Negative effects of winter and spring warming on the regeneration of forest spring geophytes

The climate is changing rapidly, provoking species to shift their ranges poleward and upslope. We currently lack a mechanistic understanding of the effect of warmer temperatures on plants, especially for seasonally distinct patterns. Spring geophytes are emblematic forest plants that have a short aboveground lifecycle in the first half of the year and are thus particularly sensitive to winter and spring warming. We set up a warming experiment with separate and combined winter and spring warming on seedlings of three European spring geophytes: Anemone nemorosa, Hyacinthoides non-scripta and Ornithogalum pyrenaicum. Seedling emergence and plant height were recorded at the end of winter and spring treatment, when also biomass of the root, shoot and storage organ was determined. We found negative effects of combined winter and spring warming on seedling emergence. The weight of the storage organ proved to be the best indicator of seedling performance and was negatively affected by separate winter warming in Anemone and by spring warming in Hyacinthoides. Successful seedling emergence was jeopardized by the absence of a cold period, while seedling performance seemed to be negatively influenced directly by higher temperatures through a phenological shift. Our findings confirm that warmer winter and spring temperatures could hamper regeneration of spring geophytes.

Integrating experiments to predict interactive cue effects on spring phenology with warming

Climate change has advanced plant phenology globally 4-6 d °C^-1 on average. Such shifts are some of the most reported and predictable biological impacts of rising temperatures. Yet as climate change has marched on, phenological shifts have appeared muted over recent decades - failing to match simple predictions of an advancing spring with continued warming. The main hypothesis for these changing trends is that interactions between spring phenological cues - long-documented in laboratory environments - are playing a greater role in natural environments due to climate change. Here, we argue that accurately linking shifts observed in long-term data to underlying phenological cues is slowed by biases in observational studies and limited integration of insights from laboratory studies. We synthesize seven decades of laboratory experiments to quantify how phenological cue-space has been studied and how treatments compare with shifts caused by climate change. Most studies focus on one cue, limiting our ability to make accurate predictions, but some well-studied forest species offer opportunities to advance forecasting. We outline how greater integration of controlled-environment studies with long-term data could drive a new generation of laboratory experiments, built on physiological insights, that would transform our fundamental understanding of phenology and improve predictions.

Extirpated prairie species demonstrate more variable phenological responses to warming than extant congeners

PREMISE

Shifting phenology in response to climate is one mechanism that can promote population persistence and geographic spread; therefore, species with limited ability to phenologically track changing environmental conditions may be more susceptible to population declines. Alternatively, apparently nonresponding species may demonstrate divergent responses to multiple environmental conditions experienced across seasons.

METHODS

Capitalizing on herbarium records from across the midwestern United States and on detailed botanical surveys documenting local extinctions over the past century, we investigated whether extirpated and extant taxa differ in their phenological responses to temperature and precipitation during winter and spring (during flowering and the growing season before flowering) or in the magnitude of their flowering time shift over the past century.

RESULTS

Although warmer temperatures across seasons advanced flowering, extirpated and extant species differed in the magnitude of their phenological responses to winter and spring warming. Extirpated species demonstrated inconsistent phenological responses to warmer spring temperatures, whereas extant species consistently advanced flowering in response to warmer spring temperatures. In contrast, extirpated species advanced flowering more than extant species in response to warmer winter temperatures. Greater spring precipitation tended to delay flowering for both extirpated and extant taxa. Finally, both extirpated and extant taxa delayed flowering over time.

CONCLUSIONS

This study highlights the importance of understanding phenological responses to seasonal warming and indicates that extirpated species may demonstrate more variable phenological responses to temperature than extant congeners, a finding consistent with the hypothesis that appropriate phenological responses may reduce species' likelihood of extinction.

Accelerated rate of vegetation green-up related to warming at northern high latitudes

Mid- to high-latitude vegetation are experiencing changes in their seasonal cycles as a result of climate change. Although the rates of seasonal growth from winter dormancy to summer maturity have accelerated because of changes in environmental conditions, less attention has been paid to the rate of vegetation green-up (RVG) and its dynamics, which could advance vegetation maturity. We analyzed the long-term changes in RVG and the drivers at high northern latitudes for 35 years (1982-2016) using satellite-retrieved leaf area index data based on partial correlation analyses and multivariable linear regression. The rates tended to increase significantly with time, particularly at high latitudes above 60°N in North America (1.8% mon^-1 decade^-1 , p < .01) and Eurasia (1.0% mon^-1 decade^-1 , p < .01). The increasing trend in North America was mostly because of increased heat accumulation in spring (1.2% mon^-1 decade^-1 ), that is, more rapid green-up owing to warming, with an increased carbon dioxide concentration (0.6 mon^-1 decade^-1 ). The trend in Eurasia, however, was induced by warming, increased carbon dioxide concentration, and stronger radiation, 1.0%, 0.7%, and 0.5% mon^-1 decade^-1 , respectively, but was partly counteracted by earlier pregreen-up dates of -1.2% mon^-1 decade^-1 , that is, earlier initiation of growth which counteracted green-up rate acceleration. The results suggested that warming was the predominant factor influencing the accelerated RVG at high latitudes; however, Eurasian vegetation exhibited different green-up dynamics, mitigating the influence of warming with the earlier pregreen-up. Our findings imply that high-latitude warming will drive vegetation seasonality toward rapid green-up and early maturity, leading to the reinforcement of climate-vegetation interactions; however, the consequences will be more distinct in North America owing to the absence of alleviation by earlier pregreen-up.

