Does climate-related in situ variability of Scots pine (Pinus sylvestris L.) needles have a genetic basis? Evidence from common garden experiments

Phylotranscriptomic and ecological analyses reveal the evolution and morphological adaptation of Abies

Coniferous forests are under severe threat of the rapid anthropogenic climate warming. Abies (firs), the fourth-largest conifer genus, is a keystone component of the boreal and temperate dark-coniferous forests and harbors a remarkably large number of relict taxa. However, the uncertainty of the phylogenetic and biogeographic history of Abies significantly impedes our prediction of future dynamics and efficient conservation of firs. In this study, using 1,533 nuclear genes generated from transcriptome sequencing and a complete sampling of all widely recognized species, we have successfully reconstructed a robust phylogeny of global firs, in which four clades are strongly supported and all intersectional relationships are resolved, although phylogenetic discordance caused mainly by incomplete lineage sorting and hybridization was detected. Molecular dating and ancestral area reconstruction suggest a Northern Hemisphere high-latitude origin of Abies during the Late Cretaceous, but all extant firs diversified during the Miocene to the Pleistocene, and multiple continental and intercontinental dispersals took place in response to the late Neogene climate cooling and orogenic movements. Notably, four critically endangered firs endemic to subtropical mountains of China, including A. beshanzuensis, A. ziyuanensis, A. fanjingshanensis and A. yuanbaoshanensis from east to west, have different origins and evolutionary histories. Moreover, three hotspots of species richness, including western North America, central Japan, and the Hengduan Mountains, were identified in Abies. Elevation and precipitation, particularly precipitation of the coldest quarter, are the most significant environmental factors driving the global distribution pattern of fir species diversity. Some morphological traits are evolutionarily constrained, and those linked to elevational variation (e.g., purple cone) and cold resistance (e.g., pubescent branch and resinous bud) may have contributed to the diversification of global firs. Our study sheds new light on the spatiotemporal evolution of global firs, which will be of great help to forest management and species conservation in a warming world.

Polar day syndrome: differences in growth, photosynthetic traits and sink-size patterns between northern and southern Finnish silver birch (Betula pendula Roth) provenances in native and non-native photoperiods

Continuous light (CL) is available throughout the polar day for plants in the Arctic during the growing season, whereas provenances of the same species experience a very different environment with non-CL (NCL) just a few latitudes to the south. Both provenances need to acclimate to climate warming, yet we lack comprehensive understanding of how their growth, photosynthesis and leaf traits differ. Further, the provenances presumably have morphological and physiological adaptations to their native environments and therefore differ in response to photoperiod. We tested the height growth, leaf longevity, biomass accumulation, biomass allocation and rates of gas exchange of northern (67°N) and southern (61°N) Finnish silver birch (Betula pendula Roth) origins in CL- and NCL-treatments in a 4-month chamber experiment. Irrespective of photoperiod, 67°N had higher area-based photosynthetic rate (Anet), stomatal conductance (gs) and relative height growth rate (RGR), but lower stomatal density and fewer branches and leaves than 61°N. Photoperiod affected height growth cessation, biomass and photosynthetic traits, whereas leaf longevity and many leaf functional traits remained unchanged. In CL, both provenances had lower gs, higher RGR, increased shoot:root ratio and increased sink sizes (more branching, more leaves, increased total plant dry weight) compared with NCL. In NCL, 67°N ceased height growth earlier than in CL, which altered biomass accumulation and distribution patterns. Northern conditions impose challenges for plant growth and physiology. Whether a provenance inhabits and is adapted to an area with or without CL can also affect its response to the changing climate. Northern birches may have adapted to CL and the short growing season with a 'polar day syndrome' of traits, including relatively high gas exchange rates with low leaf biomass and growth traits that are mainly limited by the environment and the earlier growth cessation (to avoid frost damage).

Variation, coordination, and trade-offs between needle structures and photosynthetic-related traits across five Picea species: consequences on plant growth

BACKGROUND

Picea species are distributed and planted world-wide due to their great ecological and economic values. It has been reported that Picea species vary widely in growth traits in a given environment, which reflects genetic and phenotypic differences among species. However, key physiological processes underlying tree growth and the influencing factors on them are still unknown.

