Diffuse-porous Species Research Articles

Effect of freeze-thaw treatments with different conditions on frost fatigue in three diffuse-porous trees.

In addition to inducing xylem embolism, freeze-thaw events can cause frost fatigue phenomena. Freezing temperature, freezing times, number of freeze-thaw cycles and frost drought can affect the level of freeze-thaw-induced embolism, but it is unknown whether there is an effect on frost fatigue. We assessed whether these frost-related factors changed frost fatigue in the three diffuse-porous species by simulating freeze-thaw treatments under different conditions. We also proposed a new metric, embolism area, in place of embolism resistance, to more accurately quantify the shift of the vulnerability curve after experiencing freeze-thaw-induced embolism and refilling. Frost fatigue caused vulnerability curves of all species to change from S-shaped to double S-shaped or even R-shaped curves. When exposed to a freeze-thaw event, Acer truncatum showed strong resistance to frost fatigue; in contrast, Populus (I-101×84K) and Liriodendron chinense were more vulnerable. Changing freezing temperature and times did not impact the response to frost fatigue in the three species, but a greater number of freeze-thaw cycles and more severe frost drought significantly exacerbated their fatigue degree. Considering that frost fatigue may be a widespread phenomenon among temperate diffuse-porous species, more work is needed in the future to reveal the mechanisms of frost fatigue.

Vegetation growth responses to climate change: A cross-scale analysis of biological memory and time lags using tree ring and satellite data.

Vegetation growth is affected by past growth rates and climate variability. However, the impacts of vegetation growth carryover (VGC; biotic) and lagged climatic effects (LCE; abiotic) on tree stem radial growth may be decoupled from photosynthetic capacity, as higher photosynthesis does not always translate into greater growth. To assess the interaction of tree-species level VGC and LCE with ecosystem-scale photosynthetic processes, we utilized tree-ring width (TRW) data for three tree species: Castanopsis eyrei (CE), Castanea henryi (CH, Chinese chinquapin), and Liquidambar formosana (LF, Chinese sweet gum), along with satellite-based data on canopy greenness (EVI, enhanced vegetation index), leaf area index (LAI), and gross primary productivity (GPP). We used vector autoregressive models, impulse response functions, and forecast error variance decomposition to analyze the duration, intensity, and drivers of VGC and of LCE response to precipitation, temperature, and sunshine duration. The results showed that at the tree-species level, VGC in TRW was strongest in the first year, with an average 77% reduction in response intensity by the fourth year. VGC and LCE exhibited species-specific patterns; compared to CE and CH (diffuse-porous species), LF (ring-porous species) exhibited stronger VGC but weaker LCE. For photosynthetic capacity at the ecosystem scale (EVI, LAI, and GPP), VGC and LCE occurred within 96 days. Our study demonstrates that VGC effects play a dominant role in vegetation function and productivity, and that vegetation responses to previous growth states are decoupled from climatic variability. Additionally, we discovered the possibility for tree-ring growth to be decoupled from canopy condition. Investigating VGC and LCE of multiple indicators of vegetation growth at multiple scales has the potential to improve the accuracy of terrestrial global change models.

Wood anatomical spectrum of co-occurring species in early and late-successional tropical dry forest communities

Key messageAnalysis of wood anatomical traits revealed that drought tolerance predominates in early-successional communities, and vessel variability is relevant for species’ ecological success in seasonally dry tropical environments.Successional tropical dry forests harbor a diverse array of species subjected to a strong seasonal precipitation regime. Considering that wood encompasses diverse functional properties related to stem hydraulics and mechanical support, in this study, we asked which quantitative anatomical traits are exhibited by co-occurring species in early (EC)- and late (LC)-successional communities. We used generalized linear mixed-effects models to estimate and compare means and standard deviations of wood traits (vessel area, vessel density, vessel grouping, and fiber dimensions) between eight EC and between 13 LC co-occurring species. Wood traits were highly heterogeneous. High vessel redundancy, a property associated with water safety involved in the prevention of hydraulic failure, prevails among species coexisting in EC but is maintained through succession by the same species in LC. Highly variable fiber dimensions indicate that species coexisting in LC possess different characteristics associated with mechanical resistance. Low wood density species typical of LC, which are absent from EC, displayed wood anatomical features associated with drought-evasion mechanisms. This result implies that deforestation not only reduces taxonomic but also functional diversity. Future research should adopt a standard deviation analysis approach, as this will allow confirming the high variability in vessel diameter, even in diffuse porous species, as a key feature for the ecological success of plants facing succession-driven changes in seasonally dry environments.

