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•Maximum temperatures have increased more than minimum temperatures.•Spring phenology has advanced at a faster rate than the date of the last frost.•The risk of frost injury to trees ...has increased at higher elevations in Switzerland.•The risk of frost injury to trees has remained unchanged at lower elevations.•Planting summer-adapted trees should be carefully considered regarding frost risk.
Winters and early springs are predicted to become warmer in temperate climates under continued global warming, which in turn is expected to promote earlier plant development. By contrast, there is no consensus about the changes in the occurrence and severity of late spring frosts. If the frequency and severity of late spring frosts remain unchanged in the future or change less than spring phenology of plants does, vulnerable plant organs (dehardened buds, young leaves, flowers or young fruits) may be more exposed to frost damage. Here we analyzed long-term temperature data from the period 1975–2016 in 50 locations in Switzerland and used different phenological models calibrated with long-term series of the flowering and leaf-out timing of two fruit trees (apple and cherry) and two forest trees (Norway spruce and European beech) to test whether the risk of frost damage has increased during this period. Overall, despite the substantial increase in temperature during the study period, the risk of frost damage was not reduced because spring phenology has advanced at a faster rate than the date of the last spring frost. In contrast, we found that the risk of frost exposure and subsequent potential damage has increased for all four species at the vast majority of stations located at elevations higher than 800m while remaining unchanged at lower elevations. The different trends between lower and higher elevations are due to the date of the last spring frost moving less at higher altitudes than at lower altitudes, combined with stronger phenological shifts at higher elevations. This latter trend likely results from a stronger warming during late compared to earlier spring and from the increasing role of other limiting factors at lower elevations (chilling and photoperiod). Our results suggest that frost risk needs to be considered carefully when promoting the introduction of new varieties of fruit trees or exotic forest tree species adapted to warmer and drier climates or when considering new plantations at higher elevations.
Forests are increasingly exposed to extreme global warming‐induced climatic events. However, the immediate and carry‐over effects of extreme events on forests are still poorly understood. Gross ...primary productivity (GPP) capacity is regarded as a good proxy of the ecosystem's functional stability, reflecting its physiological response to its surroundings. Using eddy covariance data from 34 forest sites in the Northern Hemisphere, we analyzed the immediate and carry‐over effects of late‐spring frost (LSF) and growing season drought on needle‐leaf and broadleaf forests. Path analysis was applied to reveal the plausible reasons behind the varied responses of forests to extreme events. The results show that LSF had clear immediate effects on the GPP capacity of both needle‐leaf and broadleaf forests. However, GPP capacity in needle‐leaf forests was more sensitive to drought than in broadleaf forests. There was no interaction between LSF and drought in either needle‐leaf or broadleaf forests. Drought effects were still visible when LSF and drought coexisted in needle‐leaf forests. Path analysis further showed that the response of GPP capacity to drought differed between needle‐leaf and broadleaf forests, mainly due to the difference in the sensitivity of canopy conductance. Moreover, LSF had a more severe and long‐lasting carry‐over effect on forests than drought. These results enrich our understanding of the mechanisms of forest response to extreme events across forest types.
The immediate and carry‐over effects of drought and late‐spring frost between needle‐leaf forests and broadleaf forests were reported. GPP capacity to drought differed between needle‐leaf and broadleaf forests, mainly due to the difference in the sensitivity of canopy conductance. LSF had a more severe and long‐lasting carry‐over effect on forests compared to drought.
Resistance and resilience to multiple stresses depend on competition among trees, thus they can be improved by means of silvicultural practices reducing stand density. However, we often ignore ...whether thinning is an appropriate practice to increase the resistance and resilience to late-spring frosts. These climatic events can damage tree species, especially when leaf emergence is advanced by warmer springs. Here, we compared the effect of a late-spring frost on crown damage and stem radial growth in beech trees (Fagus sylvatica) from areas thinned and not thinned. The study area is a mixed forest of F. sylvatica, Quercus petraea and Q. pyrenaica, and is located in central Spain, near the southern limit of beech distribution. The proportion of trees damaged by frost was not significantly different in thinned vs non-thinned areas. However, severely damaged trees in non-thinned areas showed higher growth reductions and a longer legacy effect than severely damaged trees in thinned plots. Severely damaged trees were those with earlier leaf and stem growth phenology in the frost year. These trees, compared to slightly damaged trees, delayed growth cessation and had a longer growth period in the frost year, and the year afterwards in non-thinned areas. Growth recovery was size-dependent in non-thinned areas, in which bigger trees recovered growth faster. The positive effect of thinning in attenuating frost impact on growth and accelerating growth recovery adds up to other beneficial effects of thinning for stress tolerance. Similar results could occur in other deciduous species highly sensitive to spring temperatures, which are likely to increase with global warming.
