management and active restoration. Traditional restoration practices outplant one-year-old, nursery-produced oak seedlings grown in 250–350 cm³ containers, but establishment and growth results are typically poor. This work examines holm oak (Quercus ilex L. subsp. ballota (Desf.) Samp.) grown in a non-conventional container size (24 l) and age (6–7 years) with three mulch treatments (control, stone, and straw). In an open Dehesa of SE Spain, 106 oak saplings were planted in March 2010, and survival, diameter at breast height (DBH), and leaf-nutrient concentrations were analyzed. Forty months after planting, all treatments showed high survival (81 %) but only straw-mulched saplings differed significantly (94 %) from control (74 %). DBH increased over time but showed no significant differences among mulch treatments. Saplings with high initial DBH showed the greatest growth and change in DBH at the end of the study period. Leaf-nutrient concentrations changed significantly in the year following outplanting. Bi-monthly foliar nutrient concentration data show decreases in P, K, Zn, and B and sharp increases in Ca and Fe. In this work, we provide some evidence concerning the viability of non-conventional oak size for restoring, regenerating, or building up new agroforestry or silvopastoral systems. A combination of saplings with more than 10 mm of DBH and straw mulch is recommended.     

Impact of targeted sheep grazing on herbage and holm oak saplings in a silvopastoral wildfire prevention system in south-eastern Spain

Several wildfire prevention programs in southern Europe are currently using livestock grazing for the maintenance of fuelbreaks. This silvopastoral management is valued for being sustainable and effective in reducing fuel loads, but few studies have analyzed other impacts linked to fuelbreak grazing. This paper reports on an experiment performed within the wildfire prevention program in Andalusia (southern Spain) with the aim of clarifying and quantifying the effect of fuelbreak grazing on herbage biomass, ground cover, herbage species composition, and growth of holm oak saplings. The study site, located in a semiarid Mediterranean environment, was grazed by a shepherded sheep flock from February to June in three consecutive years at a similar stocking rate. Livestock consumed between 33 and 68 % of herbage production in the different years, and the greatest fuel reduction (remaining dry matter of 200?kg?ha^?1) was registered in Year 2, when rainfall and herbage production was lowest. Ground cover was significantly affected by grazing: on average, the percentage of bare soil increased three-fold, while herbage cover was reduced by a quarter. The botanical composition of herbage varied remarkably between years, but very little between Grazed and Non-Grazed areas within each year. Non-browsed holm oak saplings became progressively larger than browsed ones, differences only reaching clear statistical significance at the end of the three experimental years. At this time, the volume of browsed saplings was 47–56 % smaller than that of non-browsed holm oaks, even though the former had also grown significantly in the course of the experiment.     

Leaf phenology,photosynthesis, and the persistence of saplings and shrubs in a mature northern hardwood forest

We quantified leaf phenologies of saplings and overstory trees of sugar maple (Acer saccharum Marsh.) and American beech (Fagus grandifolia Ehrh.), and the shrub hobblebush viburnum (Viburnum alnifolium Marsh.) in a 72-year-old northern hardwood forest. Seasonal changes in irradiance in the shrub layer, and in the leaf CO(2) exchange of viburnum, and sugar maple and beech saplings were also measured. Leaf expansion occurred earlier in the spring and green leaves were retained later in the autumn in saplings and shrubs than in overstory trees. During the spring light phase (before overstory closure), large CO(2) gains by all three shrub-layer species occurred as a result of a combination of relatively large leaf area, high photosynthetic capacity, and high irradiance. Throughout the summer shade phase, photosynthetic capacity at a given irradiance remained relatively constant, but CO(2) gain was typically limited by low irradiances. Even though irradiance in the shrub layer increased during the autumn light phase as the overstory opened, CO(2) gains were modest compared to springtime values because of declining leaf area and photosynthetic capacity in all three species. The CO(2) gains during the spring light phase, and to a lesser extent during the autumn light phase, may be important to the carbon balance and long-term persistence of saplings and shrubs in the usually light-limited shrub layer of a northern hardwood forest. Therefore, for some late-successional species, leaf phenology may be an important characteristic that permits their long-term persistence in the shrub layer of mature northern hardwood forests.     

