Seasonality of cavitation and frost fatigue in Acer mono Maxim.

Although cavitation is common in plants, it is unknown whether the cavitation resistance of xylem is seasonally constant or variable. We tested the changes in cavitation resistance of Acer mono before and after a controlled cavitation–refilling and freeze–thaw cycles for a whole year. Cavitation resistance was determined from ‘vulnerability curves’ showing the percent loss of conductivity versus xylem tension. Cavitation fatigue was defined as a reduction of cavitation resistance following a cavitation–refilling cycle, whereas frost fatigue was caused by a freeze–thaw cycle. A. mono developed seasonal changes in native embolisms; values were relatively high during winter but relatively low and constant throughout the growing season. Cavitation fatigue occurred and changed seasonally during the 12‐month cycle; the greatest fatigue response occurred during summer and the weakest during winter, and the transitions occurred during spring and autumn. A. mono was highly resistant to frost damage during the relatively mild winter months; however, a quite different situation occurred during the growing season, as the seasonal trend of frost fatigue was strikingly similar to that of cavitation fatigue. Seasonality changes in cavitation resistance may be caused by seasonal changes in the mechanical properties of the pit membranes.     

Does freezing and dynamic flexing of frozen branches impact the cavitation resistance of Malus domestica and the Populus clone Walker?

Frost damage to the xylem conduits of trees is a phenomenon of eco-physiological importance. It is often documented in terms of the percentage loss of conductivity (PLC), an indicator of air filling of the conduits. However, trees that refill their conduits in spring could be impacted more by damage to the conduits that reduce cavitation resistance, making them more susceptible to future drought events. We investigated whether ice formation, dynamic flexing of frozen branches or freeze–thaw events could reduce the cavitation resistance (cause “frost fatigue”) in first-year shoots of apple (Malus domestica) and clonal hybrid cottonwood (Walker). Frost fatigue was measured in terms of P₅₀ (the negative xylem pressure required to cause a 50 % loss of conductivity). All treatment groups showed significant frost fatigue, with the exception of the pre-flushed, constantly frozen poplar branches. The P₅₀ following freeze treatments was approximately 50 % of the pre-freeze values. The effect tended to be greater in freeze–thawed branches. Dynamic bending of the branches had no effect on either PLC or P₅₀. In three out of four cases, there was a significant correlation between P₅₀ and PLC. Frost fatigue occurred in both apple and poplar, two unrelated species with different drought and frost tolerances, suggesting that it may be a widespread phenomenon that could impact the ecophysiology of temperate forests.     

The impact of subcanopy light environment on the hydraulic vulnerability of Rhododendron maximum to freeze-thaw cycles and drought

The vulnerability of xylem to embolism development in Rhododendron maximum L., an evergreen diffuse-porous shrub, was investigated in relation to the frequency of winter freeze–thaw cycles in high and low light sites of the Eastern US. Though the frequency of freeze–thaw cycles during the winter was lower in North Carolina than in Virginia, the hydraulic conductivity of 3-year-old branches was reduced by up to 60% by winter embolism development in North Carolina compared to less than 30% in Virginia. Generally, small vessel diameters and volumes were associated with a significant resistance to embolism formation resulting from repeated freeze–thaws of xylem sap. In stems grown in high light sites (gaps), larger vessel volumes, and greater diameter growth of stems were associated with a significantly higher degree of freeze–thaw embolism development than in those grown in the low light sites. Thus, the growth patterns of R. maximum stems, under conditions of higher light availability, rendered them more susceptible to freeze–thaw-induced embolisms. Vulnerability to drought-induced embolism in stems was not affected by light environment. Rhododendron maximum was relatively sensitive to drought-induced embolism because 50% loss of hydraulic conductivity occurred at a water potential of -2.2 MPa. The distribution and gas exchange of R. maximum are constrained by the dual effects of freeze-thaw cycles and drought on vascular function.     

