Selection on leaf ecophysiological traits in a desert hybrid Helianthus species and early-generation hybrids

Leaf ecophysiological traits related to carbon gain and resource use are expected to be under strong selection in desert annuals. We used comparative and phenotypic selection approaches to investigate the importance of leaf ecophysiological traits for Helianthus anomalus, a diploid annual sunflower species of hybrid origin that is endemic to active desert dunes. Comparisons were made within and among five genotypic classes: H. anomalus, its ancestral parent species (H. annuus and H. petiolaris), and two backcrossed populations of the parental species (designated BC2ann and BC2pet) representing putative ancestors of H. anomalus. Seedlings were transplanted into H. anomalus habitat at Little Sahara Dunes, Utah, and followed through a summer growing season for leaf ecophysiological traits, phenology, and fitness estimated as vegetative biomass. Helianthus anomalus had a unique combination of traits when compared to its ancestral parent species, suggesting that lower leaf nitrogen and greater leaf succulence might be adaptive. However, selection on leaf traits in H. anomalus favored larger leaf area and greater nitrogen, which was not consistent with the extreme traits of H. anomalus relative to its ancestral parents. Also contrary to expectation, current selection on the leaf traits in the backcross populations was not consistently similar to, or resulting in evolution toward, the current H. anomalus phenotype. Only the selection for greater leaf succulence in BC2ann and greater water-use efficiency in BC2pet would result in evolution toward the current H. anomalus phenotype. It was surprising that the action of phenotypic selection depended greatly on the genotypic class for these closely related sunflower hybrids grown in a common environment. We speculate that this may be due to either phenotypic correlations between measured and unmeasured but functionally related traits or due to the three genotypic classes experiencing the environment differently as a result of their differing morphology.     

Plasticity in stomatal size and density of potato leaves under different irrigation and phosphorus regimes

The morphological features of stomata including their size and density could be modulated by environmental cues; however, the underlying mechanisms remain largely elusive. Here, the effect of different irrigation and phosphorus (P) regimes on stomatal size (SS) and stomatal density (SD) of potato leaves was investigated. The plants were grown in split-root pots under two P fertilization rates (viz., 0 and 100 mg kg⁻¹ soil, denoted as P0 and P1, respectively) and subjected to full (FI), deficit (DI), and partial root-zone drying (PRD) irrigation regimes. Results showed that SS and SD were unresponsive to P but significantly affected by the irrigation treatment. FI plants had the largest SS, followed by DI, and PRD the smallest; and the reverse was the case for SD. Compared to FI and DI, PRD plants had significantly lower values of specific leaf area (SLA) and leaf carbon isotope discrimination (Δ¹³C) under P0. Midday leaf water potential (Ψ_leaf) and stomatal conductance (g_s) was similar for DI and PRD, which was significantly lower than that of FI. Leaf contents of C, N, K, Ca and Mg were higher in PRD than in DI plants, particularly under P0. When analyzed across the three irrigation regimes, it was found that the P1 plants had significantly higher leaf contents of P and Mg, but significantly lower leaf K content compared to the P0 plants. Linear correlation analyses revealed that SS was positively correlated with Ψ_leaf and Δ¹³C; whereas SD was negatively correlated with Ψ_leaf, Δ¹³C and SLA, and positively correlated with leaf C, N and Ca contents. And g_s was positively correlated with SS but negatively correlated with SD. Collectively, under low P level, the smaller and denser stomata in PRD plants may bring about a more efficient stomatal control over gas exchange, hereby potentially enhance water-use efficiency as exemplified by the lowered leaf Δ¹³C under fluctuating soil moisture conditions.     

