We investigated how deciduous trees can adjust their freezing resistance in response to temperature during the progress of the ecodormancy phase, from midwinter to budburst.
We regularly sampled ...twigs of four different temperate deciduous tree species from January to the leaf-out date. Using computer-controlled freezers and climate chambers, the freezing resistance of buds was measured directly after sampling and also after the application of artificial hardening and dehardening treatments, simulating cold and warm spells. The thermal time to budburst in forcing conditions (c. 20°C) was also quantified at each sampling as a proxy for dormancy depth.
Earlier flushing species showed higher freezing resistance than late flushing species at either similar bud development stage or similar dormancy depth. Overall, freezing resistance and its hardening and dehardening potential dramatically decreased during the progress of ecodormancy and became almost nil during budburst.
Our results suggest that extreme cold events in winter are not critical for trees, as freezing resistance can be largely enhanced during this period. By contrast, the timing of budburst is a critical component of tree fitness. Our results provide quantitative values of the freezing resistance dynamics during ecodormancy, particularly valuable in process-based species distribution models.
Temperate climates are defined by distinct temperature seasonality with large and often unpredictable weather during any of the four seasons. To thrive in such climates, trees have to withstand a ...cold winter and the stochastic occurrence of freeze events during any time of the year. The physiological mechanisms trees adopt to escape, avoid, and tolerate freezing temperatures include a cold acclimation in autumn, a dormancy period during winter (leafless in deciduous trees), and the maintenance of a certain freezing tolerance during dehardening in early spring. The change from one phase to the next is mediated by complex interactions between temperature and photoperiod. This review aims at providing an overview of the interplay between phenology of leaves and species-specific freezing resistance. First, we address the long-term evolutionary responses that enabled temperate trees to tolerate certain low temperature extremes. We provide evidence that short term acclimation of freezing resistance plays a crucial role both in dormant and active buds, including re-acclimation to cold conditions following warm spells. This ability declines to almost zero during leaf emergence. Second, we show that the risk that native temperate trees encounter freeze injuries is low and is confined to spring and underline that this risk might be altered by climate warming depending on species-specific phenological responses to environmental cues.
Summary
Attempts at explaining range limits of temperate tree species still rest on correlations with climatic data that lack a physiological justification. Here, we present a synthesis of a ...multidisciplinary project that offers mechanistic explanations. Employing climatology, biogeography, dendrology, population and reproduction biology, stress physiology and phenology, we combine results from in situ elevational (Swiss Alps) and latitudinal (Alps vs. Scandinavia) comparisons, from reciprocal common garden and phytotron studies for eight European broadleaf tree species.
We show that unlike for low‐stature plants, tree canopy temperatures can be predicted from weather station data, and that low‐temperature extremes in winter do not explain range limits. At the current low‐temperature range limit, all species recruit well. Transplants revealed that the local environment rather than elevation of seed origin dominates growth and phenology. Tree ring width at the range limit is not related to season length, but to growing season temperature, with no evidence of carbon shortage. Bud break and leaf emergence in adults trees are timed in such a way that the probability of freezing damage is almost zero, with a uniform safety margin across elevations and taxa. More freezing‐resistant species flush earlier than less resistant species.
Synthesis: we conclude that the range limits of the examined tree species are set by the interactive influence of freezing resistance in spring, phenology settings, and the time required to mature tissue. Microevolution of spring phenology compromises between demands set by freezing resistance of young, immature tissue and season length requirements related to autumnal tissue maturation.
We conclude that the range limits of the examined tree species are set by the interactive influence of freezing resistance in spring, phenology settings, and the time required to mature tissue. Microevolution of spring phenology compromises between demands set by freezing resistance of young, immature tissue and season length requirements related to autumnal tissue maturation.
In temperate climates, seedlings and saplings have often been assumed to be more sensitive to late‐spring freezes than conspecific adult trees. Yet, no data are available to compare the freezing ...resistance of juvenile and adult trees at their phenologically most sensitive stage, that is, during leaf‐out. Emerging leaves of seedlings, saplings and adult trees were collected in spring 2013 in seven temperate tree species in a mature mixed forest in the foothills of the Swiss Jura Mountains. Freezing resistance of these emerging leaves was assessed using different target temperatures (−13 to +4 °C) in seven computer‐controlled freezers. Additionally, we assessed the risk that species encounter freeze damages based on temperature data recorded since 1898. The different study species showed contrasting freezing resistance, with the LT₅₀ (median lethal freezing temperature) of emerging leaves ranging from −3.5 ± 0.2 °C (Fraxinus excelsior) to −8.3 ± 0.2 °C (Prunus avium). Within species, juvenile trees (seedlings or saplings) were found to be as sensitive to freezing temperatures as mature trees when the same developmental stage of foliage was compared. Based on phenological observations made during spring 2012, long time series of temperatures indicate a very low risk of freeze damage at the study site, especially for adult trees. Synthesis. We conclude that seedlings and saplings are more prone to freeze damage than adult trees because of their earlier flushing rather than due to a higher sensitivity to freezing as such. Our study highlights that the timing of spring phenology has evolved in such a way that it minimizes the risk of freeze damage according to the species‐specific LT₅₀. Early flushing species are among the most freezing‐resistant species during flushing, whereas late flushing species are among the least resistant. We conclude that for the examined species the species‐specific freezing resistance during leaf emergence could be extracted from either adult or juvenile trees, as long as it is estimated at a same phenological stage.
