Waterlogging in late dormancy and the early growth phase affected root and leaf morphology in Betula pendula and Betula pubescens seedlings

Separating the effects of air and soil temperature on silver birch. Part II. The relation of physiology and leaf anatomy to growth dynamics

The aboveground parts of boreal forest trees grow earlier in the growing season, the roots mostly later. The idea was to examine whether root growth followed soil temperature, or whether shoot growth also demanded most resources in the early growing season (soil temperature vs internal sink strengths for resources). The linkage between air and soil temperature was broken by switching the soil temperature. We aimed here to identify the direct effects of different soil temperature patterns on physiology, leaf anatomy and their interactions, and how they relate to the control of the growth dynamics of silver birch (Betula pendula Roth). Sixteen 2-year-old seedlings were grown in a controlled environment for two 14-week simulated growing seasons (GS1, GS2). An 8-week dormancy period interposed the GSs. In GS2, soil temperature treatments were applied: constant 10 °C (Cool), constant 18 °C (Warm), early growing season at 10 °C switched to 18 °C later (Early Cool Late Warm) and 18 °C followed by 10 °C (Early Warm Late Cool) were applied during GS2. The switch from cool to warm enhanced the water status, net photosynthesis, chlorophyll content index, effective yield of photosystem II (ΔF/Fm') and leaf expansion of the seedlings. Warm treatment increased the stomatal number per leaf. In contrast, soil cooling increased glandular trichomes. This investment in increasing the chemical defense potential may be associated with the decreased growth in cool soil. Non-structural carbohydrates were accumulated in leaves at a low soil temperature showing that growth was more hindered than net photosynthesis. Leaf anatomy differed between the first and second leaf flush of silver birch, which may promote tree fitness in the prevailing growing conditions. The interaction of birch structure and function changes with soil temperature, which can further reflect to ecosystem functioning.

Impacts of elevation on plant traits and volatile organic compound emissions in deciduous tundra shrubs

The northernmost regions of our planet experience twice the rate of climate warming compared to the global average. Despite the currently low air temperatures, tundra shrubs are known to exhibit high leaf temperatures and are increasing in height due to warming, but it is unclear how the increase in height will affect the leaf temperature. To study how temperature, soil moisture, and changes in light availability influence the physiology and emissions of climate-relevant volatile organic compounds (VOCs), we conducted a study on two common deciduous tundra shrubs, Salix glauca (separating males and females for potential effects of plant sex) and Betula glandulosa, at two elevations in South Greenland. Low-elevation Salix shrubs were 45% taller, but had 37% lower rates of net CO₂ assimilation and 63% lower rates of isoprene emission compared to high-elevation shrubs. Betula shrubs showed 40% higher stomatal conductance and 24% higher glandular trichome density, in the low-elevation valley, compared to those from the high-elevation mountain slope. Betula green leaf volatile emissions were 235% higher at high elevation compared to low elevation. Male Salix showed a distinct VOC blend and emitted 55% more oxygenated VOCs, compared to females, possibly due to plant defense mechanisms. In our light response curves, isoprene emissions increased linearly with light intensity, potentially indicating adaptation to strong light. Leaf temperature decreased with increasing Salix height, at 4 °C m^-1, which can have implications for plant physiology. However, no similar relationship was observed for B. glandulosa. Our results highlight that tundra shrub traits and VOC emissions are sensitive to temperature and light, but that local variations in soil moisture strongly interact with temperature and light responses. Our results suggest that effects of climate warming, alone, poorly predict the actual plant responses in tundra vegetation.

Is Plant Life-History of Biseasonal Germination Consistent in Response to Extreme Precipitation?

