Norway spruce productivity in southern Sweden is equally limited by nitrogen and phosphorous

Abstract

Nutrient limitation of forest growth, especially nitrogen (N) deficiency, is widespread in the boreal region. N fertilisation has thus become a common silvicultural practice in Fennoscandian Norway spruce stands, but to what extent phosphorus (P) is co-limiting productivity and how initial basal area affects the growth response to N addition remains unresolved. To address these questions, two experiments were established in mid-rotation Norway spruce stands in southern Sweden where decades of high atmospheric N deposition have reduced the severity of N-limitation. In a P experiment initiated in 2011, we tested P addition alone (two applications of 200 kg P ha⁻¹) and in a second study also starting in 2011 (NP experiment), a single dose of N was administered alone (200 kg ha⁻¹ in thinned and unthinned stands, hereafter called N and N-unthinned treatments) and in combination with P (N+P = one-time 200 kg N ha⁻¹, two applications of 200 kg P ha⁻¹ in thinned stands). P addition alone increased PAI (periodic annual increment) significantly by 21 % during the first, moister assessment period up to 2014 and by 18 % in the drier 2015–2019 period, resulting in a 10 % increase in final stem volume yield. In the NP experiment, significant PAI increases under favourable meteorological conditions up to 2014 occurred in all fertilisation treatments. The strongest effects were seen in the N-unthinned treatment while no significant additive effect resulted from the joint addition of N and P (N: +20 %, N-unthinned: +38 %, N+P: +23 %). In the drier 2015–2019 period, only the N+P treatment caused significantly greater PAI (+29 %). Final stem volume yield in the NP experiment significantly increased by 10 %, 39 % and 16 % in the N, N-unthinned and N+P plots, respectively. In both experiments, foliar P and thus P/N rose drastically in response to P addition alone or in combination with N. Minor increases in leaf area index (LAI) only occurred in P-containing treatments. Our findings indicate that Norway spruce productivity in southern Sweden is constrained to a similar extent by both N and P. Sustainable nutrient management in Norway spruce growing regions with high N loading (like southern Sweden) should prioritise P over N supply.