Minimum temperature is assumed to be an important driver of tree species range limits. We investigated during which period of the year trees are most vulnerable to freezing damage and whether the ...pressure of freezing events increases with increasing elevation.
We assessed the course of freezing resistance of buds and leaves from winter to summer at the upper elevational limits of eight deciduous tree species in the Swiss Alps. By reconstructing the spring phenology of these species over the last eight decades using a thermal time model, we linked freezing resistance with long-term minimum temperature data along elevational gradients.
Counter-intuitively, the pressure of freeze events does not increase with elevation, but deciduous temperate tree species exhibit a constant safety margin (5–8.5 K) against damage by spring freeze events along elevational gradients, as a result of the later flushing at higher elevation. Absolute minimum temperatures in winter and summer are unlikely to critically injure trees.
Our study shows that freezing temperatures in spring are the main selective pressure controlling the timing of flushing, leading to a shorter growing season at higher elevation and potentially driving species distribution limits. Such mechanistic knowledge is important to improve predictions of tree species range limits.
Summary
Within the same forest stand, temperate deciduous trees generally exhibit a distinct pattern in leaf‐out timing, with some species flushing earlier than other species. This study aimed to ...explain the timing of leaf‐out of various temperate tree species in relation to the risk of freezing damage to leaves.
We combined long‐term series of leaf‐out date (14–32 years) of five temperate tree species located in both low and high elevations in Switzerland, daily minimum temperatures recorded at the same sites and species‐specific freezing resistance (LT50) of emerging leaves. We calculated temperature safety margins (the temperature difference between absolute minimum temperature during leaf‐out and species‐specific LT50 values), and date safety margins (time lag between the last day when temperature falls below species‐specific LT50 values and the date of leaf‐out).
Leaf‐out occurred when the probability to encounter freezing damage approaches zero, irrespective of climatic conditions (low vs. high elevation) and species (early‐ and late‐flushing species). In other words, trees leaf out precisely at the beginning of the probabilistically safe period. Interestingly, the temperature safety margins did not differ significantly between low and high elevation. Yet, the date safety margin was smaller at high elevation, presumably due to a faster increase in temperature during the leaf‐out period at high elevation.
When species‐specific freezing resistance is taken into account, the time of leaf‐out converges among species towards a marginal risk of freezing damage. Thus, leaf‐out time has likely evolved in a way that the risk of freezing damage is minimized over a large spectrum of climatic conditions. Species with a small safety margin against freezing temperature, like Fagus sylvatica, appear to employ photoperiod co‐control of spring phenology, whereas species with a large safety margin depend largely on temperature for the right timing of leaf‐out.
Our results offer a new avenue to explain the differences in leaf‐out timing among co‐occurring tree species. They further suggest that in a warming climate, tree species can expand their distribution range to the extent their phenology matches the stochasticity of freezing temperatures in spring.
Lay Summary
Phenological events, such as the initiation and the end of seasonal growth, are thought to be under strong evolutionary control because of their influence on tree fitness. Although numerous studies ...highlighted genetic differentiation in phenology among populations from contrasting climates, it remains unclear whether local adaptation could restrict phenological plasticity in response to current warming. Seedling populations of seven deciduous tree species from high and low elevations in the Swiss Alps were investigated in eight common gardens located along two elevational gradients from 400 to 1,700 m. We addressed the following questions: are there genetic differentiations in phenology between populations from low and high elevations, and are populations from the upper elevational limit of a species' distribution able to respond to increasing temperature to the same extent as low-elevation populations? Genetic variation of leaf unfolding date between seedlings from low and high populations was detected in six out of seven tree species. Except for beech, populations from high elevations tended to flush later than populations from low elevations, emphasizing that phenology is likely to be under evolutionary pressure. Furthermore, seedlings from high elevation exhibited lower phenological plasticity to temperature than low-elevation provenances. This difference in phenological plasticity may reflect the opposing selective forces involved (i.e. a trade-off between maximizing growing season length and avoiding frost damages). Nevertheless, environmental effects were much stronger than genetic effects, suggesting a high phenological plasticity to enable tree populations to track ongoing climate change, which includes the risk of tracking unusually warm springs followed by frost.
