Density Dependent Refueling of Migratory Songbirds During Stopover Within an Urbanizing Coastal Landscape

Refueling performance is the primary currency of a successful migration as birds must maintain energy stores to achieve an optimal travel schedule. Migrating birds can anticipate heightened energy demand, not to mention increased uncertainty that energy demands will be satisfied, especially within an urbanizing landscape following long-distance flights. We tested the expectation that refueling performance of songbirds is reduced as densities increase at stopover sites in an urbanizing coastline of the Gulf of Mexico. We measured the density of migrating birds, their refueling performance, and arthropod abundance in two large tracts of contiguous forest paired with two small isolated patches embedded within residential settings throughout spring migration over the course of 2 years. Refueling performance declined with increasing migrant densities, even though the overall daily densities of birds stopping in these landscapes were relatively low and arthropod densities were low throughout. Habitat patch size alone did not account for differences in refueling performance, but smaller habitat patches more often concentrated migrants in higher densities where they experienced reduced refueling performance. We found support for density-dependent refueling performance during spring migration through a region where overall passage and stopover densities are low; suggesting that larger contiguous forest tracks within urban landscapes provide higher quality habitat for refueling and that effect is likely even more pronounced in landscapes within higher density migratory corridors. The nutritional challenges encountered during migration influence the overall pace of migration and changes in access to food resources due to increasing urbanization may ultimately impact optimal travel schedules

Seasonal patterns and controls on net ecosystem CO2 exchange in a boreal peatland complex

We measured seasonal patterns of net ecosystem exchange (NEE) of CO2 in a diverse peatland complex underlain by discontinuous permafrost in northern Manitoba, Canada, as part of the Boreal Ecosystems Atmosphere Study (BOREAS). Study sites spanned the full range of peatland trophic and moisture gradients found in boreal environments from bog (pH 3.9) to rich fen (pH 7.2). During midseason (July‐August, 1996), highest rates of NEE and respiration followed the trophic sequence of bog (5.4 to −3.9 μmol CO2 m−2 s−1) \u3c poor fen (6.3 to −6.5 μmol CO2 m−2 s−1) \u3c intermediate fen (10.5 to −7.8 μmol CO2 m−2 s−1) \u3c rich fen (14.9 to −8.7 μmol CO2m−2 s−1). The sequence changed during spring (May‐June) and fall (September‐October) when ericaceous shrub (e.g., Chamaedaphne calyculata) bogs and sedge (Carex spp.) communities in poor to intermediate fens had higher maximum CO2 fixation rates than deciduous shrub‐dominated (Salix spp. and Betula spp.) rich fens. Timing of snowmelt and differential rates of peat surface thaw in microtopographic hummocks and hollows controlled the onset of carbon uptake in spring. Maximum photosynthesis and respiration were closely correlated throughout the growing season with a ratio of approximately 1/3 ecosystem respiration to maximum carbon uptake at all sites across the trophic gradient. Soil temperatures above the water table and timing of surface thaw and freeze‐up in the spring and fall were more important to net CO2 exchange than deep soil warming. This close coupling of maximum CO2 uptake and respiration to easily measurable variables, such as trophic status, peat temperature, and water table, will improve models of wetland carbon exchange. Although trophic status, aboveground net primary productivity, and surface temperatures were more important than water level in predicting respiration on a daily basis, the mean position of the water table was a good predictor (r2 = 0.63) of mean respiration rates across the range of plant community and moisture gradients. Q10 values ranged from 3.0 to 4.1 from bog to rich fen, but when normalized by above ground vascular plant biomass, the Q10 for all sites was 3.3

Geographic Position and Landscape Composition Explain Regional Patterns of Migrating Landbird Distributions during Spring Stopover along the Northern Coast of the Gulf of Mexico

