Growth, foliar nutrition and δ13C responses of red alder (Alnus rubra) to phosphorus additions soon after planting on moist sites

In south-coastal British Columbia, a low availability of phosphorus (P) may limit the early growth of young red alder (Alnus rubra Bong.), even on sites classified as productive for red alder. However, it remains unclear as to what P addition rates best alleviate P deficiencies on such sites and how long effects of P additions on growth persist.We applied P 1–3 months after planting at rates up to 60 g P tree−1 and assessed growth and foliar elemental contents over three growing seasons at three sites with site productivity classed as good for red alder. Foliar δ13C was also determined in year 1 in the two sites on Vancouver Island and in year 2 in the site on the British Columbia mainland coast in order to better understand the relationships among foliar nutritional status, leaf water use efficiency (WUE), and growth.P additions at planting significantly increased height (11–15%), diameter (26%) and stem volume (62–64%) through 3 years. Maximum growth rates were achieved at P addition rates of 30 g tree−1 and at foliar P concentrations of 2.2–2.5 g kg−1. Growth did not increase further at addition rates of 60 g P tree−1. Stem growth increases were accompanied by increased individual leaf mass, first-year foliar concentrations of N, P, Ca, Mg, and S, and foliar δ13C, the latter suggesting that WUE increased with P additions. Foliar concentrations of P in unfertilized trees were at deficient levels, based on earlier studies, and increases in first-year foliar P concentrations and stem growth through year 3 were consistent with responses in earlier single-tree plot experiments. Longer-term measurements are required to define the duration of growth response to P additions in these otherwise-productive sites.

► P added at planting increased stem growth of red alder for 3 years on fertile sites.
► Growth increases were similar at addition rates of 30 and 60 g tree−1 of P.
► Stem growth increased with foliar mass, δ13C, and concentrations of P, N, K, or S.
► P additions may increase short-term water use efficiency and N uptake of red alder.
► Knowing P requirements can improve site selection and growth of red alder.