The site-scale processes affect species distribution predictions of forest landscape models

Abstract

Forest landscape models (FLMs) are important tools for testing basic ecological theory and for exploring forest changes at landscape and regional scales. However, the ability of these models to accurately predict changes in tree species’ distributions and their spatial pattern may be significantly affected by the formulation of site-scale processes that simulate gap-level succession including seedling establishment, tree growth, competition, and mortality. Thus, the objective of this study is to evaluate the effects of site-scale processes on landscape-scale predictions of tree species’ distributions and spatial patterns.We compared the deviations and similarity in species distribution (quantified by species’ percent cover) as well as its spatial pattern derived from two FLMs: (1) an age cohort model with simplified site-scale processes based on the presence or absence of age cohorts (a representative version: LANDIS 6.0), and (2) a stand density model with detailed site-scale processes based on stand density (a representative version: LANDIS PRO 7.0), which have the same framework but different site-scale process formulations.We found that site-scale processes affected the simulated species’ percent cover and spatial pattern. The importance of site-scale processes to individual species’ predictions depended on species’ ecological traits such as shade tolerance, growth rate, seed dispersal, and other factors. For early-successional species, simulated distributions were insensitive to the formulation of site-scale processes. Conversely, for shade-tolerant, middle-to late-successional species simulated distributions were highly sensitive to the formulation of site-scale processes. Species’ shade tolerance may accentuate this simulation sensitivity. In addition, because the stand density model incorporated additional quantitative information, their simulation results had a higher year-to-year variation than those from the age cohort model. The degree of spatial aggregation of species’ distributions was insensitive to the formulation of site-scale processes, whereas patch size and arrangement (landscape composition) for the species distribution were sensitive. Results from this study revealed the differences in simulation results between these two models with different site-scale process formulations, which may help narrow down prediction uncertainties and point to areas where representations of site-scale processes need to be enhanced in the future.