Exotic earthworm effects on hardwood forest floor, nutrient availability and native plants: a mesocosm study

A greenhouse mesocosm experiment, representing earthworm-free North American Acer-dominated forest floor and soil conditions, was used to examine the individual and combined effects of initial invasion by three European earthworm species (Dendrobaena octaedra, Lumbricus rubellus and Lumbricus terrestris) on the forest floor and upper soil horizons, N and P availability, and the mortality and biomass of four native understory plant species (Acer saccharum, Aquilegia canadensis, Aralia racemosa, and Carex pensylvanica). All the three earthworm species combined caused larger impacts on most variables measured than any single earthworm species. These included loss of O horizon mass, decreased thickness of the O horizon and increased thickness of the A horizon, and higher availability of N and P. The latter finding differs from field reports where nutrients were less available after invasion, and probably represents an initial transient increase in nutrient supply as earthworms consume and incorporate the O horizon into the A horizon. Earthworms also increased mortality of plants and decreased total mesocosm plant biomass, but here the impact of all the three earthworm species was no greater than that of L. terrestris and/or L. rubellus alone. This study corroborates field studies that European earthworm invasions alter North American forest ecosystem processes by initiating a cascade of impacts on plant community composition and soil properties.     

Effects of Exotic Earthworms on Soil Phosphorus Cycling in Two Broadleaf Temperate Forests

We compared the biogeochemical cycling of phosphorus (P) in northern hardwood forest plots invaded by exotic earthworms versus adjacent uninvaded reference plots. In three of the six pairs of plots, earthworm invasion resulted in significantly more total P in the upper 12 cm of soil. The finding of increased amounts of unavailable and occluded inorganic P forms in the invaded plots suggests that earthworm activity mobilized unweathered soil particles from deeper layers of the soil, increasing the stocks of total P in surface soils. In two pairs of plots, the earthworm-invaded soils had less total P than the reference soils. In these plots, earthworm activity resulted in augmented rates of P cycling and alteration of the physical structure of the soil that increased loss of P in leaching water, reducing the total amount of P. We hypothesize that the different effects of earthworm invasion on the soil P cycle result from unique characteristics of the ecological groups of earthworms dominating each site. The invaded plots with increased total P were dominated by the anecic species Lumbricus terrestris, a large earthworm that constructs deep, vertical burrows and is effective at moving soil materials from and to deeper layers of the profile. In contrast, the earthworm-invaded plots where the total P in the surface soil decreased were dominated by the epi-endogeic species L. rubellus, which feeds and lives in the upper organic layers of the soil. In these plots, earthworms significantly increased the amount of readily exchangeable P in the soil, increasing the loss of this element in leaching water.     

Spatial and Temporal Dynamics of Exotic Earthworm Communities Along Invasion Fronts in a Temperate Hardwood Forest in South-Central New York (USA)

The invasion of North American forests by exotic earthworms is producing profound ecosystem changes, such as alterations in soil nutrient cycling, and redistribution and loss of soil organic matter. However, the present and future extent of these invasions is difficult to evaluate without a better understanding of the factors that control the distribution and abundance of earthworms in previously non-invaded habitats. In this study, the species composition and short-term dynamics of three exotic earthworm invasion fronts were studied at a northern hardwood forest in south-central New York State (USA). Belt transects were established at each of the three locations to sample from earthworm-invaded areas through transition zones and into invasion front areas. Lumbricus rubellus, L. terrestrisandOctolasion tyrtaeum were the most common species, but their distribution was not homogeneous along the transects. Whereas, L. rubellus was the only species with relatively high adult densities at transition zones and invasion fronts, L. terrestris and O. tyrtaeum occurred mostly in the heavily earthworm-invaded areas and were rare at the invasion fronts. The density of earthworms along the transects decreased by 60–87 from June 2001 to October 2002 and then recovered in 2003 to values similar to those of 2001. This decrease was apparently caused by reduced recruitment of immature earthworms, probably related to the severe drought periods that the study area experienced in 2001 and 2002. Our data suggest that climate and topography, through their effects on soil moisture patterns, can be critical factors controlling the distribution and spread of exotic earthworms in previously non-invaded habitats.     

