BOTANY VOLUME 4 - ECOLOGY - 2007

13. PLANTS IN THEIR HABITAT SPACE

13.3. Temperature Tolerance

13.3.1. Frost Resistance

Among the climatic factors that determine the distribution of plants across the Earth's surface, Water supply and tolerance to extreme low temperatures are the most critical. Frost acts as the primary "environmental filter" that a plant species must pass through before it can establish itself in any frost-prone region. Plants that successfully pass this Selection process are considered hardy, meaning they generally face no substantial threats from the environment (natural flora). Plants with insufficiently hardy shoots can survive critical periods either as seeds (annuals) or when their renewal Organs are located underground (e.g., geophytes). These coping mechanisms are often referred to as the escape strategy regarding frost impact (see Fig. 12.2). In any case, frost resistance is defined as the Prevention of lethal ice crystal formation within the Cytoplasm. To understand The Mechanism of frost resistance, recall that water in plant tissue is distributed across two compartments: outside the protoplast membrane in the so-called apoplast and xylem—primarily within Cells, potentially in intercellular spaces (containing very weakly soluble substances)—and in the symplast (with osmotic potential values in the turgescent state ranging from -1.2 to -2.5 MPa).

There are two distinct mechanisms of frost resistance.

Freeze prevention, or supercooling tolerance (English supercooling), is the persistent inhibition of ice formation at sub-zero temperatures. In the leaves of many plants, these values reach -5 °C, in some high-altitude plants down to -12 °C, and in the xylem parenchyma of certain temperate woody species down to approximately -40 °C. The decisive factors in delaying freezing are the absence of crystallization nuclei and the transition of water into a metastable state. If the critical Temperature is exceeded, instantaneous tissue freezing occurs, which is fatal to the cells.

Tolerance to sub-zero temperatures is functionally a specialized form of drought resistance. Ice formation begins in the Tissues where water has the lowest osmotic pressure—specifically, in the apoplast. Consequently, the intercellular spaces fill with ice (without tissue damage), while the symplast progressively releases water. This process requires high water permeability and, therefore, mobility of the Plasmalemma, which remains undamaged at very low temperatures—a genetic trait that also governs strong acclimation properties (frost hardening, membrane lipid properties). To varying degrees of Cell dehydration and protoplast tolerance to dehydration, osmotic parameters play a role, most notably protective substances that stabilize cell membranes (soluble CARBOHYDRATES and so-called stress Proteins), which produce a uniform effect of resistance to both frost and drought.

While the absence of crystal nuclei is crucial for freeze prevention, a certain slowing of the ice formation process is beneficial for freeze tolerance. Lowering the freezing point through the accumulation of osmotic pressure is only significant in relation to dehydration induced by ice formation. Turgid cells on their own produce a very minor effect, since lowering the freezing point by 1.9 K requires 1 mole of osmotic pressure, which corresponds to an additional Osmotic Pressure of 2.24 MPa (in most plants, this would roughly correspond to a doubling of pressure).

Of particular ecological and practical significance is the fact that plant frost hardiness is heavily dependent on external and internal factors; therefore, a specific species cannot be characterized by a single critical temperature. The lowest temperature a plant can actually withstand at a given potential of resistance is determined by the following five impact factors:

✵ acclimation circumstances (season of the year, preceding temperature conditions; Fig. 13.6);

✵ developmental phase (active, growing, unformed, or young tissue is less hardy than formed, less active, older tissue);

✵ organ or tissue type (roots tolerate temperature drops to a much lesser degree than leaves; depending on the developmental phase and plant species, the cambium may be more or less hardy than wood parenchyma, leaf buds than flower buds, etc.);

✵ moisture supply (plants requiring constant Hydration are less hardy than drought-tolerant ones);

✵ nutrient supply (optimally nourished plants are hardier than over-fertilized ones or those experiencing mineral deficiencies).

Class="center">Fig. 13.6. Annual course of day length and daily minimum and maximum temperatures (top — average three-day temperature for the city of Bayreuth) and frost resistance of one-year-old needles of Pinus sylvestris (bottom). Arrows indicate the first occurrence of night frosts. Gray tones from left to right show the phases: frost sensitivity, frost resistance, full (maximum) frost hardiness, and the phase of declining frost hardiness in spring

The critical damage temperature for leaf tissue in the temperate zone during the growing season lies between -2 and -8 °C, with forbs being more sensitive than C₃ grasses and sedges, and the leaves of summer-green plants more sensitive than those of evergreen trees. The most dangerous situations are late spring frosts after bud burst and, more generally, extreme temperature fluctuations (a sharp drop in temperature following preceding mild weather). In native plants typical of a given habitat, flowers and leaves are primarily damaged by frost, and the cambium less frequently, but this never threatens the survival of the entire plant. Radiation cooling during clear nights following the passage of a cyclone front is particularly hazardous. Maximum winter frost tolerance of above-ground organs in temperate zone plants (when well-hardened) lies between -25 and -40 °C, for adult Mediterranean woody plants between -10 and -14 °C (for container-grown plants, the critical temperature can be -2 °C). Sensitive species survive frosts under the protection of snow and thanks to the subterranean position of their renewal buds. Some tropical plants suffer damage already at temperatures of 0 to 7 °C; this is referred to as chilling injury.

13.3.2. Heat Tolerance

Heat tolerance varies significantly less; in higher plants, critical temperatures lie within 50 to 55 °C (high temperature values are found in rosette-forming, prostrate, sclerophyllous plants, and succulents; maximum values of 60 °C occur in certain C₄ grasses, savanna palms, and cacti). Heat damage is largely determined by Morphology, distance from the soil, self-shading of the SHOOT base, and water supply. Seedlings on dry, dark, bare soils are in a particularly vulnerable position, as their surface can heat up to over 75 °C under strong solar radiation. In such habitats, plants require rapid seedling development during the cooler season and subsequent shading of the soil by their own leaves or other vegetation. Many species can endure extremely hot conditions through transpirational cooling (up to 10 K; see 13.1.2). If roots fail to reach sufficient soil water reserves (roots reaching 30 m in depth are not uncommon; see 13.7.5.1), plants shed their leaves or survive the period solely in the form of seeds.

