A dynamic reference model: a framework for assessing biodiversity restoration goals in a fire‐dependent ecosystem

Abstract

The use of reference models as templates of historical or natural conditions to assess restoration progress is inherently logical; however, difficulties occur in application because of the need to incorporate temporal variation in ecosystems caused by disturbance and succession, as well as seasonal, interannual, or decadal variability. The landscape-scale restoration of the globally threatened and fire-dependent longleaf pine ecosystem in the southeastern United States is an example in which restoration efforts are even more complicated by the limited availability of extant reference sites. This study uses the dynamic reference conceptual framework to assess the direction and rate of recovery with respect to biodiversity restoration goals using a 15-year vegetation data set from an experimental restoration treatment in fire-excluded, hardwood-encroached longleaf pine sandhills. We compared ground-cover vegetation response to midstory hardwood removal through herbicide application, mechanical removal, and fire only. Nonmetric multidimensional scaling ordinations and proportional similarity analyses suggest that, while vegetation changed in all treatments over time, no differences in species composition or hardwood density in the ground cover were attributable to hardwood reduction treatments after 15 years with frequent prescribed fire. Furthermore, the results of this study indicate that considerable variability is associated with reference sites over time. Sites identified in 1994 as attainable restoration targets had become a moving target themselves, changing in magnitude consistent with alterations in restoration plots attributable to treatment effects and shaped by the modest increase in fire frequency imposed since 1998. In a broad restoration context, this study demonstrates a conceptual framework to better understand and integrate the range of spatial and temporal variation associated with the best available reference sites. It also illustrates a practical tool for statistically defining reference sites and for measuring restoration success in continually changing conditions that should be widely applicable to other ecosystems and restoration goals.

Land-Use History and Contemporary Management Inform an Ecological Reference Model for Longleaf Pine Woodland Understory Plant Communities

“…Our regional and location-specific ecological reference models suggest some strategies for restoration, but these should be interpreted with caution. For example, transitioning sites between Classes 4/5 and 6 in our regional model might simply entail mechanical thinning of overstory trees, a strategy that can increase plant diversity in some contexts [49] but has limited effects in others [20], [50]. Other transitions might require multiple restoration strategies.…”

“…basal area), and fire frequency (number of burns 1991–2009, time since last fire). We investigated Pinus and non- Pinus overstory separately because these groups respond differently to fire frequency ( Pinus is more fire tolerant; [35], [36]) and because land managers often remove hardwoods during restoration [8], [20]. We also included soil order (Entisol, Inceptisol, Spodosol, Ultisol) and soil water holding capacity in the classification, which are two aspects of soils that broadly structure longleaf pine communities [26].…”

“…Agricultural legacies can persist for decades following abandonment, leading to reduced understory diversity and modified community composition on post-agricultural sites [14]–[16]. Fire suppression leads to increased tree abundance, canopy cover, and an accumulation of leaf litter and duff (i.e., forest floor), each of which may reduce understory diversity and modify understory community composition [17]–[20]. Historically, lightning and human-ignited surface fires burned longleaf pine woodlands as frequently as every 1–6 years, but today fire suppression is widespread and, where fires do occur, they are implemented through prescribed burning [21].…”

Specialist and generalist amphibians respond to wetland restoration treatments

“…When setting restoration goals, careful consideration should be given to historical and present vegetation structure and other site factors in relation to reference conditions (Martin and Kirkman , Kirkman et al ). Trying to restore fire‐maintained conditions on a site that would not naturally support frequent fire makes no more sense than expecting species that are not adapted to fire‐maintained conditions to persist in fire‐maintained habitat.…”

Community reorganization in forest understories buried by volcanic tephra

“…, Suding and Hobbs , Matthews and Spyreas , Kirkman et al. ). In order to assess the presence and degree of community divergence during succession, one needs details about the pre‐disturbance community and a long timescale; changes of vegetation are often slow and may not be linear.…”