Frank S. Gilliam

Global assessment of nitrogen deposition effects on terrestrial plant diversity: a synthesis

Atmospheric nitrogen (N) deposition is a recognized threat to plant diversity in temperate and northern parts of Europe and North America. This paper assesses evidence from field experiments for N deposition effects and thresholds for terrestrial plant diversity protection across a latitudinal range of main categories of ecosystems, from arctic and boreal systems to tropical forests. Current thinking on the mechanisms of N deposition effects on plant diversity, the global distribution of G200 ecoregions, and current and future (2030) estimates of atmospheric N-deposition rates are then used to identify the risks to plant diversity in all major ecosystem types now and in the future. This synthesis paper clearly shows that N accumulation is the main driver of changes to species composition across the whole range of different ecosystem types by driving the competitive interactions that lead to composition change and/or making conditions unfavorable for some species. Other effects such as direct toxicity of nitrogen gases and aerosols, long-term negative effects of increased ammonium and ammonia availability, soil-mediated effects of acidification, and secondary stress and disturbance are more ecosystem- and site-specific and often play a supporting role. N deposition effects in mediterranean ecosystems have now been identified, leading to a first estimate of an effect threshold. Importantly, ecosystems thought of as not N limited, such as tropical and subtropical systems, may be more vulnerable in the regeneration phase, in situations where heterogeneity in N availability is reduced by atmospheric N deposition, on sandy soils, or in montane areas. Critical loads are effect thresholds for N deposition, and the critical load concept has helped European governments make progress toward reducing N loads on sensitive ecosystems. More needs to be done in Europe and North America, especially for the more sensitive ecosystem types, including several ecosystems of high conservation importance. The results of this assessment show that the vulnerable regions outside Europe and North America which have not received enough attention are ecoregions in eastern and southern Asia (China, India), an important part of the mediterranean ecoregion (California, southern Europe), and in the coming decades several subtropical and tropical parts of Latin America and Africa. Reductions in plant diversity by increased atmospheric N deposition may be more widespread than first thought, and more targeted studies are required in low background areas, especially in the G200 ecoregions.

The Ecological Significance of the Herbaceous Layer in Temperate Forest Ecosystems

ABSTRACT Despite a growing awareness that the herbaceous layer serves a special role in maintaining the structure and function of forests, this stratum remains an underappreciated aspect of forest ecosystems. In this article I review and synthesize information concerning the herb layers structure, composition, and dynamics to emphasize its role as an integral component of forest ecosystems. Because species diversity is highest in the herb layer among all forest strata, forest biodiversity is largely a function of the herb-layer community. Competitive interactions within the herb layer can determine the initial success of plants occupying higher strata, including the regeneration of dominant overstory tree species. Furthermore, the herb layer and the overstory can become linked through parallel responses to similar environmental gradients. These relationships between strata vary both spatially and temporally. Because the herb layer responds sensitively to disturbance across broad spatial and temporal scales, its dynamics can provide important information regarding the site characteristics of forests, including patterns of past land-use practices. Thus, the herb layer has a significance that belies its diminutive stature.

Effects of nitrogen deposition and empirical nitrogen critical loads for ecoregions of the United States

Human activity in the last century has led to a significant increase in nitrogen (N) emissions and atmospheric deposition. This N deposition has reached a level that has caused or is likely to cause alterations to the structure and function of many ecosystems across the United States. One approach for quantifying the deposition of pollution that would be harmful to ecosystems is the determination of critical loads. A critical load is defined as the input of a pollutant below which no detrimental ecological effects occur over the long-term according to present knowledge. The objectives of this project were to synthesize current research relating atmospheric N deposition to effects on terrestrial and freshwater ecosystems in the United States, and to estimate associated empirical N critical loads. The receptors considered included freshwater diatoms, mycorrhizal fungi, lichens, bryophytes, herbaceous plants, shrubs, and trees. Ecosystem impacts included: (1) biogeochemical responses and (2) individual species, population, and community responses. Biogeochemical responses included increased N mineralization and nitrification (and N availability for plant and microbial uptake), increased gaseous N losses (ammonia volatilization, nitric and nitrous oxide from nitrification and denitrification), and increased N leaching. Individual species, population, and community responses included increased tissue N, physiological and nutrient imbalances, increased growth, altered root : shoot ratios, increased susceptibility to secondary stresses, altered fire regime, shifts in competitive interactions and community composition, changes in species richness and other measures of biodiversity, and increases in invasive species.

