Vegetation

By default, the terrestrial carbon scheme contains three pools of live vegetation and two soil pools. The number of vegetation and soil pools can be modified at runtime, but the default settings are designed to approximate the flows, stocks, and turnover times of carbon in Earth’s vegetation and soils.

Conceptually, the movement of carbon through these pools is handled by Longterm as follows. The global net primary productivity (NPP) is a function of the atmospheric CO₂ concentration, and fixed fractions of that NPP flow into each of the respective pools of vegetative carbon. Each of those pools has a residence time for its carbon, with the rate of outflow (representing shedding and mortality) governed by first-order kinetics. The vegetative carbon pools operate in parallel, each receiving its designated fraction of NPP and each losing carbon at a rate proportional to its stock.

Although global NPP is specified as a function of atmospheric concentration of CO₂, the fraction of NPP that goes to each vegetation carbon pool is fixed. For example, in the default preindustrial steady state, 1 PgC/yr out of 60 PgC/yr total NPP goes to the 100-year carbon pool; therefore, whatever the value of the global NPP evolves to be in the climate simulation, 1/60 of the NPP will flow to that 100-year pool. The governing equation for the mass of carbon in vegetation pool \(i\) is

The first term on the right-hand side represents the net fixing of carbon by the vegetation through photosynthesis, with \(\alpha_{\mathrm{veg},i}\) being the fraction of NPP destined for carbon pool \(i\) (e.g., 1/60 for the 100-yr pool in the default configuration). The second term on the right-hand side represents the movement of carbon from live biomass to dead biomass via plant mortality and shedding of leaves, branches, and similar material.

The net primary productivity (NPP) is a function of the atmospheric CO\(_2\) concentration,

where [CO₂] is the atmospheric concentration of CO₂, [CO₂]₀ is the corresponding preindustrial value, and NPP₀ is the preindustrial net primary productivity. The maximum NPP achieved when the CO₂ concentration is high enough to not be a limiting factor. The preindustrial value of net primary productivity, NPP₀, is determined by summing the preindustrial vegetation carbon masses divided by their decay timescales. As NPP deviates from NPP₀, it is apportioned among the vegetation carbon pools in the same proportions.

Although the vegetative pools operate in parallel, the soil pools operate in serial. All vegetative pools decay to the first soil pool, which represents a combination of woody debris, litter, and near-surface soil. The loss of carbon from that soil pool is governed by first-order kinetics with a residence time that is sensitive to the global mean temperature T_s. The outflows from the soil pool are partitioned into two destinations: a constant fraction of the outflow is oxidized and sent directly to the atmosphere, with the remaining outflow sent to the next soil pool. This cascade continues to the last soil pool, which sends all of its outflow of carbon to the atmosphere.

The rate at which soil carbon decays, moving to the next soil pool and/or to the atmosphere as CO₂, is set by a timescale \(\tau\)_soil that depends on global-mean temperature according to a Q₁₀ value,

with a default value of Q₁₀ = 1.5 and with default values of \(\tau\)_soil,0 for each soil pool as given in the table below.