Climatic drivers and ecological implications of variation in the time interval between leaf-out and flowering

Stabilizing mechanisms enable dioecious trees to maintain synchrony in spring budburst under climate warming

Climate change could reduce dioecious plant fitness if the phenology of males and females responds differently to temperature. However, the extent to which spring phenological responses to climate differ between sexes in wind-pollinated dioecious trees remains poorly understood. Here, we combined ground observations with climate-controlled experiments to investigate sexual differences in spring budburst in Ginkgo biloba, Fraxinus chinensis, and Eucommia ulmoides. In 96% of in situ cases, male trees initiated budburst earlier than females, on average by 3.0 ± 0.4 d. This disparity was more pronounced in warmer regions. The experiment indicated that background climate is a key predictor of sexual disparity in budburst, with the largest differences observed in twigs originating from regions with higher mean annual temperatures and precipitation. However, these disparities declined in areas where mean annual temperatures exceeded 17.1°C, indicating nonlinear trends. This pattern aligns with the warming treatments, where sexual disparities decreased under spring warming of 2-10°C. These results suggest that while sexual disparities can be larger in warmer climates, dioecious trees possess stabilizing mechanisms, including photoperiod and chilling requirements, to maintain synchrony under warming conditions. Our findings enhance understanding of sex-specific phenological responses to climate change, with important implications for future species conservation and ecosystem management.

How phenology interacts with frost tolerance in Southeastern Himalayan Rhododendron species

The frost resistance of new foliage and flowers and their relationship with the phenology of leaf-out and flowering are essential for explaining plant species distribution in seasonally cold climates. In this study, we performed a congeneric, elevational comparison of phenology with frost resistance in evergreen Rhododendron species in the Southeastern Himalayas. A comparison of the microclimate with long-term meteorological records of low temperature extremes permitted the calculation of a realistic, long-term margin of safety for 12 Rhododendron species. Surprisingly, frost resistance and phenological events were matching for leaf-out time (not flowering) in higher elevation species only. Flower-leaf sequence (FLS) and frost resistance were linked for species at higher elevation and the earliest flowering species at lower elevation only. Despite a selection of FLS by elevation, flowers (including petals, filaments and ovaries) were still prone to frost damage during the early growing season at both lower and higher elevations, while new leaves were generally safe on long-term scales, regardless of phenology and elevation. In contrast to lower montane elevation, where severe frost is rare in spring, treeline elevation species maintain safety margins over centennial time-scales by adjusting leaf-out phenology. Our data show an evolutionary priority of leaf survival over flower survival. Both, physiological acclimation and phylogenetic components contribute to these adjustments. Rare extreme frost events restrict the upper range limit of the examined Rhododendron species by affecting new foliage. It is essential to know the actual temperature extremes at organ level rather than relying on weather station records.

Plant phenology response to nitrogen addition decreases community biomass stability in an alpine meadow

Phenology is a sensitive indicator of plant responses to environmental changes, and its shifts could impact community structure and function. However, the effects of phenological shifts on community stability are poorly understood. We conducted a 4-yr N enrichment and precipitation change experiment to assess their effects on community stability through phenological responses. To do so, we measured phenological duration and overlap (based on leaf-out and flowering phenology of 55 species) in an alpine meadow on the Tibetan Plateau. N enrichment extended the vegetative stage of grasses, sedges, and community by 4.62, 4.72, and 11.74 d, respectively, but shortened that of forbs by 6.14 d and increased the overlap of flowering among individuals within the community. Meanwhile, N enrichment decreased species richness, asynchrony, and stability of sedges. Furthermore, N enrichment decreased community stability by decreasing asynchrony but was not associated with richness. Interestingly, N enrichment also decreased sedges stability by extending their vegetative stage and increasing the overlap of flowering, consequently reducing community stability. Our findings imply that N enrichment reduces phenological compensation and thus threatens grassland stability, which highlights the importance of phenological niches in understanding the maintenance of grassland stability under ongoing climate change.

Limited life-history plasticity in marginal population of an invasive foundation species: Unraveling the genetic underpinnings and ecological implications

Plant's life history can evolve in response to variation in climate spatio-temporally, but numerous multiple-species studies overlook species-specific (especially a foundation species) ecological effects and genetic underpinnings. For a species to successfully invade a region, likely to become a foundation species, life-history variation of invasive plants exerts considerable ecological and evolutionary impacts on invaded ecosystems. We examined how an invasive foundation plant, Spartina alterniflora, varied in its life history along latitudinal gradient using a common gardens experiment. Two common gardens were located at range boundary in tropical zone and main distribution area of S. alterniflora in temperate zone in China. Within each population/garden, we measured the onset time of three successive phenological stages constituting the reproductive phase and a fitness trait. In the low-latitude garden with higher temperature, we found that reproductive phase was advanced and its length prolonged compared to the high-latitude garden. This could possibly due to lower plasticity of maturity time. Additionally, plasticity in the length of the reproductive phase positively related with fitness in the low-latitude garden. Marginal population from tropic had the lowest plasticity and fitness, and the poor capacity to cope with changing environment may result in reduction of this population. These results reflected genetic divergence in life history of S. alterniflora in China. Our study provided a novel view to test the center-periphery hypothesis by integration across a plant's life history and highlighted the significance in considering evolution. Such insights can help us to understand long-term ecological consequences of life-history variation, with implications for plant fitness, species interaction, and ecosystem functions under climate change.