Ecological genomics of variation in bud-break phenology and mechanisms of response to climate warming in Populus trichocarpa

Spring bud-break phenology is a critical adaptive feature common to temperate perennial woody plants. Understanding the molecular underpinnings of variation in bud-break is important for elucidating adaptive evolution and predicting outcomes relating to climate change. Field and controlled growth chamber tests were used to assess population-wide patterns in bud-break from wild-sourced black cottonwood (Populus trichocarpa) genotypes. We conducted a genome-wide association study (GWAS) with single nucleotide polymorphisms (SNPs) derived from whole genome sequencing to test for loci underlying variation in bud-break. Bud-break had a quadratic relationship with latitude, where southern- and northern-most provenances generally broke bud earlier than those from central parts of the species' range. Reduced winter chilling increased population-wide variation in bud-break, whereas greater chilling decreased variation. GWAS uncovered 16 loci associated with bud-break. Phenotypic changes connected with allelic variation were replicated in an independent set of P. trichocarpa trees. Despite phenotypic similarities, genetic profiles between southern- and northern-most genotypes were dissimilar based on our GWAS-identified SNPs. We propose that the GWAS-identified loci underpin the geographical pattern in P. trichocarpa and that variation in bud-break reflects different selection for winter chilling and heat sum accumulation, both of which can be affected by climate warming.

Current spring warming as a driver of selection on reproductive timing in a wild passerine

Evolutionary adaptation as a response to climate change is expected for fitness-related traits affected by climate and exhibiting genetic variance. Although the relationship between warmer spring temperature and earlier timing of reproduction is well documented, quantifications and predictions of the impact of global warming on natural selection acting on phenology in wild populations remain rare. If global warming affects fitness in a similar way across individuals within a population, or if fitness consequences are independent of phenotypic variation in key-adaptive traits, then no evolutionary response is expected for these traits. Here, we quantified the selection pressures acting on laying date during a 24-year monitoring of blue tits in southern Mediterranean France, a hot spot of climate warming. We explored the temporal fluctuation in annual selection gradients and we determined its temperature-related drivers. We first investigated the month-specific warming since 1970 in our study site and tested its influence on selection pressures, using a model averaging approach. Then, we quantified the selection strength associated with temperature anomalies experienced by the blue tit population. We found that natural selection acting on laying date significantly fluctuated both in magnitude and in sign across years. After identifying a significant warming in spring and summer, we showed that warmer daily maximum temperatures in April were significantly associated with stronger selection pressures for reproductive timing. Our results indicated an increase in the strength of selection by 46% for every +1°C anomaly. Our results confirm the general assumption that recent climate change translates into strong selection favouring earlier breeders in passerine birds. Our findings also suggest that differences in fitness among individuals varying in their breeding phenology increase with climate warming. Such climate-driven influence on the strength of directional selection acting on laying date could favour an adaptive response in this trait, since it is heritable.

Gas exchange, growth, and defense responses of invasive Alliaria petiolata (Brassicaceae) and native Geum vernum (Rosaceae) to elevated atmospheric CO2 and warm spring temperatures

PREMISE OF STUDY

Global increases in atmospheric CO2 and temperature may interact in complex ways to influence plant physiology and growth, particularly for species that grow in cool, early spring conditions in temperate forests. Plant species may also vary in their responses to environmental changes; fast-growing invasives may be more responsive to rising CO2 than natives and may increase production of allelopathic compounds under these conditions, altering species' competitive interactions.

METHODS

We examined growth and physiological responses of Alliaria petiolata, an allelopathic, invasive herb, and Geum vernum, a co-occurring native herb, to ambient and elevated spring temperatures and atmospheric CO2 conditions in a factorial growth chamber experiment.

KEY RESULTS

At 5 wk, leaves were larger at high temperature, and shoot biomass increased under elevated CO2 only at high temperature in both species. As temperatures gradually warmed to simulate seasonal progression, G. vernum became responsive to CO2 at both temperatures, whereas A. petiolata continued to respond to elevated CO2 only at high temperature. Elevated CO2 increased thickness and decreased nitrogen concentrations in leaves of both species. Alliaria petiolata showed photosynthetic downregulation at elevated CO2, whereas G. vernum photosynthesis increased at elevated temperature. Flavonoid and cyanide concentrations decreased significantly in A. petiolata leaves in the elevated CO2 and temperature treatment. Total glucosinolate concentrations and trypsin inhibitor activities did not vary among treatments.

CONCLUSIONS

Future elevated spring temperatures and CO2 will interact to stimulate growth for A. petiolata and G. vernum, but there may be reduced allelochemical effects in A. petiolata.