RESULTS

Here, we examined needle structures, needle chemical components, physiological characteristics and growth traits across five Picea species in a common garden in Tianshui, Gansu province in China: Picea glauca, P. mariana, P. likiangensis, P. koraiensis, and P. crassifolia, among which P. glauca and P. mariana were introduced from North America, P. likiangensis was from Lijiang, Yunan province in China, P. koraiensis was from Yichun, Heilongjiang province in China, and P. crassifolia was native to the experimental site. It was found that nearly all traits varied significantly among species. Tissue-level anatomical characteristics and leaf mass per area (LMA) were affected by needle size, but the variations of them were not associated with the variations in photosynthetic and biochemical capacity among species. Variations in area-based maximum photosynthesis (Pnmax) were affected by stomatal conductance (gs), mesophyll conductance (gm) and biochemical parameters including maximum carboxylation rate (Vcmax), and maximum electron transport rate (Jmax). The fraction of N allocated to different photosynthetic apparatus displayed contrasting values among species, which contributed to the species variations in photosynthetic nitrogen use efficiency (PNUE) and Pnmax. Additionally, all growth traits were positively correlated with Pnmax and PNUE.

CONCLUSION

Needle structures are less important than needle biochemical parameters in determining the variations in photosynthetic capacity across the five Picea species. Pnmax and PNUE are closedly associated with the fraction of N allocated to photosynthetic apparatus (Pphoto) compared with leaf N content per area (Narea). The tremendous growth differences among the five Picea species were substantially related to the interspecies variation in Pnmax and PNUE.

Climate of origin has no influence on drought adaptive traits and the drought responses of a widely distributed polymorphic shrub

Climate has a significant influence on species distribution and the expression of functional traits in different plant species. However, it is unknown if subspecies with different climate envelopes also show differences in their expression of plant functional traits or if they respond differently to drought stress. We measured functional traits and drought responses of five subspecies of a widely distributed, cosmopolitan polymorphic shrub, Dodonaea viscosa (L.) Jacq., in an experiment with 1-year-old plants. Functional traits, such as leaf size, specific leaf area, turgor loss point (ΨTLP), maximum stomatal conductance and maximum plant hydraulic conductance, differed among the five subspecies. However, while the were some differences among traits, these were not related to their climate of origin, as measured by mean annual temperature, mean annual precipitation and mean annual aridity index. Drought response was also not related to climate of origin, and all subspecies showed a combination of drought avoiding and drought tolerance responses. All subspecies closed their stomata at very high water potentials (between -1.0 and -1.3 MPa) and had large hydraulic safety margins (drought avoidance). All subspecies adjusted their ΨTLP via osmotic adjustment, and subspecies with inherently lower ΨTLP showed greater osmotic adjustment (drought tolerance). All subspecies adjusted their midday water potentials in response to drought but subspecies from more arid environments did not show greater adjustments. The results indicated that climate niche was not related to plant trait expression or response to drought. The combination of drought avoidance and drought tolerance behavior seems to be a successful strategy for this widely distributed species that occupies many different climate zones and ecosystems. Hence, the wide distribution of D. viscosa seems to be related to plasticity of trait expression and drought response rather than long-term genetic adaptations to different environmental conditions.

Higher biomass partitioning to absorptive roots improves needle nutrition but does not alleviate stomatal limitation of northern Scots pine

Harsh environmental conditions affect both leaf structure and root traits. However, shoot growth in high-latitude systems is predominately under photoperiod control while root growth may occur for as long as thermal conditions are favorable. The different sensitivities of these organs may alter functional relationships above- and belowground along environmental gradients. We examined the relationship between absorptive root and foliar traits of Scots pine trees growing in situ along a temperate-boreal transect and in trees grown in a long-term common garden at a temperate latitude. We related changes in foliar nitrogen, phosphorus, specific leaf area, needle mass and ¹³ C signatures to geographic trends in absorptive root biomass to better understand patterns of altered tree nutrition and water balance. Increased allocation to absorptive fine roots was associated with greater uptake of soil nutrients and subsequently higher needle nutrient contents in the northern provenances compared with more southern provenances when grown together in a common garden setting. In contrast, the leaf δ¹³ C in northern and southern provenances were similar within the common garden suggesting that higher absorptive root biomass fractions could not adequately increase water supply in warmer climates. These results highlight the importance of allocation within the fine-root system and its impacts on needle nutrition while also suggesting increasing stomatal limitation of photosynthesis in the context of anticipated climatic changes.

