Cambial Phenology Research Articles

Research progress on cambium activity and radial growth dynamics monitoring of coniferous species

The radial growth of trees plays a crucial role in determining forest carbon sequestration capacity. Understanding the growth dynamics of trees and their response to environmental factors is essential for predicting forest's carbon sink potential under future climate change. Coniferous forest trees are particularly sensitive to climate change, with growth dynamics responding rapidly to environmental shifts. We collected and analyzed data from 99 papers published between 1975 and 2023, and examined the effects of exogenous factors (such as temperature, water, and photoperiod) and endogenous factors (including tree age and species) on cambial activity and radial growth in conifers. We further explored the mechanisms underlying these effects. The results showed that climate warming had the potential to advance the onset while delayed the end of xylem differentiation stages in conifers in temperate and boreal regions. Water availability played a crucial role in regulating the timing of cambial phenology and wood formation by influencing water potential and cell turgor. Additionally, the photoperiod not only participated in regulating the start and end times of growth, but also influenced the timing of maximum growth rate occurrence. Future climate warming was expected to extend the growing season, leading to increase in growth of conifers in boreal regions and expanding forests to higher altitudes or latitudes. However, changes in precipitation patterns and increased evapotranspiration resulting from temperature increases might advance the end of growing season and reduce growth rate in arid areas. To gain a more comprehensive understanding of the relationship between radial growth and climatic factors, it is necessary to develop process-based models to elucidate the physiological mechanisms underlying wood formation and the response of trees to climatic factors.

Process-Based Modeling of Phenology and Radial Growth in Pinus tabuliformis in Response to Climate Factors over a Cold and Semi-Arid Region.

(1) Background: Climate change significantly impacts the phenology and dynamics of radial tree growth in alpine dryland forests. However, there remains a scarcity of reliable information on the physiological processes of tree growth and cambial phenology in response to long-term climate change in cold and semi-arid regions. (2) Methods: We employed the process-based Vaganov-Shashkin (VS) model to simulate the phenology and growth patterns of Chinese pine (Pinus tabuliformis) in the eastern Qilian Mountains, northeastern Tibetan Plateau. The model was informed by observed temperature and precipitation data to elucidate the relationships between climate factors and tree growth. (3) Results: The simulated tree-ring index closely aligned with the observed tree-ring chronology, validating the VS model's effectiveness in capturing the climatic influences on radial growth and cambial phenology of P. tabuliformis. The model outputs revealed that the average growing season spanned from mid-April to mid-October and experienced an extension post-1978 due to ongoing warming trends. However, it is important to note that an increase in the duration of the growing season did not necessarily result in a higher level of radial growth. (4) Conclusions: While the duration of the growing season was primarily determined by temperature, the growth rate was predominantly influenced by water conditions during the growing season, making it the most significant factor contributing to ring formation. Our study provides valuable insights into the potential mechanisms underlying tree growth responses to climate change in cold and semi-arid regions.

The intra-annual rhythm of Pinus sylvestris growth-climate responses under a warming climate at its southern distribution limits

Warming climate has posed a pressing threat on boreal forests and an improved understanding of the intra-annual pattern of climatic influences on the tree growth can help interpret the response of boreal forest to climate change. Here, we systematically examined the growth-climate relationship, cambium phenology, and xylem cell dynamics of Pinus sylvestris (PS) to disentangle its intra-annual growth rhythm to external drivers and internal physiological process at its southern distribution limits across Eurasia. We showed the intra-annual rhythm of PS to temperature and precipitation is anti-phase and synchronous at its southern limits. Temperature had both promoting and inhibiting effects on PS growth over growing season where the growth-temperature response gradually changed from positive in winter-early spring to negative in late spring-summer, and then returned to positive in autumn. Precipitation enhanced PS growth in late spring-summer. Temperature dominated the intra-annual growth-climate response rhythm, and the response shifted to negative when temperature reached 13.72 °C in spring-summer and shifted to positive when temperature reached 17.41 °C in summer-autumn in Shenyang, respectively. A high water demand for the rapid earlywood cell formation in spring-summer and heat requirement for cambial cell division in summer-autumn caused the shifts in growth-temperature response, respectively. Climatic warming advances and prolongs the time of summer water availability that limits PS growth at its southern distribution limits and thus warming climate may pose a greater threat to the southern population of PS.