Freeze/thaw stress in<Emphasis Type="Italic"> Ceanothus</Emphasis> of southern California chaparral

Freeze/thaw stress was examined in chaparral shrubs of the genus Ceanothus to determine the interactive effects of freezing and drought and to consider which is the more vulnerable component, the living leaves (symplast) or the non-living water transport system (apoplast). We hypothesized that where Ceanothus species co-occurred, the more inland species C. crassifolius would be more tolerant of low temperatures than the coastal species C. spinosus, both in terms of leaf survival (LT(50), or the temperature at which there is 50% loss of function or viability) and in terms of resistance to freezing-induced embolism (measurements of percent loss hydraulic conductivity due to embolism following freeze/thaw). Cooling experiments on 2 m long winter-acclimated shoots resulted in LT(50) values of about -10 degrees C for C. spinosus versus -18 degrees C for C. crassifolius. Freeze-thaw cycles resulted in no change in embolism when the plants were well hydrated (-0.7 to -2.0 MPa). However, when plants were dehydrated to -5.0 MPa, C. spinosus became 96% embolized with freeze/thaw, versus only 61% embolism for C. crassifolius. Stems of C. crassifolius became 90% and 97% embolized at -6.6 and -8.0 MPa, respectively, meaning that even in this species, stems could be more vulnerable than leaves under conditions of extreme water stress combined with freeze/thaw events. The dominance of C. crassifolius at colder sites and the restriction of C. spinosus to warmer sites are consistent with both the relative tolerance of their symplasts to low temperatures and the relative tolerance of their apoplasts to freeze events in combination with drought stress.     

Xylem dysfunction caused by water stress and freezing in two species of co-occurring chaparral shrubs

Water transport from the roots to leaves in chaparral shrubs of California occurs through xylem vessels and tracheids. The formation of gas bubbles in xylem can block water transport (gas embolism), leading to shoot dieback. Two environmental factors that cause gas embolism formation in xylem conduits are drought and freezing air temperatures. We compared the differential vulnerabilities of Rhus laurina and Ceanothus megacarpus, co-dominant shrub species in the coastal regions of the Santa Monica Mountains of southern California, to both water stress-induced and freezing-induced embolism of their xylem. Rhus laurina has relatively large xylem vessel diameters, a deep root system, and a large basal burl from which it vigorously resprouts after wildfire or freezing injury. In contrast, Ceanothus megacarpus has small-diameter vessels, a shallow root system, no basal burl and is a non-sprouter after shoot removal by wildfire. We found that R. laurina became 50% embolized at a water stress of –3 MPa and 100% embolized by a freeze–thaw cycle at all hydration levels. In contrast, C. megacarpus became 50% embolized at a water stress of –9 MPa and 100% embolized by freeze–thaw events only at water potentials lower than –3 MPa. Reducing thaw rates from 0·8 °C min^?1 to 0·08 °C min^?1 (the normal thaw rate measured in situ) had no effect on embolism formation in R. laurina but significantly reduced embolism occurrence in well-hydrated C. megacarpus (embolism reduced from 74 to 35%). These results were consistent with the theory of gas bubble formation and dissolution in xylem sap. They also agree with field observations of differential shoot dieback in these two species after a natural freeze–thaw event in the Santa Monica Mountains.     

Investigations Concerning Cavitation and Frost Fatigue in Clonal 84K Poplar Using High-Resolution Cavitron Measurements

