Katarina Čufar

Old World megadroughts and pluvials during the Common Era

An atlas of megadroughts in Europe and in the Mediterranean Basin during the Common Era provides insights into climate variability. Climate model projections suggest widespread drying in the Mediterranean Basin and wetting in Fennoscandia in the coming decades largely as a consequence of greenhouse gas forcing of climate. To place these and other “Old World” climate projections into historical perspective based on more complete estimates of natural hydroclimatic variability, we have developed the “Old World Drought Atlas” (OWDA), a set of year-to-year maps of tree-ring reconstructed summer wetness and dryness over Europe and the Mediterranean Basin during the Common Era. The OWDA matches historical accounts of severe drought and wetness with a spatial completeness not previously available. In addition, megadroughts reconstructed over north-central Europe in the 11th and mid-15th centuries reinforce other evidence from North America and Asia that droughts were more severe, extensive, and prolonged over Northern Hemisphere land areas before the 20th century, with an inadequate understanding of their causes. The OWDA provides new data to determine the causes of Old World drought and wetness and attribute past climate variability to forced and/or internal variability.

Seasonal Dynamics of Wood Formation in Pinus Halepensis from Dry and Semi-Arid Ecosystems in Spain

The seasonal dynamics of cambial activity, wood formation, occurrence of false rings (FR), and tangential bands of resin canals (RC) were investigated in Pinus halepensis from three Mediterranean dry and semi-arid ecosystems in Spain. We collected intact tissues of phloem, cambium, and outer xylem at monthly intervals throughout 2004 from each time six trees at the three sites.Cell divisions in the cambium in all trees started before our first sampling in mid-March and ceased between November and December. Cambial activity was characterized by two maxima; one in spring and another in autumn.Trees still grew in summer but at a very low rate. The first solitary RC were formed in May or June and tangential bands of RC in June or July. In general, tangential bands of RC were observed in wider growth rings. The formation of tangential bands of RC seems to be induced by drought in the second half of the growing season. FR were formed as a result of cambial reactivation in autumn and were observed in growth rings that contained more than 50 cells in a radial row. It appears that summer drought and early autumn precipitation play an important role in false-ring formation.

Regular cambial activity and xylem and phloem formation in locally heated and cooled stem portions of Norway spruce

The effect of heating (23–25°C) and cooling (9–11°C) on regular cambial activity and xylem and phloem formation in the stem portion of Norway spruce was investigated. Adult trees were sampled at 21-day intervals during the 2005 vegetation period. Continuously elevated temperatures increased the rate of cell division in the first part of the growing season, but did not significantly prolong cambial activity at the end of the vegetation period in the heated tree. Low-temperature treatments shortened regular cambial activity and slowed down cell production. The xylem growth ring was wider in the heated sample and narrower in the cooled sample compared to the control. The temperature in the cambial region was only negligibly transferred along the stem from the site of its application. In general, the temperature in the cambium was affected by a long-term rise or drop in air temperatures. Both experiments affected the structure and width of phloem growth increments. The tangential band of the axial parenchyma was not continuous in the cooled sample. The number of late phloem cells was reduced in the cool-treated sample and increased in the heat-treated sample. Our experiments confirmed the effect of constantly increased or decreased temperatures on regular cambial activity in Norway spruce.

Tree-ring variation, wood formation and phenology of beech (Fagus sylvatica) from a representative site in Slovenia, SE Central Europe

Long-term variation in tree-ring widths (1873–2006) and intra-annual dynamics of cambial activity and tree-ring formation in 2006 were studied in mature beech (Fagus sylvatica L.) trees at a typical forest site near Ljubljana (46°N, 14°40′E, 400 m a.s.l.) and related to leaf phenology and climate data. Tree-ring widths were negatively affected by minimum March and maximum August temperatures and favoured by May and July precipitation. Precipitation of the previous August and temperature of the previous November also had a positive effect. Leaf unfolding was affected by March and April temperatures, occurring later if they were low. Leaf yellowing was positively affected by minimum July temperatures and negatively by September precipitation. In 2006, leaf unfolding occurred on 16 April and was immediately followed by reactivation of cambium at breast height of the trees. One week later, the cambium obtained its maximum width (around 11 cell layers) and the rate of division increased until the end of May/beginning of June. By the end of June, 75% of the tree-ring was formed. Cambial cell divisions stopped from the end of July to mid-August. The average time of cambial activity was 100 days. Leaf yellowing occurred at the end of October, i.e. nearly 2 months after the cessation of cambial cell division. We discuss the usefulness of a combination of long-term (tree-ring width and phenology) and short-term (wood formation at a cellular level) data to understand better the environmental signals registered by a tree during growth.

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.

Age dependence of xylogenesis and its climatic sensitivity in Smith fir on the south-eastern Tibetan Plateau.

An age effect on growth trends and climate/growth relationships of trees can possibly be discovered by analysing the seasonal dynamics of xylem development. The aims of this study, therefore, were to compare xylem formation of young (43 ± 4 years) and old (162 ± 26 years) Smith fir (Abies georgei var. smithii (Viguie & Gaussen) W. C. Cheng & L. K. Fu) trees in the Sygera Mountains, south-eastern Tibetan Plateau and, to identify the association between wood formation and climate. The seasonal radial growth dynamics of young and old trees was monitored on microcores collected at weekly intervals during two growing seasons. Transverse sections through phloem, cambium and outermost xylem of 9-12 μ m thickness were observed with a light microscope under bright field and polarized light to follow the cambial activity and differentiation of the developing xylem. Young trees were characterized by an earlier onset of xylogenesis, a longer growing season and a higher growth rate, resulting in a higher number of xylem cells. Both young and old trees responded fast to changes of the minimum air temperature, confirming that this factor was dominant by controlling Smith fir growth on the south-eastern Tibetan Plateau.

