Initial period of sexual maturity determines the greater growth rate of male over female in the dioecious tree Juniperus communis subsp. communis

Differential growth rate, water use efficiency and climate sensitivity between males and females of Ilex aquifolium in north-western Spain

BACKGROUND AND AIMS

Dioecious plant species, i.e., those in which male and female functions are housed in different individuals, are particularly vulnerable to global environmental changes. For long-lived plant species, such as trees, long-term studies are imperative to understand how growth patterns and their sensitivity to climate variability differentially affect the sexes.

METHODS

Here, we explore long-term intersexual differences in wood traits, namely radial growth rates, water use efficiency quantified as stable carbon isotope abundance of wood cellulose, and their climate sensitivity in Ilex aquifolium trees growing in a natural population in NW Spain.

KEY RESULTS

We found that sex differences in secondary growth rates were variable over time, with males outperforming females in both radial growth rates and water use efficiency in recent decades. Summer water stress significantly reduced the growth of female trees in the following growing season, while the growth of male trees was primarily favoured by cloudy and rainy conditions the previous fall and winter combined with low cloud cover and warm conditions in summer. Sex-dependent lagged correlations between radial growth and water availability were found, with a strong association between tree growth and cumulative water availability in females at 30 months and in males at 10 months.

CONCLUSIONS

Overall, our results point to greater vulnerability of female tress to increasing drought, which could lead to sex-ratio biases threatening population viability in the future.

Consequences of the Reproductive Effort of Dioecious Taxus baccata L. Females in a Generative Bud Removal Experiment-Important Role of Nitrogen in Female Reproduction

Dioecious species differ in the pattern and intensity of male and female reproductive investments. We aimed to determine whether female shoots deprived of generative buds show biochemical features, indicating their less-pronounced reproductive effort. For this purpose, the same branches of mature Taxus baccata females were deprived of generative organs. In the second and third years of the experiment, measurements were made in every season from the control and bud-removed shoots of females and control males. Bud removal caused an increase in nitrogen concentration almost to the level detected in the needles of male specimens, but only in current-year needles. Moreover, differences between male and control female shoots were present in the C:N ratio and increment biomass, but they disappeared when bud removal was applied to females. Additionally, between-sex differences were observed for content of phenolic compounds, carbon and starch, and SLA, independent of the female shoot reproductive effort. The study revealed that nitrogen uptake in seeds and arils may explain the lower nitrogen level and consequently the lower growth rate of females compared to males. At the same time, reproduction did not disturb carbon level in adjacent tissues, and two hypotheses explaining this phenomenon have been put forward.

Responses of Early Distribution and Developmental Traits of Male and Female Trees to Stand Density in Fraxinus mandshurica Rupr. Plantation

Density plays an important role in tree growth and development. Exploring the growth of males and females in the early stage of gender differentiation and the distribution pattern in different densities are beneficial to assess the influence earlier caused by density of the productivity and reproductive potential of dioecious plantations. We observed the numbers, distribution pattern and phenotypic traits of the males and females of Fraxinus mandshurica Rupr. in four initial densities (D1: 1.5 × 3 m; D2: 2 × 2 m; D3: 1.5 × 1.5 m; D4: 1 × 1 m). The results showed that the number of males and females gradually decreased with the increase in stand density, and they were randomly distributed in each density. With the increase in density, the total volume of males and females increased first and then decreased, and the highest value appeared in the D2 (2 × 2 m) density. The phenotypic traits of males and females had no significant difference within the same density. With increasing density, the crown width and the diameter of males and females all gradually decreased. There was a larger leaf area, larger specific leaf weight, and less leaf index for males, but for females, there was a relatively stable leaf area, larger leaf biomass, and less living under branch height. In the current stage, the D2 (2 × 2 m) planting density has advantages in the number and quality of males and females. Our results emphasize that suitable stand density can promote the volume of timber accumulation and reproduction of males and females in plantations with dioecious trees.

