Effect of sex ratio,habitat factors and neighborhood competition on stem growth in the dioecious tree Fraxinus mandshurica

Growth rates of male and female trees are often different in a dioecious species. In this study, we analyzed sex ratios and the effect of gender, neighborhood competition and habitat factors on the stem growth of dioecious Fraxinus mandshurica trees in a secondary conifer and broad-leaved mixed forest in the Changbai Mountains of northeastern China. The sex ratio in the 5.2-ha study area does not deviate significantly from the expected 1:1 ratio, except for trees in the large diameter classes. For dbh >40 cm, the sex ratio is male-biased. This result suggests that males have a faster rate of stem growth than females, which is usually explained by the higher cost of reproduction in the fruit-bearing females. An analysis of the dbh distributions of two successive measurements showed that the rate of stem growth of the (27) females drops off with increasing dbh and remains below that of the (35) males. A causal model was used to analyze factors affecting the rate of stem growth, showing that these rates are affected significantly and positively by soil moisture and tree size in both genders and that within-gender competition is mainly for nitrogen. Our study suggests that neighborhood competition does not affect stem growth significantly, which is a rather surprising result.     

Growth,plant quality and leaf damage patterns in a dioecious tree species: is gender important?

Frequently, female plants allocate more resources to reproductive structures and defense-related secondary compounds in comparison with male plants that invest more resources to growth, reflecting trade-offs between reproduction, growth and defense. Therefore, differences in herbivory can be expected between genders. In this study, over two years, we analyzed the differences in plant chemical defense, nutritional quality, plant size and herbivory between genders in the dioecious tree, Spondias purpurea in a Mexican tropical dry forest. We estimated the total leaf area and the area consumed by folivory using a digital image of each leaf. The nutritional quality was estimated as water content, and the concentration of chlorophyll and total nonstructural carbohydrates. The secondary metabolites analyzed were total content of soluble phenolics, flavonoids, protein precipitation capacity of tannins, gallotannins, soluble proanthocyanidins, hydrolyzable tannins and ellagitannins. Our results differ from most of studies that analyze the differential herbivory patterns in dioecious plants. We found that female trees had higher levels of herbivory than male trees of S. purpurea. In the same way, female trees showed higher size and nutritional quality than males, while chemical defense was higher in male trees. The higher percentage of folivory in female trees of S. purpurea is associated with greater nutritional quality and lower chemical defenses. Our results show that male-biased herbivory might not be universal in dioecious species. Therefore, studies of fitness components affected by herbivory are necessary to understand the evolution of dioecy and the importance of herbivores as selective agents on breeding system features.     

Physiological performance and xylem water isotopic composition underlie gender‐specific responses in the dioecious shrub Corema album

Gender‐specific requirements of reproduction in dioecious species can lead to different physiological responses in male and female plants, made in relation to environmental constraints, and influencing growth, survival and population structure. Gender‐related physiological differences and seasonal responses, indicating the existence of compensatory mechanisms of reproduction, were examined during a drought year in the dioecious shrub species Corema album. To integrate aboveground and belowground physiological responses, chlorophyll fluorescence, leaf gas exchange, water potential and xylem water isotopic composition were monitored throughout the diurnal cycle and annual phenological sequence of the species. Sampling was carried out in Doñana Natural Park (SW Spain) in Mediterranean‐type climate conditions. The gender which bore greater reproductive effort showed higher physiological stress. Intersexual differences in leaf water potential were interpreted as arising from each gender's maximum reproductive allocation; lower values were found during flowering in males and during fruit production in females. Cold temperatures during winter fostered photoinhibitory responses that were most evident in male individuals, as a response to their relatively higher investment in reproduction during flowering. Net assimilation rate was not influenced by reproductive status; however, females tended to show higher values of this parameter at midday. The integrated analysis of photosynthetic variables and water relations indicated a gender effect in the physiological response at midday. The oxygen isotopic composition of xylem water showed a lack of dependence on the water table during the drought period, and indicated intersexual differences in water catchment. Females reached deeper soil layers, suggesting mechanisms compensating for their higher reproductive effort, and giving new evidence of physiological gender dimorphism in the belowground responses of a woody species.     

