The resurgent frequency of extreme weather events and their strongly distinctive spatial patterns lead to a growing interest in phenology as an indicator of tree susceptibility. Using a long-term chronology of observations collected in situ, we predicted and investigated the spatial patterns and environmental drivers of spring leaf phenology across maple stand polygons dominated by Acer saccharum Marsh. and/or Acer rubra L. in eastern North America for 2000–2018. Model’ calibration was based on Bayesian ordinal regressions relating the timing of the phenological events' observations to the MODIS vegetation indices EVI, NDVI and LAI. DAYMET data have been extracted to compute temperature and precipitation during spring phenology. Model accuracy increased as the season progressed, with prediction uncertainty spanning from 9 days for bud swelling to 4 days for leaf unfolding. NDVI and LAI were the best predictors for the onset and ending of spring phenology, respectively. Bud swelling occurred at the end of March in the early stands and at the onset of May in the late stands, while leaf unfolding was completed at the beginning of April for the early and in mid-June for the late stands. Early and late stands polarized towards a south-west–north-east gradient. In the south-western regions, which are also the driest, total precipitation and minimum temperature explained respectively 73 % and 25 % of the duration of spring phenology. In the north-eastern regions, precipitation and minimum temperature explained 62 % and 26 % of the duration of spring phenology. Our results suggest high vulnerability to extreme weather events in stands located in the south-west of the species distribution. The increasing incidence of drought in these locations might affect spring phenology, decreasing net primary production in these stands. Warmer nights might expose the buds to late frosts, events that are expected to become more frequent in the coming years.

Wood density is the product of carbon allocation for structural growth and reflects the trade-off between mechanical support and water conductivity. We tested a conceptual framework based on the assumption that micro-density depends on direct and indirect relationships with endogenous and exogenous factors. The dynamics of wood formation, including timings and rates of cell division, cell enlargement, and secondary wall deposition, were assessed from microcores collected weekly between 2002 and 2016 from five black spruce stands located along a latitudinal gradient in Quebec, Canada. Cell anatomy and micro-density were recorded by anatomical analyses and X-ray measurements. Our structural equation model explained 80% of micro-density variation within the tree-ring with direct effects of wall thickness (σ = 0.61), cell diameter (σ = −0.51), and photoperiod (σ = −0.26). Wood formation dynamics had an indirect effect on micro-density. Micro-density increased under longer periods of cell-wall deposition and shorter durations of enlargement. Our results fill a critical gap in understanding the relationships underlying micro-density variation in conifers. We demonstrated that short-term responses to environmental variations could be overridden by plastic responses that modulate cell differentiation. Our results point to wood formation dynamics as a reliable predictor of carbon allocation in trees. © 2021 The Authors New Phytologist

Under climate change, the increasing occurrence of late frost combined with advancing spring phenology can increase the risk of frost damage in trees. In this study, we tested the link between intra-specific variability in bud phenology and frost exposure and damages. We analysed the effects of the 2021 late frost event in a black spruce (Picea mariana (Mill.) BSP) common garden in Québec, Canada. We hypothesised that the timing of budbreak drives the exposure of vulnerable tissues and explains differences in frost damage. Budbreak was monitored from 2015 to 2021 in 371 trees from five provenances originating between 48° and 53° N and planted in a common garden at 48° N. Frost damages were assessed on the same trees through the proportion of damaged buds per tree and related to the phenological phases by ordinal regressions. After an unusually warm spring, minimum temperatures fell to −1.9°C on May 28 and 29, 2021. At this moment, trees from the northern provenances were more advanced in their phenology and showed more frost damage. Provenances with earlier budbreak had a higher probability of damage occurrence according to ordinal regression. Our study highlights the importance of intra-specific variability of phenological traits on the risk of frost exposure. We provide evidence that the timings of bud phenology affect sensitivity to frost, leading to damages at temperatures of −1.9°C. Under the same conditions, the earlier growth reactivation observed in the northern provenances increases the risks of late frost damage on the developing buds.

