Shredder abundance and leaf breakdown in an Appalachian Mountain stream

SUMMARY.

• 1 Breakdown rates of dogwood (Cornus florida L.), red maple (Acer rubrum L.) and white oak (Quercus alba L.) leaves were investigated at two first-order and two second-order sites in an Appalachian Mountain stream.
• 2 Leaves exposed in mesh bags were sampled on eight occasions over a 207 day period and breakdown rates were compared using an exponential decay model.
• 3 There was a consistent ranking in leaf breakdown rate within each site, i.e. dogwood > red maple > white oak, and all species broke down faster at second-than at first-order sites.
• 4 Our data suggest that differences in species-specific leaf breakdown rates were largely a function of shredder abundance on the leaves.

     

Xylem ray parenchyma cells in boreal hardwood species respond to subfreezing temperatures by deep supercooling that is accompanied by incomplete desiccation

It has been accepted that xylem ray parenchyma cells (XRPCs) in hardwood species respond to subfreezing temperatures either by deep supercooling or by extracellular freezing. Present study by cryo-scanning electron microscopy examined the freezing responses of XRPCs in five boreal hardwoods: Salix sachalinensis Fr. Schmit, Populus sieboldii Miq., Betula platyphylla Sukat. var japonica Hara, Betula pubescens Ehrh., and red osier dogwood (Cornus sericea), in which XRPCs have been reported to respond by extracellular freezing. Cryo-scanning electron microscopy observations revealed that slow cooling of xylem to -80 degrees C resulted in intracellular freezing in the majority of XRPCs in S. sachalinensis, an indication that these XRPCs had been deep supercooled. In contrast, in the majority of XRPCs in P. sieboldii, B. platyphylla, B. pubescens, and red osier dogwood, slow cooling to -80 degrees C produced slight cytorrhysis without clear evidence of intracellular freezing, suggesting that these XRPCs might respond by extracellular freezing. In these XRPCs exhibited putative extracellular freezing; however, deep etching revealed the apparent formation of intracellular ice crystals in restricted local areas. To confirm the occurrence of intracellular freezing, we rewarmed these XRPCs after cooling and observed very large intracellular ice crystals as a result of the recrystallization. Thus, the XRPCs in all the boreal hardwoods that we examined responded by deep supercooling that was accompanied with incomplete desiccation. From these results, it seems possible that limitations to the deep-supercooling ability of XRPCs might be a limiting factor for adaptation of hardwoods to cold climates.     

CHANGES IN THE ULTRASTRUCTURE OF XYLEM PARENCHYMA CELLS OF PEACH (PRUNUS PERSICA) AND RED OAK (QUERCUS RUBRA) IN RESPONSE TO A FREEZING STRESS

An ultrastructural investigation was conducted of xylem parenchyma cells of peach (Prunus persica [L.] Batsch.) cv. Harbrite and red oak (Quercus rubra L.) in response to a freezing stress. Freezing curves of xylem tissues, as determined by differential thermal analysis, were used to predict temperatures at which both living and dead cells would be observed. Tissues were exposed to low temperatures (-15 to -35 C) and fixed in a frozen state at -10C and at thawing. Current models of the freezing behavior of supercooled plant cells suggest that xylem parenchyma cells behave as individual water droplets. This implies that cells are unresponsive to the presence of low temperature and extracellular ice until internal nucleation triggers lethal, intracellular freezing. For these reasons, deep supercooling has been described as an avoidance mechanism. Results of this study confirmed earlier reports that xylem parenchyma cells freeze as individuals or in small groups. Individual cells, however, did not exhibit a neutral response. Instead, a range of responses was observed that included internal and external vesiculation, deep invaginations of the plasma membrane, and the formation of electron-dense deposits external to the plasmalemma. In general, our observations suggested that the cells responded to a dehydrative stress. Results are discussed in context of the biophysical data associated with deep supercooling phenomena and compared to responses of cells that exhibit extracellular freezing.     

