Black ash (Fraxinus nigra) mortality caused by the invasive emerald ash borer (EAB) is of concern to land managers in the upper Great Lakes region, given the large areas of ash-dominated forest and potential alteration of wetland hydrology following loss of this foundation tree species. The importance of changes in evapotranspiration (ET) following black ash mortality is currently unknown and is the focus of this study. Sap flux density rates were evaluated at three black ash stands with differing moisture regimes within the Chippewa National Forest, Minnesota, USA using the Granier thermal dissipation method. Sapwood area and sap flux density were combined to determine sap flow. Tree level sap flux density estimates were comparable to other reported values and averaged 4.59, 2.31, and 1.62m3m-2day-1, respectively, for the very wet, wet, and moderately wet field sites. However, black ash exhibited small sapwood area in general, resulting in lower overall sap flow values. Scaled stand-level transpiration followed a similar trend as the tree-level estimates; mean daily transpiration over 10 weeks was 1.62 (80% of PET), 1.15 (53% of PET), and 0.90 (42% of PET) mm for the very wet, wet, and moderately wet site, respectively. Sap flux density was positively related to vapor pressure deficit when soil moisture was at or near saturation and negatively related when soil moisture content was lower. There was also a significant positive relationship between sap flux density and relative soil moisture saturation at the stand scale. Our results indicate that hydrologic regime has substantial influence on sap flow with highest transpiration when soil moisture is at saturation, underscoring the unique ecological role that black ash plays in these wetland forest types. The effects of EAB-induced black ash mortality on overall ET and related hydrologic processes will likely be greatest in the wettest hydrologic regimes.
Bibliographical noteFunding Information:
This project was funded by the Minnesota Environment and Natural Resources Trust Fund , the Minnesota Forest Resources Council , and the USDA Forest Service Northern Research Station . We would like to thank Steve Sebestyen and Donna Olson for providing data used in the PET calculations. We would also like to thank Joe Shannon for his assistance with project development and data interpretation, and Christopher Looney for providing the LAI estimates reported in Table 1 . Assistance with field work and data collection was provided by David Rudolph.
© 2015 Elsevier B.V.
- Agrilus planipennis
- Forest management
- Sapwood depth
- Soil moisture