Earthworm invasion into previously earthworm-free temperate and boreal forests

Earthworms are keystone detritivores that can influence primary producers by changing seedbed conditions, soil characteristics, flow of water, nutrients and carbon, and plant–herbivore interactions. The invasion of European earthworms into previously earthworm-free temperate and boreal forests of North America dominated by Acer, Quercus, Betula, Pinus and Populus has provided ample opportunity to observe how earthworms engineer ecosystems. Impacts vary with soil parent material, land use history, and assemblage of invading earthworm species. Earthworms reduce the thickness of organic layers, increase the bulk density of soils and incorporate litter and humus materials into deeper horizons of the soil profile, thereby affecting the whole soil food web and the above ground plant community. Mixing of organic and mineral materials turns mor into mull humus which significantly changes the distribution and community composition of the soil microflora and seedbed conditions for vascular plants. In some forests earthworm invasion leads to reduced availability and increased leaching of N and P in soil horizons where most fine roots are concentrated. Earthworms can contribute to a forest decline syndrome, and forest herbs in the genera Aralia, Botrychium, Osmorhiza, Trillium, Uvularia, and Viola are reduced in abundance during earthworm invasion. The degree of plant recovery after invasion varies greatly among sites and depends on complex interactions with soil processes and herbivores. These changes are likely to alter competitive relationships among plant species, possibly facilitating invasion of exotic plant species such as Rhamnus cathartica into North American forests, leading to as yet unknown changes in successional trajectory.     

Minerals in the rhizosphere: overlooked mediators of soil nitrogen availability to plants and microbes

Non-native earthworms are a continued source of environmental change in the northeastern United States that may affect trace metals in the plant-soil system, with largely unknown effects. We assessed earthworm impacts on exchangeable and strong acid extractable (total) concentrations and pools of Al, Fe, Cu, Zn, Mo, Pb in non-point source polluted, forest soil horizons (Organic, A, and B) and foliar metals concentrations in young (<?3 years) Acer saccharum and Polystichum acrostichoides at four proximal forests in the Finger Lakes Region of New York. We observed decreasing total trace metal Organic horizon pools and increasing total trace metal A horizon concentrations as a function of increasing earthworm biomass. Earthworms had limited effects on exchangeable concentrations in A and B horizons and total metal concentrations in the B horizon. Foliar trace metal concentrations in Acer were better explained by earthworm biomass than soil concentrations but foliar concentrations for Polystichum were poorly predicted by both earthworm biomass and soil metal concentrations. Our results suggest that earthworms can affect trace metal uptake by some plants, but not by increasing soil trace metal exchangeability or from changing soil properties (pH, %SOM, or cation exchange capacity). Instead, non-native earthworms may indirectly alter understory plant uptake of trace metals.     

Calcium Additions and Microbial Nitrogen Cycle Processes in a Northern Hardwood Forest

Evaluating, and possibly ameliorating, the effects of base cation depletion in forest soils caused by acid deposition is an important topic in the northeastern United States. We added 850 kg Ca ha⁻¹ as wollastonite (CaSiO₃) to an 11.8-ha watershed at the Hubbard Brook Experimental Forest (HBEF), a northern hardwood forest in New Hampshire, USA, in fall 1999 to replace calcium (Ca) leached from the ecosystem by acid deposition over the past 6 decades. Soil microbial biomass carbon (C) and nitrogen (N) concentrations, gross and potential net N mineralization and nitrification rates, soil solution and stream chemistry, soil:atmosphere trace gas (CO₂, N₂O, CH₄) fluxes, and foliar N concentrations have been monitored in the treated watershed and in reference areas at the HBEF before and since the Ca addition. We expected that rates of microbial C and N cycle processes would increase in response to the treatment. By 2000, soil pH was increased by a full unit in the Oie soil horizon, and by 2002 it was increased by nearly 0.5 units in the Oa soil horizon. However, there were declines in the N content of the microbial biomass, potential net and gross N mineralization rates, and soil inorganic N pools in the Oie horizon of the treated watershed. Stream, soil solution, and foliar concentrations of N showed no response to treatment. The lack of stimulation of N cycling by Ca addition suggests that microbes may not be stimulated by increased pH and Ca levels in the naturally acidic soils at the HBEF, or that other factors (for example, phosphorus, or Ca binding of labile organic matter) may constrain the capacity of microbes to respond to increased pH in the treated watershed. Possible fates for the approximately 10 kg N ha⁻¹ decline in microbial and soil inorganic pools include components of the plant community that we did not measure (for example, seedlings, understory shrubs), increased fluxes of N₂ and/or N storage in soil organic matter. These results raise questions about the factors regulating microbial biomass and activity in northern hardwood forests that should be considered in the context of proposals to mitigate the depletion of nutrient cations in soil.     

