Implications of Woody Bioenergy Feedstock Production for Water Supply and Hydrologic Regulation Services

Woody bioenergy plantations are generally managed to maximize biomass production.  This can be accomplished through various management approaches, such as species selection, short rotation lengths, genetic breeding programs, planting density, species combinations, and chemical and fertilizer application.  One of the consequences of maximizing productivity in bioenergy plantations is a trade-off between increased carbon uptake by rapidly growing trees (positive for carbon sequestration and climate change amelioration) and increased water use by plantations relative to the original native vegetation or pre-existing vegetation the original vegetation (negative for downstream water supply).  Moreover, the particular management regime employed for bioenergy production can also affect soil hydraulic properties that determine partitioning of throughfall between soil and groundwater recharge versus runoff, thereby influencing peak and dry season flows.  However, few studies have examined the impacts of establishing bioenergy plantations on these hydrologic services, and the trade-offs compared to alternative management practices.  In this presentation, we show initial results from research in Wisconsin, USA that quantifies water use in two aspen plantations (ages 10 and 24) and a mature aspen-dominated forest stand (age 33) to understand potential consequences of increased bioenergy production in the region on water supply. Findings indicated that average stand transpiration in the 10 and 24 year old stands was 25K L d^-1 and 16K L d^-1 ha^-1, respectively, compared to 62K L d^-1 ha^-1 for the mature stand.  These differences in stand water use reflect both the higher estimated sapwood area for the 10 year old stand (25.7 m² ha^-1) compared to the 24 year old and mature forest stand (12.0 and 10.2 m² ha^-1, respectively), combined with the much higher mean sap velocity for the mature forest stand (44.1 cm h^-1) compared to the 10 and 24 year old stands  (9.0 and 15.0 cm h^-1, respectively).  These results highlight the importance of considering potential carbon-water tradeoffs when designing and managing tree plantations for bioenergy production, and underscore the need to balance both biomass accumulation and water use. We discuss our findings within the larger context of understanding the potential implications of stand water use in bioenergy plantations across different stand ages, species, and management regimes for downstream water supply and hydrologic regulation services.