Rich but not poor conditions determine sex-specific differences in growth rate of juvenile dioecious plants

Causes of secondary sexual dimorphism (SSD) in dioecious plants are very poorly understood, especially in woody plants. SSD is shown mainly in mature plants, but little is known about whether secondary sexual dimorphism can occur in juveniles. It is also assumed that stress conditions intensify differences between the sexes due to the uneven reproductive effort. Therefore, the following research hypotheses were tested: (1) secondary sexual dimorphism will be visible in juveniles; (2) unfavourable soil conditions are the cause of more pronounced differences between the sexes. Rooted shoots of the common yew (Taxus baccata L.) and common juniper (Juniperus communis L.), previously harvested from parental individuals of known sex were used in the study. During two growing seasons vegetation periods and four times a year, comprehensive morphological features of whole plants were measured. Some SSD traits were visible in the analysed juveniles. Contrary to expectations, differences were more pronounced in the fertilized treatment. Both species reacted to fertilization in different ways. Female yew had a clearly higher total plant mass, root mass, and mean root area when fertilized, whereas male juniper had a higher root mass when fertilized. Differences between the sexes independent of the fertilization treatment were seen, which can be interpreted as sexual adaptations to a continued reproduction. Female yews and male junipers made better use of fertile habitats. Our study showed that SSD may be innate, and sexual compensatory mechanisms could generate uneven growth and development of both sexes. Because the SSD pattern was rather different in both species, it was confirmed that SSD is connected with the specific life histories of specific species rather than a universal strategy of dioecious species.

Sexual Dimorphism in the Chemical Composition of Male and Female in the Dioecious Tree, Juniperus communis L., Growing under Different Nutritional Conditions

We hypothesized that female and male individuals of the dioecious tree species, Juniperus communis, exhibit different strategies of resource allocation when growing under stress conditions. To test this hypothesis, we performed a two-year pot experiment on plants exposed to different levels of nutrient availability. Analysis of the plants revealed a higher concentration of carbohydrates, carbon, and phenolic compounds in needles of female plants, indicating that females allocate more resources to storage and defense than males. This difference was independent of nutrient availability. Differences in carbohydrates levels between the sexes were most often significant in June, during the most intensive phase of vegetative growth in both sexes, but could also be attributed to female resources investment in cone development. A higher level of nitrogen and other macroelements was observed in males than in females, which may have been connected to the accumulation of resources (nitrogen) for pollen grain production in males or greater allocation of these elements to seeds and cones in females. The interaction between sex and soil fertilization for the C:N ratio may also indicate sex-specific patterns of resource allocation and utilization, which is impacted by their availability during specific periods of J. communis annual life cycle.

Sex determines xylem anatomy in a dioecious conifer: hydraulic consequences in a drier world

Increased drought frequency and severity may reshape tree species distribution in arid environments. Dioecious tree species may be more sensitive to climate warming if sex-related vulnerability to drought occurs, since lower performance of one sex may drive differential stress tolerance, sex-related mortality rates and biased sex ratios. We explored the effect of sex and environment on branch hydraulic (hydraulic conductivity and vulnerability to embolism) and trunk anatomical traits in both sexes of the dioecious conifer Juniperus thurifera L. at two sites with contrasting water availability. Additionally, we tested for a trade-off between hydraulic safety (vulnerability to embolism) and efficiency (hydraulic conductivity). Vulnerability to embolism and hydraulic conductivity were unaffected by sex or site at branch level. In contrast, sex played a significant role in xylem anatomy. We found a trade-off between hydraulic safety and efficiency, with larger conductivities related to higher vulnerabilities to embolism. At the anatomical level, females' trunk showed xylem anatomical traits related to greater hydraulic efficiency (higher theoretical hydraulic conductivity) over safety (thinner tracheid walls, lower Mork's Index), whereas males' trunk anatomy followed a more conservative strategy, especially in the drier site. Reconciling the discrepancy between branch hydraulic function and trunk xylem anatomy would require a thorough and integrated understanding of the tree structure-function relationship at the whole-plant level. Nevertheless, lower construction costs and higher efficiency in females' xylem anatomy at trunk level might explain the previously observed higher growth rates in mesic habitats. However, prioritizing efficiency over safety in trunk construction might make females more sensitive to drought, endangering the species' persistence in a drier world.