Carbon isotopes and water use efficiency: sense and sensitivity

We revisit the relationship between plant water use efficiency and carbon isotope signatures (δ¹³C) of plant material. Based on the definitions of intrinsic, instantaneous and integrated water use efficiency, we discuss the implications for interpreting δ¹³C data from leaf to landscape levels, and across diurnal to decadal timescales. Previous studies have often applied a simplified, linear relationship between δ¹³C, ratios of intercellular to ambient CO₂ mole fraction (C _i/C _a), and water use efficiency. In contrast, photosynthetic ¹³C discrimination (Δ) is sensitive to the ratio of the chloroplast to ambient CO₂ mole fraction, C _c/C _a (rather than C _i/C _a) and, consequently, to mesophyll conductance. Because mesophyll conductance may differ between species and over time, it is not possible to determine C _c/C _a from the same gas exchange measurements as C _i/C _a. On the other hand, water use efficiency at the leaf level depends on evaporative demand, which does not directly affect Δ. Water use efficiency and Δ can thus vary independently, making it difficult to obtain trends in water use efficiency from δ¹³C data. As an alternative approach, we offer a model available at to explore how water use efficiency and ¹³C discrimination are related across leaf and canopy scales. The model provides a tool to investigate whether trends in Δ indicate changes in leaf functional traits and/or environmental conditions during leaf growth, and how they are associated with trends in plant water use efficiency. The model can be used, for example, to examine whether trends in δ¹³C signatures obtained from tree rings imply changes in tree water use efficiency in response to atmospheric CO₂ increase. This is crucial for predicting how plants may respond to future climate change. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Morphological and physiological variation in western redcedar (<Emphasis Type="Italic">Thuja plicata</Emphasis>) populations under contrasting soil water conditions

Adaptation to precipitation conditions may induce genetic diversity that changes morphological and physiological traits. This hypothesis was investigated in the seedlings of seven western redcedar (Thuja plicata Donn ex D. Don) populations, which were collected along a precipitation transect from the Pacific coast to the southern interior of British Columbia, Canada. The experimental seedlings were either well-watered or soil-droughted and measured for growth, gas exchange rates, transpiration efficiency, and carbon isotope discrimination during or at the end of the third growing season. Significant variation was found in most of these morphological and physiological traits among the populations. Much of this variation occurred under well-watered, but not so much under droughted conditions. Mean height increments and transpiration efficiency showed a significant linear relationship, but biomass increments exhibited a quadratic relationship with precipitation on the origin site of these populations. Measurements of water use efficiency obtained from instantaneous gas exchange measurements, carbon isotope discrimination, and transpiration efficiency were intercorrelated in the seedlings. However, neither did any of these measurements consistently rank the populations, nor were they indicative of adaptation to climatic precipitation conditions in these western redcedar populations.     

Ecological and ecophysiological patterns in a pre-altiplano shrubland of the Andean Cordillera in northern Chile

We report on community structural, ecophysiological, phenological, and morphological measurements made on woody plant species in the high elevation pre-altiplano zone on the western slopes of the Andean Cordillera of northern Chile. Notwithstanding extreme conditions of low rainfall, high atmospheric vapour demand and diurnal temperature fluctuation, a diversity of habitats (associated with drainage and slope aspect), appreciable local plant species richness (28 woody perennial plant species in a small area), and an array of adaptive morphological ecophysiological and phenological traits are present among woody species in these shrublands. Family diversity was low with four families accounted for 82% of the species. A range of gas exchange and watering use efficiency strategies was present ranging from highly efficient CAM species with a carbon isotope discrimination (Δ) of 3.7–7.5‰ through C₃ species with varying stomatal and gas exchange characteristics with a Δ of 14.4 to 19.8‰. Drought-deciduous small-leaved amphistomatic species from arid slopes generally had high stomatal conductance and high carbon assimilation rates during the rainy season. These drought deciduous species were largely leafless and, with one exception had low water potentials, during the dry season. Wash and less xeric site species commonly had broader evergreen to semi-evergreen leaves, higher dry season water potentials, and relatively consistent and moderate rates of gas exchange throughout the year. For all species, intrinsic water use efficiency (as estimated from the inverse of c _i :c _a ratio) correlated positively with mean leaf width (broader leaves had a lower higher intrinsic WUE) and dry season water potential. The charismatic high altitude tree, Polylepis rugulosa (Roaceae), had a population structure that suggests highly episodic establishment of seedlings, likely in sequences of wet years. Little of the area of these significant shrublands is currently protected. It would be desirable to add areas of pre-altiplano shrublands to adjacent national parks to ensure the persistence of these important communities. This revised version was published online in June 2006 with corrections to the Cover Date.     

