Forestry & Forest Sciences · Foundations · also Forest Tree Physiology, Physiology of Woody Plants, Plant Physiology
How trees grow, take up water and nutrients, fix carbon, allocate it to wood, roots and reproduction, and respond to drought, heat, cold and shade.
In the exams: ICAR AICE-JRF 59 Unit 3 (growth substances) and Unit 12 (enhanced CO2); ICAR AIEEA-PG Group 09 Unit I plant physiology basics
Tree physiology is the study of how trees function: how they move water from soil to leaves tens of metres above the ground, fix carbon dioxide into sugars, spend that carbon on respiration, leaves, wood, roots and seeds, take up nutrients with the help of fungi and bacteria, lay down wood in annual or seasonal rings, time their growth and dormancy to the seasons, reproduce, and survive drought, salt, flooding, frost and a changing atmosphere. Trees share the basic physiology of all vascular plants, but their size, longevity and woody structure create problems and solutions that herbaceous plants do not face: a tall tree must pull water against gravity through a long, vulnerable pipe system; a long-lived tree must survive decades of variable weather; and a tree invests most of its carbon in a stem that both supports and conducts.
This manual follows the tree from water to wood to reproduction. It begins with water relations and the cohesion–tension mechanism, and with xylem embolism, now seen as central to drought mortality. It then covers photosynthesis in trees and the light responses that underlie shade tolerance, respiration and carbon allocation, mineral nutrition and mycorrhizal symbioses, cambial activity and wood formation, and patterns of shoot and root growth. Plant growth regulators are treated with their practical use in vegetative propagation. The later chapters cover dormancy, phenology and cold hardiness; reproductive physiology from flowering to seed development; stress physiology under drought, salinity, flooding and frost; and the effects of elevated carbon dioxide and warming on tree growth.
The physiology explained here underpins silviculture (why shade-tolerant species regenerate under canopy), nursery practice (rooting cuttings, hardening seedlings), tree improvement (inducing flowering in seed orchards), forest health (drought and frost damage) and climate-change science (forest carbon uptake). Plant anatomy and wood structure are covered more fully in the wood anatomy subject and general plant physiology in the botany subject.
Examinations test this subject through definitions and mechanisms (cohesion–tension, embolism, C3 photosynthesis, ectomycorrhizae and arbuscular mycorrhizae, earlywood and latewood, reaction wood, endodormancy, recalcitrant seeds), the roles of plant growth regulators and their use in propagation, and short calculations: water potential components, gravitational potential with height, Hagen–Poiseuille scaling of flow with vessel radius, water-use efficiency, carbon-use efficiency, Q10 respiration, nutrient resorption efficiency and loss of hydraulic conductivity. Forestry entrance, postgraduate and doctoral fellowship examinations, forestry licensure examinations and arborist certification all draw on it, the last especially for water relations, roots, mycorrhizae and stress.
Water moves from places of higher to lower water potential (Ψ), measured in megapascals (MPa). Pure free water at atmospheric pressure has Ψ = 0, and plant water potentials are normally negative. Water potential is the sum of components: Ψ = Ψs + Ψp + Ψg (+ Ψm), where the solute or osmotic potential Ψs is negative and becomes more negative as solutes increase; the pressure potential Ψp is positive in turgid living cells and negative (tension) in the xylem; the gravitational potential Ψg increases by about 0.01 MPa per metre of height; and the matric potential Ψm reflects binding to surfaces in soils and cell walls. A cell with Ψs = −1.2 MPa and Ψp = +0.5 MPa therefore has Ψ = −0.7 MPa. In a 50 m tree, gravity alone requires a water potential about 0.5 MPa lower at the top than at the base, before any friction losses. Transpiration, the evaporation of water from leaves through stomata, drives water movement. Its rate depends on the vapour pressure deficit (VPD) between leaf and air, stomatal conductance, boundary-layer conductance and the energy available. The cohesion–tension theory, proposed by Dixon and Joly in 1894, explains how water reaches the top of tall trees without pumping. Evaporation from cell walls in the leaf creates curved menisci and tension; because water molecules cohere strongly through hydrogen bonding and adhere to xylem walls, the tension pulls a continuous column of water up through the xylem from the roots. The column is under negative pressure, often −1 to −3 MPa in transpiring trees and lower in drought-adapted species. Root pressure, generated osmotically in roots, can push sap up in some species when transpiration is low, causing guttation, and spring stem pressure in maples and birches drives sap flow before leaves emerge; neither can supply transpiration in tall trees. Water potential is measured with a pressure chamber (Scholander bomb); predawn values approximate soil water potential in the root zone, and midday values show the stress experienced during transpiration.