Future climate is projected to increase in the intensity and frequency of extreme precipitation events, and the resulting ecological consequences are often more serious than those of normal precipitation events. In particular, in desert ecosystems, due to the low frequency and strong fluctuation of extreme precipitation, the destructive consequences for desert plants caused by extreme precipitation have not received enough attention for some time. Based on statistics of extreme precipitation events (1965–2018) in the Gurbantunggut Desert, we investigated the effects of extreme precipitation (+0%, CK; +50%, W1; +100%, W2; +200%, W3; maintenance of field capacity, W4) on the plant life-history of the spring-germinated (SG) and autumn-germinated (AG) ephemeral plant Erodium oxyrhynchum by monitoring seedling emergence, survival, phenology, organ size, biomass accumulation, and allocation. The results showed that extreme precipitation caused about 2.5% seedling emergence of E. oxyrhynchum in autumn 2018 and 3.0% seedling emergence in early spring 2019, which means that most seeds may be stored in the soil or have died. Meanwhile, extreme precipitation significantly improved the survival, organ size, and biomass accumulation of SG and AG plants, and W3 was close to the precipitation threshold of SG (326.70 mm) and AG (560.10 mm) plants corresponding to the maximum individual biomass; thus, AG plants with a longer life cycle need more water for growth. Conversely, W4 caused AG plants to enter the leaf stage in advance and led to death in winter, which indicates that extreme precipitation may not be good for AG plants. Root and reproduction biomass allocation of SG and AG plants showed a significantly opposite trend under extreme precipitation treatments, which might be related to their different life-history strategies. Therefore, when only taking into account the changing trend of extreme precipitation from the Coupled Model Intercomparison Project 6 (CMIP6) climate projections data, we speculate that extreme precipitation may promote the growth of SG and AG plants from the beginning to the middle of this century, but extreme precipitation in autumn exceeding a certain threshold may adversely affect the survival of AG plants at the end of the century.

Dynamics of above- and belowground responses of silver birch saplings and soil gases to soil freezing and waterlogging during dormancy

Winter precipitation and soil freeze-thaw events have been predicted to increase in boreal regions with climate change. This may expose tree roots to waterlogging (WL) and soil freezing (Fr) more than in the current climate and therefore affect tree growth and survival. Using a whole-tree approach, we studied the responses of silver birch (Betula pendula Roth.) saplings, growing in mineral soil, to 6-week Fr and WL in factorial combinations during dormancy, with accompanying changes in soil gas concentrations. Physiological activation (dark-acclimated chlorophyll fluorescence and chlorophyll content index) and growth of leaves and shoot elongation and stem diameter growth started earlier in Fr than NoFr (soil not frozen). The starch content of leaves was temporarily higher in Fr than NoFr in the latter part of the growing season. Short and long root production and longevity decreased, and mortality increased by soil Fr, while there were no significant effects of WL. Increased fine root damage was followed by increased compensatory root growth. At the beginning of the growing season, stem sap flow increased fastest in Fr + WL, with some delay in both NoWL (without WL) treatments. At the end of the follow-up growing season, the hydraulic conductance and impedance loss factor of roots were higher in Fr than in NoFr, but there were no differences in above- and belowground biomasses. The concentration of soil carbon dioxide increased and methane decreased by soil Fr at the end of dormancy. At the beginning of the growing season, the concentration of nitrous oxide was higher in WL than in NoWL and higher in Fr than in NoFr. In general, soil Fr had more consistent effects on soil greenhouse gas concentrations than WL. To conclude, winter-time WL alone is not as harmful for roots as WL during the growing season.

Different Waterlogging Depths Affect Spatial Distribution of Fine Root Growth for Pinus thunbergii Seedlings

The increase of waterlogged environments at forests and urban greenery is of recent concern with the progress of climate change. Under waterlogging, plant roots are exposed to hypoxic conditions, which strongly affect root growth and function. However, its impact is dependent on various factors, such as waterlogging depth. Therefore, our objective is to elucidate effects of different waterlogging depths on Pinus thunbergii Parl., which is widely used for afforestation, especially at coastal forests. We conducted an experiment to examine growth and morphology of fine roots and transpiration using 2-year-old seedlings under three treatments, (1) control (no waterlogging), (2) partial waterlogging (partial-WL, waterlogging depth = 15 cm from the bottom), and (3) full waterlogging (full-WL, waterlogging depth = from the bottom to the soil surface, 26 cm). As a result, fine root growth and transpiration were both significantly decreased at full-WL. However, for partial-WL, fine root growth was significantly increased compared to control and full-WL at the top soil, where it was not waterlogged. Additionally, transpiration which had decreased after 4 weeks of waterlogging showed no significant difference compared to control after 8 weeks of waterlogging. This recovery is to be attributed to the increase in fine root growth at non-waterlogged top soil, which compensated for the damaged roots at the waterlogged bottom soil. In conclusion, this study suggests that P. thunbergii is sensitive to waterlogging; however, it can adapt to waterlogging by plastically changing the distribution of fine root growth.

