Arid environments select for larger seeds in pines (Pinus spp.)

Comparative study of seed germination, phenology, and reproductive fitness in Melilotus sulcatus populations from Israel

BACKGROUND

Melilotus sulcatus (Fabaceae) is an annual weed species prevalent in the Mediterranean Basin and Asia, characterized by a hard seed coat, indicating physical seed dormancy and long seed-bank longevity. This weed exhibits high phenotypic plasticity, thriving across various climatic regions in Israel. This study investigates seed germination of M. sulcatus populations under different temperatures. Moreover, we have studied the phenology, and reproductive biology of M. sulcatus populations in a common garden experiment. Seeds of M. sulcatus were collected from onion fields at Kibbutz Grofit located in the hyper-arid Southern Arava region, with ~ 30 mm average annual rainfall, and from Kibbutz Yifat, in the Jezreel Valley, where annual rainfall is 450-650 mm. Progeny generations were grown in pollen-proof cages at Newe Ya'ar Research Center also located at the Jezreel Valley.

RESULTS

Differences in seed area and average weight were observed among populations and generations, with field populations producing heavier seeds compared to progeny populations. The optimal temperature (T0) for seed germination varied among populations, with the highest T0 recorded for the Grofit field population (23.40℃) collected at the warm site and the lowest for the Yifat field population (17.67℃) collected at the cold site, while the two progeny populations showed similar T0 values. Phenology and reproductive biology were tested using seeds of the two field populations. Despite the vigorous growth of the Grofit population, there were no significant differences in average final weight (4.59 g for Grofit and 4.23 g for Yifat) and height (51.8 cm for Grofit and 50.3 cm for Yifat) between populations. Grofit progeny plants exhibited a statistically significantly higher number of seeds per pod, combined with larger seed area and average weight.

CONCLUSIONS

Overall, our findings underscore the adaptive strategies employed by M. sulcatus populations in response to their environment. Vigorous plant growth, a higher number of seeds per pod, and larger seed area and average weight were observed in individuals from the Grofit population compared to the Yifat population. The greater fitness of Grofit population, originated from hyper-arid conditions, highlights the significant impact of environmental factors on species adaptation.

Effects of snow on reproduction of perennial Thalictrum dioicum: Plants survive but seedlings fail to recruit with reduced snow cover

PREMISE

Future reductions in snow cover are expected in temperate climates, likely leading to more soil-freezing events and damage to plant tissues. However, whether and how plants can compensate for this damage may depend on the timing of damage and on plant allocations to seed size and number. We need more information about how seed production, germination, and seedling recruitment might respond to changes in snow cover.

METHODS

We manipulated snow cover over three seasons in a common garden experiment with four treatments: (1) "control," where snowpack was left unmanipulated throughout the winter season; (2) "late addition," where snowpack was experimentally increased at the end of the winter season in order to delay the onset of spring; (3) "late removal," where snowpack was experimentally reduced at the end of the winter season in order to advance the onset of spring; and (4) "freeze," a consistent removal treatment, where snowpack was experimentally reduced following every substantial snowfall in order to induce freeze-thaw events in the soil. In all treatments, we measured survival, growth, reproduction, and recruitment of a native perennial herb, Thalictrum dioicum.

RESULTS

Reduced snow cover minimally influenced adult survival. Instead, individuals that experienced reduced snow cover throughout the winter produced more massive seeds, whereas individuals that experienced a single snow removal at the end of the season produced less massive seeds. Seedling recruitment was lower in the removal treatments than in the control, as a result of failure to germinate in the freeze treatment and seedling mortality in the late removal treatment.

CONCLUSIONS

Both reduced snow cover throughout the winter and a single late snow removal in the spring reduced seedling recruitment, but for different reasons, suggesting that a holistic approach to the life cycle is needed to understand responses to shifting climates.

