Seasonal and spatial carbon assimilation patterns in black spruce; assessing temperature impacts at canopy level

Background/Question/Methods

In black spruce (Picea mariana), as in many evergreen trees, the maintenance of multiple foliar cohorts is considered an adaptive trait to conserve nutrients. In addition, the retention of older needles enables early season carbon assimilation prior to and during shoot flushing. This may be especially important for black spruce at the southern edge of the boreal forest where bud break occurs in May-June and the new flushes are fully matured only late in the growing season. Future warming may induce earlier bud break and affect both carbon assimilation and nutrient partitioning patterns. The study site is an ombrotrophic Picea mariana–Sphagnum bog in northern Minnesota that will be exposed to elevated CO₂and multiple temperatures within the SPRUCE project (http://mnspruce.ornl.gov).

Our objectives were to identify seasonal and spatial carbon assimilation patterns in black spruce, and to assess temperature impacts on carbon exchange at the cohort, branch and whole-tree levels under warmer climate scenarios. Photosynthetic capacity based on gas exchange, specific leaf area (SLA) and nitrogen content (N) were quantified by season with a focus on canopy position and needle age [current- (C), one-year-old- (C₊₁) and two-year-old needles (C₊₂)]. Photosynthetic response curves were measured to model net carbon assimilation differences.

Results/Conclusions

The seasonal maximum assimilation rate (A_max) of C-needles increased with shoot maturation from 7.8 μmol CO₂ m^-2 s^-1 (65% lower than C₊₁) in May to its maximum of 11.8 μmol CO₂ m^-2 s^-1 in July. Photosynthetic capacity and foliar N were affected by canopy position and needle age. In July, photosynthesis on a projected leaf area basis showed A_max, respiration (R_day), maximum rate of Rubisco carboxylase activity (V_cmax), maximum rate of photosynthetic electron transport (J_max) and triose phosphate utilization  (TPU) to be 36, 28, 60 70 and 73% greater for branches in the top of the trees compared to the bottom, independent of cohort. In October, temperature response curves for C-needles showed optimum at 24-26°, with a rapid decline at temperatures above 35°C.

In conclusion, canopy position affected photosynthetic capacity more than cohort age. This heterogeneity within the canopy was likely driven by light stratification and nutrient partitioning patterns. The photosynthetic capacity of overwintering foliage (C+1 yr.) reached its seasonal A_max earlier than C yr. needles (June compared to July). Thus a future warming during spring, induce earlier bud break and accelerated shoot development, which may reduce older cohorts’ importance in the trees’ early season carbon budget.