Making the little things count: modeling the development of understory trees in complex stands

Forest growth models are useful for asking “What if? ” questions when evaluating silvicultural treatments intended to increase the complexity of future stands. What if we thinned to level A or B? How would it aff ect the growth rates of understory trees? How many trees would survive? To answer these types of questions, a growth model needs to accurately predict the growth and survival of understory trees. Some users of the Forest Vegetation Simulator (FVS) growth model have commented that model predictions for understory trees do not match their fi eld observations or data. To study the relationships which govern growth of understory trees, we assembled a large database from silvicultural experiments and operational inventory data. Th is database provided an opportunity to look at the major factors that aff ect the growth and survival of understory Douglas-fi r (Pseudotsuga menziesii), Western Hemlock (Tsuga heterophylla), and Western Redcedar (Th uja plicata). Tree attributes like diameter and crown ratio were the best predictors of tree growth, followed by measures of stand density and competition. We found that the potential and average growth of all three species decreased as the density of larger trees increased, but the growth of Douglas-fi r was reduced the most by increasing overstory density. Similarly, competition with