Spring wood phenology responds more strongly to chilling temperatures than bud phenology in European conifers.

A comparative assessment of bud and wood phenology could aid a better understanding of tree growth dynamics. However, the reason for asynchronism or synchronism in leaf and cambial phenology remains unclear. To test the assumption that the temporal relationship between the budburst date and the onset date of wood formation is due to their common or different responses to environmental factors, we constructed a wood phenology dataset from previous literature, and compared it with an existing bud phenology dataset in Europe. We selected three common conifers (Larix decidua Mill., Picea abies (L.) H. Karst. and Pinus sylvestris L.) in both datasets and analyzed 909 records of the onset of wood formation at 47 sites and 238,720 records of budburst date at 3051 sites. We quantified chilling accumulation (CA) and forcing requirement (FR) of budburst and onset of wood formation based on common measures of CA and FR. We then constructed negative exponential CA-FR curves for bud and wood phenology separately. The results showed that the median, variance and probability distribution of CA-FR curves varied significantly between bud and wood phenology for three conifers. The different FR under the same chilling condition caused asynchronous bud and wood phenology. Furthermore, the CA-FR curves manifested that wood phenology was more sensitive to chilling than bud phenology. Thus, the FR of the onset of wood formation increases more than that of budburst under the same warming scenarios, explaining the stronger earlier trends in the budburst date than the onset date of woody formation simulated by the process-based model. Our work not only provides a possible explanation for asynchronous bud and wood phenology from the perspective of organ-specific responses to chilling and forcing, but also develops a phenological model for predicting both bud and wood phenology with acceptable uncertainties.

Site and provenance interact to influence seasonal diameter growth of Pseudotsuga menziesii

Phenology of diameter growth in trees has been studied for many years but generally using a limited number of sites and genotypes. In this project provenances of Douglas-fir (Pseudotsuga menziesii var. menziesii) planted across a wide range of environments were used to evaluate diameter growth and phenology to an extreme heat event and to seasonal conditions. Sampling was done in nine common gardens in western Oregon and Washington, USA that included a coastal site, a low elevation interior site, and high elevation interior site on three latitude bands. In 2021, three provenances at four sites were monitored for late-season diameter growth following an extreme heat event in June. In 2022, five provenances were evaluated for full-seasonal diameter growth at four growth intervals across all nine sites. Growth after the 2021 extreme heat event varied by planting site, with the greatest growth observed at mild (cooler/wetter) sites and the least growth observed at arid (hotter/drier) sites. Comparisons of diameter growth over the late season period (i.e., July–October) showed that 2022 growth was 1.25-fold to 3.70-fold higher, presumably due to premature growth cessation in 2021. In 2022, diameter growth rates showed significant variation by site and provenance; the interaction between site and provenance was significant in all but the first growth interval. Intra-seasonal growth rates were not consistent across sites; there was substantial diameter growth early in the season on sites with continental climates, little diameter growth by any provenance after early August at three high elevation sites, and most sources continued growth later in the season at coastal sites. One source differed more in phenology across sites than the others, apparently due to its susceptibility to a needle-cast disease. Cambial phenology varied with genetic and site factors that control the timing of growth and intrinsic growth rates. In addition, the interaction between genotype and site resulted in some provenances performing better than others on some sites as a function of disease susceptibility or response to environmental factors. Understanding phenology of seed sources under a range of site conditions should help predict diameter growth under different climate conditions in the future.

Warming-induced phenological mismatch between trees and shrubs explains high-elevation forest expansion.

Despite the importance of species interaction in modulating the range shifts of plants, little is known about the responses of coexisting life forms to a warmer climate. Here, we combine long-term monitoring of cambial phenology in sympatric trees and shrubs at two treelines of the Tibetan Plateau, with a meta-analysis of ring-width series from 344 shrubs and 575 trees paired across 11 alpine treelines in the Northern Hemisphere. Under a spring warming of +1°C, xylem resumption advances by 2-4 days in trees, but delays by 3-8 days in shrubs. The divergent phenological response to warming was due to shrubs being 3.2 times more sensitive than trees to chilling accumulation. Warmer winters increased the thermal requirement for cambial reactivation in shrubs, leading to a delayed response to warmer springs. Our meta-analysis confirmed such a mechanism across continental scales. The warming-induced phenological mismatch may give a competitive advantage to trees over shrubs, which would provide a new explanation for increasing alpine treeline shifts under the context of climate change.