Xylem embolism and bubble formation during freezing suggest complex dynamics of pressure in Betula pendula stems.

Freeze-thaw-induced embolism is a key limiting factor for perennial plants in frost-exposed environments. Gas bubbles are formed during freezing and expand during thawing resulting in xylem embolism. However, when water freezes, its volume increases by 9%, generating local pressures, which can affect the formation of bubbles. To characterize local dynamic of pressure-tension and physical state of the sap during freeze-thaw cycles, we simultaneously used ultrasonic acoustic emissions analysis and synchrotron-based high resolution computed tomography on the diffuse-porous species Betula pendula. Visualization of individual air-filled vessels was performed to measure freeze-thaw induced embolism after successive freeze-thaw cycles down to -10°C or -20°C during the leafy and the leafless periods. We also measured the distribution of gas bubbles in frozen xylem of Betula pendula, and made additional continuous monitoring of embolism spreading during one freeze-thaw cycle using a dedicated cooling system that allowed X-ray scanning during freezing and thawing. Experiments confirmed that ultrasonic emissions occurred after the onset of ice formation, together with bubble formation, whereas the development of embolism took place after thawing in all cases. The pictures of frozen tissues indicated that the positive pressure induced by the volumetric increase of ice can provoke inward flow from the cell wall toward the lumen of the vessels. We found no evidence that wider vessels within a tissue were more prone to embolism although the occurrence of gas bubbles in larger conduits would make them prone to earlier embolism. These results highlight the need to monitor local pressure as well as ice and air distribution during xylem freezing to understand the mechanism leading to frost-induced embolism.

Wood decomposition, carbon, nitrogen, and pH values in logs of 8 tree species 14 and 15 years after a catastrophic windthrow in a mesic broad-leaved forest in the East European plain

There are limited studies of decomposition of coarse woody debris (CWD) in natural conditions of multi-species mesic broad-leaved forests. We investigated 49 lying logs of 7 angiosperm and 1 gymnosperm species 14 and 15 years after a mass windthrow in an old-growth broad-leaved forest. We observed high heterogeneity of logs in terms of decomposition stages: up to 4 stages in one transverse disk and up to 5 stages in longitudinal fragments of one log. Due to the high heterogeneity of logs, we investigated traits of wood samples rather than individual logs. Bulk density, carbon and nitrogen concentrations, and pH values were examined for 188 CWD samples of 8 species at 5 decomposition stages (from stage 1 least to stage 5 most decayed wood) and 24 control samples of live trees (3 replicates for each species; zero stage of decay). Changes of deadwood volume, carbon and nitrogen stocks over 14 and 15 years after the windthrow were estimated by re-surveying transects established in 2010. From the first to the fifth decay stage, the mean CWD density decreased for all species and averaged 54 ± 16 kg m−3 (SD) at the 5th stage of decay. Regarding decomposition rates, the tree species were arranged in descending rate from diffuse-porous (Populus tremula, Betula pendula, Tilia cordata, and Acer platanoides) to ring-porous (Ulmus glabra, Fraxinus excelsior, and Quercus robur) angiosperms and the coniferous Picea abies was after the diffuse-porous species series and before the ring-porous species. Carbon concentration ranged from 39 to 54 %, but did not change significantly during decomposition (mean value for all species in all decay stages was 45.8 ± 2.5 %). Nitrogen concentration (percentage of dry mass) increased with increasing decay stage and the mean value was 0.2 ± 0.1 % at the zero stage and 0.9 ± 0.5 % at the last decay stage. Despite the increase in N concentration, the mass N per volume decreased during decomposition for all samples considered together due to a significant decrease in wood density. The change in C/N values during deadwood decay reflected the transition of organic matter from the CWD to the SOM (soil organic matter) in forest ecosystems: the ratio C/N varied from 829 (Picea, zero stage) to 16 (Ulmus, last stage) with the mean values 286 ± 149 and 75 ± 45 at the zero and the last stages, respectively. We observed a quadratic trend in the variation of wood pH values by stages: pH mainly decreased from the initial values to the minimum in the middle stages of decay, and then increased in the advanced stages. The mean deadwood volume in the surveyed plots decreased 1.4-fold in the 14–15 years after the windthrow, but the mean stocks of carbon and nitrogen decreased 3 and 2-fold, respectively. Over time, the different decomposition rate of fallen logs leads to a decrease in the proportion of C and N stored in diffuse-porous species at the expense of an increase in this proportion in ring-porous species. This result is worth considering in a carbon management strategy: planting, maintaining stands, and retaining deadwood of ring-porous species, such as Quercus, Ulmus and Fraxinus, promote relatively long-term carbon storage.

