Mary A. Arthur

Forest Ecosystem Responses to Exotic Pests and Pathogens in Eastern North America

Abstract The forests of eastern North America have been subjected to repeated introductions of exotic insect pests and pathogens over the last century, and several new pests are currently invading, or threatening to invade, the region. These pests and pathogens can have major short- and long-term impacts on forest ecosystem processes such as productivity, nutrient cycling, and support of consumer food webs. We identify six key features of the biology of exotic animal pests and the ecology of their hosts that are critical to predicting the general nature and severity of those impacts. Using three examples of introduced pests and pathogens in eastern forest ecosystems, we provide a conceptual framework for assessing potential ecosystem-scale effects.

Control of Nitrogen Loss from Forested Watersheds by Soil Carbon:Nitrogen Ratio and Tree Species Composition

AbstractLeaching losses of nitrate from forests can have potentially serious consequences for soils and receiving waters. In this study, based on extensive sampling of forested watersheds in the Catskill Mountains of New York State, we examine the relationships among stream chemistry, the properties of the forest floor, and the tree species composition of watersheds. We report the first evidence from North America that nitrate export from forested watersheds is strongly influenced by the carbon:nitrogen (C:N) ratio of the watershed soils. We also show that variation in soil C:N ratio is associated with variation in tree species composition. This implies that N retention and release in forested watersheds is regulated at least in part by tree species composition and that changes in species composition caused by introduced pests, climate change, or forest management could affect the capacity of a forest ecosystem to retain atmospherically deposited N.

Challenges of measuring forest floor organic matter dynamics:: Repeated measures from a chronosequence

Accurate estimates of the retention of carbon in forest soils following forest disturbances are essential to predictions of global carbon cycling. The belief that 50% of soil carbon is lost in the first 20 years after clearcutting is largely based on a chronosequence study of forest floors in New Hampshire northern hardwoods (Covington, 1981). We resampled forest floors in 13 stands in a similar chronosequence after an interval of 15 years. The three youngest stands, which were predicted to lose organic matter over this time, did not exhibit the 40‐50% losses predicted by the original chronosequence. The oldest stands had about twice as much organic mass in the forest floor as those cut recently, but this pattern could be explained equally well by historical changes in the nature of forest harvest as by the age of the stands. For example, mechanized logging probably causes more mechanical disturbance to the forest floor than horse logging, burying more organic matter into the mineral soil. Markets for forest products and the intensity of harvest removals have also changed over time, possibly contributing to lower organic matter in the forest floor in young stands. In any chronosequence study, effects of change in the nature of the treatment over time can easily be misinterpreted as change with time since treatment. Repeated sampling of the chronosequence provides controls for some of these effects. In the case of forest floor organic matter, however, high spatial variation makes it difficult to distinguish whether the observed variation is more likely due to changes in treatment over time or to differences in time since treatment. Because of the large amounts of carbon involved, small changes in rates of soil organic matter storage may be quite important in global climate change, but they will remain difficult to detect, even with very intensive sampling. # 2000 Elsevier Science B.V. All rights reserved.

SINGLE AND REPEATED FIRES AFFECT SURVIVAL AND REGENERATION OF WOODY AND HERBACEOUS SPECIES IN AN OAK-PINE FOREST

response, on an oak-pine ridgetop in eastern Kentucky. Overstory (>10 cm dbh) species composition was dominated by Quercus spp., whereas the midstory (2-10 cm dbh) was dominated by red maple and blackgum. Oak regeneration was well-represented in the herb layer, but was not nearly as prolific as red maple. Single and repeated fire reduced stems 2-10 cm dbh by 60 to 80%; stump sprouting by individual trees was promoted by fire. On the twice-burned site there were fewer red maple and blackgum sprouts per hectare than on a site burned in 1995, despite greater numbers of sprouts per tree. Species richness in all strata was increased by fire, from 19 species on the unburned site to 35 species on the twice-burned site. Our data suggest that periodic prescribed fires may reduce regeneration of red maple and other non-oak species sufficiently to promote chestnut oak regeneration; regeneration of more shade intolerant species, including scarlet oak and the hard pines, would require a disturbance of greater intensity.