Ecosystem Consequences of Exotic Earthworm Invasion of North Temperate Forests

The invasion of north temperate forests by exotic species of earthworms is an important issue that has been overlooked in the study and management of these forests. We initiated research to address the hypothesis that earthworm invasion will have large consequences for nutrient retention and uptake in these ecosystems. In this special feature of Ecosystems, we present five papers describing results from our experiment. In this paper, we (a) introduce our experimental approach and conceptual model of how earthworms influence forest ecosystem processes, (b) describe the characteristics of the study areas and earthworm communities at our two study locations, and (c) provide a brief overview and synthesis of the main findings. The most dramatic effect of earthworm invasion was the loss of the forest floor at an undisturbed forest site, which altered the location and nature of nutrient cycling activity in the soil profile. Invasion changed soil total carbon (C) and phosphorus (P) pools, carbon–nitrogen (C:N) ratios, the loss and distribution of different soil P fractions, and the distribution and function of roots and microbes. Response to invasion varied with site characteristics and earthworm species. Our results suggest that exotic earthworm invasion is a significant factor that will influence the structure and function of northern temperate forest ecosystems over the next few decades. Regional evaluations of these forests will need to consider the presence or absence of earthworms along with other important ecosystem drivers, such as pollution, climate, and underlying soil characteristics.     

Influence of Earthworm Invasion on Redistribution and Retention of Soil Carbon and Nitrogen in Northern Temperate Forests

We analyzed soil organic matter distribution and soil solution chemistry in plots with and without earthworms at two sugar maple (Acer saccharum)–dominated forests in New York State, USA, with differing land-use histories to assess the influence of earthworm invasion on the retention or loss of soil carbon (C) and nitrogen (N) in northern temperate forests. Our objectives were to assess the influence of exotic earthworm invasion on (a) the amount and depth distribution of soil C and N, (b) soil ¹³C and ¹⁵N, and (c) soil solution chemistry and leaching of C and N in forests with different land-use histories. At a relatively undisturbed forest site (Arnot Forest), earthworms eliminated the thick forest floor, decreased soil C storage in the upper 12 cm by 28%, and reduced soil C:N ratios from 19.2 to 15.3. At a previously cultivated forest site with little forest floor (Tompkins Farm), earthworms did not influence the storage of soil C or N or soil C:N ratios. Earthworms altered the stable isotopic signature of soil at Arnot Forest but not at Tompkins Farm; the alteration of stable isotopes indicated that earthworms significantly increased the loss of forest floor C but not N from the soil profile at Arnot Forest. Nitrate (NO₃^–) concentrations in tension and zero-tension lysimeters were much greater at Tompkins Farm than Arnot Forest, and earthworms increased NO₃^– leaching at Tompkins Farm. The results suggest that the effect of earthworm invasion on the distribution, retention, and solution chemistry of soil C and N in northern temperate forests may depend on the initial quantity and quality of soil organic matter at invaded sites.     

Earthworm invasions of ecosystems devoid of earthworms: effects on soil microbes

Recent studies document North American earthworm invasions and their profound effects on the structure of the soil profile, which is the habitat for soil microorganisms (mainly fungi and bacteria). Dramatic alterations made to these layers during earthworm invasion significantly change microbial community structure and therefore microbial activities such as C transformations. Understanding the impacts of earthworm invasion on the microbes themselves will give insight into earthworm effects on microbial activities. Bacterial and actinomycete communities in earthworm guts and casts have not been studied in environments recently invaded by earthworms. Earthworm invasion tended to decrease fungal species density and fungal species diversity and richness. The presence of earthworms decreased zygomycete species abundance probably due to disruption of fungal hyphae. Physical disruption of hyphae may also explain decreased mycorrhizal colonization rates, decreased mycorrhizal abundance and altered mycorrhizal morphology in the presence of earthworms. Mixing of organic layers into mineral soil during earthworm invasion tended to decrease microbial biomass in forest floor materials while increasing it in mineral soil. In newly invaded forest soils, microbial respiration and the metabolic quotient tended to decline. In forests where either the microbial community has had time to adapt to earthworm activities, or where the destruction of the forest floor is complete, as in invasions by the Asian Amynthas hawayanus, the presence of earthworms tends to increase the metabolic quotient indicating a shift to a smaller, more active microbial community.     

