Canopy Nitrogen Addition and Soil Warming Affect Conifer Seedlings’ Phenology but Have Limited Impact on Growth and Soil N Mineralization in Boreal Forests of Eastern Canada

Abstract

The increase in soil organic matter mineralization rate in boreal forests that may result from global warming is a major concern as it could release large amounts of C to the atmosphere. On the other hand, this may also release N to the soil and stimulate tree growth, which could partly offset the C losses from the soil. The long-term interaction between increased N deposition and soil warming on soil organic N mineralization and tree growth is still uncertain in the boreal zone. In this study, soil temperature was increased by +2-4 • C from April to July with heating cables for a period of 9 years and N was applied above the canopy of mature stands from June to September for 7-9 years in two eastern Canada boreal forests [Bernatchez (BER) and Simoncouche (SIM)]. We assessed the effects of these treatments on organic N mineralization rate and on the growth, phenology and foliar N of Abies balsamea (L.) Mill. (balsam fir; BF) and Picea mariana (Mill.) BSP (black spruce; BS) seedlings. The soil warming (SW) treatment had no significant effect on N mineralization rates, whereas canopy N addition (CNA) decreased gross N mineralization rate by 23% and forest floor's alkyl/O-alkyl C ratio by ∼15% relative to unfertilized plots. Foliar δ 15 N in the control plots was markedly lower in BS than in BF and at BER than at SIM (-4.8 and-2.9 in BS at BER and SIM, respectively; 1.6 and 3.8 in BF at BER and SIM, respectively) likely due to a higher contribution of 15 N-depleted N derived from mycorrhizal fungi in BS and at BER, the colder and the more N-depleted site. The treatments had non-significant effects on seedling growth and foliar chemistry but SW caused a premature bud outbreak and faster bud development for both species at both sites. Overall, our results show that

Understory functional groups and fire history but not experimental warming drive tree seedling performance in unmanaged boreal forests

“…In contrast to the prediction of our first hypothesis, experimental warming of air and soil temperature did not significantly increase seedling survival or growth. This is consistent with the results of Marty et al (2020) who found no effect of experimental warming on seedling growth, but is in contrast to Nissinen et al (2020) and Okano et al (2021) who observed a positive effect on growth, and Wright et al (2018) who found a negative effect on survival. These divergent responses can origin from different magnitudes of warming in different experiments, ranging from 0.4 • C in our experiment, over 2 • C in the experiment of Marty et al (2020) to 3.4 • C in the experiment of Wright et al (2018).…”

“…This is consistent with the results of Marty et al (2020) who found no effect of experimental warming on seedling growth, but is in contrast to Nissinen et al (2020) and Okano et al (2021) who observed a positive effect on growth, and Wright et al (2018) who found a negative effect on survival. These divergent responses can origin from different magnitudes of warming in different experiments, ranging from 0.4 • C in our experiment, over 2 • C in the experiment of Marty et al (2020) to 3.4 • C in the experiment of Wright et al (2018). Boreal forests are expected to experience a 1.5-4 • C increase of temperature (Gauthier et al, 2015;IPCC, 2021) suggesting that the magnitude of warming in our study was comparatively small and therefore may not have triggered a seedling response.…”