Annual migration of landbirds across the Gulf of Mexico (GOM) presents a unique opportunity to examine extrinsic processes operating at various spatial scales in determining animal distributions. Our objectives were to comprehensively quantify bird stopover densities across the northern GOM coast and model broad-scale factors explaining distributional patterns. We used weather surveillance radars to measure reflectivity of birds aloft at onset of nocturnal migratory flights and estimate bird stopover densities during four springs (2009-2012) for 6.7 million ha along the GOM. We aggregated bird densities to one longitudinal degree and 3 km of proximity to coast. Boosted Regression Tree models revealed that stopover density was related to year, longitude, proximity to coast, and amount of hardwood forest cover in the landscape. Average longitudinal patterns supported previous studies of broad-scale trans-Gulf migrant arrivals with highest density in Louisiana (92-93A degrees W) and lowest in Alabama (88-89A degrees W). Florida (83-84A degrees W) supported a second peak in migrant density, suggesting an eastern trans-Gulf route or contribution from trans-Caribbean migrants. Longitudinal patterns in migrant distributions varied strongly between years and appear generally related to variability in GOM wind patterns. Densities increased with proximity to coast, highlighting constraints on migrants to travel inland, especially in Florida\u27s panhandle. Despite this, density was positively related to amount of forest cover more steeply along the immediate coast. Broad-scale stopover distributions of migrating landbirds along the GOM coast are heavily influenced by geographic constraints in the context of the GOM acting as a barrier to landbird migration

Interannual variability in the peatland-atmosphere carbon dioxide exchange at an ombrotrophic bog

Eddy covariance measurements of net ecosystem carbon dioxide (CO2) exchange (NEE) were taken at an ombrotrophic bog near Ottawa, Canada from 1 June 1998 to 31 May 2002. Temperatures during this period were above normal except for 2000 and precipitation was near normal in 1998 and 1999, above normal in 2000, and well below normal in 2001. Growing period maximum daytime uptake (−0.45 mg CO2 m−2 s−1) was similar in all years and nighttime maximum respiration was typically near 0.20 mg CO2 m−2 s−1, however, larger values were recorded during very dry conditions in the fourth year of study. Winter CO2 flux was considerably smaller than in summer, but persistent, resulting in significant accumulated losses (119–132 g CO2 m−2 period−1). This loss was equivalent to between 30 and 70% of the net CO2uptake during the growing season. During the first 3 years of study, the bog was an annual sink for CO2 (∼−260 g CO2 m−2 yr−1). In the fourth year, with the dry summer, however, annual NEE was only −34 g CO2 m−2 yr−1, which is not significantly different from zero. We examined the performance of a peatland carbon simulator (PCARS) model against the tower measurements of NEE and derived ecosystem respiration (ER) and photosynthesis (PSN). PCARS ER and PSN were highly correlated with tower-derived fluxes, but the model consistently overestimated both ER and PSN, with slightly poorer comparisons in the dry year. As a result of both component fluxes being overestimated, PCARS simulated the tower NEE reasonably well. Simulated decomposition and autotrophic respiration contributed about equal proportions to ER. Shrubs accounted for the greatest proportion of PSN (85%); moss PSN declined to near zero during the summer period due to surface drying

Plant biomass and production and CO2 exchange in an ombrotrophic bog

Summary Above-ground biomass was measured at bog hummock, bog hollow and poor-fen sites in Mer Bleue, a large, raised ombrotrophic bog near Ottawa, Ont., Canada. The average above-ground biomass was 587 g m−2 in the bog, composed mainly of shrubs and Sphagnum capitula. In the poor fen, the average biomass was 317 g m−2, comprising mainly sedges and herbs and Sphagnum capitula. Vascular plant above-ground biomass was greater where the water table was lower, with a similar but weaker relationship for Sphagnum capitula and vascular leaf biomass. Below-ground biomass averaged 2400 g m−2 at the bog hummock site, of which 300 g m−2 was fine roots (\u3c 2 mm diameter), compared with 1400 g m−2 in hollows (fine roots 450 g m−2) and 1200 g m−2 at the poor-fen site. Net Ecosystem Exchange (NEE) of CO2 was measured in chambers and used to derive ecosystem respiration and photosynthesis. Under high light flux (PAR of 1500 µmol m−2 s−1), NEE ranged across sites from 0.08 to 0.22 mg m−2 s−1 (a positive value indicates ecosystem uptake) in the spring and summer, but fell to –0.01 to –0.13 mg m−2 s−1 (i.e. a release of CO2) during a late-summer dry period. There was a general agreement between a combination of literature estimates of photosynthetic capacity for shrubs and mosses and measured biomass and summer-time CO2 uptake determined by the eddy covariance technique within a bog footprint (0.40 and 0.35–0.40 mg m−2 s−1, respectively). Gross photosynthesis was estimated to be about 530 g m−2 year−1, total respiration 460 g m−2 year−1, and export of DOC, DIC and CH4 10 g m−2 year−1, leaving an annual C sequestration rate of 60 g m−2 year−1. Root production and decomposition are important parts of the C budget of the bog. Root C production was estimated to be 161–176 g m−2 year−1, resulting in fractional turnover rates of 0.2 and 1 year−1 for total and fine roots, respectively

