Linking Tree Growth Response to Measured Microclimate – A Field Based Approach

Abstract:

The general relationship between climate and tree growth is a well established and important tenet shaping both paleo and future perspectives of forest ecosystem growth dynamics. Across much of the American west, water limits growth via physiological mechanisms that tie regional and local climatic conditions to forest productivity in a relatively predictable way, and these growth responses are clearly evident in tree ring records. However, within the annual cycle of a forest landscape, water availability varies across both time and space, and interacts with other potentially growth limiting factors such as temperature, light, and nutrients. In addition, tree growth responses may lag climate drivers and may vary in terms of where in a tree carbon is allocated. As such, determining when and where water actually limits forest growth in real time can be a significant challenge. Despite these challenges, we present data suggestive of real-time growth limitation driven by soil moisture supply and atmospheric water demand reflected in high frequency field measurements of stem radii and cell structure across ecological gradients. The experiment was conducted at the Lubrecht Experimental Forest in western Montana where, over two years, we observed intra-annual growth rates of four dominant conifer species: Douglas fir, Ponderosa Pine, Engelmann Spruce and Western Larch using point dendrometers and microcores. In all four species studied, compensatory use of stored water (inferred from stem water deficit) appears to exhibit a threshold relationship with a critical balance point between water supply and demand. The occurrence of this point in time coincided with a decrease in stem growth rates, and the while the timing varied up to one month across topographic and elevational gradients, the onset date of growth limitation was a reliable predictor of overall annual growth. Our findings support previous model-based observations of nonlinearity in the relationship between climate and annual ring formation, and suggest a rather immediate growth response to critical micro-meteorological conditions occurring at different times across the landscape by linking the timing and magnitude of tree growth responses to in situ measurements of environmental conditions.