Tuula Jyske

Woody biomass production lags stem-girth increase by over one month in coniferous forests

Wood is the main terrestrial biotic reservoir for long-term carbon sequestration(1), and its formation in trees consumes around 15% of anthropogenic carbon dioxide emissions each year(2). However, the seasonal dynamics of woody biomass production cannot be quantified from eddy covariance or satellite observations. As such, our understanding of this key carbon cycle component, and its sensitivity to climate, remains limited. Here, we present high-resolution cellular based measurements of wood formation dynamics in three coniferous forest sites in northeastern France, performed over a period of 3 years. We show that stem woody biomass production lags behind stem-girth increase by over 1 month. We also analyse more general phenological observations of xylem tissue formation in Northern Hemisphere forests and find similar time lags in boreal, temperate, subalpine and Mediterranean forests. These time lags question the extension of the equivalence between stem size increase and woody biomass production to intra-annual time scales(3, 4, 5, 6). They also suggest that these two growth processes exhibit differential sensitivities to local environmental conditions. Indeed, in the well-watered French sites the seasonal dynamics of stem-girth increase matched the photoperiod cycle, whereas those of woody biomass production closely followed the seasonal course of temperature. We suggest that forecasted changes in the annual cycle of climatic factors(7) may shift the phase timing of stem size increase and woody biomass production in the future.

The effect of artificially induced drought on radial increment and wood properties of Norway spruce

We studied experimentally the effects of water availability on height and radial increment as well as wood density and tracheid properties of Norway spruce (Picea abies (L.) Karst.). The study was carried out in two long-term N-fertilization experiments in Southern Finland (Heinola and Sahalahti). At each site, one fertilized and one control plot was covered with an under-canopy roof preventing rainwater from reaching the soil. Two uncovered plots were monitored at each site. The drought treatment was initiated in the beginning of growing season and lasted for 60-75 days each year. The treatment was repeated for four to five consecutive years depending on the site. Altogether, 40 sample trees were harvested and discs sampled at breast height. From the discs, ring width and wood density were measured by X-ray densitometry. Tracheid properties were analysed by reflected-light microscopy and image analysis. Reduced soil water potential during the growing season decreased annual radial and height increment and had a small influence on tracheid properties and wood density. No statistically significant differences were found in the average tracheid diameter between the drought-treated and control trees. The average cell wall thickness was somewhat higher (7-10%) for the drought treatment than for the control, but the difference was statistically significant only in Sahalahti. An increased cell wall thickness was found in both early- and latewood tracheids, but the increase was much greater in latewood. In drought-treated trees, cell wall proportion within an annual ring increased, consequently increasing wood density. No interaction between the N fertilization and drought treatment was found in wood density. After the termination of the drought treatment, trees rapidly recovered from the drought stress. According to our results, severe drought due to the predicted climate change may reduce Norway spruce growth but is unlikely to result in large changes in wood properties.

Comparison of phloem and xylem hydraulic architecture in Picea abies stems

The hydraulic properties of xylem and phloem differ but the magnitude and functional consequences of the differences are not well understood. Phloem and xylem functional areas, hydraulic conduit diameters and conduit frequency along the stems of Picea abies trees were measured and expressed as allometric functions of stem diameter and distance from stem apex. Conductivities of phloem and xylem were estimated from these scaling relations. Compared with xylem, phloem conduits were smaller and occupied a slightly larger fraction of conducting tissue area. Ten times more xylem than phloem was annually produced along the stem. Scaling of the conduit diameters and cross-sectional areas with stem diameter were very similar in phloem and xylem. Phloem and xylem conduits scaled also similarly with distance from stem apex; widening downwards from the tree top, and reaching a plateau near the base of the living crown. Phloem conductivity was estimated to scale similarly to the conductivity of the outermost xylem ring, with the ratio of phloem to xylem conductivity being c. 2%. However, xylem conductivity was estimated to increase more than phloem conductivity with increasing tree dimensions as a result of accumulation of xylem sapwood. Phloem partly compensated for its smaller conducting area and narrower conduits by having a slightly higher conduit frequency.

The effects of artificial soil frost on cambial activity and xylem formation in Norway spruce

