Computation and visualization of regional-scale forest disturbance and associated dissolved nitrogen export from Shenandoah National Park,Virginia

Long-term watershed research conducted in Shenandoah National Park (SNP) in Virginia and elsewhere in the eastern U.S. indicates that annual export of dissolved nitrogen (N) from gaged forested watersheds to surface waters increases dramatically in response to vegetation disturbances. Dissolved N leakage is a common, well-documented response of small forested watersheds to logging in the larger region, while recent defoliation outbreaks of the gypsy moth ( Lymantria dispar) larva in the deciduous forests of SNP have been shown to generate similar biogeochemical responses. A recent modeling analysis further suggests that a parsimonious, empirical, unit N export response function (UNERF) model can explain large percentages of the temporal variation in annual N export from a group of small gaged forested watersheds in the years following disturbance. The empirical UNERF modeling approach is completely analogous to the unit hydrograph technique for describing storm runoff, with the model representing annual N export as a linear deterministic process both in space and in time. The purposes of this analysis are to (1) test the applicability of the UNERF model using quarterly streamwater nitrate data from a group of ungaged watersheds in SNP; (2) demonstrate a park-wide application of a regional UNERF model that references the geographic distributions of bedrock geology and the timing and extent of gypsy moth defoliation over the entire SNP area; and (3) visualize the temporal and spatial patterns in vegetation disturbance and annual dissolved N export through the use of computer animation software. During water year 1992, the year of peak defoliation, our modeling study suggests that park-wide export had transiently increased by 1700% from a baseline rate of about 0.1 kg/ha/year. SNP forests appear to be characteristic of other N-limited second-growth forests in the eastern U.S. that leak little N under undisturbed conditions, despite receiving relatively large inputs of N from atmospheric deposition sources. Vegetation disturbances can apparently cause major changes in N input-output balances with potentially important ramifications for low-order forest streams and downstream receiving waters.     

Evidence for a Regime Shift in Nitrogen Export from a Forested Watershed

In this study, we document a functional regime shift in stream inorganic nitrogen (N) processing indicated by a major change in N export from a forested watershed. Evidence from 36 years of data following experimental clearcut logging at Coweeta Hydrologic Laboratory, NC, suggests that forest disturbance in this area can cause elevation of dissolved inorganic N (DIN) loss lasting decades or perhaps longer. This elevation of N export was apparently caused by an initial pulse of organic matter input, reduced vegetation uptake, increased mineralization of soil organic N, and N fixation by black locust-associated bacteria following clearcut logging. In forested reference watersheds at Coweeta, maximum DIN concentration occurs in summer when base flow is low, but the clearcut watershed shifted to a pattern of maximum winter DIN concentration. The seasonal pattern of DIN concentration and export from reference watersheds can be explained by terrestrial and in-stream processes, but following clearcutting, elevated DIN availability saturated both terrestrial and in-stream uptake, and the N export regime became dominated by hydrologic transport. We suggest that the long-term elevation of stream DIN concentration and export along with the changes in seasonality of DIN export and the relationship between concentration and discharge represent a functional regime shift initiated by forest disturbance.     

Impact of a first-order riparian zone on nitrogen removal and export from an agricultural ecosystem

Riparian zones are reputed to be effective at preventing export of agricultural groundwater nitrogen (N) from local ecosystems. This is one impetus behind riparian zone regulations and initiatives. However, riparian zone function can vary under different conditions, with varying impacts on the regional (and ultimately global) environment. Rates of groundwater delivery to the surface appear to have significant effects on the N-removing capabilities of a riparian zone. Research conducted at a first-order agricultural watershed with a well-defined riparian zone in the Maryland coastal plain indicates that more than 2.5 kg/day of nitrate-N can be exported under moderate-to-high stream baseflow conditions. The total nitrate-N load that exits the system increases with increasing flow not simply because of the greater volume of water export. Stream water nitrate-N concentrations also increase by more than an order of magnitude as flow increases, at least during baseflow. This appears to be largely the result of changes in dominant groundwater delivery mechanisms. Higher rates of groundwater exfiltration lessen the contact time between nitrate-carrying groundwater and potentially reducing riparian soils. Subsurface preferential flow paths, in the wetland and adjacent field, also strongly influence N removal. Simple assumptions regarding riparian zone function may be inadequate because of complexities observed in response to changing hydrologic conditions.     