Interaction of age and elevation on xylogenesis in Juniperus przewalskii in a cold and arid region

Exogenous factors (such as temperature and water availability) as well as endogenous factors (such as tree age) have effects on cambial phenology and intra-annual tree growt dynamics. However, how these factors interactively influence this physiological process remains unclear. Here, we compared biweekly cambial activity, timing and duration of xylogenesis in adult (< 250 yr) and old (> 400 yr) trees of Juniperus przewalskii Kom. along an altitudinal gradient on the northeastern Tibetan Plateau. We found that the onset of cell production was 10–11 days earlier in adult than in old trees, both at high and low elevations. However, the end of cell production was mainly correlated with water availability varying with elevation, regardless of age. In addition to earlier growth onset, adult trees were characterized by a longer growing season and higher growth rates, resulting in a higher number of xylem cells. We found that at relatively wet high elevations, tree age exerts a more dominant control on cell production, whereas at relatively dry low elevations, the effect of tree age on tree growth became weak or negligible. Our study demonstrates that age interacts with environmental constraints to determine intra-annual growth dynamics in this cold and arid region.

Cambial phenology and wood formation of Korean pine in response to climate change in Changbai Mountain, Northeast China

Korean pine (Pinus koraiensis Sieb. et Zucc.) is a highly ecological and economic species due to its rarity, varied usage and favorable effect on the environment. To better understand the adaptation strategies of intra-annual radial growth of Korean pine to local environmental regimes, we continuously monitored the cambial activity and wood formation in northeastern China weekly from April to October across the two consecutive years of 2020 and 2021 using Trephor. The results indicated that the warmer spring in 2021 resulted in an earlier onset of cambial activity compared to that of 2020 (DOY 118 vs DOY129), but the cessation of wood formation was similar (DOY 250 vs DOY 248). In both years, similar thresholds were estimated using logistic regression, xylem growth was active when the daily mean air temperature was 11.5 °C. Sensitivity analysis revealed that 86% of xylem growth was attributed to the growth rate and 14% to its duration. The total number of xylem cells was positively correlated with air temperature, soil temperature and relative humidity, and negatively correlated with vapor pressure deficit and soil water content in both years. Temperature is the main factor limiting the growth of Korean pine. Our findings complement a description of cambial phenology and wood formation dynamics of Korean pine at cellular levels and provide a better understanding of the climate-growth relationship, which will enable the improvement of climate-growth models used to predict climate change.

Response of model-based cambium phenology and climatic factors to tree growth in the Altai Mountains, Central Asia

• The principal growth limiting factor shifts from temperature to soil moisture from RU to FY. • Cambial dynamic was bimodal pattern in AL, right-skewed unimodal in FY and unimodal in RU. • The length of growing seasons had a significant positive trends in two sites from Northwest China – Chinese Altai Mountains for the last six decades. • The intra-seasonal growth rate regulated tree growth rather than length of growing season. It is important to study the relationships between tree growth and phenology and climate factors in Central Asia for the sustainable development of forest ecology and the exploration of carbon sink potential in the region in a context of global change. Herein, the process-based Vaganov-Shashkin model was used to simulate tree-ring growth rates, to assess a cambial phenology (start and end of growing season) of larch trees ( Larix sibirica ) based on three sites located on three different latitudes in the Altai-Sayan Mountains in Central Asia over 1960–2018. It was shown that the principal factor limiting growth rates was temperature in RU (located in southern central Siberia), cambial dynamic was unimodal. In RU site, although the length of growing season did not change significantly over the past 59 years, it was correlated with tree radial growth. Moisture was the principal factor limiting the growth rates of AL and FY (located in Chinese Altai Mountains), and cambium dynamics were bimodal and unimodal, respectively. Temperature and precipitation before the start of the growing season lengthen the growing season, but this did not promote the tree radial growth. In conclusion, we determined that the climate-influenced intra-seasonal growth rate regulated tree growth rather than length of growing season in this area. In this study, the VS-simulation showed high potential develop a long-term reconstruction of cambial phenology in the Altai Mountains, which was conducive to a more comprehensive understanding of the exogenous-endogenous factors effects on tree-ring growth.

