Variation in Density and Diversity of Species of Phytophthora in Two Forest Stream Networks

Monitoring occurrence and distribution of Phytophthora species, including Phytophthora ramorum, in forest ecosystems can be achieved in several ways including sampling symptomatic plants, infested soils, and infested streams. Collecting plant and soil samples can be laborious and time consuming due to the distance surveyors need to travel. Not all forests are available for survey because of limited accessibility and stand density. Species of Phytophthora are well adapted to aquatic habitats and more diverse populations of Phytophthora spp. are found in forest streams than in nearby soils and symptomatic riparian plants. The current protocol for the National P. ramorum Early Detection Survey, conducted by the U.S. Department of Agriculture Forest Service (USDA FS), adopted sampling forest streams using baiting and filtration procedures. One assumption being made in these monitoring efforts is that species present in a stream network are representative of those present in the land area drained by that network. Therefore, careful selection of stream networks is essential to optimize the sampling effort with limited available resources, and strategic selection of sample sites within a stream network should maximize detection of the species of Phtyophthora present A stream network in a natural ecosystem consists of a main stream, its tributaries, and a drainage point of the main stream. In this study, our hypothesis was that the occurrence and diversity of Phytophthora spp. at the drainage point of the main stream represents the overall population of Phytophthora spp. within the upstream network. If our hypothesis is true, a stream network could be surveyed effectively at the drainage point without additional sampling of the upstream tributaries.