Resource units (RU) are the main entities for which resource levels for plants (i.e. light, water, nutrients) are calculated. The chosen approach is to model resource dynamics at the resolution of RU and competition of individual trees for resources separately. The default RU are defined as a regular 100x100m grid over the landscape, although more complex RU definitions are generally possible in iLand.
For every resource unit the stocked area (SA, m2) is updated in every simulation year. It is calculated at a 10x10m resolution (i.e. to a precision of 1% for a 1ha resource unit), where all pixels containing >0 individuals are accounted for as stocked. SA plays an important role in the computation of resource unit light interception. The dynamic calculation of SA reduces erroneous effects of within-RU heterogeneity due to, e.g., disturbances, on the RU-level calculations of the light regime.
Seidl, R., Rammer, W., Scheller, R.M., Spies, T.A. 2012. An individual-based process model to simulate landscape-scale forest ecosystem dynamics. Ecol. Model. 231, 87-100.