ACCUMULATION AND DEPLETION OF BASE CATIONS IN FOREST FLOORS IN THE NORTHEASTERN UNITED STATES

Loss of base cations from forest soils can be accelerated by acid rain, by forest regrowth following harvest removals, and by declining inputs of base cations from atmospheric deposition. Calcium losses from forest floors have been reported at several sites in the northeastern United States. To test for loss of base cations from forest floors at the Hubbard Brook Experimental Forest in New Hampshire (USA), we analyzed samples collected on seven dates between 1976 and 1997. Calcium and magnesium contents of the forest floor did not decline significantly; a change >0.9%/yr would have been detectable. Concentrations of Ca were 40% higher in 1969-1970 than in the current study, but the difference is partly due to changes in collection methods. Magnesium concentrations were too variable to detect a loss of <47% over the 21-yr interval. To determine whether base- cation losses were associated with forest growth, we resampled a chronosequence of north- ern hardwood stands in the White Mountains of New Hampshire. The 13 stands did not show consistent changes in Ca, Mg, and potassium over the 15-yr interval. Losses of these cations were most pronounced in stands logged more than 25 yr earlier. Younger stands, contrary to our expectation that rapid forest growth should cause cation depletion, all gained base cations in the forest floor. Early in stand development these forest floors appeared to accumulate biomass along with living vegetation, rather than serving as a net source of nutrients. Finally, in a regional survey of 28 mature stands in the northeastern United States, some lost significant forest-floor Ca and Mg between 1980 and 1990, while others gained. The average change in Ca and Mg content was not significant; a loss of 1.4%/yr would have been detectable. Forest floors in the region are not currently experiencing rapid losses of base cations, though losses may have preceeded the onset of these three studies.

Plant and soil natural abundance δ15N: indicators of relative rates of nitrogen cycling in temperate forest ecosystems

Watersheds within the Catskill Mountains, New York, receive among the highest rates of nitrogen (N) deposition in the northeastern United States and are beginning to show signs of N saturation. Despite similar amounts of N deposition across watersheds within the Catskill Mountains, rates of soil N cycling and N retention vary significantly among stands of different tree species. We examined the potential use of δ15N of plants and soils as an indicator of relative forest soil N cycling rates. We analyzed the δ15N of foliage, litterfall, bole wood, surface litter layer, fine roots and organic soil from single-species stands of American beech (Fagus grandifolia), eastern hemlock (Tsuga canadensis), red oak (Quercus rubra), and sugar maple (Acer saccharum). Fine root and organic soil δ15N values were highest within sugar maple stands, which correlated significantly with higher rates of net mineralization and nitrification. Results from this study suggest that fine root and organic soil δ15N can be used as an indicator of relative rates of soil N cycling. Although not statistically significant, δ15N was highest within foliage, wood and litterfall of beech stands, a tree species associated with intermediate levels of soil N cycling rates and forest N retention. Our results show that belowground δ15N values are a better indicator of relative rates of soil N cycling than are aboveground δ15N values.

Do Nutrient Limitation Patterns Shift from Nitrogen Toward Phosphorus with Increasing Nitrogen Deposition Across the Northeastern United States

Atmospheric nitrogen (N) deposition is altering biogeochemical cycling in forests and interconnected lakes of the northeastern US, and may shift nutrient limitation from N toward other essential elements, such as phosphorus (P). Whether this shift is occurring relative to N deposition gradients across the northeastern US has not been investigated. We used datasets for the northeastern US and the Adirondack sub-region to evaluate whether P limitation is increasing where N deposition is high at two geographic scales, based on N:P mass ratios. Using a model-selection approach, we determined that foliar N for dominant tree species and lake dissolved inorganic N (DIN) increased coincident with increasing N deposition, independent of relationships between foliar N or lake DIN and precipitation or temperature. Foliar P also increased with N deposition across the northeastern US for seven of eight deciduous species, but changed less across the Adirondacks. Foliar N:P therefore declined at the highest levels of N deposition for most deciduous species across the region (remaining nearly constant for most conifers and increasing only for black cherry and hemlock), but increased across all species in the Adirondacks. Ratios between DIN and total P (DIN:TP) in lakes were unrelated to N deposition regionally but increased across the Adirondacks. Thus, nutrient limitation patterns shifted from N toward P for dominant trees, and further toward P for predominantly P-limited lakes, at the sub-regional but not regional scale. For the northeastern US overall, accumulated N deposition may be insufficient to drive nutrient limitation from N toward P; alternatively, elements other than P (for example, calcium, magnesium) may become limiting as N accumulates. The consistent Adirondack foliar and lake response could provide early indication of shifts toward P limitation within the northeastern US, and together with regional patterns, suggests that foliar chemistry could be a predictor of lake chemistry in the context of N deposition across the region.