Water flows through xylem conduits. Conifers rely on tracheids, single cells usually a few millimetres long and tens of micrometres wide, connected by bordered pits whose torus–margo membranes act as valves; most angiosperms have vessels, columns of cells with perforated end walls that can be centimetres to metres long and up to several hundred micrometres wide. By the Hagen–Poiseuille equation, flow through a tube is proportional to the fourth power of its radius, so doubling vessel radius increases conductance sixteen-fold; wide vessels are very efficient. Ring-porous species such as oaks and ashes produce very wide earlywood vessels each spring and rely on them for most conduction, while diffuse-porous species such as maples, birches and poplars have more uniform vessels. Efficiency has a cost: under tension, xylem can embolise. Embolism occurs when air is pulled into a water-filled conduit through pores in the pit membrane from a neighbouring air-filled space, a process called air-seeding, when tension exceeds a threshold. The conduit then fills with air and stops conducting, and the embolism can spread. Freezing and thawing also cause embolism, because air dissolved in sap forms bubbles when it freezes, and wide conduits are much more vulnerable to freeze–thaw embolism than narrow ones, which is one reason conifers with narrow tracheids dominate cold regions. Vulnerability curves plot the percentage loss of conductivity (PLC = (Kmax − K)/Kmax × 100) against xylem water potential; P50, the water potential at which 50 percent of conductivity is lost, ranges from about −1 MPa in some tropical and riparian species to below −10 MPa in some dryland conifers. The hydraulic safety margin is the difference between the minimum water potential a species experiences and its P50; studies have found that many forest species worldwide operate with narrow safety margins. Some species refill embolised conduits by root or stem pressure, others replace lost capacity by growing new xylem. Sapwood area is related to supported leaf area, as stated in the pipe model theory, and the ratio of sapwood area to leaf area (the Huber value) increases in drier sites.
Almost all trees use the C3 pathway, in which carbon dioxide is fixed by the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) to form a three-carbon compound in the Calvin–Benson cycle. A few woody species have C4 photosynthesis, such as some tree-sized Euphorbia species and the saxaul shrubs of Central Asian deserts, and some succulent trees and epiphytes use crassulacean acid metabolism (CAM), but these are exceptions. Rubisco also reacts with oxygen, initiating photorespiration, which releases previously fixed carbon dioxide and wastes energy; photorespiration increases with temperature and falls when carbon dioxide concentration rises, which is why elevated carbon dioxide stimulates C3 photosynthesis. Biochemical models, of which the Farquhar–von Caemmerer–Berry model is the standard, treat photosynthesis as limited either by Rubisco activity (at low carbon dioxide) or by the regeneration of ribulose bisphosphate through electron transport (at high carbon dioxide or low light). Carbon dioxide reaches chloroplasts through stomata, whose aperture balances carbon gain against water loss. Water-use efficiency (WUE) at the leaf level is the ratio of photosynthesis to transpiration, A/E; a leaf fixing 10 µmol CO₂ m⁻² s⁻¹ while transpiring 4 mmol H₂O m⁻² s⁻¹ has an instantaneous WUE of 2.5 µmol CO₂ per mmol H₂O. Intrinsic WUE is A divided by stomatal conductance, removing the effect of VPD. Rubisco discriminates against the heavier carbon isotope ¹³C, and the discrimination is smaller when stomata are more closed relative to photosynthetic demand; so leaves and wood with less negative δ¹³C values indicate higher intrinsic WUE, a relationship used to study drought responses in tree rings. Photosynthesis has a temperature optimum that acclimates to growth temperature, and is reduced by photoinhibition under excess light, especially when combined with cold or drought.
A light response curve plots net photosynthesis against photosynthetically active radiation (PAR). In darkness net photosynthesis is negative, equal to dark respiration. As light increases, net photosynthesis rises linearly, with a slope called the apparent quantum yield, and crosses zero at the light compensation point, where photosynthesis just balances respiration. At higher light the curve bends over and reaches a plateau, the light-saturated rate Amax, at the light saturation point. Shade tolerance is the ability of a species to survive and grow in low light, and it is one of the most important properties in silviculture because it determines whether a species can regenerate under a canopy. Shade-tolerant species, such as many firs, hemlocks, beeches, maples and many late-successional tropical trees, typically have low dark respiration rates, low light compensation points and relatively low Amax, together with leaves that are thin, have a high specific leaf area, more chlorophyll relative to nitrogen and long life spans; they survive long periods as suppressed seedlings (advance regeneration) and respond when gaps open. Shade-intolerant (light-demanding) pioneer species, such as many pines, larches, birches, poplars, eucalypts, teak and pioneer tropical trees such as Cecropia and Macaranga, have high Amax and respiration, grow fast in the open and die quickly under shade. Within one tree, sun leaves at the top of the crown are thicker, with more palisade layers and higher Amax per unit area, while shade leaves lower in the crown are thinner and more efficient at low light. Shade tolerance can change with age and site; some species are more tolerant as seedlings than as larger trees, and tolerance is greater on fertile, moist sites. Survival in shade depends on carbon balance and on allocation to storage and defence as well as on leaf physiology, so whole-plant traits matter more than any one leaf measurement. Silvicultural systems, from clear-felling to selection, are designed around these differences.
