ABSTRACT
In forestry, the productivity of forest ecosystems refers to the rate of producing matter [e.g., dry mass kg ha-1 yr-1] in a tree population/community for timber and biomass. This equals the Net Primary Productivity (NPP); i.e., Gross Primary Production (GPP) minus photosynthates used for maintenance and growth processes. Over time, the mass in a tree population/community occupying a given site develops as a function of the birth (regeneration), growth and death of trees:
( ) ( 1) ( ) ( ) ( )M t M t G t S t D t (15.1) where M(t) is the mass of trees on the site at time t, G(t) is growth of trees, S(t) is the mass of new seedlings and D(t) is the mass of dying trees. This is further:
( ) ( ) ( ) ( 1) ( ) ( ) ( ) ( ( ) ( ))M t h t s n t m t n t g t k t m t g t (15.2)
where h(t) is the density of new seedlings, n(t) is the density of trees, and k(t) is the density of trees dying while —s(t)is the mean mass of seedlings,
—m(t)is the mean mass of trees, and —g(t)is the
mean growth of trees. The stock of trees at a given moment is the balance based on the mass in the previous moment plus the growth of trees and the mass of new trees and their growth minus the mass of dying trees and their growth (Table 15.1).
