Stephen G. Pallardy

The Physiological Ecology of Woody Plants

How Woody Plants Grow. Physiological and Environmental Requirements for Tree Growth. Establishment and Growth of Tree Stands.Radiation. Temperature. Soil Properties and Mineral Nutrition. Water Stress. Soil Aeration, Compaction, And Flooding. Air Pollution. Carbon Dioxide. Fire. Wind. Cultural Practices. Each Chapter Includes References. Index.

Acclimation and Adaptive Responses of Woody Plants to Environmental Stresses

AbstractThe predominant emphasis on harmful effects of environmental stresses on growth of woody plants has obscured some very beneficial effects of such stresses. Slowly increasing stresses may induce physiological adjustment that protects plants from the growth inhibition and/or injury that follow when environmental stresses are abruptly imposed. In addition, short exposures of woody plants to extreme environmental conditions at critical times in their development often improve growth. Furthermore, maintaining harvested seedlings and plant products at very low temperatures extends their longevity. Drought tolerance: Seedlings previously exposed to water stress often undergo less inhibition of growth and other processes following transplanting than do seedlings not previously exposed to such stress. Controlled wetting and drying cycles often promote early budset, dormancy, and drought tolerance. In many species increased drought tolerance following such cycles is associated with osmotic adjustment that involves accumulation of osmotically active substances. Maintenance of leaf turgor often is linked to osmotic adjustment. A reduction in osmotic volume at full turgor also results in reduced osmotic potential, even in the absence of solute accumulation. Changes in tissue elasticity may be important for turgor maintenance and drought tolerance of plants that do not adjust osmotically.Water deficits and nutrient deficiencies promote greater relative allocation of photosynthate to root growth, ultimately resulting in plants that have higher root:shoot ratios and greater capacity to absorb water and minerals relative to the shoots that must be supported.At the molecular level, plants respond to water stress by synthesis of certain new proteins and increased levels of synthesis of some proteins produced under well-watered conditions. Evidence has been obtained for enhanced synthesis under water stress of water-channel proteins and other proteins that may protect membranes and other important macromolecules from damage and denaturation as cells dehydrate. Flood tolerance: Both artificial and natural flooding sometimes benefit woody plants. Flooding of orchard soils has been an essential management practice for centuries to increase fruit yields and improve fruit quality. Also, annual advances and recessions of floods are crucial for maintaining valuable riparian forests. Intermittent flooding protects bottomland forests by increasing groundwater supplies, transporting sediments necessary for creating favorable seedbeds, and regulating decomposition of organic matter. Major adaptations for flood tolerance of some woody plants include high capacity for producing adventitious roots that compensate physiologically for decay of original roots under soil anaerobiosis, facilitation of oxygen uptake through stomata and newly formed lenticels, and metabolic adjustments. Halophytes can adapt to saline water by salt tolerance, salt avoidance, or both. Cold hardiness: Environmental stresses that inhibit plant growth, including low temperature, drought, short days, and combinations of these, induce cold hardening and hardiness in many species. Cold hardiness develops in two stages: at temperatures between 10° and 20°C in the autumn, when carbohydrates and lipids accumulate; and at subsequent freezing temperatures. The sum of many biochemical processes determines the degree of cold tolerance. Some of these processes are hormone dependent and induced by short days; others that are linked to activity of enzyme systems are temperature dependent. Short days are important for development of cold hardiness in species that set buds or respond strongly to photoperiod. Nursery managers often expose tree seedlings to moderate water stress at or near the end of the growing season. This accelerates budset, induces early dormancy, and increases cold hardiness. Pollution tolerance: Absorption of gaseous air pollutants varies with resistance to flow along the pollutant’s diffusion path. Hence, the amount of pollutant absorbed by leaves depends on stomatal aperture, stomatal size, and stomatal frequency. Pollution tolerance is increased when drought, dry air, or flooding of soil close stomatal pores. Heat tolerance: Exposure to sublethal high temperature can increase the thermotolerance of plants. Potential mechanisms of response include synthesis of heat-shock proteins and isoprene and antioxidant production to protect the photosynthetic apparatus and cellular metabolism. Breaking of dormancy: Seed dormancy can be broken by cold or heat. Embryo dormancy is broken by prolonged exposure of most seeds to temperatures of 1° to 15°C. The efficiency of treatment depends on interactions between temperature and seed moisture content. Germination can be postponed by partially dehydrating seeds or altering the temperature during seed stratification. Seed-coat dormancy can be broken by fires that rupture seed coats or melt seedcoat waxes, hence promoting water uptake. Seeds with both embryo dormancy and seed-coat dormancy may require exposure to both high and low temperatures to break dormancy. Exposure to smoke itself can also serve as a germination cue in breaking seed dormancy in some species.Bud dormancy of temperate-zone trees is broken by winter cold. The specific