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Key message The 2024 ENSO event advanced the timing of spring phenological phases of native shrubs significantly more than non-native shrubs and native trees in a temperate deciduous woodland fragment in Wisconsin, USA. This suggests that, as spring temperatures warm, shrubs will likely play a pivotal role in forest dynamics including contributing to an earlier onset to the growing period and an early start to CO 2 assimilation. Context The 2023/2024 El Niño Southern Oscillation (ENSO) event brought warmer than average temperatures to the Midwest USA. This presented a unique opportunity to examine how short-term warming might impact the phenology of temperate deciduous forest vegetation. Aim To quantify the impact of an ENSO-driven warm spring on the phenology of temperate deciduous forest vegetation in order to assess how trees and shrubs respond to short-term temperature anomalies. Methods Spring phenology was recorded twice weekly (2018–2024) on 5 dominant tree species and 5 native and 4 non-native shrub species, in a woodland fragment on the University of Wisconsin Milwaukee campus. In addition, phenological transition dates were extracted from daily Green Chromatic Coordinate (GCC) data from a PhenoCam installed at the site. Results In 2024, the average spring (March–May) temperature (8.7 ± 0.57 °C) was significantly warmer than the 2018–2023 average (6.7 ± 0.28 °C). Compared to the average of the previous 6 years, the timing of budburst in 2024 occurred significantly ( p  < 0.001) earlier on DOY 76, 82, and 98 for native shrubs, non-native shrubs, and trees, representing advances of 20, 17, and 18 days, respectively. The advance was greater in shrubs than trees suggesting that any future advance to the start of the growing-season in temperate deciduous forests resulting from warmer spring temperatures will likely be driven by early leafing species like shrubs. Notably, the rise in GCC in 2024 (DOY 116) occurred following budburst, indicating that PhenoCam imagery may not fully capture early vegetation phenology. Conclusion Early leafing shrubs, and in particular native species, were more sensitive to warmer temperatures early in the season than non-native shrubs and native trees. Therefore, as temperatures warm in the future, the onset of growth in temperate deciduous forests is likely to be driven by the early spring phenophases of early leafing species.

The timing and duration of autumn leaf phenology marks important transitions in temperate deciduous forests, such as, start of senescence, declining productivity and changing nutrient cycling. Phenological research on temperate deciduous forests typically focuses on upper canopy trees, overlooking the contribution of other plant functional groups like shrubs. Yet shrubs tend to remain green longer than trees, while non-native shrubs, in particular, tend to exhibit an extended growing season that confers a competitive advantage over native shrubs. We monitored leaf senescence and leaf fall (2017–2020) of trees and shrubs (native and non-native) in an urban woodland fragment in Wisconsin, USA. Our findings revealed that, the start of leaf senescence did not differ significantly between vegetation groups, but leaf fall started (DOY 273) two weeks later in shrubs. Non-native shrubs exhibited a considerably delayed start (DOY 262) and end of leaf senescence (DOY 300), with leaf-fall ending (DOY 315) nearly four weeks later than native shrubs and trees. Overall, the duration of the autumn phenological season was longer for non-native shrubs than either native shrubs or trees. Comparison of the timing of spring phenophases with the start and end of leaf senescence revealed that when spring phenology in trees starts later in the season senescence also starts later and ends earlier. The opposite pattern was observed in native shrubs. In conclusion, understanding the contributions of plant functional groups to overall forest phenology requires future investigation to ensure accurate predictions of future ecosystem productivity and help address discrepancies with remote sensing phenometrics.

Phenological research in temperate-deciduous forests typically focuses on upper canopy trees, due to their overwhelming influence on ecosystem productivity and function. However, considering that shrubs leaf out earlier and remain green longer than trees, they play a pivotal role in ecosystem productivity, particularly at growing season extremes. Furthermore, an extended growing season of non-native shrubs provides a competitive advantage over natives. Here, we report spring phenology, budburst, leaf-out, and full-leaf unfolded (2017–2021) of a range of co-occurring species of tree (ash, American basswood, red oak, white oak, and boxelder) and shrub (native species: chokecherry, pagoda dogwood, nannyberry, American wild currant and Eastern wahoo, and non-native species: buckthorn, honeysuckle, European privet, and European highbush cranberry) in an urban woodland fragment in Wisconsin, USA, to determine how phenology differed between plant groups. Our findings show that all three spring phenophases of shrubs were 3 weeks earlier (p < 0.05) than trees. However, differences between shrubs groups were only significant for the later phenophase; full-leaf unfolded, which was 6 days earlier (p < 0.05) for native shrubs. The duration of the spring phenological season was 2 weeks longer (p < 0.05) for shrubs than trees. These preliminary findings demonstrate that native shrubs, at this site, start full-leaf development earlier than non-native species suggesting that species composition must be considered when generalizing whether phenologies differ between vegetation groups. A longer time series would be necessary to determine future implications on ecosystem phenology and productivity and how this might impact forests in the future, in terms of species composition, carbon sequestration, and overall ecosystem dynamics.

