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1. Land Use Change Influences Ecosystem Function in Headwater Streams of the Lowland Amazon Basin

   https://doi.org/10.3390/w13121667  

   Jankowski, Kathi Jo; Deegan, Linda A.; Neill, Christopher; Sullivan, Hillary L.; Ilha, Paulo; Maracahipes-Santos, Leonardo; Marques, Nubia; Macedo, Marcia N. (June 2021, Water)

   null (Ed.)

   Intensive agriculture alters headwater streams, but our understanding of its effects is limited in tropical regions where rates of agricultural expansion and intensification are currently greatest. Riparian forest protections are an important conservation tool, but whether they provide adequate protection of stream function in these areas of rapid tropical agricultural development has not been well studied. To address these gaps, we conducted a study in the lowland Brazilian Amazon, an area undergoing rapid cropland expansion, to assess the effects of land use change on organic matter dynamics (OM), ecosystem metabolism, and nutrient concentrations and uptake (nitrate and phosphate) in 11 first order streams draining forested (n = 4) or cropland (n = 7) watersheds with intact riparian forests. We found that streams had similar terrestrial litter inputs, but OM biomass was lower in cropland streams. Gross primary productivity was low and not different between land uses, but ecosystem respiration and net ecosystem production showed greater seasonality in cropland streams. Although we found no difference in stream concentrations of dissolved nutrients, phosphate uptake exceeded nitrate uptake in all streams and was higher in cropland than forested streams. This indicates that streams will be more retentive of phosphorus than nitrogen and that if fertilizer nitrogen reaches streams, it will be exported in stream networks. Overall, we found relatively subtle differences in stream function, indicating that riparian buffers have thus far provided protection against major functional shifts seen in other systems. However, the changes we did observe were linked to watershed scale shifts in hydrology, water temperature, and light availability resulting from watershed deforestation. This has implications for the conservation of tens of thousands of stream kilometers across the expanding Amazon cropland region. 

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2. Riparian canopy openings on mountain streams: Landscape controls upon temperature increases within openings and cooling downstream

   https://doi.org/10.1002/hyp.13706  

   Coats, William Alan; Jackson, Charles Rhett (February 2020, Hydrological Processes)

   Abstract How much stream temperatures increase within riparian canopy openings and whether stream temperatures cool downstream of these openings both have important policy implications. Past studies of stream cooling downstream of riparian openings have found mixed results including rapid, slow, and no cooling. We collected longitudinal profiles of stream temperatures above, within, and below riparian forest openings along stream segments within otherwise forested riparian conditions to evaluate how sensitivity of stream temperatures to riparian conditions varied across landscape factors. We conducted these temperature surveys across openings in 12 wadeable streams within and near the Upper Little Tennessee River Basin in western North Carolina and northeastern Georgia. Basin areas ranged from 74 to 6,913 ha, and bankfull channel widths varied from 3.4 to 16.4 m. Stream temperatures were collected every 15 min using HOBO® data loggers for 2 weeks in each stream, repeated later in summer in some streams. Reference temperatures were highest in stream reaches at low elevations and with large drainage areas. Stream temperature increases in the middle of riparian gaps were highest when streams drained small high‐elevation watersheds, and increases at the end of openings were highest when the opening length was large relative to watershed size. Downstream from openings, cooling rates were greatest in small, high‐elevation headwater streams and also increased with larger increases in canopy cover. Stream segments that warmed the most within openings also featured higher cooling rates downstream. The data show that stream temperature sensitivity to canopy change is highly dependent on network position and watershed size. A better understanding of stream temperature responses to riparian vegetation may be useful to land managers and landowners prioritizing riparian forest restoration. 

