Dr. Frank S Gilliam

Endangered coastal ecosystems, such as biodiverse longleaf pine savannas, have historically been resistant and resilient to the impacts of tropical cyclones. But changing hurricane regimes, coupled with little remaining habitat and detrimental management actions, threaten their persistence. We review the hurricane ecology of these systems and summarize risk factors across scales. We categorize extant longleaf pine habitat, 41% of which is privately owned, into risk categories based on coastal, inland, and continental hurricane regimes. The majority of habitat (85%) experiences inland hurricane regimes (6-year average return intervals). Considering increasing exposure to more intense cyclones, we review the ecological risk of linked disturbances, including fire, insect outbreaks, and management actions, such as salvage logging. Our adaptive management recommendations emphasize the need to maintain longleaf pine ecosystem resistance and resilience based on ecological research accounting for changing ecosystem dynamics and comprehensive postcyclone ecosystem responses to develop climate adaptation strategies and response plans.

A major dimension of pattern and process in ecological systems is the way in which species interact. In the study of forest communities, the phenomenon of linkage among forest strata (e.g., overstory and herbaceous layer) has been well investigated and arises when forest strata interact in ways that lead to causal connections between them. Whereas trees alter the light regime of forest herb communities, the herb layer can direct survivorship among seedlings of overstory species. Less studied, however, is linkage between forest strata and forest soil microbiomes. This review examines ways in which forest vegetation and soil microbiomes exert reciprocating effects on each other that can lead to linkage, beginning with a brief literature review of several phenomena relevant to how these effects occur. Because of the coincidence of the ubiquity of soil microbes with their almost infinitely small size, their interactions—both above and belowground in nature—with forest vegetation are particularly intimate. Although the most direct link, and certainly one that likely first comes to mind, is through root/microbe interactions, foliar surfaces and internal foliar tissues can support a diverse microbiome. Following the overview of potential mechanisms, examples from two separate forest studies of how linkage was demonstrated will be summarized. In each of these studies, linkage was evident through significant correlations among axis scores generated by canonical correspondence run separately for forest vegetation and soil microbial communities.

Power line rights-of-way (ROWs), which are extensive throughout the United States, create persistent canopy openings in forest stands. We utilized a ROW running north/south through mixed longleaf pine (Pinus palustris)/hardwood stands to examine impacts of persistent canopy openings on stand structure and composition, light availability to the understory, and soil texture and fertility, addressing the following questions: (1) how does light vary with respect to the persistent canopy opening? (2) what differences are there in structure and composition of forest stands relative to the ROW? (3) how does light availability to the forest interior vary between sites? (4) how does soil vary between sites? Sampling was carried out on the campus of the University of West Florida (UWF), Pensacola, Florida, within each of three sample sites: west of the ROW (“West”), east of the ROW (“East”), and adjacent to the East area (“Control”). Spatio-temporal patterns of light contrasted sharply between stands west versus east of the ROW, creating an asymmetry in light regimes. All sample sites had low densities of large pines and numerous stems of hardwoods, particularly southern evergreen oaks (Quercus spp.). In contrast to the Control site, wherein longleaf pine had highest importance value (IV), sand laurel oak (Q. hemisphaerica) had highest IVs in stands adjacent to the ROW. Light and silt content of the soil were negatively related across the three sites. Canonical correspondence analysis suggests a sharp contrast in overall species composition between West and East sites, suggesting that the asymmetry of light drives asymmetry of forest composition.

Elucidating the impacts of chronic atmospheric nitrogen (N) deposition on soil organic carbon (SOC) is crucial for predicting the dynamics of terrestrial C sinks, particularly in N-rich subtropical forests. Experiments using understory N addition (UN) have provided valuable insights into these impacts, but unavoidably neglect processes such as interception and absorption of N within forest canopy. We assessed the effects of long-term (11-yr) fertilization via both canopy N addition (CN) and UN on SOC in a subtropical forest. Our results showed significantly different responses of SOC between the approaches, with UN displaying greater effects on SOC than CN. Specifically, both low and high rates of UN substantially increased the concentrations of particulate organic C (POC), whereas the high rate of CN significantly increased those of mineral-associated organic C (MAOC) rather than POC. Long-term CN and UN treatments had distinct effects on plant- and microbial-derived C processes. UN treatments significantly increased soil available N and improved the litter quality, enhancing the formation of POC, and suppressing microbial decomposition of POC due to the significant decreases in soil pH. However, CN treatments significantly improved litter quality and mitigated soil acidification, thus stimulating microbial C utilization and accelerating the microbial transformation of POC to MAOC. Our findings imply that the underlying mechanisms of natural N deposition influencing forest SOC may differ from those obtained from UN, and conventional fertilization experiments may overestimate the benefits of elevated N deposition to forest SOC.

