Along rivers with regulated stream flow species such as eastern redcedar (Juniperus virginiana) and Russian olive (Elaeagnus angustifolia) have spread in extent and in density in Nebraska and elsewhere in the Great Plains, affecting water balance and evapotranspiration in riparian areas and reducing stream flow.  Removing these woody species has been proposed as a way to reduce vegetation water losses and restore stream flow.  One of the challenges is the extrapolation of tree-level measurements to the stand-level in these uneven-aged stands.  Our objective is to test if a relationship between sap flux measurements and tree diameter exists before extrapolating results.  We are using preliminary data to test whether sap flux varied significantly among species, between sthinning treatments, and among tree diameter to refine ongoing monitoring.  In three thinned plots, invasive Juniper and Elaeagnus trees were removed.  In three control plots no trees were removed.  Sample trees were selected to represent population surveyed in three control plots and three thinned plots.  Granier sap flux sensors were installed in sample trees selected to represent the age structure of each species.

Results/Conclusions

Significant differences were observed between eastern cottonwood (Populus deltoides) and Juniperus, but not between thinning treatments in Populus deltoides trees.  Before frost, sap flux of the overstory Populus trees exceeded that of the understory Juniperus trees.  In Week 36 (between September 3-9), Populus tree sap flux averaged 16.4 ± 2.1 g m^-2 s^-1 tree^-1 day^-1 in control plots and 20.5 ± 1.0 g m^-2 s^-1 in thinned plots while Juniperus tree sap flux averaged 9.9 ± 0.5 g m^-2 s^-1.  During Week 41 (October 8-14) temperatures dropped below freezing for four consecutive days and the average sap flux of Populus trees dropped to 0.8 ± 0.1 L g m^-2 s^-1 in control plots and 2.6 ± 0.4 g m^-2 s^-1 in thinned plots while Juniperus trees transpired an average of 2.4 ± 0.6 g m^-2 s^-1.  Populus trees still retained many of their leaves September 29.  Monitoring leaf fall in future years will allow us to determine whether Populus in thinned plots retain leaves longer and thus extend transpiration later than those in control plots.

Sap flux velocity was correlated with diameter in Populus.  This suggests that if Populus tree growth increases in response to removing understory Juniperus and Elaeagnus trees, tree-level Populus transpiration rates in thinned stands will increase over time and stand-level transpiration between treatments may remain the same.