Size mediated climate–growth relationships in Pinus halepensis and Pinus pinea

Functional processes in trees undergo changes as the tree size increases, which may affect the response of trees to environmental factors. We tested tree-ring response to climate in four groups of trees, including large and small Pinus halepensis and Pinus pinea trees using cluster, principal component (PC) and dendroclimatological analysis. The trees were of the same age and growing on a plantation in the semiarid coastal area of southern Spain. Cluster and PC analyses showed a clear separation into four groups of trees. Autocorrelation and mean sensitivity showed significant differences between the two size classes. PCA recognised four representative principal components where PC1 represented the tree-ring—climate variability common to both species and sizes, whereas PC2, PC3 and PC4 represented a species-specific and size-dependent response of trees to climate. The differences between the two size classes were greater than those between the two species. The results suggest that future tree-ring studies should include trees stratified by size. Only this would make it possible to produce unbiased predictions on the consequences of climate change and to devise suitable mitigation strategies for preserving Mediterranean forest ecosystems.

A meta-analysis of cambium phenology and growth: linear and non-linear patterns in conifers of the northern hemisphere.

BACKGROUND AND AIMS Ongoing global warming has been implicated in shifting phenological patterns such as the timing and duration of the growing season across a wide variety of ecosystems. Linear models are routinely used to extrapolate these observed shifts in phenology into the future and to estimate changes in associated ecosystem properties such as net primary productivity. Yet, in nature, linear relationships may be special cases. Biological processes frequently follow more complex, non-linear patterns according to limiting factors that generate shifts and discontinuities, or contain thresholds beyond which responses change abruptly. This study investigates to what extent cambium phenology is associated with xylem growth and differentiation across conifer species of the northern hemisphere. METHODS Xylem cell production is compared with the periods of cambial activity and cell differentiation assessed on a weekly time scale on histological sections of cambium and wood tissue collected from the stems of nine species in Canada and Europe over 1-9 years per site from 1998 to 2011. KEY RESULTS The dynamics of xylogenesis were surprisingly homogeneous among conifer species, although dispersions from the average were obviously observed. Within the range analysed, the relationships between the phenological timings were linear, with several slopes showing values close to or not statistically different from 1. The relationships between the phenological timings and cell production were distinctly non-linear, and involved an exponential pattern. CONCLUSIONS The trees adjust their phenological timings according to linear patterns. Thus, shifts of one phenological phase are associated with synchronous and comparable shifts of the successive phases. However, small increases in the duration of xylogenesis could correspond to a substantial increase in cell production. The findings suggest that the length of the growing season and the resulting amount of growth could respond differently to changes in environmental conditions.

Plasticity in Dendroclimatic Response across the Distribution Range of Aleppo Pine (Pinus halepensis)

We investigated the variability of the climate-growth relationship of Aleppo pine across its distribution range in the Mediterranean Basin. We constructed a network of tree-ring index chronologies from 63 sites across the region. Correlation function analysis identified the relationships of tree-ring index to climate factors for each site. We also estimated the dominant climatic gradients of the region using principal component analysis of monthly, seasonal, and annual mean temperature and total precipitation from 1,068 climatic gridpoints. Variation in ring width index was primarily related to precipitation and secondarily to temperature. However, we found that the dendroclimatic relationship depended on the position of the site along the climatic gradient. In the southern part of the distribution range, where temperature was generally higher and precipitation lower than the regional average, reduced growth was also associated with warm and dry conditions. In the northern part, where the average temperature was lower and the precipitation more abundant than the regional average, reduced growth was associated with cool conditions. Thus, our study highlights the substantial plasticity of Aleppo pine in response to different climatic conditions. These results do not resolve the source of response variability as being due to either genetic variation in provenance, to phenotypic plasticity, or a combination of factors. However, as current growth responses to inter-annual climate variability vary spatially across existing climate gradients, future climate-growth relationships will also likely be determined by differential adaptation and/or acclimation responses to spatial climatic variation. The contribution of local adaptation and/or phenotypic plasticity across populations to the persistence of species under global warming could be decisive for prediction of climate change impacts across populations. In this sense, a more complex forest dynamics modeling approach that includes the contribution of genetic variation and phenotypic plasticity can improve the reliability of the ecological inferences derived from the climate-growth relationships.

Number of Cells in Xylem, Phloem and Dormant Cambium in Silver Fir (Abies Alba), in Trees of Different Vitality

We investigated the anatomical structure of phloem and xylem growth rings as well as the dormant cambium in relation to vitality in 81 adult silver fir trees (Abies alba Mill.). Specifically, we investigated the number of cells produced in the current phloem growth ring (PR), xylem growth ring (XR) and their ratio, the number of cells in the dormant cambium (CC), and the structure of the PR, which included characterisation of the early phloem (EP), the late phloem (LP), and the presence, absence, and continuity of tangential bands of axial parenchyma (AP). EP was relatively stable with respect to number and types of cells as PR width changed, but LP was quite variable. The CC of more vital trees produced more xylem than phloem cells. The ratio of XR to PR number decreased with decreasing vitality of trees and in the most severely affected trees (4% of the study group), more cells were formed in the PR than in the XR. The number of cells in phloem, xylem and dormant cambium is positively correlated. The use of width and structure of phloem and the ratios between PR, XR and CC can provide information on tree conditions and, consequently, can be a useful tool for forest management.