Response of galling invertebrates on Salix lanata to reindeer herbivory

Browsing and defoliation often increase the densities of insect herbivores on woody plants. Densities of herbivorous invertebrates were estimated in a long-term grazing manipulation experiment. More then 30-yr-old fences allow us to compare densities of invertebrate herbivores on Salix lanata in areas heavily grazed and areas lightly grazed by reindeer. The number of gall-forming insects ( Pontania glabrifons) and gall-forming mites were higher on the heavily grazed shrubs than on lightly grazed shrubs. In contrast to most short-term studies, the heavily grazed S. lanata had shorter current annual shoots. No difference in leaf size, leaf nitrogen content, or C:N ratio between grazing intensities were detected. However, the enhanced natural δ15N value indicates that heavily grazed shrubs get a higher proportion of their N directly from reindeer faeces. Leaf weight per unit area and relative fluctuating asymmetry of leaf shape increased in heavily grazed S. lanata . Enhanced relative fluctuating asymmetry might indicate higher susceptibility to herbivores. Long-term grazing seems to increase the density of invertebrate herbivory in the same way as short-term grazing, even if the plant responses differ substantially.     

Variations in leaf traits and susceptibility to insect herbivory within a Salix miyabeana population under field conditions

Variations in leaf traits (toughness, total nitrogen and total phenolic concentrations) and susceptibility to herbivory in Salix miyabeana were studied among individual trees within a population under field conditions. Leaf quality clearly decreased as season progressed, i.e. increases in leaf toughness and total phenolics and decrease in leaf nitrogen. Seasonal pattern and extent of herbivore attack were similar between years. Significant correlation between leaf traits and susceptibility to herbivore attack was detected, while effects of sex and plant size on leaf traits and herbivory were less clear. There was a negative correlation between total nitrogen and total phenolics, and a positive correlation between leaf toughness and total phenolics among trees. Trees with high quality leaves tended to suffer from frequent herbivore attack and leaf damage. Such a clear relationship between leaf traits and susceptibility to herbivory may be related with a life-history strategy of willows, which are rapid-growing pioneer species and generally respond to herbivorous damage not by induced resistance but by compensative growth. This revised version was published online in August 2006 with corrections to the Cover Date.     

Composition, Stand Structure, and Biomass Estimates of “Willow Rings” on the Canadian Prairies

Many small wetlands are scattered across the Canadian Prairies. These wetlands are often surrounded by a ring of phreatophytic shrubs, of which willows are the dominant genus. Even though many of these willow rings are currently degraded, due to the lack of disturbance factors such as prairie wild fires and bison herds, they are still of great value as a biomass resource without carbon debt. Our study had the objectives to (1) study the distribution of five native willows (Salix bebbiana, Salix discolor, Salix eriocephala, Salix interior, and Salix petiolaris) in 12 willow wetland communities in relation to a moisture gradient and (2)examine the effects of age of the willow ring on biomass potential, stand structure, and species distribution. Annual biomass production ranged from 1.9 to 16.2 odt ha^?1 year^?1 for the 12 sites that were between 9 and 34 years old. Current standing biomass for the 12 willow rings with a total area of 1.5 ha was estimated at 239.2 odt. With age, stand structure changed from multiple small-diameter stems to fewer large-diameter stems, and at the same time the species distribution shifted from one dominated by S. petiolaris to one being dominated by S. discolor. Additionally, the sympatric willows showed a distribution along a moisture gradient, with S. eriocephala, S. interior, and S. petiolaris in moist locations and S. bebbiana and S. discolor in drier locations.     

Sex ratio variation in dioecious plant species: A comparative ecological study of six species of Lindera (Lauraceae)

Sex ratio variation was investigated in natural populations of six dioecious shrub species of Lindera in Japan. Interspecific differences in sex ratio were examined in relation to patterns of population structure, floral dimorphism, fruit production and intersexual differences in herbivory. Sex ratios tended towards equality or bias in favour of males, except for populations of L. glauca , in which no male plants were found. Sex switching in individual plants was not observed. Although male flowers were generally larger in size than flowers of conspecific females, costs of flowering did not appear to influence sex ratio. However, a relation was found between high allocation to fruit production and increasing male fraction of populations. In addition, girths of flowering plants tended to be larger in females, which may indicate delay in age of the first reproduction compared to males. No intersexual difference was found in degree of leaf damage due to herbivory. Results of this study are discussed in the light of other studies on sex ratio variation in dioecious plant species.     

