Can phosphorus additions increase long-term growth and survival of red alder (Alnus rubra Bong.) on periodically dry sites?

Abstract

Deficiencies of nutrients, especially phosphorus (P), and growing-season moisture may limit the growth and survival of nitrogen-fixing red alder (Alnus rubra Bong.), a commercially – important species in coastal forests of the Pacific Northwest and British Columbia. Future growing seasons are projected to be warmer and drier, potentially reducing P availability and plantation survival and growth on sites currently considered acceptable for alder management. We examined the effects of P additions on growth, survival, and nutrition in a young alder plantation on a moderately fertile and periodically droughty site on eastern Vancouver Island. Alder was planted in fall 1999 (C99 cohort) and fill-planted in spring 2001 (C01 cohort). We applied triple superphosphate (0-45-0) beginning in spring 2001 and repeatedly to year 6. Initial rates were 0, 15, or 30 g P tree−1 for the P0, P1, and P2 regimes respectively, and cumulative rates over 6 years were 0, 41, and 288 g P tree−1. Moisture availability was inferred from monthly precipitation data and periodic measurements of soil moisture content. Growth and mortality were measured yearly through year 13 and foliar elemental concentrations, yearly through year 6.Through 13 years, P-fertilized trees were taller (+1.6 m, +16%) than unfertilized trees, had greater diameter at breast height (dbh) (+1.5 cm; +14%) and, through year 6, greater foliar P concentrations. Treatment effects on growth were significant in year one and maintained thereafter but did not differ between the P1 and P2 regimes. Effects of P additions on current annual basal area increment (CAIba) increased with early growing-season precipitation. Total mortality was ca. 30%, two-fold greater in the fill-planted (C01) than in the C99 cohort, and greatest in dry summers (2003, 2004), but did not vary with P treatment. Effects of P additions on height and dbh were less for crop (largest-diameter 400 trees ha−1) than for non-crop trees. Through 13 years, P additions did not affect the diameter-density curve. Effects of P additions on growth were less through 3 years but greater through 8–10 years than in plantations on sites classified as moister and more fertile. P additions early in plantation development can increase the productivity of red alder in periodically dry sites, but the increases are greater during moister growing seasons.