Δ13C and tree-ring width reflect different drought responses in Quercus ilex and Pinus halepensis

Holm oak (Quercus ilex L.) and Aleppo pine (Pinus halepensis Mill) are representative of two different functional types of trees extensively found in the Mediterranean: evergreen sclerophyllous and drought-adapted conifers. The former is considered a partially drought-tolerant species, whereas the latter is a typically drought-avoiding, water-saving species. We postulated that contrasting strategies in response to water deficits in Q. ilex and P. halepensis would lead to a differential sensitivity to changes in water availability. To test this hypothesis, we compared the response of both species in growth rate (measured as radial increments) and intrinsic water use efficiency [WUE_i, as inferred from carbon isotope discrimination (¹³C) in wood samples] among sites from different provenance regions in NE Spain. We found significant differences in ¹³C and growth among provenance regions, partly explained by contrasting water availability. Wood ¹³C was positively related with precipitation and the ratio between precipitation and potential evapotranspiration (P / E). However, these relationships were stronger in P. halepensis (for P / E, r ²=0.67, P <0.001) than in Q. ilex (r ²=0.42, P <0.01). In addition, radial growth was positively related with precipitation and ¹³C in P. halepensis (r ²=0.32 and r ²=0.35, respectively, P <0.01), but not in Q. ilex. We concluded that P. halepensis was more sensitive than Q. ilex to water availability, showing faster increase in WUE_i in response to water stress. We also found that the effect of north/south aspect on ¹³C and growth was site-specific, and unrelated to climatic variables.Due to an error in the citation line, this revised PDF (published in December 2003) deviates from the printed version, and is the correct and authoritative version of the paper.     

Leaf functional traits of Neotropical savanna trees in relation to seasonal water deficit

The seasonal savannas (cerrados) of Central Brazil are characterized by a large diversity of evergreen and deciduous trees, which do not show a clear differentiation in terms of active rooting depth. Irrespective of the depth of the root system, expansion of new foliage in deciduous species occurs at the end of the dry season. In this study, we examined a suite of leaf traits related to C assimilation, water and nutrients (N, P) in five deciduous and six evergreen trees that were among the dominant families of cerrado vegetation. Maximum CO₂ assimilation on a mass basis (A_mass) was significantly correlated with leaf N and P, and specific leaf area (SLA; leaf area per unit of leaf mass). The highest leaf concentrations of both nutrients were measured in the newly mature leaves of deciduous species at the end of the dry period. The differences in terms of leaf N and P between evergreen and deciduous species decreased during the wet season. Deciduous species also invested less in the production of non-photosynthetic leaf tissues and produced leaves with higher SLA and maintained higher water use efficiency. Thus, deciduous species compensated for their shorter leaf payback period by maintaining higher potential payback capacity (higher values of A_mass) and lower leaf construction costs (higher SLA). Their short leafless period and the capacity to flush by the end of the dry season may also contribute to offset the longer payback period of evergreen species, although it may involve the higher cost of maintaining a deep-root system or a tight control of plant water balance in the shallow-rooted ones.     

Carbon isotope discrimination,water-use efficiency,growth, and mortality in a natural shrub population

In order to scale up from the ecophysiological characters of individual plants to population-level questions, we need to determine if character patterns in natural populations are stable through time, and if the characters are related to growth and survival. We investigated these questions in a 3-year study for one character, integrated water-use efficiency (WUE) as estimated by carbon isotope discrimination () in a population of the Great Basin shrub, Chrysothamnus nauseosus. WUE was a conservative character for a given plant within and across seasons, and a previously documented difference between two size classes (represented by juveniles and adults) was maintained; smaller juveniles had a lower WUE than larger adults. The lower WUE of juveniles was often accompanied by higher rates of photosynthesis and stomatal conductance as compared to adults even though juveniles generally had more negative xylem pressure potentials. Although many discussions of the role of WUE in natural populations have been based on the expectation that higher WUE (lower ) is generally associated with less growth, we found no such relation-ship for juvenile plants in this population (i.e was not positively correlated with height increase). In addition, juvenile plant mortality was not correlated with . Although there were stable patterns of WUE for plants in this population, the positive correlation between WUE and size, and the lack of a negative correlation between WUE and height growth, make it unlikely that the WUE of an individual plant will be related in a simple manner to its growth and survival in the population.     

Morphological and molecular characterization of a natural hybrid betweenOrchis laxiflora andO. morio (Orchidaceae)

A putative natural hybrid betweenOrchis laxiflora andO. morio (Orchidaceae) from southern Italy, formerly known asO. alata, was characterized both on morphological and molecular grounds in order to confirm its hybrid status and to trace its maternal lineage. The morphological characters of the putative hybrid showed intermediacy between those of the parent species, and restriction fragment length polymorphism (RFLP) analysis of a region of the nuclear ribosomal DNA confirmed its hybrid origin. Chloroplast DNA RFLP analysis indicated thatO. morio provided the maternal genome.     

