Biomass allocation and nutrient use efficiency in response to water stress: insight from experimental manipulation of balsam fir, concolor fir and white pine transplants

Plants under water stress alter their normal morphological and physiological processes. However, biomass allocation and nutrient use efficiency remain largely unknown under the interactive effect of irrigation and fertilization for many economically important conifer species. In this study, we compared biomass allocation and resources use efficiency of two-year-old (plug+1) containerized balsam fir [Abies balsamea (L) Mill.] ‘Cooks’ and concolor fir [Abies concolor (Gord. & Glend.) Lindl. Ex Hildebr.] ‘Cibola’, and one-year-old bare-root white pine [Pinus strobus L.] transplants grown under variable watering and fertilization conditions. In a controlled greenhouse setting, transplants were submitted to three irrigation levels and two fertilization rates in a factorial experimental design. Fir species, mainly concolor fir, had a greater capacity to utilize water resulting in decreased stem water potential than white pine, leading to increased aboveground growth and shoot-to-root (S/R) ratio. White pine transplants had a higher assimilatory nutrient efficiency, nutrient use efficiency (NUE), root weight ratio, and a lower nitrogen availability index than the two fir species, suggesting a better ability to tolerate water stress. The high rates of fertilization increased nutrient concentration more than growth, suggesting luxury consumption. These results indicate that white pine responds to drought stress by closing their stomata and decreasing their S/R ratio to limit physiological activities, while the two fir species exhibit increased water uptake via expanded fine root networks, increasing the NUE and S/R ratio. However, there was no clear trend for the irrigation and fertilization interaction for NUE for three species, making it difficult to generalize responses.