Causes of secondary sexual differences in plants — Evidence from extreme leaf dimorphism in Leucadendron (Proteaceae)

In extreme cases leaves in male plants of the dioecious genus Leucadendron (Proteaceae) are up to an order of magnitude smaller than female leaves. This secondary sexual dimorphism (SSD) in leaf size has previously been suggested to be due to intra-male sexual selection, leading to an increase in male allocation to reproduction in dimorphic species. After critically evaluating previous data provided to support this hypothesis, I suggest on both theoretical grounds and on re-analysis that this argument is unlikely and unsupported. Leaf size dimorphism could theoretically evolve directly due to disruptive ecological selection between genders, leading to niche dimorphism either within or between habitats. I test this ecological causation hypothesis by providing data on specific leaf area (sla) and water use efficiency (δ ¹³C) of leaves from males and females of several Leucadendron species. Results confirm the expectation of minimal gender differences. I argue that leaf dimorphism is a consequence of selection on flower size and architecture.     

Roots, shoots and reproduction: sexual dimorphism in size and costs of reproductive allocation in an annual herb

Females tend to be smaller than males in woody dioecious plant species, but they tend to be larger in herbs. The smaller size of females in woody species has been attributed to higher reproductive costs, yet no satisfactory explanation has been provided for their larger size in herbs. Because herbs have higher nitrogen concentrations in their tissues than woody plants, and because pollen is particularly rich in nitrogen, we predicted that male growth would be more compromised by reproduction than female growth. To test this hypothesis, we conducted three experiments on the annual dioecious herb Mercurialis annua. First, we compared the timing of reproduction between males and females and found that males started flowering earlier than females; early flowering is expected to compromise growth more than later flowering. Second, we compared plants allowed to flower with those prevented from flowering by experimental debudding and found that males incurred a higher reproductive cost than females in terms of both biomass and, particularly, nitrogen. Third, we grew plants under varying levels of nitrogen availability and found that although sexual size dimorphism was unaffected by nitrogen, females, but not males, decreased their relative allocation to both roots and reproduction under high nitrogen availability. We propose that males deal with the high cost of pollen production in terms of nitrogen by allocating biomass to nitrogen-harvesting roots, whereas females pay for carbon-rich seeds and fruits by investing in photosynthetic organs. Sexual dimorphism would thus seem to be the outcome of allocation to above- versus below-ground sinks that supply resources (carbon versus nitrogen) limiting the female and male reproduction differentially.     

Gender-related differences in adaptability to drought stress in the dioecious tree <Emphasis Type="Italic">Ginkgo biloba</Emphasis>

Ginkgo (Ginkgo biloba) as a precious relict plant is cultivated around the world, and it is also a typical dioecious tree. Drought is a major environmental stress that limits the growth and development of ginkgo. Although many studies have examined the impact of drought on ginkgo, few have investigated gender-related under drought treatment in the species. In our research, we examined comparative morphology, physiology and the ultrastructure of mesophyll cell in male and female ginkgoes to determine which gender shows superior adaptability to drought stress. Two-year-old cutting-propagated male and female ginkgoes suffered to drought treatment. The experiments showed that drought significantly limited growth and development, disrupted photosynthesis, and destoried the antioxidant protection system in both male and female ginkgoes. When the gender differences in the species were compared, females showed better growth, activities of SOD and POD, concentrations of chl t, chl a/b ratio and proline, P _n, C _i, g _s, qP and NPQ under drought, but lower concentrations of H₂O₂ and O₂ ^?, and relative electrolyte leakage. In the aspect of cell ultrastructure, female plants showed a slower rate of cell breakdown and chloroplast decomposition under drought stress than males. The results indicate that female plants of ginkgo show superior growth performance and self-protective mechanisms and higher photosynthetic capacity than male plants under drought stress. Thus, we conclude that female individuals of ginkgo possess better adaptability to drought stress than male individuals.     