Respiration oxidises carbohydrates to release energy (ATP) and carbon skeletons for growth and maintenance, giving off carbon dioxide. In trees it occurs in every living cell: leaves, the living cells of the inner bark and cambium, ray and axial parenchyma in the sapwood, roots and reproductive organs. Heartwood is dead and does not respire. It is useful to divide respiration into growth respiration, the cost of synthesising new tissue, roughly proportional to growth rate and dependent on tissue composition (lipids and lignin are expensive, sugars cheap), and maintenance respiration, the cost of keeping existing living tissue functioning, which is proportional to living biomass and its nitrogen content and rises with temperature. The temperature sensitivity of respiration is often summarised by Q₁₀, commonly around 2 in short-term measurements: a respiration rate of 2.0 µmol m⁻² s⁻¹ at 15 °C becomes about 4.0 at 25 °C with Q₁₀ = 2. Over longer periods respiration acclimates to growth temperature, so long-term increases are smaller than short-term Q₁₀ values suggest. Respiration by the whole tree or stand (autotrophic respiration, Ra) consumes a large share of gross primary production (GPP): net primary production NPP = GPP − Ra. The ratio NPP/GPP is the carbon-use efficiency (CUE). An influential analysis suggested CUE was close to 0.5 across many forests, but later work found it varies, roughly from 0.3 to 0.7, being lower in old, tropical or nutrient-poor forests and higher in young, fertile or managed ones. For a stand with GPP of 20 t C ha⁻¹ per year and autotrophic respiration of 11 t C ha⁻¹ per year, NPP is 9 t C ha⁻¹ and CUE 0.45. As trees grow taller and larger, the proportion of living tissue that must be maintained and the hydraulic constraints both change, part of the explanation for the decline of stand growth with age.
Carbon allocation is the distribution of assimilated carbon among respiration, growth of different organs, storage, reproduction, defence and exudation to roots and symbionts. Allocation is not fixed; it responds to the environment in ways that tend to relieve the most limiting resource, an idea known as functional balance. Trees on dry or nutrient-poor sites allocate proportionally more carbon below ground, to fine roots and mycorrhizal fungi, while trees on fertile, moist sites allocate more to foliage and stems; global analyses of forest carbon budgets show that the fraction of GPP going below ground is highest in infertile forests. Fertilisation of plantations increases wood production partly by increasing GPP and partly by shifting allocation from roots to stems. A widely taught priority scheme, associated with Waring, places maintenance of foliage and fine roots first, then shoot extension and buds, then stem diameter growth, then storage and resistance to pests; this explains why diameter growth is a sensitive indicator of tree vigour and why stressed trees with little spare carbon are vulnerable to bark beetles and pathogens. Non-structural carbohydrates (NSC), mainly starch and soluble sugars, are stored in roots, stems and branches and buffer the tree against periods of negative carbon balance, such as spring leaf-out in deciduous trees, defoliation by insects, drought and shade. Whether storage is merely a passive overflow or an actively regulated sink remains debated, but trees rarely deplete NSC completely. Allocation to reproduction can be large in mast years, when stem growth is often reduced. Allometric relations, such as root-to-shoot ratio and leaf area to sapwood area, summarise allocation and are used in growth models and carbon accounting. Heartwood formation, the death of parenchyma in the inner sapwood with deposition of extractives, removes maintenance costs from old xylem and often increases decay resistance.
Trees need the same essential mineral elements as other plants. Macronutrients, required in larger amounts, are nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg) and sulphur (S); micronutrients are iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), boron (B), molybdenum (Mo), chlorine (Cl) and nickel (Ni). Nitrogen is the element most often limiting growth in temperate and boreal forests, phosphorus in many old, highly weathered tropical and subtropical soils and in some plantations, and boron deficiency causes dieback of leaders in pines and eucalypts on some soils. Nutrients differ in mobility within the plant. Mobile nutrients (N, P, K, Mg) are withdrawn from older tissues when supply is short, so deficiency symptoms appear first in older leaves; immobile nutrients (Ca, B, Fe, Mn) cannot be withdrawn, so deficiency appears first in young leaves and shoot tips. Trees conserve nutrients by resorption: before leaves are shed, a substantial part of their N, P and K is withdrawn into stems and roots for reuse. Resorption efficiency is (green-leaf concentration − senesced-leaf concentration) / green-leaf concentration × 100; if green leaves contain 2.0 percent N and freshly fallen leaves 0.9 percent, resorption efficiency is 55 percent. This internal recycling supplies a large share of the annual nutrient requirement of mature trees, which is why forests can grow on poor soils and why harvest removal of foliage and branches depletes sites more than stem-only harvest. Nutrient status is assessed by foliar analysis, comparing concentrations in standardised samples (for example, current-year needles from the upper crown, collected in the dormant season for conifers) with critical values below which growth is reduced; these values differ by species, region and sampling protocol. Liebig's law of the minimum states that growth is limited by the scarcest resource relative to need; ratios such as N:P and diagnostic systems compare balance among nutrients.
Most tree roots form mycorrhizae, mutualistic associations with fungi in which the plant supplies carbohydrates and the fungus supplies nutrients and water from a much larger soil volume. Two types dominate in trees. Ectomycorrhizae (ECM), formed mainly by basidiomycete and ascomycete fungi, enclose short roots in a fungal mantle and penetrate between cortical cells to form a Hartig net, without entering cells; external hyphae and rhizomorphs explore the soil, and many ECM fungi produce the mushrooms and truffles seen in forests. ECM associations are characteristic of the pine family (pines, spruces, firs, larches, Douglas-fir), the beech family (oaks, beeches, chestnuts), birches, eucalypts and the dipterocarps that dominate many Southeast Asian rainforests. Arbuscular mycorrhizae (AM), formed by fungi of the Glomeromycota, penetrate root cortical cells and form tree-like arbuscules where exchange occurs; they are the most widespread type, found in most tropical trees, many temperate hardwoods such as maples and ashes, teak, and many agroforestry and fruit trees. ECM fungi can access organic nitrogen and phosphorus through enzymes, which helps explain their dominance in cold, organic-rich soils, while AM fungi mainly improve uptake of inorganic phosphate. Mycorrhizae also improve drought tolerance and protect roots from pathogens. Historically, plantations of pines established in regions without native ECM fungi, in parts of Africa, the Caribbean and elsewhere, failed until soil or fungi from established pine stands were introduced, a lesson that led to routine nursery inoculation. Nitrogen fixation adds N to many tree species through root nodules: rhizobia in legumes such as acacias, Leucaena, Robinia and Prosopis, and the actinomycete Frankia in actinorhizal trees such as alders, Casuarina, Myrica and Elaeagnus. These species are used for land reclamation, agroforestry and mixed plantations, though their fixation rates vary widely and depend on phosphorus and molybdenum supply.
