Andrews LTER

Understanding Variations in Recovery of Clear-cuts Proves Useful for Guiding Forest Management

LTER graduate student Yang Zhiqiang recently defended his Ph.D. thesis, in which he used air photos to determine how quickly clear-cut forests were returning to conifer cover. He found that there as a great deal of variation in the time required for conifers to become the dominant cover. For example, conifers might dominate a clear cut in 10 years, but in some cases it takes more than 50 years. What does this mean? For forest management practices, which have no doubt increased the rate that conifers dominate forests, there is still a wide diversity of situations out there to consider. For carbon and timber volume dynamics it means there is still a great deal of room for increasing rates of conifer reforestation in the region. For biodiversity it means that open-, shrub-, and hardwood-dominated stages of succession can be found for a fairly long time after clear cutting.

Rivers of Air May Influence Watershed Dynamics

Also at Andrews LTER, we've learned a lot about "airsheds" in Watershed 1 and Watershed 2 over the past year, which may be generally applicable to other airsheds within small, deeply incised watersheds. We've learned that there is a predictable, very deep (greater than 30 meters), and well-mixed "river" of air flowing down the watersheds. This airstream occurs almost every night and sometimes during the day as well. It is at least partially uncoupled from the upper atmosphere, creating different microclimatic conditions within the airstream than above it, raising the possibility that the ecosystem "within" the airshed may respond differently to variations in climate than ecosystems that are more tightly associated with the atmosphere. The air carries with it gases (such as respired CO2) and other aerosols that it "picks up" as it moves across the land surface, offering eventual possibilities for sampling air chemistry much as we currently sample stream chemistry as an indicator of biogeochemical processes within the watershed. Interesting points from this research: 1. The use of watersheds as air sheds, or using the topography to delimit the system under some circumstances is an interesting new concept for ecological research, just as watersheds have been used for a century of so for hydrologic studies and for 40 years or so (e.g. Likens and Bormann) for biogeochemical cycling studies. 2. this work accounts for terrestrial-atmosphere exchange of gasses in steep-terrain-conditions (which flux tower folks have previously avoided) and uses this information to measure gas exchange and ecosystem respiration.

Cross-site Study Links Micro-climate to Plant Physiology in Watersheds

Julia Jones (Prof., Geosciences, OSU and Andrews LTER coPI) is using the new HydroDB data harvester (see LTER Network News, Fall 2003) to compare streamflow in clear-cut forest sites across several east and several west coast locations. Using a daily time-step over one- and five-year intervals, Jones is comparing streamflow response to forest removal at deciduous versus coniferous forest sites, as well as among sites with and without snow-packs.

Streamflow response to forest removal is positively related to the age difference in age (measured as time since last major disturbance) between the forests in the treated and control basin. The difference in age explains 45% of the variation in streamflow change in years 1 to 5 after forest removal (y = 138 ln(x) - 332, r2 = 0.45) and 71% of the variation in streamflow change in years 15 to 25 after forest removal (y = 142 ln(x) - 524, r2 = 0.71). Data are from the Andrews (OR), Caspar Creek (CA), Coweeta (NC), Coyote (OR), Fernow (WV), and Hubbard Brook (NH) Experimental Forests, stored on the hydro-DB data harvester maintained by the USFS PNW Station and Andrews LTER.

This research breaks ground for ecohydrology because it gives a basis for linking climate controls to the ecophysiology of plants in the watershed, to streamflow, and to effects of streamflow variability on aquatic ecosystems. Findings based on long-term records from Andrews, Coweeta, and Hubbard Brook sites (as well as Caspar Creek, Coyote Creek, and Fernow Experimental Forests) indicate that conversion of older forests to young forests can create persistent changes in snow accumulation and melt, affecting spring runoff, as well as summer streamflow deficits 10-25 years after forest conversion, potentially influencing stream habitat and biogeochemistry.

The paper has been accepted for publication in Water Resources Research, an interdisciplinary journal integrating research in the social and natural sciences.