Establishing a baseline to monitor future climate-change-effects on peripheral populations of Abies alba (Mill.) in central Apennines

Understanding tree species responses to climate change is crucial for preserving biodiversity especially in Southern Europe hot spots where Abies alba is widely spread. Three Apennine silver fir populations, Pigelleto (PIG), La Verna (LV) and Bocca Trabaria (BT), ensured gene flows in interglacial periods between the two phylogenetically different groups of northern and southern Apennines. These stands were analysed (nuclear and chloroplast SSRs) with the aim to establish a baseline for their future management in view of the expected changes. The three forests were tested for the Centre-Periphery Hypothesis (CPH) compared to forty-five Italian populations. At the same time, permanent areas were surveyed within LV and PIG on dominant (a) and dominated or natural regeneration (r) tree layers, and on age classes. In two consecutive years, spring cambial phenology activity was also weekly monitored on microcores, and critical phenology dates recorded. The stands matched CPH only partially, showing different phylogenetic history and their bridging between northern and southern groups of silver fir populations was confirmed. LV was distinct from PIG and BT. The within-population variance component was significantly high, and no narrow relatedness was observed between dominant and dominated/regeneration spatially closer trees, and genetic parameters were comparable in both layers at LV and PIG. In both stands, older age classes ensured natural regeneration. Cambium phenology was highly variable within populations, consistently to other Mediterranean conifers, and highly sensitive to local and year’s conditions and monitoring will improve population’s adaptive capacity detection. Shelterwood-system silvicultural treatments are suggested on small areas to drive the demographic and panmictic balance towards an uneven-aged more resilient structure, and iterated monitoring will help to adapt the forest management to the isotherm shift.

Localized stem heating from the rest to growth phase induces latewood-like cell formation and slower stem radial growth in Norway spruce saplings.

Recent climate projections predict a more rapid increase of winter temperature than summer and global temperature averages in temperate and cold environments. As there is relatively little experimental knowledge on the effect of winter warming on cambium phenology and stem growth in species growing in cold environments, the setting of manipulative experiments is considered of primary importance, and they can help to decipher the effect of reduced winter chilling and increased forcing temperatures on cambium reactivation, growth and xylem traits. In this study, localized stem heating was applied to investigate the effect of warming from the rest to the growth phase on cambium phenology, intra-annual stem growth dynamics and ring wood features in Picea abies (L.) H.Karst. We hypothesized that reduced winter chilling induces a postponed cambium dormancy release and decrease of stem growth, while high temperature during cell wall lignification determines an enrichment of latewood-like cells. The heating device was designed to maintain a +5°C temperature delta with respect to air temperature, thus allowing an authentic scenario of warming. Continuous stem heating from the rest (November) to the growing phase determined, at the beginning of radial growth, a reduction of the number of cell layers in the cambium, higher number of cell layers in the wall thickening phase and an asynchronous stem radial growth when comparing heated and ambient saplings. Nevertheless, heating did not induce changes in the number of produced cell layers at the end of the growing season. The analyses of two-photon fluorescence images showed that woody rings formed during heating were enriched with latewood-like cells. Our results showed that an increase of 5°C of temperature applied to the stem from the rest to growth might not influence, as generally reported, onset of cambial activity, but it could affect xylem morphology of Norway spruce in mountain environments.

A Bimodal Pattern and Age-Related Growth of Intra-Annual Wood Cell Development of Chinese Fir in Subtropical China.

Age plays an important role in regulating the intra-annual changes in wood cell development. Investigating the effect of age on intra-annual wood cell development would help to understand cambial phenology and xylem formation dynamics of trees and predict the growth of trees accurately. Five intermediate trees in each stand (total of 5 stands) in five age groupings of Chinese fir (Cunninghamia lanceolata Hook.) plantations in subtropical China were monitored on micro-cores collected weekly or biweekly from January to December in 2019. We modeled the dynamics of wood cell development with a mixed effects model, analyzed the age effect on intra-annual wood cell development, and explored the contribution of rate and duration of wood cell development on intra-annual wood cell development. We found a bimodal pattern of wood cell development in all age classes, and no matter the date of peak or the maximal number of cells the bimodal patterns were similar in all age classes. In addition, compared with the older trees, the younger trees had the longest duration of wood cell development because of the later end of wood cell development and a larger number of wood cells. The younger trees had the faster growth rate than the older trees, but the date of the maximal growth rate in older trees was earlier than younger trees, which led to the production of more wood cells in the younger trees. Moreover, we found that the number of cells in wood cell formation was mostly affected by the rate (92%) rather than the duration (8%) of wood cell formation.