Interannual variability in the peatland-atmosphere carbon dioxide exchange at an ombrotrophic bog

Eddy covariance measurements of net ecosystem carbon dioxide (CO2) exchange (NEE) were taken at an ombrotrophic bog near Ottawa, Canada from 1 June 1998 to 31 May 2002. Temperatures during this period were above normal except for 2000 and precipitation was near normal in 1998 and 1999, above normal in 2000, and well below normal in 2001. Growing period maximum daytime uptake (−0.45 mg CO2 m−2 s−1) was similar in all years and nighttime maximum respiration was typically near 0.20 mg CO2 m−2 s−1, however, larger values were recorded during very dry conditions in the fourth year of study. Winter CO2 flux was considerably smaller than in summer, but persistent, resulting in significant accumulated losses (119–132 g CO2 m−2 period−1). This loss was equivalent to between 30 and 70% of the net CO2 uptake during the growing season. During the first 3 years of study, the bog was an annual sink for CO2 (∼−260 g CO2 m−2 yr−1). In the fourth year, with the dry summer, however, annual NEE was only −34 g CO2 m−2 yr−1, which is not significantly different from zero. We examined the performance of a peatland carbon simulator (PCARS) model against the tower measurements of NEE and derived ecosystem respiration (ER) and photosynthesis (PSN). PCARS ER and PSN were highly correlated with tower-derived fluxes, but the model consistently overestimated both ER and PSN, with slightly poorer comparisons in the dry year. As a result of both component fluxes being overestimated, PCARS simulated the tower NEE reasonably well. Simulated decomposition and autotrophic respiration contributed about equal proportions to ER. Shrubs accounted for the greatest proportion of PSN (85%); moss PSN declined to near zero during the summer period due to surface drying

Contemporary carbon balance and late Holocene carbon accumulation in a northern peatland

Northern peatlands contain up to 25% of the world's soil carbon (C) and have an estimated annual exchange of CO2-C with the atmosphere of 0.1-0.5 Pg yr-1 and of CH4-C of 10-25 Tg yr-1. Despite this overall importance to the global C cycle, there have been few, if any, complete multiyear annual C balances for these ecosystems. We report a 6-year balance computed from continuous net ecosystem CO2 exchange (NEE), regular instantaneous measurements of methane (CH4) emissions, and export of dissolved organic C (DOC) from a northern ombrotrophic bog. From these observations, we have constructed complete seasonal and annual C balances, examined their seasonal and interannual variability, and compared the mean 6-year contemporary C exchange with the apparent C accumulation for the last 3000 years obtained from C density and age-depth profiles from two peat cores. The 6-year mean NEE-C and CH4-C exchange, and net DOC loss are -40.2±40.5 (±1SD), 3.7±0.5, and 14.9±3.1 g m-2yr-1, giving a 6-year mean balance of -21.5±39.0 g m-2yr-1 (where positive exchange is a loss of C from the ecosystem). NEE had the largest magnitude and variability of the components of the C balance, but DOC and CH4 had similar proportional variabilities and their inclusion is essential to resolve the C balance. There are large interseasonal and interannual ranges to the exchanges due to variations in climatic conditions. We estimate from the largest and smallest seasonal exchanges, quasi-maximum limits of the annual C balance between 50 and -105 g m-2yr-1. The net C accumulation rate obtained from the two peatland cores for the interval 400-3000 bp (samples from the anoxic layer only) were 21.9±2.8 and 14.0±37.6 g m-2yr-1, which are not significantly different from the 6-year mean contemporary exchange

A Multi-Year Record of Methane Flux at the Mer Bleue Bog, Southern Canada

The Mer Bleue peatland is a large ombrotrophic bog with hummock-lawn microtopography, poor fen sections and beaver ponds at the margin. Average growing-season (May-October) fluxes of methane (CH4) measured in 2002-2003 across the bog ranged from less than 5 mg m-2 d-1 in hummocks, to greater than 100 mg m-2 d-1 in lawns and ponds. The average position of the water table explained about half of the variation in the season average CH4 fluxes, similar to that observed in many other peatlands in Canada and elsewhere. The flux varied most when the water table position ranged between -15 and -40 cm. To better establish the factors that influence this variability, we measured CH4 flux at approximate