水分和氮磷水平对小麦碳同位素分辨率和水分利用效率的影响

 依据盆栽试验数据,利用植物稳定性碳同位素分辨率的理论模型,研究了水分和氮磷营养对小麦叶片碳同位素分辨率(Carbon-isotope discrimination △)的影响。结果表明：水分差异引起碳同位素分辨率较大变异,碳同位素分辨率随土壤相对含水量(Soil relative water content)的提高而提高,在土壤相对含水量为60％～70％条件下碳同位素分辨率最高。缺水时磷水平提高,碳同位素分辨率提高。水分利用效率(Water use efficiency)与碳同位素分辨率关系受土壤水分和养分水平的影响。缺水条件下水分利用效率与碳同位素分辨率之间为负相关,充分供水下为正相关;在低氮水平下的关系不明显,施氮150kg·hm-2时相关性显著。     

Water use efficiency and carbon isotope composition of plants in a cold desert environment

Summary The effects of the availabilities of water and nitrogen on water use efficiency (WUE) of plants were investigated in a sagebrush steppe. The four species studied wereArtemisia tridentata (shrub),Ceratoides lanata (suffrutescent shrub),Elymus lanceolatus (rhizomatous grass), andElymus elymoides (tussock grass). Water and nitrogen levels were manipulated in a two-by-two factorial design resulting in four treatments: control (no additions), added water, added nitrogen, and added water and nitrogen. One instantaneous and two long-term indicators of WUE were used to testa priori predictions of the ranking of WUE among treatments. The short-term indicator was the instantaneous ratio of assimilation to transpiration (A/E). The long-term measures were 1) the slope of the relationship between conductance to water vapor and maximum assimilation and 2) the carbon isotope composition (¹³C) of plant material. Additional water decreased WUE, whereas additional nitrogen increased WUE. For both A/E and ¹³C, the mean for added nitrogen alone was significantly greater than the mean for added water alone, and means for the control and added water and nitrogen fell in between. This ranking of WUE supported the hypothesis that both water and nitrogen limit plant gas exchange in this semiarid environment. The short- and long-term indicators were in agreement, providing evidence in support of theoretical models concerning the water cost of carbon assimilation.     

青海祁连棱果沙棘自然杂交带三种沙棘 光合特性日变化分析

杂交带是研究物种形成及进化的天然实验室。为探究棱果沙棘自然杂交带三种沙棘的生理生态适应性,该研究在光照充足的7月中旬分别测定了该区域三种沙棘雌、雄株的光合特性及相关环境因子的日变化。结果表明:(1)同种沙棘雌、雄株间光合日变化规律基本一致,不同沙棘光合特性的日变化规律存在较大差异。中国沙棘(Hippophae rhamnoides ssp. sinensis)净光合速率(Pn)日变化呈明显的双峰曲线,峰值出现在光合有效辐射(PAR)和空气温度(Ta)相对较高的10:00和14:00,最高达到(19.53±5.35)μmol·m~(-2)·s~(-1);棱果沙棘(Hippophae goniocarpa)和肋果沙棘(H. neurocarpa ssp. neurocarpa) Pn日变化均呈近双峰曲线,在PAR和Ta相对较低的8:00和16:00有较高的值,最高值分别为(13.43±3.43)和(15.27±2.43)μmol·m~(-2)·s~(-1)。(2)三种沙棘水分利用效率(WUE)与Pn的日变化规律一致,但中国沙棘具有最高的WUE,达到(6.72±0.95)μmol·mmol~(-1),棱果沙棘和肋果沙棘的WUE日变化最高分别为(4.03±1.08)和(4.93±0.86)μmol·mmol~(-1)。(3)三种沙棘蒸腾速率(Tr)、气孔导度(Gs)、胞间二氧化碳浓度(Ci)和气孔限制值(Ls)的日变化规律相似,其中Gs均在10:00后一致下降,在12:00左右均出现不同程度的气孔关闭现象,这是其发生光合午休的主要原因之一。杂交带三种沙棘光合特性日变化差异主要表现在中国沙棘和其他两种沙棘之间,而杂交种棱果沙棘与亲本种肋果沙棘的变化规律基本一致,其Pn与WUE可能受到了Ta、PAR和空气相对湿度(RH)等环境因子的影响。     