Variation in Onset of Leaf Unfolding and Wood Formation in a Central African Tropical Tree Species

A diversity of phenological strategies has been reported for tropical tree species. Defoliation and seasonal dormancy of cambial activity inform us on how trees cope with water stress during the dry season, or maximize the use of resources during the rainy season. Here, we study the matching between leaf phenology (unfolding and shedding) and cambial activity for Prioria balsamifera, a key timber species in the Democratic Republic of Congo. In particular, we (i) evaluated the seasonality of cambial activity and synchrony of phenology among trees in response to climate and (ii) identified the seasonality of leaf phenology and its relation with cambial phenology. The study was conducted in the Luki Man and Biosphere Reserve, located in the Mayombe forest at the southern margin of the Congo Basin. Historic defoliation data were collected every ten days using weekly crown observations whereas recent observations involved time-lapse cameras. Cambial pinning was performed on ten trees during 20 months and radius dendrometers were installed on three trees during 13 months. Tree rings were measured on cores from 13 trees and growth synchrony was evaluated. We found that P. balsamifera defoliates annually with a peak observed at the end of the dry season and the beginning of the rainy season. The new leaves unfolded shortly after shedding of the old leaves. The peak defoliation dates varied across years from September 12 to November 14 and the fraction of number of trees that defoliated at a given time was found to be negatively correlated with annual rainfall and temperature; during the dry season, when precipitation and temperatures are the lowest. Wood formation (radial growth), was found to be highly seasonal, with cambial dormancy occurring during the dry season and growth starting at the beginning of the rainy season. Individual ring-width series did not cross date well. The within species variability of leaf phenology and cambial rhythms provides indication about resistance of the population against climatic changes.

Research progress on cambial activity of trees and the influencing factors

Tree growth is the main way of carbon sequestration in forest ecosystems, which is influenced by climatic and non-climatic factors. The long-term location monitoring of cambial phenology and wood formation dynamics (xylogenesis) is an important method to clarify the responses of radial growth to climate change. Here, we reviewed studies on cambial phenology and xylogenesis by microcoring method. Firstly, we reviewed the effects of climatic factors on cambial phenology. The onset and cessation of xylogenesis were determined by temperature in cold and humid conditions. Temperature and water availability collectively modulated the onset of xylogenesis under dry conditions, and the later determined the end of xylogenesis. The radial increment was regulated by rate and duration of cell production, with the maximum of growth rate occurring around the summer solstice. Short-term N addition did not affect wood formation dynamics. Secondly, we reviewed the roles of biological factors in regulating xylogenesis. The onset of xylogenesis differred among species, ages, and inter-specific competition. Seasonal dynamics of non-structural carbohydrates were coupled with wood formation. Finally, we reviewed the response mechanisms of xylogenesis to the interaction of climatic and biological factors. In conclusion, we put forward problems in current studies and prospected future development to provide reference for further scientific research.

Effects of moisture and heat coupling on xylem growth of Pinus tabuliformis in Shenyang, China

Based on the micro-core technology, we can accurately examine the tree cambium phenology and the radial growth at the cellular scale, and reveal the relationship between tree growth and climate. Pinus tabuliformis is one of the constructive species in forests of northern China. We investigated the growth pattern of P. tabuliformis in Shenyang and the changes in cambium and xylem cells throughout the growing season (April to November) in 2020. Results showed that the dividing activity in cambium started in early April and ended at the end of September. Xylem began to grow from the appearance of enlargement cells (mid April) to the end of the disappearance of lignified cells (late October), with a growth trajectory of 'S' shape curve. Approximately 53 xylem cells per row in radical direction were produced in 2020. The maximum growth rate (0.55 ind/row/day) occurred at the end of May, while the change of earlywood and latewood cells occurred at the end of July. When the minimum temperature above 0 ℃ in Shenyang, the cambium began to divide. The minimum critical temperature that affected the beginning and ending of xylem growth was 2-3 ℃. Precipitation promoted the growth in the growing season. The high temperature and insufficient water supply at the end of July were the main factors driving the differentiation of xylem cells to form earlywood and latewood.