Forest Invasibility in Communities in Southeastern New York

While biological invasions have been the subject of considerable attention both historically and recently, the factors controlling the susceptibility of communities to plant invasions remain controversial. We surveyed 44 sites in southeastern New York State to examine the relationships between plant community characteristics, soil characteristics, and nonnative plant invasion. Soil nitrogen mineralization and nitrification rates were strongly related to the degree of site invasion (F= 30.2, P < 0.0001 and F= 11.8, P < 0.005, respectively), and leaf C : N ratios were negatively correlated with invasion (R ²= 0.22, P < 0.0001). More surprisingly, there was a strong positive relationship between soil calcium levels and the degree of site invasion (partial r= 0.70, P < 0.01), and there were also positive relationships between invasion and soil magnesium and phosphorus. We found, in addition, a positive factor-ceiling relationship between native species diversity and invasive species diversity. This positive relationship between native and invasive diversity contradicts earlier hypotheses concerning the relationships between species diversity and invasion, but supports some recent findings. Cluster analysis distinguished two broad forest community types at our sites: pine barrens and mixed hardwood communities. Invaders were significantly more abundant in mixed hardwood than in pine barrens communities (Mann–Whitney U = 682.5, P < 0.0001). Even when evaluating the mixed hardwood communities alone, invasion remained significantly positively correlated with soil fertility (calcium, magnesium, and net nitrogen mineralization rates). Soil texture and pH were not useful predictors of the degree to which forests were invaded. Nitrogen and calcium are critical components of plant development, and species better able to take advantage of increased nutrient availability may out-perform others at sites with higher nutrient levels. These results have implications for areas such as the eastern United States, where anthropogenic changes in the availability of nitrogen and calcium are affecting many plant communities.     

Effects of position, understorey vegetation and coarse woody debris on tree regeneration in two environmentally contrasting forests of north-western Patagonia: a manipulative approach

Aim To investigate the differential effects of position within gaps, coarse woody debris and understorey cover on tree seedling survival in canopy gaps in two old‐growth Nothofagus pumilio (Poepp. & Endl.) Krasser forests and the response of this species to gaps in two forests located at opposite extremes of a steep rainfall gradient. Location Nahuel Huapi National Park, at 41° S in north‐western Patagonia, Argentina. Methods In both study sites, seedlings were transplanted to experimental plots in gaps in three different positions, with two types of substrate (coarse woody debris or forest floor), and with and without removal of understorey vegetation. Survival of seedlings was monitored during two growing seasons. Soil moisture and direct solar radiation were measured once in mid‐summer. Seedling aerial biomass was estimated at the end of the experiment. Results Mid‐summer soil water potential was lowest in the centre of gaps, in plots where the understorey had been removed, and highest at the northern edges of gaps. Direct incoming radiation was highest in gap centres and southern edges, and lowest at northern edges. Seedling mortality was highest in gap centres, in both sites. Coarse woody debris had a positive effect on seedling survival during summer in the mesic forest and during winter in the xeric forest. The removal of understorey cover had negative effects in gap centres during summer. Seedling final aerial biomass was positively affected by understorey removal and by soil substrate in both sites. In the dry forest gaps, seedling growth was highest in northern edges, whereas it was highest in gap centres in the mesic forest. Overall growth was positively related to survival in the xeric forest, and negatively related in the mesic forest. Main conclusions Survival and growth were facilitated by the shade of gap‐surrounding trees only in the xeric forest. Understorey vegetation of both forests facilitated seedling survival in exposed microsites but competed with seedling growth. Nurse logs were an important substrate for seedling establishment in both forests; however, causes of this pattern differed between forests. Water availability positively controls seedling survival and growth in the xeric forest while in the mesic forest, survival and growth are differentially controlled by water and light availability, respectively. These two contrasting old‐growth forests, separated by a relatively short distance along a steep rainfall gradient, had different yet unexpected microenvironmental controls on N. pumilio seedling survival and growth. These results underscore the importance of defining microscale limiting factors of tree recruitment in the context of large‐scale spatial variation in resources.     