Ecological drivers of flower-leaf sequences: aridity and proxies for pollinator attraction select for flowering-first in the American plums

Across temperate forests, many tree species produce flowers before their leaves emerge. This flower-leaf phenological sequence, known as hysteranthy, is generally described as an adaptation for wind pollination. However, this explanation does not address why hysteranthy is also common in biotically pollinated taxa. We quantified flower-leaf sequence variation in the American plums (Prunus, subg. Prunus sect. Prunocerasus), a clade of insect-pollinated trees, using herbaria specimens and Bayesian hierarchical modeling. We tested two common, but rarely interrogated hypotheses - that hysteranthy confers aridity tolerance and/or pollinator visibility - by modeling the associations between hysteranthy and related traits. To understand how these phenology-trait associations were sensitive to taxonomic scale and flower-leaf sequence classification, we then extended these analyses to all Prunus species in North America. Our findings across two taxonomic levels support the hypotheses that hysteranthy may help temporally partition hydraulic demand to reduce water stress and increase pollinator visibility - thereby reducing selective pressure on inflorescence size. Our results provide foundational insights into the evolution of flower-leaf sequences in the genus Prunus, with implications for understanding these patterns in biotically pollinated plants in general. Our approach suggests a path to advance these hypotheses to other clades, but teasing out drivers fully will require new experiments.

Responses of C:N:P stoichiometric correlations among plants, soils and microorganisms to warming: A meta-analysis

Plants, soils and microorganisms play important roles in maintaining stable terrestrial stoichiometry. Studying how nutrient balances of these biotic and abiotic players vary across temperature gradients is important when predicting ecosystem changes on a warming planet. The respective responses of plant, soil and microbial stoichiometric ratios to warming have been observed, however, whether and how the stoichiometric correlations among the three components shift under warming has not been clearly understood and identified. In the present study, we have performed a meta-analysis based on 600 case studies from 74 sites or locations to clarify whether and how warming affects plant, soil and microbial stoichiometry, respectively, and their correlations. Our results indicated that: (1) globally, plants had higher C:N and C:P values compared to soil and microbial pools, but their N:P distributions were similar; (2) warming did not significantly alter plant, soil and microbial C:N and C:P values, but had a noticeable effect on plant N:P ratios. When ecosystem types, duration and magnitude of warming were taken into account, there was an inconsistent and even inverse warming response in terms of the direction and magnitude of changes in the C:N:P ratios occurring among plants, soils and microorganisms; (3) despite various warming responses of the stoichiometric ratios detected separately for plants, soils and microorganisms, the stoichiometric correlations among all three parts remained constant even under different warming scenarios. Our study highlighted the complexity of the effect of warming on the C:N:P stoichiometry, as well as the absence and importance of simultaneous measurements of stoichiometric ratios across different components of terrestrial ecosystems, which should be urgently strengthened in future studies.

Molecular Mechanisms of Seasonal Gene Expression in Trees

In trees, the annual cycling of active and dormant states in buds is closely regulated by environmental factors, which are of primary significance to their productivity and survival. It has been found that the parallel or convergent evolution of molecular pathways that respond to day length or temperature can lead to the establishment of conserved periodic gene expression patterns. In recent years, it has been shown in many woody plants that change in annual rhythmic patterns of gene expression may underpin the adaptive evolution in forest trees. In this review, we summarize the progress on the molecular mechanisms of seasonal regulation on the processes of shoot growth, bud dormancy, and bud break in response to day length and temperature factors. We focus on seasonal expression patterns of genes involved in dormancy and their associated epigenetic modifications; the seasonal changes in the extent of modifications, such as DNA methylation, histone acetylation, and histone methylation, at dormancy-associated loci have been revealed for their actions on gene regulation. In addition, we provide an outlook on the direction of research on the annual cycle of tree growth under climate change.

Climate change: Shifts in time between flowering and leaf-out are complex and consequential

A new study investigated how time intervals between flowering and leaf-out in woody plants are impacted by climate change. Climate change has shifted the timing of both stages, but its impact on the interval between them is complex and variable.