Boreal forests are particularly vulnerable to climate change, yet we know little about how tree reproduction will be impacted. Experiments elevating CO2 (eCO2) found increased production of flowers, fruits, seeds, and seed mass in plants. However, with climate change temperatures will also increase, influencing soil nutrient mineralization and soil moisture. Here we ask, how will cone production of two boreal forest tree species, black spruce (Picea mariana) and eastern tamarack (Larix laricina), be influenced by climate change? We collected field data at the SPRUCE (Spruce and Peatland Response Under Changing Environment) project located at the S1 bog at 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 and eCO2, +0 and +500 ppm, respectively) treatments. Each summer from 2017-2019, we quantified new cone production on each tree within experimental plots. We tested the treatment effects of warming and eCO2 on mean cone production and cone attributes including size, and interpret the results in the context of belowground nutrient and soil moisture availability.
Results/Conclusions
Across years, black spruce cone production was three to seven times higher under eCO2 compared to ambient CO2, while there was little impact of temperature. We also detected an interaction between temperature and eCO2 under the highest temperature treatment, where trees produced the fewest cones on average under the +9°C with eCO2 in both black spruce and tamarack; this combination of treatments also produced tamarack cone sizes twice that of other treatments. The warmest (+9C) treatments had both higher soil nutrients and lower soil moisture, which may divert allocation from reproduction to fine roots. Our empirical experimental findings suggest climate change will influence conifer cone production, which is informative for future projections of the ecological impacts of climate change, because allocation is central to the carbon-nutrient budget of the stand and regeneration is key to forest composition.