Mapping forest-cover disturbances in Papua New Guinea with AVHRR data

Abstract. Land management and land-use planning in a forested country such as Papua New Guinea, which is subject to various anthropogenic pressures, requires an accurate mapping of forest-cover disturbances. The central hypothesis of this study was that remote sensing indicators of forest-cover conditions can be used to measure and map the impact of long-term forest-cover disturbances. This was tested with single year NOAA's Advanced Very-High Resolution Radiometer sensor (AVHRR) data at 1.1 km resolution. First, an ordinal scale of forest-cover disturbance was defined from field observations, a set of thematic maps and high spatial resolution satellite sensor data. Secondly, we analysed the relationship between the forest-cover disturbance scale and several biophysical indicators derived from AVHRR data at two seasons. Thirdly, a statistical analysis identified the optimal combination of biophysical indicators and observation dates to discriminate between the forest disturbance levels defined previously. A forest-cover disturbance map was then produced over part of Papua New Guinea. Finally, a regionalization of the study area in terms of aggregate degree of disturbance was produced and the spatial patterns of forest disturbances were interpreted for each region in terms of broad processes of forest-cover change. The overall accuracy of the forest-cover disturbance map was 79%. Nine regions, homogeneous in regard to the distribution and spatial pattern of disturbance categories, were identified.     

Seasonal Variations of Dissolved Nitrogen and DOC:DON Ratios in an Intermittent Mediterranean Stream

Seasonal variations of dissolved inorganic nitrogen (DIN) (NO₃–N and NH₄–N) and dissolved organic nitrogen (DON) were determined in Fuirosos, an intermittent stream draining an unpolluted Mediterranean forested catchment (10.5 km²) in Catalonia (Spain). The influence of flow on streamwater concentrations and seasonal differences in quality and origin of dissolved organic matter, inferred from dissolved organic carbon to nitrogen ratios (DOC:DON ratios), were examined. During baseflow conditions, nitrate and ammonium had opposite behaviour, probably controlled by biological processes such as vegetation uptake and mineralization activity. DON concentrations did not have a seasonal trend. During storms, nitrate and DON increased by several times but discharge was not a good predictor of nutrient concentrations. DOC:DON ratios in streamwater were around 26, except during the months following drought when DOC:DON ratios ranged between 42 and 20 during baseflow and stormflow conditions, respectively. Annual N export during 2000–2001 was 70 kg km⁻¹ year⁻¹, of which 75% was delivered during stormflow. The relative contribution of nitrogen forms to the total annual export was 57, 35 and 8% as NO₃–N, DON and NH₄–N, respectively.     

Landscape Controls on Organic and Inorganic Nitrogen Leaching across an Alpine/Subalpine Ecotone,Green Lakes Valley,Colorado Front Range

Here we report measurements of organic and inorganic nitrogen (N) fluxes from the high-elevation Green Lakes Valley catchment in the Colorado Front Range for two snowmelt seasons (1998 and 1999). Surface water and soil samples were collected along an elevational gradient extending from the lightly vegetated alpine to the forested subalpine to assess how changes in land cover and basin area affect yields and concentrations of ammonium-N (NH₄-N), nitrate-N (NO₃-N), dissolved organic N (DON), and particulate organic N (PON). Streamwater yields of NO₃-N decreased downstream from 4.3 kg ha⁻¹ in the alpine to 0.75 kg ha⁻¹ at treeline, while yields of DON were much less variable (0.40–0.34 kg ha⁻¹). Yields of NH₄-N and PON were low and showed little variation with basin area. NO₃-N accounted for 40%–90% of total N along the sample transect and was the dominant form of N at all but the lowest elevation site. Concentrations of DON ranged from approximately 10% of total N in the alpine to 45% in the subalpine. For all sites, volume-weighted mean concentrations of total dissolved nitrogen (TDN) were significantly related to the DIN:DON ratio (R ² = 0.81, P < 0.001) Concentrations of NO₃-N were significantly higher at forested sites that received streamflow from the lightly vegetated alpine reaches of the catchment than in a control catchment that was entirely subalpine forest, suggesting that the alpine may subsidize downstream forested systems with inorganic N. KCl-extractable inorganic N and microbial biomass N showed no relationship to changes in soil properties and vegetative cover moving downstream in catchment. In contrast, soil carbon–nitrogen (C:N) ratios increased with increasing vegetative cover in catchment and were significantly higher in the subalpine compared to the alpine (P < 0.0001) Soil C:N ratios along the sample transect explained 78% of the variation in dissolved organic carbon (DOC) concentrations and 70% of the variation in DON concentrations. These findings suggest that DON is an important vector for N loss in high-elevation ecosystems and that streamwater losses of DON are at least partially dependent on catchment soil organic matter stoichiometry. Received 26 July 2001; accepted 6 May 2002.     

