Immediate Response Mechanisms to Account for Sustained Tree Growth Following Intensive Biomass Removal.

Tree-based metrics of productivity have limitations when attempting to predict long-term sustainability of forest stands. However key soil variables, as affected directly by management, regulate the capacity of a site to maintain a sustainable carrying capacity over many rotations. The aim of this project is to identify the apparent mechanisms of resilience seen in tree growth, following intensive biomass removal through key soil indices. Both soil porosity and site organic matter content are known to change following forest management activities.

Our study addresses the following questions: (i) do sites lacking residual organic matter content (forest residuals) have higher soil moisture and temperature compared to sites with an intact O-horizon and slash, (ii) do sites with higher soil moisture and temperature have higher rates of heterotrophic respiration, (iii) do sites with higher heterotrophic respiration have higher rates of N-mineralization?

We are measuring soil temperature and volumetric water content on an hourly basis at 10, 20, 30, and 100cm depth. Zero-tension lysimeters and throughfall collectors will be used to quantify inputs from the O-horizon and atmosphere respectively. CO2 respiration measurements on a variable-depth collar system identifies auto/heterotrophic contributions. Density fractionation followed by stable isotope analysis on soil samples will be used as a proxy for carbon and nitrogen mineralization.

We expect soil microclimate conditions to accelerate heterotrophic activity on sites with complete biomass removal, compared to sites with slash left on the surface. We predict these sites will also have higher mineralization rates that may offset, to varying degrees, the nutrient pool reduction from biomass removal. Identifying the degree to which these mechanisms buffer the loss of nutrients from forest residuals will be examined with potential impacts to long-term sustainable productivity.