Flowering dogwood (Cornus florida L.) as mediator of calcium cycling: new insights are revealed by analysis of foliar partitioning

Flowering dogwood (Cornus florida L.) is an important understory tree species that is thought to enhance ecological calcium (Ca) cycling and soil Ca availability through high foliar Ca concentrations and rapid leaf litter decomposition. Calcium is an essential macronutrient in plants, important for stabilizing cell walls and plasma membranes. It is also an ubiquitous intracellular second messenger, helping plants sense and physiologically respond to numerous environmental cues. Analyses of total foliar Ca can be dominated by chemically sequestered Ca, which is not readily available for cellular processes. Thus, analyses of specific foliar partitions of Ca are more closely tied to Ca-dependent processes such as signal transduction. To further develop our understanding of the role of flowering dogwood in ecological Ca cycling, we evaluated foliar Ca partitioning via sequential acidic extractions. We compared Ca partitioning in flowering dogwood to that of white oak (Quercus alba L.), and found significantly more labile Ca in dogwood and a much greater proportion of Ca sequestration in oak. We compared foliar Ca partitioning in white oak at sites with dogwood to that of oaks at dogwood-absent sites, and found significantly greater labile Ca in oaks where dogwood was present. We also investigated the phenological patterns of Ca partitioning and sequestration in flowering dogwood foliage, and found preferential partitioning of Ca into the more labile and physiologically accessible pools throughout the growing season, with minimal Ca sequestration. This work helps elucidate the mechanisms and consequences associated with Ca cycling by flowering dogwood in forested systems.     

Units of freezing of deep supercooled water in woody xylem

The low temperature exotherms (LTE) of 1-year-old twigs of Haralson apple (Malus pumila Mill.), shagbark hickory (Carya ovata [Mill.] K. Koch), green ash (Fraxinus pennsylvanica Marsh), honey locust (Gleditsia triacanthos L.), American chestnut (Castanea dentata [Marsh] Borkh.), and red oak (Quercus rubra L.) were determined by differential thermal analysis (DTA). In one type of experiment freezing during a DTA experiment was halted for up to 2.5 hours after part of the supercooled water had frozen at temperatures between −25 and −42 C. Upon resumption of cooling the freezing started within 2 C of the stopping temperature. In a second type of experiment living and dead cells were microscopically observed in the same ray after partial freezing in the DTA apparatus. In another experiment, the LTE persisted even after tangential and radial sectioning of the twig to 0.13 millimeters. In a final experiment the LTE of a single multiseriate ray of red oak had the same shape as the LTE of wood with many uniseriate rays.     

Response of Xylem Ray Parenchyma Cells of Red Osier Dogwood (Cornus sericea L.) to Freezing Stress (Microscopic Evidence of Protoplasm Contraction)

Freezing behavior of wood tissue of red osier dogwood (Cornus sericea L.) cannot be explained by current concepts of freezing resistance. Previous studies indicated that water in wood tissue presumably froze extracellularly. However, it was observed that xylem ray parenchyma cells within these tissues could survive temperatures as low as -80[deg]C and the walls of these cells did not collapse during freezing (S.R. Malone and E.N. Ashworth [1991] Plant Physiol 95: 871-881). This observation was unexpected and is inconsistent with the current hypothesis of cell response during freezing. Hence, the objective of our study was to further examine the mechanism of freezing resistance of wood tissue of red osier dogwood. We studied freezing stress response of xylem ray parenchyma cells of red osier dogwood using freeze substitution and transmission electron microscopy. Wood samples were collected in winter, spring, and summer of 1992. Specimens were cooled from 0[deg]C to -60[deg]C at 5[deg]C/h. Freezing stress did not affect the structural organization of wood tissue. However, the xylem ray parenchyma cells showed two unique responses to a freezing stress: protoplasm contraction and protoplasm fragmentation. Protoplasm contraction was evident at all freezing temperatures and in tissues collected at different times of the year. Cells with fragmented protoplasm, however, were noticed only in tissues collected in spring and summer. Protoplasm contraction in winter tissue occurred without apparent damage to the protoplasm. In contrast, protoplasm contraction in spring and summer tissues was accompanied by substantial damage. No evidence of intracellular ice formation was observed in parenchyma cells exposed to freezing stress. Differences in protoplasm contraction and appearance of cells with fragmented protoplasm likely indicated seasonal changes in cold hardiness of the wood tissue of red osier dogwood. We speculate that the appearance of fragmented protoplasm may indicate that cells are being injured by an alternative mechanism in spring and summer.     