Microclimate moderates plant responses to macroclimate warming

Significance Around the globe, climate warming is increasing the dominance of warm-adapted species—a process described as “thermophilization.” However, thermophilization often lags behind warming of the climate itself, with some recent studies showing no response at all. Using a unique database of more than 1,400 resurveyed vegetation plots in forests across Europe and North America, we document significant thermophilization of understory vegetation. However, the response to macroclimate warming was attenuated in forests whose canopies have become denser. This microclimatic effect likely reflects cooler forest-floor temperatures via increased shading during the growing season in denser forests. Because standing stocks of trees have increased in many temperate forests in recent decades, microclimate may commonly buffer understory plant responses to macroclimate warming. Recent global warming is acting across marine, freshwater, and terrestrial ecosystems to favor species adapted to warmer conditions and/or reduce the abundance of cold-adapted organisms (i.e., “thermophilization” of communities). Lack of community responses to increased temperature, however, has also been reported for several taxa and regions, suggesting that “climatic lags” may be frequent. Here we show that microclimatic effects brought about by forest canopy closure can buffer biotic responses to macroclimate warming, thus explaining an apparent climatic lag. Using data from 1,409 vegetation plots in European and North American temperate forests, each surveyed at least twice over an interval of 12–67 y, we document significant thermophilization of ground-layer plant communities. These changes reflect concurrent declines in species adapted to cooler conditions and increases in species adapted to warmer conditions. However, thermophilization, particularly the increase of warm-adapted species, is attenuated in forests whose canopies have become denser, probably reflecting cooler growing-season ground temperatures via increased shading. As standing stocks of trees have increased in many temperate forests in recent decades, local microclimatic effects may commonly be moderating the impacts of macroclimate warming on forest understories. Conversely, increases in harvesting woody biomass—e.g., for bioenergy—may open forest canopies and accelerate thermophilization of temperate forest biodiversity.

Symptoms of nitrogen saturation in two central Appalachian hardwood forest ecosystems

By synthesizing more than twenty years of research at the Fernow Experimental Forest, we have documented 7 symptoms of nitrogen saturation in two adjacent watersheds. The symptoms include: 1) high relative rates of net nitrification, 2) long-term increases in stream-water concentrations of nitrate and base cations, 3) relatively high nitrate concentrations in solution losses, 4) little seasonal variability in stream-water nitrate concentrations, 5) a high discharge of nitrate from a young aggrading forest, 6) a rapid increase in nitrate loss following fertilization of a young aggrading forest, and 7) low retention of inorganic nitrogen when compared with other forested sites. These data support current conceptual models of nitrogen saturation and provide a strong, and perhaps the best, example of nitrogen saturation in the United States.

Effects of long-term fire exclusion on tree species composition and stand structure in an old-growth Pinus palustris (Longleaf pine) forest

Frequent fire is an integral component of longleaf pine ecosystems, creating environmental conditions favoring survival and growth of juvenile pines. This study examined stand structure, species composition, and longleaf pine regeneration in an old-growth tract of longleaf pine forest (Boyd Tract) experiencing long-term (>80 yr) fire exclusion in the Sandhills of North Carolina. Sampling of woody stems (i.e., ≥2.5 cm diameter at breast height) and tallies of longleaf pine seedlings were carried out in plots established randomly on upland, mesic areas and lowland, xeric areas within the Boyd Tract. Dominant woody species in mesic plots were black oak, hickories, and large, sparse longleaf pines. Xeric plots had high densities of turkey oak with the large longleaf pines, as well as higher frequencies of smaller longleaf stems. These differences between areas were associated with higher clay content of upland soils and higher sand content of lowland soils. Age-class frequency distributions for fire-suppressed longleaf pine following the last wildfire at the Boyd Tract approximately 80 yr ago contrasted sharply with data from an old-growth longleaf tract in southern Georgia (Wade Tract) that has been under a long-term frequent fire regime. Post-burn recruitment for the Boyd Tract wildfire appears to have been initially high on both site types. Longleaf pine recruitment diminished sharply on the mesic site, but remained high for ∼60 yr on the xeric site. Currently, longleaf pine regeneration is minimal on both site types; several plots contained no seedlings. Sharp contrasts in longleaf pine dominance and stand structure between the Boyd and Wade Tracts demonstrate the importance of large-scale disturbance, especially hurricanes and fire, in shaping the structure and function of longleaf pine ecosystems of the southeastern United States. In particular, long-term exclusion of fire on the Boyd Tract has altered stand structure dramatically by permitting hardwoods to occupy at high densities the characteristically large gaps between longleaf stems that are maintained by fire and other disturbances.

Temporal and spatial variation of nitrogen transformations in nitrogen-saturated soils of a central Appalachian hardwood forest

We studied temporal and spatial patterns of soil nitrogen (N) dynamics from 1993 to 1995 in three watersheds of Fernow Experimental Forest, W.V.: WS7 (24-year-old, untreated); WS4 (mature, untreated); and WS3 (24- year-old, treated with (NH4)2SO4 since 1989 at the rate of 35 kg N·ha–1·year–1). Net nitrification was 141, 114, and 115 kg N·ha–1·year–1, for WS3, WS4, and WS7, respectively, essentially 100% of net N mineralization for all watersheds. Temporal (seasonal) patterns of nitrification were significantly related to soil moisture and ambient temperature in untreated watersheds only. Spatial patterns of soil water NO3– of WS4 suggest that microenvironmental variability limits rates of N processing in some areas of this N-saturated watershed, in part by ericaceous species in the herbaceous layer. Spatial patterns of soil water NO3– in treated WS3 suggest that later stages of N saturation may result in higher concentrations with less spatial variability. Spatial variability in soil N variables was lower in treated WS3 versus untreated watersheds. Nitrogen additions have altered the response of N-processing microbes to environmental factors, becoming less sensitive to seasonal changes in soil moisture and temperature. Biotic processes responsible for regulating N dynamics may be compromised in N-saturated forest ecosystems.