Secondary growth, the increase in stem and root diameter, comes from two lateral meristems. The vascular cambium, a thin cylinder of dividing cells between xylem and phloem, produces secondary xylem (wood) towards the inside and secondary phloem (inner bark) towards the outside, much more xylem than phloem. It contains elongated fusiform initials, which give rise to the axial cells of wood (tracheids, vessel elements, fibres and axial parenchyma), and short ray initials, which produce the radial ray cells that store carbohydrates and conduct radially. Periclinal divisions, parallel to the surface, add cells to the xylem and phloem, while anticlinal divisions increase the number of initials so that the cambial circumference keeps pace with the growing stem. The cork cambium (phellogen) forms the periderm, the protective outer bark, and is renewed repeatedly as the stem expands. Newly formed xylem cells pass through stages of expansion, secondary wall thickening with deposition of cellulose, hemicelluloses and lignin, and, for conducting and supporting cells, programmed cell death. Cambial activity is controlled by hormones and environment. In temperate trees, reactivation in spring is linked to auxin moving basipetally (downwards) from expanding buds and young leaves, together with rising temperature; it begins below the buds and spreads down the stem, earlier in ring-porous than in many diffuse-porous species. Cytokinins from roots, gibberellins and ethylene also influence division and differentiation. Cambial activity stops in late summer or autumn under short days and lower temperatures, or earlier under drought. Water availability strongly controls cell expansion, so drought reduces ring width even when photosynthesis continues. In the dry season of the seasonal tropics, cambial dormancy is induced mainly by water deficit, not by temperature.
A growth ring is the layer of wood produced in one growth period. In temperate and boreal trees, earlywood formed in spring has wide, thin-walled cells suited to conduction, and latewood formed later in the season has narrow, thick-walled cells giving density and strength; the abrupt boundary between one year's latewood and the next year's earlywood makes rings visible. The transition from earlywood to latewood is associated with declining auxin supply as shoot growth ends, increasing water deficit and changes in day length. Ring-porous species concentrate large vessels in the earlywood, diffuse-porous species spread them through the ring. In the tropics, distinct annual rings form where there is a pronounced dry season or annual flooding, as in teak and many dry-forest species, while trees in aseasonal humid climates often lack clear annual rings. Rings are not always annual: false rings (intra-annual density fluctuations) form after a mid-season drought followed by rain, and rings can be missing in very stressed years or in parts of a stem. Dendrochronology, founded by the astronomer A.E. Douglass in the early twentieth century, uses patterns of wide and narrow rings, matched between trees by cross-dating, to date wood exactly and reconstruct past climate, fires, insect outbreaks and growth trends. Reaction wood forms in leaning stems and branches to restore their orientation. In conifers, compression wood forms on the lower side; it has rounded, thick-walled tracheids with more lignin, appears darker and shrinks strongly along the grain, causing warping in sawn timber. In angiosperms, tension wood forms on the upper side, with fibres that have a gelatinous inner layer rich in cellulose; it gives woolly surfaces when sawn. Juvenile wood, formed in the first years near the pith and in the crown, has shorter cells, lower density in many conifers and a higher microfibril angle than mature wood, so fast-grown short-rotation plantation timber contains a high proportion of it.
Shoot growth combines the formation of new leaf primordia in the bud and the extension of internodes. In determinate (fixed or preformed) growth, all the leaves and internodes of next year's shoot are formed in the bud during the previous growing season; the shoot expands quickly in spring in a single flush and then sets a terminal bud. Many pines, spruces, firs, oaks, beeches and walnuts grow in this way, and their shoot length depends largely on conditions in the year the bud was formed. In indeterminate (free) growth, some leaves are preformed in the overwintering bud and further leaves are initiated and expanded during the season (neoformed growth), so shoots continue to elongate as long as conditions are favourable, until shortening days or drought stop them; poplars, willows, birches, alders, larches, sweetgum and many eucalypts behave this way. Some species have intermediate patterns. Recurrent flushing, in which a shoot forms and expands several successive buds in one season, occurs in some southern pines and in many tropical trees, whose growth is rhythmic, with periods of flushing alternating with rest even under constant climate. Lammas shoots are second flushes in late summer from buds that would normally have opened next spring, common in young vigorous trees of oaks and some conifers. Branches can develop from buds after a rest period (proleptic) or immediately as the parent shoot grows (sylleptic). Crown form reflects apical dominance, the inhibition of lateral bud outgrowth by the terminal bud in the current shoot, and apical control, the control by the leader over the growth of lateral branches over several years. Strong apical control produces the excurrent, conical crowns of many conifers, while weak control produces the decurrent, spreading crowns of many broadleaves such as oaks and elms. Architectural models of tree form, developed by Hallé and Oldeman from tropical trees, classify these patterns.
