Storage vs. growth: Seasonal nitrogen allocation in Pseudotsuga menziesii

Background/Question/Methods Variation in nitrogen (N) availability in soil throughout the season may influence evergreen N allocation. In temperate forests, snow insulates soil and promotes N mineralization throughout the winter. During snowmelt, a pulse of N from this winter microbial activity is readily available, but conifers may not use it. Studies suggest that evergreens use N stored from the previous season before bud break. This implies an asynchrony between the timing of N uptake and soil N availability. Our objective is to determine if trees use stored or newly acquired N for current growth. If trees use stored N for the current season’s growth, then we would expect a decrease in N in the previous season’s growth. However, if trees use newly acquired N for the current season’s growth, then total N in the previous season’s growth would remain constant across the season. To test these predictions, we used a mass balance approach. From April until November, we collected Pseudotsuga menziesii leaf samples from high and low elevations at Lubrecht Experimental Forest. The samples, which were divided between leaves that grew in 2015, buds that broke in 2016, and leaves that flushed in 2016, were analyzed for total N content. Using relationships between growth fractions’ whole tree dry mass and tree diameter at breast height, we determined total N per kg dry mass for each growth fraction.

Results/Conclusions A decrease occurred in total N in 2015 leaves until 2016 bud break (33.4 to 25.9 ug N/kg dry mass and 34.2 to 25.5 ug N/kg dry mass at low and high elevations, respectively), then total N increased in 2015 leaves after 2016 bud break (25.9 to 38.3 ug N/kg soil and 25.5 to 36.57 ug N/kg dry mass at low and high elevations, respectively). In 2016 leaves, total N content was highest directly after bud break (21.9 ug N/kg dry mass and 16.6 ug N/kg dry mass at low and high elevations, respectively) and declined exponentially throughout the season (10.1 ug N/kg dry mass and 6.8 ug N/kg dry mass in November at low and high elevations, respectively). 2016 buds gained more than 200 ug N/kg dry mass from the start of sampling in April until bud break in early-mid June. This evidence suggests that trees use stored rather than newly acquired N to support the current season’s growth. This data will help us to further asses N use in long-lived evergreens.