Sex determination and sex ratio in the dioecious shrub Salix viminalis L.

 Various ecological factors (e.g. herbivory, difference between males and females in colonising ability) have been invoked to explain female-biased sex ratios in populations of willow species. It was implicitly assumed that genetic factors would lead to a balanced sex ratio in the absence of ecological disturbances. In an experiment carried out in a homogeneous environment and in the absence of herbivores the progeny sex ratio of 13 crosses of basket willow (Salix viminalis L.) was observed to range from extreme female bias to extreme male bias. The observed sex ratio cannot be explained by the presence of sex chromosomes without assuming that additional loci are also involved in the sex determination. Alternatively, the sex ratios in this study can be explained by a sex determination mechanism governed by multiple independent loci. Received: 1 February 1996 / Accepted: 14 June 1996     

Growth, herbivory and disease in relation to gender in Salix viminalis L.

Many species of dioecious plants show sex-related differences in growth rate and rates of attack by various herbivores and diseases. The common pattern is for males to grow faster than females and to be less well defended against herbivores. In willows (Salix spp.), the predominance of female-biased sex ratios has been ascribed in part to differential feeding by herbivores. In this study of Salix viminalis, seven families grown on agricultural land showed no gender-related variation in shoot biomass or rates of herbivory by insects (lepidopterans and cecidomyiids). However, Melampsora rust disease was found to be more severe on females than on males when the plants were in a non-reproductive stage. After flowering and seed-set females tended to be more affected in some families but less affected in others. Although, on average, there was a female bias in the sex ratio of S. viminalis, sex ratios differed significantly between families. These ratios were not related to any of the recorded biotic agents, but rather to relationships between families. These results are interpreted in terms of resource allocation between reproduction, growth and defence, and causes for divergence from the expected patterns are discussed. The results may have implications for S. viminalis breeding strategies where the aim is to produce biofuel. For instance, these findings suggest that gender can be ignored when selecting for a high growth rate and resistance to Melampsora and certain insect pests. Received: 2 November 1996 / Accepted: 8 February 1997     

Sexually differential tolerance to water deficiency of Salix paraplesia—A female‐biased alpine willow

Salicaceae plants are dioecious woody plants. Previous studies have shown that male individuals are more tolerant to water deficiency than females for male‐biased poplars. However, Salix paraplesia is a female‐biased species in nature. It is still unknown whether female willows are more tolerant to drought stress than males. To better understand the sexually different tolerance to water deficiency in willows, a greenhouse experiment combined with a field investigation was conducted, and physiological traits were tested in male and female S. paraplesia under a drought‐stressed condition (50% of soil water capacity). Our field investigation showed that S. paraplesia was a species with female‐biased sex ratio along altitude gradients (2,400 m, 2,600 m and 2,800 m) in their natural habitats. Our results showed that the height growth, biomass accumulation, total chlorophyll pigment content (TChl), and the net photosynthetic rate were higher in female willows than in males at the low and middle altitudes (2,400 m and 2,600 m) rather than at a high altitude (2,800 m) under well‐watered conditions. Under drought‐stressed conditions, the growth, biomass, and photosynthesis were greatly inhibited in both sexes, while females showed higher biomass and TChl content and suffered less negative effects than did males. Particularly, females that originated from a high altitude showed lower leaf relative electrolyte leakage, malondialdehyde content, and less disorder of chloroplast ultrastructures but a higher peroxidase activity (POD) than that of males. Therefore, S. paraplesia females exhibited a better drought tolerance and self‐protective ability than males from high altitude. There is a reason to speculate that the population structure of S. paraplesia at a high altitude would be likely to further female biases with the increased drought intensity in the alpine regions.     