Morphological and physiological responses of two contrasting Malus species to exogenous abscisic acid application

This study characterized morphological and physiological responses of two Malus species to exogenous abscisic acid (ABA) application under both well-watered and drought-stressed conditions. Exogenous ABA was sprayed onto the leaves of potted 1-year-old seedlings of M. sieversii and M. hupehensis, originated from regions with low annual rainfall and high annual rainfall, respectively. The results demonstrated that exogenous ABA application significantly decreased height growth (H), total biomass (TB), total leaf area (LA), net photosynthesis (A) and stomatal conductance (g _s), and significantly increased root/shoot ratio (RS), specific leaf area (SLA), endogenous ABA concentration, water use efficiency (WUE) and carbon isotope composition (δ¹³C) under both well-watered and drought-stressed conditions. However, distinct interspecific differences were found in ABA-induced morphological and physiological responses. Compared with M. hupehensis, M. sieversii was more responsive to exogenous ABA application, resulting in larger decreases in H, LA, A and g _s, and larger increases in RS, SLA, WUE_L, WUE_i, ABA and δ¹³C. These results suggest strong evidence for different maintenance of fitness under stressful conditions between species of Malus. In addition, application of exogenous ABA appears to enhance the tolerance of two Malus species to drought-stress.     

Genetic differentiation in carbon isotope discrimination and gas exchange in Pseudotsuga menziesii

Patterns of genetic variation in gas-exchange physiology were analyzed in a 15-year-old Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) plantation that contains 25 populations grown from seed collected from across the natural distribution of the species. Seed was collected from 33°30 to 53°12 north latitude and from 170 m to 2930 m above sea level, and from the coastal and interior (Rocky Mountain) varieties of the species. Carbon isotope discrimination () ranged from 19.70() to 22.43() and was closely related to geographic location of the seed source. The coastal variety (20.50 (SE=0.21)) was not significantly different from the interior variety (20.91 (0.15)). Instead, most variation was found within the interior variety; populations from the southern Rockies had the highest discrimination (21.53 (0.20)) (lowest water-use efficiency). Carbon isotope discrimination (), stomatal conductance to water vapor (g), the ratio of intercellular to ambient CO₂ concentration (c_i/c_a), and intrinsic water-use efficiency (A/g) were all correlated with altitude of origin (r=0.76, 0.73, 0.74, and –0.63 respectively); all were statistically significant at the 0.01 level. The same variables were correlated with both height and diameter at age 15 (all at P0.0005). Observed patterns in the common garden did not conform to our expectation of higher WUE, measured by both A/g and , in trees from the drier habitats of the interior, nor did they agree with published in situ observations of decreasing g and with altitude. The genetic effect opposes the altitudinal one, leading to some degree of homeostasis in physiological characteri tics in situ.     

Long-term steady-state labelling of wheat plants by use of natural 13CO2/12CO2 mixtures in an open,rapidly turned-over system

A photosynthate labelling method is presented which takes advantage of the natural difference in carbon-isotope composition () which exists between atmospheric CO₂ (-8) and commercially available compressed CO₂. Carbon dioxide with -4.0 and –27.9%., respectively, has been used for labelling. A plant growth cabinet served as the labelling compartment. CO₂-free air was continuously injected at a rate of up to 54m³·h^–1. Dilution of cabinet CO₂ by CO₂-free air was counterbalanced by addition of CO₂ with known constant . Since the labelling-cabinet atmosphere was continuously exchanged at a high rate, photosynthetic carbon-isotope discrimination was fully expressed. In order to study the distribution of carbon acquired by the plant during a defined growth period, the of CO₂ was modified by replacing, for example, atmospheric CO₂ by CO₂ with –27.9%. and the weight and 5 of plant carbon pools was monitored over time. In such an experiment the change of CO₂ was followed by a rapid change of the of sucrose in mature flag-leaf blades of wheat (Triticum aestivum L.). The 5 of sucrose stabilized near –51%., indicating complete exchange by current photosynthate. In contrast 83% of the total carbon in mature flag-leaf blades was not exchanged after 14 d continuous labelling. Differential labelling of pre- and post-anthesis photosynthate indicated that 13% of grain carbon originated from pre-anthesis photosynthesis. Carbon-isotope discrimination and its consideration in experimentation and labelling data evaluation are discussed in detail. Since the air supplied to the labelling cabinet is dry and free of CO₂, carbon-isotope discrimination and carbon turnover and partitioning can be studied over a wide range of CO₂ concentrations (0–2600 cm³ · m^–3) and vapor-pressure deficits.Abbreviation and Symbol PPFD photosynthetic photon flux density - carbon-isotope composition Dr. G. Schleser (Forschungszentrum Jülich, FRG) and Professor S. Hoernes (Mineralogisch-Petrologisches Institut, Universität Bonn) for valuable help and advice during the initial stages of the project and Professor W. Kühbauch (Institut für Pflanzenbau, Universität Bonn) for continuing support. Technical assistance of Ute Labusch, Petra Biermann, Ludwig Schmitz and Thomas Gebbing is gratefully acknowleged.