Effects of Succession on Nitrogen Export in the West-Central Cascades, Oregon

This study examined impacts of succession on N export from 20 headwater stream systems in the west central Cascades of Oregon, a region of low anthropogenic N inputs. The seasonal and successional patterns of nitrate (NO₃−N) concentrations drove differences in total dissolved N concentrations because ammonium (NH₄−N) concentrations were very low (usually < 0.005 mg L⁻¹) and mean dissolved organic nitrogen (DON) concentrations were less variable than nitrate concentrations. In contrast to studies suggesting that DON levels strongly dominate in pristine watersheds, DON accounted for 24, 52, and 51% of the overall mean TDN concentration of our young (defined as predominantly in stand initiation and stem exclusion phases), middle-aged (defined as mixes of mostly understory reinitiation and older phases) and old-growth watersheds, respectively. Although other studies of cutting in unpolluted forests have suggested a harvest effect lasting 5 years or less, our young successional watersheds that were all older than 10 years still lost significantly more N, primarily as NO₃−N, than did watersheds containing more mature forests, even though all forest floor and mineral soil C:N ratios were well above levels reported in the literature for leaching of dissolved inorganic nitrogen. The influence of alder may contribute to these patterns, although hardwood cover was quite low in all watersheds; it is possible that in forested ecosystems with very low anthropogenic N inputs, even very low alder cover in riparian zones can cause elevated N exports. Only the youngest watersheds, with the highest nitrate losses, exhibited seasonal patterns of increased summer uptake by vegetation as well as flushing at the onset of fall freshets. Older watersheds with lower N losses did not exhibit seasonal patterns for any N species. The results, taken together, suggest a role for both vegetation and hydrology in N retention and loss, and add to our understanding of N cycling by successional forest ecosystems influenced by disturbance at various spatial and temporal scales in a region of relatively low anthropogenic N input.     

Nitrogen accumulation in forests. Exposure monitoring by mosses

At present, there is still little information on nitrogen (N) accumulation in forests contrasting with the crucial importance of N in forest ecosystems. This work analyzes the N bioaccumulation in mosses from forested areas from Lower Saxony and North Rhine-Westphalia (two of 16 federal states of Germany), the Weser Ems Region (part of Lower Saxony), and the Euro Region Nissa (covering the Czech Republic, Germany, Poland). The studies involved samples collected from 190 sites between 1998 and 2005. Different spatial scales and regional differences in land use were chosen to assess the factors affecting N bioaccumulation in forested areas. A continuous reduction of N bioaccumulation was found from Lower Saxony (a region where agriculture is most predominant) to North Rhine-Westphalia (mostly urban). The Weser Ems Region (an agricultural region) showed a higher N concentration in mosses than the Euroregion Nissa (a former industrial region). Statistical analyses performed at the different spatial scales revealed that the areas showing greater agricultural and livestock spatial densities favor N bioaccumulation in mosses. N concentration in mosses was moderately correlated with the N concentration in the leaves and needles of the surrounding trees. No significant relationships were found regarding the crown density of forest trees or N deposition estimations from a combination of atmospheric models and deposition measurements.     

Inorganic Nitrogen Losses from a Forested Ecosystem in Responseto Physical,Chemical, Biotic,and Climatic Perturbations

