Greater seed production in elevated CO2 is not accompanied by reduced seed quality in loblolly pine

Background/Question/Methods In herbaceous species, elevated CO2 often increases seed production, but usually leads to decreased seed quality. However, the effects of increased atmospheric CO2 on tree fecundity remain uncertain, despite the importance of reproduction to the composition of future forests. We determined how seed quantity and quality differed for pine trees grown for 12 years in ambient and elevated (ambient plus 200–μl l-1) CO2, at the Duke Forest FACE (free-air CO2 enrichment) site. We also related inter-annual variation in reproductive effort to yearly measurements of aboveground net primary productivity (ANPP), precipitation (P), potential evapotranspiration (PET) and water availability (P-PET). Results/Conclusions The number of mature, viable seeds was doubled per unit basal area in high CO2 plots from 1997 to 2008 (p<0.001), but there was no CO2 effect on mean seed mass, nutrient content, germination or seedling growth. Inter-annual variation in seed production was positively related to ANPP, with a similar percentage of ANPP diverted to reproduction across years. Seed production was negatively related to potential evapotranspiration (PET)(p<0.005) and positively correlated with water availability (p<0.05), but showed no relationship with precipitation (p=0.88). This study adds to the few findings that, unlike herbaceous crops, woody plants may benefit from future atmospheric CO2 by producing larger numbers of seeds without suffering degraded seed quality. Differential reproductive responses between functional groups and species could facilitate woody invasions or lead to changes in forest community composition as CO2 rises.