Tree root systems include a taproot in some species, especially on deep, well-drained soils (many oaks, pines, teak in youth), large horizontal lateral roots near the surface, sinker roots that grow down from laterals, and a mass of fine roots, usually defined as roots less than 2 mm in diameter, concentrated in the upper soil and the forest floor where nutrients and water are richest. Most roots of most trees are in the upper metre of soil, though some roots reach far deeper and supply water during drought. Root architecture is plastic: shallow soils, high water tables, compacted layers and nursery practices can prevent taproot development, and container-grown seedlings can develop spiralling roots that later destabilise the tree. Fine roots and their mycorrhizal partners turn over rapidly, some within months, and their production and death are a large share of forest net primary production and soil carbon input. Root growth is seasonal but not synchronised with shoot growth: in many temperate trees root growth begins before bud break, slows when shoot extension is fastest and peaks again in late summer and autumn, continuing as long as soil temperature and moisture allow. This timing matters in the nursery and at planting: root growth potential (or root regeneration potential), the ability of a seedling to produce new roots in a standard test, is used as an index of seedling quality and predicts survival after outplanting. Roots of neighbouring trees of the same species can graft together, sharing water, carbohydrates and also pathogens. Adventitious roots arise from stems or other non-root tissues, as in rooted cuttings, mangrove prop roots and flooded trees. Structural roots anchor the tree, and windthrow risk depends on rooting depth, soil and the size and shape of the root plate. For arborists, the critical root zone around urban trees must be protected from trenching, compaction and grade changes.
Plant hormones are organic compounds produced in small amounts that regulate growth and development at sites away from or near where they are made; synthetic compounds with similar effects are called plant growth regulators (PGRs). Auxins, chiefly indole-3-acetic acid (IAA), are made in shoot tips and young leaves and move polarly towards the base. They promote cell elongation, maintain apical dominance, stimulate cambial activity and vascular differentiation, and initiate adventitious and lateral roots; synthetic auxins indole-3-butyric acid (IBA) and naphthaleneacetic acid (NAA) are the standard rooting hormones, and 2,4-D, at higher doses, is a herbicide. Gibberellins (GAs) promote stem elongation, break some seed dormancies and influence flowering; in many members of the pine family, applications of the less polar gibberellins GA₄ and GA₇ (usually as a GA4/7 mixture) promote cone formation in seed orchards. Inhibitors of gibberellin synthesis such as paclobutrazol reduce shoot growth and are used to control tree size and in some cases promote flowering. Cytokinins, such as zeatin and the synthetic benzylaminopurine (BAP), are made mainly in roots; they promote cell division, shoot formation and release of lateral buds, and delay senescence. Abscisic acid (ABA) closes stomata under water stress and promotes bud and seed dormancy and cold acclimation. Ethylene, a gas, promotes abscission of leaves and fruits, fruit ripening, responses to wounding and flooding (including aerenchyma formation) and is involved in reaction wood formation. Other signalling compounds include brassinosteroids, jasmonates, salicylic acid and strigolactones, which regulate branching, defence and symbiosis. Hormones act in balances rather than alone: in tissue culture, Skoog and Miller showed in 1957 that a high cytokinin-to-auxin ratio favours shoot formation, a high auxin-to-cytokinin ratio favours roots, and intermediate ratios favour callus growth.
Vegetative (clonal) propagation reproduces a selected genotype exactly, capturing both additive and non-additive genetic gains, and is the basis of clonal forestry in eucalypts, poplars, willows, acacia hybrids, some conifers and many fruit and ornamental trees. The main methods are rooted cuttings, grafting and budding, layering and air layering, and tissue culture (micropropagation and somatic embryogenesis). Rooting of stem cuttings depends on genotype, the physiological age of the stock plant, the type of cutting, auxin treatment and the rooting environment. The ability to root declines strongly as trees mature; cuttings from juvenile seedlings, from basal sprouts of coppiced stumps and from hedged stock plants maintained in a juvenile state root far better than those from mature crowns. This loss of juvenility with age and position in the tree, and the persistence of mature traits in propagules taken from mature parts (topophysis, such as plagiotropic, branch-like growth of cuttings taken from side branches of conifers), are major constraints. Auxins, usually IBA alone or with NAA, are applied as a powder, a quick dip in a concentrated solution or a long soak in a dilute solution; effective concentrations differ widely by species and cutting type and are found by trial. Mist or fog systems maintain high humidity so that leafy cuttings photosynthesise without wilting, with bottom heat to stimulate rooting. Some species, such as poplars and willows, root so easily from leafless hardwood cuttings that no hormone is needed. Grafting joins a scion of the selected tree to a rootstock and is used to establish clonal seed orchards and to propagate mature trees that do not root; compatibility between scion and stock is required, and delayed incompatibility can appear years later in some conifers. Micropropagation multiplies shoots on media with cytokinins, then roots them with auxins, followed by acclimatisation to greenhouse conditions; somatic embryogenesis, producing embryos from somatic cells, allows very large-scale clonal production and cryopreservation of conifer clones. Clonal forestry must maintain enough genetic diversity among deployed clones to limit risk from pests and diseases.
