Effects of variable nitrogen fertilization on growth, gas exchange, and biomass partitioning in black spruce and tamarack seedlings

Abstract

To compare the ability of black spruce ( Picea mariana (Mill.) BSP) and tamarack ( Larix laricina (Du Roi) K. Koch) to adjust to variable edaphic conditions, as found in natural peatlands, we varied N fertilization of seedlings in a growth chamber experiment over two growing seasons and examined growth, biomass partitioning, and gas exchange. Seedlings from both species received either high N (100 µg·L–1) or low N (10 µg·L–1) in consecutive growing seasons as follows: (i) low N and low N (LL), (ii) low N and high-N (LH), (iii) high N and low N (HL), and (iv) high N and high N (HH). Both species had greater shoot and total dry weight after 1 year in the high-N treatment as compared with seedlings grown for 1 year under low-N. For tamarack, these differences were larger and they also exhibited a positive effect of fertilization on net assimilation and water use efficiency. Only black spruce exhibited a positive growth response following the move to higher N fertilization in the second year (LL versus LH), whereas only tamarack exhibited a negative growth response following the move to lower N fertilization (HH versus HL). Still, tamarack had greater total biomass at the end of 2 years than did black spruce, irrespective of fertilizer treatment. Both species had greater total biomass in the HL treatment than in the LH treatment. Tamarack seems able to take advantage of favorable nutrient conditions, but it also experiences more dramatic growth declines under poor or deteriorating conditions. While black spruce grows more slowly than tamarack, it is somewhat buffered from declines in growth under poor or deteriorating conditions. Each species appears to be adapted in its own way to the edaphic heterogeneity that exists in natural boreal peatlands.