Managed Flooding for Riparian Ecosystem Restoration

Abstract

The dams and levels built along the middle of the Rio Grande in New Mexico has reorganized the region's riparian ecosystem. Managed flooding caused the the decrease in the germination and establishment of native cotton-woods and willows.; FROM INTRO: Students from the Department of Biology at the University of New Mexico have been monitoring several stands of riparian forest, or "bosque," as riparian forests along the Rio Grande are known, for approximately ten years. Monitoring of these riparian forests was prompted by the concern that without long-term data sets, these natural areas could not be managed properly. The riparian forest monitored by these students was established mainly during the last large flood on the middle Rio Grande, which took place in 1941-1942. Now, however, most of the forest along this reach no longer experiences overbank flooding. In the absence of flooding and with the invasion of non-native trees, these tracts of riparian forest have become tangled with woody debris [ILLUSTRATION FOR FIGURE 2 OMITTED]. The organic matter that has accumulated on the floor of riparian forests along the middle Rio Grande now averages over 50 x [10.sup.6] g/ha in some areas (Molles et al. 1996), which increases fire danger (Stuever 1997) and immobilizes substantial quantities of plant nutrients.; ; Monitoring has produced several data sets, including leaf fall, which has been used as an index of forest production. Autumn leaf fall has been measured in three sections of a 760 m long segment of riparian forest within the Rio Grande Nature Center in Albuquerque, New Mexico. From 1989 to 1996, leaf fall dropped steadily, declining by over 40% in just eight years. This record of leaf fall quantifies what had previously only been suspected: The bosque within Albuquerque is senescing rapidly (Figure 3). This finding gives a sense of urgency to research on riparian restoration along the middle Rio Grande.; ; Little is known about the potential importance of flooding for maintaining established riparian forest. To investigate how the absence of a flood pulse on the middle Rio Grande has affected the structure of microbial and animal populations on the forest floor and how the flood pulse affects the rates of ecosystem processes such as forest floor respiration and decomposition of plant litter, we created a series of managed floods in a riparian forest at Bosque del Apache National Wildlife Refuge near San Antonio, New Mexico. A major objective of our research is to evaluate the potential of managed floods as a tool for riparian restoration by measuring key population, community, and ecosystem responses to the reestablishment of flooding. In this article, we present some of our findings that bear on this approach to riparian restoration.