In boreal ecosystems, phenological events display seasonal patterns. These patterns allow for the development of tissues during the short time window available for growth in cold climates. Primary and secondary growth, two expensive processes for plants, are supposedly modulated in time to optimize allocation of carbon to bud and woody tissues. We aimed to assess the phenology of primary and secondary meristems, testing their relationship over the closed black spruce stands of the commercially exploited forest region in Quebec, Canada.

Location

Quebec, Canada.

Time period

2002–2016.

Major taxa studied

Gymnospermae.

Methods

We combined weekly scaled field observations with Moderate Resolution Imaging Spectroradiometer (MODIS) time series of the normalized difference vegetation index (NDVI) to extract timings of photosynthesis and meristem growth in five black spruce [Picea mariana (Mill.) B.S.P.] stands located along a latitudinal gradient and to assess their relationship. We then tested empirical models based on geographical position and seasonal temperatures to predict wood phenology (i.e., the onset and ending of earlywood and latewood formation), and compared its spatial patterns with existing predictions of bud phenology for the same study area.

Results

Photosynthesis started at the beginning of May, 3 weeks before bud reactivation and the onset of wood growth. Latewood formation started in mid-July, after shoot elongation was completed. For wood phenology models, the residual standard error ranged from 1 week to 12 days. Growth dynamics spatialized across the boreal forest of Quebec varied with the transition between the subarctic and humid continental climate.

Main conclusions

Shoot elongation and latewood formation were temporally separated, providing evidence of a trade-off in structural carbon allocation between primary and secondary growth in trees. Spatial patterns of wood phenology predicted for the black spruce polygons are consistent with spatial patterns of bud phenology, demonstrating synchronized temporal dynamics of meristems at the regional scale.

New insights into the intra-annual dynamics of tree-ring formation can improve our understanding of tree-growth response to environmental conditions at high-resolution time scales. Obtaining this information requires, however, a weekly monitoring of wood formation, sampling that is extremely time-intensive and scarcely feasible over vast areas. Estimating the timing of cambial and xylem differentiation by modeling thus represents an interesting alternative for obtaining this important information by other means. Temporal dynamics of cambial divisions can be extracted from the daily tree-ring growth rate computed by the Vaganov–Shashkin (VS) simulation model, assuming that cell production is tightly linked to tree-ring growth. Nonetheless, these predictions have yet to be compared with direct observations of wood development, i.e., via microcoring, over a long time span. We tested the performance of the VS model by comparing the observed and predicted timing of wood formation in black spruce [Picea mariana (Mill.)]. We obtained microcores over 15 years at 5 sites along a latitudinal gradient in Quebec (Canada). The measured variables included cell size and the timing of cell production and differentiation. We calibrated the VS model using daily temperature and precipitation recorded by weather stations located on each site. The predicted and observed timing of cambial and enlarging cells were highly correlated (R² = 0.8); nonetheless, we detected a systematic overestimation in the predicted timing of cambial cells, with predictions delayed by 1–20 days compared with observations. The growth rate of cell diameter was correlated with the predicted growth rate assigned to each cambial cell, confirming that cell diameter developmental dynamics have the potential to be inferred by the tree-ring growth curve of the VS model. Model performances decrease substantially in estimating the end of wood formation. The systematic errors suggest that the actual relationships implemented in the model are unable to explain the phenological events in autumn. The mismatch between the observed and predicted timing of wood formation in black spruce within our study area can be reduced by better adapting the VS model to wet sites, a context for which this model has been rarely used.

One of the key issues of the distribution of tree species is their ability to track environmental changes. European beech (Fagus sylvatica L.) is a species highly sensitive to extreme climatic events, because of its high phenotypic plasticity. In this study, we aim to determine the variability in stem size between and within marginal beech populations. Marginal populations of beech growing under uniform environmental conditions of provenance trial offer unique opportunity to detect adaptive differentiations driven by natural selection. In this work, we studied stem size variation recorded by automatic band dendrometers in four beech marginal populations growing in a common garden in the south-eastern distribution range of beech in Poland over the period 2016–2018. Strong climatic effects and weak provenance differences in seasonal stem size variation were observed. The provenances exhibited similar climate-related seasonal stem circumference variation. A high within-provenance variation was confirmed. Temperature of spring as well as temperature and precipitation of autumn were detected as key climatic parameters mostly for onset and end of stem size variation. Maximum stem size was mostly affected by the later end of its variation, which positively affected its duration. Climatic distance between beech provenances and provenance trial had a negligible effect on the variability in seasonal stem size variation between provenances. The evidence of weak inter-provenance and high intra-provenance variation in stem size changes observed in the south-eastern distribution range indicates that an individual-based approach could be a suitable strategy, when selecting for phenotypic plasticity.