Dormancy is the temporary suspension of visible growth of any structure containing a meristem. A widely used classification by Lang distinguishes three kinds. Paradormancy is growth inhibition caused by signals from another part of the plant, as when apical dominance suppresses lateral buds. Endodormancy is inhibition within the bud itself, which cannot be broken by favourable conditions until a chilling requirement has been met. Ecodormancy is inhibition imposed by unfavourable environment, such as cold, after endodormancy has been released; buds resume growth once temperatures are warm enough for long enough. In many temperate and boreal trees, growth cessation and bud set in late summer are triggered mainly by shortening days. The critical day length varies with the origin of the population: populations from high latitudes stop growing at longer day lengths than those from lower latitudes, so seed sources moved south often stop growing too early and grow slowly, while those moved north may grow late and suffer autumn frost. This photoperiodic ecotypic variation is a key reason for using local or climatically matched seed sources. Release from endodormancy needs a period of chilling, usually counted as hours or units at temperatures between about 0 and 7 °C, though models differ; once chilling is met, bud burst follows after accumulation of forcing heat, often expressed as growing degree-days above a base temperature such as 5 °C. Phenology, the timing of seasonal events such as bud burst, leaf colouring, flowering and leaf fall, therefore integrates photoperiod, chilling and forcing. Long-term records show earlier spring leaf-out and later autumn senescence in many regions with warming, lengthening the growing season, but warmer winters can reduce chilling and delay or spread out bud burst in some species, and earlier leaf-out raises the risk of late spring frost damage. In seasonally dry tropics, leaf flush and leaf fall are driven more by water availability, and some species flush before the rains using stored water.
Cold hardiness, the ability to survive low temperatures, changes through the year. Trees acclimate in autumn in two stages: first, short days trigger growth cessation and an initial increase in hardiness; then low, non-freezing and freezing temperatures bring full hardiness. In spring they deacclimate rapidly as temperatures rise, so a frost after bud burst can kill new shoots even of very hardy species. Freezing injury is caused mainly by ice forming inside cells, which ruptures membranes and is almost always lethal. Trees survive freezing by two main strategies. In freeze tolerance through extracellular freezing, ice forms in the cell walls and intercellular spaces; water moves out of the cells towards the ice, and the cells survive the resulting dehydration because their membranes and contents have been modified during acclimation, through accumulation of sugars, dehydrins and other protective compounds. Fully hardened boreal trees such as birches, willows, larches and spruces survive extreme cold in this way. In deep supercooling, water in certain tissues, notably xylem ray parenchyma and some buds, remains liquid well below 0 °C, but only to a limit of about −40 °C, the temperature at which pure water freezes spontaneously; species that rely on deep supercooling, including many temperate hardwoods such as oaks and some maples, have northern range limits that coincide roughly with the −40 °C minimum temperature isotherm. Chilling injury, damage at low temperatures above freezing, affects many tropical species of lowland origin. Other forms of cold damage include frost cracks in stems from rapid cooling, sunscald on thin bark in late winter, winter desiccation of evergreen foliage when frozen soil prevents water uptake, frost heaving of seedlings in wet soils, and frost rings in the wood. Cold hardiness is measured by freezing samples to different temperatures and assessing damage by electrolyte leakage or visual browning, and is used in nursery management to decide when seedlings can be lifted and stored.
Trees pass through a juvenile phase during which they cannot flower, whatever the conditions. Its length varies from one or two years in some eucalypts, acacias and pioneer species to several decades in some oaks, beeches and firs, and it is a major constraint on tree breeding. After the transition to the mature phase, flowering requires floral induction, usually in the season before the flowers open; the buds that will flower are determined months in advance, so flowering in a given year depends on weather and tree condition in the previous year. Many trees of temperate zones initiate flower buds in summer, which then overwinter. Sex expression varies: trees may be hermaphrodite, with perfect flowers containing both stamens and pistils (eucalypts, limes, many tropical trees); monoecious, with separate male and female flowers or cones on the same tree (pines, spruces, oaks, birches, walnuts); or dioecious, with male and female trees (poplars, willows, ginkgo, yews, many tropical species). Pollination is by wind (anemophily) in conifers and many temperate broadleaves such as oaks, birches, poplars and alders, which produce vast amounts of light pollen, usually before or as leaves expand; or by animals (zoophily), including insects, birds and bats, in most tropical trees and many temperate ones such as limes, cherries and maples. Many tropical trees are pollinated by specialised animals and depend on them for seed set. Most trees are predominantly outcrossing, with mechanisms that reduce self-fertilisation, such as dichogamy (separation in time of pollen release and stigma receptivity), self-incompatibility in many angiosperms and, in conifers, the death of inbred embryos, which produces empty seeds. Inbreeding depression is strong in conifers, which matters in seed orchards with few clones. In seed orchards, flowering is stimulated by gibberellin GA4/7 in the pine family, girdling or strangulation of stems, root pruning, moderate drought, nitrogen fertiliser and top-grafting onto mature trees.
