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Disturbance events are important drivers of ecosystem patterns and processes (Turner, 1989), and understanding the patterns associated with such events allows for a greater understanding of the variability within a system. Particularly concerning are recent tree die-off events at up to regional scales, apparently triggered by a combination of drought and heat – previously referred to as ‘global-change-type drought’ (Breshears et al., 2005) – in some cases associated with pests and pathogens (Allen et al., 2010). The physiological mechanisms associated with drought-induced mortality remain a major uncertainty, and appear to be related to complex interrelationships between plant hydraulics and carbon metabolism (McDowell et al., 2008, 2011; Zeppel et al., 2012). Nonetheless, the fundamental drivers of such die-off patterns often appear to be a combination of reduced precipitation, warmer temperature and associated increased atmospheric demand. There is a need for improved relationships, even if empirical rather than mechanistic, that can aid in prediction of forest vulnerability to future climate. While controlled experiments are being used to untangle the mechanisms of drought-induced tree mortality (Adams et al., 2009; Plaut et al., 2012), simultaneously broad-scale analyses are needed that utilize extensive available data sets on climate drivers that affect tree die-off. Indeed, recent synthesis of ecosystem responses to severe drought reveals strong cross-biome patterns associated with precipitation (Ponce Campos et al., 2013).
Among the most studied examples of drought-induced tree mortality is the die-off of the pinyon pine (Pinus edulis) in the southwestern USA during drought occurring around the year 2000. In the semiarid southwestern USA, drought disturbance has altered vegetation patterns and forest and woodland dynamics (Allen & Breshears, 1998; Mueller et al., 2005), and has persisted in areas since the mid-1990s (Breshears et al., 2005; Shaw et al., 2005), with extreme drought conditions occurring between 2002 and 2004. Drought, coupled with increased temperatures (Breshears et al., 2005; Adams et al., 2009) and an associated bark beetle (Ips confusus) outbreak in pinyon pine, a co-dominant tree species of the pinyon–juniper woodlands (Juniperus spp.), caused widespread pinyon die-off occurring between 2002 and 2003 (Breshears et al., 2005).
Most regional climate models predict increasing future temperatures and decreased precipitation in the southwestern USA (Seager et al., 2007; Overpeck & Udall, 2010). Persistent drought conditions and increased temperatures will continue to alter vegetation patterns as dominant overstory species trees continue to die (Adams et al., 2009; Allen et al., 2010), especially as water budgets become more stressed (Seager et al., 2007). Temperatures during the drought of 2002 and 2003 were higher than in the previous recorded drought of similar magnitude, which occurred in the 1950s (Breshears et al., 2005). The elevated temperatures combined with decreased precipitation increase the vapor pressure deficit (VPD), causing substantial negative impacts on the physiology of the tree, and increasing the potential for carbon starvation or xylem cavitation (Adams et al., 2009; McDowell et al., 2011; Choat et al., 2012), ultimately leading to death.
Elevated temperature can cause increased die-off risks through physiological mechanisms in trees; however, temperatures also affect insect physiology and life cycles (Raffa et al., 2008). Warmer temperatures can increase the populations of many outbreak-type insect species (Swetnam & Lynch, 1993; Logan et al., 2003; Bigler et al., 2006). These outbreaks occur as vegetation becomes stressed and host plant defense mechanisms are unable to keep herbivorous insects from attacking (Waring & Cobb, 1992). The effect of temperature and the associated change in atmospheric demand (as reflected in vapor pressure deficit) has recently been shown to contribute to patterns of regional variability in die-off (Weiss et al., 2009, 2012). However, other recent studies suggest that this effect should be related to precipitation amount. Edaphic variables such as soil topography (Kleinman et al., 2012) and soil water-holding capacity (WHC) (Peterman et al., 2012) have been shown to be important controls on the patterns of pinyon die-off. Edaphic variables controlling die-off appear to be related to factors that control available moisture over varying temporal scales, and have also been shown to control overall canopy cover in water-limited woodlands (Kerkhoff et al., 2004). Precipitation patterns probably underlie these responses, affecting plant-available water and associated plant water stress.
