Longleaf Pine Patch Dynamics Influence Ground-Layer Vegetation in Old-Growth Pine Savanna

Abstract

Old-growth longleaf pine savannas are characterized by diverse ground-layer plant communities comprised of graminoids, forbs, and woody plants. These communities co-exist with variable-aged patches containing similar-aged trees of longleaf pine (Pinus palustris Mill.). We tested the conceptual model that physical conditions related to the cycle of longleaf pine regeneration (stand structure, soil attributes, fire effects, and light) influence plant species’ composition and spatial heterogeneity of ground-layer vegetation. We used a chrono-sequence approach in which local patches represented six stages of the regeneration cycle, from open areas without trees (gaps) to trees several centuries old, based on a 40-year population study and increment cores of trees. We measured soil characteristics, patch stand structure, fuel loads and consumption during fires, plant productivity, and ground-layer plant species composition. Patch characteristics (e.g., tree density, basal diameter, soil carbon, and fire heat release) indicated a cyclical pattern that corresponded to the establishment, growth, and mortality of trees over a period of approximately three centuries. We found that plants in the families Fabaceae and Asteraceae and certain genera were significantly associated with a particular patch stage or ranges of patch stages, presumably responding to changes in physical conditions of patches over time. However, whole-community-level analyses did not indicate associations between the patch stage and distinct plant communities. Our study indicates that changes in composition and the structure of pine patches contribute to patterns in spatial and temporal heterogeneity in physical characteristics, fire regimes, and species composition of the ground-layer vegetation in old-growth pine savanna.

Patchy Fires Promote Regeneration of Longleaf Pine (Pinus palustris Mill.) in Pine Savannas

“…Unlike growth rates, survival of juveniles prior to fire appears not to be strongly influenced by canopy cover, or even may be positively influenced during droughts [24,63,64,75,76] (but see [66]). Evidence for effects of soil moisture and nutrients on juvenile growth and survival are mixed [24,34,62,64,77]. Although these local environmental conditions probably have an influence in the absence of fire, it is likely that their greatest influence is through predisposing juveniles to survive or die during subsequent fires.…”

“…The positive association of longleaf juveniles with areas re-opened by death of canopy trees suggests legacy effects of canopy pines on the vegetation fuel structure and/or changes in fire behavior following tree death. While the tree is living, bunchgrasses and woody plants tend to be reduced somewhat by competition with canopy trees [24,77], mulching effects of pine needles [30,77,78], and damage from increased fire severity associated with deposition of pine needles, bark, branches, and cones [31,32,34,66,72]. Once the canopy tree has died, reduced ground layer fuels and a decrease in needle deposition likely reduce fire severity and spread, thereby increasing juvenile survival.…”

“…Certain canopy tree species produce flammable litter [26][27][28] and impose particularly strong limits to regeneration beneath tree canopies [29][30][31][32]. This effect may be amplified by the diffuse canopies of savanna tree species, allowing persistence of flammable grasses beneath the canopy [33,34]. As a result, juvenile trees may not survive under or in the vicinity of canopy trees.…”

“…The authors have observed that clusters of longleaf pine regeneration often occur where a large canopy tree has recently died. This pattern might be attributed to the thinning of ground-layer vegetation in the relatively high fire severity environment beneath large canopy trees [30,32,34,50,66] combined with the reduction in pine needle litter following the death of such trees. Thus, there are competing hypotheses regarding spatial patterns of longleaf pine regeneration.…”

Frequent fire slows microbial decomposition of newly deposited fine fuels in a pyrophilic ecosystem

“…Natural fires in this site tended to occur every 1-3 years, generally during a fire season that spanned dry springs to wet summers, when annual thunderstorms first occurred (Platt et al 2015, Rother et al 2018. Historical "open-woods burning" and more recently prescribed fires, have maintained the open savanna/woodland physiognomy (Platt et al 1988, Gilliam and Platt 1999, Mugnani et al 2019. The ground layer vegetation and litter on the site has burned annually-biennially (return intervals averaging 1.5 years) during prescribed fires between March and June using drip torches, 1-2 weeks after rain at relative humidity of 50-60% and winds 10-20 km/hr.…”