Climatic regulation of leaf and cambial phenology in Quercus pubescens: Their interlinkage and impact on xylem and phloem conduits

Increased frequency and severity of stressful events affects the growth patterns and functioning of trees which adjust their phenology to given conditions. Here, we analysed environmental effects (temperature, precipitation, VPD and SWC) on the timing of leaf phenology, seasonal stem radial growth patterns, and xylem and phloem anatomy of Quercus pubescens in the sub-Mediterranean in the period 2014–2019, when various adverse weather events occurred, i.e. spring drought in 2015, summer fire in 2016 and summer drought in 2017. Results showed that the timings of leaf and cambium phenology do not occur simultaneously in Q. pubescens, reflecting different environmental and internal constraints. Although year-to-year variability in the timings of leaf and cambial phenology exists, their chronological sequence is fairly fixed. Different effects of weather conditions on different stages of leaf development in spring were observed. Common climatic drivers (i.e., negative effect of hot and dry summers and a positive effect of increasing moisture availability in winter and summer) were found to affect the widths of xylem and phloem increments with more pronounced effect on late formed parts. A legacy effect of the timing of leaf and cambial phenology of the previous growing season on the timing of phenology of the following spring was confirmed. Rarely available phloem data permitted a comprehensive insight into the interlinkage of the timing of cambium and leaf phenology and adjustment strategies of vascular tissues in Mediterranean pubescent oak to various environmental constraints, including frequent extreme events (drought, fire). Our results suggest that predicted changes in autumn/winter and spring climatic conditions for this area could affect the timings of leaf and stem cambial phenology of Q. pubescens in the coming years, which would affect stem xylem and phloem structure and hydraulic properties, and ultimately its performance.

Growing faster, longer or both? Modelling plastic response of Juniperus communis growth phenology to climate change

AbstractAimPlant growth and phenology respond plastically to changing climatic conditions in both space and time. Species‐specific levels of growth plasticity determine biogeographical patterns and the adaptive capacity of species to climate change. However, a direct assessment of spatial and temporal variability in radial growth dynamics is complicated, because long records of cambial phenology do not exist.LocationSixteen sites across European distribution margins of Juniperus communis L. (the Mediterranean, the Arctic, the Alps and the Urals).Time period1940–2016.Major taxa studiedJuniperus communis.MethodsWe applied the Vaganov–Shashkin process‐based model of wood formation to estimate trends in growing season duration and growth kinetics since 1940. We assumed that J. communis would exhibit spatially and temporally variable growth patterns reflecting local climatic conditions.ResultsOur simulations indicate regional differences in growth dynamics and plastic responses to climate warming. The mean growing season duration is the longest at Mediterranean sites and, recently, there has been a significant trend towards its extension of up to 0.44 days/year. However, this stimulating effect of a longer growing season is counteracted by declining summer growth rates caused by amplified drought stress. Consequently, overall trends in simulated ring widths are marginal in the Mediterranean. In contrast, durations of growing seasons in the Arctic show lower and mostly non‐significant trends. However, spring and summer growth rates follow increasing temperatures, leading to a growth increase of up to 0.32 %/year.Main conclusionsThis study highlights the plasticity in growth phenology of widely distributed shrubs to climate warming: an earlier onset of cambial activity that offsets the negative effects of summer droughts in the Mediterranean and, conversely, an intensification of growth rates during the short growing seasons in the Arctic. Such plastic growth responsiveness allows woody plants to adapt to the local pace of climate change.