In summary, several studies have documented the impacts of drought, increased temperatures, and bark beetle infestation on die-off in pinyon–juniper woodlands since the die-off event of 2002 and 2003 (Negron & Wilson, 2003; Breshears et al., 2005; Mueller et al., 2005; Shaw et al., 2005; Huang et al., 2010; Clifford et al., 2011; Royer et al., 2011; Gaylord et al., 2013), but no studies have examined pinyon die-off along a selected precipitation gradient. In this study, we identified spatial gradients of die-off using an extensive network of field sites and extrapolated to the entire area using remote sensing and asked the question: How do environmental patterns alter pinyon pine die-off dynamics over a region that experienced a gradient of mortality? In addition to correlating die-off patterns to regional climatic variation, we discuss our results in the context of a recently reported relationship of pinyon die-off to soil WHC (Peterman et al., 2012) and VPD (Williams et al., 2013), as well as stand-level variables identified in other studies (Negron & Wilson, 2003; Floyd et al., 2009) that could potentially promote die-off in pinyon pine.
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We show a pattern of pinyon die-off along a precipitation gradient during a drought, where the most notable pattern was a strong threshold response of die-off to precipitation; sites above a 600-mm threshold of cumulative precipitation during the drought period had little to no die-off, while sites < 600 mm were highly variable but included areas with high levels of die-off. Furthermore, this threshold response in die-off had a distinct south to north pattern, where reduced precipitation occurred in more northerly sites. Data from weather stations at the extreme north and south ends of the study region showed that the northern portion experienced earlier drying and a more severe drought than the southern portion (Fig. 2). The tree responses below the c. 600-mm threshold provide a key insight regarding a survival threshold. Decreases in precipitation across the gradient of our sites were c. 100 mm over 2 yr. This precipitation-related threshold for survival is also notable in the context of recent syntheses highlighting the point that many tree species persist at the brink of tolerable water potential deficits (Choat et al., 2012). Notably, we also detected a die-off threshold related to VPD, with mortality occurring at a warm season (May–August) mean VPD of c. 1.7 kPa. This result reinforces the important interaction between atmospheric demand and drought, highlighted experimentally (Adams et al., 2009) and in recent regional analysis and projection (Williams et al., 2013). In addition to the patterns in cumulative climate metrics, there was also evidence for such thresholds in seasonal data, for both precipitation and VPD (Figs S4, S5).
Because precipitation and VPD were important climatic metrics of die-off, we examined WHC as a further link to understand the patterns of mortality. However, we did not find a strong relationship between die-off and soil WHC, as was detected in Peterman et al. (2012). Our sites covered a much smaller spatial extent and our methods of pinyon die-off detection was different from those used in Peterman et al. (2012), which may explain the differences in our results. Thus, over the extent of pinyon pine distribution, existing WHC data may be an important predictor of pinyon die-off, but at more local scales, where WHC varies less, a combination of other variables apparently had greater control over pinyon die-off. The relatively coarse detail of the SSURGO data set compared with the spatial scale used in our study may limit our ability to detect a WHC effect; there was not much variability in the data set and most of our sites are near the upper end of the classifications used in Peterman et al. (2012; W. Peterman, pers. comm.).
We also did not find any compelling relationships between tree die-off and stand characteristics that would explain either the threshold response or variation in die-off among sites that received < 600 mm precipitation. We found a significant, but weak increase in die-off with decreasing stand density. This finding is consistent with two other studies of the die-off event (Floyd et al., 2009; Clifford et al., 2011), but contrasts with results from other studies that have reported positive associations between tree density and increased pinyon die-off (Negron & Wilson, 2003; Weisberg et al., 2007; Greenwood & Weisberg, 2008). As discussed in Floyd et al. (2009) and Clifford et al. (2011), there are several reasons why die-off might increase or decrease with density, including the level of tree mortality, the tree species involved, and the range of stand densities.
We extrapolated field and Landsat estimates of die-off to estimate regional impacts on carbon, and land surface–atmospheric flux and feedback. Regarding carbon loss, our estimates are lower than, but comparable to, those of Huang et al. (2010), implying that, while the stand characteristics differ between Colorado and New Mexico (Floyd et al., 2009), drought and die-off had similar effects on carbon loss. We expect that the regional loss in canopy cover will correspond to an increase in shortwave albedo, and a suppression of longwave radiation, similar to the impact of changes in semi-arid forests on climate systems at the same latitude (Rotenberg & Yakir, 2010). The offset of carbon loss to a cooling effect resulting from increased albedo and suppressed longwave radiation could be further quantified with respect to total climate forcing with the inclusion of flux data (beyond the scope of this study). We note that the relationship between landsat-based canopy cover and dNBR to field-based canopy cover may be vastly improved with higher resolution imagery (Greenwood & Weisberg, 2009; Royer et al., 2011). Consequently, the relationship between climate variables and spectral signatures produced by remote sensing would potentially align more closely to field-based climate variables.
