BOTANY, VOLUME 4 - ECOLOGY - 2007

13. PLANTS WITHIN THEIR HABITAT SPACE

The provision of solar radiation, Water, and mineral nutrients, alongside the synthesis of carbon essential for growth and biomass production, forms the vital link between plants and the PHYSICOCHEMICAL PROPERTIES OF their surrounding environment. The biochemical and Physiological foundations of this connection were discussed in Chapter 6 (and partly in Chapters 7 and 9). This chapter examines the responses of individual plants, plant communities, and ecosystems to natural fluctuations in solar radiation, moisture supplies, and nutrient availability, as well as the carbon balance from an ecological perspective. Finally, it explores biological interactions, human impacts on vegetation, and plant utilization.

13.1. Radiation and Energy Budget

The radiation regime and energy budget are determined by climate both directly and indirectly—via evaporation, cloud formation, precipitation, and local or global Temperature effects. Under a full midday sun in summer in Central Europe, the Earth's surface receives up to 900 W of energy per 1 m². What happens to this enormous amount of energy within the ecosystem, and indeed on each individual leaf, is crucial for understanding plant life.

13.1.1. Radiation Quantity and Radiation Balance

The total amount of solar radiation reaching the Earth's surface is termed global radiation. It comprises both direct and diffuse radiation and, due to atmospheric reflection and absorption in mid-latitudes, amounts to only about half the radiation intensity measured in the direction of the Sun outside the atmosphere (the "solar constant" = approximately 1400 W m²). Roughly half of the global radiation falls within the visible spectrum (light wavelength 380 — 780 nm), which largely corresponds to the spectral range of photosynthetically active radiation spanning wavelengths from 380 to 710 nm (photosynthetically active radiation, PAR, PhAR — predominantly characteristic of the 400 — 700 nm wavelength range; W m²). Since photosynthetically active radiation (see 6.4.1, Box 6.2) consists of photons (light particles) carrying varying amounts of energy (quantum load: blue photons are energy-rich, red photons are energy-poor), neutral, non-filtering radiation sensors will always primarily capture the shorter-wave, energy-rich spectral regions. So-called photon sensors filter the light, and all photons in the 400 — 700 nm range are registered roughly equally, effectively acting as photon counters. Due to the direct stoichiometric relationship between absorbed photons in the 400 — 700 nm spectral range and photosynthetic CO₂ fixation, biology adopts photosynthetically active photon flux density as the standard (photosynthetically active photon flux density — PPFD, or simply PFD, usually measured in μmol photons per 1 m² per 1 s^-1; occasionally the unit "Einstein" is used, where 1 E = 1 mol of photons, which is inconsistent with SI units). It is widespread yet incorrect to denote photon flux density data (improperly called quantum sensors) in μmol • m^-2 • s^-1 as PAR or PhAR (see above). Conversely, PFD is unsuitable for calculating the Energy balance, as photon flux density is not an energy unit.

Diffuse components of global radiation penetrate plant communities significantly better (deeper) than direct radiation, which casts sharp shadows. Depending on leaf shape and size, plants can increase the proportion of diffuse radiation within the community (for instance, needles or fine pinnate acacia leaves enhance the proportion of scattered light). Part of the global radiation is reflected by the surface it strikes, and the vegetation cover exerts a significant influence on this reflection. Reflection from light-leaved desert shrubs amounts to about 20% of the radiation, from spruce forests only 10%, from bare soil up to 30%, and from freshly fallen snow up to 80%. The remaining fraction, the radiation balance, represents The amount of energy absorbed by the leaves and, consequently, by the community (Fig. 13.1). During the day, this value is always positive; at night, it is zero or negative. A negative nocturnal radiation balance arises from the plant's own thermal radiation.

Class="center">Fig. 13.1. Leaf energy budget. Minus the energy reflected (R), emitted (IR), and utilized in photochemical processes (Ph), the major part of the incoming global radiation (G)—the radiation balance energy (S)—must be dissipated by the leaf, since its mass is too small to accumulate substantial energy. Depending on water supply, this energy can either be expended as latent heat of vaporization (Transpiration, Tr, "senseless/latent heat flux") or lost to the air via convective Heat transfer (K, "sensible heat flux")

All bodies (even gases) emit thermal energy proportional to the fourth power of their absolute temperature. The balance of this thermal radiation depends on the temperature of opposing or surrounding bodies. A warm body in a cold environment radiates more than it absorbs. For plants under a clear sky on a clear night, heat loss via radiation into the cold surrounding space is significant, and leaf temperatures drop by 3 — 5 K compared to the air, which can lead to supercooling (K, Kelvin, used for temperature differences independently of the degree scale). Cloud cover or fog prevents this effect.

