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A positive relationship between drought and insect attacks on trees is widely accepted (e.g. Mattson & Haack, 1987). Much of the evidence for this positive relationship is based on correlative observations of insect activity and tree mortality after drought. Experimental manipulations of water availability to mature trees that include measurements of insect defense mechanisms and tree mortality are rare. Furthermore, drought stress in trees may have a negative or no effect on insect populations and effects often vary by insect feeding guild, damage agent status (i.e. primary or secondary), and intensity and duration of drought (Larsson, 1989; Huberty & Denno, 2004; Jactel et al., 2012). An improved understanding of drought impacts on tree resistance to insect attacks is critical because climate models forecast more frequent and severe droughts in the future across many subtropical regions (Seager et al., 2007), and forests globally appear susceptible to increases in mortality during drought (e.g. Allen et al., 2010). Here, we report the first experimental evidence regarding the role of drought in insect resistance mechanisms, insect attacks, and mortality of mature trees of the two dominant trees of woodlands of the southwestern USA, piñon pine (Pinus edulis) and one-seed juniper (Juniperus monosperma).
Bark beetles (Coleoptera: Curculionidae), which include the most lethal insects to trees in western forests (Raffa et al., 2008), often attack and kill drought-stressed trees (Huberty & Denno, 2004; Jactel et al., 2012). Drought has well-established physiological impacts on trees, including compromised hydraulic function, reduced carbohydrate production, and possible disruption of carbohydrate supply to sinks (McDowell et al., 2008; McDowell & Sevanto, 2010; Sala et al., 2010; Ryan, 2011). Reduced carbon uptake as a result of stomatal closure and diminished transport to sinks could diminish tree bark beetle defense by reducing carbon available for resin production (McDowell et al., 2008; Sala et al., 2010), which is acknowledged as the primary defense against bark beetles (Berryman, 1972; Christiansen et al., 1987; Herms & Mattson, 1992; Strom et al., 2002; Franceschi et al., 2005). Furthermore, the growth-differentiation-balance hypothesis (GDBH) predicts that moderately water-stressed trees may actually be more resistant to bark beetle attacks because moderate water stress constrains growth sinks more than defensive compound sinks, leading to a relative increase in resin production (Loomis, 1932; Lorio, 1986; Herms & Mattson, 1992; Dunn & Lorio, 1993; Stamp, 2003). The framework also predicts that long-term, severe droughts eventually reduce defense because chronic depletion of carbohydrates reduces carbon availability for resin production (McDowell et al., 2011). Similarly, reduced phloem transport during drought could reduce carbon available for resin production (Dunn & Lorio, 1992; Sala et al., 2010). Despite substantial research on carbon allocation within trees (e.g. Litton et al., 2007), the role of drought in resin defense of trees is poorly understood.
Mortality of piñon was unusually high throughout the southwestern USA during a severe drought in 2000–2003 (Breshears et al., 2005; Shaw et al., 2005; Kleinman et al., 2012). In 2002, Arizona and New Mexico had one of the driest and warmest years on record and by 2003 c. 774 711 ha of piñon woodlands had evidence of bark beetle activity (United States Department of Agriculture, aerial surveys (USDA 2002–2010); Kleinman et al., 2012). Regional (Arizona, Utah, Colorado and Nevada) piñon mortality increased at least two-fold during this drought (Shaw et al., 2005; Williams et al., 2012) and certain areas had > 90% mortality (Breshears et al., 2005; Floyd et al., 2009). Mortality was lower for juniper than for piñon during this drought (Shaw et al., 2005; Floyd et al., 2009; Koepke et al., 2010).
Juniper and piñon have been useful model organisms for studies of tree mortality during drought because they often grow together, but their different survival and hydraulic strategies are representative of a broad range of species in semi-arid regions globally (Mueller et al., 2005a; McDowell et al., 2008; Adams et al., 2009; Koepke et al., 2010). Piñon is more isohydric than juniper (McDowell et al., 2008; Plaut et al., 2012) and minimizes xylem cavitation during drought via stomatal closure to reduce water loss. Juniper, by contrast, can withstand negative xylem pressures with little cavitation during drought and continues carbon assimilation (Linton et al., 1998).
Trees can be attacked by many pathogens and guilds of herbivores. The most lethal or abundant herbivores can determine which components of tree defense are most crucial to tree survival. The most lethal insect to piñon in recent years is Ips confusus (LeConte), the piñon ips bark beetle, which generally attacks stressed or recently dead trees (Wood, 1982b; Rogers, 1995; Raffa et al., 2008). In contrast to piñon, little is known about the insect guild of junipers. Insects that can damage junipers include bark beetles and wood borers (Coleoptera: Buprestidae & Cerambycidae) (Furniss & Carolin, 1977; Itami & Craig, 1989), but data are scarce regarding the amount of tree mortality attributable to these agents.
