A review of forest gap models

Forest gap models, initially conceived in 1969 as a special case of individ ual-tree based models, have become widely popular among forest ecologists f or addressing a large number of applied research questions, including the i mpacts of global change on long-term dynamics of forest structure, biomass, and composition. However, they have been strongly criticized for a number of weaknesses inherent in the original model structure. In this paper, I re view the fundamental assumptions underlying forest gap models, the structur e of the parent model JABOWA, and examine these criticisms in the context o f the many alternative formulations that have been developed over the past 30 years. Four assumptions originally underlie gap models: (1) The forest is abstract ed as a composite of many small patches of land, where each can have a diff erent age and successional stage; (2) patches are horizontally homogeneous, i.e., tree position within a patch is not considered; (3) the leaves of ea ch tree are located in an indefinitely thin layer (disk) at the top of the stem; and (4) successional processes are described on each patch separately , i.e., there are no interactions between patches. These simplifications ma de it possible to consider mixed-species, mixed-age forests, which had been difficult previously mainly because of computing limitations. The structure of JABOWA is analysed in terms of the functional relationship s used for formulating the processes of tree establishment, growth, and mor tality. It is concluded that JABOWA contains a number of unrealistic assump tions that have not been questioned strongly to date. At the same time, som e aspects of JABOWA that were criticized strongly in the past years are int ernally consistent given the objectives of this specific model. A wide variety of formulations for growth processes, establishment, and mor tality factors have been developed in gap models over the past 30 years, an d modern gap models include more robust parameterizations of environmental influences on tree growth and population dynamics as compared to JABOWA. Ap proaches taken in more recent models that led to the relaxation of one or s everal of the four basic assumptions are discussed. It is found that the or iginal assumptions often have been replaced by alternatives; however, no sy stematic analysis of the behavioral effects of these conceptual changes has been attempted to date. The feasibility of including more physiological detail (instead of using re latively simple parameterizations) in forest gap models is discussed, and i t is concluded that we often lack the data base to implement such approache s for more than a few commercially important tree species. Hence, it is imp ortant to find a compromise between using simplistic parameterizations and expanding gap models with physiology-based functions and parameters that ar e difficult to estimate. While the modeling of tree growth has received a l ot of attention over the past years, much less effort has been spent on imp roving the formulations of tree establishment and mortality, although these processes are likely to be just as sensitive to global change as tree grow th itself. Finally, model validation issues are discussed, and it is found that there is no single data source that can reliably be used for evaluatin g the behavior of forest gap models; instead, I propose a combination of se nsitivity analyses, qualitative examinations of process formulations, and q uantitative tests of gap models or selected submodels against various kinds of empirical data to evaluate the usefulness of these models for assessing their utility for predicting the impacts of global change on long-term for est dynamics.