Induction of Frost Hardiness in Stem Cortical Tissues of Cornus stolonifera Michx. by Water Stress: II. Biochemical Changes

A decrease of protein, RNAs, and starch, and an increase of sugar were observed in 3-day water-stressed red osier dogwood plants (Cornus stolonifera Michx.) when the frost hardiness increased from −3 to −6 C. As the frost hardiness increased to −11 C after 7 days of treatment, the starch continuously decreased, however, the proteins and RNAs increased with a continuous increase of sugar. Further water stress treatment had little effect on the changes of these chemicals. Control plants in short days showed similar gradual biochemical changes in patterns. From the results of frost hardiness increases, the pattern of biochemical changes, and the mechanism of the increased freezing resistance, it appears that the water stress and short days accomplished essentially the same physiological end(s) in inducing frost hardiness in red-osier dogwood.     

Induction of Frost Hardiness in Stem Cortical Tissues of Cornus stolonifera Michx. by Water Stress: I. Unfrozen Water in Cortical Tissues and Water Status in Plants and Soil

Water supply and day length were varied in cold hardiness studies of red osier dogwood plants (Cornus stolonifera Michx.). The frost killing temperature, the content and freezing of stem cortical tissue water along with soil moisture content and tension were evaluated. Seven days of water stress in long and short day photoperiod regimes caused a rapid decrease in soil moisture content and plant water potential. During the same period, the frost hardiness increased from −3 to −11 C. Further water stress treatment had little effect. Control plants in short days showed only a gradual decrease in plant water potential and only gradually increased in frost hardiness while control plants in long days were unchanged. Freezing studies using nuclear magnetic resonance showed that increased hardiness in water-stressed plants resulted from both an increased tolerance of freezing and an increased avoidance of freezing, the latter resulting from higher solute concentration in the tissue solutions. The short day controls also showed similar changes; however, the changes were smaller over the 21 days of the study.     

Ultrastructural Evidence That Intracellular Ice Formation and Possibly Cavitation Are the Sources of Freezing Injury in Supercooling Wood Tissue of Cornus florida L

Although cellular injury in some woody plants has been correlated with freezing of supercooled water, there is no direct evidence that intracellular ice formation is responsible for the injury. In this study we tested the hypothesis that injury to xylem ray parenchyma cells in supercooling tissues is caused by intracellular ice formation. The ultrastructure of freezing-stress response in xylem ray parenchyma cells of flowering dogwood (Cornus florida L.) was determined in tissue prepared by freeze substitution. Wood tissue was collected in the winter, spring, and summer of 1992. Specimens were cooled from 0 to -60[deg]C at a rate of 5[deg]C h-1. Freezing stress did not affect the structural organization of wood tissue, but xylem ray parenchyma cells suffered severe injury in the form of intracellular ice crystals. The temperatures at which the ice crystals were first observed depended on the season in which the tissue was collected. Intracellular ice formation was observed at -20, -10, and -5[deg]C in winter, spring, and summer, respectively. Another type of freezing injury was manifested by fragmented protoplasm with indistinguishable plasma membranes and damaged cell ultrastructure but no evidence of intracellular ice. Intracellular cavitation may be a source of freezing injury in xylem ray parenchyma cells of flowering dogwood.     

Investigation of the photosynthetic activity of bark phelloderm of arboreous plants using the fluorescent method

Seasonal changes in the characteristics of chlorophyll fluorescence were studied in the bark of several species of trees originating in various climatic zones: Siberian cedar (Pinus sibirica), larch (Larix sibirica), eastern arborvitae (Thuja occidentalis), pendent white birch (Betula pendula), wild black cherry (Padus virginiana), horse chestnut (Aesculus hippocastanum), red oak (Quercus rubra), Manchurian catalpa (Catalpa bungei), linden (Tilia cordata), goat willow (Salix caprea), Amur cherry (Padus maackii), and apple Korichnaya (Malus domestrica B.). Tree bark has a sufficient amount of chlorophyll for measuring the parameters of chlorophyll fluorescence throughout the year. The relative yield of the variable fluorescence of chlorophyll (F _v/F _m) can be used to assess seasonal changes in the physiological state of various trees.     

