Land application of treated municipal wastewater or reclaimed water on established forests is a cost-effective and environmentally friendly disposal alternative, particularly in the Inland Northwest, where tree growth is limited by soil water and nutrient availability. This collaborative research includes five water reuse facilities in northern Idaho that have been applying Class C reclaimed water on native coniferous forests for variable time periods up to several decades. Forest responses to treated wastewater application has been studied extensively in deciduous hardwood forests of eastern United States. However, literature on response of coniferous western forests is significantly lacking. With a multi-decadal time-scale, the chronosequence of facilities (5, 10, 20, 30 and 40 years of application) provides an opportunity to study forest productivity and soil responses to reclaimed water. To address the effect on productivity, we utilized dendrochronological methods to assess average annual increment in tree ring widths in western red cedar (Thuja plicata), Douglas fir (Pseudotsuga menziesii) and Ponderosa pine (Pinus ponderosa) across five plots in both control and wastewater management units at each facility. Trees from each diameter class for each species were randomly selected, cored, prepared and analyzed using WinDENRO for annual growth responses. Approximately 800 trees were cored.
Results/Conclusions
Analysis of preliminary cores from two of the five sites indicated a significant effect (p<0.05) of reclaimed water on average annual increment in growth rings. Significant interaction was observed between treatment in effect with respect to tree species and was most significant for western red cedar. Tukey’s pairwise contrasts also showed that western red cedar is more responsive to reclaimed water application. Knowledge of maximum productivity potential and corresponding soil responses will lead to improved forest management and cultural practices. The study also presents an important opportunity to link greater forest production with improved water quality. Nutrient saturation and leaching have been attributed to prolonged land-application of reclaimed water and chronic nitrogen amendment. Nutrient loss through leaching occurs when nutrient absorption capacity of forests is exceeded. Thus, it is essential to ensure that the land-application forest systems do not eventually become nutrient saturated and start leaching into groundwater sources and surface waters. Understanding long-term capacity of forests to accumulate biomass and filtering nutrients will allow water reuse facility managers to formulate sound forest management strategies to prevent nutrient contamination and consequent environmental degradation, while presenting possibilities of generating revenue through timber production.