Both drought and freezing-thawing of stems induce a loss of hydraulic conductivity (percentage loss of conductivity [PLC]) in woody plants. Drought-induced PLC is often accompanied by physical damage to pit membranes, causing a shift in vulnerability curves (cavitation fatigue). Hence, if cavitated stems are flushed to remove embolisms, the next vulnerability curve is different (shifted to lower tensions). The 84K poplar (Populus alba × Populus glandulosa) clone has small vessels that should be immune from frost-induced PLC, but results demonstrated that freezing-thawing in combination with tension synergistically increased PLC. Frost fatigue has already been defined, which is similar to cavitation fatigue but induced by freezing. Frost fatigue caused a transition from a single to a dual Weibull curve, but drought-fatigued stems had single Weibull curves shifted to lower tensions. Studying the combined impact of tension plus freezing on fatigue provided evidence that the mechanism of frost fatigue may be the extra water tension induced by freezing or thawing while spinning stems in a centrifuge rather than direct ice damage. A hypothesis is advanced that tension is enhanced as ice crystals grow or melt during the freeze or thaw event, respectively, causing a nearly identical fatigue event to that induced by drought.Water transport in xylem conduits of trees occurs while water is under tension (negative pressure; Tyree and Zimmermann, 2002). The xylem water-transport system is vulnerable to cavitation and embolism because tensile water is metastable, so if a gas bubble appears in a conduit it will rapidly expand to fill the conduit whenever the fluid tension is 0.1 MPa or greater, where a tension of 0.1 MPa is equivalent to vacuum pressure. A cavitation event occurs whenever a tensile water column breaks, which results in a water vapor-filled void. Because of Henry’s law of gas solubility in water, this vapor void will eventually equilibrate with air at atmospheric pressure, at which point the conduit is fully embolized (Tyree and Zimmermann, 2002). Embolism has been identified as a limiting factor of primary production (Hubbard et al., 2001). As a result, tree growth and fitness are probably negatively impacted temporarily or seriously limited permanently if embolism is extensive (Christensen-Dalsgaard and Tyree, 2013).The two main factors causing cavitation and embolism are drought and frost (Mayr et al., 2003; Christensen-Dalsgaard and Tyree, 2013). Drought-induced cavitation is caused by the high xylem tension attributed to water stress. The high tension in the sap forces air bubbles into functional conduits from neighboring embolized ones through shared pit membranes (Jarbeau et al., 1995; Sperry et al., 1996; Hacke et al., 2001; Stiller and Sperry, 2002; Christman et al., 2012) according to an air-seeding mechanism (Sperry and Tyree, 1988; Cochard et al., 1992). Hence, the continuity of water flow is disrupted due to cavitation. Frost-induced cavitation, on the other hand, occurs when dissolved gases in the sap freeze out and create bubbles during ice formation because air is not soluble in ice (Mayr et al., 2003; Christensen-Dalsgaard and Tyree, 2013, 2014) but remains entrapped between ice crystals. Once the sap melts and tension is regenerated, the entrapped bubbles may expand to embolize the conduits instead of dissolving (Pittermann and Sperry, 2006). Current thinking is that freezing-induced embolism occurs when the tension exceeds a critical value determined by the surface tension of the bubbles, which mainly depends on the xylem water potential and the bubble radius (Yang and Tyree, 1992; Tyree et al., 1994; Hacke and Sperry, 2001). Larger bubbles may form in conduits with a larger diameter, so species with larger conduits are more vulnerable to frost-induced embolism (Langan et al., 1997; Davis et al., 1999; Pittermann and Sperry, 2006). Furthermore, enhanced loss of hydraulic conductivity (K_h) of trees may occur when stems are subjected to a combination of frost and drought causing low xylem water potential (Mayr et al., 2003; Willson and Jackson, 2006) and repeated freeze-thaw cycles (Sperry and Sullivan, 1992; Cox and Zhu, 2003; Mayr et al., 2003). However, even trees with small conduits are found to suffer severe embolism in winter (Sperry et al., 1988; Améglio et al., 2002) due mostly to freezing-drying of stems.Resilient species are those that suffer no significantly different cavitation resistance before and after a cavitation-refilling cycle (Hacke et al., 2001; Christensen-Dalsgaard and Tyree, 2013). In contrast, species that are weakened by cavitation or frost are said to suffer cavitation fatigue or frost fatigue. Cavitation or frost fatigue is quantified by how much the vulnerability curve is shifted before versus after a fatigue-inducing event, and it is typically reported as a shift in P₅₀, which is either the pressure (negative value) or tension (positive value) that produces 50% loss of K_h. In the rest of this article, we will use T₅₀ to indicate the tension at 50% loss of conductivity or T_x to indicate the tension that induces x% loss of conductivity. Vulnerability curves (VCs) are usually measured by a centrifuge technique (Alder et al., 1997; Cochard et al., 2005), but most researchers measure just five or six points to determine a VC. High-resolution VC curves with nine to 27 points per curve can be collected quickly using the Cochard rotor. Recent studies have successfully used high-resolution VC to characterize the detailed shape of VCs, revealing dual Weibull curves (e.g. r- and s-shaped or dual s-shaped curves; Cai et al., 2014; Wang et al., 2014a) because a complex shape to a VC cannot be identified with just a few points. Furthermore, we used a centrifuge to induce tension while simultaneously freezing in order to study the combined impact of tension and freezing-thawing on frost fatigue and freeze-thaw-induced embolism.In this article, we intend to investigate whether drought and freeze-thaw cycles could have an effect on the cavitation resistance in terminal shoots from adult trees of 84K poplar (Populus alba × Populus glandulosa), with high-resolution analysis of VCs and an artificial freeze-thaw simulation technique. Populus spp. are known to be water-demanding, drought-sensitive species with T₅₀ ranging from 1.07 to 2.5 MPa (Fichot et al., 2015) and vulnerable to winter damage (Feng et al., 2010). Among poplars, 84K poplar is known by foresters to be relatively resistant to water stress, low temperature, diseases, and insects (Zhou et al., 2007). As the main afforestation species in Shaanxi, Gansu, and Qinghai Province, 84K poplar is of great ecological importance.     