Understanding the relationship between leaf phenology and physiological properties has important implications for improving ecosystem models of biogeochemical cycling. However, previous studies have investigated such relationships only at the ecosystem level, limiting the biological interpretation and application of the observed relationships due to the complex vegetation structure of forest ecosystems. Additionally, studies focusing on transpiration are generally limited compared to those on photosynthesis. Thus, we investigated the relationship between stem sap flux density (SFD) and crown leaf phenology at the individual tree level using the heat dissipation method, unmanned aerial vehicle (UAV)-based observation, and ground-based visual observation across 17 species in a cool temperate forest in Japan, and assessed the potential of UAV-derived phenological metrics to track individual tree-level sap flow phenology. We computed five leaf phenological metrics (four from UAV imagery and one from ground observations) and evaluated the consistency of seasonality between the phenological metrics and SFD using Bayesian modelling. Although seasonal trajectories of the leaf phenological metrics differed markedly among the species, the daytime total SFD (SFDday) estimated by the phenological metrics was significantly correlated with the measured ones across the species, irrespective of the type of metric. Crown leaf cover derived from ground observations (CLCground) showed the highest ability to predict SFDday, suggesting that the seasonality of leaf amount rather than leaf color plays a predominant role in sap flow phenology in this ecosystem. Among the UAV metrics, Hue had a superior ability to predict SFDday compared with the other metrics because it showed seasonality similar to CLCground. However, all leaf phenological metrics showed earlier spring increases than did sap flow in more than half of the individuals. Our study revealed that UAV metrics could be used as predictors of sap flow phenology for deciduous species in cool, temperate forests. However, for a more accurate prediction, phenological metrics representing the spring development of sap flow must be explored.

Autumn phenology, determined mainly by temperature and photoperiod, is essential for ecosystem carbon sequestration. Usually, the variations in the maximum rate of Rubisco (Vcmax) and the maximum rate of ribulose-bisphosphate regeneration (Jmax) are taken as the mechanism regulating the seasonal pattern of photosynthetic rates and autumn phenology. In this study, we used Quercus mongolicus seedlings as an example to examine the photosynthetically physiological mechanism of leaf coloration onset (LCO) responding to different warming and photoperiod treatments based on experimental data acquired from large artificial climate simulation chambers. The results indicated that: (1) LCO and the net CO2 assimilation rate (Pn), transpiration rate (Tr), stomatal conductance (Gs), Vcmax, and Jmax of Quercus mongolicus seedlings were significantly affected by the changes of photoperiod. (2) LCO was significantly correlated only with the Pn approach, supporting the view that leaf senescence is the result of a trade-off between nutrient resorption and reserves. (3) The major variation in stomatal conductance (Gs) is the mechanism by which photoperiod regulates the seasonal pattern of photosynthetic rates, implying that both limitations of stomatal and photosynthetical capacity (Vcmax and Jmax, non-stomatal limitation) are able to modulate LCO. Our study riches the knowledge of phenology and provides a reference for phenological modelling and ecosystem carbon estimation.

The transport of water, sugar, and nutrients in trees is restricted to specific vascular pathways, and thus organs may be relatively isolated from one another (i.e. sectored). Strongly sectored leaf-to-leaf pathways have been shown for the transport of sugar and signal molecules within a shoot, but not previously for water transport. The hydraulic sectoriality of leaf-to-leaf pathways was determined for current year shoots of six temperate deciduous tree species (three ring-porous: Castanea dentata, Fraxinus americana, and Quercus rubra, and three diffuse-porous: Acer saccharum, Betula papyrifera, and Liriodendron tulipifera). Hydraulic sectoriality was determined using dye staining and a hydraulic method. In the dye method, leaf blades were removed and dye was forced into the most proximal petiole. For each petiole the vascular traces that were shared with the proximal petiole were counted. For other shoots, measurements were made of the leaf-area-specific hydraulic conductivity for the leaf-to-leaf pathways (kLL). In five out of the six species, patterns of sectoriality reflected phyllotaxy; both the sharing of vascular bundles between leaves and kLL were higher for orthostichous than non-orthostichous leaf pairs. For each species, leaf-to-leaf sectoriality was determined as the proportional differences between non-orthostichous versus orthostichous leaf pairs in their staining of shared vascular bundles and in their kLL; for the six species these two indices of sectoriality were strongly correlated (R2=0.94; P <0.002). Species varied 8-fold in their kLL-based sectoriality, and ring-porous species were more sectored than diffuse-porous species. Differential leaf-to-leaf sectoriality has implications for species-specific co-ordination of leaf gas exchange and water relations within a branch, especially during fluctuations in irradiance and water and nutrient availability.