13.3.3. Fire Ecology

In many PARTS OF THE world, fires represent a vital ecological factor for ecosystem development and the establishment of their characteristic species assemblages. Major biomes have acquired their distinctive appearance thanks to fire (savannas, semi-arid shrublands, Mediterranean vegetation, prairies, as well as boreal forests). The usual natural cause of fire is lightning. The wide range of typical Plant Adaptations to existence under these conditions proves that fire was an ecologically potent factor long before humans increased the frequency of fires (Fig. 13.7). Pyrophytes (plants specialized for fire) often maintain viable seed banks in the soil or canopy for long periods, are capable of sprouting from stumps (underground stems capable of regeneration, known as xylopodia), are equipped with protective bark (trees), or possess a largely subterranean apical meristem (grasses, geophytes). In tussock grasses and rosette-forming plants (e.g., Xanthorrhoea, Yucca, Espeletia), protection against fire is frequently provided by a "grass tunic" or dead leaf bases. Phenorhythmicity is often closely linked to the onset of fires (e.g., shedding leaves during critical drought periods). In many species of Pinus, Eucalyptus, the Proteaceae family, and others, fruits open only after exposure to fire; only then do the seeds achieve full germinability and disperse. This ensures regeneration at a favorable moment when competition for light and ROOT competition are reduced, and litter that hinders germination is converted into nutrient-rich ash.

Fig. 13.7. Plant responses to fire. Fire protection: B — with thick bark (Pinus halepensis, Western Mediterranean); D — with a dense "skirt" of dead leaf bases (Xanthorrhoea sp., Western Australia, in the Background on the right); E, G — by burying the meristem underground, as in most grasses (G — immediately after fire; E — 10 days later). Post-fire regeneration: D — via the canopy seed bank and the splitting of woody fruits only under METABOLISM/18.html">The Influence of high temperatures (Hakea sp.); F — through rapid germination in a nutrient-enriched and unshaded layer (Eucalyptus sp., eastern Australia); A, C — via underground (C — Arbutus andrachne, Eastern Mediterranean) or above-ground (A — Pinus canariensis, Tenerife) sprouting. H — surface fires, which merely mineralize the litter layer, cause only slight damage to vegetation, and are especially beneficial for ecosystem nutrient enrichment

Obligatory fire cycles vary, ranging from annual (savanna), short-interval (other grassland biomes), 30- to 40-year (Mediterranean scrub), to centennial (up to several hundred years) in boreal forests. Combustion is facilitated by a highly flammable, thick litter layer. In open dry forests and savannas, the result is generally not destructive crown fires (with temperatures up to 1,000 °C and the destruction of all woody plants), but rather rapidly moving surface fires (temperatures in the litter layer and upper soil horizon remain at 70 to 100 °C for a short time, barely exceeding 500 °C at a height of 0.5 to 1.0 m; Fig. 13.8). Consequently, the renewal organs of woody and herbaceous plants suffer little damage. The same holds true for fires in steppes and tropical grassland communities. The decisive factor is the average duration of the fire front passing through a given Location. During a surface fire, this often fails to 2 minutes, which is insufficient for lethal scorching to reach the sensitive meristem.

Fig. 13.8. Maximum soil surface temperatures across various vegetation types during the passage of a fire front. The duration of the fire at a single location and the peak temperatures both increase with a higher proportion of woody plants. Under dry conditions, the maximum thermal impact is also heightened

To prevent hazardous wildfires near populated areas, controlled Burns are frequently conducted in advance during cool and humid weather (such as traditional burning in California). Similarly, in nature reserve management, acknowledging the ecological significance of fire, managers sometimes permit or even deliberately set fires rather than suppressing them entirely. Ecosystems that lack natural fires, or experience them very rarely, suffer severe damage from arson (see 13.6.1). Because the species comprising such biomes possess no fire resistance, these fires result in heavily degraded secondary communities. In place of tropical rainforests, tall-grass savannas emerge, whose high flammability impedes forest regeneration. A fire frequency exceeding a certain threshold will also lead to degradation, even in fire-adapted ecosystems.

The remarkable age of large forest trees in many Regions of the Earth is a direct result of their fire resistance. A well-known example is the giant sequoia (Sequoiadendron giganteum) in California, which can live up to 2,000 years thanks exclusively to its thick bark (Fig. 13.9). Beyond regulating species composition and life forms within an ecosystem, the primary importance of fire lies in maintaining mineral nutrient cycling in habitats where low moisture availability slows decomposition rates, or where the ground layer increasingly hinders the supply of nutrients from litter to the canopy vegetation (e.g., mosses in boreal forests, see 15.2.14).

Fig. 13.9. Thick bark ensures longevity (note the bark remnants (Bo) with a thickness of 15 cm in B). The ancient 2,000-year-old trunk of a giant sequoia — Sequoiadendron giganteum (A, C — Sierra Nevada in California) demonstrates successful coexistence with fire (D — fire in a eucalyptus forest). Heat is dissipated within the energy-poor lamellar Structure OF THE Sequoiadendron bark (E), much like a stack of newspaper. Black arrows (B) indicate charred and subsequently overgrown areas, while white arrows point to growth rings corresponding in time to major historical events (1 — coronation of Charlemagne, 2 — Columbus reaching America, 3 — end of World War I)