•Frost damage results in reduced growth for two consecutive years.•Thinning reduces the frost impact on growth and shortens the legacy effect.•Thinning had no effect on the proportion of frost-damaged trees.•Frost-damaged trees were those with earlier leaf and stem growth phenology.•Damaged trees had longer growth period in the frost year and the year afterwards.
Climate change is increasing the frequency of extreme climate events, causing profound impacts on forest function and composition. Late frost defoliation (LFD) events, the loss of photosynthetic ...tissues due to low temperatures at the start of the growing season, might become more recurrent under future climate scenarios. Therefore, the detection of changes in late-frost risk in response to global change emerges as a high-priority research topic. Here, we used a tree-ring network from southern European beech (Fagus sylvatica L.) forests comprising Spain, Italy and the Austrian Alps, to assess the incidence of LFD events in the last seven decades. We fitted linear-mixed models of basal area increment using different LFD indicators considering warm spring temperatures and late-spring frosts as fixed factors. We reconstructed major LFD events since 1950, matching extreme values of LFD climatic indicators with sharp tree-ring growth reductions. The last LFD events were validated using remote sensing. Lastly, reconstructed LFD events were climatically and spatially characterized. Warm temperatures before the late-spring frost, defined by high values of growing-degree days, influenced beech growth negatively, particularly in the southernmost populations. The number of LFD events increased towards beech southern distribution edge. Spanish and the southernmost Italian beech forests experienced higher frequency of LFD events since the 1990s. Until then, LFD events were circumscribed to local scales, but since that decade, LFD events became widespread, largely affecting the whole beech southwestern distribution area. Our study, based on in-situ evidence, sheds light on the climatic factors driving LFD occurrence and illustrates how increased occurrence and spatial extension of late-spring frosts might constrain future southern European beech forests' growth and functionality. Observed alterations in the climate-phenology interactions in response to climate change represent a potential threat for temperate deciduous forests persistence in their drier/southern distribution edge.
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•Earlier leaf phenology by warmer springs may boost late frost defoliation (LFD).•We reconstructed LFD events since 1950 in southern Fagus sylvatica distribution.•Drier and higher populations suffer more LFD events.•LFD events frequency has increased at the southernmost beech forests.•Recent LFD events have larger geographical extent.
Abstract
Background and Aims
Conifers are key components of many temperate and boreal forests and are important for forestry, but species differences in stem growth responses to climate are still ...poorly understood and may hinder effective management of these forests in a warmer and drier future.
Methods
We studied 19 Northern Hemisphere conifer species planted in a 50-year-old common garden experiment in the Netherlands to (1) assess the effect of temporal dynamics in climate on stem growth, (2) test for a possible positive relationship between the growth potential and climatic growth sensitivity across species, and (3) evaluate the extent to which stem growth is controlled by phylogeny.
Key results
Eighty-nine per cent of the species showed a significant reduction in stem growth to summer drought, 37 % responded negatively to spring frost and 32 % responded positively to higher winter temperatures. Species differed largely in their growth sensitivity to climatic variation and showed, for example, a four-fold difference in growth reduction to summer drought. Remarkably, we did not find a positive relationship between productivity and climatic sensitivity, but instead observed that some species combined a low growth sensitivity to summer drought with high growth potential. Both growth sensitivity to climate and growth potential were partly phylogenetically controlled.
Conclusions
A warmer and drier future climate is likely to reduce the productivity of most conifer species. We did not find a relationship between growth potential and growth sensitivity to climate; instead, some species combined high growth potential with low sensitivity to summer drought. This may help forest managers to select productive species that are able to cope with a warmer and drier future.
Frost events during the active growth period of plants can cause extensive frost damage with tremendous economic losses and dramatic ecological consequences. A common assumption is that climate ...warming may bring along a reduction in the frequency and severity of frost damage to vegetation. On the other hand, it has been argued that rising temperature in late winter and early spring might trigger the so called “false spring”, that is, early onset of growth that is followed by cold spells, resulting in increased frost damage. By combining daily gridded climate data and 1,489 k in situ phenological observations of 27 tree species from 5,565 phenological observation sites in Europe, we show here that temporal changes in the risk of spring frost damage with recent warming vary largely depending on the species and geographical locations. Species whose phenology was especially sensitive to climate warming tended to have increased risk of frost damage. Geographically, compared with continental areas, maritime and coastal areas in Europe were more exposed to increasing occurrence of frost and these late spring frosts were getting more severe in the maritime and coastal areas. Our results suggest that even though temperatures will be elevated in the future, some phenologically responsive species and many populations of a given species will paradoxically experience more frost damage in the future warming climate. More attention should be paid to the increased frost damage in responsive species and populations in maritime areas when developing strategies to mitigate the potential negative impacts of climate change on ecosystems in the near future.