Biomass Harvest from Natural Willow Rings around Prairie Wetlands

The study followed the harvest of natural willow from three wetlands using a prototype modified agricultural round baler nicknamed a Bio-Baler. The study reports fuel characteristics and combustion testing of biomass harvested from natural willow rings. Composition of native willow species in the harvested willow rings was determined. We specifically measured regrowth of the biomass and number of regenerated stems per stump 1 year after harvest to determine how different willow species responded to mechanical biomass cutting with the Bio-Baler. The results of combustion testing for the natural willow were essentially similar to those with “conventional” wood chips or planted willows. The ash content was approximately 1.65%, slightly lower than for planted willow plantations. The calorific value of the natural willow was 19.6 MJ kg^?1 (dry basis) similar to what is expected for wood and planted willow. Four Salix species (Salix bebbiana Sarg., Salix petiolaris Sm., Salix eriocephala Michx., and Salix discolor Muhl) were identified in the willow rings. Stem biomass increased for all species except S. bebbiana after willow was harvested with the Bio-Baler. Overall, willow regeneration was not affected by mechanical harvesting compared to hand cutting. Regenerated stem density was 93 stems per square meter for mechanically harvested stumps compared to 105 stems per square meter for hand-pruned stumps. Based on the results, biomass harvested from natural willow rings has acceptable fuel characteristics when compared to purpose-grown willows and mechanical harvest with a Bio-Baler does not have a negative effect on willow regeneration.     

Interplant volatile signaling in willows: revisiting the original talking trees

The importance of interplant volatile signaling in plant–herbivore interactions has been a contentious issue for the past 30 years. We revisit willows as the system in which evidence for interplant signaling was originally found, but then questioned. We established three well-replicated experiments with two willow species (Salix exigua and Salix lemmonii) to address whether the receipt of an interplant signal from a neighboring willow reduces herbivore damage. Additionally we tested whether this signal is volatile in nature, and whether plants signal better to themselves than they do to other individuals. In all three experiments, we found evidence that cues from a damaged neighbor reduce subsequent herbivory experienced by willows. In one experiment, we showed that bagging of clipped tissue, which prevents the exchange of volatile signals, removed the effect of neighbor wounding. This was consistent with results from the other two experiments, in which clipping potted neighbors connected only through airborne volatile cues reduced damage of receivers. In one year, we found evidence that the perception of volatile signals from genetically identical clones was more effective at reducing foliar damage to a neighbor than signals from a genetically different individual. However, this trend was not significant in the following year. In three well-replicated experiments, we found strong evidence for the importance of interplant volatile cues in mediating herbivore interactions with willows.     

Cumulative effects of wild ungulate and livestock herbivory on riparian willows

We examined the effects of wild ungulates (deer and elk) and domestic sheep browsing on the growth, structure, and reproductive effort of two common willow species, Salix boothii and S. geyeriana, in a montane northeast Oregon riparian zone. With the use of exclosures, large herbivore effects on willows were studied in an area browsed by native mammals only and an adjacent area in which domestic sheep also lightly grazed during summer months. Growth variables were repeatedly measured on individual plants over a 5-year period to understand physiognomic and flowering responses of native willows to different levels of browsing pressure. At the beginning of the study, all willows were intensely browsed but were significantly taller in the area browsed only by native mammals than in the area also grazed by sheep (69 versus 51 cm, respectively). Willows inside exclosures responded with pronounced increases in height, crown area, and basal stem diameters while the stature of browsed plants outside exclosures stayed constant or declined. In the area browsed by both sheep and wild herbivores, the size of browsed plants remained at pre-treatment levels (<60 cm in height) for the duration of the study. There was no significant difference in growth rates of enclosed willows, indicating that current herbivory was the primary cause of growth retardation in the study area. Foliar area was strongly correlated with basal stem numbers for enclosed plants but much less so for browsed plants. Willows inside exclosures had more than twice as much foliar area per stem. Stem diameters were a positive function of crown area: stem-number ratios, suggesting lower photosynthetic potential was correlated with diminished radial growth among browsed plants. No flowering was observed until 2 years after exclusion when plants inside all exclosures and browsed willows in the wild ungulate area responded with a large pulse in flowering. Browsed plants in the sheep + wild ungulate area did not flower. The number of catkins produced per plant was significantly associated with willow height and plants <70 cm in height did not flower, thus suggesting a size threshold for reproduction in these species. Our results suggest that even relatively light levels of domestic livestock grazing, when coupled with intense wild ungulate browsing, can strongly affect plant structure and limit reproduction of riparian willows.     

