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.
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•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.

Nitrogen (N) deposition can alter N availability, soil pH, and microbial activity and composition in soils, which directly and indirectly affects soil N transformations and influences forest ecosystem processes and functions. Understanding the effects of N deposition on soil N transformations in forest ecosystems has important implications for the ecological and environmental impacts of N deposition. In this chapter, the effects of N deposition on net and gross soil N transformations are synthesized based on the published literature. Results show that N deposition increases both net and gross soil N mineralization and nitrification rates, but do not affect gross NH4+ immobilization rates and gross NO3− immobilization rates. Responses of soil N transformations to N deposition varied depending on climate (such as mean annual precipitation, mean annual temperature, climate region), environmental factors (especially ambient atmospheric N deposition rate), soil properties (soil C:N, soil pH, soil horizon), and experimental N addition (e.g., chemical form of N, N addition duration). The duration of N addition plays an important role affecting the responses of soil N transformations to such additions. Thus, the temporal changes in the effects of N deposition are specifically reviewed based on several case studies. In future research, the observations under long-term time scales are needed to clarify the effects of N deposition on soil N transformations in forest ecosystems. In addition, the role of microbes in the responses of soil N transformations to N deposition also should be investigated.

Historical increases in emissions and deposition of oxidized and reduced nitrogen (N) provided the impetus for global-scale research on the effects of excess N in terrestrial and aquatic ecosystems. Much of the eastern U.S. has been susceptible to negative effects of excess N. The Clean Air Act and associated rules have led to decreases in emissions and deposition of oxidized N, especially in the eastern U.S., representing a research challenge and opportunity for ecosystem ecologists. This chapter predicts changes in the structure and function of North American forest ecosystems in response to decreased N deposition. Hysteresis is a property of a system wherein output is not a strict function of corresponding input, incorporating a time lag, particularly when responses to decreasing input vary from responses to increasing input. A conceptual hysteretic model predicts varying lag times in recovery of soil acidification and nutrient leaching, surface water nitrogen concentrations and export, plant diversity, soil microbial communities, and forest carbon and N cycling toward pre-N-impact conditions. These processes are expected to respond notably to reductions in N deposition, most showing a degree of hysteresis, with the greatest delays in the response occurring in those tightly linked to “slow pools” of N in wood and soil organic matter. Some responses, especially nitrate concentrations in stream flow, have already become apparent in regions of northeastern U.S. Because experimental studies of declines in N loads in forests of North America are lacking and due to the expected hysteresis, it is difficult to generalize from experimental results to patterns expected from declining N deposition. Responses to declining N will be long-term and difficult to distinguish from concurrent environmental changes affecting the N cycle, e.g., elevated atmospheric CO2, climate change, reductions in acidity, invasive species, and vegetation responses to disturbance.