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A pyrenoid-based carbon-concentrating mechanism is present in terrestrial bryophytes of the class Anthocerotae

It has been widely accepted that carbon assimilation in bryophytes is exclusively based on the conventional C₃ photosynthetic pathway. The occurrence of biochemical CO₂-concentrating mechanisms (C₄ or Crassulacean acid metabolism), which have developed in plants in the last 20–100 million years, has been discounted for bryophytes from studies of the carbon isotope composition (¹³C) of organic material. In contrast cyanobacteria and many algae show active accumulation of dissolved inorganic carbon via biophysical CO₂-concentrating mechanisms which are also found in the photobiont partners in certain lichens. The presence of a pyrenoid, a granular particle within the chloroplast, has been linked with CO₂-concentrating mechanism activity in green algae and lichens and we now show that such a mechanism is categorically associated with the occurrence of a pyrenoid in bryophytes belonging to the class of Anthocerotae. These observations have significant evolutionary implications for the development of terrestrial photosynthesis during the colonisation of the land, raising the intriguing question of why the pyrenoid-based CO₂-concentrating mechanism did not persist in the terrestrial environment.Abbreviations and Symbols CCM carbon-concentrating mechanism - DIG dissolved inorganic carbon (CO₂+HCO ₃ ^- +CO ₂ ^- ) - DW dry weight - K_0.5 external concentration of CO₂ at which half-maximal rates of CO₂ assimilation are reached - Rubisco ribulose-l,5-bisphosphate carboxylase-oxygenase - carbon isotope discrimination (%) - ¹³C carbon isotope ratio (%) This work was supported by the Natural Environment Research Council (GR3/8813) and the Leverhulme Trust. We thank Prof. A. Roy Perry (National Museum of Wales, Cardiff), Dr. B. Coppins and Mr. D. Long (Royal Botanic Garden Edinburgh) for access to herbarium specimens and Mr. M. Fletcher for providing living bryophytes.     

Herbivory and tree mortality across a pinyon pine hybrid zone

We examined the abundances of three common insect herbivores on pure and hybrid pinyon pines along a 250-km transect in west-central Arizona, United States. Using six morphological traits, we developed a hybrid index to classify trees as pure Pinus californiarum, hybrid, or pure Pinus edulis. The insects (the stem-boring moth, Dioryctria albovittella, the scale insect, Matsucoccus acalyptus, and several species of pitch moths that produce wounds on the trunk and branches) exhibited different distributional patterns across tree types. Stem-boring moths were significantly more abundant on trees at hybrid sites compared to trees at pure sites. In addition, within hybrid sites, hybrids supported significantly more moth larvae than pure trees of either species. These two patterns support the hybrid susceptibility hypothesis in which hybrid breakdown results in increased susceptibility to herbivory. In contrast to stem-borers, there were significantly more pitch moth wounds on trees at pure P. californiarum sites than at hybrid and pure P. edulis sites. Within the hybrid zone, pitch moth abundance was equal on pure P. californiarum and hybrids, and both were significantly greater than on pure P. edulis. These within-site comparisons support the dominance hypothesis where hybrid resistance differs from one tree species, but not the other. Scale insects exhibited the most restricted distribution; over the 250 km transect they were found only in the hybrid zone. This supports the hybrid susceptibility and/or the stress hypothesis (i.e., species at the edge of their range suffer greater stress and are more susceptible to herbivory). We summed the mean numbers of these three common herbivores across sites and found that hybrid sites supported 2.1 and 3.9 times more herbivores than pure P. californiarum and P. edulis sites, respectively. Furthermore, tree mortality was on average, 35 times greater within the hybrid zone compared to pure zones of each species and was associated with the cumulative abundance of herbivores (r ²=0.646). Regardless of whether this mortality is due to insect infestation, stress or a combination of both, these results suggest that hybrid zones are important arenas of natural selection.     