No evidence of age-related decline in propagated Acer pseudoplatanus and Fraxinus excelsior plants.

Although a substantial body of evidence suggests that large and old trees have reduced metabolic levels, the search for the causes behind this observation has proved elusive. The strong coupling between age and size, commonly encountered in the field, precludes the isolation of the potential causes. We used standard propagation techniques (grafting and air-layering) to decouple the effects of size from those of age in affecting leaf structure, biochemistry and physiology of two broadleaved trees, Acer pseudoplatanus (a diffuse-porous species) and Fraxinus excelsior (a ring-porous species). The first year after establishment of the propagated plants, some of the measurements suggested the presence of age-related declines in metabolism, while other measurements either did not show any difference or suggested variability across treatments not associated with either age or size. During the second year after establishment, only one of the measured properties (specific leaf area) continued to show some evidence of an age-mediated decline (although much reduced compared to the field), whereas, for some properties (particularly for F. excelsior), even the opposite trend of age-related increases was apparent. We concluded that (1) our plants suffered from grafting shock during year 1 and they gradually recovered during year 2; (2) the results over two years do not support the statement that age directly mediates ageing in either species but instead suggest that size directly mediates ageing processes; and (3) neither shoots nor roots of A. pseudoplatanus showed any evidence of senescence. This article is protected by copyright. All rights reserved.

Dynamic variation of non-structural carbohydrates in branches and leaves of temperate broad-leaved tree species over a complete life history

The composition of non-structural carbohydrates (NSC), one of the photosynthetic products of plants, reflects the survival strategy of a plant. Although several studies have investigated variation of NSC content in woody plants over a set time scale, few studies have considered the dynamic variation of NSC over a continuous life history. In this study, the leaves, new twigs, and old branches of seven temperate broad-leaved tree species (diffuse-porous species:Betula platyphylla,Betula costata,Tilia amurensis,Acer pictumsubsp.mono; ring-porous species:Ulmus davidianavar.japonica,Ulmus laciniata,Fraxinus mandshurica) were observed at three life history stages (seedling, sapling, and mature tree) to measure the dynamic changes of NSC and its influencing factors throughout the entire life cycles of these species. The results showed that life history, wood type, and environmental factors (soil nitrogen and phosphorus content, soil pH) significantly affected the NSC content in leaves and branches (including both new twigs and old branches). As plants grew, the NSC content in the leaves and branches generally showed an upward trend, meaning the total non-structural carbohydrate (TNC) content and soluble sugar (SS) content increased significantly, and the starch (ST) content was relatively stable. Lastly, there was no significant difference in NSC content between the canopy layers of mature trees. This indicates that the influence of life stage on NSC content in leaves and branches of plants may be dominated by genetics instead of being regulated by light factors.

Growth response, climate sensitivity and carbon storage vary with wood porosity in a southern Appalachian mixed hardwood forest

Disturbance regimes are often a complex suite of interacting agents that drive forest dynamics. Changes to any one or many of the agents will potentially affect future forest services such as productivity and carbon storage potential. Differences in sensitivity to disturbance events between diffuse-porous and ring-porous tree species, however, are currently unclear despite having important ecological and management implications. We used a dendroecological approach to identify whether diffuse and ring-porous species differ in their disturbance history, response to climate influence, and carbon storage potential in a mature Quercus-Carya stand in the Appalachian Mountains, USA. Several major abrupt growth increases indicating disturbance were identified during the history of the stand in the 1860s, 1930s, and 1960s. While both functional groups showed sensitivity to climate variables, growth reductions following drought events were more often significant for ring-porous species compared to diffuse-porous species. The decadal growth responses to drought events were similar among functional groups, and age classes, but indicated reduced growth following successive events. For the inventory year of 2015, the stand-wide aboveground live carbon content was 96.2 Mg C ha−1, with 58.3 Mg C ha−1 captured in ring-porous species and 37.9 Mg C ha−1 captured in diffuse-porous species. Our results suggest that understanding how different species and functional groups respond to forest disturbance and climate variability is critical for evaluating future management scenarios and prediction of climate change feedbacks.