Nitrogen Limits an Invasive Perennial Shrub in Forest Understory

Plant invasions can harm communities by domination of one or more vegetation layers. We studied whether Japanese barberry (Berberis thunbergii DC.) is limited by soil acidity or nitrogen availability in its domination of relatively undisturbed forest understories. In two sites, one more acid than the other, we applied lime, urea, or a sawdust–sugar mix to replicate plots in established barberry populations. We predicted that the acid site would be pH or cation limited, while the less acid site was N limited, unless N availability was inherently higher before treatment. Barberry above-ground net primary production (NPP) was estimated by a combination of harvest and allometric analysis. Foliar N increased in the urea treatment and was proportional to incubation estimates of net N mineralization and nitrification. Foliar Ca and P were unaffected by the treatments. Foliar K was proportional to foliar N. The more acid site had higher foliar Mn, but otherwise the sites differed little. Barberry NPP was proportional to pre-treatment biomass. The ratio of net production to pre-treatment woody biomass (relative production rate) increased with foliar N and soil N availability and decreased when soil N was immobilized by sawdust and sugar. There was no effect of soil pH or cation status on barberry growth, although a correlation with foliar K was reflected by the maintenance of a constant K : N ratio. Although more severely acid sites may be less invasible than those studied here, N availability is the primary limitation to invasive dominance in this landscape. This revised version was published online in July 2006 with corrections to the Cover Date.     

The wave towards a new steady state: effects of earthworm invasion on soil microbial functions

Earthworms are ecosystem engineers that cause a long cascade of ecological effects when they invade previously earthworm-free forests. However, the consequences of earthworm invasion for soil microbial functions are poorly understood. Here, we used two well-studied invasion fronts of European earthworms in northern North American hardwood forests previously devoid of earthworms in order to investigate three stages of earthworm invasion: uninvaded, the front of the leading edge of earthworm invasion and locations invaded at least 10 years previously. Soil microbial biomass, respiration and metabolic quotient were measured. Earthworms had marked effects on soil microbial biomass (−42%) and respiration (−32%). At both sites, impacts were most pronounced at the leading edge of the invasion front, significantly decreasing soil microbial C use efficiency. This was most likely due to the disturbance of the soil microbial community caused by water stress. Based on these results, we hypothesize that effects of earthworm invasion on native soil ecosystem functioning are most pronounced at the peak of the invasion wave. After experiencing this wave, ecosystems possibly enter a new steady state with altered biotic compositions and functions.     

Earthworms, litter and soil carbon in a northern hardwood forest

The important role of soil carbon (C) in the global C cycle has stimulated interest in better understanding the mechanisms regulating soil C storage and its stabilization. Exotic earthworm invasion of northern forest soils in North America can affect soil C pools, and we examined their effects on these mechanisms by adding ¹³C labeled leaf litter to adjacent northern hardwood forests with and without earthworms. Two types of labeled litter were produced, one with the ¹³C more concentrated in structural (S) components and the other in non-structural (NS) components, to evaluate the role of biochemical differences in soil C stabilization. Earthworm invasions have reduced soil C storage in the upper 20 cm of the soil profile by 37 %, mostly by eliminating surface organic horizons. Despite rapid mixing of litter into mineral soil and its incorporation into aggregates, mineral soil C has not increased in the presence of earthworms. Incorporation of litter C into soil and microbial biomass was not affected by biochemical differences between S versus NS labeled litter although NS litter C was assimilated more readily into earthworm biomass and S litter C into fungal hyphae. Apparently, the net effect of earthworm mixing of litter and forest floor C into mineral soil, plus stabilization of that C in aggregates, is counterbalanced by earthworm bioturbation and possible priming effects. Our results support recent arguments that biochemical recalcitrance is not a major contributor to the stabilization of soil C.     