On the Distribution and Origin of Salix in the World

1. The distribution of Salix species among the continents. There are about 526 species of Salix in the world, most of which are distributed in the Northern Hemisphere with only a few species in the Southern Hemisphere. In Asia, there are about 375 species, making up 71.29 percent of the total in the world, including 328 endemics; in Europe, about 114 species, 21.67 percent with 73 endemics; in North America, about 91 species, 17.3 percent with 71 endemics; in Africa, about 8 species, 1.5 percent, with 6 endemics. Only one species occurs in South America. Asia, Europe and North America have 8 species in common (excluding 4 cultivated species). There are 34 common species between Asia and Europe, 14 both between Europe and North America and between Asia and North America, 2 between Asia and Africa. Acording to the Continental Drift Theory, the natural circumstances which promoted speciation and protected newly originated and old species were created by the orogenic movement of the Himalayas in the middle and late Tertiary. Besides, the air temperature was a little higher in Asia than in Europe and North America (except its west part) and the dominant glaciers were mountainous in Asia during the glacial epoch in the Quaternary Period. Then willows of Europe moved southwards to Asia. During the interglacial period they moved in opposite direction. Such a to-and-fro willow migration between Asia and Europe and between and North America occurred so often that it resulted in the diversity of willow species in Asia. Those species of willows common among the continents belong to the Arctic flora. 2. The multistaminal willows are of the primitive group in Salix. Asia has 28 species of multistaminal willows, but Europe has only one which is also found in Asia. These 28 species are divided into two groups, “northern type” and “southern type”, according to morphology of the ovary. The boundary between the two forms in distribution is at 40°N. The multistaminal willows from south Asia, Africa and South America are very similar to each other and may have mutually communicated between these continents in the Middle or Late Cretaceous Period. The southern type willows in south Asia are similar to the North American multistaminal willows but a few species. The Asian southern type willows spreaded all over the continents of Europe, Asia and North America through the communication between them before the Quaternany Period. Nevertheless, it is possible that the willows growing in North America immigranted through the middle America from South America. The Asian northern type multistaminal willows may have originated during the ice period. The multistaminal willows are more closed to populars in features of sexual organs. They are more primitive than the willows with 1-3 stamens and the most primitive ones in the genus. 3. The center of origin and development of willows Based on the above discussion it is reasonable to say that the region between 20°-40°N in East Asia is the center of the origin and differentiation of multistaminal willows. It covers Southern and Southwestern China and northern Indo-China Pennisula.     

Long-term sexual allocation in herbivore resistant and susceptible pinyon pine (Pinus edulis)

Chronic herbivory by the stem-boring moth (Dioryctria albovittella) alters the sexual expression of a monoecious tree, pinyon pine (Pinus edulis) by reducing female function and increasing male function. Observations and long-term moth removal experiments show that 55% of susceptible trees can lose all female function. Moth herbivory has little effect on male function in young trees, but has an important effect on older trees, where moth-susceptible trees produced 1.5 times more pollen than moth-resistant trees. Susceptible trees were 6.5 times more likely to exhibit male-only function than either resistant trees or susceptible trees that have had their moths experimentally removed. This herbivore-induced sex change is caused primarily by differential moth attack and the resulting mortality of the shoots that bear female reproductive structures. Moth attack rates were positively correlated with individual stem biomass (female stems >non-reproductive stems >male stems). Moth attack also increased conelet abortion on unattacked shoots, indicating that moths indirectly reduce female function. Moth-induced altering of sexual function is also expressed at the population level. Male function is relatively greater in stands with high moth densities on stressful soils than in stands with few moths. Under certain conditions, sexual selection theory suggests that the negative effects on female function could be overcome with greater investment in male function. Because susceptible trees produce large amounts of pollen and are more abundant than moth-resistant trees, frequency-dependent selection may counteract selection against susceptible genotypes. Plant-herbivore interaction studies typically examine negative impacts of herbivory on female function, but not positive effects on male function. Here we demonstrate that herbivory may have important effects on the evolutionary ecology of pinyon by both promoting male function and depressing female function.     