13.1.2. Energy Budget and Microclimate

A leaf can "unload its concerns" regarding the energy acquired via the radiation balance and absorbed by the leaf itself through four mechanisms: (1) thermal radiation; (2) photochemical energy fixation (maximum 1 — 2% of total PAR); (3) water transpiration Tr; (4) energy dissipation via thermal convection K (loss through the heated surrounding air). By day, only the latter two components are significant for the leaf's energy budget Q (heat accumulation is negligible due to the small mass of the leaves):

Q = Tr + K; Tr = g∆wv; K = h∆Tq,

where g is the diffusive conductance of the leaf epidermis for water vapor (essentially stomatal conductance); ∆w is the gradient of the molar ratios of water vapor and air between the internal leaf Tissues and the ambient air (at sea level and 0.1 MPa air pressure, this numerically corresponds to the vapor pressure gradient); v is the latent heat of vaporization (at 20 °C equal to 2.45 kJ/g); h is the capacity of the leaf boundary layers to transfer heat into the air (a function of leaf width and wind speed); ∆T is the temperature difference between the leaf and the air; q is the heat capacity of air. Conductances are the reciprocals of the corresponding resistances (water vapor diffusion resistance and heat dissipation resistance).

Through g and h, plants exert physiological and morphological influences—via their leaves—on their own microclimate as well as the surrounding environment, while simultaneously depending on water supply. Cooling can only dissipate energy when soil moisture is sufficiently high ("latent" heat flux; the leaf temperature itself remains close to the ambient air temperature or 1 — 2 K below it). In the event of water scarcity and closed Stomata, the energy flux is forcibly shifted toward thermal convection ("sensible heat flux"), which can lead to heat stress and death if morphological adaptations facilitating heat dissipation are lacking. Plants of hotter and drier habitats often feature leaves oriented perpendicularly to the sun's rays, which are small and highly reflective. This achieves reduced radiation absorption and strong thermal coupling with the air (low aerodynamic boundary-layer resistance), thereby avoiding overheating. Knowing the value of Q, air humidity and temperature, wind speed (meteorological parameters), as well as g and leaf width, one can calculate leaf temperature.

In plant communities, aerodynamic obstacles further complicate gas and heat exchange. The denser and lower the community, the greater its independence from atmospheric conditions and, consequently, the more heat and moisture it retains. This is most pronounced in low-growing alpine plants (especially cushion plants), within the canopy layer of which conditions resembling a humid tropical forest can develop under bright sun—bearing little resemblance to the readings of any weather station. Under such canopy closure effects, the direct influence of stomata on transpiration diminishes. The community Structure itself becomes the determining factor.

Broadly similar relationships apply to the energy budget of the entire ecosystem. The Role of leaf and community transpiration is fulfilled by evapotranspiration ET (or total evaporation V, including soil evaporation and wetting surfaces; in closed communities and with moistened soil surfaces, leaf transpiration accounts for over 80%). With increased evaporation, the ecosystem remains relatively cool; with decreased evaporation, it warms up. The energy equivalent expressed by the ratio K : V is termed the Bowen ratio, β. It is generally considered that if β is less than 1, the vegetation does not experience water stress. Under arid conditions or in the presence of a soil crust, β increases toward infinity—meaning that once all water is depleted, nearly all energy must be channeled into heating the air (a small portion is temporarily stored in the soil as heat; Fig. 13.2). The value of β can be determined non-destructively using meteorological Methods (measuring radiation balance and vertical climatic gradients above the plant canopy).

