Ecohydrological impacts of woody-plant encroachment: seasonal patterns of water and carbon dioxide exchange within a semiarid riparian environment

Across many dryland regions, historically grass‐dominated ecosystems have been encroached upon by woody‐plant species. In this paper, we compare ecosystem water and carbon dioxide (CO₂) fluxes over a grassland, a grassland–shrubland mosaic, and a fully developed woodland to evaluate potential consequences of woody‐plant encroachment on important ecosystem processes. All three sites were located in the riparian corridor of a river in the southwest US. As such, plants in these ecosystems may have access to moisture at the capillary fringe of the near‐surface water table. Using fluxes measured by eddy covariance in 2003 we found that ecosystem evapotranspiration (ET) and net ecosystem exchange of carbon dioxide (NEE) increased with increasing woody‐plant dominance. Growing season ET totals were 407, 450, and 639 mm in the grassland, shrubland, and woodland, respectively, and in excess of precipitation by 227, 265, and 473 mm. This excess was derived from groundwater, especially during the extremely dry premonsoon period when this was the only source of moisture available to plants. Access to groundwater by the deep‐rooted woody plants apparently decouples ecosystem ET from gross ecosystem production (GEP) with respect to precipitation. Compared with grasses, the woody plants were better able to use the stable groundwater source and had an increased net CO₂ gain during the dry periods. This enhanced plant activity resulted in substantial accumulation of leaf litter on the soil surface that, during rainy periods, may lead to high microbial respiration rates that offset these photosynthetic fluxes. March–December (primary growing season) totals of NEE were ?63, ?212, and ?233 g C m^?2 in the grassland, shrubland, and woodland, respectively. Thus, there was a greater disparity between ecosystem water use and the strength of the CO₂ sink as woody plants increased across the encroachment gradient. Despite a higher density of woody plants and a greater plant productivity in the woodland than in the shrubland, the woodland produced a larger respiration response to rainfall that largely offset its higher photosynthetic potential. These data suggest that the capacity for woody plants to exploit water resources in riparian areas results in enhanced carbon sequestration at the expense of increased groundwater use under current climate conditions, but the potential does not scale specifically as a function of woody‐plant abundance. These results highlight the important roles of water sources and ecosystem structure on the control of water and carbon balances in dryland areas.     

Long‐term impacts of invasive species on a native top predator in a large lake system

1. Declining abundances of forage fish and the introduction and establishment of non‐indigenous species have the potential to substantially alter resource and habitat exploitation by top predators in large lakes. 2. We measured stable isotopes of carbon (δ¹³C) and nitrogen (δ¹⁵N) in field‐collected and archived samples of Lake Ontario lake trout (Salvelinus namaycush) and five species of prey fish and compared current trophic relationships of this top predator with historical samples. 3. Relationships between δ¹⁵N and lake trout age were temporally consistent throughout Lake Ontario and confirmed the role of lake trout as a top predator in this food web. However, δ¹³C values for age classes of lake trout collected in 2008 ranged from 1.0 to 3.9‰ higher than those reported for the population sampled in 1992. 4. Isotope mixing models predicted that these changes in resource assimilation were owing to the replacement of rainbow smelt (Osmerus mordax) by round goby (Neogobius melanostomus) in lake trout diet and increased reliance on carbon resources derived from nearshore production. This contrasts with the historical situation in Lake Ontario where δ¹³C values of the lake trout population were dominated by a reliance on offshore carbon production. 5. These results indicate a reduced capacity of the Lake Ontario offshore food web to support the energetic requirements of lake trout and that this top predator has become increasingly reliant on prey resources that are derived from nearshore carbon pathways.     

Tidal geomorphology affects phytoplankton at the transition from forested streams to tidal rivers

1. Coastal rivers can have long tidally influenced reaches that are affected by tides but do not contain saline water. These tidal freshwater reaches have steep geomorphic gradients where the river transitions from narrow, heavily shaded streams to wide, unshaded channels. The influence of these gradients on river ecosystem production is poorly understood. 2. We characterised gradients in irradiance, geomorphology, water clarity and chlorophyll a along 9‐ to 16‐km tidal freshwater reaches of the Newport and White Oak Rivers in North Carolina, USA, and examined the effect of nutrient enrichment on phytoplankton growth in the Newport River. Underwater irradiance was modelled at 2–4 week intervals along both rivers using measurements of the above‐canopy irradiance, canopy cover, water column light attenuation (K_d) and water depth. Suspended material (TSS), dissolved organic carbon (DOC) and chlorophyll a were sampled at 2‐week interval at five sites on the Newport River and on four dates at four sites on the White Oak River over the course of one year. 3. Phytoplankton nutrient limitation was assessed at three locations along the tidal gradient. River water was collected during March, April, June and October, and incubated in 10‐L plastic outdoor containers under ambient water temperature and sunlight. Additions of inorganic nitrogen and phosphorus served as treatments; growth rate during the 4 days of incubation was calculated from the change in chlorophyll a concentration over time. 4. Canopy cover decreased from more than 90% to <10% over the length of both tidal freshwater rivers. Water column irradiance and phytoplankton biomass increased as tree canopy cover decreased and channel width increased. Channel width exceeded predictions for non‐tidal rivers by threefold because of tidal influence. TSS and DOC decreased significantly along the length of the Newport River, but no significant gradients were observed in the White Oak River. K_d did not vary along the tidal gradient of either river. 5. Mesocosm experiments indicated that inorganic nitrogen and phosphorus enhanced the growth of phytoplankton advected from the non‐tidal river into the tidal freshwater river during spring and summer. Phytoplankton in the tidal freshwater reach were generally not nutrient limited. 6. Tidal hydrology (in the absence of saltwater) directly affected the morphology of the channel and indirectly affected biological growth and production. The significant increase in river width, irradiance and phytoplankton biomass distinguished these tidal freshwater ecosystems from their upstream (non‐tidal fluvial) counterparts, while the strong influence of riparian shading distinguished them from the saline estuaries downstream. Future development of ecosystem and biogeochemical models for tidal freshwater rivers will benefit from the linkages between geomorphology and biological processes identified here.