Physiological acclimation to drought stress in Solidago canadensis

Solidago canadensis is an invasive species from North America that is spreading across Europe, Australia and temperate Asia. We hypothesized that the species' wide ecological amplitude is also based on its potential in hydraulic acclimation, and analyzed hydraulic and anatomical properties along a transect with decreasing soil humidity. Stem hydraulic conductivity, vulnerability to drought‐induced embolism, stomatal closure during dehydration and xylem‐anatomical parameters were quantified at three sites. At the humid site, specific hydraulic conductivity of stems (1.0 ± 0.2 kg m^–1 MPa^–1 s^–1) was about twofold higher, and leaf‐specific conductivity about 1.5 times higher (3.1 ± 0.5 kg m^–1 MPa^–1 s^–1) than at the dry site. Water potential (Ψ) at 50% loss of conductivity was ?3.7 ± 0.1 MPa at the dry site and ?3.1 ± 0.2 MPa at the humid site (September). Vulnerability to drought‐induced embolism decreased along the transect and over the vegetation period. At drier sites, stomata started closing at lower Ψ while complete stomatal closure was reached at less negative Ψ (12% of maximum stomatal conductance: –2.5 ± 0.0 and ?3.0 ± 0.2 MPa at the dry and humid site). The safety margin between stomatal closure and 50% loss of conductivity was 1.2 and 0.2 MPa at the dry and humid sites. The observed variability indicated an efficient acclimation in hydraulic conductivity and safety: plants at dry sites exhibited lower specific hydraulic conductivity, higher embolism resistance and broader safety margins, signifying a trade‐off between the hydraulic safety and efficiency. The observed intraspecific plasticity in hydraulic and anatomical traits may help to explain the invasive potential of this species.     

Diurnal and seasonal variation in root xylem embolism in neotropical savanna woody species: impact on stomatal control of plant water status

Vulnerability to water-stress-induced embolism and variation in the degree of native embolism were measured in lateral roots of four co-occurring neotropical savanna tree species. Root embolism varied diurnally and seasonally. Late in the dry season, loss of root xylem conductivity reached 80% in the afternoon when root water potential (psi root) was about -2.6 MPa, and recovered to 25-40% loss of conductivity in the morning when psi root was about -1.0 MPa. Daily variation in psi root decreased, and root xylem vulnerability and capacitance increased with rooting depth. However, all species experienced seasonal minimum psi root close to complete hydraulic failure independent of their rooting depth or resistance to embolism. Predawn psi root was lower than psi soil when psi soil was relatively high (> -0.7 MPa) but became less negative than psi soil, later in the dry season, consistent with a transition from a disequilibrium between plant and soil psi induced by nocturnal transpiration to one induced by hydraulic redistribution of water from deeper soil layers. Shallow longitudinal root incisions external to the xylem prevented reversal of embolism overnight, suggesting that root mechanical integrity was necessary for recovery, consistent with the hypothesis that if embolism is a function of tension, refilling may be a function of internal pressure imbalances. All species shared a common relationship in which maximum daily stomatal conductance declined linearly with increasing afternoon loss of root conductivity over the course of the dry season. Daily embolism and refilling in roots is a common occurrence and thus may be an inherent component of a hydraulic signaling mechanism enabling stomata to maintain the integrity of the hydraulic pipeline in long-lived structures such as stems.     

Xylem embolism in response to freeze-thaw cycles and water stress in ring-porous, diffuse-porous, and conifer species

