Effects of root plastic responses to spatial and temporal nutrient heterogeneity on plant inter- and intra-specific competition

Background/Question/Methods: Root foraging plasticity enables plants to increase the nutrient-uptake efficiency as well as enhance plant growth when encountering heterogeneous soil nutrient distribution. The responses of root foraging plasticity to different heterogeneous soil conditions may affect plant competition. The objective of this study is to investigate how the responses of root foraging plasticity to spatial and temporal nutrient heterogeneity will affect inter- and intra-specific competition of plants. We established a garden-plot experiment where sweetgum and loblolly pine were planted in the plots of 0.5m². In each plot two focal plants of the same or different species were planted with associate plants surrounded. A small area of the soil between the two plants was fertilized to create the nutrient patch. Nutrients were added through either applying slow-release fertilizer on the surface of the patch (spatial heterogeneity) or sprinkling nutrient solution once a week to the patch (temporal heterogeneity). Minirrhizotrons were installed between the focal plants and the root growth was surveyed periodically. All plants were harvested two growing seasons later. Fine roots (diameter < 2mm) were separated from root samples. The measurements include aboveground biomass, fine root biomass in nutrient patch and overall fine root biomass. 

Results/Conclusions:  1) Sweetgum grew larger and proliferated more roots than loblolly pine in all treatments. This result showed that sweetgum is the stronger competitor under the experimental conditions.  2) Sweetgum grew larger and proliferated more roots in inter-specific competition set-ups than it did in intra-specific ones. This may suggest a more severe intra-specific competition of sweetgum than it competed with loblolly pine.  3) The responses of root foraging plasticity to root competition of the two species were significantly different. Loblolly pine showed strong morphological plasticity than sweetgum did, differing from previous studies when the two species were grew individually that sweetgum had strong plasticity. This indicated that loblolly pine may alter its nutrient foraging responses to heterogeneity under competition, from foraging scaler to precise forager. This change of foraging plasticity may reflect the adaptation of the inferior competitors for quick occupation of certain nutrient-rich patches to enhance its growth.  4) Our results also revealed shifting of root abundance of the two species at different soil layers, indicating niche differentiation in root nutrient foraging.