PS 26-62 - Experimental warming and elevated CO₂ changes reproductive output of black spruce

Background/Question/Methods

Boreal forests are vulnerable to climate change, and tree reproduction will impact future forests. Impacts of warming and CO₂ on tree reproduction have been investigated separately, but not together. Here we ask, how will cone production of black spruce (Picea mariana), a species with a large distribution across the boreal forest, be influenced by climate change? We collected five years of field data at the SPRUCE (Spruce and Peatland Response Under Changing Environment) project located in the Marcell Experimental Forest in Minnesota, USA. SPRUCE experimentally manipulates above and below-ground warming and CO₂ in ten open-topped octagonal plots (12-m wide, 8-m high) with paired temperature (from +0°C (unheated control) to +9°C, in 2.25°C increments), and CO₂ (ambient/elevated CO₂ at +0 / +500 ppm, respectively) treatments. Above-ground warming began in 2015 and CO₂ manipulations began in 2016. Each summer from 2017-2021, we quantified new cone production on each black spruce tree within experimental plots. We tested the treatment effects of warming and CO₂ on black spruce cone production including the proportion of trees within each treatment plot that reproduce, and used model selection to test for differences in reproduction across treatments. We interpret the results in the context of whole ecosystem manipulations.

Results/Conclusions

Across all years, black spruce cone production was more prevalent in trees in the elevated CO₂ treatment than in ambient CO₂. Model selection of cones per tree across treatments showed that the best model (with a AIC weight of >0.9) had temperature, CO₂, their interaction, and year in the model. Black spruce trees do produce cones every year, and cone production was highest in 2019. Across years, cones per tree were on average three to ten times higher under elevated CO₂ compared to ambient CO₂, while there was a smaller impact of temperature, except at the highest temperature treatment where cone production was extremely low under both CO₂ conditions. While CO₂ can have a fertilizing effect that is increasing reproduction, extreme warming decreases soil moisture, which likely impacts nutrient uptake and allocation within trees. Our empirical experimental findings show that warming and CO₂ influences conifer cone production, which is informative for future projections of the ecological impacts of climate change.