Afforestation Site Suitability Calculator

Site suitability scoring is an exercise in matching the autecology of candidate tree species to the resources a piece of land can actually supply. The eight parameters used here stand in for four controlling resources: water, rooting volume, energy, and workability. Rainfall carries the largest weight, 25 percent, because moisture is the first limiting factor across most afforestation contexts; roughly 600 mm per year is the practical floor for unirrigated temperate planting, 800 to 1500 mm is the comfortable band, and above about 2000 mm waterlogging and nutrient leaching start to pull the score back down. Soil depth is graded against a 2 m reference, and below 30 cm the rooting volume is so restricted that the score collapses. The pH curve follows soil chemistry rather than plant preference directly: below pH 4.5 aluminium and manganese reach phytotoxic concentrations, while above pH 8 iron, zinc, and manganese become unavailable and induce chlorosis. Slope scoring falls sharply past 15 percent, the practical limit for mechanised planting, and again past 30 percent where terracing becomes necessary. The main limitation is that fixed weights are generic. A single national weight set cannot represent frost pockets, slope aspect, salinity, subsoil compaction, browsing pressure by deer or livestock, or land tenure, any one of which can dominate outcomes on a given parcel. The resulting index is ordinal, useful for ranking candidate parcels, not a yield prediction. The growth rate and the 0.47 carbon fraction applied to it are indicative defaults, and any project-level decision requires field soil pits, local climate normals, and species trials.

Structured land suitability grading was formalised by the Food and Agriculture Organization in the Framework for Land Evaluation, published as Soils Bulletin 32 in 1976. That document introduced the ordered suitability classes S1 highly suitable, S2 moderately suitable, S3 marginally suitable, and N not suitable, a scheme that the five-band scale used by most modern scoring tools still mirrors. FAO extended the framework specifically to trees with Land Evaluation for Forestry, issued as Forestry Paper 48 in 1984. The underlying idea was older: European forest science had classified growing sites for well over a century through site index curves and the German tradition of Standortskartierung, and Russian forest typology developed by Georgy Morozov in the early twentieth century grouped stands by habitat conditions rather than species alone. The arithmetic of combining unlike parameters came from decision analysis. Weighted linear combination and the Analytic Hierarchy Process, developed by Thomas Saaty in the 1970s, supplied a defensible way to derive and check parameter weights. FAO and the International Institute for Applied Systems Analysis built the Agro-Ecological Zones methodology from 1978 onward, systematising climate and soil matching at continental scale. From the 1990s these methods migrated into geographic information systems, where raster layers for rainfall, elevation, slope, and soil could be scored and overlaid directly. More recently, planners have begun substituting projected climate normals for historical ones, since trees planted now must tolerate conditions decades ahead.