Nitrate leaching to streams is a sensitive indicator of the biogeochemical status of forest ecosystems. Two primary theories predicting long-term (decadal) changes in nitrate loss rates (N saturation theory and the nutrient retention hypothesis) both predict increasing dissolved inorganic nitrogen (DIN) losses for watershed 6 (W6), the biogeochemical reference watershed at the Hubbard Brook Experimental Forest (HBEF). Measured values, however, have declined substantially since measurements began in the mid-1960s. Are these theories wrong, or are there other important controls on DIN losses at the annual to decadal time scale that have obscured the tendency toward higher losses over time? We tested the individual and combined effects of several forms of disturbance on DIN loss rates from northern hardwood forests by comparing predictions from a relatively simple model of forest carbon, nitrogen, and water dynamics (PnET-CN) with the long-term record of annual DIN loss from W6 at HBEF. Perturbations tested include interannual climate variation, changes in atmospheric chemistry (CO₂, O₃, N deposition), and physical and biotic disturbances (two harvests, a hurricane salvage, and a defoliation event). No single disturbance caused changes in DIN losses to mimic measured values. Only when run with all of the disturbances combined did the model-predicted pattern of interannual change in DIN loss approach the measured record. Single-disturbance simulations allow an estimation of the role of each in the total pattern of DIN loss. We conclude that DIN losses from W6 were elevated in the 1960s by a combination of recovery from extreme drought and a significant defoliation event. N deposition alone, in the absence of other disturbances, would have increased DIN losses by 0.35 g N m^?2y^?1. These findings indicate that predictions of DIN losses must take into account the full spectrum of disturbance events and changes in environmental conditions impacting the systems examined.     

哀牢山中山湿性常绿阔叶林不同干扰强度下土壤无机氮的变化

在云南哀牢山中山湿性常绿阔叶林地区,选取了木果柯原始林、栎类次生林和人工茶叶地3种群落类型代表人为干扰强度从小到大的梯度,研究了人为干扰强度对土壤NH4^+—N、NO3^-—N等特征的影响．结果表明,3种群落的土壤无机氮含量(0～15cm)存在显著差异：表现为随干扰强度增加,土壤有机质、全N降低,C／N比增高,NO3^-—N流失的潜力在增加,说明干扰不利于土壤肥力的保持和群落正向演替．同一群落类型下不同空间位置土壤的有机质、全N、C／N比、pH值和NH4^+—N基本一致,但NO3^-—N有较大变化,表明土壤中NO3^-—N的不稳定性．此外,NO4^+—N为无机氮的主要存在形式,约占无机氮总量的95.5％～99.2％     

Nitrogen exports at multiple-scales in a southern Chilean watershed (Patagonian Lakes district)

We evaluated nitrogen (N) export for various catchments in the San Pedro River watershed of South-central Chile (39°20′ to 40°12′S) during the dry season (February to March). We measured concentrations and export of the various N species at 16 points from the Andean headwaters to the lowland portion of the watershed: eight main nested points along the main watershed and eight secondary points on tributaries. We expected that, given a downstream increase in pastureland and decrease in native pristine forest cover, inorganic forms of N (DIN) would increase downstream, while conversely, dissolved organic nitrogen (DON) would decrease compared with concentrations in the forested headwaters. Nitrogen concentrations did not show statistically significant differences among the nested catchments. However, there were statistically significant differences in N concentrations associated with land cover among the tributaries. The results suggest that in the presence of base flow, natural landscape properties (barren land, lakes and rivers), explained most of the spatial variation in the N exports, while anthropogenic disturbance was not detectable. There was a negative relationship between DIN export and the coverage of lakes and rivers, suggesting that lakes might be acting as N traps. On the other hand, DIN, DON and total N exports were positively associated to barren land. Total nitrogen export during this 60-day dry season was less than 20 kg km⁻² and the annual export was not larger than 100 kg km⁻². This study documents the as yet pristine conditions of rivers in southern Chile.     

Influence of nutrients,disturbances and site conditions on carbon stocks along a boreal forest transect in central Canada

The interacting influence of disturbances and nutrient dynamics on aboveground biomass, forest floor, and mineral soil C stocks was assessed as part of the Boreal Forest Transect Case Study in central Canada. This transect covers a range of forested biomes–-from transitional grasslands (aspen parkland) in the south, through boreal forests, and into the forested subarctic woodland in the north. The dominant forest vegetation species are aspen, jack pine and spruce. Disturbances influence biomass C stocks in boreal forests by determining its age-class structure, altering nutrient dynamics, and changing the total nutrient reserves of the stand. Nitrogen is generally the limiting nutrient in these systems, and N availability determines biomass C stocks by affecting the forest dynamics (growth rates and site carrying capacity) throughout the life cycle of a forest stand. At a given site, total and available soil N are determined both by biotic factors (such as vegetation type and associated detritus pools) and abiotic factors (such as N deposition, soil texture, and drainage). Increasing clay content, lower temperatures and reduced aeration are expected to lead to reduced N mineralization and, ultimately, lower N availability and reduced forest productivity. Forest floor and mineral soil C stocks vary with changing balances between complex sets of organic carbon inputs and outputs. The changes in forest floor and mineral soil C pools at a given site, however, are strongly related to the historical changes in biomass at that site. Changes in N availability alter the processes regulating both inputs and outputs of carbon to soil stocks. N availability in turn is shaped by past disturbance history, litter fall rate, site characteristics and climatic factors. Thus, understanding the life-cycle dynamics of C and N as determined by age-class structure (disturbances) is essential for quantifying past changes in forest level C stocks and for projecting their future change.     