This study examined the well‐known “increased frost damage” hypothesis by Cannell (1985), based on long‐term phenological records and climate data. Temporal changes in the risk of spring frost damage with recent warming vary largely depending on the species and geographical locations. Species whose phenology was especially sensitive to climate warming tended to have increased risk of frost damage. Geographically, compared with continental areas, maritime and coastal areas in Europe were more exposed to increasing occurrence of frost and these late spring frosts were getting more severe in the maritime and coastal areas.
•Phenology dynamics and frost risk of apples were investigated under climate change.•Apples phenophase expect to advance, but the advance rate would be slower.•Frost frequency expects to decrease, ...while frost intensity expects to increase.•The larger advance in apple phenophase, the more frost risk is expected.
Changes in apple phenology associated with climate change have attracted extensive attention. However, it is poorly known whether the phenological dynamics responding to climate change would increase the severity and frequency of frost in apple trees. Here, we investigated the variation of phenophase (budburst and fruit-setting) and the frost risk for apple trees in the Loess Plateau combined with phenology models driven by Global Climate Models (GCMs) under two emission scenarios (SSP2-RCP4.5 and SSP5-RCP8.5 for two time periods 2050s and 2090s, respectively). The results showed that apple budburst and fruit-setting are expected to be advanced to varying degrees, but the rate of advance is decreasing (budburst 0.04–0.14 d∙y−1, fruit-setting 0.12–0.22 d∙y−1). The combinations of high emission scenarios and ‘far’ time periods (SSP5-RCP8.5, 2090s) in budburst and fruit-setting advance larger than conservative emission scenarios and ‘near’ time periods (SSP2-RCP4.5, 2050s). Furthermore, although frost frequency is expected to decrease about 0.09–0.36 d under both SSP2-RCP4.5 and SSP5-RCP8.5, frost intensity tends to increase about 0.004–0.008 °C·d−1 (except SSP5-RCP8.5, 2090s). Due to the different directions of changes in frequency and intensity of frost under future time periods, overall frost risk showed regional differences. The unchanged or decreased frost risk distributed in the northern and southern areas of the study areas, while the increased frost risk mainly distributed across the central areas of the study areas. Finally, the advancement of two phenophase was disproportionately related to frost risks in the geographic distributions. Areas with large advance in phenophase were themselves expected to be at greater frost risk. Our findings will help to promote local preventative interventions for reasonably reducing the risk of late spring frosts in future climate warming scenarios.
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•In autumn plantings, Scots pine seedlings are sensitive to harsh winter conditions.•Norway spruce seedlings can be planted even in October when suitable sites are selected.•Norway spruce container ...seedlings should be planted properly in mineral soil mounds.•The risk of spring frost damage may increase in spruce seedlings planted in summer and early autumn.
Due to the lack of labor and the mechanization of planting work, seedlings should be planted during the entire period of unfrozen soil, from early spring to late autumn. In experimental studies, it is difficult to investigate all the risk factors related to different planting windows. In Finland, the winter of 2015–2016 was harsh for seedlings. First temperatures were above zero, then they suddenly dropped to <−30 °C without any protective snow cover. These conditions offered a good opportunity to study the success of autumn plantings and clarify which factors best predict the field performance of autumn planted conifer seedlings. In the survey, 22 regeneration sites planted with Scots pine (Pinus sylvestris L.) and 71 with Norway spruce (Picea abies (L.) Karst.) seedlings were inventoried in the summer of 2016. Scots pine seedlings were more sensitive, and the regeneration result was good only in a few sites. With Norway spruce, the results were better although both good and poor results were found. For Norway spruce, we modeled the probability of severe damage and leader growth damage separately for summer (July and August), September and October plantings. The most important factors affecting the probability of damage was the quality of the planting and the planting spot as well as the soil type. If a planting hole was open or a seedling was planted in a planting spot other than a mineral-covered mound, the risk of damage was high. In fine and course textured soils, as well as medium coarse soils with a high risk of frost heaving, the probability of damage was also high. Low precipitation after planting and during the following spring also increased the probability of leader growth damage. Planting in the summer and early September exposed seedlings to leader growth damage, especially when the seedlings were planted in sites with a high risk of frost damage in autumn and spring. The results indicate that the planting of Scots pine seedlings in the autumn should be avoided. Norway spruce seedlings can be planted even in October when the work quality is good and suitable sites are selected for plantings.
