Contrasting acclimation responses to elevated CO₂ and warming between an evergreen and a deciduous boreal conifer

Abstract

Rising atmospheric carbon dioxide (CO 2 ) concentrations may warm northern latitudes up to 8°C by the end of the century. Boreal forests play a large role in the global carbon cycle, and the responses of northern trees to climate change will thus impact the trajectory of future CO 2 increases. We grew two North American boreal tree species at a range of future climate conditions to assess how growth and carbon fluxes were altered by high CO 2 and warming. Black spruce (Picea mariana, an evergreen conifer) and tamarack (Larix laricina, a deciduous conifer) were grown under ambient (407 ppm) or elevated CO 2 (750 ppm) and either ambient temperatures, a 4°C warming, or an 8°C warming. In both species, the thermal optimum of net photosynthesis (T optA ) increased and maximum photosynthetic rates declined in warm-grown seedlings, but the strength of these changes varied between species. Photosynthetic capacity (maximum rates of Rubisco carboxylation, V cmax , and of electron transport, J max ) was reduced in warmgrown seedlings, correlating with reductions in leaf N and chlorophyll concentrations.Warming increased the activation energy for V cmax and J max (E aV and E aJ , respectively) and the thermal optimum for J max . In both species, the T optA was positively correlated with both E aV and E aJ , but negatively correlated with the ratio of J max /V cmax . Respiration acclimated to elevated temperatures, but there were no treatment effects on the Q 10 of respiration (the increase in respiration for a 10°C increase in leaf temperature). A warming of 4°C increased biomass in tamarack, while warming reduced biomass in spruce.We show that climate change is likely to negatively affect photosynthesis and growth in black spruce more than in tamarack, and that parameters used to model photosynthesis in dynamic global vegetation models (E aV and E aJ ) show no response to elevated CO 2 . K E Y W O R D Sacclimation, boreal conifers, climate change, evergreen and deciduous, Larix laricina, Picea mariana, V cmax and J max

Peatland warming strongly increases fine-root growth

“…Peat drying and increases in nutrients could increase tree recruitment and growth ( 29 ), and we see some evidence of this in the increased larch fine-root biomass ( SI Appendix , Fig. S3 ) and aboveground growth ( 24 ).…”

“…Shrubs may be taking a "do-it-yourself" approach by increasing the length production of new roots for foraging, while larch may be mainly "outsourcing" resource acquisition by investing in mycorrhizal fungal partners (22); both strategies have implications for total belowground C input, decomposition rates (23), and ultimately, peatland C storage. Lastly, the tree fine-root response may be mirroring aboveground C gain; parallel investigations have found that seedlings of the two tree species differ in their aboveground responses to warming (positive for larch, negative for spruce) (24). Investigations of PFT-specific root-fungal interactions and aboveground-belowground linkages in the bog are underway, but for the remainder of the manuscript, we focus on the shrub fine root-length response given that it was the strongest and earliest response and has implications for peatland C function through negative effects of shrubs on the growth of keystone Sphagnum mosses.…”

Reduced photosynthetic thermal acclimation capacity under elevated ozone in poplar (Populus tremula) saplings

“…The observed shift in T optA , 0.6°C per 1°C warming in AO 3 , is consistent with previously reported values that typically are in the range of 0.3–0.5°C per °C change in growth temperature (Battaglia et al, 1996; Berry & Björkman, 1980). However, even greater T optA shifts have been reported (e.g., Dusenge et al, 2020; Sendall et al, 2015), suggesting that considerable differences exist in the physiological capacities for photosynthetic thermal acclimation among trees species. In addition, variations in the degree and duration of temperature treatments and in the surrounding environmental factors likely explain some of the reported differences.…”

“…Much work has recently been carried out on determining the impact of growth temperature on T optA (e.g., Aspinwall et al, 2016; Crous et al, 2018; Kumarathunge et al, 2019; Scafaro et al, 2017; Slot & Winter, 2017b). Other environmental factors, such as atmospheric CO 2 concentrations (e.g., Crous et al, 2013; Dusenge et al, 2020; Ghannoum, Phillips, Sears, et al, 2010; Kurepin et al, 2018), light availability (Niinemets et al, 1999), and water limitation (Kumarathunge et al, 2020), are also known to both independently affect the T optA and to modify its acclimation response to warming. However, while tropospheric ozone (O 3 ) is recognized as an important regional air pollutant with strong effects on plant gas exchange (e.g., Ainsworth et al, 2012; Dai et al, 2017; Feng et al, 2016), its impact on photosynthetic thermal acclimation potential is yet to be determined.…”

High‐resolution minirhizotrons advance our understanding of root‐fungal dynamics in an experimentally warmed peatland

“…Observed differences in mycelial morphology between the two ends of the temperature gradient at SPRUCE may be related to ECM host performance (Fernandez et al, 2017). At SPRUCE, multiple data streams indicate that L. laricina individuals may be better acclimating to warming than P. mariana individuals (Dusenge et al, 2020;Peters et al, unpublished data;Peters (JP) Fernandez et al (2019). Accordingly, ECM rhizomorph formers may recycle mycelium to minimize nitrogen immobilization in their biomass, thereby providing the nitrogen surplus to their plant hosts (Clemmensen et al, 2015).…”