A second spontaneous hybrid in the genusLeucaena (Leguminosae, Mimosoideae)

The detection of hybridity inLeucaena is discussed in relation to: (i) traditional criteria, (ii) molecular criteria and (iii) models to predict hybrid leaf morphology. Morphological, geographical and molecular evidence for the occurrence of interspecific hybrids betweenL. leucocephala andL. diversifolia in south-central Mexico, northern Guatemala, Jamaica, Dominican Republic, the Philippines and Papua New Guinea is presented. Predicted mean hybrid leaf trait values calculated from parent material are compared with data from putative hybrids and shown to be similar. The origin of these hybrids is discussed and shown to be the result of artificial sympatry resulting from indigenous, and recent exotic, domestication of the parent species. The hybrid is described asL. ×spontanea.     

Fitness of natural willow hybrids in a pioneer mosaic hybrid zone

Hybrid fitness is an important parameter to predict the evolutionary consequences of a hybridization event and to characterize hybrid zones. We studied fitness parameters of F₁ and later‐generation hybrids between the lowland species Salix purpurea and the alpine S. helvetica that have recently emerged during colonization of an alpine glacier forefield. Fruit production (number of capsules per catkin and fruit set) did not differ between hybrids and parents, but the number of seeds per capsule of F₁ hybrids was slightly lower than that of later‐generation hybrids and of the parents. Germination rates and seedling growth were tested on three substrates (pH 4.5, 7.0, and 8.0). Germination rates of seeds collected from F₁ hybrids were lower on acid and neutral substrates, but equal at pH 8.0 compared to all other groups, while the seeds from later‐generation hybrids performed as well as the parents on all three substrates. In seedling growth, the colonizer S. purpurea performed better than all other taxa on all three substrates, while hybrids resembled the subalpine species S. helvetica. Results suggest that endogenous selection acts against F₁ hybrids, but favors fitter genotypes in later‐generation hybrids. Exogenous selection via soil pH appears to be weak during seedling establishment. The pioneer vegetation on the glacier forefield may offer sufficient niche space for hybrid seedlings. Owing to the relatively high fitness of the hybrids and the scattered distribution of hybrids and parental individuals on the glacier forefield, this hybrid zone can be assigned to a mosaic model, probably facilitating gene flow and introgression between the parental species. As establishment of the hybrid zone appears to be linked to a colonization process, we propose to call it a pioneer mosaic hybrid zone.     

Soil water exploitation after fire: competition between Bromus tectorum (cheatgrass) and two native species

Summary Causes for the widespread abundance of the alien grass Bromus tectorum (cheatgrass) after fire in semiarid areas of western North America may include: (1) utilization of resources freed by the removal of fireintolerant plants; and (2) successful competition between B. tectorum and individual plants that survive fire. On a site in northwestern Nevada (USA), measurements of soil water content, plant water potential, aboveground biomass production, water use efficiency, and B. tectorum tiller density were used to determine if B. tectorum competes with either of two native species (Stipa comata and Chrysothamnus viscidiflorus) or simply uses unclaimed resources. Soil water content around native species occurring with B. tectorum was significantly lower (P<0.05) than around individuals without B. tectorum nearby. Native species had significantly more negative plant water potential when they occurred with B. tectorum. Aboveground biomass was significantly higher for native species without B. tectorum. However, the carbon isotope ratio of leaves for native species with B. tectorum was not significantly different from individuals without B. tectorum. Thus, B. tectorum competes with native species for soil water and negatively affects their wate status and productivity, but the competition for water does not affect water use efficiency of the native species. These adverse effects of B. tectorum competition on the productivity and water status of native species are also evident at 12 years after a fire. This competitive ability of B. tectorum greatly enhances its capability to exploit soil resources after fire and to enhance its status in the community.