We studied the effects of artificial soil frost on cambial activity and xylem formation on 47-year-old Norway spruce [Picea abies (L.) Karst.] trees grown on medium fertile site type (with moraine soil) in eastern Finland (62°42′N; 29°45′E). Different soil frost treatments applied were: (1) natural snow accumulation and melting (control, CTRL); (2) artificial removal of snow from soil surface during two consecutive winters (OPEN); and (3) snow clearing and insulation (FROST), which was in other ways similar to OPEN, but the ground was insulated in early spring to delay soil thawing. Each treatment was replicated in three blocks, and two sample trees in each plot were repeatedly microcored during growing seasons of 2006–2007 for the analysis of the onset, cessation and the duration of xylem formation. The phases of tracheid differentiation (tracheids in radial enlargement, secondary cell wall formation, and mature tracheids) were measured from the microcores of 2007. The intra-ring growth and wood density variables were analysed based on X-ray densitometry. In FROST in 2006, xylem formation started a week later than in the other treatments. In 2007, no difference was found between the treatments. The discrepancy in results between the two study years may be explained by between-years variation in weather, i.e., the winter was colder in 2005/2006 than in 2006/2007. No effects of soil frost treatments on tracheid differentiation and on most of the intra-ring growth and density variables were discovered. Our results suggest that the delayed thawing of moraine soil may slightly affect the onset, timing and duration of xylem formation in Norway spruce. However, the effects of delayed soil frost may depend also on the soil type and become more evident with increasing water holding capacity of the soil.

Variation of tracheid length within annual rings of Scots pine and Norway spruce

Abstract Variation of tracheid length was studied within individual annual rings of Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.) from the pith to the bark. The material consisted of six Scots pine and six Norway spruce trees growing on sites of both low and high fertility. Microtome sections of 0.25 mm thick were cut from annual rings 7, 20 and 50 counted from the pith outwards, i.e., juvenile, transition and mature wood, respectively. After maceration, tracheid lengths were separately measured in each sample. In juvenile wood of Scots pine, tracheids were on average 17% longer in the latewood than in earlywood. However, in juvenile wood, the first formed earlywood tracheids were slightly longer than those in the middle of the earlywood zone. In the transition and mature wood of Scots pine, the increase in tracheid length was more gradual from earlywood to latewood, and no significant differences were found between earlywood and latewood. In Norway spruce, tracheids were 2–4% longer in the latewood than in earlywood. In general, tracheid length is highly variable within annual rings and the variation can differ from ring-to-ring even within the same tree.

Seasonal variation in formation, structure, and chemical properties of phloem in Picea abies as studied by novel microtechniques

AbstractMain conclusionPhloem production and structural development were interlinked with seasonal variation in the primary and secondary metabolites of phloem. Novel microtechniques provided new perspectives on understanding phloem structure and chemistry. To gain new insights into phloem formation in Norway spruce (Picea abies), we monitored phloem cell production and seasonal variation in the primary and secondary metabolites of inner bark (non-structural carbohydrates and phenolic stilbene glucosides) during the 2012 growing season in southern and northern Finland. The structure of developing phloem was visualised in 3D by synchrotron X-ray microtomography. The chemical features of developing phloem tissues isolated by laser microdissection were analysed by chemical microanalysis. Within-year phloem formation was associated with seasonal changes in non-structural carbohydrates and phenolic extractive contents of inner bark. The onset of phloem cell production occurred in early and mid-May in southern and northern Finland, respectively. The maximal rate of phloem production and formation of a tangential band of axial phloem parenchyma occurred in mid-June, when total non-structural carbohydrates peaked (due to the high amount of starch). In contrast, soluble sugar content dropped during the most active growth period and increased in late summer and winter. The 3D visualisation showed that the new axial parenchyma clearly enlarged from June to August. Sub-cellular changes appeared to be associated with accumulation of stilbene glucosides and soluble sugars in the newest phloem. Stilbene glucosides also increased in inner bark during late summer and winter. Our findings may indicate that stilbene biosynthesis in older phloem predominantly occurs after the formation of the new band(s) of axial parenchyma. The complementary use of novel microtechniques provides new perspectives on the formation, structure, and chemistry of phloem.

Intra-annual tracheid formation of Norway spruce provenances in southern Finland

We studied the intra-annual wood formation in a Norway spruce provenance experiment in southern Finland from 2004–2008. Two Finnish provenances, northern and southern, as well as German and Hungarian provenances were included. Timing of tracheid formation and differentiation, and tracheid dimensions were determined from periodically extracted microcores. The aim was to determine the differences between the years and provenances in the timing of the xylogenesis and in the xylem characteristics. Year-to-year variation was high both in timing of tracheid formation and xylem characteristics, while between-provenance differences were small. The onset of tracheid formation varied from early May to late June in different trees in different years. The onset of tracheid formation was not closely related to the annual variations of temperature sum. In all the years, daily temperatures exceeded the threshold +5°C for several weeks before the onset of tracheid formation. The highest tracheid formation rate occurred after the summer solstice in all years and generally coincided with the highest daily temperatures during the growing season. Tracheid production ceased early in 2006 due to a mid-summer drought. Cell differentiation continued late in autumn as non-mature tracheids were still observed around mid-September. No clear differences between the provenances in the timing of tracheid formation were observed, although the Finnish provenances tended to initiate tracheid formation slightly earlier than the other provenances. The tree-ring widths of the Finnish provenances were also wider, while tracheid diameter of the German provenance was slightly smaller. Our results indicate that between-tree variation in the timing of wood formation is high compared with the latitude effect of seed source.