太白山牛皮桦-巴山冷杉混交林空间格局及关联性研究

采用改进的点格局分析法——邻域密度函数（Neighborhood Density Function,NDF）对太白山地区桦林-冷杉林交错区内主要树种的空间分布格局、种间空间关联性及其更新空间关联性进行了分析。结果表明：各树种所有个体整体上都呈随机分布,划分不同层级后,则各林层个体分布格局各异,各树种在较低的林层呈明显聚集分布,随林层增高,中、高层内个体聚集分布的空间尺度降低。两个主要竞争种牛皮桦（Betula albo-sinensis var．septentrionalis）和巴山冷杉（Abies fargesii）整体表现出显著空间负相关,巴山冷杉与玉皇柳（Salix yuhuangshanensis）未表现出明显相关性,而牛皮桦与玉皇柳间则有空间负相关的趋势。牛皮桦的更新格局仅依靠自身倒木扰动产生的机会,且被限定在较小尺度上。巴山冷杉在小尺度上会限制自身更新,此外与牛皮桦在小尺度上竞争激烈,可占据利用其空间位置进行更新。从空间格局分析角度认识牛皮桦和巴山冷杉的生态学特性,为进一步研究其共存机制,认识太白山桦林在秦岭植被垂直带中的作用和地位提供参考依据。     

Aberrant chromosomal sex-determining mechanisms in mammals, with special reference to species with XY females

Both mouse and man have the common XX/XY sex chromosome mechanism. The X chromosome is of original size (5-6% of female haploid set) and the Y is one of the smallest chromosomes of the complement. But there are species, belonging to a variety of orders, with composite sex chromosomes and multiple sex chromosome systems: XX/XY1Y2 and X1X1X2X2/X1X2Y. The original X or the Y, respectively, have been translocated on to an autosome. The sex chromosomes of these species segregate regularly at meiosis; two kinds of sperm and one kind of egg are produced and the sex ratio is the normal 1:1. Individuals with deviating sex chromosome constitutions (XXY, XYY, XO or XXX) have been found in at least 16 mammalian species other than man. The phenotypic manifestations of these deviating constitutions are briefly discussed. In the dog, pig, goat and mouse exceptional XX males and in the horse XY females attract attention. Certain rodents have complicated mechanisms for sex determination: Ellobius lutescens and Tokudaia osimensis have XO males and females. Both sexes of Microtus oregoni are gonosomic mosaics (male OY/XY, female XX/XO). The wood lemming, Myopus schisticolor, the collared lemming, Dirostonyx torquatus, and perhaps also one or two species of the genus Akodon have XX and XY females and XY males. The XX, X*X and X*Y females of Myopus and Dicrostonyx are discussed in some detail. The wood lemming has proved to be a favourable natural model for studies in sex determination, because a large variety of sex chromosome aneuploids are born relatively frequently. The dosage model for sex determination is not supported by the wood lemming data. For male development, genes on both the X and the Y chromosomes are necessary.     

Home range size and activity of the wood lemming, Myopus schisticolor

We present the results from a radiotelemetric study on space use and activity in a natural population of the wood lemming Myopus schisticolor. Male home ranges were larger than females, and the ratio female home range to male home range was smaller than expected compared to other small rodents. Males moved distances 4-12 times longer than females. We argue that this difference in mobility gives a higher probability of capturing males than females in snap trap studies. If there is a sex biased trappability, this might explain the increasing female biased sex ratio during the snow free season (1:1 in spring, 3:1 [female:male| in autumn), as a higher mortality of males during the reproductive season.     

Tritrophic interactions: willows, herbivorous insects and insectivorous birds

Insectivorous birds can increase plant growth by consuming herbivorous insects and reducing insect damage. However, plant traits such as the level of chemical defense may affect the quantity and quality of insects, and alter the foraging behavior of birds. Therefore, I predicted that plant traits can also modify the effect of birds on leaf damage and plant growth. This study compared the effect of insectivorous birds on the herbivory and growth of two chemically different willow species, weakly defended Salix phylicifolia and strongly defended S. myrsinifolia under two fertilization levels. Half of the willows were protected from birds using a translucent gill-net, which did not limit access by insects. The effect of birds on the densities of leaf-chewing insects and leaf damage was considerable on unfertilized S. phylicifolia but less obvious on fertilized ones. The effect of bird predation was negligible on S. myrsinifolia, which had very low insect densities in all treatments. Birds increased the growth of the experimental willows, but the effect was clear only in unfertilized S. phylicifolia. I suggest that birds avoided foraging on willows with low populations of insects and little visible damage. The study shows that bird predation can alter the patterns of insect densities we see on willows, emphasizing the importance of considering multitrophic effects when studying plant-insect interactions. Received: 25 May 1999 / Accepted: 9 August 1999     

