Plant phenological models typically relate temperatures to plant developmental stages. Air temperatures are usually used to develop these models, but may differ significantly from the temperature of the plant itself. This is especially true for situations involving strong sunlight. In this study, we use a numerical finite difference model of stem heat transfer to simulate the temperature of stems at various locations within a forest opening. We compare heat sums based on stem and air temperatures with observed annual stem growth increment, to determine whether stem or air temperatures have a greater correlation with growth.