chilling requirement varies widely with species and genotype, type of bud (e.g., vegetative or floral bud), depth of dormancy, temperature, duration of chilling, stage of plant development, and daylength. Interruption of a cold regime by high temperature may negate the effect of sustained chilling or breaking of bud dormancy. Near-lethal heat stress may release buds from both endodormancy and ecodormancy. Pollen shedding: Dehiscence of anthers and release of pollen result from dehydration of walls of anther sacs. Both seasonal and diurnal pollen shedding are commonly associated with shrinkage and rupture of anther walls by low relative humidity. Pollen shedding typically is maximal near midday (low relative humidity) and low at night (high relative humidity). Pollen shedding is low or negligible during rainy periods. Seed dispersal: Gymnosperm cones typically dehydrate before opening. The cones open and shed seeds because of differential shrinkage between the adaxial and abaxial tissues of cone scales. Once opened, cones may close and reopen with changes in relative humidity. Both dehydration and heat are necessary for seed dispersal from serotinous (late-to-open) cones. Seeds are stored in serotinous cones because resinous bonds of scales prevent cone opening. After fire melts the resinous material, the cone scales can open on drying. Fires also stimulate germination of seeds of some species. Some heath plants require fire to open their serotinous follicles and shed seeds. Fire destroys the resin at the valves of follicles, and the valves then reflex to release the seeds. Following fire the follicles of some species require alternate wetting and drying for efficient seed dispersal. Stimulation of reproductive growth: Vegetative and reproductive growth of woody plants are negatively correlated. A heavy crop of fruits, cones, and seeds is associated with reduced vegetative growth in the same or following year (or even years). Subjecting trees to drought during early stages of fruit development to inhibit vegetative growth, followed by normal irrigation, sometimes favors reproductive growth. Short periods of drought at critical times not only induce formation of flower buds but also break dormancy of flower buds in some species. Water deficits may induce flowering directly or by inhibiting shoot flushing, thereby limiting the capacity of young leaves to inhibit floral induction. Postharvest water stress often results in abundant return bloom over that in well-irrigated plants. Fruit yields of some species are not reduced or are increased by withholding irrigation during the period of shoot elongation. In several species, osmotic adjustment occurs during deficit irrigation. In other species, increased fruit growth by imposed drought is not associated largely with osmotic adjustment and maintenance of leaf turgor. Seedling storage: Tree seedlings typically are stored at temperatures just above or below freezing. Growth and survival of cold-stored seedlings depend on such factors as: date of lifting from the nursery; species and genotype; storage temperature, humidity, and illumination; duration of storage; and handling of planting stock after storage. Seedlings to be stored over winter should be lifted from the nursery as late as possible. Dehydration of seedlings before, during, and after storage adversely affects growth of outplanted seedlings. Long-term storage of seedlings may result in depletion of stored carbohydrates by respiration and decrease of root growth potential. Although many seedlings are stored in darkness, a daily photoperiod during cold storage may stimulate subsequent growth and increase survival of outplanted seedlings. For some species, rapid thawing may decrease respiratory consumption of carbohydrates (over slowly thawed seedlings) and decrease development of molds. Pollen storage: Preservation of pollen is necessary for insurance against poor flowering years, for gene conservation, and for physiological and biochemical studies. Storage temperature and pollen moisture content largely determine longevity of stored pollen. Pollen can be stored successfully for many years in deep freezers at temperatures near −15°C or in liquid nitrogen (−196°C). Cryopreservation of pollen with a high moisture content is difficult because ice crystals may destroy the cells. Pollens of many species do not survive at temperatures below −40°C if their moisture contents exceed 20–30%. Pollen generally is air dried, vacuum dried, or freeze dried before it is stored. To preserve the germination capacity of stored pollen, rehydration at high humidity often is necessary. Seed storage:

The 2007 Eastern US Spring Freeze: Increased Cold Damage in a Warming World?

ABSTRACT Plant ecologists have long been concerned with a seemingly paradoxical scenario in the relationship between plant growth and climate change: warming may actually increase the risk of plant frost damage. The underlying hypothesis is that mild winters and warm, early springs, which are expected to occur as the climate warms, may induce premature plant development, resulting in exposure of vulnerable plant tissues and organs to subsequent late-season frosts. The 2007 spring freeze in the eastern United States provides an excellent opportunity to evaluate this hypothesis and assess its large-scale consequences. In this article, we contrast the rapid prefreeze phenological advancement caused by unusually warm conditions with the dramatic postfreeze setback, and report complicated patterns of freeze damage to plants. The widespread devastation of crops and natural vegetation occasioned by this event demonstrates the need to consider large fluctuations in spring temperatures a real threat to terrestrial ecosystem structure and functioning in a warming climate.

