The project team evaluated the combined long-term impacts of management, site characteristics, climate, and species traits on the long-term, local and outward recovery of understory plant communities in longleaf pine savannas through three complementary technical objectives. First, they evaluated if short-term recovery led to long-term recovery, as defined by species initial establishment and ongoing persistence. Second, they evaluated whether the understory species that established locally, spread outwardly. Third they identified whether trait characteristics were related to species’ establishment, persistence, and spread.
The project team used three complementary approaches during 2017-2018. First, they resampled presence of 25 species, within 192 plots (half seeded in 2011, originally sampled 2010-2013) from 18 sites at Fort Bragg (Department of Defense [DoD], NC), 12 sites at Fort Stewart (DoD, GA), and 18 sites at Savannah River Site (Department of Energy [DOE], SC). Second, they quantified the spread of each seeded species (up to 9 meters) away from the 48 sites. Third, they collected trait data on the 25 seeded species. They evaluated establishment and persistence using multi-species dynamic occupancy models. They evaluated spatial spread using multi-species Weibull-kernel based models.
The greatest limitation to successful local recovery (Objective 1) was the ability of a species to establish (i.e., for a species to successfully colonize a site where it had not previously been found). Once established, all seeded species had high probabilities of persisting across all years spanned by the study. After establishing, most species also exhibited spatial spread beyond the area where they were initially seeded (Objective 2), highlighting the potential for self-perpetuating recovery once populations are established. The greatest determinant of establishment was the addition of seeds of understory species in the initial experiment in early 2011. Sites with lower total basal area of canopy trees and shallow leaf litter had greater establishment as well as persistence. The project team found that annual climatic conditions were also important. Persistence for all species was greater with higher levels of winter precipitation, whereas precipitation had species-specific effects on establishment. Establishment and persistence of all species was associated with cooler temperatures during the spring and summer.
Differences in functional traits explained some variation in establishment and spread, but not persistence (Objectives 1, 2, 3). Species with taller adult plants and smaller seed/dispersule ratios established better. Wind-dispersed species with smaller specific leaf area (SLA), slower germination rates and higher probabilities of germination spread at higher rates. For non-wind-dispersed species, spread was greater for taller species and those with greater SLA and faster germination.
DoD and DOE are faced with the challenge of promoting successful recovery of understory plant communities in a way that allows continued execution of mission-related activities. Achieving this goal requires understanding how to maximize the establishment, persistence, and spatial spread that are the hallmarks of recovery. Through long-term sampling of a large-scale experiment, the project team found that seed additions are a primary means DoD/DOE managers can use to promote establishment, persistence, and subsequent species spread. These findings further benefit DoD/DOE goals by revealing that some management activities (e.g., prescribed burn regimes, low-density canopy trees) can also be particularly helpful. The results also benefit recovery efforts by revealing a strong link between climatic conditions (e.g., temperature and precipitation) and establishment and persistence. This knowledge will help DoD/DOE plan optimal recovery efforts under current climatic conditions, and also highlights how it will be important to understand the role of future non-stationary climate on establishment, persistence, and spread in order to promote continued long-term recovery.