Forest Ecology Notes

Forest Ecology

Energy in Ecosystems

• Forest ecology evaluates the production, transfer, and storage of energy in ecosystems.
• Energy is crucial for life, and all aspects of life are associated with energy.
• Ecology and economics, along with energy, form a unified system.
• The primary energy source in an ecosystem is light.

Light as an Energy Source

• Light is the primary energy source in all ecosystems.
• Photoautotrophs (primary producers) use photosynthesis to capture solar energy and store it in organic molecules like sugars and carbon.
• Gross Primary Productivity (GPP) is the rate of photosynthetic carbon production per unit area and time, measured at the ecosystem scale.

Photosynthesis

• Light energy from the sun converts carbon dioxide ($CO<em>2$) and water ($H</em>2O$) into glucose.
• Chlorophyll, a pigment in chloroplasts, absorbs light energy.
• Rubisco, an enzyme, catalyzes the reaction of carbon dioxide ($CO_2$) into sugar.

Carbon Flow in Ecosystems

• Carbon entering ecosystems via GPP is accumulated, respired, released by disturbance, or transported laterally.
• Respiration releases energy from sugars, yielding heat, carbon dioxide ($CO<em>2$), and water ($H</em>2O$).
• Every living organism respires, including plants.
• The efficiency of converting light energy into sugars is approximately 1% in field conditions.

Photosynthetically Active Radiation (PAR)

• PAR refers to photosynthetically active radiation.
• It includes the range of light wavelengths between 400 and 700 nanometers (nm) used for photosynthesis.

Light Availability and Photosynthesis

• Light availability influences photosynthetic rate.
• Net photosynthesis is the difference between photosynthesis and respiration (carbon gain minus carbon loss).
• Light compensation point is the point at which photosynthesis equals respiration (carbon gain equals carbon loss).
• Light saturation point is the PAR level above which no further increase in photosynthesis occurs.

Adaptations to Light Environments

• Species have differing adaptations to sun and shade environments.
• Shade-intolerant species are adapted to high light environments.
• Shade-tolerant species are adapted to low light environments.

Shade and Photosynthetic Rate

• In shady environments, low light limits the rate of photosynthesis.
  • Shade-adapted species produce less RUBISCO.
  • They require less energy, resulting in lower leaf respiration.
  • Their light saturation point is lower.
• Moving a shade-tolerant plant to a high light environment:
  • Limits its maximum rate of photosynthesis.
  • Results in less carbon gain and slower growth compared to a shade-intolerant species.

Comparative Study of Tree Species

• A study of 9 tree species from northeastern North America, representing a range of shade tolerance, was conducted in a greenhouse under constant environmental conditions.
• Shade-tolerant trees exhibited lower photosynthesis, respiration, and growth rates than shade-intolerant trees, regardless of the light environment.

Leaf Morphology and Specific Leaf Area (SLA)

• Shade-tolerant and shade-intolerant species also show differences in leaf morphology.
• Specific Leaf Area (SLA) is the ratio of leaf surface area (cm²) to weight (g), indicating the surface area of leaf produced per gram of biomass allocated to leaf production.
• Shade-tolerant plants generally have greater SLA.

SLA and Light Capture

• Greater SLA increases the surface area for light capture per unit of biomass.
• Leaf thickness decreases as SLA increases.
• Plants exhibit phenotypic plasticity of SLA in response to light levels in the environment (e.g., open vs. under canopy).

Leaf Adaptations

• Leaves on different parts of a plant can be adapted to different environments.
• Species adapted to bright habitats may incur costs or have specializations that are disadvantageous in shaded habitats.

Sun Leaves vs. Shade Leaves

• Sun leaves become light-saturated at much higher light intensities.
• The maximum photosynthetic rates are much higher for sun leaves compared to shade leaves.
• Table 2.2 Contrasts between sun leaves and shade leaves of Fagus sylvatica (beech, Fagaceae)

Interdependence of Environmental Factors

• Features of the physical environment are interdependent.
• Solar radiation influences:
  • The amount of photosynthetically active radiation (PAR).
  • The temperature of the leaf and surrounding air.
• Air temperature affects relative humidity.
• Relative humidity affects the rates of transpiration and evaporation of water in the soil.