Conflict over multiple-partner mating between males and females of the polygynandrous common lizards

The optimal number of mate partners for females rarely coincides with that for males, leading to a potential sexual conflict over multiple-partner mating. This suggests that the population sex ratio may affect multiple-partner mating and thus multiple paternity. We investigate the relationship between multiple paternity and the population sex ratio in the polygynandrous common lizard (Lacerta vivipara). In six populations the adult sex ratio was biased toward males, and in another six populations the adult sex ratio was biased toward females, the latter corresponding to the average adult sex ratio encountered in natural populations. In males the frequency and the degree of polygyny were lower in male-biased populations, as expected if competition among males determines polygyny. In females the frequency of polyandry was not different between treatments, and polyandrous females produced larger clutches, suggesting that polyandry might be adaptive. However, in male-biased populations females suffered from reduced reproductive success compared to female-biased populations, and the number of mate partners increased with female body size in polyandrous females. Polyandrous females of male-biased populations showed disproportionately more mating scars, indicating that polyandrous females of male-biased populations had more interactions with males and suggesting that the degree of multiple paternity is controlled by male sexual harassment. Our results thus imply that polyandry may be hierarchically controlled, with females controlling when to mate with multiple partners and male sexual harassment being a proximate determinant of the degree of multiple paternity. The results are also consistent with a sexual conflict in which male behaviors are harmful to females.     

Sex-different response in growth traits to resource heterogeneity explains male-biased sex ratio

In dioecious plants, differences in growth traits between sexes in a response to micro-environmental heterogeneity may affect sex ratio bias and spatial distributions. Here, we examined sex ratios, stem growth traits and spatial distribution patterns in the dioecious clonal shrub Aucuba japonica var. borealis, in stands with varying light intensities. We found that male stems were significantly more decumbent (lower height/length ratio) but female stems were upright (higher height/length ratio). Moreover, we found sex-different response in stem density (no. of stems per unit area) along a light intensity gradient; in males the stem density increased with increases in canopy openness, but not in females. The higher sensitivity of males in increasing stem density to light intensity correlated with male-biased sex ratio; fine-scale sex ratio was strongly male-biased as canopy openness increased. There were also differences between sexes in spatial distributions of stems. Spatial segregation of sexes and male patches occupying larger areas than female patches might result from vigorous growth of males under well-lit environments. In summary, females and males showed different growth responses to environmental variation, and this seemed to be one of possible causes for the sex-differential spatial distributions and locally biased sex ratios.     

Distribution of Willows (Salix) in China

The distribution of willows in China is studied in this paper. The origin of this genus and its distribution in the world are also discussed. There are 255 species of willows in China, taking up 46% of the total number of species of this genus in the world. They belong to 37 sections, which represent almost all forms. China is therefore the richest area in species and sections of willows in the world. This is mostly caused by the uplifting of Qinghai-Xizang (Tibet) plateau. The willows in China are mainly distributed in northwestern, northeastern and southwestern regions. The northwest is a part of central Asian flora. The northeast is a part of northeastern Asian flora. They both have some of the Europe-Siberia and Arctic-Alpine elements. Qinghai-Xizang plateau is another important distribution center of willows, which has no close relation with other floristic regions. As one of typical genera of the Holarctic flora, Salix has probably originated in the tropical mountains of Southeast Asia.     

Sex change in tree species: long-term monitoring of sex expression in Acer rufinerve

We monitored sex expression in Acer rufinerve from 1986 to 1999, in order to study branch-autonomous sex changes in tree species. During this observation period, 70 of 338 stems (20.7 %) changed sexual expression. Fifty of these sex-changed stems exhibited monoecism (having both female and male branches) in the course of the sex change, while the remaining stems changed directly from male to female or vice versa. A sex change resulting in monoecism was called a partial sex change and a total male/female change was referred to as a complete sex change. The mean diameter at breast height of stems that partially changed sex was significantly greater than that of stems that changed sex completely. Thus, it was primarily large stems with many branches that underwent partial sex changes. These findings suggest that sex change is a branch autonomous event in A. rufinerve and underline the importance of taking branching structure into account when studying sex change in trees.