var fromDate = new DateTime(2015, 1, 1);
var toDate = new DateTime(2020, 1, 1);
var resDict = new ConcurrentDictionary<string, ApiState>();
            Parallel.For(0, nbStands, i =>
            {
                var growthCalculator = new GrowthCalculator(controlCategory: null, heightCalibrationWeight: 1.0);
                var compartment = new ApiCompartment(fromDate, _evenAgedClass[i], _soilMoistCode[i], _vegType[i],
                     _eastCoord, _northCoord, _altitude, _siSpecies[i], _siteIndex[i], GISHelpers.GetSWEREFf99_Projection());
                compartMent.Id = _standId[i];
                compartment.GrowthCalculator = gc;
                addTreeLayerData(compartment, i); // Some method that populates a compartment with tree layer data
                ApiState res = compartment.Grow(fromDate, toDate, growthCalculator);
                resDict.TryAdd(compartment.Id, res.DeepClone());  // The apiState DeepClone() copies all data, but not any reference to the ApiCompartment
            });