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3. The long shadow of woody encroachment: An integrated approach to modeling grassland songbird habitat

   https://doi.org/10.1002/eap.2954  

   Silber, Katy M.; Hefley, Trevor J.; Castro‐Miller, Henry N.; Ratajczak, Zak; Boyle, W. Alice (February 2024, Ecological Applications)

   Abstract Animals must track resources over relatively fine spatial and temporal scales, particularly in disturbance‐mediated systems like grasslands. Grassland birds respond to habitat heterogeneity by dispersing among sites within and between years, yet we know little about how they make post‐dispersal settlement decisions. Many methods exist to quantify the resource selection of mobile taxa, but the habitat data used in these models are frequently not collected at the same location or time that individuals were present. This spatiotemporal misalignment may lead to incorrect interpretations and adverse conservation outcomes, particularly in dynamic systems. To investigate the extent to which spatially and temporally dynamic vegetation conditions and topography drive grassland bird settlement decisions, we integrated multiple data sources from our study site to predict slope, vegetation height, and multiple metrics of vegetation cover at any point in space and time within the temporal and spatial scope of our study. We paired these predictions with avian mark‐resight data for 8 years at the Konza Prairie Biological Station in NE Kansas to evaluate territory selection for Grasshopper Sparrows (Ammodramus savannarum), Dickcissels (Spiza americana), and Eastern Meadowlarks (Sturnella magna). Each species selected different types and amounts of herbaceous vegetation cover, but all three species preferred relatively flat areas with less than 6% shrub cover and less than 1% tree cover. We evaluated several scenarios of woody vegetation removal and found that, with a targeted approach, the simulated removal of just one isolated tree in the uplands created up to 14 ha of grassland bird habitat. This study supports growing evidence that small amounts of woody encroachment can fragment landscapes, augmenting conservation threats to grassland systems. Conversely, these results demonstrate that drastic increases in bird habitat area could be achieved through relatively efficient management interventions. The results and approaches reported pave the way for more efficient conservation efforts in grasslands and other systems through spatiotemporal alignment of habitat with animal behaviors and simulated impacts of management interventions. 

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4. High-light demanding and rare tree species define tropical riparian tree communities

   Hudspeth, Virginia; Pastor, Alberto; Heartsill-Scalley, T. (August 2023, Acta Cientifica)

   Riparian zones are areas of transition between aquatic and terrestrial ecosystems, and their vegetation provides many important ecosystem services that are becoming of greater importance due to the changing climate. To understand riparian conditions along headwater tropical streams in terms of species composition and structure, we surveyed riparian tree communities and physical conditions in adjacent streams in the Luquillo Experimental Forest. We determined riparian substrate condition visually by estimating ground and vegetation attributes and measuring slope and elevation in a series of riparian plots at three stream sites. We determined riparian forest composition by identifying stem species and recording their height and diameter in plots. Our results demonstrated that elevation and slope were the main differences among sites, while all sites had similar ground and vegetation conditions. Species composition in all sites was characterized by hydrophytic species. However, although the most abundant species were generalists, we also found many species that were only observed once or twice per site. Overall, our results indicate that although recent and major hurricanes influence structure and composition at this site, riparian areas still maintain a large proportion of hydrophytic and rare tree species. 

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5. Land-Use Type as a Driver of Large Wildfire Occurrence in the U.S. Great Plains

   https://doi.org/10.3390/rs12111869  

   Donovan, Victoria M.; Wonkka, Carissa L.; Wedin, David A.; Twidwell, Dirac (June 2020, Remote Sensing)

   Wildfire activity has surged in North America’s temperate grassland biome. Like many biomes, this system has undergone drastic land-use change over the last century; however, how various land-use types contribute to wildfire patterns in grassland systems is unclear. We determine if certain land-use types have a greater propensity for large wildfire in the U.S. Great Plains and how this changes given the percentage of land covered by a given land-use type. Almost 90% of the area burned in the Great Plains occurred in woody and grassland land-use types. Although grassland comprised the greatest area burned by large wildfires, woody vegetation burned disproportionately more than any other land-use type in the Great Plains. Wildfires were more likely to occur when woody vegetation composed greater than 20% of the landscape. Wildfires were unlikely to occur in croplands, pasture/hay fields, and developed areas. Although these patterns varied by region, wildfire was most likely to occur in woody vegetation and/or grassland in 13 of 14 ecoregions we assessed. Because woody vegetation is more conducive to extreme wildfire behaviour than other land-use types in the Great Plains, woody encroachment could pose a large risk for increasing wildfire exposure. Regional planning could leverage differential wildfire activity across land-uses to devise targeted approaches that decrease human exposure in a system prone to fire. 

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