Our study evaluated the impacts of water availability on insect attacks, resin defenses, canopy condition, and mortality of co-occurring piñon and one-seed juniper over 3 yr via a large-scale precipitation manipulation experiment performed in situ on mature trees. Based on differences in mortality during nonexperimental drought (Shaw et al., 2005; Koepke et al., 2010) and physiological differences in carbon uptake and water relations between these species (e.g. McDowell et al., 2008), we hypothesized that 3 yr of experimental drought would decrease resin defense and increase bark beetle attacks and mortality of piñon, but have little impact on juniper. Moreover, we compared treatment effects on the carbon isotope ratio (δ13C) of leaf sugars and bole resin of piñon to assess the role of recent photosynthate in resin synthesis. Finally, we examined the relationship between resin volume and other parameters of defense/vigor in piñon to understand possible tradeoffs.
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Our study provides the first experimental evidence that drought predisposes mature piñons to bark beetle attacks and improves understanding of mechanisms of drought impacts on insect resistance of piñons. One or 2 yr of drought with 45% reduction in ambient precipitation increased bark and twig beetle attacks and tree mortality for piñon. In addition, we found that junipers were stressed, but not killed, by 3 yr of drought.
Interestingly, experimental drought had variable impacts on the insect resistance characteristics we measured on piñon. Based on previous reports of reduced photosynthesis of piñon during drought (McDowell et al., 2008; Breshears et al., 2009) and evidence of 7 months of near-zero gas exchange of piñon in the drought treatments at our study site (Plaut et al., 2012), we hypothesized that piñons in the H2O− treatment would be carbon limited and thus have lower resin flows in response to wounding than in other treatments. Support for this hypothesis differed for bole and twig resin. For bole resin, the H2O− treatment had little impact on flow. We should note that, similar to other studies of southwestern pines (Gaylord et al., 2007, 2011), piñon bole resin flow was extremely variable in our study (coefficient of variation = 127%), and thus a larger sample size would be needed to detect treatment effects. In addition, our sampling methodology did not allow us to examine within-tree variation of bole resin flow.
Our finding of no treatment effect on piñon bole resin δ13C in combination with lower δ13C for leaf sugars in the H2O+ treatment suggests a lag between drought impacts on carbon assimilation and resin synthesis, because most bole resin was constitutive and formed from previously assimilated carbon. A larger difference in δ13C between pre- and post-treatment values of leaf sugars (≈ 2.5‰) than resin (≈ 0.6‰) provides additional support for this interpretation. Our interpretation that pine bole resin is largely constitutive and formed from old assimilates has been previously reported for seedlings (Lewinsohn et al., 1991; Guérard et al., 2007), but this is the first report for mature trees.
The results for twig resin, that is, less flow in both the H2O− and the H2O+ treatments than in other treatments on some dates, were more consistent with our hypothesis and with predictions of the GDBH framework of a bell-shaped relationship between carbon-based defense and drought stress (Herms & Mattson, 1992; Stamp, 2003). The bell-shaped relationship is expected because severely water stressed trees, such as in the H2O− treatment, are not able to supply photosynthate for resin production, whereas well-hydrated trees, such as in the H2O+ treatment, preferentially allocate photosynthate to growth rather than defense. This tradeoff results in the moderately water-stressed trees (the A and C treatments in our study, which experience seasonal water stress from the ambient precipitation cycle) having the greatest resin production and intermediate growth. Contrary to our prediction, however, we found no relationship between twig resin volume and tree radial growth (Fig. S2). Interestingly, we found a weakly significant, bell-shaped relationship between bole resin volume and tree radial growth (Fig. S1b). In conclusion, our findings of no relationship between twig resin flow and bole resin flow, and differing treatment effects on resin flow from the bole and twigs, strongly suggest that resin synthesis and flow are compartmentalized within piñon trees, and argue against attempts to infer the defense capability of an entire tree from resin flow measured on a single location or tissue.