Vulnerability to xylem embolism by freeze-thaw cycles and water stress was quantified in ring-porous (Quercus gambelii Nutt.), diffuse-porous (Populus tremuloides Michx., Betula occidentalis Hook.), and conifer species (Abies lasiocarpa Nutt., Juniperus scopulorum Sarg.). Embolism was measured by its reduction of xylem hydraulic conductivity; it was induced by xylem tension (water-stress response) and by a tension plus a freeze-thaw cycle (freeze response). Conifers showed little (Juniperus) or no (Abies) freeze response even to repeated cycles. In contrast, Quercus embolized more than 90% by freezing at tensions below 0.2 MPa, whereas similar embolism without freezing required tensions above 4.5 MPa. Diffuse-porous trees (Betula, Populus) showed an intermediate freeze response. The magnitude of the freeze response was correlated with conduit volume but occurred at higher tensions than predicted from theory. Large early-wood vessels (2.8 × 10⁻⁹ m³) in oak were most vulnerable to embolism by freezing, small vessels in Populus and Betula were intermediate (approximately 7 × 10⁻¹¹ m³), and tracheids in conifers (about 3 × 10⁻¹³ m³) were most resistant. The same trend was found within a stem: embolism by freeze-thawing occurred preferentially in wider conduits. The water-stress response was not correlated with conduit volume; previous work indicates it is a function of interconduit pit membrane structure. Native embolism levels during winter corroborated laboratory results on freezing: Quercus embolized 95% with the first fall freeze, Populus and Betula showed gradual increases to more than 90% embolism by winter's end, and Abies remained below 30%.     

Measurement of vulnerability to water stress‐induced cavitation in grapevine: a comparison of four techniques applied to a long‐vesseled species

Among woody plants, grapevines are often described as highly vulnerable to water‐stress induced cavitation with emboli forming at slight tensions. However, we found native embolism never exceeded 30% despite low xylem water potentials (Ψ_x) for stems of field grown vines. The discrepancy between native embolism measurements and those of previous reports led us to assess vulnerability curve generation using four separate methods and alterations (i.e. segment length and with/without flushing to remove embolism prior to measurement) of each. Centrifuge, dehydration and air‐injection methods, which rely on measurement of percentage loss of hydraulic conductivity (PLC) in detached stems, were compared against non‐invasive monitoring of xylem cavitation with nuclear magnetic resonance (NMR) imaging. Short segment air‐injection and flushed centrifuge stems reached >90 PLC at Ψ_x of‐0.5 and ?1.5 MPa, respectively, whereas dehydration and long‐segment air‐injection measurements indicated no significant embolism at Ψ_x > ?2.0 MPa. Observations from NMR agreed with the dehydration and long segment air‐injection methods, showing the majority of vessels were still water‐filled at Ψ_x > ?1.5 MPa. Our findings show V. vinifera stems are far less vulnerable to water stress‐induced cavitation than previously reported, and dehydration and long segment air‐injection techniques are more appropriate for long‐vesseled species and organs.     

Spatial extent of winter thaw events in eastern North America: historical weather records in relation to yellow birch decline

An algorithm (Weather Reader) was developed and used to analyze daily weather records from all existing Canadian and American weather stations of eastern North America (in excess of 2100 stations), from 1930 through 2000. Specifically, the Weather Reader was used to compile daily minimum, mean, and maximum air temperatures for weather stations with at least 30 years of data, and was used to calculate accumulated degree days for winter thaw–freeze events relevant to yellow birch (Betula alleghaniensis Britt.) from beginning to end. A thaw–freeze event relevant to yellow birch was considered to take place when (i) the station daily maximum temperature reached or exceeded +4°C after being below freezing for at least 2 months of the winter, (ii) sufficient growing degree days accumulated (>50 growing degree days) to cause the affected yellow birch trees to prematurely deharden, and (iii) the daily minimum temperature dropped below ?4°C causing roots and/or shoots of dehardened trees to experience freeze‐induced injury and possibly dieback. The threshold temperature of +4°C represents the daily temperature above which biological activity occurs in yellow birch. The station growing degree day summaries were subsequently spatially interpolated with the Kriging function in GS+^? and mapped in ArcView^? GIS in order to display the geographic extent of the most severe thaw–freeze events. The ArcView^? maps were then compared with the extent of historically observed yellow birch decline. It was found that the years 1936, 1944, and 1945 were particularly uncharacteristic in terms of region‐wide winter thaw–freeze extremes, and also in terms of observed birch decline events during 1930–1960. An overlay of suspected accumulated birch decline based on thaw–freeze mapping and observed decline maps prepared by Braathe (1995), Auclair (1987) , and Auclair et al. (1997) for 1930–1960 demonstrated similar geographic patterns. The thaw–freeze projection for 1930–1960 was shown to coincide with 83% of the birch decline map appearing in Braathe (1995) and 55% of the geographic range of yellow birch in eastern North America. Thaw–freeze mapping was also applied to two significant events in 1981. Greatest impact was recorded to occur mostly in southern Quebec and Ontario, and several American Great Lake States, specifically in northern Michigan and New York, where the greatest growing degree day accumulation prior to refreeze in late February (February 28th) was projected to have occurred; and in southern Quebec, most of Atlantic Canada, and Maine, prior to a late spring frost in mid‐April (April 17).     