Impacts of seasonal air and soil temperatures on photosynthesis in Scots pine trees

Seasonal courses of light-saturated rate of net photosynthesis (A360) and stomatal conductance (gs) were examined in detached 1-year-old needles of Scots pine (Pinus sylvestris L.) from early April to mid-November. To evaluate the effects of soil frost and low soil temperatures on gas exchange, the extent and duration of soil frost, as well as the onset of soil warming, were manipulated in the field. During spring, early summer and autumn, the patterns of A360 and gs in needles from the control and warm-soil plots were generally strongly related to daily mean air temperatures and the frequency of severe frost. The warm-soil treatment had little effect on gas exchange, although mean soil temperature in the warm-soil plot was 3.8 degrees C higher than in the control plot during spring and summer, indicating that A360 and gs in needles from control trees were not limited by low soil temperature alone. In contrast, prolonged exposure to soil temperatures slightly above 0 degrees C severely restricted recovery of A360 and especially gs in needles from the cold-soil treatment during spring and early summer; however, full recovery of both A360 and gs occurred in late summer. We conclude that inhibition of A360 by low soil temperatures is related to both stomatal closure and effects on the biochemistry of photosynthesis, the relative importance of which appeared to vary during spring and early summer. During the autumn, soil temperatures as low as 8 degrees C did not affect either A360 or gs.     

两个山茶品种光合荧光参数日变化对春季低温的响应

为了了解山茶在低温下光合作用的机理及其耐寒性,在室外自然低温条件下,测定了抗寒性不同的2个山茶品种("Red Aurora"抗寒性高于"April Pink")光合和荧光参数的日变化,并对其进行比较。结果表明:2个山茶品种叶片的净光合速率日变化均呈双峰曲线,具有明显的"午休"现象。山茶品种Red Aurora光合午休是受非气孔因素的影响,而April Pink光合午休既受气孔限制方面的影响,也受非气孔限制方面的影响;2个山茶品种的净光合速率与气孔导度和蒸腾速率呈极显著正相关,与胞间CO2浓度呈极显著负相关;2个山茶品种在春季低温下均不同程度地发生光抑制,随着白天温度升高和光照时间的延长,Red Aurora光抑制现象得到一定程度的恢复,而April Pink基本没有恢复。2个品种PSⅡ电子传递量子效率,光化学淬灭系数都表现出"升高-降低-升高"波动的变化趋势,Red Aurora的变化幅度大于April Pink。与抗寒性弱的April Pink相比,Red Aurora具有较强的光能利用能力和避免强光低温伤害的能力。     

Forest stand dynamics and sudden oak death: Mortality in mixed-evergreen forests dominated by coast live oak

Sudden oak death (SOD), caused by the recently discovered non-native invasive pathogen, Phytophthora ramorum, has already killed tens of thousands of native coast live oak and tanoak trees in California. Little is known of potential short and long term impacts of this novel plant–pathogen interaction on forest structure and composition. Coast live oak (Quercus agrifolia) and bay laurel (Umbellularia californica) form mixed-evergreen forests along the northern California coast. This study measured tree mortality over a gradient of disease in three time periods. Direct measurements of current mortality were taken during 2004, representing a point-in-time estimate of present and ongoing mortality. Past stand conditions, c. 1994, were estimated using a stand reconstruction technique. Future stand conditions, c. 2014, were calculated by assuming that, given a lack of host resistance, live trees showing signs of the disease in 2004 would die. Results indicate that coast live oaks died at a rate of 4.4–5.5% year⁻¹ between 1994 and 2004 in highly impacted sites, compared with a background rate of 0.49% year⁻¹, a ten-fold increase in mortality. From 2004 to 2014, mortality rates in the same sites were 0.8–2.6% year⁻¹. Over the entire period, in highly impacted sites, a 59–70% loss of coast live oak basal area was predicted, and coast live oak decreased from 60% to 40% of total stand basal area, while bay laurel increased from 22% to 37%. Future stand structures will likely have greater proportions of bay laurel relative to coast live oak.     