Ring- and diffuse-porous tree species from a cold temperate forest diverge in stem hydraulic traits, leaf photosynthetic traits, growth rate and altitudinal distribution.

In cold and humid temperate forests, low temperature, late frost and frequent freeze-thaw cycles are the main factors limiting tree growth and survival. Ring- and diffuse-porous tree species differing in xylem anatomy coexist in these forests, but their divergent adaptations to these factors have been poorly explored. To fill this knowledge gap, we compared four ring-porous and four diffuse-porous tree species from the same temperate forest in Northeast China by quantifying their leaf and stem functional traits, their stem growth rates using tree ring analysis and their resistance to cold represented by upper altitude species distribution borders from survey data. We found that the ring-porous trees were characterized by traits related to more rapid water transport, carbon gain and stem growth rates than those of the diffuse-porous species. Compared with the diffuse-porous species, the ring-porous species had a significantly higher shoot hydraulic conductance (Ks-shoot, 0.52 vs 1.03kgm-1s-1MPa-1), leaf photosynthetic rate (An, 11.28 vs 15.83μmolm-2s-1), relative basal area increment (BAIr, 2.28 vs 0.72cmyear-1) and stem biomass increment (M, 0.34 vs 0.09kgyear-1m-1). However, the observed upper elevational distribution limit of the diffuse-porous species was higher than that of the ring-porous species and was associated with higher values of conservative traits, such as longer leaf life span (R2=0.52). Correspondingly, BAIr and M showed significant positive correlations with acquisitive traits such as Ks-shoot (R2=0.77) and leaf photosynthetic rate (R2=0.73) across the eight species, with the ring-porous species occurring at the fast-acquisitive side of the spectrum and the diffuse-porous species located on the opposite side. The observed contrasts in functional traits between the two species groups improved our understanding of their differences in terms of growth strategies and adaptive capabilities in the cold, humid temperate forests.

Calibration of Thermal Dissipation Probes for Xylem Sap Flow in the Wood of a Diffuse-Porous and a Conifer Species under Cyclic Heating

The most popular sap flow measurement technique uses thermal dissipation probes. Differences in wood characteristics and the natural temperature gradient between probes have affected the accuracy and applicability of the sap flow equation. In addition, the continued heat of the probe can also cause thermal damage to tree tissue. The objectives of this study were to use cyclic heating and calibrate the probes with two species: Pinus bungeana Zucc. And Salix matsudana Koidz., two typical diffuse-porous species. This experiment evaluated a thermal dissipation probe in three heating modes: continuous heating, 10 min heating and 50 min cooling (10/50), and 30 min heating and 30 min cooling (30/30). The heating modes were evaluated on two species. Temperature differences between the heating needle and the control needle under different heating modes and transpiration water consumption (whole-tree weighing method) were observed simultaneously. The sap flow estimation equation under cyclic heating mode was established by analyzing the relationship between the sap flow rate and the values obtained from whole-tree weighing. The results showed that the original equation underestimated sap flow rate of P. bungeana and S. matsudana by 67% and 60%. Under the cyclic heating modes, the modified equations were different from the original equation, and their accuracy was improved. After verification, the corrected equations [Fd = 0.0264K0.738 (P. bungeana, 30/30, R2 = 0.67), Fd = 0.0722K1.113(S. matsudana, 30/30, R2 = 0.60), Fd is the sap flow density, K is temperature coefficient] reduced the influence of the natural temperature gradient on the estimation of sap flow rate, thereby significantly improving the accuracy of sap flow rate estimation. The resulting equation may be more suitable for actual field observations of sap flow in the two tested species. The cyclic heating mode has the potential to measure plant transpiration for extended periods in the field.