Cavitation resistance of peduncle,petiole and stem is correlated with bordered pit dimensions in Magnolia grandiflora

Variation in resistance of xylem to embolism among flowers, leaves, and stems strongly influences the survival and reproduction of plants. However, little is known about the vulnerability to xylem embolism under drought stress and their relationships to the anatomical traits of pits among reproductive and vegetative organs. In this study, we investigated the variation in xylem vulnerability to embolism in peduncles, petioles, and stems in a woody plant, Magnolia grandiflora. We analyzed the relationships between water potentials that induced 50% embolism (P₅₀) in peduncles, petioles, and stems and the conduit pit traits hypothesized to influence cavitation resistance. We found that peduncles were more vulnerable to cavitation than petioles and stems, supporting the hypothesis of hydraulic vulnerability segmentation that leaves and stems are prioritized over flowers during drought stress. Moreover, P₅₀ was significantly correlated with variation in the dimensions of inter-vessel pit apertures among peduncles, petioles and stems. These findings highlight that measuring xylem vulnerability to embolism in reproductive organs is essential for understanding the effect of drought on plant reproductive success and mortality under drought stress.     

Embolism spread in the primary xylem of Polystichum munitum: implications for water transport during seasonal drought

Xylem network structure and function have been characterized for many woody plants, but less is known about fern xylem, particularly in species endemic to climates where water is a limiting resource for months at a time. We characterized seasonal variability in soil moisture and frond water status in a common perennial fern in the redwood understory of a costal California, and then investigated the consequences of drought‐induced embolism on vascular function. Seasonal variability in air temperature and soil water content was minimal, and frond water potential declined slowly over the observational period. Our data show that Polystichum munitum was protected from significant drought‐induced hydraulic dysfunction during this growing season because of a combination of cavitation resistant conduits (Air‐seeding threshold (ASP) = ?1.53 MPa; xylem pressure inducing 50% loss of hydraulic conductivity (P₅₀) = ?3.02 MPa) and a soil with low moisture variability. High resolution micro‐computed tomography (MicroCT) imaging revealed patterns of embolism formation in vivo for the first time in ferns providing insight into the functional status of the xylem network under drought conditions. Together with stomatal conductance measurements, these data suggest that P. munitum is adapted to tolerate drier conditions than what was observed during the growing season.     

Extreme climatic events affect populations of Asian chestnut gall wasps,Dryocosmus kuriphilus,but do not stop the spread

1. Global climate change affects the frequency of extreme weather events that can influence plant–insect interactions.
2. We evaluated how the late-spring frost and severe drought that occurred in Spain in 2017 affected interactions between the invasive gall insect, Dryocosmus kuriphilus, and the native tree, Castanea sativa. We assessed effects on insect survival, fertility, population growth, and effects through changes in tree palatability and in other pests and pathogens.
3. Late-spring frost reduced D. kuriphilus to 25–40% of previous abundance. Wasp populations recovered rapidly (>7-fold in 3 years), consistent with density-dependence in population dynamics.
4. Larvae affected by freeze or drought were smaller. Female fecundity was affected by the freeze 1 year later.
5. Late-spring frosts and severe drought affected leaf size and physiology. Water content was higher within galls, but nitrogen was higher within galls in non-freeze plots after weather conditions improved.
6. Freezing also influenced the secondary chemistry of leaves. Phenol concentrations were lower, and terpenes higher, in frozen plots, while condensed tannins remained the same. Condensed tannins were reduced to half in the drought year.
7. Freezing had limited effects on damage from other pests and pathogens.
8. Our work expands understanding of how climate and weather affects forest pests.

     

Polytrichum Strictum as a Solution to Frost Heaving in Disturbed Ecosystems: A Case Study with Milled Peatlands

Substrate instability is a common problem in many disturbed ecosystems. In the case of milled harvested peatlands, the pioneer moss Polytrichum strictum is commonly found; it is well adapted to tolerate the harsh microclimatic conditions and peat instability of these sites. A field experiment was used to determine the effectiveness of P. strictum against frost heaving, a major type of disturbance on bare peat. Wooden dowels and fir trees (Abies balsamea) placed in a P. strictum carpet experienced almost no frost heaving, whereas heaving was severe on bare peat. Reintroduced P. strictum fragments thinly spread on bare peat reduced but did not eliminate frost heaving. Straw mulch (a protective cover often required in peatland restoration) effectively reduced heaving in the fall, but was less effective in the spring because it had partially decomposed. The P. strictum carpet, P. strictum fragments, and straw mulch reduced frost heaving by reducing the number of freeze–thaw cycles, by slowing the rate of ground thaw in the spring, and by reducing the unfrozen water content of the peat during the spring thaw. Different species of Polytrichum mosses should be considered for the restoration or regeneration of disturbed ecosystems where soil stability is problematic.     