Water-use efficiency and whole-plant performance of nine tropical tree species at two sites with contrasting water availability in Panama

Across their natural distributions, tropical tree species are regularly exposed to seasonal droughts of varying intensities. Their ability to tolerate drought stress plays a vital role in determining growth and mortality rates, as well as shaping the functional composition of tropical forests. In order to assess the ability of species to acclimate to contrasting levels of drought stress, physiological and structural traits involved in drought adaptation—wood C isotope discrimination (δ¹³C), wood specific gravity, and wood C content—of 2-year-old saplings of nine tropical tree species were evaluated in common garden experiments at two study sites in Panama with contrasting seasonality. We assessed co-variation in wood traits with relative growth rates (RGR_BD), aboveground biomass, and basal diameter and the plasticity of wood traits across study sites. Overall, species responded to lower water availability by increasing intrinsic water-use efficiency, i.e., less negative wood δ¹³C, but did not exhibit a uniform, directional response for wood specific gravity or wood C content. Trait plasticity for all wood traits was independent of RGR_BD and tree size. We found that the adaptive value of intrinsic water-use efficiency varied with water availability. Intrinsic water-use efficiency increased with decreasing RGR_BD at the more seasonal site, facilitating higher survival of slower growing species. Conversely, intrinsic water-use efficiency increased with tree size at the less seasonal site, which conferred a competitive advantage to larger individuals at the cost of greater susceptibility to drought-induced mortality. Our results illustrate that acclimation to water availability has negligible impacts on tree growth over short periods, but eventually could favor slow-growing species with conservative water-use strategies in tropical regions experiencing increasingly frequent and severe droughts.     

Gender-specific costs of reproduction on vegetative growth and physiological performance in the dioecious shrub Corema album

Background and Aims

Reproductive costs imply trade-offs in resource distribution at the physiological level, expressed as changes in future growth and/or reproduction. In dioecious species, females generally endure higher reproductive effort, although this is not necessarily expressed through higher somatic costs, as compensatory mechanisms may foster resource uptake during reproduction.

Methods

To assess effects of reproductive allocation on vegetative growth and physiological response in terms of costs and compensation mechanisms, a manipulative experiment of inflorescence bud removal was carried out in the sexually dimorphic species Corema album. Over two consecutive growing seasons, vegetative growth patterns, water status and photochemical efficiency were measured to evaluate gender-related differences.

Key Results

Suppression of reproductive allocation resulted in a direct reduction in somatic costs of reproduction, expressed through changes in growth variables and plant physiological status. Inflorescence bud removal was related to an increase in shoot elongation and water potential in male and female plants. The response to inflorescence bud removal showed gender-related differences that were related to the moment of maximum reproductive effort in each sexual form: flowering in males and fruiting in females. Delayed costs of reproduction were found in both water status and growth variables, showing gender-related differences in resource storage and use.

Conclusions

Results are consistent with the existence of a trade-off between reproductive and vegetative biomass, indicating that reproduction and growth depend on the same resource pool. Gender-related morphological and physiological differences arise as a response to different reproductive resource requirements. Delayed somatic costs provide evidence of gender-related differences in resource allocation and storage. Adaptive differences between genders in C. album may arise through the development of mechanisms which compensate for the cost of reproduction.     

Genders in Juniperus thurifera have different functional responses to variations in nutrient availability

? Differences in reproductive investment can trigger asymmetric, context-dependent, functional strategies between genders in dioecious species. However, little is known about the gender responses of dioecious species to nutrient availability. ? We experimentally fertirrigated a set of male and female Juniperus thurifera trees monthly for 2 yr. Water potential, photosynthesis rate and stomatal conductance were measured monthly for 2 yr, while shoot nitrogen (N) concentration, carbon isotopic composition (δ(13) C), branch growth, trunk radial growth and reproductive investment per branch were measured yearly. ? Control males had lower gas exchange rates and radial growth but greater reproductive investment and higher water use efficiency (WUE; as inferred from more positive δ(13) C values) than females. Fertirrigation did not affect water potential or WUE but genders responded differently to increased nutrient availability. The two genders similarly increased shoot N concentration when fertilized. The increase in shoot N was associated with increased photosynthesis in males but not in females, which presented consistently high photosynthetic rates across treatments. ? Our results suggest that genders invest N surplus in different functions, with females presenting a long-term strategy by increasing N storage to compensate for massive reproductive masting events, while males seem to be more reactive to current nutrient availability, promoting gas-exchange capacity.     