Biological nitrogen fixation is a fundamental part of ecosystem functioning. Anthropogenic nitrogen deposition and climate change may, however, limit the competitive advantage of nitrogen-fixing plants, leading to reduced relative diversity of nitrogen-fixing plants. Yet, assessments of changes of nitrogen-fixing plant long-term community diversity are rare. Here, we examine temporal trends in the diversity of nitrogen-fixing plants and their relationships with anthropogenic nitrogen deposition while accounting for changes in temperature and aridity. We used forest-floor vegetation resurveys of temperate forests in Europe and the United States spanning multiple decades. Nitrogen-fixer richness declined as nitrogen deposition increased over time but did not respond to changes in climate. Phylogenetic diversity also declined, as distinct lineages of N-fixers were lost between surveys, but the "winners" and "losers" among nitrogen-fixing lineages varied among study sites, suggesting that losses are context dependent. Anthropogenic nitrogen deposition reduces nitrogen-fixing plant diversity in ways that may strongly affect natural nitrogen fixation.

Arbuscular mycorrhizal fungi (AMF) increase the ability of plants to obtain nitrogen (N) from the soil, and thus can affect emissions of nitrous oxide (N2O), a long-lived potent greenhouse gas. However, the mechanisms underlying the effects of AMF on N2O emissions are still poorly understood, particularly in agroecosystems with different forms of N fertilizer inputs. Utilizing a mesocosm experiment in field, we examined the effects of AMF on N2O emissions via their influence on maize root traits and denitrifying microorganisms under ammonia and nitrate fertilizer input using 15N isotope tracer. Here we show that the presence of AMF alone or both maize roots and AMF increased maize biomass and their 15N uptake, root length, root surface area, and root volume, but led to a reduction in N2O emissions under both N input forms. Random forest model showed that root length and surface area were the most important predictors of N2O emissions. Additionally, the presence of AMF reduced the (nirK + nirS)/nosZ ratio by increasing the relative abundance of nirS-Bradyrhizobium and Rubrivivax with ammonia input, but reducing nosZ-Azospirillum, Cupriavidus and Rhodopseudomonas under both fertilizer input. Further, N2O emissions were significantly and positively correlated with the nosZ-type Azospirillum, Cupriavidus and Rhodopseudomonas, but negatively correlated with the nirS-type Bradyrhizobium and Rubrivivax. These results indicate that AMF reduce N2O emissions by increasing root length to explore N nutrients and altering the community composition of denitrifiers, suggesting that effective management of N fertilizer forms interacting with the rhizosphere microbiome may help mitigate N2O emissions under future N input scenarios.
[Display omitted]
•The effects of AMF and roots on N2O emissions were studied with 15N isotope tracer.•AMF altered maize root traits, leading to a decrease in soil N2O emissions.•AMF reduced (nirK + nirS)/nosZ ratio and altered denitrifier community composition.•N fertilizer management interacting with the microbiome may reduce N2O emissions.

Globally increased nitrogen (N) to phosphorus (P) ratios (N/P) affect the structure and functioning of terrestrial ecosystems, but few studies have addressed the variation of foliar N/P over time in subtropical forests. Foliar N/P indicates N versus P limitation in terrestrial ecosystems. Quantifying long-term dynamics of foliar N/P and their potential drivers is crucial for predicting nutrient status and functioning in forest ecosystems under global change. We detected temporal trends of foliar N/P, quantitatively estimated their potential drivers and their interaction between plant types (evergreen vs. deciduous and trees vs. shrubs), using 1811 herbarium specimens of 12 widely distributed species collected during 1920-2010 across China's subtropical forests. We found significant decreases in foliar P concentrations (23.1%) and increases in foliar N/P (21.2%). Foliar N/P increased more in evergreen species (22.9%) than in deciduous species (16.9%). Changes in atmospheric CO
concentrations (
), atmospheric N deposition and mean annual temperature (MAT) dominantly contributed to the increased foliar N/P of evergreen species, while
, MAT, and vapor pressure deficit, to that of deciduous species. Under future Shared Socioeconomic Pathway (SSP) scenarios, increasing MAT and
would continuously increase more foliar N/P in deciduous species than in evergreen species, with more 12.9%, 17.7%, and 19.4% versus 6.1%, 7.9%, and 8.9% of magnitudes under the scenarios of SSP1-2.6, SSP3-7.0, and SSP5-8.5, respectively. The results suggest that global change has intensified and will progressively aggravate N-P imbalance, further altering community composition and ecosystem functioning of subtropical forests.

