Biomass is defined as the total mass of living or recently deceased organic material within a specific area or ecosystem. Measuring this mass provides foundational data for numerous scientific and commercial applications. In ecology, biomass measurements reveal the productivity and energy flow between different trophic levels within a food web. Quantifying the carbon stored in forests and other ecosystems is necessary for global resource management and meeting climate change mitigation goals. Furthermore, biomass measurement is a prerequisite for the renewable energy sector, where it dictates the potential energy yield of feedstocks like wood, agricultural residues, and dedicated energy crops.
Defining and Standardizing Biomass Measurement
Comparability in biomass studies requires adherence to strict measurement standards, involving the distinction between wet mass and dry mass. Wet mass, or fresh weight, is the total mass of the organism as it exists in nature, including its highly variable water content. Because the water content in plants and microorganisms can fluctuate widely based on environmental conditions, wet mass is generally considered unreliable for scientific comparison.
The scientific standard for measurement is dry mass, which is the weight of the organic material after all moisture has been removed. This standardized value represents the actual structural material and stored carbon compounds in the organism. Biomass is typically expressed in standardized units that relate mass to the area or volume sampled, such as grams per square meter (\(text{g}/text{m}^2\)) or kilograms per hectare (\(text{kg}/text{ha}\)), allowing for direct comparison.
Direct Measurement Techniques
Direct measurement, often referred to as destructive sampling, involves physically harvesting and processing the material to obtain a precise mass. This method is the most accurate for validating other techniques, but it is labor-intensive and permanently removes the sampled material from the ecosystem. The process begins with establishing sampling plots, such as a one square meter quadrat for grasses or a designated area for crops.
After the material is collected, it is sorted into components like leaves, stems, and roots, and its fresh weight is recorded. Subsamples of each component are then taken to the laboratory and placed in a drying oven, typically at a temperature of \(60^circtext{C}\) to \(105^circtext{C}\). The samples are dried until they reach a constant weight, indicating that all moisture has been driven off. For small organisms like microbial cultures, biomass is determined by filtering or centrifuging the cells, washing them, and then drying the resulting pellet to a constant weight.
Indirect Measurement Techniques
When destructive sampling is impractical, such as in protected forests or for large trees, researchers use indirect measurement techniques relying on mathematical models to estimate mass. The most widely adopted of these are allometric equations, which establish a statistical relationship between easily measurable tree dimensions and total biomass. These models are developed by destructively sampling a subset of trees to create a regression curve that links simple parameters to actual dry mass.
The most common input parameter for allometric equations is the tree’s Diameter at Breast Height (DBH), measured at \(1.3\) meters above the ground. Tree height is often included to improve the accuracy of the estimate. Once an allometric equation is developed for a specific species and region, researchers can rapidly measure DBH and height for hundreds of trees in a plot and input these values into the model to estimate the total aboveground biomass. For very small organisms, a non-destructive proxy like turbidity, which measures how much light a cell suspension scatters, can be correlated to dry weight using a standard curve, allowing for quick, repeated biomass estimates.
Large-Scale Measurement Through Remote Sensing
To scale biomass estimates across vast geographic areas, scientists rely on remote sensing technology. Light Detection and Ranging (LiDAR) actively pulses a laser beam toward the ground, measuring the time it takes for the light to return. This process generates a three-dimensional point cloud of the landscape.
In forested areas, LiDAR data allows for the measurement of canopy height, vertical structure, and density, which are strong predictors of wood volume and biomass. By combining these structural metrics with ground-truthed data from allometric equations, researchers can create highly accurate, large-scale biomass maps. Spectral analysis uses the reflection of light wavelengths to estimate vegetation density. The Normalized Difference Vegetation Index (NDVI), for example, measures the contrast between near-infrared and visible red light reflected by plants. This index can be correlated with aboveground biomass, proving useful for monitoring the productivity of grasslands and agricultural fields.