Simulated Nitrogen Deposition Causes a Decline of Intra- and Extraradical Abundance of Arbuscular Mycorrhizal Fungi and Changes in Microbial Community Structure in Northern Hardwood Forests

Increased nitrogen (N) deposition caused by human activities has altered ecosystem functioning and biodiversity. To understand the effects of altered N availability, we measured the abundance of arbuscular mycorrhizal fungi (AMF) and the microbial community in northern hardwood forests exposed to long-term (12 years) simulated N deposition (30 kg N ha⁻¹ y⁻¹) using phospholipid fatty acid (PLFA) analysis and hyphal in-growth bags. Intra- and extraradical AMF biomass and total microbial biomass were significantly decreased by simulated N deposition by 36, 41, and 24%, respectively. Both methods of extraradical AMF biomass estimation (soil PLFA 16:1ω5c and hyphal in-growth bags) showed comparable treatment responses, and extraradical biomass represented the majority of total (intra-plus extraradical) AMF biomass. N deposition also significantly affected the microbial community structure, leading to a 10% decrease in fungal to bacterial biomass ratios. Our observed decline in AMF and total microbial biomass together with changes in microbial community structure could have substantial impacts on the nutrient and carbon cycling within northern hardwood forest ecosystems.     

Morella cerifera invasion and nitrogen cycling on a lowland Hawaiian lava flow

Invasive plants that fix nitrogen can alter nutrient availability and thereby community dynamics and successional trajectories of native communities they colonize. Morella cerifera (Myricaceae) is a symbiotic nitrogen fixer originally from the southeastern U.S. that is colonizing native-dominated vegetation on a young lava flow near Hilo, Island of Hawai‘i, where it increases total and biologically available soil nitrogen and increases foliar nitrogen concentrations in associated individuals of the native tree Metrosideros polymorpha. This invasion has the potential to alter the few remaining native-dominated lowland forest ecosystems in windward Hawai‘i.     

Urbanization and fragmentation have opposing effects on soil nitrogen availability in temperate forest ecosystems

Nitrogen (N) availability relative to plant demand has been declining in recent years in terrestrial ecosystems throughout the world, a phenomenon known as N oligotrophication. The temperate forests of the northeastern U.S. have experienced a particularly steep decline in bioavailable N, which is expected to be exacerbated by climate change. This region has also experienced rapid urban expansion in recent decades that leads to forest fragmentation, and it is unknown whether and how these changes affect N availability and uptake by forest trees. Many studies have examined the impact of either urbanization or forest fragmentation on nitrogen (N) cycling, but none to our knowledge have focused on the combined effects of these co-occurring environmental changes. We examined the effects of urbanization and fragmentation on oak-dominated (Quercus spp.) forests along an urban to rural gradient from Boston to central Massachusetts (MA). At eight study sites along the urbanization gradient, plant and soil measurements were made along a 90 m transect from a developed edge to an intact forest interior. Rates of net ammonification, net mineralization, and foliar N concentrations were significantly higher in urban than rural sites, while net nitrification and foliar C:N were not different between urban and rural forests. At urban sites, foliar N and net ammonification and mineralization were higher at forest interiors compared to edges, while net nitrification and foliar C:N were higher at rural forest edges than interiors. These results indicate that urban forests in the northeastern U.S. have greater soil N availability and N uptake by trees compared to rural forests, counteracting the trend for widespread N oligotrophication in temperate forests around the globe. Such increases in available N are diminished at forest edges, however, demonstrating that forest fragmentation has the opposite effect of urbanization on coupled N availability and demand by trees.     

Impacts of moso bamboo (Phyllostachys pubescens) invasion on dry matter and carbon and nitrogen stocks in a broad‐leaved secondary forest located in Kyoto,western Japan