After pollination, fertilisation follows in a time that differs greatly among taxa. In angiosperms it is usually within days, and double fertilisation produces a diploid embryo and a triploid endosperm. In pines, the reproductive cycle usually spans about two years: pollination occurs in the spring of the first year, fertilisation about thirteen months later, and seeds mature in the autumn of the second year, while in spruces, firs and larches seeds mature in the year of pollination. In gymnosperms the nutritive tissue of the seed is the haploid megagametophyte, not an endosperm formed by fertilisation. Seed development passes through histodifferentiation, reserve accumulation and maturation drying in orthodox seeds. Seed production varies greatly between years. Masting is the synchronous, intermittent production of large seed crops by a population of trees, separated by years of little or no seed; beeches, oaks, many conifers and the dipterocarps of Southeast Asian rainforests, which flower in general flowering events across many species triggered by drought or cool spells, are classic examples. Masting may satiate seed predators, improve pollination efficiency in wind-pollinated species, and is linked to resource accumulation and weather cues; it complicates seed collection and natural regeneration planning. Seeds differ in storage behaviour. Orthodox seeds, such as those of most pines, eucalypts, acacias and teak, tolerate drying to low moisture contents (about 5 to 10 percent) and can be stored for years at low temperatures. Recalcitrant seeds, such as those of many dipterocarps, rubber, mango, many oaks and chestnuts and many tropical rainforest species, are shed at high moisture and die if dried below a fairly high moisture content, so they can be stored only briefly; intermediate seeds tolerate partial drying but not cold storage when dry. Seed dormancy also varies: hard, impermeable seed coats in legumes such as acacias require scarification, and the physiological dormancy of many temperate conifers and broadleaves is broken by cold, moist stratification.
Stress is any environmental condition that reduces growth or survival below the potential. Plants respond by avoidance, keeping tissues away from the stress (for example by deep roots or shedding leaves), or by tolerance, enduring it (for example by osmotic adjustment or embolism-resistant xylem). Drought develops as soil water potential falls. The first response is usually stomatal closure, mediated by abscisic acid and leaf water status, which reduces transpiration and carbon uptake. Species differ along a continuum: more isohydric species close stomata early and keep leaf water potential relatively constant, avoiding embolism but limiting carbon gain; more anisohydric species keep stomata open longer and let water potential fall, maintaining photosynthesis at a greater hydraulic risk. Other responses include osmotic adjustment, accumulating solutes that lower osmotic potential and maintain turgor; reduced leaf area by shedding or smaller leaves; increased root growth relative to shoots; and leaf traits such as thick cuticles and sunken stomata. Drought-induced tree mortality has been documented on every forested continent and appears to be increasing with hotter droughts. Two main mechanisms are recognised and can interact: hydraulic failure, when embolism exceeds a critical level and the plant can no longer supply water to its tissues (often around 60 percent loss of conductivity in conifers and higher in angiosperms), and carbon starvation, when prolonged stomatal closure depletes carbohydrate reserves; weakened trees are also more susceptible to insects and pathogens. Salinity imposes an osmotic effect, lowering soil water potential so that uptake is harder, and an ionic effect, the toxicity of sodium and chloride accumulating in leaves. Tolerance mechanisms include exclusion of salts at the root, compartmentation of ions in vacuoles, accumulation of compatible solutes and secretion by salt glands. Mangroves illustrate the range: Rhizophora excludes most salt at the root, while Avicennia also secretes salt from leaf glands. Trees used on saline land include species of Prosopis, Tamarix, Casuarina and salt-tolerant eucalypts and acacias, though tolerance varies with provenance.
Flooding cuts off oxygen to roots because oxygen diffuses about ten thousand times more slowly in water than in air, and soil microbes rapidly consume what remains. Roots switch to anaerobic respiration, producing far less energy and toxic products such as ethanol, while reduced soil compounds (ferrous iron, manganese, sulphides) accumulate. Stomata close and leaves may wilt, yellow and drop even though water is abundant. Flood-tolerant trees, such as bald cypress, many willows, tupelos, alders, some ashes and the trees of floodplain and swamp forests in the Amazon and elsewhere, have adaptations: aerenchyma, tissue with large air spaces that carries oxygen from shoots to roots; hypertrophied lenticels on submerged stems; adventitious roots formed near the water surface; pneumatophores, the aerial roots of mangroves such as Avicennia; and metabolic tolerance of anaerobic conditions. Ethylene, which accumulates in flooded tissues, triggers many of these responses. Timing matters: flooding during dormancy causes less damage than during the growing season, and duration and water movement (stagnant versus flowing) affect survival. Frost injury occurs when tissues freeze before they have hardened or beyond their tolerance. Late spring frosts kill flushing shoots and flowers, especially in frost pockets, low-lying areas where cold air drains and collects on clear, calm nights (radiation frost); early autumn frosts injure shoots that have not set bud. Frost damage to young plantations can be reduced by choosing species and provenances with appropriate phenology, avoiding frost hollows, retaining a shelterwood canopy that reduces radiative cooling, and controlling weeds that trap cold air near the ground. Combined stresses are common in nature: drought with heat, salinity with waterlogging in irrigated lands, and frost after a warm spell, and they can cause greater damage than either stress alone. Air pollutants such as ozone and acid deposition add further stress in some regions.
