Climate Warming Impacts on Distributions of Scots Pine (Pinus sylvestris L.) Seed Zones and Seed Mass across Russia in the 21st Century

Research highlights: We investigated bioclimatic relationships between Scots pine seed mass and seed zones/climatypes across its range in Russia using extensive published data to predict seed zones and seed mass distributions in a changing climate and to reveal ecological and genetic components in the seed mass variation using our 40-year common garden trial data. Introduction: seed productivity issues of the major Siberian conifers in Asian Russia become especially relevant nowadays in order to compensate for significant forest losses due to various disturbances during the 20th and current centuries. Our goals were to construct bioclimatic models that predict the seed mass of major Siberian conifers (Scots pine, one of the major Siberian conifers) in a warming climate during the current century. Methods: Multi-year seed mass data were derived from the literature and were collected during field work. Climate data (January and July data and annual precipitation) were derived from published reference books on climate and climatic websites. Our multiple regression bioclimatic models were constructed based on the climatic indices of growing degree days > 5 °C, negative degree days < 0 °C, and annual moisture index, which were calculated from January and July temperatures and annual precipitation for both contemporary and future climates. The future 2080 (2070–2100) January and July temperatures and annual precipitation anomalies were derived from the ensemble of twenty CMIP5 (the Coupled Model Intercomparison Project, Phase 5) global circulation models (GCMs) and two scenarios using a mild RCP (Representative Concentration Pathway) 2.6 scenario and an extreme RCP 8.5 scenario. Results: Site climate explained about 70% of the seed mass variation across the Scots pine range. Genetic components explained 30% of the seed mass variation, as per the results from our common garden experiment in south central Siberia. Seed mass varied within 3.5 g (min) and 10.5 g (max) with the mean 6.1 g (n = 1150) across Russia. Our bioclimatic seed mass model predicted that a July temperature elevated by 1 °C increased seed mass by 0.56 g, and a January temperature elevated by 5 °C increased seed mass by 0.43 g. The seed mass would increase from 1 g to 4 g in the moderate RCP 2.6 and the extreme RCP 8.5 climates, respectively. Predicted seed zones with heavier seed would shift northwards in a warming climate. However, the permafrost border would halt this shifting due to slower permafrost thawing; thus, our predicted potential for Scots pine seed zones and seed mass would not be realized in the permafrost zone in a warmed climate. Our common garden experiment in central Siberia showed that trees of northerly origins produced lighter seeds than local trees but heavier ones than the trees at the original site. Trees of southerly origins produced heavier seeds than local trees but lighter seeds than the trees at the original site. Conclusions: The findings from this study could serve as blueprints for predicting new landscapes with climatic optima for Pinus sylvestris to produce better quality seeds to adjust to a warming climate.

Analysis of six-decadal seed mass and emergence records in mast species shows little inter-annual variability

Patterns of crop production in mast species do not track crop-year climate, but instead are regulated by climate cues in prior-years. Whether the pattern of year-to-year seed mass variation is coupled in time with mast seeding, maintaining seed mass-number trade-offs, and coherently driven by similar climate cues as other seed traits (e.g. seed germination) remains unknown. Using ca. 6,000 long-term seed inventory data over the years 1955-2015 in conifers, this retrospective study revealed the temporal patterns of mast species’ seed mass and its associated trait, seed germination. To pinpoint their ecological drivers, pairwise correlation analysis was performed between each trait and seasonal climates in crop year and four prior-years. Using climate variables key to each trait, regression models were constructed to project trait values. Findings showed minor seed mass variation among years, which rejects the generality of seed mass-number trade-offs in many plant species. This result reasonably arises as the economies of scale (compensating benefits) theory are often used to account for mast seeding but not for seed mass. Moreover, final germination fraction also varied little over time, but exhibited an increasing tendency. In addition, we found that temperature-based climate variables drive seed mass, number, and germination variation, but these variables in different seasons of crop year or prior-years did not have equal influences on trait variability. Finally, regression models showed that the number of frost-free days and evapotranspiration are crucial to the three traits and climate in autumn is a critical season, followed by summer and winter. This study holds considerable promise for explaining reproductive strategies of taxonomic groups with mast seeding characteristics in allocating reproductive resources to different life-history traits using ecological signals.

Viability and vigour loss during storage of Rudbeckia mollis seeds having different mass: an intra-specific perspective

Recent evidence points to relationships between intra-specific seed mass variation and viability loss in response to ageing stress. However, little is known about how seed quality may change temporally in response to such stress. Here we examined seed–water relations of mass-separated Rudbeckia mollis seeds to better understand physiological status among mass classes. We then evaluated seed viability and vigour changes in response to various storage conditions or post-storage vigour tests (a 41°C, 75% RH stress for up to 45 d). We found similar pre-storage physiology among mass classes. However, seeds of lower mass deteriorated up to 1.5-fold faster than heavier seeds under certain conditions. Stressing seeds after storage resulted in distinct vigour differences among mass classes. For example, vigour in lower mass seeds tended to decline more compared to heavier seeds following storage in a climate-controlled room. Alternatively, vigour loss varied among mass classes following storage in a non-climate-controlled shed. Our results highlight the importance of distinguishing between pre-sowing storage and post-storage vigour effects when quantifying relative levels of viability loss among seeds of different mass. Furthermore, differential responses to storage and ageing stress among mass classes may have important implications for post-storage regeneration and subsequent population dynamics.