Canopy Photosynthesis, Productivity, and Decomposition

Canopy

A canopy is the above-ground portion of a plant or plant community, consisting of leaves, stems, and branches arranged in 3D space

Leaf Area Index (LAI)

Total leaf area per unit ground area LAI> 1 indicates overlapping leaves

Foliage Inclination

Erectophilic: Vertical leaves (grasses; better light penetration)
Planophilic: horizontal leaves (broadleaf plants; higher light absorption)

Light Penetration

Beer’s law (I=I0​e−kL)

• I = Light at depth L,

• k = Extinction coefficient (lower for vertical leaves).

Primary Productivity

The amount of carbon transformed from CO2 to organic carbon per unit area in a specified time period

3 types of productivity

Gross, Net and standing crop biomass

Gross primary productivity (GPP)

Total CO2 fixed by photosynthesis

Net Primary productivity (NPP)

GPP- plant respiration (NPP=GPP-R)

Standing Crop Biomass

Total biomass at a given time

Methods of determining NPP

Directly measure how much carbon dioxide is absorbed by vegetation

Eddy covariance: Large scale CO2 Flux measurements
Harvest techniques: weigh biomass over time

Remote Sensing (NDVI): uses reflectance (near infrared vs visible light) to estimate greenness (satellite)

Chambers are only used for small canopies

Decomposition

Breakdown of detritus (dead organic matter) into simpler compounds (CO₂, nutrients)

Key Steps to decomposition

Leaching: Loss of water-soluble compounds

Fragmentation: Physical breakdown by detritivores (e.g., earthworms)

Chemical alteration: Microbial action (fungi/bacteria) on lignin/cellulose

Mineralization: Conversion to inorganic nutrients (e.g., NH₄⁺, PO₄³⁻)

Soil organic matter (SOM): Stable carbon storage (slow turnover)

Microbial Respiration = use of organic matter as energy source by fungi and bacteria--results in efflux of CO2

Mineralization = conversion of organic to inorganic nutrients

Nutrient immobilization = incorporation of mineral nutrients into microbial biomass (unavailable to plants)

What Factors affect Decomposition

Litter quality:

Low C:N ratio which leads to faster decomposition like fresh leaves

High Lignin which leads to slower decomposition like woody material

Environment:

Temperature/Moisture: faster in warm and moist climates

Photodegradation: UV light breaks down lignin in drylands like deserts

Microbial community: determines decomposition pathways (fungi and bacteria)

Photofacilitation: Photodegradation can also alter litter chemistry making it more digestible for microbes

Carbon storage and Net Ecosystem production NEP

NEP: net carbon gain by an ecosystem

Positive: Carbon sink; growing forests

Negative: Carbon source; disturbed ecosystem

Soil Carbon pools

Fast turnover: microbial biomass (years)

Slow Turnover: Humic substances (millennia)