Atmospheric carbon dioxide has risen from about 280 parts per million before industrialisation to more than 420 ppm in the mid-2020s. Because C3 photosynthesis is not saturated at current concentrations and photorespiration falls as carbon dioxide rises, elevated CO₂ directly increases leaf photosynthesis, often by 30 to 50 percent in experiments at about 550 to 600 ppm, and reduces stomatal conductance, often by about 20 percent, raising water-use efficiency. The best evidence on whole trees and forests comes from free-air CO₂ enrichment (FACE) experiments, which release CO₂ around plots of trees in the open, avoiding the artefacts of chambers. Early FACE experiments in young, fast-growing temperate plantations, such as pine and sweetgum stands in the south-eastern United States, aspen and birch in the northern United States and poplar plantations in Italy, found an average increase in net primary production of about 23 percent. Several findings qualified this. Responses depended on nitrogen supply: at the sweetgum site, the initial stimulation declined as nitrogen became limiting, an example of progressive nitrogen limitation. Extra carbon was often allocated to fine roots and turned over quickly rather than stored in wood. In a mature eucalypt woodland on phosphorus-poor soil in Australia, elevated CO₂ increased leaf photosynthesis by about a fifth but did not increase above-ground growth, the extra carbon apparently being respired or exuded below ground. Photosynthetic capacity may decline with long exposure (acclimation or down-regulation), especially when nutrients or sinks are limiting. FACE experiments in mature temperate forest in Europe and in the Amazon rainforest are testing responses of old forests. Tree-ring studies worldwide show that intrinsic water-use efficiency has risen with CO₂, but have mostly not detected the corresponding growth increases, suggesting other limits. At global scale, satellite records show greening of vegetation, which models attribute largely to CO₂ fertilisation along with land management and climate, and the land carbon sink has grown, but how long it will continue is uncertain.
Rising temperature affects trees through many processes at once. It lengthens the growing season in cold regions by advancing spring and delaying autumn, increasing growth where temperature limits it, for example at many alpine and boreal treelines, where trees have advanced upslope or poleward in some places, though not everywhere. It increases respiration and photorespiration, although both acclimate partly over time, and raises the vapour pressure deficit of the air, increasing transpiration demand and drought stress even when rainfall is unchanged. In warm regions, higher temperatures can push leaves beyond their photosynthetic optimum, and heat waves can cause direct damage. Warmer winters reduce chilling and frost-hardiness, increase survival of overwintering insect pests such as bark beetles, and allow pests to extend their ranges; together with hotter droughts and more frequent fires, these effects have caused widespread forest dieback in western North America, Europe, the Mediterranean and Australia. In parts of the boreal zone, satellite records show both greening and browning, the latter where drought stress outweighs longer seasons. Interactions are central: elevated CO₂ may partly offset drought stress through improved water-use efficiency, but this benefit appears limited during severe drought; nitrogen deposition can amplify growth responses where nitrogen limits growth; and ozone can reduce them. Long-lived trees cannot quickly adapt genetically to rapid change, so provenance trials, which reveal how populations from different climates grow in a common environment, are used to guide assisted migration, the deliberate planting of seed sources adapted to projected future climates. Tree physiology is therefore central to forecasting the future forest carbon sink and to choosing species and seed sources for planting today, and physiological knowledge is built into the process-based models used in climate-impact assessments.
A student uses this subject to explain the why behind silviculture and nursery practice: why spruce regenerates under shade and pine does not, why cuttings from old trees fail to root, why seedlings need hardening before planting, and why ring-porous oaks suffer more from freeze–thaw. Calculations to master are water potential components, gravitational potential with height, Hagen–Poiseuille scaling, WUE, CUE, Q₁₀, resorption efficiency and percentage loss of conductivity. Common mistakes are confusing endodormancy with ecodormancy and compression with tension wood.
A forester or nursery manager five years into practice applies physiology daily: scheduling irrigation and lifting by seedling water potential and cold hardiness, choosing IBA treatments and stock-plant management for clonal cuttings, inoculating nursery beds with mycorrhizal fungi, diagnosing nutrient deficiencies from foliage, timing GA4/7 treatments or girdling in seed orchards, and choosing species and seed sources that match site frost, drought, flooding or salinity.
A senior professional with twenty-five years of experience uses physiology to manage risk across estates and programmes: matching provenances to future climates, setting thinning regimes that reduce drought stress, deciding when to salvage drought- or beetle-killed stands, planning restoration on saline or waterlogged land, and judging claims about forest carbon uptake under rising CO₂ in carbon projects and national strategies.
A researcher with a PhD and long experience works at the frontier: hydraulic traits and drought mortality, carbon allocation and storage, phenology models, mycorrhizal ecology and nutrient economies, wood formation genomics, and FACE and warming experiments. Such researchers build the physiological parts of forest models, contribute to international assessments, supervise doctoral students and examine fellowship candidates, so they must distinguish well-established mechanisms from active debates such as carbon starvation versus hydraulic failure.
This manual gives mechanisms and representative ranges rather than species-by-species values; P50 values, critical foliar nutrient concentrations, chilling requirements, IBA concentrations and juvenile-phase lengths vary with species, provenance and method and must be taken from specific studies. Molecular physiology (signalling pathways, gene regulation), phloem transport, volatile emissions, and the physiology of fruit trees and bamboos are covered only briefly or not at all. The relative importance of hydraulic failure and carbon starvation in drought mortality, and the long-term response of mature forests to elevated CO₂, remain active research questions.
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