•We examined changes in the late spring frost damage of flower buds for some deciduous fruit species.•We confirmed that DTA is a valid method in estimating of flower bud in deciduous fruit species ...frost tolerance.•CDP in flower buds of deciduous fruit species that failed to produce two distinct exotherms was questioned for the first time by DTA after and during budbreak.•This is the first comprehensive report indicated that the starting time of ice formation and time spans of ice propagation in whole flower buds by using the DTA.
The fine details of freezing behavior in flower buds during deacclimation and their exothermic characteristics are poorly understood with the use of differential thermal analysis (DTA). Understanding how spring frosts affect cold tolerance of flower buds is a fundamental question in deciduous fruit species and will be useful in developing cold-tolerant varieties and species. The occurrence of cell death point (CDP), bud death time (BDT) and exothermic characteristics in apple (Malus domestica L. 'Golden Delicious'), pear (Pyrus communis L. 'Santa Maria'), quince (Cydonia oblonga L. a native wild 'Type- 24'), sour cherry (Prunus cerasus L. 'Kütahya'), wild apricot (Prunus armeniaca L. 'Zerdali-24'), nectarine (Prunus persica var. nectarine L. 'Armking'), peach (Prunus persica L. 'Loring'), plum (Prunus salicina L. 'Formosa') and cherry (Prunus avium L. 'native wild type') flower buds in pink-white bloom and at the full bloom stages were determined using DTA. During controlled freezing of flower buds in deciduous fruit species one distinct freezing events (single exothermic peak) was detected by DTA, and it was considered the cell death point of the flower buds. The fruit species were classified as most (sour cherry, peach, quince and cherry), moderately hardy (apple and pear) and least hardy (wild apricot, plum and nectarine) according to tolerance to low temperatures. The highest mean BDT values of flower buds were observed in pear and nectarine species compared to other species. The flower buds of nectarine and pear species showed large and wider single exothermic peaks compared to quince, sour cherry, wild apricot, plum, apple, cherry and peach species. The single exotherms in the flower buds of the fruit species support the hypothesis that supercooling in buds is lost mainly starting with bud swell and usually spreads rapid ice nucleation into the flower buds due to established xylem continuity between the woody tissues and the bud scales. This hypothesis, however, reflects the complexity of the freezing process in flower buds, and so indicates that comprehensive observational studies are essential to understanding freezing tolerance traits in deciduous fruit species, after and during budbreak.
•We applied machine learning to EVI profiles to iden6fy SFD pixels.•We analyzed the influence of topographic factors on SFD pixels distribu6on.•We analyzed the produc6vity loss of SFD pixels compared ...to the previous 15 years.•Eleva6on proved to be the main topographic driver for SFD pixels distribu6on.•A produc6vity loss of 14% occurred compared to the previous 15 years.
In common beech forests the most damaging frosts are those that occur at the end of spring. At that time the fresh new leaves are at a vulnerable stage and risk to be readily killed by the freezing temperatures. The ability to identify late spring frost spatial dynamics is a key issue for understanding forest patterns and processes linked to such extreme event. The aim of this study is to detect, map and quantify the vegetation anomalies that occurred in the mono-specific beech forest of the Lazio, Abruzzo and Molise National Park (Italy) after an exceptional spring frost recorded on the 25th of April 2016. Results showed that, beech forests at lower elevations that had an early greening process were subject to spring frost damage (SFD pixels) and their productivity performance strongly decreased with respect to the previous 15 years; to the contrary the beech forests located at higher elevations did not suffer the spring frost effects (NSFD pixels) thanks to their delayed leaf unfolding phase. The duration of the effects of freezing stress for the SFD pixels was about two months, until the end of June, confirmed by Net Ecosystem Exchange measurements. This greening hiatus led to an average 14% loss of productivity compared to the previous 15 years. Elevation had a significant role on the probability of occurrence of SFD pixels. Productivity loss in SFD pixels was more severe at elevations in the range 1500–1700 m, on steeply terrains and North aspects. This study represents a step forward the systematic use of automated techniques to study areas subject to stress or anomalies from multitemporal satellite imagery and to identify break points and recovery of the greening process.
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