Die-off indirect effects
Direct hydrological consequences of tree mortality include changes in E, T, I, and snow accumulation and melt dynamics. Considering these changes to the water balance, tree mortality may indirectly affect other aspects of hydrological functioning, such as infiltration and flow path partitioning, which would subsequently result in changes in soil moisture status, groundwater recharge dynamics, and streamflow volume and timing. For example, if tree mortality decreases overall ETI, as discussed above, and precipitation remains the same, then more water is available for these other components as an indirect effect of tree die-off. Whether this water enters the soil or becomes overland flow will depend on the infiltration capacity of the soil, which could be increased through inputs of dead material like needle litter from dying trees and creation of macropores when dead trees fall, or decreased if soil organic material is washed away following tree mortality.
Subsurface connections such as fractured bedrock that create flow paths to groundwater are not directly affected by tree mortality (although there could be potential effects from roots in rock; Schwinning, 2010), yet affect whether water that enters the soil following tree mortality will contribute to groundwater recharge (for non-die-off effects, see Wilcox, 2002; Seyfried and Wilcox, 2006; Wilcox et al., 2006, Wilcox and Huang, 2010). Published studies on the indirect effects of tree mortality on flow path partitioning are rare, and assessments of groundwater recharge from die-off are almost non-existent. At one intensively studied experimental watershed in southern Germany, Grosse Ohe, an isotopic tracer model was used to partition discharge response for a catchment that underwent a ∼53% forest cover loss though Norway spruce (Picea abies) mortality (Beudert et al., 2007). Tree mortality caused a 39% decrease in ET associated with a 135% increase in runoff and a 125% increase in groundwater flow. Additionally, NO3 concentrations of soil water increased from 10 to 200 mg/l at 40 cm in depth and to 130 mg/l at 100 cm, demonstrating that tree die-off can affect water quality. In a watershed in Colorado experiencing extensive mortality of lodgepole pine NO3, NH4, and total N increased in soils under stands of dead trees, but this did not translate to elevated NO3 in stream water in the near-term following die-off (Clow et al., 2011).
Increased groundwater recharge can translate to increased streamflow volumes and higher flows during low flow periods, and in areas where the water table is already close to the ground surface, it may increase the potential for saturation excess overland flow or unfavourable conditions for seedling establishment and tree growth. This combination of higher antecedent wetness conditions, elevated groundwater levels, and more area available for overland flow generation could lead to changes in the timing and magnitude of responses to rainfall and snowmelt. Streamflow, which integrates overland, subsurface, and groundwater flows differently depending on watershed properties, may ultimately be indirectly affected by tree mortality. Quantified as water yield, streamflow will increase if tree mortality decreases watershed ETI, and conversely decrease if ETI is increased.
The variation in water yield responses reported for die-off ecohydrology studies, all in watersheds with conifer mortality involving bark beetle outbreaks, demonstrates a wide range of possible hydrological responses to tree die-off (Table II). In two northern Colorado river drainages in the United States annual water yield increased by 10% following a bark beetle outbreak that killed up to 80% of trees in the late 1930s and early 1940s, mostly from nonproportional streamflow increases during wet years (Bethlahmy, 1974, 1975). At Jack Creek in southern Montana, United States, a mountain pine beetle outbreak killed 35% of trees across the watershed from 1975 to 1977 (Potts, 1984). This event caused a 15% increase in annual water yield, a 2–3 week advance in the onset of snowmelt-driven flows, and a 10% increase in low flows over a period of 5 years post-mortality. The research in southern Germany at Gross Ohe showed that peak flows in a catchment with ∼53% tree mortality increased by a factor of 2·2 relative to an adjacent catchment mostly unaffected by die-off (Beudert et al., 2007).
In contrast, research examining the streamflow response to mortality of lodgepole pine and subalpine fir (Abies lasiocarpa) in eight Colorado, US catchments found that streamflow relative to precipitation was unchanged in seven catchments where canopy cover loss averaged 43%, and even decreased by 31% in one catchment that had 50% tree mortality (Somor, 2010; unpublished manuscript). In addition, an assessment of water yield changes in the southwestern United States after extensive piñon pine die-off found that five semi-arid basins, ranging in size from ∼1000 to ∼5000 km2 and which lost 11–21% of tree cover, had on average ∼50% less water yield post-mortality after correcting for precipitation changes (Guardiola-Claramonte, 2009; Guardiola-Claramonte et al., in press). In addition, these basins had significantly delayed streamflow generation compared to similar unaffected basins. The responses of these watersheds were attributed to rapid post-die-off understorey growth detected in a remote sensing analysis for the same basins (Rich et al., 2008). This increase in understorey cover was speculated to have reduced overland flow (consistent with Zou et al., 2010) and increased infiltration, T, and I, increasing overall ETI in these dry, low-elevation watersheds (Guardiola-Claramonte, 2009; Guardiola-Claramonte et al., in press).
The extensive mountain pine beetle outbreak across British Columbia, Canada in the last decade has driven development of a process-based hydrological model to estimate peak- and low-flow responses across the mostly ungauged Fraser River Basin (Carver et al., 2009a,b; Weiler et al., 2009). Incorporating changes in ETI and snow dynamics with flow components and discharge responses, the model suggests that with complete mortality of all trees in the watershed, snowmelt-induced peak flows increase with area of forest affected, up to a maximum of 140%, with a 26% increase predicted at the Fraser River outlet. High variability in these projections across the basin revealed potential nonlinear thresholds in the hydrological response and specific effects of differences in runoff generation processes (i.e., Hortonian overland flow, saturation excess overland flow, subsurface flow) among the watersheds with increasing mortality.
