A forest is a dynamic system whose characteristics vary over time. Williams (1998), Kangas and Maltamo (Eds, 2006) and Ko¨hl et al. (2006) have provided general introductions to the subject of estimating periodic changes and growth in the forest. Here, we restrict our attention to spatial means at time points t1 < t2. To simplify the notation we shall consider only one response variable in order to drop the upper index. A particular forest F with area λ(F ) is assumed to be the same at both time points. Of course, this area is itself a key issue for monitoring over time, but the temporal change in F is assessed by different means (e.g. remote sensing or GIS).