Sexual dimorphism and cost of reproduction in the dioecious shrub Lindera benzoin (Lauraceae)>

We examined sexual dimorphism in reproductive allocation and its effects on growth and subsequent reproduction in a natural population of the dioecious woody shrub Lindera benzoin (L.) Blume. In addition to comparing natural patterns of growth and reproduction in a marked population of 251 females and 87 males, we experimentally examined the effect of reduced reproduction on future growth and reproduction in female plants and examined sexual dimorphism in carbon/nutrient balance. Our results suggest that females of L. benzoin bear greater reproductive costs in terms of both biomass and minerals. These costs were measurable in terms of current biomass and allocation of nitrogen to reproduction, as well as subsequent growth, reproduction, and tissue carbon/nutrient balance. Based upon the results of a fruit-thinning experiment and path analysis, fruit production in 1991 appeared to have direct negative effects on fruit production in 1992, an effect that was not necessarily mediated through effects on plant growth. We discuss our results in the context of other intrinsic and extrinsic factors that can influence growth, reproductive costs, and mortality in this species.     

A multi-proxy dendroecological analysis of two tropical species (Hymenaea spp., Leguminosae) growing in a vegetation mosaic

Tree species use a variety of strategies to obtain resources. As a result, semi-deciduous forest species and cerrado species can grow in close proximity and in the same climate, while occupying very different vegetation types. The aim of this study was to understand the dynamic responses of Hymenaea courbaril, a forest species, and Hymenaea stigonocarpa, a cerrado species, to annual climatic variation and increasing atmospheric CO₂ concentrations under the same macroclimatic conditions. To that goal, we constructed chronologies of tree-ring width, vessel area, and intrinsic water-use efficiency (calculated from tree-ring δ¹³C content) for Hymenaea trees growing in a mosaic of the two vegetation types. Our analyses revealed that both species responded to climatic variation in similar ways, but with different intensities and at different times of year. Climate models showed that precipitation had a stronger effect on tree-ring width and earlywood vessel area of H. courbaril and temperature was slightly more determinant for H. stigonocarpa. In addition, both species showed increasing intrinsic water-use efficiency over the last five decades, but only individuals with reduced growth rate presented this trend, suggesting that those specimens in favorable growth conditions do not respond to the atmospheric CO₂ enrichment. Despite the trend in water-use efficiency found in some individuals, it did not reflect in a higher growth rate. The differences between the two species documented by us may be due to divergent sources of hydrological stress in the two vegetation types.     

The changes in water-use efficiency and stoma density of Leymus chinensis along Northeast China Transect

Leymus chinensis is a keystone species in the temperate zone grassland of China. Along the NECT (Northeast China Transect) in 2001, water-use efficiency of L. chinensis was analyzed with δ¹³C, and changes in the stoma density of its leaves were observed and computed under a microscope. Results showed that the ecological plasticity modulation of the stoma density of L. chinensis and its water-use efficiency were two important mechanisms for its broad ecological adaptability. From east to west along the NECT, the δ¹³C of the species varied from -27.49‰ to -23.57‰, consisting with the reduction of annual precipitation, soil water and annual average temperature, but increased with the increase of the elevation of sampling sites. The stoma density increased from 96.9/mm² to 169.5/mm², indicating that the water-use efficiency for the species was improved along the gradient. The linear coefficient between the two parameters was significant (R² = 0.7338). The results of a stepwise regression analysis showed that the soil water was the first marked factor for determining the stoma density, and the next was the annual precipitation, which suggested that the water factors were the primary ecological factors influencing the stoma density of L. chinensis. The findings in this study showed that the responses of the stoma density and the water-use efficiency for L. chinensis to environmental changes were very complicated. They may be the outcome operated synthetically by all environmental factors in the long-term adaptation to different ecological environments, including human activity, for L. chinensis.     

A comparison of growth and reproduction, under laboratory conditions, of males and females of a dioecious fig tree

The interaction between Ficus spp. (Moraceae) and their pollinating wasps (Chalcidoidae: Agaonidae) is a highly co-evolved mutualism. Approximately half of all fig species are monoecious and produce a mixture of wasps and seeds within the same fig. In functionally dioecious fig trees male and female functions are separate. Figs on male trees produce wasps and pollen, whereas figs borne on female plants produce only seeds. Dioecious fig phenology provides an excellent opportunity to investigate the effect of sexual specialization on the obligate fig?Cfig wasp interaction and the non-pollinators associated with the system. Here we describe laboratory studies of phenological variation between two sexes in terms of vegetative growth and fig production in a dioecious fig tree Ficus montana. We also describe reproductive output in terms of wasp production in males and seeds in females. Intrasexual asynchrony was observed for the plants, with synchrony between the sexes with year-round production of figs. Male plants grew more rapidly, but leaf phenology was very similar. Crop sizes and development times were the same for males and females. Seasonal effects were strong for leaf phenology and fig initiation, but had a very limited effect on fig composition. The results show that the phenological differences described for other dioecious figs do not apply to all species.     