The objectives of this study were to (1) characterize stomatal response of six deciduous tree species to non-hydraulic, root-sourced signals of soil drying, and (2) test whether species sensitivity to non-hydraulic signalling is allied with their drought avoidance and tolerance profiles. Saplings were grown with roots divided between two pots. Three treatments were compared: one half of the root system watered and half droughted (WD), one half of the root system watered and half severed (WS), both halves watered (WW). Drying about half of the root system caused nonhydraulic declines in stomatal conductance (gs) in all species, with gs of WD plants reduced to from 40% to 60% of WS controls. Declines in stomatal conductance were closely related to declining soil matric potential (ψm) between −0.01 and −0.10 MPa. Soil ψm required to cause declines in gs of WD plants to 80% of WS controls varied from a high of −0.013 to a low of −0.044 MPa. Stomatal inhibition varied somewhat with leaf age in half of the species. Leaf osmotic potentials during soil drying were mostly similar among treatments. Although stomatal sensitivity to the nonhydraulic, root-sourced signal (characterized as decline in gs per unit decline in soil ψm) was not closely correlated with previously identified lethal leaf water potentials or capacity for osmotic adjustment, species having the highest stomatal sensitivity also had the least dehydration tolerance. This suggests that stomatal sensitivity to non-hydraulic root signals may be mechanistically linked to a limited extent with other characteristics defining relative species drought tolerance.

As climate change drives increased drought in many forested regions, mechanistic understanding of the factors conferring drought tolerance in trees is increasingly important. The dendrochronological record provides a window through which we can understand how tree size and traits shape growth responses to droughts. We analyzed tree-ring records for 12 species in a broadleaf deciduous forest in Virginia (USA) to test hypotheses for how tree height, microenvironment characteristics, and species’ traits shaped drought responses across the three strongest regional droughts over a 60-yr period. Drought tolerance (resistance, recovery, and resilience) decreased with tree height, which was strongly correlated with exposure to higher solar radiation and evaporative demand. The potentially greater rooting volume of larger trees did not confer a resistance advantage, but marginally increased recovery and resilience, in sites with low topographic wetness index. Drought tolerance was greater among species whose leaves lost turgor (wilted) at more negative water potentials and experienced less shrinkage upon desiccation. The tree-ring record reveals that tree height and leaf drought tolerance traits influenced growth responses during and after significant droughts in the meteorological record. As climate change-induced droughts intensify, tall trees with drought-sensitive leaves will be most vulnerable to immediate and longer-term growth reductions.

As the climate changes, warmer spring temperatures are causing earlier leaf-out 1 – 3 and commencement of CO 2 uptake 1 , 3 in temperate deciduous forests, resulting in a tendency towards increased growing season length 3 and annual CO 2 uptake 1 , 3 – 7 . However, less is known about how spring temperatures affect tree stem growth 8 , 9 , which sequesters carbon in wood that has a long residence time in the ecosystem 10 , 11 . Here we show that warmer spring temperatures shifted stem diameter growth of deciduous trees earlier but had no consistent effect on peak growing season length, maximum growth rates, or annual growth, using dendrometer band measurements from 440 trees across two forests. The latter finding was confirmed on the centennial scale by 207 tree-ring chronologies from 108 forests across eastern North America, where annual ring width was far more sensitive to temperatures during the peak growing season than in the spring. These findings imply that any extra CO 2 uptake in years with warmer spring temperatures 4 , 5 does not significantly contribute to increased sequestration in long-lived woody stem biomass. Rather, contradicting projections from global carbon cycle models 1 , 12 , our empirical results imply that warming spring temperatures are unlikely to increase woody productivity enough to strengthen the long-term CO 2 sink of temperate deciduous forests. Warmer spring temperatures affect the timing of stem diameter growth of temperate deciduous trees but have little effect on annual growth.

Accurate individual tree segmentation (ITS) is fundamental to forest management and to the studies of forest ecosystem. Unmanned Aerial Vehicle Light Detection and Ranging (UAV-LiDAR) shows advantages for ITS and tree height estimation at stand and landscape scale. However, dense deciduous forests with tightly interlocked tree crowns challenge the performance for ITS. Available LiDAR points through tree crown and appropriate algorithm are expected to attack the problem. In this study, a new UAV-LiDAR dataset that fused leaf-off and leaf-on point cloud (FULD) was introduced to assess the synergetic benefits for ITS and tree height estimation by comparing different types of segmentation algorithms (i.e., watershed segmentation, point cloud segmentation and layer stacking segmentation) in the dense deciduous forests of Northeast China. Field validation was conducted in the four typical stands, including mixed broadleaved forest (MBF), Mongolian oak forest (MOF), mixed broadleaf-conifer forest (MBCF) and larch plantation forest (LPF). The results showed that the combination of FULD and the layer stacking segmentation (LSS) algorithm produced the highest accuracies across all forest types (F-score: 0.70 to 0.85). The FULD also showed a better performance on tree height estimation, with a root mean square error (RMSE) of 1.54 m at individual level. Compared with using the leaf-on dataset solely, the RMSE of tree height estimation was reduced by 0.22 to 0.27 m, and 12.3% more trees were correctly segmented by the FULD, which are mainly contributed by improved detection rate at nearly all DBH levels and by improved detection accuracy at low DBH levels. The improvements are attributed to abundant points from the bole to the treetop of FULD, as well as each layer point being included for segmentation by LSS algorithm. These findings provide useful insights to guide the application of FULD when more multi-temporal LiDAR data are available in future.