Other relevant research on indirect effects. As noted previously, the wide range of responses documented in the small set of die-off hydrology literature calls for drawing on the forest harvest literature to further consider likely indirect effects of forest die-off. Early reviews of paired catchment studies in US watersheds found harvest responses to be highly variable (Bosch and Hewlett, 1982). However, more recent reviews have focused on organizing responses by climate and treatment characteristics (e.g. Brown et al., 2005). Harvesting intensity is a primary determinant of water yield response, with canopy cover removal thresholds of 20–25% suggested to enable statistical detection of a response across several environments (Bosch and Hewlett, 1982; Stednick, 1996; Brown et al., 2005; Troendle et al., 2010). However, for cases with a reduction of less than 20% forest cover, lack of significant hydrological responses could be due to low statistical power from short post-treatment records or the effects of different harvesting techniques (McMinn and Hewlett, 1975).
Locations with greater annual precipitation tend to show greater hydrological sensitivity to forest treatments compared to drier locales (Bosch and Hewlett, 1982; Stednick, 1996; Troendle et al., 2010). Similarly, results from high elevation lodgepole pine forest also show that at a given site, water yield in wetter years is more sensitive to harvest (Troendle and King, 1987). A global analysis of catchment studies comparing the ET of forested versus non-forested watersheds with similar climates demonstrated that potential water yield changes could be predicted from annual precipitation (Zhang et al., 2001). This assessment suggested that little change should be expected with a shift from forest to grassland for sites below 500 mm of annual rainfall because potential ET at these sites is a large proportion of precipitation. The largest increases in water yield have occurred when coniferous forest cover was removed in mesic environments, while removal of ‘scrub’ cover elicited the smallest response across a variety of climates (Brown et al., 2005; see also hypotheses in Huxman et al., 2005).
The seasonality of precipitation and streamflow in a watershed determined the season that experienced the greatest shifts in water yield after treatment (Brown et al., 2005). For example, in watersheds where most precipitation occurred in summer, forest removal increases in annual water yield were driven by proportionally higher increases in summer water yield. Likewise, for watersheds dominated by snowmelt peak flows, tree harvest increased annual water yield through higher and earlier snowmelt peak flow (Brown et al., 2005; Zou et al., 2010).
Ecohydrological responses to die-off vary substantially and are much more inconsistent than well-reviewed responses to forest harvest (Figure 2). In particular, initial responses of decreased streamflow following canopy loss from die-off in drier forests are not seen in response to forest harvest, suggesting caution should be used in overgeneralizing from these results. Specifically, the finding of decreased streamflow following mortality in the piñon-juniper ecosystem (Guardiola-Claramonte, 2009; Guardiola-Claramonte et al., in press) contrasts with previous tree removal research in watersheds of the same vegetation type. At the Beaver Creek experimental watershed in northern Arizona, two watershed treatments where 100% of tree cover was removed by harvest resulted in unchanged flows (Clary et al., 1974; Baker, 1984). A third watershed was treated with an herbicide targeted at junipers that removed 83% of tree cover. This increased annual streamflow initially by 65% in 4 years post-treatment (Clary et al., 1974) and by 157% over 8 years post-treatment (Baker, 1984). The larger watersheds considered in the recent die-off study also include some higher-elevation, mesic forests (Guardiola-Claramonte, 2009; Guardiola-Claramonte et al., in press). Changes in precipitation dynamics at high elevations could have exerted a disproportionate influence on whole basin water yield numbers following piñon die-off (Guardiola-Claramonte, 2009; Guardiola-Claramonte et al., in press). On the other hand, at Beaver Creek, in addition to killing junipers, the herbicide treatment also initially damaged piñon pines and led to a shift in the understorey from perennial to annual grasses (possibly further depressing T); harvested watersheds also were subjected to some burning of slash (Clary et al., 1974; Baker, 1984). Other assessments of water yield response to tree removal in piñon-juniper watersheds found that ETI still accounted for almost all precipitation following treatment, and that flows did not increase unless slash was burned (Gifford, 1975; Wright et al., 1976). Assessment of other shrub-dominated watersheds suggests that subsurface characteristics which permit deep drainage of soil water are key to determining if shrub removal leads to increased streamflow (Wilcox, 2002; Seyfried and Wilcox, 2006; Wilcox et al., 2006).
Figure 2. The relationship between canopy cover reduction and annual water yield change for die-off hydrology studies that measured or estimated water yield. Points represent individual studies from Table II. For Somor (2010) the response of seven catchments where water yield was unchanged (diamond) and the single catchment with decreased flows (triangle pointing down) are shown separately. Values shown for Carver et al. (2009b) are modelled peak flow maximum (triangle) and Frasier River outlet peak flow changes (plus sign). The grey area represents the range of hypothesized water yield responses to die-off. Arrows indicate that for Beudert et al. (2007) values reported are for peak flows and annual water yield change is likely lower, and that for Bethlahmy (1974, 1975) canopy cover reduction was up to 80% and average canopy cover reduction is likely much lower. Also shown is the relationship between canopy cover reduction by harvest and water yield increase (dashed line), calculated from the dataset of Stednick (1996)
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In summary, studies of die-off effects on indirectly affected hydrological processes are limited in scope and not always consistent with relevant harvest study results, perhaps due to a variety of factors. Inconsistencies in measuring and reporting canopy cover loss among die-off studies may also be contributing to the variability in responses: it is easier to measure the impact of an externally applied forest harvest than to quantify a tree mortality event that varies greatly with space and time. We hypothesize that the interaction of three influences—annual precipitation, level of canopy loss, and belowground characteristics—determines many of the ecohydrological differences among studies of responses to tree cover loss both within and between die-off and harvest responses, all of which will vary with successional dynamics.