Less pronounced drought responses in ring-porous than in diffuse-porous temperate tree species

Tree species differ in their physiological responses to drought, but the underlying causes are often unclear. Here we explored responses of radial growth to centennial drought events and sap flow (Fs) to seasonal drought in four mixed forests on either moist or drier sites in northwestern Switzerland. While the diffuse-porous species (Fagus sylvatica, Prunus avium, Tilia platyphyllos) showed marked growth reductions in 1976 and 2003, two known marker years for severe drought, growth of the two ring-porous species (Quercus petraea and Fraxinus excelsior) was less severely affected. During a dry early to midsummer, diffuse-porous species strongly reduced Fs at the two drier sites but not (or less so) at the two moister sites. Regardless of soil moisture availability, the deep-rooting, ring-porous trees invariably down-regulated Fs to 60–70% of their maxima in response to vapour pressure deficit (VPD) and maintained similar fluxes across sites, irrespective of upper soil moisture conditions. A generalised additive model of normalised Fs as a function of VPD and soil matric potential yielded a drought-sensitivity ranking of Fs led by the two insensitive ring-porous species followed by the diffuse-porous trees (ordered by increasing sensitivity: Fraxinus excelsior < Quercus petraea < Prunus avium < Acer pseudoplatanus < Fagus sylvatica < Tilia platyphyllos). In conclusion, ring-porous tree species exhibited stronger VPD-driven stomatal control over Fs, and tree-ring formation was less sensitive to severe drought than in their neighbouring diffuse-porous species. The Fs regulation explained the greater drought tolerance of the ring-porous trees.

Intra-Annual Growth and Its Response to Climatic Factors of Two Salix Species under Warm Temperate Environment

Monitoring cambial activity and intra-annual growth dynamics is an effective method for identifying tree growth response to climate change. However, there have been few pieces of research on intra-annual wood formation of diffuse-porous species under monsoonal warm temperate environment. Here, we monitored weekly the cambial activity and xylem differentiation of Salix babylonica and Salix matsudana by collecting microcores during the 2018 growing season. Two willow species exhibited similar cambial activity and xylem differentiation processes, of which the onset and cessation of xylem growth was from middle/late March to early/middle November. The onset and cessation of cambial activity were slightly earlier for Salix matsudana (19 March and 12 October) than for Salix babylonica (26 March and 17 October), which peaked on 2 June and 31 May, respectively. Salix babylonica showed wider xylem increment and higher growth rate than that of Salix matsudana, of which the intra-annual xylem width was 8525 ± 1201 µm and 7603 ± 826 µm, respectively, fitted by Gompertz function. Moreover, the maximum growth rate of Salix babylonica and Salix matsudana was 79.75 μm day−1 and 66 μm day−1, respectively, occurring on 4 June (DOY155) and 26 May (DOY146). Both temperature and water availability were important factors influenced the xylem growth for two species, which Salix matsudana had a stronger response to temperature but not to water availability than Salix babylonica. These results suggested that Salix babylonica seem to grow better under moist warm temperate environment due to it being a more conservative response to the climate than Salix babylonica. These observed species-specific differences at the intra-annual scale may help researchers more accurately anticipate the species suitability in temperate forests.

Xylem porosity, sapwood characteristics, and uncertainties in temperate and boreal forest water use

Sapwood characteristics, such as sapwood area as well as thermal and hydraulic conductivity, are linked to species-specific hydraulic function and resource allocation to water transport tissues (xylem). These characteristics are often unknown and thus a major source of uncertainty in sap flow data processing and transpiration estimates because bulk rather than species-specific values are usually applied. Here, we analyzed the sapwood characteristics of fifteen common tree species in eastern North America from different taxonomic (i.e., angiosperms and gymnosperms) and xylem porosity groups (i.e., tracheid-bearing, diffuse- or ring-porous species) and we assessed how uncertainties in sapwood characteristics involved in sap flow calculations are propagated in tree water use estimates. We quantified their sapwood area changes with stem diameter (allometric scaling) and thermal conductivity. We combined these measurements with species-specific values of wood density and hydraulic conductivity found in the literature and assessed the role of wood anatomy in orchestrating their covariation. Using an example sap flow dataset from tree species with different xylem porosity, we assessed the sensitivity of tree water use estimates to sapwood characteristics and their interactions. Angiosperms (ring- and diffuse-porous species), with specialized vessels for water transport, showed a steeper relationship (scaling) between tree stem diameter and sapwood area in comparison to gymnosperms (tracheid-bearing species). Gymnosperms (angiosperms) were characterized by lower (higher) wood density and higher (lower) sapwood moisture content, resulting in non-significant differences in sapwood thermal conductivity between taxonomic and xylem porosity groups. Clustering of species sapwood characteristics based on taxonomic or xylem porosity groups and constraining these parameters could facilitate more accurate sap flow calculations and tree water use estimates. When combined with an increasing number of sap flow observations, these findings should improve tree- and landscape-level transpiration estimates, leading to more robust partitioning of terrestrial water fluxes.