Herbaceous Layer Cover and Biomass in a Young Versus a Mature Stand of a Central Appalachian Hardwood Forest

GILLIAM, F. S. AND N. L. TURRILL (Department of Biological Sciences, Marshall University, Huntington, WV 25755). Herbaceous layer cover and biomass in a young versus a mature stand of a central Appalachian hardwood forest. Bull. Torrey Bot. Club 120: 445-450. 1993.-This study examined herb layer vegetation (all vascular plants c 1 m in height) of two montane watersheds of different stand ages in the Femow Experimental Forest, in north-central West Virginia (WS3, -20 yr; WS4, >80 yr). Mean herb layer cover was 19.3 and 26.4% for WS3 and WS4, respectively. Herb layer biomass was significantly (P < 0.00 1) correlated with herb cover for both watersheds. Mean herb layer biomass was 8.1 and 11.6 g/m2 for WS3 and WS4, respectively. Herb cover responded positively to elevation on both watersheds. Within WS3, herb cover was only weakly correlated with canopy characteristics, but was positively correlated with soil pH, organic matter, sand content, cation exchange capacity, Ca, K, Mg, and N03-N. Within WS4, herb cover was negatively correlated with understory basal area and density, but not significantly correlated with any soil variables except Ca. From these data we suggest a hypothesis that in early forest succession in these ecosystems, early herb layer development is influenced greatly by allogenic factors (such as soil fertility) but that autogenic factors, such as canopy closure, become more important as the stand matures and becomes more stratified.

Disturbance effects on herbaceous layer vegetation and soil nutrients in Populus forests of northern lower Michigan

. Recent disturbance models have identified changes in resource availability as factors that control plant community response. Soil nutrient resources typically are assumed to change following forest disturbance, usually with nutrient availablity increasing initially and subsequently decreasing through later stages of succession. We examined the effects of disturbance (clearcut harvesting with a brief recovery period) on soil organic matter, pH and extractable soil nutrients in successional aspen forests of northern lower Michigan to determine relationships of these variables to changes in herbaceous layer vegetation. Two site types were identified: drymesic (glacial outwash sands, low in organic matter) and mesic (calcareous clay till, high in organic matter). Extractable nutrient concentrations were 1.5 to 3 times higher in the A1 horizon of mesic sites than those of dry-mesic sites. Soil pH and cations increased after disturbance on mesic sites, but not on dry-mesic sites. Patterns of change with disturbance were less pronounced in lower horizons on both site types. Herblayer species diversity increased after disturbance on mesic sites, but with decreases in the importance of shade-tolerant tree species and Maianthemum canadense. Species characteristic of open habitats (e.g. Pteridium aquilinum, Rubus spp., Fragaria virginiana, and Diervilla lonicera), increased in importance. Soil factors, species composition and diversity on dry-mesic sites changed little after disturbance, with Pteridium aquilinum and ericaceous species remaining dominant in both mature (55–82 yr) and disturbed (≤ 15 yr) stands. These results suggest that soil nutrient resources do not always change through secondary succession and that patterns of change can be distinctly site-dependent. Disturbance response patterns in the herbaceous layer of these aspen forests are also site-dependent.

Interactive Effects of Nitrogen and Phosphorus on Soil Microbial Communities in a Tropical Forest

Elevated nitrogen (N) deposition in humid tropical regions may exacerbate phosphorus (P) deficiency in forests on highly weathered soils. However, it is not clear how P availability affects soil microbes and soil carbon (C), or how P processes interact with N deposition in tropical forests. We examined the effects of N and P additions on soil microbes and soil C pools in a N-saturated old-growth tropical forest in southern China to test the hypotheses that (1) N and P addition will have opposing effects on soil microbial biomass and activity, (2) N and P addition will alter the composition of the microbial community, (3) the addition of N and P will have interactive effects on soil microbes and (4) addition-mediated changes in microbial communities would feed back on soil C pools. Phospholipid fatty acid (PLFA) analysis was used to quantify the soil microbial community following four treatments: Control, N addition (15 g N m−2 yr−1), P addition (15 g P m−2 yr−1), and N&P addition (15 g N m−2 yr−1 plus 15 g P m−2 yr−1). These were applied from 2007 to 2011. Whereas additions of P increased soil microbial biomass, additions of N reduced soil microbial biomass. These effects, however, were transient, disappearing over longer periods. Moreover, N additions significantly increased relative abundance of fungal PLFAs and P additions significantly increased relative abundance of arbuscular mycorrhizal (AM) fungi PLFAs. Nitrogen addition had a negative effect on light fraction C, but no effect on heavy fraction C and total soil C. In contrast, P addition significantly decreased both light fraction C and total soil C. However, there were no interactions between N addition and P addition on soil microbes. Our results suggest that these nutrients are not co-limiting, and that P rather than N is limiting in this tropical forest.