Deforestation of watersheds of Panama: nutrient retention and export to streams

A series of eight watersheds on the Pacific coast of Panama where conversion of mature lowland wet forest to pastures by artisanal burning provided watershed-scale experimental units with a wide range of forest cover (23, 29, 47, 56, 66, 73, 73, 91, and 92 %). We used these watersheds as a landscape-scale experiment to assess effects of degree of deforestation on within-watershed retention and hydrological export of atmospheric inputs of nutrients. Retention was estimated by comparing rainfall nutrient concentrations (volume-weighted to allow for evapotranspiration) to concentrations in freshwater reaches of receiving streams. Retention of rain-derived nutrients in these Panama watersheds averaged 77, 85, 80, and 62 % for nitrate, ammonium, dissolved organic N, and phosphate, respectively. Retention of rain-derived inorganic nitrogen, however, depended on watershed cover: retention of nitrate and ammonium in pasture-dominated watersheds was 95 and 98 %, while fully forested watersheds retained 65 and 80 % of atmospheric nitrate and ammonium inputs. Watershed forest cover did not affect retention of dissolved organic nitrogen and phosphate. Exports from more forested watersheds yielded DIN/P near 16, while pasture-dominated watersheds exported N/P near 2. The differences in magnitude of exports and ratios suggest that deforestation in these Panamanian forests results in exports that affect growth of plants and algae in the receiving stream and estuarine ecosystems. Watershed retention of dissolved inorganic nitrogen calculated from wet plus dry atmospheric deposition varied from 90 % in pasture- to 65 % in forest-dominated watersheds, respectively. Discharges of DIN to receiving waters from the watersheds therefore rose from 10 % of atmospheric inputs for pasture-dominated watersheds, to about 35 % of atmospheric inputs for fully forested watersheds. These results from watersheds with no agriculture or urbanization, but different conversion of forest to pasture by burning, show significant, deforestation-dependent retention within tropical watersheds, but also ecologically significant, and deforestation-dependent, exports that are biologically significant because of the paucity of nutrients in receiving tropical stream and coastal waters.     

Modelling Spatial Interactions Among Fire,Spruce Budworm,and Logging in the Boreal Forest

In the boreal forest, fire, insects, and logging all affect spatial patterns in forest age and species composition. In turn, spatial legacies in age and composition can facilitate or constrain further disturbances and have important consequences for forest spatial structure and sustainability. However, the complex three-way interactions among fire, insects, and logging and their combined effects on forest spatial structure have seldom been investigated. We used a spatially explicit landscape simulation model to examine these interactions. Specifically, we investigated how the amount and the spatial scale of logging (cutblock size) in combination with succession, fire, and spruce budworm outbreaks affect area burned and area defoliated. Simulations included 30 replicates of 300 years for each of 19 different disturbance scenarios. More disturbances increased both the fragmentation and the proportion of coniferous species and imposed additional constraints on the extent of each disturbance. We also found that harvesting legacies affect fire and budworm differently due to differences in forest types consumed by each disturbance. Contrary to expectation, budworm defoliation did not affect area burned at the temporal scales studied and neither amount of logging nor cutblock size influenced defoliation extent. Logging increased fire size through conversion of more of the landscape to early seral, highly flammable forest types. Although logging increased the amount of budworm host species, spruce budworm caused mortality was reduced due to reductions in forest age. In general, we found that spatial legacies do not influence all disturbances equally and the duration of a spatial legacy is limited when multiple disturbances are present. Further information on post-disturbance succession is still needed to refine our understanding of long-term disturbance interactions.     