Strategy by latitude? Higher photosynthetic capacity and root mass fraction in northern than southern silver birch (Betula pendula Roth) in uniform growing conditions

Growth of northern trees is limited by short growing seasons. In multi-year trials, northern trees usually grow less than southern ones but can have higher gas exchange, whereas differences in biomass allocation and its relation to photosynthesis are less known. We characterized silver birch (Betula pendula Roth) provenances from southern (latitude 61°) and northern (latitude 67°) Finland in uniform chamber conditions. In a time-series experiment, we measured traits related to growth, biomass allocation and photosynthesis, and determined gas exchange responses to temperature and light. We found provenance differences in photosynthetic capacity and growth. The northern provenance allocated relatively more to roots, having a higher root mass fraction and lower shoot:root ratio than the southern provenance. On the other hand, the northern provenance had fewer leaves and lower total leaf dry weight (DW) than the southern provenance. The northern provenance attained higher rates of net photosynthesis (Anet) and higher stomatal conductance (gs) in all measured temperatures and higher photosynthesis at the optimum temperature (Aopt) than the southern provenance, but there was no difference in the optimum temperature of photosynthesis (Topt, 18.3 °C for the southern provenance vs 18.9 °C for the northern one). Photosynthetic light response curves showed no between-provenance differences. In a time-series, the northern provenance had higher Anet than the southern provenance, but gs was similar. The northern provenance had higher maximum quantum yield of photosystem II photochemistry (Fv/Fm) than the southern provenance. There were no differences between provenances in height, total plant DW, shoot DW, root DW or shoot mass fraction. Our results suggest that the provenances occupy a common thermal niche, or can at least relatively quickly acclimate to a common growth temperature. Thus, carbon assimilation of these northern trees may not be significantly affected by rising temperatures alone. In an equal photoperiod and optimal conditions, we found different one-season biomass accumulation strategies: southern trees grow with more leaves, while northern trees reach similar total assimilation (total DW, height) with more efficient photosynthetic capacity per leaf area (higher gas exchange, higher Fv/Fm) and relatively more investment in the below-ground fraction of the plant.

Combined high leaf hydraulic safety and efficiency provides drought tolerance in Caragana species adapted to low mean annual precipitation

Clarifying the coordination of leaf hydraulic traits with gas exchange across closely-related species adapted to varying rainfall can provide insights into plant habitat distribution and drought adaptation. The leaf hydraulic conductance (K_leaf ), stomatal conductance (g_s ), net assimilation (A), vein embolism and abscisic acid (ABA) concentration during dehydration were quantified, as well as pressure-volume curve traits and vein anatomy in 10 Caragana species adapted to a range of mean annual precipitation (MAP) conditions and growing in a common garden. We found a positive correlation between Ψ_leaf at 50% loss of K_leaf (K_leaf P₅₀ ) and maximum K_leaf (K_leaf-max ) across species. Species from low-MAP environments exhibited more negative K_leaf P₅₀ and turgor loss point, and higher K_leaf-max and leaf-specific capacity at full turgor, along with higher vein density and midrib xylem per leaf area, and a higher ratio of K_leaf-max : maximum g_s . Tighter stomatal control mediated by higher ABA accumulation during dehydration in these species resulted in an increase in hydraulic safety and intrinsic water use efficiency (WUE_i ) during drought. Our results suggest that high hydraulic safety and efficiency combined with greater stomatal sensitivity triggered by ABA production and leading to greater WUE_i provides drought tolerance in Caragana species adapted to low-MAP environments.