Let it snow! Winter conditions affect growth of birch seedlings during the following growing season

Air temperatures and precipitation are predicted to increase in the future, especially at high latitudes and particularly so during winter. In contrast to air temperatures, changes in soil temperatures are more difficult to predict, as the fate of the insulating snow cover is crucial in this respect. Soil conditions can also be affected by rain-on-snow events and warm spells during winter, resulting in freeze-thaw cycles, compacted snow, ice encasement and local flooding. These adverse conditions during winter could counteract the otherwise positive effects of climate change on forest growth and productivity. For studying the effects of different winter and snow conditions on young Downy birch (Betula pubescens Ehrh.) seedlings, we carried out a laboratory experiment with birch seedlings subjected to four different winter scenarios: snow covering the seedlings (SNOW), compressed snow and ice encasement (ICE), flooded and frozen soil (FLOOD) and no snow at all (NO SNOW). After the winter treatments we simulated a spring and early summer period of 9.5 weeks, and monitored the growth by measuring shoot and root biomass of the seedlings, and starch and soluble sugar concentrations. We also assessed the stress experienced by the seedlings by measuring leaf chlorophyll fluorescence and gas exchange. Although no difference in mortality was observed between the treatments, the seedlings in the SNOW and ICE treatments had significantly higher shoot and root biomass compared with those in the FLOOD and NO SNOW treatments. We found higher starch concentrations in roots of the seedlings in the SNOW and ICE treatments, compared with those in the FLOOD and NO SNOW treatments, although photosynthesis did not differ. Our results suggest a malfunction of carbohydrate distribution in the seedlings of the FLOOD and NO SNOW treatments, probably resulting from decreased sinks. The results underline the importance of an insulating and protecting snow cover for small tree seedlings, and that future winters with changed snow pattern might affect the growth of tree seedlings and thus possibly species composition and forest productivity.

Photosynthesis, nutrient accumulation and growth of two Betula species exposed to waterlogging in late dormancy and in the early growing season

Increased risk of soil waterlogging in winter and spring at northern latitudes will potentially affect forest production in the future. We studied gas exchange, chlorophyll content index, chlorophyll fluorescence, nutrient concentration and biomass accumulation in 1-year-old silver (Betula pendula Roth) and pubescent birch (Betula pubescens Ehrh.) seedlings. We hypothesized that B. pubescens has different physiological mechanisms that make it tolerate waterlogging better than B. pendula. The treatments were: (i) no waterlogging throughout the experiment; (ii) 4-week waterlogging during dormancy (dormancy waterlogging 'DW'); (iii) 4-week waterlogging during the early growing season (growth waterlogging 'GW'); and (iv) 4-week DW followed by 4-week GW during the early growing season ('DWGW'). Stomatal conductance and light-saturated net assimilation rate were reduced by GW in both species, and in B. pubescens also by DW. However, recovery was seen during the follow-up growing season. In B. pendula, DW, GW and DWGW temporarily resulted in reduced stem biomass, and GW and DWGW caused reduced leaf biomass. In B. pubescens, the stem biomass was decreased in GW and DWGW. Leaf nitrogen (N) and phosphorus (P) concentrations were generally low, and increased by GW, while potassium, calcium, magnesium and to some extent, boron and zinc concentrations decreased in both species and additionally manganese in B. pendula. The increases in N and P are mostly due to a concentration effect due to smaller leaf biomass, yet suggest that their uptake was not impaired. The decreases in cation concentrations are likely to be connected to impaired root functioning, which was not yet fully recovered from GW. We conclude that morphological acclimation to waterlogging of the leaves and roots rather than photosynthesis explains why B. pubescens is able to grow better in wetter areas than B. pendula.