The effect of increasing salinity and forest mortality on soil nitrogen and phosphorus mineralization in tidal freshwater forested wetlands

Tidal freshwater wetlands are sensitive to sea level rise and increased salinity, although little information is known about the impact of salinification on nutrient biogeochemistry in tidal freshwater forested wetlands. We quantified soil nitrogen (N) and phosphorus (P) mineralization using seasonal in situ incubations of modified resin cores along spatial gradients of chronic salinification (from continuously freshwater tidal forest to salt impacted tidal forest to oligohaline marsh) and in hummocks and hollows of the continuously freshwater tidal forest along the blackwater Waccamaw River and alluvial Savannah River. Salinification increased rates of net N and P mineralization fluxes and turnover in tidal freshwater forested wetland soils, most likely through tree stress and senescence (for N) and conversion to oligohaline marsh (for P). Stimulation of N and P mineralization by chronic salinification was apparently unrelated to inputs of sulfate (for N and P) or direct effects of increased soil conductivity (for N). In addition, the tidal wetland soils of the alluvial river mineralized more P relative to N than the blackwater river. Finally, hummocks had much greater nitrification fluxes than hollows at the continuously freshwater tidal forested wetland sites. These findings add to knowledge of the responses of tidal freshwater ecosystems to sea level rise and salinification that is necessary to predict the consequences of state changes in coastal ecosystem structure and function due to global change, including potential impacts on estuarine eutrophication.     

Riparian vegetated buffer strips in water-quality restoration and stream management

• 1 A review is presented of the literature on riparian vegetated buffer strips (VBS) for use in stream-water-quality restoration and limitations associated with their use are discussed. The results are also presented of recent investigations on the effectiveness of a forested and a grass vegetated buffer strip for reducing shallow subsurface inputs of nutrients from agriculture to a stream in central Illinois, U.S.A.
• 2 Because riparian zones link the stream with its terrestrial catchment, they can modify, incorporate, dilute, or concentrate substances before they enter a lotic system. In small to mid-size streams forested riparian zones can moderate temperatures, reduce sediment inputs, provide important sources of organic matter, and stabilize stream banks.
• 3 Several questions on the utility and efficiency of vegetated buffer strips for stream restoration still remain unanswered, including: what types (grass v forest) are most efficient; do they become nutrient saturated; are they only temporary sinks; how does species composition influence effectiveness; and, what is the optimal width of buffer to facilitate nutrient reduction under different conditions?
• 4 Water samples were collected (1989–90) from lysimeters located at three depths (60, 120, and > 120cm) in an upland area planted in conventional row crops (corn and soybean) and in three adjacent riparian buffer treatments, a 39m wide grass buffer. a 16 m wide mature forested buffer, and a buffer planted in row-crops to the stream bank. Concentrations of dissolved and total phosphorus and nitrate-N in each sample were determined following major precipitation events over a seventeen month period.
• 5 Both the forested and grass VBS reduced nitrate-N concentrations in shallow groundwater (up to 90% reduction). On an annual basis the forested VBS was more effective at reducing concentrations of nitrate-N than was the grass VBS, but was less efficient at retaining total and dissolved P.
• 6 During the dormant season, both grass and forested buffer strips released dissolved and total P to the groundwater. The VBS apparently acted as a nutrient sink for much of the year, but also released accumulated nutrients during the remaining portion of the year. Periodic harvesting of plant biomass may reduce the amount of P released during the dormant season.
• 7 VBSs are not as effective in agriculture areas with tile drained fields. Alternative restoration practices such as discharging drain tiles into wetlands constructed parallel to the stream channel may prove to be a more effective means of controlling non-point-source agricultural inputs of nutrients in such areas.

     

Stormflow Dynamics of Dissolved Organic Carbon and Total Dissolved Nitrogen in a Small Urban Watershed