Grapevine petioles are more sensitive to drought induced embolism than stems: evidence from in vivo MRI and microcomputed tomography observations of hydraulic vulnerability segmentation

The ‘hydraulic vulnerability segmentation’ hypothesis predicts that expendable distal organs are more susceptible to water stress‐induced embolism than the main stem of the plant. In the current work, we present the first in vivo visualization of this phenomenon. In two separate experiments, using magnetic resonance imaging or synchrotron‐based microcomputed tomography, grapevines (Vitis vinifera) were dehydrated while simultaneously scanning the main stems and petioles for the occurrence of emboli at different xylem pressures (Ψ_x). Magnetic resonance imaging revealed that 50% of the conductive xylem area of the petioles was embolized at a Ψ_x of ?1.54 MPa, whereas the stems did not reach similar losses until ?1.9 MPa. Microcomputed tomography confirmed these findings, showing that approximately half the vessels in the petioles were embolized at a Ψ_x of ?1.6 MPa, whereas only few were embolized in the stems. Petioles were shown to be more resistant to water stress‐induced embolism than previously measured with invasive hydraulic methods. The results provide the first direct evidence for the hydraulic vulnerability segmentation hypothesis and highlight its importance in grapevine responses to severe water stress. Additionally, these data suggest that air entry through the petiole into the stem is unlikely in grapevines during drought.     

Frost fatigue and spring recovery of xylem vessels in three diffuse‐porous trees in situ

Frost has been shown to cause frost fatigue (reduced cavitation resistance) in branch segments in the lab. Here, we studied the change in cavitation resistance and percent loss of conductivity (PLC) from fall to spring over 2 consecutive years in three diffuse‐porous species in situ. We used the cavitron technique to measure P₂₅, P₅₀ and P₉₀ (the xylem pressure causing a 25, 50 and 90% conductivity loss) and PLC and stained functioning vessels. Cavitation resistance was reduced by 64–87% (in terms of P₅₀), depending on the species and year. P₂₅ was impacted the most and P₉₀ the least, changing the vulnerability curves from s‐ to r‐shaped over the winter in all three species. The branches suffered an almost complete loss of conductivity, but frost fatigue did not necessarily occur concurrently with increases in PLC. In two species, there was a trade‐off between conduit size and vulnerability. Spring recovery occurred by growth of new vessels, and in two species by partial refilling of embolized conduits. Although newly grown and functioning conduits appeared more vulnerable to cavitation than year‐old vessels, cavitation resistance generally improved in spring, suggesting other mechanisms for partial frost fatigue repair.     

Effect of seasonal freeze–thaw cycle on net nitrogen mineralization of soil organic layer in the subalpine/alpine forests of western Sichuan,China