Seasonal variations in leaf δ13C values: implications for different water-use strategies among species growing on continuous dolomite outcrops in subtropical China

Plant species growing in shallow-soil habitat are likely to experience water deficit especially in seasonally dry or arid regions. However, only scarce studies focused on their water-use strategies. The current study aimed to reveal water-use strategies of different species growing on continuous dolomite outcrops (a typical shallow-soil habitat) in subtropical China that relied on different water sources, and to investigate the differences between narrow endemic and widespread species, based on season variations in leaf δ¹³C values. Leaf samples of six plant species (Radermachera sinica, Sapium rotundifolium, Sterculia euosma, Schefflera octophylla, Alchornea trewioides, and Vitex negundo, in different life-forms and leaf phenologies) were collected for carbon isotope measurements in the wet and dry seasons, respectively. Contrary to the expectation, the evergreen big shrub species, S. octophylla, which always relied on deep water sources, exhibited the most positive δ¹³C values (high water-use efficiency, WUE), indicating more conservative water-use strategies. While the two deciduous small shrubs, A. trewioides and V. negundo, which always relied on shallow water sources, exhibited the most negative δ¹³C values (low WUE). This result was associated with their short life spans, indicating an opportunistic water-use strategy. Leaf δ¹³C values of almost all (except for S. octophylla) the selected species were significantly (P < 0.05) higher in the dry season than in the wet season. This indicated that it was a common strategy for species in rocky karst habitat to improve their WUE in dry season. Despite the similar water sources utilized by the selected three tree species, the widespread one (R. sinica) exhibited greater improvement in leaf δ¹³C values than the narrow endemic ones (S. rotundifolium and S. euosma). This suggested that the widespread tree species had more flexible water-use strategies. It was further speculated that broad spatial distribution of widespread species may contribute to their highly plastic responses to changes in environmental conditions rather than always maintaining high WUE.     

When, how and how much: Gender-specific resource-use strategies in the dioecious tree Juniperus thurifera

BACKGROUND AND AIMS: In dioecious species male and female plants experience different selective pressures and often incur different reproductive costs. An increase in reproductive investment habitually results in a reduction of the resources available to other demands, such as vegetative growth. Tree-ring growth is an integrative measure that tracks vegetative investment through the plant's entire life span. This allows the study of gender-specific vegetative allocation strategies in dioecious tree species thoughout their life stages. METHODS: Standard dendrochronological procedures were used to measure tree-ring width. Analyses of time-series were made by means of General Mixed Models with correction of autocorrelated values by the use of an autoregressive covariance structure of order one. Bootstrapped correlation functions were used to study the relationship between climate and tree-ring width. KEY RESULTS: Male and female trees invest a similar amount of resources to ring growth during the early life stages of Juniperus thurifera. However, after reaching sexual maturity, tree-ring growth is reduced for both sexes. Furthermore, females experience a significantly stronger reduction in growth than males, which indicates a lower vegetative allocation in females. In addition, growth was positively correlated with precipitation from the current winter and spring in male trees but only to current spring precipitation in females. CONCLUSIONS: Once sexual maturity is achieved, tree rings grow proportionally more in males than in females. Differences in tree-ring growth between the genders could be a strategy to respond to different reproductive demands. Therefore, and responding to the questions of when, how and how much asked in the title, it is shown that male trees invest more resources to growth than female trees only after reaching sexual maturity, and they use these resources in a different temporal way.     