At high elevation, temperate trees generally exhibit adaptive genetic differentiation in their morphological and physiological traits. On account of this directional selection, we hypothesized that ...tree populations growing near their upper cold elevational limits exhibit lower phenotypic plasticity of growth and leaf morphological traits in response to temperature changes than populations growing at lower elevations. Seedlings of six common deciduous tree species originating from low and high elevations were transplanted into eight common gardens along two elevational gradients in the Swiss Alps. The aim of the experiment was to evaluate the genetic differentiation in growth and leaf morphology between populations from low and high elevations and to quantify the phenotypic plasticity of these traits to temperature changes. In contrast to growth that decreased with increasing elevation, leaf mass per area (LMA) showed no significant change with elevation of common garden, except for a decrease in Laburnum alpinum for both low‐ and high‐elevation provenances. Interestingly, leaf density was found to decrease with elevation of the gardens for all species. Genetic differentiation between low‐ and high‐elevation populations was found in both leaf morphology and growth: high‐elevation populations tended to have slower growth rate than low‐elevation populations, while no consistent trend was found for LMA across species. Interestingly, for Acer pseudoplatanus and Fraxinus excelsior high‐elevation populations exhibited a lower phenotypic plasticity of growth in response to temperature compared to lower populations, whereas no interactions between the elevation of a provenance and the elevation of the garden was detected for the four other species. Hence, during young life stages, the expected increase in tree growth in future warmer climates might be lower in populations living in the coldest part of the species distribution range in temperate species such as Acer pseudoplatanus and Fraxinus excelsior, but similar in other tree species, disregarding other environmental changes.
International consensus on best practices for calculating and reporting vestibular function is lacking. Quantitative vestibulo-ocular reflex (VOR) gain using a video head impulse test (HIT) device ...can be calculated by various methods.
To compare different gain calculation methods and to analyze interactions between artifacts and calculation methods.
We analyzed 1300 horizontal HIT traces from 26 patients with acute vestibular syndrome and calculated the ratio between eye and head velocity at specific time points (40 ms, 60 ms) after HIT onset ('velocity gain'), ratio of velocity slopes ('regression gain'), and ratio of area under the curves after de-saccading ('position gain').
There was no mean difference between gain at 60 ms and position gain, both showing a significant correlation (r2 = 0.77, p < 0.001) for artifact-free recordings. All artifacts reduced high, normal-range gains modestly (range -0.06 to -0.11). The impact on abnormal, low gains was variable (depending on the artifact type) compared to artifact-free recordings.
There is no clear superiority of a single gain calculation method for video HIT testing. Artifacts cause small but significant reductions of measured VOR gains in HITs with higher, normal-range gains, regardless of calculation method. Artifacts in abnormal HITs with low gain increased measurement noise. A larger number of HITs should be performed to confirm abnormal results, regardless of calculation method.
Atherosclerosis expression varies across not only coronary, cerebrovascular, and peripheral arteries but also within the peripheral vascular tree. The underlying pathomechanisms of distinct ...atherosclerosis phenotypes in lower extremity peripheral artery disease (PAD) is poorly understood. We investigated the association of cardiovascular risk factors (CVRFs) and atherosclerosis distribution in a targeted approach analyzing symptomatic patients with isolated anatomic phenotypes of PAD.
In a cross-sectional analysis of consecutive patients undergoing first-time endovascular recanalization for symptomatic PAD, data of patients with isolated anatomic phenotypes of either proximal (iliac) or distal (infrageniculate) atherosclerosis segregation were extracted. We performed a multivariable logistic regression model with backward elimination to investigate the association of proximal and distal PAD with CVRFs.
Of the 637 patients (29% females) with endovascular recanalization, 351 (55%) had proximal and 286 (45%) had distal atherosclerosis. Female sex odds ratio (OR) 0.33, 95% confidence interval (CI) 0.20-0.54,
= 0.01, active smoking (OR 0.16, 95% CI 0.09-0.28,
< 0.001), and former smoking (OR 0.33, 95% CI 0.20-0.57,
< 0.001) were associated with proximal disease. Diabetes mellitus (DM) (OR 3.25, 95% CI 1.93-5.46,
< 0.001), chronic kidney disease (CKD) (OR 1.18, 95% CI 1.08-1.28,
< 0.001), and older age (OR 1.31, 95% CI 1.06-1.61,
= 0.01) were associated with distal disease.
Female sex, particularly in the context of smoking, is associated with clinically relevant, proximal atherosclerosis expression. Our additional findings that distal atherosclerosis expression is associated with DM, CKD, and older age suggest that PAD has at least two distinct atherosclerotic phenotypes with sex-specific and individual susceptibility to atherogenic risk factors.
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