We propose a set of four meetings on the causes and consequences of seasonal changes in water fluxes in headwater drainages. Analysis of long-term discharge records for several headwater basins has revealed seasonally specific changes in flow regimes regimes at two-dozen sites across the U.S. and Canada, which may be attributed to climate change and ecological responses to past disturbances (Jones et al. 2012). For example, polar/alpine sites are experiencing increased flows at the margins of the formerly frozen periods, whereas forested sites with snowpacks are experiencing changes in spring snowmelt timing. In contrast, vegetated sites without snow/ice are experiencing alterations in summer streamflow: both decreasing flows as would be expected from warmer temperatures and increased evapotranspiration, but also in some cases increasing flows associated with forest succession from past disturbances. The intent of this working group is to address the question: How do these seasonal hydrologic changes affect ecological patterns and processes? Our strategy will be to hold an introductory session (1) to provide an overview of the issue and the work plan, then have a set of three sub-group meetings tackle specific ecological themes. Sub-groups will focus on: (2) chemical responses to changing seasonality of flows; (3) evapotranspiration and woody invader responses; and (4) organismal and community responses. Sub-groups will identify available information to evaluate flow-response relationships, with the intent of developing a manuscript, or, if appropriate, a proposal to pursue the question further.
Summary of sessions:
(1) The overview session will review the evidence for changes in temperature, precipitation and streamflow at the daily time scale at LTER sites.
The second session will be divided into three subgroups.
(2) Chemical responses to changing seasonality of flows. Changes in the timing of streamflow and the hydrologic flowpaths, as well as changes in biotic activity may alter biogeochemistry and temperatures of streams. What examples/evidence do we have from LTER sites of these changes? What is their magnitude and direction and what elements are affected?
(3) Evapotranspiration and woody invader responses. During the growing season, warming temperatures would be expected to increase ET, reducing streamflow or offsetting streamflow increases from earlier snowmelt or permafrost melt. However, vegetation may respond at multiple timescales and multiple scales of ecological organization, producing structural adjustments in water use that may result in no change in ET or streamflow at some times of year, despite warming. Alternatively, warming may facilitate invasive species that have greater water use, exacerbating streamflow declines. How are these processes playing out at various sites?
(4) Organismal and community responses. Changes in timing, magnitude, temperature or chemistry of streamflow would be expected to alter life history of instream organisms, such as timing of emergence of aquatic insects that may desynchronize them from consumers, altering food webs. What examples/evidence do we have from LTER sites of these changes? What organisms are affected, and how have they responded?