Phylotranscriptomics reveals the evolutionary history of subtropical East Asian white pines: further insights into gymnosperm diversification

Floristic composition within a geographic area is driven by a wide array of factors from local biotic interactions to biogeographical processes. Subtropical East Asia is a key biodiversity hotspot of the world, and harbors the most families of extant gymnosperms and a large number of endemic genera with ancient origins, but rare phylogenetic studies explored whether it served as a diversification center for gymnosperms. Here, we investigated the evolutionary and biogeographical history of subtropical East Asian white pines using an integrative approach that combines phylotranscriptomic and ecological analyses. Using 2,606 orthologous nuclear genes, we reconstructed a fully resolved and dated phylogeny of these species. Two main clades first diverged in the early Miocene, and by the late Miocene, all species appeared. Two white pines endemic to Taiwan Island experienced independent colonization events and regional extinction, which resulted in the present disjunctive distribution from mainland China. Ecological and biogeographical analyses indicate that the monsoon-driven assembly of evergreen broadleaved forests (EBLFs) might have significantly affected the diversification of subtropical East Asian white pines. Our study highlights the interactions of biotic and abiotic forces in the diversification and speciation of subtropical East Asian white pines. These findings indicate that subtropical East Asia is not only a floristic museum, but also a diversification center for gymnosperms. Our study also demonstrates the importance of phylotranscriptomics on species delimitation and biodiversity conservation, particularly for closely related species.

Selection patterns on early-life phenotypic traits in Pinus sylvestris are associated with precipitation and temperature along a climatic gradient in Europe

Understanding the dynamics of selection is key to predicting the response of tree species to new environmental conditions in the current context of climate change. However, selection patterns acting on early recruitment stages and their climatic drivers remain largely unknown in most tree species, despite being a critical period of their life cycle. We measured phenotypic selection on Pinus sylvestris seed mass, emergence time and early growth rate over 2 yr in four common garden experiments established along the latitudinal gradient of the species in Europe. Significant phenotypic plasticity and among-population genetic variation were found for all measured phenotypic traits. Heat and drought negatively affected fitness in the southern sites, but heavy rainfalls also decreased early survival in middle latitudes. Climate-driven directional selection was found for higher seed mass and earlier emergence time, while the form of selection on seedling growth rates differed among sites and populations. Evidence of adaptive and maladaptive phenotypic plasticity was found for emergence time and early growth rate, respectively. Seed mass, emergence time and early growth rate have an adaptive role in the early stages of P. sylvestris and climate strongly influences the patterns of selection on these fitness-related traits.

Enhanced seed defenses potentially relax selection by seed predators against serotiny in lodgepole pine

Serotiny, the retention of seeds in a canopy seed bank until high temperatures cause seeds to be released, is an important life history trait for many woody plants in fire-prone habitats. Serotiny provides a competitive advantage after fire but increases vulnerability to predispersal seed predation, due to the seeds being retained in clusters in predictable locations for extended periods. This creates opposing selection pressures. Serotiny is favored in areas of high fire frequency, but is selected against by predispersal seed predators. However, predation also selects for cone traits associated with seed defense that could reduce predation on serotinous cones and thereby relax selection against serotiny. This helps explain the elevated defenses in highly serotinous species. However, whether such interactions drive variation in seed defenses within variably serotinous populations has been studied rarely. We investigated the effects of phenotypic selection exerted by red squirrel (Tamiasciurus hudsonicus) predation on Rocky Mountain lodgepole pine (Pinus contorta latifolia) seeds. Squirrels preferentially harvested cones with more and larger seeds, indicating a preference for a higher food reward. We found evidence for stronger selection on trees with serotinous cones, which presumably accounts for the elevated defenses of and lower predation on serotinous compared to non-serotinous cones. Lower levels of predation on serotinous cones in turn lessen selection against serotiny by squirrels. This has important implications because the frequency of serotiny in lodgepole pine has profound consequences for post-fire communities and ecosystems widespread in the Rocky Mountains.