Systematic review of short-term growth responses of semi-mature black spruce and jack pine stands to nitrogen-based fertilization treatments

Based on a meta-analytical review of experimental results presented in the scientific literature, the objective of this study was to quantify the short-term growth responses (periodic basal area growth (ΔBA; m2/ha/decade), periodic total volume growth (ΔTV; m3/ha/decade) and periodic merchantable volume growth (ΔMV; m3/ha/decade)) of semi-mature black spruce (Picea mariana (Mill.) B.S.P.) and jack pine (Pinus banksiana Lamb.) stands to nitrogen-based fertilization treatments (one-time dosages of nitrogen (N), N and phosphorus (P), N and potassium (K), and NPK). Specifically, five fertilization treatments were assessed: (1) [84 ≤ kg N ha−1 < 140] denoted N; (2) [196 ≤ kg N ha−1 < 252] denoted 2N; (3) [196 ≤ kg N ha−1 < 252] + [84 ≤ kg P ha−1 < 140] denoted 2NP; (4) [196 ≤ kg N ha−1 < 252] + [84 ≤ kg K ha−1 < 140] denoted 2NK; (5) [196 ≤ kg N ha−1 < 252] + [84 ≤ kg P ha−1 < 140] + [84 ≤ kg K ha−1 < 140] denoted 2NPK. Analytically, the procedure consisted of three sequential steps: (1) identifying relevant fertilization studies via a systematic search of the scientific literature and associated electronic databases; (2) obtaining and critically reviewing the identified publications for their specific applicability in terms of species, experimental approach, treatments applied, growth parameters measured, and duration of response; (3) determining the mean percent change in periodic growth of the fertilized stands relative to comparable unfertilized control stands by growth parameter, treatment, site quality class and species, employing regression analyses. In total, 112 and 161 fertilized-control observational growth response pairs were derived from 24 black spruce and 37 jack pine experimental installations, respectively. Results indicated that among the 30 black spruce relationships assessed (3 response variates × 5 treatments × 2 site quality classes), black spruce stands situated on (1) good and excellent quality sites exhibited significant (p ≤ 0.05) and consistent positive responses (concurrent increases in all three growth measures) to N and 2NP treatments (i.e. mean increases of 22.6% in ΔBA, 16.9% in ΔTV and 17.9% in ΔMV); (2) poor and medium quality sites exhibited largely insignificant and (or) inconsistent responses to fertilization (i.e. a significant (p ≤ 0.05) mean increase of 12.6% in ΔBA to the 2N and 2NPK treatments (p ≤ 0.05) only). Among the 45 jack pine relationships assessed (3 response variables × 5 treatments × 3 site quality classes), jack pine stands situated on (1) good quality sites exhibited significant (p ≤ 0.05) and consistent responses to all five treatments (i.e. mean increases of 17.3% in ΔBA, 15.4% in ΔTV and 14.5% in ΔMV); (2) medium quality sites responded significantly (p ≤ 0.05) and consistently to N, 2N, 2NK and 2NPK treatments (i.e. mean increases of 28.3% in ΔBA, 22.1% in ΔTV and 22.7% in ΔMV); (3) poor quality sites exhibited significant (p ≤ 0.05) and consistent responses to 2NK and 2NPK treatments (i.e. mean increases of 23.5% in ΔBA, 19.5% in ΔTV and 19.5% in ΔMV). The delineation and selection of a single optimal treatment for a given site quality class was not possible given insignificant (p > 0.05) differences among the various treatments that exhibited significant (p ≤ 0.05) mean responses. The overall result of this review suggests that short-term productivity of black spruce stands situated on good and excellent sites and jack pine stands situated on good, medium and poor sites, could be enhanced during the later stages of stand development through N-based fertilization treatments.