Gender-related traits in the dioecious shrub Empetrum rubrum in two plant communities in the Magellanic steppe

Following the theory on costs of reproduction, sexually dimorphic plants may exhibit several trade-offs in energy and resources that can determine gender dimorphism in morphological or physiological traits, especially during the reproductive period.In this study we assess whether the sexes of the dioecious species Empetrum rubrum differ in morphological and ecophysiological traits related to water economy and photochemical efficiency and whether these differences change in nearby populations with contrasting plant communities.We conducted physiological, morphological, sex ratio, and cover measurements in E. rubrum plants in the Magellanic steppe, North-Eastern part of Tierra del Fuego (Argentina), from two types of heathlands with differing community composition.We found differences between sites in soil pH and wind speed at the canopy level. E. rubrum plants exhibited lower photosynthetic height and higher LAI (leaf area index), lower RWC (relative water content) and higher water-use efficiency (lower Δ¹³C) in the heathland with harsher environmental conditions. Gender dimorphism in the physiological response was patent for photochemical efficiency and water use (RWC and Δ¹³C discrimination), with males showing a more conservative strategy in relation to females. Accordingly, male-biased sex ratio in the stress-prone community suggested a better performance of male plants under stressful environmental conditions. The integrated analysis of all variables (photochemical efficiency, RWC, leaf dry matter content (LDMC), pigments, and Δ¹³C) indicated an interaction between gender and heathland community effects in the physiological response. We suggest that female plants may exhibit compensatory mechanisms to face their higher reproductive costs.     

高榕榕果内Eupristina属两种榕小蜂的遗传进化关系

对生长在福州地区的高榕进行长期追踪观察,发现高榕榕果内仅生活着Eupristina altissima和Eupristina sp.榕小蜂,前者为高榕的传粉小蜂,后者无传粉行为,两者雌蜂之间在体色、触角、花粉袋和花粉刷等部位存在细微的差异,而两者雄蜂之间无形态差异。通过克隆福建地区5个样地的高榕榕果内收集到的E. altissima和Eupristina sp.榕小蜂,以及细叶榕的传粉小蜂Eupristina verticillata(外群)的Cytb及COI基因,并进行碱基组成及遗传距离分析,用邻接法构建系统发育树,分析两榕小蜂群体之间的遗传进化关系,结果显示:(1)榕小蜂COI及Cytb序列碱基组成中A+T的含量(Cytb序列中A+T=75.3%,COI序列中A+T=75.5%)显著高于G+C,符合膜翅目昆虫线粒体基因碱基组成特征。(2)对两群体小蜂进行遗传距离分析显示,Cytb序列中E. altissima和Eupristina sp. 群体内各样本之间的平均遗传距离分别为0.0092和0.0030,而E. altissima与Eupristina sp. 群体间的平均距离为0.1588;COI序列中E. altissima 与Eupristina sp. 群体内各样本之间的平均遗传距离分别为0.0065和0.0205,而二者群体间的平均遗传距离为0.1043,表明两者群体间的遗传距离明显大于各自群体内各样本间的遗传距离。统计GenBank中下载的6个属34种榕小蜂Cytb序列的种间遗传距离为0.0811-0.1723,6个属28种榕小蜂COI序列的种间遗传距离为0.0939-0.1986。由此认为E. altissima和Eupristina sp.之间的遗传距离差异已经达到了种间水平,即E. altissima与Eupristina sp.为两个不同的种。(3)在形态上,两种小蜂的雌蜂之间有微小差异,而二者雄蜂之间无差异,但Cytb与COI序列分析结果一致表明:E. altissima与Eupristina sp.雄蜂之间,以及二者雌蜂之间的遗传距离均差异显著,表明形态变异滞后于基因变异。雌蜂在表型上进化快于雄蜂,可能是由于雌蜂羽化后从榕果出飞,受到外界环境因素的影响较大,且两种雌蜂在传粉功能上存在差异,故二者之间的形态差异较大,而雄蜂寿命短,又终生生活在黑暗封闭、环境变化相对恒定的榕果内,两种雄蜂在行为上不存在差异,故二者表型变异较为缓慢。E. altissima和Eupristina sp.小蜂对宿主的专一性不强,在榕-蜂协同进化过程中,可能发生过宿主转移事件。     

No differential consequences of reproduction according to sex in Juniperus communis var. depressa (Cupressaceae)