Interspecific interactions in temperate agroforestry

The ecological principles that define the competitive and complementary interactions among trees, crops, and fauna in agroforestry systems have received considerable research attention during the recent past. These principles have not yet, however, been adequately integrated and synthesized into an operational approach. This paper reviews the ecological and ecophysiological bases for interspecific interactions based on data from site-specific research and demonstration trials from temperate agroforestry systems, primarily from temperate North America. The review shows that information on ecological interactions in several temperate agroforestry systems is inadequate. It is recommended that the future research should focus on exploring new species and systems that have received little attention in the past. Priority research areas should include cultural practices and system designs to minimize interspecific competition and maximize environmental benefits such as improved water quality. Potential for genetic modification of components to increase productivity and reduce competition also needs to be explored. Process-oriented models may be used increasingly to predict resource-allocation patterns and possible benefits for a suite of site and species combinations.

Drought stress, plant water status, and floral trait expression in fireweed, Epilobium angustifolium (Onagraceae).

In a controlled environment, we artificially induced drought during flowering of Epilobium angustifolium, an animal-pollinated plant. Leaf water potential (ψ(l)) and floral traits were monitored over a 12-d period of soil moisture depletion. Soil moisture depletion induced drought stress over time, as revealed by significant treatment × day interactions for predawn and midday ψ(l). Nectar volume and flower size showed significant negative responses to drought stress, but nectar sugar concentration did not vary between treatments. Floral traits were more buffered from drought than leaf water potentials. We used path analysis to examine direct and indirect effects of ψ(l) on floral traits for plants in well-watered (control) vs. drought treatments. According to the best-fit path models, midday ψ(l) has significant positive effects on flower size and nectar volume in both environments. However, for controls midday ψ(l) also had a significant negative effect on nectar sugar concentration. Results indicate that traits influencing floral attractiveness to pollinators in E. angustifolium vary with plant water status, such that pollinator-mediated selection could indirectly target physiological or biochemical controls on ψ(l). Moreover, under mesic conditions selection for greater nectar sugar reward may be constrained by the antagonistic effects of plant water status on nectar volume and sugar concentration.

Mercury Distribution Across 14 U.S. Forests. Part I: Spatial Patterns of Concentrations in Biomass, Litter, and Soils

Results from a systematic investigation of mercury (Hg) concentrations across 14 forest sites in the United States show highest concentrations in litter layers, strongly enriched in Hg compared to aboveground tissues and indicative of substantial postdepositional sorption of Hg. Soil Hg concentrations were lower than in litter, with highest concentrations in surface soils. Aboveground tissues showed no detectable spatial patterns, likely due to 17 different tree species present across sites. Litter and soil Hg concentrations positively correlated with carbon (C), latitude, precipitation, and clay (in soil), which together explained up to 94% of concentration variability. We observed strong latitudinal increases in Hg in soils and litter, in contrast to inverse latitudinal gradients of atmospheric deposition measures. Soil and litter Hg concentrations were closely linked to C contents, consistent with well-known associations between organic matter and Hg, and we propose that C also shapes distribution of Hg in forests at continental scales. The consistent link between C and Hg distribution may reflect a long-term legacy whereby old, C-rich soil and litter layers sequester atmospheric Hg depositions over long time periods. Based on a multiregression model, we present a distribution map of Hg concentrations in surface soils of the United States.

Reliable estimation of biochemical parameters from C3 leaf photosynthesis–intercellular carbon dioxide response curves

The Farquhar-von Caemmerer-Berry (FvCB) model of photosynthesis is a change-point model and structurally overparameterized for interpreting the response of leaf net assimilation (A) to intercellular CO₂ concentration (Ci). The use of conventional fitting methods may lead not only to incorrect parameters but also several previously unrecognized consequences. For example, the relationships between key parameters may be fixed computationally and certain fits may be produced in which the estimated parameters result in contradictory identification of the limitation states of the data. Here we describe a new approach that is better suited to the FvCB model characteristics. It consists of four main steps: (1) enumeration of all possible distributions of limitation states; (2) fitting the FvCB model to each limitation state distribution by minimizing a distribution-wise cost function that has desirable properties for parameter estimation; (3) identification and correction of inadmissible fits; and (4) selection of the best fit from all possible limitation state distributions. The new approach implemented theoretical parameter resolvability with numerical procedures that maximally use the information content of the data. It was tested with model simulations, sampled A/Ci curves, and chlorophyll fluorescence measurements of different tree species. The new approach is accessible through the automated website leafweb.ornl.gov.