Seasonal variations in freezing curves of stem sections of Cornus stolonifera MICHX.I

Comparisons of freezing curves have been used to determine theviability of plant parts exposed to stress. To gain understandingof the natural seasonal variations in freezing curves, uniformtwig sections of red-osier dogwood (Cornus stolonifera MICHX.)were collected throughout the year from a single clone and subjectedto controlled freezing while the tissue temperature was recorded.The supercooling of samples ranges from –2 to –7,but the variation was random and unpredictable. There was noapparent relationship between supercooling and the season ofthe year or the hardiness of the tissue. The freezing pointdepression, as estimated by the temperature of the first freezingplateau, was always between –0.25 and –1.0 andbore no relationship to hardiness or season. The freezing curveswere basically of three types: Summer and winter curves withtwo distinct freezing points; Early autumn curves with 3distinctfreezing points and spring curves with one prominent first freezingpoint which tended to mask the second freezing point. ¹Scientific Journal Series paper No. 6628, Minnesota AgriculturalExperiment Station. This research was supported in part by agrant from the Louis W. and MAUD HILL Family Foundation. ²Present Address: Horticulture Department, University of Wisconsin,Madison, Wisconsin, U.S.A.     

Lethal freeze-dehydration injury of dogwood stem tissue does not change the activation energy of water permeability

The Arrhenius activation energy for water permeability, (ΔE_a,H2O) through stem cortical tissue of red osier dogwood (Cornus sericea L.) was determined after treatments which cause membrane rupture as well as after a lethal slow freeze and subsequent slow rewarming. The latter value was higher than the former, but was indistinguishable from the ΔE_a,H2O found for healthy tissue. It was concluded that membrane permeability to water is not altered during the first 24 to 48 hours after exposure of nonacclimated red osier dogwood to lethal freeze dehydration injury.     

Herbarium records in Arctic dwarf shrub dendrochronology: Methodological approach and perspectives

Our work aims to investigate whether herbaria resources can be used for the extension of Arctic dwarf shrub chronologies. The current use of herbaria reaches far beyond their initially aims; among the new applications, phenology observations and conservation biology can be mentioned. However, to this date, no studies on the use of herbarium specimens for dendrochronological research have been published. Examples of perennial plants from herbarium sheets that could potentially be used for such studies are dwarf shrubs, samples of which often consist of whole specimens, including the root system, the root collar and branches. Here, we present a protocol for the selection and processing of historical material. Based on the collections from Kew and Copenhagen, which are among of the largest herbaria with Arctic plants, a database of 25 areas from the Atlantic sector of the Arctic was created. Material from the following most common species was collected: grey willow (Salix glauca L.), polar willow (Salix polaris Wahlenb.), dwarf willow (Salix herbacea L.), net-leaved willow (Salix reticulata L.), arctic willow (Salix arctica Pall.), mountain avens (Dryas octopetala L.), dwarf birch (Betula nana L.). We present the preliminary results of a case study using historical samples of Salix arctica from the Thule (Qaanaaq) area, NW Greenland. Dwarf shrubs can commonly reach the age of 80–100 years or beyond, while herbaria resources may allow the extension of such series over the last centuries. Therewith, these resources may provide an excellent proxy data source on the changing natural environment beyond the northern and upper tree limits, where well-replicated proxy time-series remain sparse.     

Allocation of extracellular enzymatic activity in relation to litter composition, N deposition, and mass loss

Decomposition of plant material is a complex process that requiresinteraction among a diversity of microorganisms whose presence and activity issubject to regulation by a wide range of environmental factors. Analysis ofextracellular enzyme activity (EEA) provides a way to relate the functionalorganization of microdecomposer communities to environmental variables. In thisstudy, we examined EEA in relation to litter composition and nitrogendeposition. Mesh bags containing senescent leaves of Quercusborealis (red oak), Acer rubrum (red maple) andCornus florida (flowering dogwood) were placed on forestfloor plots in southeastern New York. One-third of the plots were sprayedmonthly with distilled water. The other plots were sprayed monthly withNH₄NO₃ solution at dose rates equivalent to 2 or 8 g N m^–2 y^–1. Mass loss, litter composition, fungal mass, and the activities ofeight enzymes were measured on 13 dates for each litter type. Dogwood wasfollowed for one year, maple for two, oak for three. For each litter type andtreatment, enzymatic turnover activities were calculated from regressions of LN(%mass remaining) vs. cumulative activity. The decomposition of dogwood litterwas more efficient than that of maple and oak. Maple litter had the lowestfungal mass and required the most enzymatic work to decompose, even though itsmass loss rate was twice that of oak. Across litter types, N amendment reducedapparent enzymatic efficiencies and shifted EEA away from N acquisition andtoward P acquisition, and away from polyphenol oxidation and towardpolysaccharide hydrolysis. The effect of these shifts on decomposition ratevaried with litter composition: dogwood was stimulated, oak was inhibited andmaple showed mixed effects. The results show that relatively small shifts intheactivity of one or two critical enzymes can significantly alter decompositionrates.     