Regulation of photosynthesis in interior spruce during water stress: changes in gas exchange and chlorophyll fluorescence

Photosynthetic response to water stress was analyzed in 1-year-old interior spruce (Picea glauca (Moench) Voss x P. engelmanni Parry hybrid complex) seedlings and emblings produced from somatic embryogenesis. Carbon dioxide uptake, oxygen evolution and chlorophyll fluorescence at 20 degrees C were monitored as predawn shoot water potential (Psi) decreased. Concurrently with stomatal closure, carbon assimilation declined rapidly as Psi decreased to -1.0 MPa. Oxygen evolution at 10,000 micro l CO(2) l(-1) declined continuously as Psi decreased to -1.6 MPa. At photon flux densities (PFD) above 50 micro mol m(-2) s(-1), photochemical efficiency of photosystem (PS) II observed during actinic light exposure (Phi(II), calculated as DeltaF/F(m)') decreased as Psi decreased. At the same PFDs, photochemical quenching (q(P)) declined with decreasing Psi and nonphotochemical quenching (q(N)) increased steadily. At PFDs below 50 micro mol m(-2) s(-1), major decreases in q(N) were not observed until Psi decreased below -1.6 MPa. We identified three phases of photosynthetic response to progressive water stress in interior spruce: a pronounced decline in gas exchange, subsequent photoprotective changes in chlorophyll fluorescence as primary photochemistry was down-regulated, and a decline in photochemical efficiency of dark-adapted needles.     

Response of photosynthesis,growth, and acorn mass of pedunculate oak to diff erent levels of nitrogen in wet and dry growing seasons

The objective was to examine the effects of optimal leaf nitrogen levels> 2.0% and suboptimal levels <2.0%,nitrogen nutrition on net photo synthetic rate,stem diameter increment,height growth increment and acorn mass of pedunculate oak during 2010 in the absence of drought stress and during 2011 under the impact of moderate drought stress.According to the results,moderate drought stress significantly reduced net photo synthetic rate,stem diameter increment and height growth increment,while...     

Local-scale diversity and adaptation along elevational gradients assessed by reciprocal transplant experiments: lack of local adaptation in silver fir populations

Key message

Silver fir transplantations along elevational gradients revealed a high diversity but no local adaptation. Populations displayed similar abilities to adapt to new environments including those due to climate change.

Context

The sustainability of forest stands depends on the ability of species and communities to adapt by combining plasticity and genetic evolution. Although well-documented at the scale of species distributions, the variability and adaptation of forest tree genetic resources are less understood at the short-distance scale.

Aims

We analysed the effects of genetic and environmental factors on the local-scale phenotypic diversity of traits related to adaptation in Abies alba. We also sought to highlight local adaptation, revealing past selection.

Methods

Six adaptive traits related to growth, phenology and survival were measured on seedlings from 57 half-sib families collected from 15 provenances and planted in a nine-site reciprocal transplant experiment distributed along three elevational gradients.

Results

Most part of the phenotypic variability was attributed to the environmental factors. Provenances and families had also significant effects on seedling performances, but the genetic variability was mostly attributed to the families. No pattern of local adaptation was observed, except in the presence of lateral branches in the driest garden.

Conclusion

The absence of local adaptation suggests a similar ability of all silver fir populations to develop in the various environments. This result provides favourable conditions for coping with the ongoing climate change without exotic resources enrichment.

     

Responses of plant diversity and species composition to the cessation of fertilization in a sandy grassland

Nitrogen was the main limiting nutrient of net primary production in the southeastern Keerqin Sandy Lands, Northeast China. Species richness declined and biomass increased after five consecutive years of nitrogen fertilization of these sandy grasslands （2004-2008）. After fertilization had been stopped for three years （2009-2011）, we surveyed vegetation on previously fertilized plots to quantify changes in commu- nity composition. Respect species richness showed an increasing trend over time since the cessation of fertilization. Respect vegetation height and coverage showed decreasing trends over time since the cessation of fertilization. Species composition changed after fertilization ceased, the dominant species shifting from Cannabis sativa, Phragmites communis and Chenopodium acuminaturn in 2008 to Cannabis sativa, Phragmites communis and Artemisia scoparia in 2011. Dominance of dominant species declined from 66.2% in 2008 to 57.5% in 2011. The importance value of annual plants in the earlier nitrogen addition plots was higher than in control plots, but the differences were not significant in 2011. The importance value of perennial plants differed significantly between treatments from 2009 to 2011. The reversion rate not only differed be- tween community characteristics, but also between functional groups in the same community characteristic. Although the residual effect of nitrogen addition on vegetation was still observed three years after fertilization ceased, the vegetation showed signs of recovery.     