Resource competition across the crop-tree interface in a maize-silver maple temperate alley cropping stand in Missouri

In order to improve the management of temperate alley cropping, it is important to study the growth and physiological responses of plants arising from competition across the crop-tree interface. Maize (Zea mays L.) was established between rows of seven-year-old silver maple (Acer saccharinum L.) trees in north-central Missouri, USA with four imposed treatments: (1) an unmodified control with a standard rate of N fertilization (179.2 kg N (as NH4NO3) ha−1), (2) trenching with root barrier installed, (3) supplemental fertilization treatment (standard N + 89.6 kg ha−1 N), and (4) a combination of trenching with root barrier and supplemental fertilization. Whereas soil N status had little effect on maize physiology and yield at the interface, competition for soil water was substantial in both years. Without a root barrier, soil water content, predawn and midday water potential, and midday net photosynthesis of maize plants adjacent to the tree row were reduced compared with those of plants in the alley center, but no differences across the maize crop were evident in the presence of a barrier. Grain yield of border row maize plants lacking an adjacent barrier was depressed compared with that for maize plants with a root barrier present (8.42 vs. 6.59 Mg ha−1 in 1997; 5.38 vs. 3.91 Mg ha−1 in 1998). However, the barrier did not completely restore yield to that in the alley center, suggesting that reductions in light near the tree row also limited production. Top ear height showed a similar pattern of response to the presence of a root barrier. Silver maple trees responded to root barrier installation with reduced annual diameter growth and reduced water status on some sample days.

Seasonal Changes in Tissue Water Relations of Three Woody Species of the Quercus-Carya Forest Type

Tissue water relations of white and northern red oaks and mockernut hickory were studied during the growing season of 1979. In all species, osmotic potentials at full saturation and turgor loss point decreased during the period of leaf maturation; subsequently, osmotic potential responded to soil moisture availability. Estimates of the bulk modulus of elasticity and the relative water content at the turgor loss point revealed that white oak possessed less elastic leaf tissue than did either northern red oak or hickory. The bulk leaf pressure potential associated with the initiation of stomatal closure was lower in white oak and hickory (0.2 MPa) than in northern red oak (0.4 MPa) and remained seasonally constant, while total leaf water potential associated with stomatal closure was lower during periods of drought as osmotic potential decreased. The drought-tolerating behavior of white oak is consistent with its frequent occurrence and success in xeric habitats. Northern red oak and mockernut hickory exhibited responses more typical of drought-avoiding species, which would result in sustained turgor-mediated processes essential for growth and high competitive ability at moderate moisture stresses characteristic of more mesic habitats.

Greenness indices from digital cameras predict the timing and seasonal dynamics of canopy‐scale photosynthesis

The proliferation of digital cameras co-located with eddy covariance instrumentation provides new opportunities to better understand the relationship between canopy phenology and the seasonality of canopy photosynthesis. In this paper we analyze the abilities and limitations of canopy color metrics measured by digital repeat photography to track seasonal canopy development and photosynthesis, determine phenological transition dates, and estimate intra-annual and interannual variability in canopy photosynthesis. We used 59 site-years of camera imagery and net ecosystem exchange measurements from 17 towers spanning three plant functional types (deciduous broadleaf forest, evergreen needleleaf forest, and grassland/crops) to derive color indices and estimate gross primary productivity (GPP). GPP was strongly correlated with greenness derived from camera imagery in all three plant functional types. Specifically, the beginning of the photosynthetic period in deciduous broadleaf forest and grassland/crops and the end of the photosynthetic period in grassland/crops were both correlated with changes in greenness; changes in redness were correlated with the end of the photosynthetic period in deciduous broadleaf forest. However, it was not possible to accurately identify the beginning or ending of the photosynthetic period using camera greenness in evergreen needleleaf forest. At deciduous broadleaf sites, anomalies in integrated greenness and total GPP were significantly correlated up to 60 days after the mean onset date for the start of spring. More generally, results from this work demonstrate that digital repeat photography can be used to quantify both the duration of the photosynthetically active period as well as total GPP in deciduous broadleaf forest and grassland/crops, but that new and different approaches are required before comparable results can be achieved in evergreen needleleaf forest.