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

Differential warming at crown scale impacts walnut primary growth onset and secondary growth rate

Trees are exposed to significant spatio-temporal thermal variations, which can induce intra-crown discrepancies in the onset and dynamics of primary and secondary growth. In recent decades, an increase in late winter and early spring temperatures has been observed, potentially accelerating bud break, cambial activation, and their coordination. Intra-crown temperature heterogeneities could lead to asymmetric tree shapes unless there is a compensatory mechanism at the crown level. An original warming experiment was conducted on young Juglans regia trees in a greenhouse. The average temperature difference during the day between warmed and control parts from February to August was 4 °C. The warming treatment advanced the date of budbreak significantly, by up to 14 d. Warming did not alter secondary growth resumption but increased growth rates, leading to higher xylem cell production (by 2-fold) and to an increase in radial increment (+80% compared with control). Meristem resumptions were asynchronous without coordination in response to temperature. Buds on warmed branches began to swell 2 weeks prior to cambial division, which was 1 week earlier than on control branches. A difference in carbon and water remobilization at the end of bud ecodormancy was noted under warming. Overall, our results argue for a lack of compensatory mechanisms at the crown scale, which may lead to significant changes in tree architecture in response to intra-crown temperature heterogeneities.

Non-linear response of Norway spruce to climate variation along elevational and age gradients in the Carpathians

Climate change, namely increased warming coupled with a rise in extreme events (e.g., droughts, storms, heatwaves), is negatively affecting forest ecosystems worldwide. In these ecosystems, growth dynamics and biomass accumulation are driven mainly by environmental constraints, inter-tree competition, and disturbance regimes. Usually, climate-growth relationships are assessed by linear correlation due to the simplicity and straightforwardness of modeling. However, applying this method may bias results, since the ecological and physiological responses of trees to environmental factors are non-linear, and usually bell-shaped. In the Eastern Carpathian, Norway spruce is at the southeasternmost edge of its natural occurrence; this region is thus potentially vulnerable to climate change. A non-linear assessment of climate-growth relationships using machine-learning techniques for Norway spruce in this area had not been conducted prior to this study. To address this knowledge gap, we analyzed a large tree-ring network from 158 stands, with over 3000 trees of varying age distributed along an elevational gradient. Our results showed that non-linearity in the growth-climate response of spruce was season-specific: temperatures from the previous autumn and current growing season, along with water availability during winter, induced a bell-shaped response. Moreover, we found that at low elevations, spruce growth was mainly limited by water availability in the growing season, while winter temperatures are likely to have had a slight influence along the entire elevational gradient. Furthermore, at elevations lower than 1400 m, spruce trees were also found to be sensitive to previous autumn water availability. Overall, our results shed new light on the response of Norway spruce to climate in the Carpathians, which may aid in management decisions.

Temporal and spatial variability of phloem structure in Picea abies and Fagus sylvatica and its link to climate

Using a unique 8-year data set (2010-2017) of phloem data, we studied the effect of temperature and precipitation on the phloem anatomy (conduit area, widths of ring, early and late phloem) and xylem-ring width in two coexisting temperate tree species, Picea abies and Fagus sylvatica, from three contrasting European temperate forest sites. Histometric analyses were performed on microcores taken from tree stems in autumn. We found high interannual variability and sensitivity of phloem anatomy and xylem-ring widths to precipitation and temperature; however, the responses were species- and site-specific. The contrasting response of xylem and phloem-ring widths of the same tree species to weather conditions was found at the two Slovenian sites generally well supplied with precipitation, while at the driest Czech site, the influence of weather factors on xylem and phloem ring widths was synchronised. Since widths of mean annual xylem and phloem increments were narrowest at the Czech site, this site is suggested to be most restrictive for the radial growth of both species. By influencing the seasonal patterns of xylem and phloem development, water availability appears to be the most important determinant of tissue- and species-specific responses to local weather conditions.

