Allochthonous input1 is an important source of food and energy to stream systems (Culp and Davies 1985, Hollingsworth 1988, Alan 1995, May et al. 1997, Webster et al. 1999, Wenger 1999, Wipfli and Gregovich 2002, Cole et al. 2003, MacKenzie and Moran 2004, Muto et al. 2009), and a primary component of organic material in small headwater streams (Lowe et al. 1986, Hollingsworth 1988).
In this project we measure vertical allochthonous inputs to small headwater streams to quantify the relationships between vertical allochthonous inputs and riparian characteristics to better understand the implications of riparian forest management. Starting in 2003, we annually monitor allochthonous mass (g) that is naturally introduced vertically into small streams, nutrient content (nitrogen, phosphorous, and potassium (mg)) and composition (mass of wood, needles, cones, leaves and other organic material (g)). The study located in a coastal temperate rainforest in Clayoquot Sound British Columbia (BC), Canada. The sample sites are relatively close geographically, all within the CWHvh1 variant and spread over benign terrain (so aspect differences are expected to be inconsequential). Rain and snow fall, wind and temperature variation is estimated to be similar across all study sites. The second growth conifer forests used in this study were selected to represent a forest that would result from currently regulated reforestation requirements following logging (conifer dominated). The old growth forests used in this study were selected for similar age, species composition, density and structure. The study is divided into four components: different variable retention (VR) timber harvest regimes, a chronosequence study of different age classes of second growth conifer forest, three deciduous riparian vegetation types (RVT), and an intact late seral (old growth) forest control.
Mass: In the VR component of the study, there appears to be a clear trend of the greatest reduction in allochthonous mass occurring with 15% retention (no RRZ) to least impact with 30% retention with a 5m riparian reserve zone (RRZ) when averaged over the last four years. The 50% retention site is almost equivalent to the vertical allochthonous input found in the old growth control site. Needles and wood made up the vast majority of the total input for the control and all VR sites with the exception of the 15% retention site without a riparian reserve. For the Chronosequence component of this study, there appears to be a clear trend to old growth mass equivalency as early as 25 years. Needles and wood made up the vast majority of the total. For the RVT portion of this study, old growth equivalency is significantly exceeded for mass of allochthonous input in both of the alder dominated sites; however the composition, as expected with these types of sites, was mostly leaves.
Nutrients: With VR, there appears to be a clear trend of the greatest reduction in mass of Nitrogen, Phosphorous and Potassium (NPK) occurring with 15% retention - no RRZ to least impact with 30% retention ? 5m RRZ when averaged over the last four years. The 50% retention site exceeded the NPK input found in the old growth control site. Needles made up the vast majority of the total mass of NPK in most cases. In the Chronosequence component there appears to be a clear trend to old growth equivalency for NPK contribution in less than 25 years. For N and P, leaves made up the majority source, whereas needles made up the vast majority of the total mass of K. Old growth equivalency is exceeded for NPK input in both of the alder dominated sites with the brush dominated site showing minimal input. The source, as expected with these types of deciduous sites, was mostly leaves. The alder dominated sites contributed notably more Potassium, Nitrogen and Phosphorous than any of the conifer sites (including the old growth control).