Linking xylem structure and function: the comparative method in from the cold.

This article is a Commentary on Savage et al. (2022), 235: 953–964.

Annual shoot length of temperate broadleaf species responses to drought

In recent years, Central European forests and trees have been severely suffered from drought events in the growing season. To examine the tree growth response to drought stress, measurements of annual shoot lengths (ASL) of various tree species with distinct ages and habitats were taken in southeast Germany. The index of relative ASL increase (RAI), which is a standardized ASL increase, was calculated to indicate the growth rate of various tree species between the non-drought (2015–2017) and the drought period (2018–2020). The results showed the ASLs of most early-young trees (< 10 years) significantly showed a decreasing tendency in the drought period, whereas the middle-young (11–15 years) and late-young trees (> 16 years) presented a relatively stable trend. The ring-porous species with anisohydric behavior were likely to have higher RAIs associated with higher drought tolerance than diffuse-porous species with isohydric behavior. However, tree growth rate did not correlate with drought tolerance (resistance) of tree species indicated by leaf turgor loss point (Ψtlp, MPa). Therefore, this study suggested parameters, such as growth rate, age phases, and microhabitats, should be considered to predict tree species’ drought tolerance other than Ψtlp. Finally, we emphasized tree shoot growth responses to drought were affected by multiple internal and external factors, and strongly depended on species- and site-specific characteristics, such as age phases, xylem structure, hydraulic strategy, microhabitat, grafting effect, and competition. ASL measurement can be recommended to be an easy, fast, and effective method to detect tree growth rate and response to environmental stress.

Measuring xylem hydraulic vulnerability for long-vessel species: an improved methodology with the flow centrifugation technique

ContextUnderstanding plant resilience and adaptation to drought is a major challenge in crop and forest sciences. Several methods have been developed to assess the vulnerability to xylem embolism. The in situ flow centrifuge (or cavitron) is the fastest technique allowing to characterise this trait for plants having vessel lengths shorter than the rotor size.AimsWe present (i) a series of changes to the earlier cavitron design, aimed at improving the accuracy and speed of measurement through automated operations, and (ii) a new development through the design of a large diameter rotor expanding the range of species that can be measured.MethodsBoth hardware and software modifications to the original design have been developed. In order to avoid artefacts caused by cut open vessels, a centrifuge with a large rotor (1 m) has been developed, and vulnerability curves obtained with this new device were compared with those obtained using reference methods.ResultsThe new set-up expands the range of conductance measurable with a cavitron and enables it to accurately determine the absolute value of conductivity even for species having very low hydraulic conductivity. The large rotor cavitron shows good agreement with the reference techniques for conifers and diffuse-porous species but also for ring-porous species having long vessels.ConclusionThe set-up described in this manuscript provides a faster, safer and more accurate method to construct vulnerability curves, compared to the original cavitron design, and extends the measurement capabilities to new species that are difficult to measure to date.Key messageRecent improvements to cavitron setup enable to measure xylem vulnerability curves for an expanded number of plant species, with longer vessels or lower hydraulic conductivity.

Limitation by vapour pressure deficit shapes different intra-annual growth patterns of diffuse- and ring-porous temperate broadleaves.

Understanding the effects of temperature and moisture on radial growth is vital for assessing the impacts of climate change on carbon and water cycles. However, studies observing growth at sub-daily temporal scales remain scarce. We analysed sub-daily growth dynamics and its climatic drivers recorded by point dendrometers for 35 trees of three temperate broadleaved species during the years 2015-2020. We isolated irreversible growth driven by cambial activity from the dendrometer records. Next, we compared the intra-annual growth patterns among species and delimited their climatic optima. The growth of all species peaked at air temperatures between 12 and 16°C and vapour pressure deficit (VPD) below 0.1kPa. Acer pseudoplatanus and Fagus sylvatica, both diffuse-porous, sustained growth under suboptimal VPD. Ring-porous Quercus robur experienced a steep decline of growth rates with reduced air humidity. This resulted in multiple irregular growth peaks of Q. robur during the year. By contrast, the growth patterns of the diffuse-porous species were always right-skewed unimodal with a peak in June between day of the year 150-170. Intra-annual growth patterns are shaped more by VPD than temperature. The different sensitivity of radial growth to VPD is responsible for unimodal growth patterns in both diffuse-porous species and multimodal growth pattern in Q. robur.