Fig. 13.2. Influence of transpiring plants on ambient temperature. Three scenarios illustrate how the reduction of green cover forces an increasingly larger share of incoming solar radiant energy to be "rejected" as thermal convection (K). Under a closed plant canopy and moist soil, more than half of the energy is consumed via heat-driven water vaporization (V) in a "cool" state, keeping the air cool—the so-called Bowen ratio: β = K/V < 1—and soil heat flux can be neglected. As soil compaction increases due to the prevalence of non-transpiring surfaces, the value of K rises, β becomes >1, and both air and soil heat up more intensely. The illustration clearly demonstrates why the air in urban green spaces is cooler than in built-up, "sealed" districts (numerical values are given as percentages of the radiated energy)

Soil surfaces dry out within a few days after rain, with direct soil evaporation being very minor. Once roots tap deep soil layers (see Table 13.3 in Section 13.7.5.1), the evaporating water is released into the atmosphere—a process impossible without the participation of plants. Plants thus connect deep water reserves with the atmosphere and retain control over this process via their stomata (see 13.5.2). These principles explain why city squares and parks act as cool islands, and why deforestation leads to temperature increases severe enough to alter regional climates (warm updrafts reduce precipitation, Fig. 13.3). Green vegetation cover impacts both the water balance (see 13.5) and the energy balance within a landscape. Through their Morphology and transpiration regulation, plants influence both their own microclimate and that of the ecosystem.

Fig. 13.3. Regional climatic consequences of deforestation. Approximately 400 years of history separate the left and right images. Explorers of the 16th century described these locations as green jungles (A — near Valencia); nowadays, As a result of deforestation, overgrazing, soil degradation, and recurring fires, this part of Venezuela is dominated by spiny shrubs (B — near Barquisimeto). The progressive elimination of transpiration cooling (see Fig. 13.2) has resulted in regional warming and a semi-arid climate. A Bowen ratio (β) of roughly 1 keeps the ecosystem on the left relatively cool (<30 °C), whereas a ratio significantly greater than 1 allows temperatures in the right-hand system to exceed 40 °C—a condition that itself triggers strong thermal updrafts and leads to reduced precipitation

An example of climate-regulating plant transpiration can be found in the studies of urban Los Angeles climate by A. H. Rosenfeld and J. J. Romm. Due to continuous urban sprawl and the associated soil sealing, urban air temperatures have risen by an average of 1 K every 15 years. If more shade trees (providing transpiration cooling) were planted between buildings and roofs were painted in lighter colors (enhancing reflection), the city could save 0.5 billion dollars annually in cooling and smog-cleaning expenses. Across all cities in the southern United States, the estimated savings would reach 5 — 10 billion dollars per year—not even accounting for the improved quality of life in such greener cities. Through the energy expended on transpiration, plants act not only as shading elements but also as climatic cooling devices.

13.1.3 Light in the Plant Community

As a photon stream penetrates through a plant canopy or a plant community—much like passing through a body of water (see Fig. 12.10)—its density gradually attenuates. The scale of this attenuation is determined by the leaf area index (LAI) of the stand, because leaves that receive insufficient light for a positive carbon balance are shed by the plant (or fail to form in such locations altogether). Similarly, photosynthetically active plankton is distributed across depths in aquatic environments. For higher plants, the lower threshold of net photosynthetic gain for a shade-adapted leaf lies at approximately 0.2% of the maximum midday photon flux density (a PFD of about 3 — 5 µmol • m^-2 • s^-1). Taking into account the leaf's nighttime carbon losses and the carbon demands of non-photosynthetic Organs, the minimum PFD requirement for a positive carbon balance rises to 0.5 — 1% of the intensity measured above the community at midday.

For a homogeneous plant stand in photometry, by analogy with the Lambert–Beer absorption law, the exponential relationship (Fig. 13.4) holds true in the formula where I and I_o are the PFD below and above the respective canopy layer, and k is the extinction coefficient.

Fig. 13.4. Typical pattern of Light absorption in a forest. For simplicity, it is assumed that in each of the 6 canopy layers, the leaf area equals 1 m² per 1 m² of soil surface (leaf area index LAI = 6). The leaf area is distributed homogeneously, and the transmitted photon flux is halved in each layer (absorption coefficient k = 0.69). According to the Lambert–Beer law, the exponential attenuation of PFD is determined by how many such leaf layers (what cumulative LAI value) are required to reach the critical threshold for a positive carbon balance in the lowest canopy layer (the minimum PFD needed for a positive carbon balance in a single leaf). The maximum possible LAI value is largely governed by the absorption coefficient, which typically ranges from 0.4 to 0.8.