Seasonal changes in the freezing behavior of xylem ray parenchyma cells in four boreal hardwood species

The freezing behavior of xylem ray parenchyma cells in several boreal hardwood species, namely, Betula platyphylla, Populus canadensis, P. sieboldii, and Salix sachalinensis, was examined by differential thermal analysis (DTA), cryo-scanning electron microscopy (Cryo-SEM), and freeze-fracture replica electron microscopy. Although DTA profiles of samples harvested in summer and in winter suggested that the xylem ray parenchyma cells in all four species responded to freezing stress by extracellular freezing, Cryo-SEM showed clearly that the xylem ray parenchyma cells in all these species responded to freezing stress by shallow supercooling in summer and by extracellular freezing in winter. It is suggested that DTA failed to reveal the true freezing behavior of xylem ray parenchyma cells because of an overlap of temperature ranges between the high-temperature exotherm and the low-temperature exotherm and/or because of the limited extent of the LTE. The seasonal changes in freezing behavior of xylem ray parenchyma cells in all these boreal species, which are results of seasonal cold acclimation, support the hypothesis that a gradual shift of freezing behavior in xylem ray parenchyma cells from shallow supercooling in hardwood species that grow in tropical zones to extracellular freezing in hardwood species that grow in cold areas might be a result of the evolutionary adaptation of hardwood species to cold climates. Copyright 1999 Academic Press.     

Freezing of water in red-osier dogwood stems in relation to cold hardiness

Studies of stem water in red-osier dogwood (Cornus stolonifera Michx.) using nuclear magnetic resonance spectroscopy indicated that most freezing occurs at temperatures above −30 C in cold-hardy and tender stems. Hardy and tender stems had about the same amount of unfrozen water at −40 C (0.28 gram of water per gram dry weight). When hardy stems were slowly cooled below −20 C, the temperature below which little additional freezing occurs, they survived direct immersion in liquid N₂ (−196 C). Fully hardy samples not slowly precooled to at least −15 C did not survive direct immersion in liquid N₂. The results support the hypothesis that cooling rate is an unimportant factor in tissue survival at and below temperatures where there is little freezable water.     

Anti-ice nucleation activity in xylem extracts from trees that contain deep supercooling xylem parenchyma cells

Boreal hardwood species, including Japanese white birch (Betula platyphylla Sukat. var. japonica Hara), Japanese chestnut (Castanea crenata Sieb. et Zucc.), katsura tree (Cercidiphyllum japonicum Sieb. et Zucc.), Siebold’s beech (Fagus crenata Blume), mulberry (Morus bombycis Koidz.), and Japanese rowan (Sorbus commixta Hedl.), had xylem parenchyma cells (XPCs) that adapt to subfreezing temperatures by deep supercooling. Crude extracts from xylem in all these trees were found to have anti-ice nucleation activity that promoted supercooling capability of water as measured by a droplet freezing assay. The magnitude of increase in supercooling capability of water droplets in the presence of ice-nucleation bacteria, Erwinia ananas, was higher in the ranges from 0.1 to 1.7 °C on addition of crude xylem extracts than freezing temperature of water droplets on addition of glucose in the same concentration (100 mosmol/kg). Crude xylem extracts from C. japonicum provided the highest supercooling capability of water droplets. Our additional examination showed that crude xylem extracts from C. japonicum exhibited anti-ice nucleation activity toward water droplets containing a variety of heterogeneous ice nucleators, including ice-nucleation bacteria, not only E. ananas but also Pseudomonas syringae (NBRC3310) or Xanthomonas campestris, silver iodide or airborne impurities. However, crude xylem extracts from C. japonicum did not affect homogeneous ice nucleation temperature as analyzed by emulsified micro-water droplets. The possible role of such anti-ice nucleation activity in crude xylem extracts in deep supercooling of XPCs is discussed.     

Water Permeability and Cold Hardiness of Cortex Cells in Cornus stolonifera Michx.-A Preliminary Report

The relationship of freezing resistance to water permeability of cortex cells was studied in stems of red osier dogwood (Cornus stolonifera Michx.). Permeability was estimated by determining the diffusion flux of tritiated water from cortex slices previously equilibrated in tritiated water. Energy of activation and diffusion time comparisons of tritiated water flux from living cortex slices and slices killed by immersion in liquid N₂ verified that intact membranes of uninjured cortex cells limited water flux.     

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%.