How does water yield respond to mountain pine beetle infestation in a semiarid forest?

Jianning Ren,Jeffrey A Hicke,Crystal A Kolden,John T Abatzoglou,Jennifer C Adam,Christina L Tague,Erin J Hanan,Mingliang Liu

doi:10.5194/hess-25-4681-2021

Abstract

Abstract. Mountain pine beetle (MPB) outbreaks in the western United States result in widespread tree mortality, transforming forest structure within watersheds. While there is evidence that these changes can alter the timing and quantity of streamflow, there is substantial variation in both the magnitude and direction of hydrologic responses, and the climatic and environmental mechanisms driving this variation are not well understood. Herein, we coupled an eco-hydrologic model (RHESSys) with a beetle effects model and applied it to a semiarid watershed, Trail Creek, in the Bigwood River basin in central Idaho, USA, to examine how varying degrees of beetle-caused tree mortality influence water yield. Simulation results show that water yield during the first 15 years after beetle outbreak is controlled by interactions between interannual climate variability, the extent of vegetation mortality, and long-term aridity. During wet years, water yield after a beetle outbreak increased with greater tree mortality; this was driven by mortality-caused decreases in evapotranspiration. During dry years, water yield decreased at low-to-medium mortality but increased at high mortality. The mortality threshold for the direction of change was location specific. The change in water yield also varied spatially along aridity gradients during dry years. In wetter areas of the Trail Creek basin, post-outbreak water yield decreased at low mortality (driven by an increase in ground evaporation) and increased when vegetation mortality was greater than 40 % (driven by a decrease in canopy evaporation and transpiration). In contrast, in more water-limited areas, water yield typically decreased after beetle outbreaks, regardless of mortality level (although the driving mechanisms varied). Our findings highlight the complexity and variability of hydrologic responses and suggest that long-term (i.e., multi-decadal mean) aridity can be a useful indicator for the direction of water yield changes after a disturbance.

Highlights

In recent decades, mountain pine beetle (MPB) outbreaks in the Western US and Canada have killed billions of coniferous trees (Bentz et al, 2010)
This was especially true during wet years; there was no significant increase during dry years (i.e., 1992, 1994, 2001, and 2004; Fig. 6a)
We tested a coupled ecohydrologic and beetle effects model of a semi-arid basin in southern Idaho to examine how the watershed hydrology responds to a beetle outbreak and how interannual climatic variability, vegetation mortality, and long-term aridity influence these responses

Summary

Introduction

Mountain pine beetle (MPB) outbreaks in the Western US and Canada have killed billions of coniferous trees (Bentz et al, 2010). It is essential to understand how ecosystems and watersheds respond to beetle outbreaks and to identify the dominant processes that drive these responses (Bennett et al, 2018). While some studies show increases in water yield following beetle outbreaks (e.g., Bethlahmy, 1974; Livneh et al, 2015; Potts, 1984), many others show no change, or even decreases (e.g., Biederman et al, 2014; Guardiola-Claramonte et al, 2011; Slinski et al, 2016). To determine which mechanisms control the change in water yield following a beetle outbreak, more quantitative approaches are needed

Results

Discussion

Conclusion