In western and central Japan, the expansion of exotic moso bamboo (Phyllostachys pubescens Mazel ex J. Houz.) populations into neighboring vegetation has become a serious problem. Although the effects of bamboo invasion on biodiversity have been well studied, shifts in nutrient stocks and cycling, which are fundamental for ecosystem functioning, are not fully understood. To explore the effects of P. pubescens invasion on ecosystem functions we examined above‐ and below‐ground dry matter and carbon (C) and nitrogen (N) stocks in a pure broad‐leaved tree stand, a pure bamboo stand, and two tree–bamboo mixed stands with different vegetation mix ratios in the secondary forest of Kyoto, western Japan. In the process of invasion, bamboo shoots offset broad‐leaved tree deaths; thus, no clear trend was apparent in total above‐ or below‐ground biomass or in plant C and N stocks during invasion. However, the ratio of above‐ground to below‐ground biomass (T/R ratio at the stand level) decreased with increasing bamboo dominance, especially in the early stages of invasion. This shift indicates that rapid bamboo rhizomatous growth is a main driver of substantial changes in stand structure. We also detected rises in the C/N ratio of forest‐floor organic matter during bamboo invasion. Thus major impacts of P. pubescens invasion into broad‐leaved forests include not only early shifts in biomass allocation, but also changes in the distribution pattern of C and N stored in plants and soil.     

The effects of gap disturbance on nitrogen cycling and retention in late-successional northern hardwood–hemlock forests

Late-successional forests in the upper Great Lakes region are susceptible to nitrogen (N) saturation and subsequent nitrate (NO₃⁻) leaching loss. Endemic wind disturbances (i.e., treefall gaps) alter tree uptake and soil N dynamics; and, gaps are particular susceptible to NO₃⁻ leaching loss. Inorganic N was measured throughout two snow-free periods in throughfall, forest floor leachates, and mineral soil leachates in gaps (300–2,000 m², 6–9 years old), gap-edges, and closed forest plots in late-successional northern hardwood, hemlock, and northern hardwood–hemlock stands. Differences in forest water inorganic N among gaps, edges, and closed forest plots were consistent across these cover types: NO₃⁻ inputs in throughfall were significantly greater in undisturbed forest plots compared with gaps and edges; forest floor leachate NO₃⁻ was significantly greater in gaps compared to edges and closed forest plots; and soil leachate NO₃⁻ was significantly greater in gaps compared to the closed forest. Significant differences in forest water ammonium and pH were not detected. Compared to suspected N-saturated forests with high soil NO₃⁻ leaching, undisturbed forest plots in these late-successional forests are not losing NO₃⁻ (net annual gain of 2.8 kg ha⁻¹) and are likely not N-saturated. Net annual NO₃⁻ losses were observed in gaps (1.3 kg ha⁻¹) and gap-edges (0.2 kg ha⁻¹), but we suspect these N leaching losses are a result of decreased plant uptake and increased soil N mineralization associated with disturbance, and not N-saturation.     

Woody Plant Encroachment by <Emphasis Type="Italic">Juniperus virginiana</Emphasis> in a Mesic Native Grassland Promotes Rapid Carbon and Nitrogen Accrual

The cover and abundance of Juniperus virginiana L. in the U.S. Central Plains are rapidly increasing, largely as a result of changing land-use practices that alter fire regimes in native grassland communities. Little is known about how conversion of native grasslands to Juniperus-dominated forests alters soil nutrient availability and ecosystem storage of carbon (C) and nitrogen (N), although such land-cover changes have important implications for local ecosystem dynamics, as well as regional C and N budgets. Four replicate native grasslands and adjacent areas of recent J. virginiana encroachment were selected to assess potential changes in soil N availability, leaf-level photosynthesis, and major ecosystem C and N pools. Net N mineralization rates were assessed in situ over two years, and changes in labile soil organic pools (potential C and N mineralization rates and microbial biomass C and N) were determined. Photosynthetic nitrogen use efficiencies (PNUE) were used to examine differences in instantaneous leaf-level N use in C uptake. Comparisons of ecosystem C and N stocks revealed significant C and N accrual in both plant biomass and soils in these newly established forests, without changes in labile soil N pools. There were few differences in monthly in situ net N mineralization rates, although cumulative annual net N mineralization was greater in forest soils compared to grasslands. Conversely, potential C mineralization was significantly reduced in forest soils. Encroachment by J. virginiana into grasslands results in rapid accretion of ecosystem C and N in plant and soil pools with little apparent change in N availability. Widespread increases in the cover of woody plants, like J. virginiana, in areas formerly dominated by graminoid species suggest an increasing role of expanding woodlands and forests as regional C sinks in the central U.S.