The greater bark and twig beetle attacks on piñons in the H2O− treatment than in other treatments may be explained by treatment effects on tree characteristics other than resin flow. For instance, I. confusus, the attacking bark beetle in our study, may select hosts via olfactory or auditory cues from drought-stressed trees (Kimmerer & Kozlowski, 1982; Wood, 1982a; Tyree et al., 1984; Mattson & Haack, 1987; Stumpf & Johnson, 1987; Tadege et al., 1999; Seybold et al., 2006). Furthermore, resin quality (e.g. terpenes and crystallization rate) or production of induced defenses, both of which could be impacted by tree water stress, may be more important than constitutive resin to beetle attack success (Raffa & Berryman, 1982; Lieutier, 2002). The importance of induced defenses for piñon, which could include the production of additional resin after constitutive resin has been drained by initial attacks, is not known, although studies of another southwestern pine (Pinus ponderosa) have shown little induction of resin flow by traumatic bole wounding or inoculation with beetle-vectored fungi (Wallin et al., 2003; Gaylord et al., 2011). Moreover, other studies have reported a positive association between sapwood resin duct abundance and pine survival during drought and bark beetle attacks (Kane & Kolb, 2010; Kläy, 2011; A. K. Macalady, unpublished data). Our study found that trees that died had smaller resin ducts than those that survived, and most duct parameters were reduced by the H2O− treatment by the third year, but not earlier (Fig. 3). In contrast to other studies (Blanche et al., 1992), our study did not find a significant relationship between constitutive resin flow and resin duct parameters (Fig. S1c-f).
We measured piñon twig resin because of previous reports that twig resin is important for tree resistance to stem-boring moths (Whitham & Mopper, 1985; Mopper et al., 1991; Brown et al., 2001; Mueller et al., 2005b). Tree damage from stem moths was minor in our study (ranging from 0 to 5% foliage impacted on a tree for any year); however, twig beetle attacks were common in dying piñons and may have contributed to mortality. To our knowledge, no studies have assessed the effect of twig resin on twig beetle performance or host selection. We found no difference in twig resin flow between trees that were attacked by twig beetles and those that were not (P = 0.32). This result suggests little functional importance of twig resin flow to tree resistance to twig beetles, although more research is needed on this topic.
Extremes of water availability in our study had no effect on juniper resin flow, which is consistent with current understanding that juniper maintains gas exchange during drought (e.g. McDowell et al., 2008; Plaut et al., 2012). We also found no evidence of bark beetles in junipers, suggesting that susceptibility to insect attacks was unchanged by water availability. Damage associated with the juniper twig pruner (twig dieback) occurred at the beginning of our study before the onset of treatments, but this damage did not increase during the study and few junipers died over the 3 yr of the study, consistent with reports that this Cerambycid is generally not associated with tree death (Furniss & Carolin, 1977). In addition, there was some evidence of Buprestids on dead junipers, as indicated by ovoid as opposed to circular exit holes; however, no western cedar borers (a Buprestid associated with juniper mortality) were collected in Lindgren funnel traps (Lindgren, 1983), which we deployed at the study site in 2010 for monitoring purposes (n = 4, two with I. confusus lure (cis-verbenol, 50/50 ipsdienol and 50/50 ipsenol), and two with a general wood borer lure (ethanol ultra high release (UHR) and alpha pinene gel UHR)) (Contech Enterprise, Victoria, BC, Canada). Although we know little about juniper resistance to insect attacks, our study and others (e.g. Floyd et al., 2009) have concluded that insect damage is not a strong contributor to juniper decline during drought.
Greater needle browning of junipers in the H2O− treatment than in other treatments over 3 yr clearly shows sublethal impacts of drought on juniper. Our finding of drought-induced canopy loss in juniper, but not whole-plant mortality, is consistent with previous nonexperimental field observations in the southwestern USA after severe drought in 2002 (Koepke et al., 2010). While our results show that mature one-seed juniper can survive at least 3 yr with 45% of ambient precipitation, the intensity and duration of drought required for substantial juniper mortality are not yet known.
Climate predictions for the southwestern USA suggest that droughts will be more frequent and severe as a consequence of global warming (e.g. Seager et al., 2007). Our results provide strong experimental evidence that 1 or more years of drought with 45% less than ambient precipitation will kill many piñons, and 3 yr of the same drought treatment will cause substantial canopy loss of one-seed juniper and suggest increased mortality in the future. Differences in mortality among species could lead to fundamental ecosystem shifts in dominant plants of woodlands and also changes in associated communities dependent on these plants (Allen & Breshears, 1998, 2007; Brown et al., 2001). Furthermore, biotic agent impacts may lag drought, which may provide opportunities for future investigators to examine the correlation between droughts and bark beetle outbreaks more fully (e.g. Williams et al., 2012). Although our study cannot address whether bark beetles were the ultimate agent of tree mortality during drought, our study shows that drought predisposes piñon trees to bark beetle attacks. Furthermore, increased temperatures, which occur often with drought in the southwestern USA, could increase bark beetle populations and tree mortality via increases in insect reproductive rate and geographic range (Ayres & Lombardero, 2000; Tran et al., 2007; Bentz et al., 2010). Increased availability of susceptible hosts for bark beetle attacks during warming and drought could lead to increased bark beetle outbreaks and further landscape-scale tree mortality.