Osmolality and Non-Structural Carbohydrate Composition in the Secondary Phloem of Trees across a Latitudinal Gradient in Europe.

Phloem osmolality and its components are involved in basic cell metabolism, cell growth, and in various physiological processes including the ability of living cells to withstand drought and frost. Osmolality and sugar composition responses to environmental stresses have been extensively studied for leaves, but less for the secondary phloem of plant stems and branches. Leaf osmotic concentration and the share of pinitol and raffinose among soluble sugars increase with increasing drought or cold stress, and osmotic concentration is adjusted with osmoregulation. We hypothesize that similar responses occur in the secondary phloem of branches. We collected living bark samples from branches of adult Pinus sylvestris, Picea abies, Betula pendula and Populus tremula trees across Europe, from boreal Northern Finland to Mediterranean Portugal. In all studied species, the observed variation in phloem osmolality was mainly driven by variation in phloem water content, while tissue solute content was rather constant across regions. Osmoregulation, in which osmolality is controlled by variable tissue solute content, was stronger for Betula and Populus in comparison to the evergreen conifers. Osmolality was lowest in mid-latitude region, and from there increased by 37% toward northern Europe and 38% toward southern Europe due to low phloem water content in these regions. The ratio of raffinose to all soluble sugars was negligible at mid-latitudes and increased toward north and south, reflecting its role in cold and drought tolerance. For pinitol, another sugar known for contributing to stress tolerance, no such latitudinal pattern was observed. The proportion of sucrose was remarkably low and that of hexoses (i.e., glucose and fructose) high at mid-latitudes. The ratio of starch to all non-structural carbohydrates increased toward the northern latitudes in agreement with the build-up of osmotically inactive C reservoir that can be converted into soluble sugars during winter acclimation in these cold regions. Present results for the secondary phloem of trees suggest that adjustment with tissue water content plays an important role in osmolality dynamics. Furthermore, trees acclimated to dry and cold climate showed high phloem osmolality and raffinose proportion.

Effects of nutrient optimization on intra-annual wood formation in Norway spruce

In the Nordic countries, growth of Norway spruce (Picea abies (L.) Karst.) is generally limited by low availability of nutrients, especially nitrogen. Optimizing forest management requires better insight on how growth responds to the environmental conditions and their manipulation. The aim of this study was to analyse the effects of nutrient optimization on timing and the rate of tracheid formation of Norway spruce and to follow the differentiation of newly formed tracheids. The study was performed during two growing seasons in a long-term nutrient optimization experiment in northern Sweden, where all essential macro- and micronutrients were supplied in irrigation water every second day from mid-June to mid-August. The control plots were without additional nutrients and water. Tracheid formation in the stem was monitored throughout the growing season by weekly sampling of microcores at breast height. The onset of xylogenesis occurred in early June, but in early summer there were no significant between-treatment differences in the onset and relative rate of tracheid formation. In both treatments, the onset of secondary cell wall formation occurred in mid-June. The maximum rate of tracheid formation occurred close to the summer solstice and 50% of the tracheids had been accumulated in early July. Optimized nutrition resulted in the formation of ∼50% more tracheids and delayed the cessation of tracheid formation, which extended the tracheid formation period by 20-50%, compared with control trees. The increased growth was mainly an effect of enhanced tracheid formation rate during the mid- and later-part of the growing season. In the second year, the increased growth rate also resulted in 11% wider tracheids. We conclude that the onset and rate of tracheid formation and differentiation during summer is primarily controlled by photoperiod, temperature and availability of nutrients, rather than supply of carbohydrates.

Gradients and dynamics of inner bark and needle osmotic potentials in Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies L. Karst)

Preconditions of phloem transport in conifers are relatively unknown. We studied the variation of needle and inner bark axial osmotic gradients and xylem water potential in Scots pine and Norway spruce by measuring needle and inner bark osmolality in saplings and mature trees over several periods within a growing season. The needle and inner bark osmolality was strongly related to xylem water potential in all studied trees. Sugar concentrations were measured in Scots pine, and they had similar dynamics to inner bark osmolality. The sucrose quantity remained fairly constant over time and position, whereas the other sugars exhibited a larger change with time and position. A small osmotic gradient existed from branch to stem base under pre-dawn conditions, and the osmotic gradient between upper stem and stem base was close to zero. The turgor in branches was significantly driven by xylem water potential, and the turgor loss point in branches was relatively close to daily minimum needle water potentials typically reported for Scots pine. Our results imply that xylem water potential considerably impacts the turgor pressure gradient driving phloem transport and that gravitation has a relatively large role in phloem transport in the stems of mature Scots pine trees.