College and university campuses with a notable arboreal component provide unique opportunities for carrying out ecological research. The University of West Florida Campus Ecosystem Study (UWF CES) was established in 2019 as interconnected research to take advantage of the extensive arborescent nature of the UWF campus, particularly concerning longleaf pine (Pinus palustris). One of these investigations established permanent plots in forested sites of two contrasting types, one dominated by longleaf pine (“pine site”) and the other dominated by hardwoods (‘hardwood site’). This study used these plots to examine the influence of forest vegetation on light availability and soil processes. Light was measured as photosynthetically active radiation (and expressed as photon flux density—PFD) with a handheld meter in each plot. Soil was sampled to 5 cm in each plot; texture was measured with the hydrometer method. Identical sampling methods were carried out in a persistent canopy opening to assess light and soil conditions under maximum solar radiation. Mean PFD was ~4× higher in pine stands than in hardwood stands; PFD was 12.8 and 3.5% of full light in the pine and hardwood stands, respectively. All soils were dominated by coarse-textured sands, but silt was significantly higher in pine stand soil and higher still in the canopy opening. Among forest stand plots, sand was negatively related to PFD, whereas clay was positively related to PFD. Across the three sites, silt was positively related to PFD. These relationships are consistent with the importance of solar radiation as one of many drivers of soil weathering.

Arbuscular mycorrhizal fungi (AMF) generally improve crop nutrient acquisition and grain yield, especially under nutrient deficiency. It is uncertain, however, whether and how AMF colonization affects maize nitrogen (N) uptake during grain filling stage and grain yield under varying soil N status. To investigate this role under the conditions of poor and rich agricultural soils, two pot experiments were conducted with AMF inoculated and non-inoculated maize growing at low (180 kg N hm⁻²) and high N (270 kg N hm⁻²) input and two different nutrient areas. Compared to the non-inoculation treatment, AMF inoculation increased maize grain yield, plant biomass, and N accumulation during the filling stage under different soil N conditions. Other responses included increasing root length, root surface area, activities of grain nitrate reductase, nitrite reductase, glutamine synthetase, glutamate synthase (GOGAT) and their gene expressions. All enzyme activities and GOGAT gene expression were significantly correlated with grain yield. Grain yield and N accumulation were significantly higher at the nutrient rich site than poor site. Inoculation with AMF significantly increased grain yield with either lower or higher N input at both sites, whereas increased efficiency was greater with lower N input than higher N input. These results showed that AMF inoculation can increase maize yield and N uptake during the filling stage through regulating root traits and grain N metabolic enzyme activities and their gene expressions independent of soil N status. This enhances our knowledge of the role of AMF in the context of conventional agricultural management.

Biodiversity in forest ecosystems is paradoxical. Whereas their most apparent component—the woody overstory—is the least diverse with respect to numbers of species, the least apparent component is the biotic community of highest diversity—the soil microbiome. Numerous factors influence the composition and diversity of soil microbial communities, which in turn exert a profound impact on plant species occupying the soil and the biogeochemistry of essential plant nutrients. Of interest in forest ecosystems is how the soil microbiome interacts with the overstory, a phenomenon referred to as linkage. This study compared the soil microbiome of two adjacent stand types—hardwood‐ and longleaf pine (Pinus palustris)‐dominated—and addressed the following questions: (1) How does soil microbiome vary with stand type? (2) Do the forest overstory community and soil microbiome exhibit linkage? Twelve 0.04‐ha circular plots were established in each stand type to assess tree community composition and structure and to sample mineral soil for three separate analyses: assessment of soil fertility, measurement of total carbon and nitrogen (N), and extraction of genomic DNA for assessment of microbiome communities. All live stems ≥2.5 cm dbh in each plot were identified to species and measured for dbh to the nearest 0.1 cm. Mineral soil was taken from a depth of 5 cm and oven‐dried at 38°C prior to analyses. Hardwood stands were dominated by flowering magnolia (Magnolia grandiflora) and southern evergreen oaks, whereas pine stands were dominated by longleaf pine and live oak (Quercus virginiana). Although soils of both stand types were highly acidic, the hardwood stands were generally higher in fertility, especially for total and available N. The overstory and soil microbial communities exhibited evidence of linkage among all sample plots combined. When assessed separately by stand type, only hardwood‐dominated stands displayed evidence of overstory/microbial linkage. These results have broader implications for future scenarios given the sensitivity of soil microbes to climate change.