Ecophysiological responses of four boreal tree species to soil nitrogen supply

The ecophysiological responses of four boreal tree species, trembling aspen
\n(Populus tremuloides Michx.), black spruce (Picea mariana (Mill.) B.S.P.), white spruce
\n(Picea glauca (Moench) Voss), and jack pine (Pinus banksiana Lamb.) were examined at
\nsix different levels (from 25 to 775 ppm) of soil nitrogen (N). At the 50th and 100th day of
\ntreatment, two-year-old conifer seedlings and six-month-old trembling aspen seedlings were
\nmeasured for gas exchange, foliar N concentration, growth and biomass allocation traits.
\nThe gas exchange and resource use efficiency responses varied with species. The
\nadditional N input had no significant effect on photosynthesis (see document) and positive
\neffect on transpiration (E, mmol/m2/s), and beyond 175 ppm N treatment it produced
\nnegative effect on whole seedling photosynthetic capacity (see document) in trembling
\naspen. A, E and At in conifers responded negatively to N treatment, except for a brief
\npositive response from 25 to 125 ppm N in jack pine. <l>PSII
\n revealed relevant relationship
\nwith A but only for jack pine and white spruce. We found explicit positive response of
\nfoliar N concentration and negative response of PNUE to increasing soil N availability.
\nHowever, the trends of PWUE between species varied across six N treatments and possibly
\ndue to luxury consumption. The PWUE was positively correlated with soil N supply in only
\nwhite spruce and was negatively correlated in other species. The sufficient soil N
\navailability for optimum gas exchange and nutritional status were at 75 ppm N for black
\nspruce and at 125 for the other three species.
\nOverall, the growth response of aspen to N was more pronounced than that of
\nconifers due to its fast growing nature. Significant growth response of aspen occurred
\nbetween the 75 to 175 ppm N treatments. Substantial growth reduction occurred when
\naspen seedlings were induced to excessively high soil N concentrations (375 and 775 ppm),
\nspecifically due to the vulnerability of these seedlings to pest damages. This is also related
\nto the significant increase in foliar N concentration at these N treatments. Greatest growth
\nwas achieved at 25 ppm N addition rate in black spruce and jack pine, and most notables at
\n125 ppm N in white spruce. However, these N levels are not conclusive when optimum soil
\nN concentration(s) for gas exchange parameters, particularly for At, in these seedlings are
\ntaken into consideration. Nonetheless, at highest soil N supply (375 and 775 ppm), the
\ngrowth of conifer seedlings was not as adversely affected as that of aspen seedlings. Our
\nresults also proved strong significance of N availability on biomass allocation between root
\nand shoot components and height growth in all species. Root production was significantly suppressed as soil N availability increased in the conifers. The opposite was found in aspen.
\nIn conifers, the proportions of total seedling biomass allocated to roots (or the root-toshoot
\nratio, R/S), at all N treatments, were highest in white spruce, followed by black
\nspruce then jack pine. The R/S ratio in aspen was comparable with, but followed an
\nopposite pattern to, white spruce. The stem and foliage productions in all studied species
\nshowed negligible positive response to increasing soil N treatments. However, with the
\nexception of aspen, the order of the species allocating highest to lowest portions of its total
\nbiomass to the foliage followed the exact opposite pattern as that of root allocation
\npercentage. Aspen showed lesser biomass allocation to the foliage than conifers. However,
\nit allocated highest percentages of total seedling biomass to the stem, at all N treatments,
\nfollowed in sequential order by black spruce, white spruce and jack pine.
\nWhen all ecophysiological parameters are considered, our results indicated that low
\nsoil N availability, such as that at 25 ppm, does not adversely affect the boreal seedling
\ngrowth and survival as much as it does at excessively high soil N availability (e.g., 375 and
\n775 ppm). Within the sufficient N regimes (i.e., 75 - 175 ppm), the most suitable soil N
\nconcentration for black spruce is at 75 ppm and at 125 ppm for other species.

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