Uncovering traits in recovering grasslands: A functional assessment of oil and gas well pad reclamation

Abstract

Oil and gas exploration has disturbed extensive areas of grassland in North America. Reclamation of this human footprint is only a first step towards ecological recovery. A trait-based approach may provide a mechanistic understanding of biological and edaphic filters influencing longer-term plant community assembly on reclaimed sites. Using taxonomic indices, trait community weighted means (CWM), and functional diversity (FDQ) combined with multivariate models, we compared biological and edaphic properties of 18 reclaimed well pads in Alberta (Canada), to proximate native grasslands. These well pads were certified reclaimed under two reclamation criteria (old, new). Consistent with practices in other regions, newer criteria emphasize using native plant species in place of historically-used introduced agronomic species. We found significant differences between reclaimed and undisturbed reference soil properties (e.g., pH, electrical conductivity), with greater differences on sites reclaimed using the older criteria (e.g., lower TOC, higher bulk density). Plant trait composition also differed between reclaimed and undisturbed sites, with a lower prevalence of short, native, xeric species, with semi-abundant seed production and large seed weight on reclaimed sites. We found a strong trait-environment relationship underlying trait composition difference. While not significantly different in overall trait composition from new sites, old sites included higher prevalence of introduced species, dispersed by animals, preferring mesic conditions, and high seed production. The increased cover of introduced species reduced trait FDQ and led to an arrested succession. New sites included higher prevalence of tall, native species preferring hydric conditions, therophytes, geophytes, and species with low dispersal capacity. The use of native seed with higher FDQ on new sites seemed to alleviate arrested succession; However, biological trait filters (e.g., tall, hydric preference) and altered edaphic properties, might continue to drive differences between reclaimed and reference sites. Our results suggest that even as practices and policies evolve, reclamation does not fully alleviate the legacy effects of this industrial disturbance. We have demonstrated how trait-based approaches can inform recovery assessment and future reclamation best practices. We must go beyond simply seeding with native species for recovery of plant communities – the traits of these native species matter too.