Incorporating high-resolution climate, remote sensing and topographic data to map annual forest growth in central and eastern Europe

To enhance our understanding of forest carbon sequestration, climate change mitigation and drought impact on forest ecosystems, the availability of high-resolution annual forest growth maps based on tree-ring width (TRW) would provide a significant advancement to the field. Site-specific characteristics, which can be approximated by high-resolution Earth observation by satellites (EOS), emerge as crucial drivers of forest growth, influencing how climate translates into tree growth. EOS provides information on surface reflectance related to forest characteristics and thus can potentially improve the accuracy of forest growth models based on TRW. Through the modelling of TRW using EOS, climate and topography data, we showed that species-specific models can explain up to 52 % of model variance (Quercus petraea), while combining different species results in relatively poor model performance (R2 = 13 %). The integration of EOS into models based solely on climate and elevation data improved the explained variance by 6 % on average. Leveraging these insights, we successfully generated a map of annual TRW for the year 2021. We employed the area of applicability (AOA) approach to delineate the range in which our models are deemed valid. The calculated AOA for the established forest-type models was 73 % of the study region, indicating robust spatial applicability. Notably, unreliable predictions predominantly occurred in the climate margins of our dataset. In conclusion, our large-scale assessment underscores the efficacy of combining climate, EOS and topographic data to develop robust models for mapping annual TRW. This research not only fills a critical void in the current understanding of forest growth dynamics but also highlights the potential of integrated data sources for comprehensive ecosystem assessments.

Stem growth phenology, not canopy greening, constrains deciduous tree growth

Canopy phenology is a widely used proxy for deciduous forest growth with various applications in terrestrial ecosystem modeling. Its use relies on common assumptions that canopy greening and stem growth are tightly coordinated processes, enabling predictions on the timing and the quantity of annual tree growth. Here, we present parallel observations of canopy and stem growth phenology and annual stem increment in around 90 deciduous forest trees with diffuse-porous (Fagus sylvatica, Acer pseudoplatanus, Carpinus betulus) or ring-porous (Quercus robur × petraea) wood anatomy. These data were collected in a mixed temperate forest at the Swiss-Canopy-Crane II site, in 4 years with strongly contrasting weather conditions. We found that stem growth resumption lagged several weeks behind spring canopy greening in diffuse-porous but not in ring-porous trees. Canopy greening and stem growth resumption showed no or only weak signs of temporal coordination across the observation years. Within the assessed species, the seasonal timing of stem growth varied strongly among individuals, as trees with high annual increments resumed growth earlier and also completed their main growth earlier. The length of main growth activity had no influence on annual increments. Our findings not only challenge tight temporal coordination of canopy and stem growth phenology but also demonstrate that longer main growth activity does not translate into higher annual increments. This may compromise approaches modeling tree growth and forest productivity with canopy phenology and growth length.

Mismatch between primary and secondary growth and its consequences on wood formation in Qinghai spruce

The connections between the primary and secondary growth of trees allows better understanding of the dynamics of carbon sequestration in forest ecosystems. The relationship between primary and secondary growth of trees could change due to the diverging responses of meristems to climate warming. In this study, the bud phenology and radial growth dynamics of Qinghai spruce (Picea crassifolia) in arid and semi-arid areas of China in 2019 and 2020 were weekly monitored to analyze their response to different weather conditions and their links with carbon sink. Xylem anatomical traits (i.e. lumen radial diameter and cell wall thickness) were quantified along cell radial files after the end of xylem lignification to calculate the early-to-latewood transition date. Winter and early spring (January-March) were warmer in 2020 with a colder April compared with 2019. Precipitation in April-June was lower in 2020 than in 2019. In 2019, bud phenology occurred earlier, while the onset of xylem formation and the early-to-latewood transition date were delayed. The duration from the beginning of split bud and exposed shoot to the early-to-latewood transition date was positively correlated with the radial width of earlywood (accounting for ~80% of xylem width) and total xylem width. The longer duration of xylem cell division did not increase xylem cell production and radial width. Moreover, the duration from bud burst to the early-to-latewood transition date in 2020 was negatively linked with early phloem cell production as compared with 2019. Our findings suggest that warm conditions in winter and early spring promote the xylogenesis of Qinghai spruce, but might delay bud burst. However, the xylem width increments largely depend on the duration from bud burst to the start of latewood cell division rather than on the earlier xylogenesis and longer duration of xylem cell differentiation induced by warm conditions.