Warming menaces high-altitude Himalayan birch forests: Evidence from cambial phenology and wood anatomy

The Himalayas are experiencing dramatic warming and drought events, which occur at a faster rate than the global average. How will the high-elevation ecosystems cope with such changing conditions? This study aims to combine cambial phenology with quantitative wood anatomy to understand how the growth dynamics affect the trade-off between safety and efficiency of water transport in the Himalayan forests. We investigated the timing of xylogenesis and its relationship with hydraulic wood anatomical traits in Himalayan birch (Betula utilis D. Don) at two altitudes in the central Himalayas. Xylogenesis was monitored weekly from April to September 2014 by collecting microcores. We measured vessel area on histological sections, calculated the hydraulic diameter, and assessed the effects of climate on xylem production rate. Xylogenesis occurred from June to September in both study sites. Moisture availability influenced positively xylem growth besides thermal conditions. The onset and duration of vessel enlargement were correlated with the hydraulic diameter. Trees with earlier onsets and longer durations of vessel enlargement exhibited larger hydraulic diameters. The close relationship between cambial phenology and hydraulic diameter suggests that earlier growth resumption induced by climate changes may result in larger vessels, which are more efficient in water transport, but vulnerable to drought-induced cavitation. The ongoing warming and drying climate conditions may menace the survival of birch forest in the central Himalayas.

Different Wood Anatomical and Growth Responses in European Beech (Fagus sylvatica L.) at Three Forest Sites in Slovenia.

European beech (Fagus sylvatica L.) adapts to local growing conditions to enhance its performance. In response to variations in climatic conditions, beech trees adjust leaf phenology, cambial phenology, and wood formation patterns, which result in different tree-ring widths (TRWs) and wood anatomy. Chronologies of tree ring width and vessel features [i.e., mean vessel area (MVA), vessel density (VD), and relative conductive area (RCTA)] were produced for the 1960–2016 period for three sites that differ in climatic regimes and spring leaf phenology (two early- and one late-flushing populations). These data were used to investigate long-term relationships between climatic conditions and anatomical features of four quarters of tree-rings at annual and intra-annual scales. In addition, we investigated how TRW and vessel features adjust in response to extreme weather events (i.e., summer drought). We found significant differences in TRW, VD, and RCTA among the selected sites. Precipitation and maximum temperature before and during the growing season were the most important climatic factors affecting TRW and vessel characteristics. We confirmed differences in climate-growth relationships between the selected sites, late flushing beech population at Idrija showing the least pronounced response to climate. MVA was the only vessel trait that showed no relationship with TRW or other vessel features. The relationship between MVA and climatic factors evaluated at intra-annual scale indicated that vessel area in the first quarter of tree-ring were mainly influenced by climatic conditions in the previous growing season, while vessel area in the second to fourth quarters of tree ring width was mainly influenced by maximum temperature and precipitation in the current growing season. When comparing wet and dry years, beech from all sites showed a similar response, with reduced TRW and changes in intra-annual variation in vessel area. Our findings suggest that changes in temperature and precipitation regimes as predicted by most climate change scenarios will affect tree-ring increments and wood structure in beech, yet the response between sites or populations may differ.

Transition Dates from Earlywood to Latewood and Early Phloem to Late Phloem in Norway Spruce

Climate change will affect radial growth patterns of trees, which will result in different forest productivity, wood properties, and timber quality. While many studies have been published on xylem phenology and anatomy lately, little is known about the phenology of earlywood and latewood formation, also in relation to cambial phenology. Even less information is available for phloem. Here, we examined year-to-year variability of the transition dates from earlywood to latewood and from early phloem to late phloem in Norway spruce (Picea abies) from three temperate sites, two in Slovenia and one in the Czech Republic. Data on xylem and phloem formation were collected during 2009–2011. Sensitivity analysis was performed to determine the specific contribution of growth rate and duration on wood and phloem production, separately for early and late formed parts. We found significant differences in the transition date from earlywood to latewood between the selected sites, but not between growth seasons in trees from the same site. It occurred in the first week of July at PAN and MEN and more than two weeks later at RAJ. The duration of earlywood formation was longer than that of latewood formation; from 31.4 days at PAN to 61.3 days at RAJ. In phloem, we found differences in transition date from early phloem to late phloem also between the analysed growth seasons; from 2.5 weeks at PAN to 4 weeks at RAJ Compared to the transition from earlywood to latewood the transition from early phloem to late phloem occurred 25–64 days earlier. There was no significant relationship between the onset of cambial cell production and the transition dates. The findings are important to better understand the inter-annual variability of these phenological events in spruce from three contrasting temperate sites, and how it is reflected in xylem and phloem anatomy.