Leaf phenology and freeze tolerance of the invasive shrub Amur honeysuckle and potential native competitors

Abstract The non-native invasive deciduous shrub Lonicera maackii causes a reduction in plant growth and species diversity under its canopy. The mechanisms of these effects are not fully understood, but an apparent difference between L. maackii and native shrub species is its extended leaf duration. We tested the hypothesis that L. maackii has a longer leaf duration than native shrub species found in the same habitats. Leaf phenology of L. maackii and the native deciduous shrubs Asimina triloba and Lindera benzoin was observed at four sites in central Kentucky (USA) from March until December, 2007. Additionally, a late spring freeze allowed for examination of freeze tolerance among the three test species. Lonicera maackii leaf development was two to three weeks earlier than the natives in March and early April. A hard freeze in early April caused significant (P < 0.05) leaf mortality to both of the native species (60–100% leaf mortality at 3 of 4 sites) while L. maackii showed no observable damage. L. maackii had a later transition to fall color and leaf abscission than the native species, which were at a significantly later stage of development (closer to leaf abscission) for a period of four to six weeks. These data suggest two advantages for L. maackii over potential native competitors: 1) greater access to carbon via a longer leaf duration, and 2) a greater capacity to withstand freezing temperatures.

Above- and Belowground Net Primary Production in a Temperate Mixed Deciduous Forest

Our current ability to detect and predict changes in forest ecosystem productivity is constrained by several limitations. These include a poor understanding of belowground productivity, the short duration of most analyses, and a need for greater examination of species- or community-specific variability in productivity studies. We quantified aboveground net primary productivity (ANPP) over 3 years (1999–2001), and both belowground NPP (BNPP) and total NPP over 2 years (2000–2001) in both mesic and xeric site community types of the mixed mesophytic forest of southeastern Kentucky to examine landscape variability in productivity and its relation with soil resource [water and nitrogen (N)] availability. Across sites, ANPP was significantly correlated with N availability (R2 = 0.58, P = 0.028) while BNPP was best predicted by soil moisture content (R2 = 0.72, P = 0.008). Because of these offsetting patterns, total NPP was unrelated to either soil resource. Interannual variability in growing season precipitation during the study resulted in a 50% decline in mesic site litter production, possibly due to a lag effect following a moderate drought year in 1999. As a result, ANPP in mesic sites declined 27% in 2000 compared to 1999, while xeric sites had no aboveground production differences related to precipitation variability. If global climate change produces more frequent occurrences of drought, then the response of mesic sites to prolonged moisture deficiency and the consequences of shifting carbon (C) allocation on C storage will become important questions.

Biotic Control of Calcium Cycling in Northern Hardwood Forests: Acid Rain and Aging Forests

Observations of declining base saturation in soils and declining calcium (Ca) in streamwater have contributed to concerns that prolonged exposure to acid rain threatens forest health and productivity. We suggest that these changes could be caused, in part, by aging of the forests. To test this possibility, we characterized Ca cycling in previously harvested, variously aged northern hardwood stands over 15–18 years. The Ca content and concentrations in the forest floor and the density of snails, which require Ca for growth, increased in young stands (less than 30 years old) and decreased in older stands (more than 30 years old) over the measurement period. Similarly, the concentrations of Ca in litterfall decreased with stand age, and hydrologic export of Ca from a young stand was higher than that from an old stand. Ecosystem budgets suggest that the supply of Ca from the mineral soil to other parts of the ecosystem is large (3.3–4.7 g Ca m 2 y 1 ) in young forest stands but negligible or negative in older stands (–1 g Ca m 2 y 1 ). This difference in Ca mobilization between young and old stands is large compared to the changes in soil Ca that can reasonably be attributed to acid precipitation (less than 1gm 2 y 1 ). We conclude that changes in soil and streamwater Ca in maturing forests do not necessarily indicate an important loss of bioavailable Ca. Trace amounts of apatite in the mineral soil may be the source of Ca needed for forest regrowth.