The magnitude of the variable absorption coefficient depends on the average leaf size and inclination angle, as well as slightly on leaf transmittance, and significantly on the solar position and the proportion of diffuse radiation. Typical values of k are 0.4 — 0.5 for stands with vertically oriented or very small leaves (e.g., graminoids, conifers; small leaves promote strong light scattering), and 0.7 — 0.8 for horizontally disposed large leaves (e.g., certain tall-herb species and broad-leaved trees). For grassland communities, k is roughly 0.5; for temperate deciduous forests, it is about 0.65. The values of k and the mean leaf inclination angle can also be determined from the differences in light attenuation at various azimuth angles (via computer analysis of sunfleck distributions in hemispherical upward-facing "fisheye" photographs).

Knowing the values of k and I₀ (the latter obtained from meteorological station data), one can predict the value of I for a given LAI and thereby estimate the photon flux density per leaf at that Location. Conversely, knowing I, I₀, and k allows one to calculate the LAI. Modern equipment utilizing concentrically segmented fisheye sensors and sophisticated algorithms can determine k and requires only two ideally simultaneous measurements—radiation readings below and above the canopy—to rapidly and non-destructively evaluate one of the most vital biological parameters of a plant community, provided that boundary conditions (above all, a homogeneous and random leaf distribution) are met.

Typical LAI values are 3 — 4 for pine forests; around 5.5 for mixed temperate deciduous forests; 7 — 8 for highly productive hay meadows; 8 for all strata of lowland tropical rainforests; and 10 for dense spruce plantations. In closed agricultural crops, by the time vegetative shoots reach maximum development, the LAI is roughly 4 depending on the variety, while in natural high-altitude meadows it is about 2. LAI values exceeding 10, occasionally cited in literature, are unrealistic. In forests, such calculations often include the light attenuation caused by tree trunks and branches; hence, the term PAI (plant area index) is frequently used, with THE CONTRIBUTION OF such non-leaf structures generally remaining below 1. Calibrating these measurements requires direct (destructive) Determination of the leaf area (harvesting). A simple and highly instructive (albeit retrospective) method for approximating LAI is the "needle-punch" technique in deciduous forests shortly after autumn leaf fall, ideally in damp weather. The average number of punctures made by a needle through a fresh layer of fallen leaves directly indicates the LAI value just prior to leaf fall, provided there has been no significant wind displacement. As a rule, the results of this method show a remarkably good agreement with calculated absorption methods.

By determining LAI values across different strata of a community, one can reveal the characteristic vertical distribution of leaf areas for many of them, and consequently the typical light absorption curve within the stand. In forests, the absorption maximum is often prominent in the upper tree crown (up to half of the PFD is absorbed in the uppermost square meter of leaf area per square meter of soil surface), whereas in grassland swards, absorption largely takes place in the lower strata.

This is also facilitated by the vertical distribution of leaf inclination angles: leaves tend to be vertically oriented in the upper canopy layers and horizontally disposed near the ground (e.g., rosette plants). The actual light availability of a leaf decreases with the cosine of the angle of incidence of radiation relative to the leaf normal (cosine law). There is a close correlation between a leaf's inclination angle and its anatomy. The more vertically oriented a leaf is, the more symmetrically its mesophyll is arranged relative to the leaf surfaces; in other words, narrow, nearly vertical leaves often lack the clear differentiation between palisade and spongy parenchyma. In agriculture and horticulture, light utilization is optimized by establishing mixed cropping systems (e.g., cucumbers or pumpkins grown with corn). The leaf inclination angle, and consequently the geometric component of radiation absorption by the canopy, play a major role in yield optimization; agronomic and management practices are therefore geared toward ensuring high LAI values. Vertical leaf orientation (as seen in Japanese rice varieties) enables high LAI values in the tropics, which favors increased yields under nitrogen Fertilization. Conversely, more horizontally oriented leaves, or drooping grass leaves (such as in Indian rice varieties), rapidly lead to self-shading, meaning that increasing LAI via fertilization does not boost yields. Even in distinctly "sun" leaves, less than half of the full midday solar radiation is generally sufficient for Photosynthesis. Leaves in the outermost canopy layer are frequently exposed to supra-optimal radiation. It has been repeatedly demonstrated that the vertical profile of radiation distribution within a plant community correlates with nitrogen distribution and the maximum photosynthetic rate Am_max (higher nitrogen content and greater Am_max values occurring in the upper canopy layers, see 13.6.3). Nitrogen fertilization can increase Am_max to such an extent that highly shade-tolerant species, such as cocoa, no longer require shade, allowing their uppermost leaves to tolerate full tropical sunlight.