Tree rings are the result of the seasonal activity of the vascular cambium, the secondary meristem of woody angiosperms and gymnosperms. During tree-ring formation, a combination of endogenous and environmental factors affects cambial division, cell differentiation and maturation; this, in turn, affects wood quantity and quality. Among the endogenous factors affecting tree-ring formation, plant hormones are recognized as determinant players in regulating many aspects of the features and fate of each xylem cell. Most of our knowledge regarding the roles of plant hormones on tree-ring formation comes from herbaceous plants, although an increasing interest involves the analysis of the hormonal patterns in tree-species. This paper reviews the state of knowledge of the role of plant hormones during tree-ring formation by focusing on experiments performed on woody species. An overview of the main plant hormones and their main activities during radial tree growth will be followed by discussion of their role in each tree-ring developmental stage and in the overall tree-ring seasonal pattern.

Bud and leaf development are important phenological events and help in defining the growing period of trees. Canopy greenness derived from PhenoCam has been used to investigate leaf phenology. Questions remain on how much the continuous records of canopy greenness represent bud developmental phases, and how growing period boundaries are related to canopy greenness and bud phenology. In this study, we compared bud phenology of black spruce [Picea mariana (Mill.) B.S.P] during 2015, 2017 and 2018 with the canopy greenness, represented by Green Chromatic Coordinate (GCC), derived from PhenoCam images of a boreal stand in Quebec, Canada. Logit models were applied to estimate the probability of observing sequential phenological phases of bud burst and bud set along with GCC. GCC showed a bell-shaped pattern, with a slow increase in spring, a peak in summer and a gradual decrease in autumn. The start and end of budburst, and bud set, occurred when GCC reached 72% and 92% (spring), and 94% (autumn) of its maximum amplitude, respectively. These GCC values are reliable thresholds indicating the growing period boundaries. Our study builds a bridge between phenological observations and automatic near-surface remote sensing, providing a statistically sound protocol for calibrating PhenoCam with field observations.

BACKGROUND AND AIMS: Secondary growth is a process related to the formation of new cells that increase in size and wall thickness during xylogenesis. Temporal dynamics of wood formation influence cell traits, in turn affecting cell patterns across the tree ring. We verified the hypothesis that cell diameter and cell wall thickness are positively correlated with the duration of their differentiation phases.

METHODS: Histological sections were produced by microcores to assess the periods of cell differentiation in black spruce [Picea mariana (Mill.) B.S.P.]. Samples were collected weekly between 2002 and 2016 from a total of 50 trees in five sites along a latitudinal gradient in Quebec (Canada). The intra-annual temporal dynamics of cell differentiation were estimated at a daily scale, and the relationships between cell traits and duration of differentiation were fitted using a modified von Bertalanffy growth equation.

KEY RESULTS: At all sites, larger cell diameters and cell wall thicknesses were observed in cells that experienced a longer period of differentiation. The relationship was a non-linear, decreasing trend that occasionally resulted in a clear asymptote. Overall, secondary wall deposition lasted longer than cell enlargement. Earlywood cells underwent an enlargement phase that lasted for 12 d on average, while secondary wall thickness lasted 15 d. Enlargement in latewood cells averaged 7 d and secondary wall deposition occurred over an average of 27 d.

CONCLUSIONS: Cell size across the tree ring is closely connected to the temporal dynamics of cell formation. Similar relationships were observed among the five study sites, indicating shared xylem formation dynamics across the entire latitudinal distribution of the species.The duration of cell differentiation is a key factor involved in cell growth and wall thickening of xylem, thereby determining the spatial variation of cell traits across the tree ring.