Insights from pinyon–juniper woodlands may have broader relevance, in that distribution is tightly linked to water balance in general, and particularly mortality events (Kerkhoff et al., 2004; Breshears et al., 2005, 2009b); consequently, they may provide insights for other water-limited systems. They can also span a large range of canopy cover from open savannah through dense woodland (Kerkhoff et al., 2004; Breshears, 2006), which may provide insights in the context of mixed woody-herbaceous ecosystems (House et al., 2003). More generally, tree die-off driven by drought and associated factors, such as pests and pathogens and anomalously warm conditions, remains a fundamental challenge to predict (Breshears & Allen, 2002; McDowell et al., 2008, 2011; Allen et al., 2010). Much effort is currently focused on resolving the mechanistic details associated with mortality, including the interrelationships between hydraulic failure, carbon starvation and biotic agents, and associated water balance and carbon metabolism dynamics (McDowell et al., 2011; Choat et al., 2012). Also emerging are mortality-related linkages that span the continuum from precipitation and soil moisture, plant water potential and conductance, photosynthesis and respiration to associated plant hydraulics and carbon metabolism. Arguably these relationships are most exhaustively documented for Pinus edulis, which could serve as a model species for conifer responses (Martinez-Vilalta et al., 2012). The amount and timing of reductions in precipitation, concurrent with anomalously warm temperatures, resulted in regional-scale die-off for this species (Breshears et al., 2005; Shaw et al., 2005) and also resulted in soil moisture levels that were below plant available thresholds for most of the time during extreme dry years preceding mortality (Breshears et al., 2009a). Consequently, plant water potential dropped below a threshold of stomatal closure (Breshears et al., 2009b), resulting in reduced conductance, transpiration and respiration (McDowell et al., 2008, 2011). This can result in die-off that appears to be related, at least in part, to carbon metabolism, based on a glasshouse experiment (Adams et al., 2009, 2013), but also highlights the interrelationship of plant hydraulics and carbon metabolism (McDowell et al., 2011; Plaut et al., 2012). Nonetheless, precipitation and related metrics such as soil WHC, in association with temperature data, are the most widely accessible data associated with the drivers of die-off events and if interpreted appropriately, may aid in revealing key aspects of drought mortality thresholds (Breshears et al., 2009a; Peterman et al., 2012), especially when analyses control for climatic gradients, as was done here.
In conclusion, climate variability has played a dominant role in the dynamics of stand development and population structure of pinyon–juniper woodlands of the southwest (Swetnam & Betancourt, 1998; Barger et al., 2009; Clifford et al., 2011). Our data suggest that within-region climate variability can be an important source of woodland heterogeneity, where extensive drought in one location of a region can lead to greater die-off and more radically altered vegetation structure than another location (Allen & Breshears, 1998). In further studies, drought- and bark beetle-induced die-off in pinyon pines was more common in larger, mature trees (Mueller et al., 2005; Clifford et al., 2008), suggesting that changes in demography and recruitment will occur in coming decades (Redmond & Barger, 2013). Through such changes in demographics, some areas such as the northern portion of our study area will probably be comprised of a much younger pinyon pine population (Mueller et al., 2005), whereas others will have less dense stands compared with areas in the southern part of the study area (Fig. 4). Furthermore, a litany of ecosystem-level and biophysical changes to forest productivity have likely occurred in the northern region (Anderegg et al., 2012; Edburg et al., 2012). Numerous studies suggest that rapid die-off alters carbon cycling (Huang et al., 2010; Hicke et al., 2012), hydrological cycles (Guardiola-Claramonte et al., 2011; Adams et al., 2012), and near-ground insolation and evapotranspiration (Royer et al., 2011) which greatly affect regional-scale Earth systems feedbacks (Adams et al., 2010). Our results refine how precipitation patterns within a region influence pinyon die-off, revealing a precipitation and VPD envelope for tree mortality and its uncertainty band where other factors probably come into play – a response type that influences stand demography and landscape heterogeneity and that is of general interest yet has rarely been documented. Understanding the processes behind the patterns of die-off and how these will impact the cascade of Earth systems feedbacks will be vital to modeling the future of forested regions in the southwestern USA under a changing climate.