INTRAGRO: A machine learning approach to predict future growth of trees under climate change

The escalating impact of climate change on global terrestrial ecosystems demands a robust prediction of the trees' growth patterns and physiological adaptation for sustainable forestry and successful conservation efforts. Understanding these dynamics at an intra-annual resolution can offer deeper insights into tree responses under various future climate scenarios. However, the existing approaches to infer cambial or leaf phenological change are mainly focused on certain climatic zones (such as higher latitudes) or species with foliage discolouration during the fall season. In this study, we demonstrated a novel approach (INTRAGRO) to combine intra-annual circumference records generated by dendrometers coupled to the output of climate models to predict future tree growth at intra-annual resolution using a series of supervised and unsupervised machine learning algorithms. INTRAGRO performed well using our dataset, that is dendrometer data of P. roxburghii Sarg. from the subtropical mid-elevation belt of Nepal, with robust test statistics. Our growth prediction shows enhanced tree growth at our study site for the middle and end of the 21st century. This result is remarkable since the predicted growing season by INTRAGRO is expected to shorten due to changes in seasonal precipitation. INTRAGRO's key advantage is the opportunity to analyse changes in trees' intra-annual growth dynamics on a global scale, regardless of the investigated tree species, regional climate and geographical conditions. Such information is important to assess tree species' growth performance and physiological adaptation to growing season change under different climate scenarios.

Priorities for Bark Anatomical Research: Study Venues and Open Questions

The bark fulfils several essential functions in vascular plants and yields a wealth of raw materials, but the understanding of bark structure and function strongly lags behind our knowledge with respect to other plant tissues. The recent technological advances in sampling and preparation of barks for anatomical studies, along with the establishment of an agreed bark terminology, paved the way for more bark anatomical research. Whilst datasets reveal bark's taxonomic and functional diversity in various ecosystems, a better understanding of the bark can advance the understanding of plants' physiological and environmental challenges and solutions. We propose a set of priorities for understanding and further developing bark anatomical studies, including periderm structure in woody plants, phloem phenology, methods in bark anatomy research, bark functional ecology, relationships between bark macroscopic appearance, and its microscopic structure and discuss how to achieve these ambitious goals.

Inter-annual and inter-species tree growth explained by phenology of xylogenesis

Wood formation determines major long-term carbon (C) accumulation in trees and therefore provides a crucial ecosystem service in mitigating climate change. Nevertheless, we lack understanding of how species with contrasting wood anatomical types differ with respect to phenology and environmental controls on wood formation. In this study, we investigated the seasonality and rates of radial growth and their relationships with climatic factors, and the seasonal variations of stem nonstructural carbohydrates (NSC) in three species with contrasting wood anatomical types (red oak: ring-porous; red maple: diffuse-porous; white pine: coniferous) in a temperate mixed forest during 2017-2019. We found that the high ring width variability observed in both red oak and red maple was caused more by changes in growth duration than growth rate. Seasonal radial growth patterns did not vary following transient environmental factors for all three species. Both angiosperm species showed higher concentrations and lower inter-annual fluctuations of NSC than the coniferous species. Inter-annual variability of ring width varied by species with contrasting wood anatomical types. Due to the high dependence of annual ring width on growth duration, our study highlights the critical importance of xylem formation phenology for understanding and modelling the dynamics of wood formation.