Variable responses to tree root exclusion by understory plant functional types in a xeric longleaf pine woodland

Plant species interactions in woodlands help maintain the coexistence of trees and groundcover, contributing to high plant diversity. Interactions can be either positive or negative and take place both above and below ground. Both competitive and facilitative interactions have been documented in longleaf pine woodlands, but the dynamics of these interactions and their effects on plant physiology are poorly understood. Our objective was to quantify the impact of tree root exclusion on physiology and growth of understory plant functional types (PFTs) that represent the diversity of longleaf pine woodlands. We used trenching to isolate understory plants from tree roots and compared soil moisture, leaf water potential (Ψ), leaf-level gas exchange, and plant growth for four understory species representing different PFTs in trenched root exclusion plots and untrenched control plots. We hypothesized that root exclusion would reduce understory plant performance compared to untrenched control plots by isolating plants from facilitative effects of canopy trees, but our hypothesis was not supported. We observed better plant performance in trenched plots for some PFTs, and no differences in plant growth for any PFT. These data suggest competitive effects of tree root presence, but results varied among PFT. The study period was unusually wet, with rainfall 53% above normal. In the context of previous studies, these results suggest that belowground competition is more important than facilitation during a wet year, and the relationship between longleaf pines and understory plants shifts from facilitation to competition depending on rainfall.