Adaptive Trade-offs

• Multiple environmental conditions lead to adaptive trade-offs.
• Adaptations for one environment may not be as effective in a different environment.

Carbon Allocation

• Carbon allocation:
  • Allocating more carbon to producing leaves and stems increases access to light and carbon dioxide ($CO_2$).
  • Reduces availability to produce roots, decreasing access to water and soil nutrients.

Energy Flow Through an Ecosystem

Heterotrophs

• Heterotrophs obtain energy and nutrients by consuming organic compounds from plants and animals.
  • Herbivores feed exclusively on plant tissue.
  • Carnivores feed exclusively on tissue of other animals.
  • Omnivores feed on both plant and animal tissue (feeding at multiple trophic levels).
  • Detritivores feed on dead plant and animal matter.

Herbivores and Plant Tissue Consumption

• Herbivores must convert plant tissue (high in carbohydrates, low in proteins) to animal tissue (high in fats/proteins).
• Herbivores are categorized by the type of plant tissues they eat:
  • Grazers: eat mainly leaves, especially grasses.
  • Browsers: eat woody material.
  • Granivores: eat seeds.
  • Frugivores: eat fruit.
  • Nectivores: eat plant nectar/sap.

Grazers, Browsers, and Cellulose Digestion

• Grazers and browsers usually have diets very high in cellulose, which are long chains of glucose molecules.
• Cellulose is indigestible because most animals do not produce enzymes to digest it.
• Symbiotic organisms (bacteria, protozoa, fungi) live in their digestive tract and digest cellulose and proteins, as well as synthesize fatty acids, amino acids, proteins, and vitamins.

Nitrogen Content and Herbivore Life Cycles

• The highest quality food for herbivores is high in nitrogen (N).
• Nitrogen is concentrated in new leaves, growing tips, and buds (declines as tissues age).
• As nitrogen increases, assimilation of plant material increases, improving growth, reproductive success, and survival.
• Herbivore life-cycles have adapted to this period of new growth and high nitrogen content (e.g., herbivorous insect larvae, vertebrate herbivores like deer).
• Herbivores select for high nitrogen plants (e.g., beavers and willows, cows and clovers).

Carnivores

• Carnivores eat other animals and do not have problems with food quality or digesting cellulose.
• They also do not have problems digesting and assimilating nutrients from prey because the chemical composition of the tissues is similar.
• Quantity is important – they must be able to find enough food.

Omnivores

• Omnivores feed on both plants and animals (feeding at multiple trophic levels).
• Their food habits often vary with seasons, stages in life cycle, size, and growth rate.

Detritivores and Decomposers

• Detritivores obtain nutrients by consuming detritus (decomposing plants/animals/feces), mostly invertebrates and arthropods.
• Decomposers are mostly microbial and make nutrients available.

Food Chains and Food Webs

• Flow of matter and energy between individuals in a community.
  • Food chain: Simplest representation of matter and energy flow from prey (consumed) to predator (consumer).
  • Food webs: More accurate representation of matter and energy flow in a community, with complex linkages between predators and prey.
• Can be simplified by grouping species into broader categories (autotroph vs heterotroph) that represent general feeding groups (trophic levels).

Energy Loss Between Trophic Levels

• Within all biological communities, energy at each trophic level is lost in the form of heat, as organisms expend energy for metabolic processes such as staying warm and digesting food.
• As a rule of thumb, only about 10% of the energy that's stored as biomass in one trophic level (per unit time) ends up stored as biomass in the next trophic level (per the same unit time).

Energy Flow Through Forests

• Sunlight is available to plants, but not all is absorbed.
• A significant amount of energy is lost as heat through respiration.
• Energy flows through grazing and detritus trophic webs.
• There is a total tree biomass storage (current annual increment).