In dioecious plant species, males and females are thought to have dissimilar allocation patterns. Females are believed to invest more in reproduction and less in growth and maintenance than males. This differential investment between sexes could result in distinct growth patterns and contrasting survival rates, thereby affecting the sex ratio of a population and the age and size distribution of males and females, possibly leading to habitat segregation according to sex. These effects might become more apparent under particularly limiting conditions, such as in nutrient-deficient soils or in climatically stressed environments. To verify these predictions, growth patterns, microsite characteristics, and age and size distribution of male and female individuals were compared, and population sex ratio was determined in three populations of the dioecious shrub Juniperus communis var. depressa (Cupressaceae, Pinophyta) along a short latitudinal gradient on the eastern coast of Hudson Bay (Northern Québec, Canada). We found that the northernmost population had a male-biased sex ratio, but that the southernmost one had a higher proportion of females. Our results failed to reveal any significant differences in radial growth patterns, mean sensitivity, annual elongation of the main axis, and size and age frequency distribution between males and females in any population. Furthermore, there was no evidence of microhabitat segregation according to sex as indicated by the lack of differences in the physicochemical characteristics of the substrate under males and females. Clearly, the expected ecological consequences of a presumed greater investment of females in reproduction were not apparent even under the very stressful conditions prevailing on subarctic dunes. Many factors could reduce differences in the cost of reproduction between males and females, such as the number and quality of reproductive structures produced annually by individuals of each sex, the possible photosynthetic activity of the immature female cones, and the complexity of the source/sink relationship within individuals. Alternatively, there may be no differences between sexes in their reproductive investment.     

GENDER-RELATED DIFFERENCES IN GAS EXCHANGE ARE NOT RELATED TO HOST QUALITY IN THE XYLEM-TAPPING MISTLETOE,PHORADENDRON JUNIPERINUM (VISCACEAE)

Male and female individuals of dioecious species often differ in morphology, physiology, growth, and habitat distribution. Where habitat distribution differences have been demonstrated, female plants generally occupy those habitats with greater resource availability (“rich” habitats). Gender-specific habitat preferences are often presumed to be a consequence of greater resource requirements, per gamete, of female reproduction. Previous work has shown that Phoradendron juniperinum, a xylem-tapping dioecious mistletoe that parasitizes Juniperus species in western North America, displays the opposite pattern: males are relatively more numerous than females in richer sites (i.e., branches with relatively high light and low evaporative demand within the host tree). We report here differences in host (“site”) quality and gas-exchange properties between the sexes. To minimize environmental variation, all measurements were made on sunlit foliage between 9:00 a.m. and 2:00 p.m. Males had significantly higher photosynthetic rates (4.0 [SE = 0.2] μmol m^-2 sec^-1) than either females (2.9 [0.3] μmol m^-2 sec^-1) or nonreproductive individuals (3.0 [0.2] μmol m-² sec^-1). Female photosynthetic rates were not statistically different from those of nonreproductive individuals. No concomitant differences in stomatal conductance were observed. Gas exchange data were independently confirmed by significant differences in carbon isotope ratio (δ¹³C). Gender-related differences were not related to host quality as measured by foliar N, foliar δ¹³C, or water potential of the host tree. The fate of the additional photosynthate in males is unknown, but we discuss the possibility that carbon costs of reproduction in males have been underestimated in past work.     

How to tell a shrub from a tree: A life‐history perspective from a South African savanna

The ecological differences between ‘shrubs’ and ‘trees’ are surprisingly poorly understood and clear ecological definitions of these two constructs do not exist. It is not clear whether a shrub is simply a small tree or whether shrubs represent a distinct life‐history strategy. This question is of special interest in African savannas, where shrubs and trees often co‐dominate, but are often treated uniformly as ‘woody plants’ even though the tree to shrub ratio is an important determinant of ecosystem functioning. In this study we use data from a long‐term fire experiment, together with a trait‐based approach to test (i) if woody species usually classified as shrubs or trees in African savanna differ in key traits related to disturbance and resource use; and (ii) if these differences justify the interpretation of the two growth forms as distinct life‐history strategies. We measured for 22 of the most common woody plant species of a South African savanna 27 plant traits related to plant architecture, life‐history, leaf characteristics, photosynthesis and resprouting capacity. Furthermore we evaluated their performance during a long‐term fire experiment. We found that woody plants authors call (i) shrubs; (ii) shrubs sometimes small trees; and (3) trees responded differently to long‐term fire treatments. We additionally found significant differences in architecture, diameter‐height‐allometry, foliage density, resprouting vigour after fire, minimum fruiting height and foliar δ¹³C between these three woody plant types. We interpret these findings as evidence for at least two different life‐history‐strategies: an avoidance/adaptation strategy for shrubs (early reproduction + adaptation to minor disturbance) and an escape strategy for trees (promoted investment in height growth + delayed reproduction).