A physiologically-based comparison of even- and multi-aged ponderosa pine stand productivity

The productivity of natural even-and multi-aged ponderosa pine (Pinus ponderosa) stand structures was compared based on leaf level physiological factors responsible for production.Leaf area index (LAI), stem volume increment, specific leaf area, leaf nitrogen, and leaf water potential were compared between the stand structures in western Montana and central Oregon.LAI and mean annual volume increment were relatively higher in the even-aged stands, suggesting higher growing space occupancy compared to multi-aged stands.Specific leaf area and leaf nitrogen content were significantly different between the top and bottom crown thirds in even-aged stands.Smaller ranges in specific leaf area and leaf nitrogen content between top and bottom thirds in multi-aged stands suggest higher foliage production efficiencies throughout the crowns compared to those in even-aged stands.Higher vertical stratification of crowns in multi-aged stands enhance the threedimensional growing space availability and also reduced the stand-level competition for light and moisture.Pre-dawn leaf water potential was comparatively higher in multi-aged structures than in even-aged throughout the growing season.A late summer pre-dawn leaf water potential lower than -1.6 MPa, in even-aged stands indicate that trees might have very limited photosynthesis due to moisture limited stomatal closure.Higher water stress in evenaged stands could be a combined effect of stand density and structure.The long-term water-use efficiency estimated from carbon isotope discrimination suggest the top third of crowns in even-aged stands were more water-use efficient compared to multi-aged structures.Prolonged periods of water stress, lower than -1.6 MPa, could result in decreased foliage and tree productivity in even-aged stands despite their higher water-use efficiency.Results from this study suggest that stand structure is an important factor influencing stand productivity.Diverse vertical stratification of tree crowns in stands reduce the competition for available growing space, especially light and soil moisture and could result in enhanced leaf, tree, and stand productivity.