Contrast in vulnerability to freezing-induced xylem embolism contributes to divergence in spring phenology between diffuse- and ring-porous temperate trees

The spring phenology and growth strategy of temperate tree species can be strongly linked to their sensitivity to frosts, which deserve more profound investigations under the background of climate warming particularly considering the advancement of spring phenology as well as the increase in frequency and intensity of spring cold waves. Spring phenologies, stem radial growth characteristics, frost sensitivity of leaves and stem hydraulic systems were studied in five diffuse-porous and five ring-porous temperate tree species under a common garden condition. The results showed that the spring leaf phenology of the diffuse-porous species was one to two weeks earlier than that of the ring-porous species. The ring-porous species had significantly higher stem hydraulic conductivity than the diffuse-porous species (1.81 and 0.95 kg·m −1 ·s −1 ·MPa −1 , P < 0.05) but were more vulnerable to freeze-thaw induced xylem embolism than the latter. After a simulated freeze-thaw event, the average percentage loss of hydraulic conductivity in the current year shoots increased from 26.0% (native embolism) to 86.7% in the ring-porous species, while it only increased from 21.3% to 38.3% in the diffuse-porous species. The spring phenology was clearly correlated with vulnerability to freeze-thaw induced embolism, with the more vulnerable ring-porous species exhibited substantially delayed phenology to reduce risks of catastrophic hydraulic dysfunction during spring frosts. Nevertheless, ring-porous species can offset the postponed onset of growth and gained even higher annual growth due to significantly higher hydraulic efficiency and leaf gas exchange rates. Contrasts between ring-porous and diffuse-porous species in resistance to freeze-thaw induced embolism suggest that they face different selective pressures from early spring frosts, which may at least be partially responsible for their divergence in spring phenology and growth strategy and can potentially lead to different responses to climate regime shifts.

Less Pronounced Drought Responses in Ring-Porous than in Diffuse-Porous Temperate Tree Species

Less Pronounced Drought Responses in Ring-Porous than in Diffuse-Porous Temperate Tree Species

Contrasting Carbon Allocation Strategies of Ring-Porous and Diffuse-Porous Species Converge Toward Similar Growth Responses to Drought.

Extreme climatic events that are expected under global warming expose forest ecosystems to drought stress, which may affect the growth and productivity. We assessed intra-annual growth responses of trees to soil water content in species belonging to different functional groups of tree-ring porosity. We pose the hypothesis that species with contrasting carbon allocation strategies, which emerge from different relationships between wood traits and canopy architecture, display divergent growth responses to drought. We selected two diffuse-porous species (Acer saccharum and Betula alleghaniensis) and two ring-porous species (Quercus rubra and Fraxinus americana) from the mixed forest of Quebec (Canada). We measured anatomical wood traits and canopy architecture in eight individuals per species and assessed tree growth sensitivity to water balance during 2008–2017 using the standardized precipitation evapotranspiration index (SPEI). Stem elongation in diffuse-porous species mainly depended upon the total number of ramifications and hydraulic diameter of the tree-ring vessels. In ring-porous species, stem elongation mainly depended upon the productivity of the current year, i.e., number of vessels and basal area increment. Diffuse-porous and ring-porous species had similar responses to soil water balance. The effect of soil water balance on tree growth changed during the growing season. In April, decreasing soil temperature linked to wet conditions could explain the negative relationship between SPEI and tree growth. In late spring, greater water availability affected carbon partitioning, by promoting the formation of larger xylem vessels in both functional groups. Results suggest that timings and duration of drought events affect meristem growth and carbon allocation in both functional groups. Drought induces the formation of fewer xylem vessels in ring-porous species, and smaller xylem vessels in diffuse-porous species, the latter being also prone to a decline in stem elongation due to a reduced number of ramifications. Indeed, stem elongation of diffuse-porous species is influenced by environmental conditions of the previous year, which determine the total number of ramifications during the current year. Drought responses in different functional groups are thus characterized by different drivers, express contrasting levels of resistance or resilience, but finally result in an overall similar loss of productivity.