Physiological and phenological responses of oak seedlings to oak forest soil in the absence of trees

Established trees influence the growth and physiology of seedlings by altering above- and belowground conditions; however, tree influences on seedling physiology via belowground interactions are not well understood. We used soil transfers to an open field to examine the belowground influences of a Quercus ellipsoidalis E.J.Hill dominated forest on Q. ellipsoidalis seedling mycorrhizal infection, nutrient uptake, growth and photosynthesis over three years. After two years, seedlings planted with large quantities of forest soil (HF treatment) had greater leaf mass and foliar N concentrations than seedlings receiving smaller quantities of forest soil (LF) and control treatments. Mycorrhizal infection was greater in the HF treatment after one year compared with the LF and control treatments, with a positive correlation of foliar N and mycorrhizal infection in Year 2. There were marked effects of treatments on seedling spring phenology with HF seedlings breaking bud up to 17 days earlier than seedlings in the other treatments. The HF seedlings also had more rapid leaf expansion and larger leaves, and an increase in net photosynthetic rates. These results highlight complex linkages between above- and belowground physiology: forest soil had substantial effects on seedling physiology, including traits such as phenology that have previously been considered to be under aboveground control. Belowground influences of trees on conspecific seedlings may play a critical role in early seedling establishment.     

Limits of tolerance to high temperatures causing direct and indirect damage to black spruce

The heat tolerance of actively growing 13-15-week-old black spruce (Picea mariana (Mill.) B.S.P.) was determined by exposing seedlings to temperatures of 40 to 60 degrees C for durations of 5 seconds to 3 hours by direct immersion in a hot water bath. Direct and indirect heat damage to needles were differentiated by assessing damage 5 minutes (direct) and 3 weeks (indirect) after exposure to high temperature. Both direct and indirect damage increased exponentially with the duration of exposure to high temperatures. However, indirect damage occurred at lower temperatures and with shorter periods of exposure than direct damage. Arrhenius plots of length of exposure versus exposure temperature revealed that the energy of activation for indirect damage was 384 kJ mol(-1), 36% higher than for direct damage. Both direct and indirect damage were less in seedlings preconditioned by a heat shock treatment (3 hours of exposure to an air temperature of 38 degrees C on each of 6 days prior to immersion in the water bath). Preconditioned seedlings withstood higher temperatures and longer durations of high temperature exposure than seedlings not preconditioned by heat shock.     

Germination characteristics and diversity of soil seed banks and above-ground vegetation in disturbed and undisturbed oak forests

The destruction of natural ecosystems is an important issue in many parts of the world. In the west of Iran, a vast area of the Zagros Mountain range is covered by typical vegetation including several rare plant species, of which many are currently considered endangered by anthropogenic activities. Despite the important role of soil seed banks to help conserve these degraded plant communities, the floristic studies in the Zagros forests have only focused on above- ground vegetation. In this study, the characteristics of soil seed banks and above-ground vegetation were examined at two forest sites： an undisturbed control （Un） and a disturbed （D） site. The objectives of this study were 1） to investigate the diversity of above-ground vegetation and soil seed banks in disturbed and undisturbed forests, 2） to examine the soil properties and the germination characteristics of the soil seed bank in disturbed and undisturbed oak forests and 3） to estimate the potential of soil seed banks in the restoration of disturbed sites. The results show that soil properties between Un and D sites were significantly different with higher values of pH, NH4-N, N~o,, CEC, OC, clay and canopy percentage in the Un site than in the D site. The Simpson diversity, Margalef richness and evenness indices differed significantly between sites, either for the soil seed banks or the above-ground vegetation. After a period of 26 weeks, the germination speed and the mean daily number of germinants were significantly different between Un and D sites. Without other conservation measures, soil seed banks alone cannot result in a full recovery after severe disturbances in the oak forests of Zagros.