| 英文摘要: | The recent climate-exacerbated mountain pine beetle infestation in the Rocky Mountains of North America has resulted in tree death that is unprecedented in recorded history. The spatial and temporal heterogeneity inherent in insect infestation creates a complex and often unpredictable watershed response, influencing the primary storage and flow components of the hydrologic cycle. Despite the increased vulnerability of forested ecosystems under changing climate1, watershed-scale implications of interception, ground evaporation, and transpiration changes remain relatively unknown, with conflicting reports of streamflow perturbations across regions. Here, contributions to streamflow are analysed through time and space to investigate the potential for increased groundwater inputs resulting from hydrologic change after infestation. Results demonstrate that fractional late-summer groundwater contributions from impacted watersheds are 30 ± 15% greater after infestation and when compared with a neighbouring watershed that experienced earlier and less-severe attack, albeit uncertainty propagations through time and space are considerable. Water budget analysis confirms that transpiration loss resulting from beetle kill can account for the relative increase in groundwater contributions to streams, often considered the sustainable flow fraction and critical to mountain water supplies and ecosystems. In Colorado alone, the mountain pine beetle (MPB) has impacted over 1.3 million hectares of pine forest2. Although evapotranspiration is generally assumed to decrease in beetle-affected watersheds, tree death also causes competing effects on evapotranspiration. By the end of the first growing season following infestation, a killed pine no longer transpires3, causing the needles to turn red (identified as red-phase) and begin to drop. Within three to four years after infestation, most trees have lost all remaining needles (grey-phase; ref. 4). The resultant loss of canopy cover increases fluxes of water and energy to the ground surface, causing changes in soil moisture dynamics and snowmelt processes5, 6 that may offset the effects of reduced evapotranspiration. Increases in soil moisture6, 7 are dependent on the net increase of water inputs due to losses of transpiration and canopy evaporation balanced against the net decrease in moisture from higher solar exposure, surface temperature and ground evaporation6. The interactions among these processes are poorly understood across scales, highlighting the need for better quantification of net transpiration changes from MPB infestation at the watershed scale. Transpiration is commonly quantified using sap flux3, eddy covariance8 or energy balance formulations9. Each of these methods presents limitations. For sap flux and energy-based approaches, upscaling stand-scale estimates requires spatially comprehensive measurements to capture the heterogeneity of vegetation and local energy balances. Eddy covariance methods provide larger-scale estimations of evapotranspiration but are less reliable in mountain environments10 and do not separately assess evaporation and transpiration. The exceptional extent of tree death from the MPB provides a unique opportunity to evaluate the contribution of tree processes to the hydrologic cycle at watershed scales, where water budget perturbations are complex and often combine non-uniquely. Here, we quantify hydrologic changes in MPB-impacted watersheds by identifying changes in streamflow contributions through a chemical and isotopic hydrograph separation analysis. The importance of transpiration loss is relative to the magnitude of the other components of the hydrologic cycle, for example, precipitation, snowmelt, evaporation and soil moisture11. In the Rocky Mountains of North America, the effect of transpiration at the watershed scale may be most apparent during late summer, when near-surface antecedent soil moisture and snow inputs approach their annual minima, and the relative importance of subsurface contributions is greatest12, 13. During this low-flow period, loss of transpiration may lead to measurable increases in recharge and groundwater contributions to streamflow, whereas loss of interception and increased ground evaporation would influence both surface and subsurface contributions to streamflow, as conceptualized in Fig. 1. The distribution of late-summer flows is not commonly studied, but may have important implications for water supply, water rights, impairment of riverine ecosystems, and water quality concerns, such as formation of disinfection by-products in water from MPB-impacted watersheds14.
| http://www.nature.com/nclimate/journal/v4/n6/full/nclimate2198.html
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