Lisa M. Ellis

BIOGEOCHEMICAL AND METABOLIC RESPONSES TO THE FLOOD PULSE IN A SEMIARID FLOODPLAIN

Flood pulse inundation of riparian forests alters rates of nutrient retention and organic matter processing in the aquatic ecosystems formed in the forest interior. Along the Middle Rio Grande (New Mexico, USA), impoundment and levee construction have created riparian forests that differ in their inter-flood intervals (IFIs) because some flood- plains are still regularly inundated by the flood pulse (i.e., connected), while other flood- plains remain isolated from flooding (i.e., disconnected). This research investigates how ecosystem responses to the flood pulse relate to forest IFI by quantifying nutrient and organic matter dynamics in the Rio Grande floodplain during three years of experimental flooding of the disconnected floodplain and during a single year of natural flooding of the connected floodplain. Surface and subsurface conditions in paired sites (control, flood) established in the two floodplain types were monitored to address metabolic and biogeochemical responses. Compared to dry controls, rates of respiration in the flooded sites increased by up to three orders of magnitude during the flood pulse. In the disconnected forest, month-long experimental floods produced widespread anoxia of four-week duration during each of the three years of flooding. In contrast, water in the connected floodplain remained well ox- ygenated (3-8 ppm). Material budgets for experimental floods showed the disconnected floodplain to be a sink for inorganic nitrogen and suspended solids, but a potential source of dissolved organic carbon (DOC). Compared to the main stem of the Rio Grande, flood- water on the connected floodplain contained less nitrate, but comparable concentrations of DOC, phosphate-phosphorus, and ammonium-nitrogen. Results suggest that floodplain IFI drives metabolic and biogeochemical responses dur- ing the flood pulse. Impoundment and fragmentation have altered floodplains from a mosaic of patches with variable IFI to a bimodal distribution. Relatively predictable flooding occurs in the connected forest, while inundation of the disconnected forest occurs only as the result of managed application of water. In semiarid floodplains, water is scarce except during the flood pulse. Ecosystem responses to the flood pulse are related to the IFI and other measures of flooding history that help describe spatial variation in ecosystem function.

Bird use of saltcedar and cottonwood vegetation in the Middle Rio Grande Valley of New Mexico, U.S.A.

Riparian vegetation provides important habitat for migrating and breeding birds in the arid south-western United States. Surveys conducted in native cottonwood (Populus fremontii var. wislizeni) and introduced saltcedar (Tamarix chinensis) vegetation during spring, summer and fall were used to compare avian use of these two vegetation types along the Middle Rio Grande Valley in central New Mexico. Bird species richness in cottonwood and saltcedar did not differ during any season, but species composition varied. Overlap in species between the two habitats was lowest in spring and increased in fall, with more species unique to cottonwood than to saltcedar in all seasons. A number of obligate riparian species readily used saltcedar, while others were restricted to areas dominated by native vegetation. Neotropical migrants showed a slight preference for cottonwood in the spring, but some migrant species were detected in saltcedar as well, possibly reflecting the dense nature of this vegetation. However, although saltcedar was used by a number of species, maintenance of native vegetation will be necessary for the persistence of many bird species in the Middle Rio Grande Valley.

Influence of annual flooding on terrestrial arthropod assemblages of a Rio Grande riparian forest

Terrestrial arthropod communities remain poorly described for riparian ecosystems of the arid southwestern United States, and the effects of extensive river regulation and habitat alteration on these potentially important invertebrates are largely unknown. Beginning in 1991, surface-active arthropods were trapped at two riparian sites along the Rio Grande, in central New Mexico, for 2 years. One site was then experimentally flooded from mid-May to mid-June for each of the next 3 years to simulate historic, low intensity flooding, after which arthropod collections were continued. These primary sites, located outside the riverside levee, and isolated from flooding for about 50 years prior to the experiment, were compared with a naturally flooded site and a second non-flooded reference. Experimental flooding and observations of the naturally flooded site indicated that flooding did not affect total taxonomic richness, nor richness of spiders, beetles or ants. However, flooding may have slightly increased the number of carabid beetle taxa present. Flooding altered the overall composition for all taxa, insects, beetles and carabid beetles. Spider taxa composition may be insensitive to flooding, while ant responses were not clear. Abundance of terrestrial isopods and spiders decreased after flooding, while overall beetle abundance did not change. Abundance of crickets and carabid beetles increased, but the response was delayed until after the second flood. Changes in taxa composition and abundance after experimental flooding were generally consistent with arthropod community structure observed at a nearby naturally flooded site. This similarity suggests that reorganization of the terrestrial arthropod community may follow restoration of flooding to this riparian ecosystem. Copyright

Short-term response of woody plants to fire in a Rio Grande riparian forest, Central New Mexico, USA

Abstract Both fire frequency and severity have increased in riparian vegetation in the southwestern United States in recent years, but the role of fire in these ecosystems is still not well understood. I monitored short-term responses of riparian vegetation to a wildfire at two established study sites. Fire severity reflected the amount of organic debris present before the fire, which reflected flooding history at the two sites: severity was lower at the site with a more extensive flooding history. Only native cottonwoods Populus deltoides located in an area that had been regularly flooded retained viable above-ground tissue 2 years after the fire, but 40–80% of trees at both sites produced shoot sprouts. No other woody species retained viable above-ground tissue. Seventy-three percent of willows Salix gooddingii were sprouting four months after the fire, but only about 55% of exotic saltcedars Tamarix ramosissima were sprouting. Sprouting of native shrubs was higher at the site with the lower fire severity. Reducing current fuel load, either by restoring flooding or by mechanical removal, is needed to lessen the impact of fires on riparian forests along the Rio Grande.

Short-Term Effects of Annual Flooding on a Population of Peromyscus Leucopus in a Rio Grande Riparian Forest of Central New Mexico

-We studied the short-term effects of experimental flooding on a population of Peromyscus leucopus in a Rio Grande riparian forest that had not flooded for over 50 y. We monitored populations at two sites for 2 yr before flooding and then for 3 yr during which we flooded one site between mid-May and mid-June each year. Considerable interannual variation in density was detected at both sites, with no clear effect of flooding on density. Some mice stayed within the forest during flooding and used trees as refugia. Survivorship decreased slightly at the flood site after the first flood but did not differ from the reference site 2 mo after flooding that year, nor in the final year. There was no difference between sites in recruitment of new individuals.