Nitrogen mineralization, a main way that soil organic nitrogen converts to mineral nitrogen, is one of the key processes in soil nitrogen cycle. The mineral nitrogen has an important role in plant growth in the growing season. It has been widely accepted that soil freezing in winter can kill a number of microorganisms, weakening soil nitrogen mineralization. However, more and more recent studies have documented that soil microorganisms still have high activity during the deep freezing period, and obvious nitrogen mineralization in winter. Seasonal freeze–thaw cycle is a common phenomenon in the subalpine/alpine forest region, which may have a strong effect on soil ecological processes. Furthermore, the changing pattern of seasonal freeze–thaw cycles might have a significant influence on soil nitrogen mineralization in this region in the scenarios of global warming. As yet, little attention has been given to nitrogen mineralization of soil organic layer as affected by changed seasonal freeze–thaw pattern, although the increasing studies have demonstrated that winter warming might give strong effects on the litter decomposition and microbial activity in the subalpine/alpine forest regions. Therefore, a method of intact soil core incubation in combination with natural environmental gradient was employed by transferring forest soils from 3582 m (A₁) of altitude to 3298 m (A₂) of altitude and 3023 m (A₃) of altitude in the subalpine/alpine forests of western Sichuan, respectively. The amounts and rates of net nitrogen mineralization in soil organic layer were measured. The incubation period included the growing season and the freeze–thaw season from May 24, 2010 to April 19, 2011. The results suggested that significant net nitrogen mineralization was only observed in soil organic layer at low altitude (A₃) during the whole incubation period. Forest soils at higher altitudes (A₁ and A₂) showed obvious soil nitrogen immobilization. In comparison with the growing season which showed remarkable nitrogen immobilization characteristic, the freeze–thaw season showed obvious nitrogen mineralization at lower altitudes (A₂ and A₃). In contrast, the nitrogen immobilization amounts at high altitude (A₁) in freeze–thaw period were less than those in the growing season. Besides, the maximum of net nitrogen mineralization amounts and rates at high altitude (A₁) in soil organic layer mainly occurred in the late stage of growing season and the onset of freezing, soil nitrogen mineralization at the middle altitude (A₂) mainly occurred in the onset of freezing and the deep freezing period, while the highest amount and rate of net nitrogen mineralization at low altitude (A₃) occurred in the early stage of thawing and the late stage of growing season. Furthermore, the amount and rate of soil net nitrogen mineralization during the freeze–thaw season were increasing with the decrease of altitude, which correlated with soil freeze–thaw cycle and freezing process at different altitudes. These results indicated that increasing soil temperature in the future could not only significantly enhance soil nitrogen mineralization in the freeze–thaw season, but also improve soil nitrogen mineralization by increasing freeze–thaw cycle times and shortening freeze–thaw period. However, the processes were significantly influenced by soil micro-environment of subalpine/alpine forest regions.     

Effects of freeze-thaw injury on parameters of distributed electrical circuits of stems and needles of Scots pine seedlings at different stages of acclimation

Frost hardening and frost injury of both the stems and needlesof Scots pine (Pinus sylvestris L.) seedlings were studied byelectrical impedance analysis. This impedance analysis was basedon equivalent circuits with distributed circuit elements (DCE).A double-DCE model for stems and two single-DCE models for needlesprovided excellent fit to the experimental impedance data. However,the single-DCE model for needles which takes into account theasymmetry of the impedance arc proved more appropriate thanthe symmetric model. Several parameters changed in proportionto frost injury. In stems, extracellular resistance and onerelaxation time decreased with increasing damage, whereas intracellularresistance remained relatively unchanged. In needles, the overallpattern for extracellular resistance and relaxation time wassimilar to that in the stem. Intracellular resistance remainedapproximately constant in the case of the symmetric DCE model.During frost hardening, both extracellular and intracellularresistance increased in stems. In needles, extracellular resistanceincreased but relaxation time decreased with hardening. Theskewness of the impedance spectra in the Cole-Cole plot forneedles increased with hardening. The coefficient for distributionof the relaxation time(s) did not change in either stems orneedles with frost hardening but some changes were found withfrost damage. Key words: Pinus sylvestris L, electrical impedance, equivalent circuit, frost hardening, frost injury     

Summer frost resistance and freezing patterns measured in situ in leaves of major alpine plant growth forms in relation to their upper distribution boundary

Summer frost resistance and ice nucleation temperatures for 33 alpine plant species were measured in situ to avoid the shortcomings of laboratory tests. Species were selected to investigate the relationship between plant stature and upper distribution boundary, and frost resistance and freezing patterns. The species tested in situ were on average 1.1 K (± 0.2, SE) frost hardier than in laboratory tests. Frost resistance (LT₅₀) ranged from ?4.5 to ?14.6 °C and appeared insufficient to protect against air temperature minima, corroborating reports of natural frost damage. All species tolerated extracellular ice formation (recorded at ?1.9 ± 0.2 °C; E1). Initial frost damage occurred at average temperatures 4.9 K below E1. In 64% of the species a second exotherm (E2) and frost damage were recorded between ?3.7 and ?9.4 °C. In the highest ranging species E2 was not detectable. Frost resistance increased with increasing upper distribution boundary (0.4 K per 100 m), corresponding well with the altitudinal decrease in air temperature minima. No relationship between plant stature and frost resistance was found. Graminoids were significantly frost hardier than other growth forms. Frost survival at high altitudes will depend not only on altitudinal increase in frost resistance but also on freezing avoidance strategies, snow cover protection and a high recuperation capacity.