An accessible, data-driven approach for robust regional calibration of
the Forest Vegetation Simulator for improved stand structure and carbon
density modeling
Abstract
Growth and yield models are an essential tool for predicting the
long-term response of forests to management and disturbance. Model
evaluation and calibration are challenging, however, given data
limitations for observing stand structural changes with age across
heterogeneous forest landscapes. Here we present an approach for
calibrating the lodgepole pine (LP) forest model in the Central Rockies
variant of the Forest Vegetation Simulator (FVS-CR) using Forest
Inventory and Analysis (FIA) plot data from the US Forest Service.
Previous evaluation showed the FVS-CR LP model is generally successful
in reproducing known patterns of stand dynamics. However, the default
model settings tend to result in unrealistic stand successional behavior
and over-estimate the density, basal area, and especially carbon density
of mature lodgepole pine forest stands as compared to expectations. Here
we develop a generalized model calibration procedure based on simulating
bare-ground re-growth of a single mixed lodgepole stand and comparing to
a forest growth chronosequence constructed from FIA plot measurements in
Colorado and Wyoming. We set parameters such as basal area increment and
maximum tree size to match corresponding characteristics observed
directly in the FIA plot data. The remaining ‘free’ parameters (notably
small tree height increment and tree mortality parameters) were then
tuned for best fit against the FIA chronosequence in terms of five
different stand characteristics: live and dead basal area, trees per
hectare, quadratic mean diameter, and average height. Improving model
fit against all five stand characteristics simultaneously required
substantial increases to mortality-related parameters, particularly for
the shade-tolerant species present in mixed stands. These parameters had
to be adjusted empirically rather than based on literature values,
suggesting some underlying model structural challenges. However, the
resulting recalibrated FVS-CR LP model achieves much better
representation of expected lodgepole stand structure and successional
behavior, and can more credibly be used to evaluate carbon storage
outcomes for different forest management choices in the region.