Plant biomass

Background & relevance

Plant biomass refers to the total amount (volume or mass) of organic matter produced by plants [Houghton et al. 2009]. Plant biomass is mostly made-up of carbon, nitrogen and phosphorus, with carbon typically being > 10 times more abundant than nitrogen and > 100 times more abundant than phosphorus [Chen & Chen 2021]. Plant biomass can be quantified for any plant species at any scale of biological organisation, from a tissue/organ, to an individual, to a multi-species community. In practice, plant biomass is most commonly split into four categories:

Aboveground biomass (AGB): living aboveground plant parts
Belowground biomass (BGB): living roots
Litter: dead plant material, including woody branches/stems < 10 cm diameter
Deadwood: dead woody branches/stems > 10 cm diameter

Some frameworks do not consider litter or deadwood to be biomass because they are non-living.

Importance

Biomass is of foundational importance to ecosystems, in that it is not highly informative on its own but influences concepts of direct value to nature teams.

Plant biomass is a stock of organic matter — primarily carbon — so it influences facets of nature that depend simply on its physical existence. Examples include carbon stocks, crop yields, soil/snow/water retention, nesting bird diversity, etc.

Plant biomass is also an engine for physiological processes so it influences ecosystem processes [Aerts & Honnay 2011]. Examples include evapotranspiration, net primary production, plant growth, soil decomposition, soil carbon stocks, etc.

We anticipate that most data teams measure plant biomass as an input parameter to calculate or model other concepts of direct value to business operations, such as risk, CO₂ fluxes, etc.

Dependencies & influences

Below are non-exhaustive lists of how plant biomass depends on and influences other concepts. Linked concepts can be existing or anticipated concepts, or may be concepts that are important but never actually operationalised.

Plant biomass depends on

Concept	Details
PAR (light)	Strong positive link between photosynthetically active radiation (light) and biomass accumulation
Temperature	Strong positive link between temperature and biomass accumulation
Precipitation	Plants need H₂O for photosynthesis; plants under drought close stomata to prevent water loss (stops photosynthesis); rainfall does not always increase water availability
Topography	Steep slopes are hard to anchor roots into; topography drives mass flows (rockfall, soil erosion, avalanches, etc.) that destroy biomass
Net primary production	Net primary production is the process of biomass accumulation through photosynthesis
Plant growth	Akin to NPP, plant growth is the process of biomass accumulation
Soil moisture	Plants get H₂O for photosynthesis from soil; plants under drought close stomata to prevent water loss (stops photosynthesis)
Soil nutrient stocks	Plants get all macro- (NPK) and micro-nutrients from soil
Plant diversity	Different species grow to different sizes and have different proportions of aboveground vs belowground biomass; most variation is among functional types (e.g. trees versus grasses)
Animal diversity	Strong links between some animal groups (e.g. pollinators, ecosystem engineers) and biomass in some ecosystems; hard to separate from general ecosystem health effects
Soil diversity	Plant biomass can be lower when soil diversity is impoverished; usually limited to specific microbial groups (e.g. mycorrhiza); jury still out
Herbivory	Mammal and insect herbivores consume plant biomass; particularly important in grazed farmland
Harvesting	Periodic harvesting of biomass as yield (crops, timber, fruit) is common in managed parcels

Plant biomass influences

Concept	Details
Avalanche risk	Limited to snowy biomes on 30º+ slopes; plant biomass is a physical avalanche barrier
Soil erosion	Landscape dependent; root biomass is a physical barrier that inhibits water-borne soil erosion
Landslide risk	Limited to steep slopes; root biomass is a physical barrier that stabilises the soil, inhibiting landslides
Flood risk	Landscape dependent; plant biomass is a physical barrier to and physiological user of water, which slows catchment discharge
Crop yield	Crop yield is derived from plant biomass
NPP	Plant biomass is the engine for plant physiology, so more biomass = more net primary production
Photosynthesis (GPP)	Plant biomass is the engine for plant physiology, so more biomass = more photosynthesis
Plant growth	Plant biomass is the engine for plant physiology, so more biomass = higher rates of plant growth
Evapotranspiration	Plant biomass is the engine for plant physiology, so more biomass = more evapotranspiration
Soil nutrient stocks	Plants get all macro- (NPK) and micro-nutrients from soil, so more plant biomass = less nutrients; relationship can be absent or reversed if soil biomass is high and is not nutrient limited
Soil carbon stock	Plant biomass is the main source of soil carbon; but relationship is complex because soil carbon is a mix of new litter, old litter (usually < 100 years) and recycled organic matter (can be 1000+ years)
Soil decomposition	Soil decomposers consume litter biomass, so more biomass = more decomposition; decomposers use root exudates as energy for decomposition and more biomass = more exudates
Soil CO₂ efflux	Soil CO₂ efflux is root respiration plus microbial respiration; more biomass = more root respiration, and more biomass = more decomposition = more microbial respiration
Soil moisture	Plants get H₂O for photosynthesis from soil, so more biomass = lower soil moisture; caveat is that biomass shades the soil surface and can inhibit evaporation
Soil biomass	Plant biomass is the primary source of carbon for soil microbes; but soil microbial biomass is more influenced by microclimate and soil carbon stock
Plant diversity	Highly context dependent; plant biomass can lower plant diversity because competition for space, light, nutrients, etc., is higher
Animal diversity	Context dependent; plant biomass provides habitat for animal species (e.g. nests, burrows, etc.) — but possibly more a function of habitat structure
Animal biomass	Plant biomass is the primary source of carbon for the whole food web; but relationship is complicated by food web dynamics, animal mobility, etc.

Data requirements

The data that is used to quantify plant biomass. Variables come from three sources: (i) market demand; (ii) what science tells us is important; and (iii) what regulatory frameworks use.

Relevance: ability of variable to represent the concept of plant biomass
Scientific use: usage of variable in academic research
Regulatory use: usage of variable in regulatory nature frameworks

Name	Unit	Scale	Relevance	Scientific use	Regulatory use	Notes
Canopy cover	`%`	`Area`	`Accepted proxy`	`Regularly used`	`Regularly used`	In non-forests, “canopy” is “vegetation”
Canopy height	`m`	`Area,Point`	`Accepted proxy`	`Regularly used`	`Regularly used`	In non-forests, “canopy” is “vegetation”
Total biomass	`Mg/ha`	`Area`	`Direct measure`	`Regularly used`	`Regularly used`	Can be subdivided into aboveground, belowground, litter, etc.
Total volume	`m³/ha`	`Area`	`Direct measure`	`Sometimes used`	`Sometimes used`	Can be subdivided into aboveground, belowground, litter, etc.
Total stand/stocking density	`#/ha`	`Area`	`Tenuous proxy`	`Rarely used`	`Sometimes used`
Plant wood density	`g/cm³`	`Organism`	`Tenuous proxy`	`Rarely used`	`Sometimes used`
Plant root density	`g/cm³`	`Organism`	`Tenuous proxy`	`Rarely used`	`Rarely used`
Plant height	`m`	`Organism`	`Accepted proxy`	`Sometimes used`	`Rarely used`
Plant basal area	`m²`	`Organism`	`Accepted proxy`	`Rarely used`	`Sometimes used`
Plant diameter at breast height	`cm`	`Organism`	`Accepted proxy`	`Sometimes used`	`Rarely used`