A hierarchical approach for simulating northern forest dynamics

Abstract

Complexity in ecological systems has challenged forest simulation modelers for years, resulting in a number of approaches with varying degrees of success. Arguments in favor of hierarchical modeling are made, especially for considering a complex environmental issue like widespread eastern hemlock regeneration failure. We present the philosophy and basic framework for the NORTHern Woodland Dynamics Simulator (NORTHWDS) Integrated Hierarchical Model System (NIHMS). NIHMS has an individual tree component (the NORTHWDS Individual Response Model (NIRM)), a mesoscale stand simulator (NORTHWDS), and a landscape model (NORTHWDS Landscape Model (NLM, presented in another paper)). NIRM predicts the behavior of a tree given the physical and biotic environment that constrains its performance, using process-response functions at a scale larger than the individual plant. The NORTHWDS model integrates both the structure of the individual tree model (including tree growth and mortality functions) with a series of ecosystem processes (e.g. competition, site biogeochemistry, small-scale disturbance, deer browsing) and even larger scale events (e.g. catastrophic windthrow) to predict long-term stand dynamics on a 5-year simulation cycle. The boundaries in time and space between the NIRM, NORTHWDS, and NLM models are not discrete, but overlap due to the multiscale expression of ecological and physiological processes. For example, the NORTHWDS model represents both the intersection between the NIRM and NLM models with additional unique mesoscale processes (e.g. intertree competition). At the highest level of NIHMS, NLM provides the environmental context for NORTHWDS, with all levels operating in an internally consistent and parsimonious manner.

Three case study scenarios are used to illustrate some of the potential applications of NIRM. Scenario 1: simulation of northern red oak survivorship, crown dynamics, diameter increment, and cumulative propagule production under different local stand densities; Scenario 2: the response of white ash along an available nitrogen gradient with respect to mortality, crown surface area, diameter growth, tree biomass, and propagule production; and Scenario 3: the survivorship and total propagule production of black spruce along a soil moisture gradient. Under Scenario 1, crown size decreased appreciably as local stand density increased from open-grown (

Citation

Bragg, Don C.; Roberts, David W.; Crow, Thomas R. 2004. A hierarchical approach for simulating northern forest dynamics. Ecological Modelling 173(2004) 31-94