We examined patterns of dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) loading to a small urban stream during baseflow and stormflow. We hypothesized that lower DOC and TDN contributions from impervious surfaces would dilute natural hydrologic flowpath (i.e., riparian) contributions during storm events in an urban watershed, resulting in lower concentrations of DOC and TDN during storms. We tested these hypotheses in a small urban watershed in Portland, Oregon, over a 3-month period during the spring of 2003. We compared baseflow and stormflow chemistry using Mann–Whitney tests (significant at p<0.05). We also applied a mass balance to the stream to compare the relative significance of impervious surface contributions versus riparian contributions of DOC and TDN. Results showed a significant increase in stream DOC concentrations during stormflows (median baseflow DOC = 2.00 mg l⁻¹ vs. median stormflow DOC = 3.46 mg l⁻¹). TDN streamwater concentrations, however, significantly decreased with stormflow (median baseflow TDN = 0.75 mg l⁻¹ vs. median stormflow TDN = 0.56 mg l⁻¹). During storms, remnant riparian areas contributed 70–74% of DOC export and 38–35% of TDN export to the stream. The observed pattern of increased DOC concentrations during stormflows in this urban watershed was similar to patterns found in previous studies of forested watersheds. Results for TDN indicated that there were relatively high baseflow nitrogen concentrations in the lower watershed that may have partially masked the remnant riparian signal during stormflows. Remnant riparian areas were a major source of DOC and TDN to the stream during storms. These results suggest the importance of preserving near-stream riparian areas in cities to maintain ambient carbon and nitrogen source contributions to urban streams.     

Effect of forest removal on the abundance of the endangered American burying beetle, Nicrophorus americanus (Coleoptera: Silphidae)

We test the hypothesis that the decline of the endangered American burying beetle (Nicrophorus americanus) from over 90% of its original range is the result of habitat loss and fragmentation of eastern North America. Forest removal at a site in southeastern Oklahoma known to have a significant population of N. americanus gave us a unique opportunity to test this hypothesis. At the local scale of this experiment, N. americanus declined significantly after forest removal while beetle numbers at adjacent forested plots did not change. Our results indicate that local disturbances such as forest removal, if occurring across relatively broad spatial scales, can cause wholesale geographic range collapse in this species.     

闽楠林土壤-凋落物-叶片碳氮磷生态化学计量对人为干扰的响应

干扰是影响森林生态系统稳定性和功能的重要因子,干扰程度直接影响天然林的生长进而影响其生态系统能量流动和养分循环过程,为此开展干扰对天然林生态系统影响研究,对于揭示干扰对天然林生态系统养分平衡特征机制具有重要意义。以福建两种人为干扰模式下(重度干扰和轻度干扰)闽楠林为研究对象,通过分析“土壤-凋落物-叶片”三个组分化学计量特征,结合养分利用效率、养分再吸收效率、内稳态理论解析干扰对闽楠林养分资源利用策略和生态适应。结果表明：(1)两种干扰模式下,叶片C、N、P含量均显著高于其土壤和凋落物,且三个组分中N和P含量均表现出重度干扰显著高于轻度干扰,但三个组分C/N、C/P和N/P呈现轻度干扰显著高于其重度干扰。(2)闽楠林叶片N、P养分利用效率表现出：重度干扰<轻度干扰,但P再吸收效率则是重度干扰高于轻度干扰,且两种干扰模式下P养分利用效率和再吸收效率显著高于N。(3)随干扰强度的增加,闽楠林叶片N呈现内稳态弱,而叶片P的内稳态强以适应低P环境。(4)凋落物与叶片两组分P、C/P、N/P存在显著正相关关系,土壤C/N分别与叶片P、C/P、N/P以及凋落物P、C/N和C/P存在显著相关关...     

Influence of Disturbance on Temperate Forest Productivity

Climate, tree species traits, and soil fertility are key controls on forest productivity. However, in most forest ecosystems, natural and human disturbances, such as wind throw, fire, and harvest, can also exert important and lasting direct and indirect influence over productivity. We used an ecosystem model, PnET-CN, to examine how disturbance type, intensity, and frequency influence net primary production (NPP) across a range of forest types from Minnesota and Wisconsin, USA. We assessed the importance of past disturbances on NPP, net N mineralization, foliar N, and leaf area index at 107 forest stands of differing types (aspen, jack pine, northern hardwood, black spruce) and disturbance history (fire, harvest) by comparing model simulations with observations. The model reasonably predicted differences among forest types in productivity, foliar N, leaf area index, and net N mineralization. Model simulations that included past disturbances minimally improved predictions compared to simulations without disturbance, suggesting the legacy of past disturbances played a minor role in influencing current forest productivity rates. Modeled NPP was more sensitive to the intensity of soil removal during a disturbance than the fraction of stand mortality or wood removal. Increasing crown fire frequency resulted in lower NPP, particularly for conifer forest types with longer leaf life spans and longer recovery times. These findings suggest that, over long time periods, moderate frequency disturbances are a relatively less important control on productivity than climate, soil, and species traits.