Irregular Shelterwood Cuttings Promote Viability of European Yew Population Growing in a Managed Forest: A Case Study from the Starohorské Mountains, Slovakia

The increasing probability of Taxus baccata (L.) decline given climate change brings forth many uncertainties for conservation management decisions. In this article, the authors present the effects of applying regeneration cuttings since the year 2000 on the viability of the understory yew population. By collecting data from a stand located at the centre of the largest population of European yew in Slovakia, containing approximately 160,000 individuals, and analysing tree-ring records from 38 sampled trees, the improved performance of yews, including stem growth, seed production, and number of regenerated individuals, was revealed. Thinning the canopy by removing 15% of the growing stock volume per decade, combined with the subsequent irregular shelterwood cuttings, was assessed as a useful strategy. Moreover, lower radial growth of females compared to males, but simultaneously their similar response to climate, suggests a possible trade-off between reproduction and growth. Release cuttings of up to 30% of the standing volume in the vicinity of the female trees, executed in the rainy summers following warmer winters, and consistent elimination of deer browsing, can further enhance the positive effects of applied cuts on yew viability. Overall, the suggested active measures could be considered as an effective option to preserve the unique biodiversity of calcareous beech-dominated forests in Central Europe.

Drought and reproductive effort interact to control growth of a temperate broadleaved tree species (Fagus sylvatica)

Interannual variation in radial growth is influenced by a range of physiological processes, including variation in annual reproductive effort, although the importance of reproductive allocation has rarely been quantified. In this study, we use long stand-level records of annual seed production, radial growth (tree ring width) and meteorological conditions to analyse the relative importance of summer drought and reproductive effort in controlling the growth of Fagus sylvatica L., a typical masting species. We show that both summer drought and reproductive effort (masting) influenced growth. Importantly, the effects of summer drought and masting were interactive, with the greatest reductions in growth found in years when high reproductive effort (i.e., mast years) coincided with summer drought. Conversely, mast years that coincided with non-drought summers were associated with little reduction in radial growth, as were drought years that did not coincide with mast years. The results show that the strength of an inferred trade-off between growth and reproduction in this species (the cost of reproduction) is dependent on environmental stress, with a stronger trade-off in years with more stressful growing conditions. These results have widespread implications for understanding interannual variability in growth, and observed relationships between growth and climate.

Seasonal variation and resin composition in the Andean tree Austrocedrus chilensis

Little is known about the changes in resin composition in South American gymnosperms associated with the different seasons of the year. The diterpene composition of 44 resin samples from seven Austrocedrus chilensis (Cupressaceae) trees, including male and female individuals, was investigated in three different seasons of the year (February, June and November). Twelve main diterpenes were isolated by chromatographic means and identified by gas chromatography-mass spectrometry and nuclear magnetic resonance (NMR). The diterpene composition was submitted to multivariate analysis to find possible associations between chemical composition and season of the year. The principal component analysis showed a clear relation between diterpene composition and season. The most characteristic compounds in resins collected in summer were Z-communic acid (9) and 12-oxo-labda-8(17),13E-dien-19 oic acid methyl ester (10) for male trees and 8(17),12,14-labdatriene (7) for female trees. For the winter samples, a clear correlation of female trees with torulosic acid (6) was observed. In spring, E-communic acid (8) and Z-communic acid (9) were correlated with female trees and 18-hydroxy isopimar-15-ene (1) with male tree resin. A comparison between percent diterpene composition and collection time showed p < 0.05 for isopimara-8(9),15-diene (2), sandaracopimaric acid (4), compound (7) and ferruginol (11).