Crown complementarity rather than crown selection contributes to stem complementarity in genetic mixtures of Pinus taeda L

Abstract

Increased genotypic diversity has been associated with increased biomass production in short-rotation tree species. Few studies have investigated potential mechanisms which might explain why genetic diversity increases productivity. We leveraged an existing study which deployed clones, control-pollinated, and open pollinated genotypes of Pinus taeda L. (loblolly pine) with large and small crown ideotypes. Building on a previous study which found increased stem volume in genotypic mixtures, we measured crown characteristics, using a drone-based LiDAR, in genetic mixtures of loblolly pine in Virginia and North Carolina to test for: (1) aboveground spatial niche partitioning and (2) if this partitioning was associated with greater stem volume. Overall, we found no evidence that differences in crown sizes by ideotype led to aboveground spatial niche partitioning, resulting in greater overall crown size or stem volume when grown in mixture. Instead, genotypic mixtures overall, as opposed to genotypic monocultures, consistently had greater crown area, crown volume, and stem volume. Additionally, competition intensity decreased under genetic mixtures despite that these conditions led to greater overall stem and crown sizes than genotypic monocultures. No increase in aboveground spatial partitioning or individual-tree LAI in genotypic mixtures suggests greater radiation-use efficiency. Increases in radiation-use efficiency are primarily driven by belowground resource acquisition which suggests these mixtures may be capable of greater belowground resource exploitation. We found mixtures to outgrow pure conditions on average 4%–5% per tree (0.002–0.008 m³ per tree) by ages 8–9.