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Tree-Ring Variation in Pinyon Predicts Likelihood of Death Following Severe Drought

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Tree-Ring Variation in Pinyon Predicts Likelihood of Death Following Severe Drought November 2000 NOTES 3237 Ecology, 81(11), 2000, pp. 3237±3243 q 2000 by the Ecological Society of America TREE-RING VARIATION IN PINYON PREDICTS LIKELIHOOD OF DEATH FOLLOWING SEVERE DROUGHT KIONA OGLE,1 THOMAS G. WHITHAM, AND NEIL S. COBB Northern Arizona University, Department of Biological Sciences and the Merriam-Powell Center for Environmental Research, Flagstaff, Arizona 86011 USA Abstract. A severe drought in northern Arizona caused widespread pinyon (Pinus edulis) mortality, exceeding 40% in some populations. We measured tree-ring widths of pinyons that survived and that died in three sites designated as ``high,'' ``medium,'' and ``low'' stress. Growth characteristics during the previous 10±15 years can be used to predict the likelihood of drought-induced death; dead trees exhibited 1.5 times greater variation in growth than live trees. A model of ring-width deviations vs. drought severity showed a loss of ``climatic sensitivity'' with age in dead trees. These differences were independent of site. We found two distinct tree types that are predisposed to die during drought; highly sensitive young trees, and insensitive older trees. As the Southwest has a dynamic climate typi®ed by severe droughts, it is important to understand how droughts act as bottleneck events to affect a dominant tree in a major vegetation type of the United States. Key words: climatic sensitivity; drought; environmental stress; mortality, likelihood following drought; Pinus edulis; pinyon pine; tree rings. INTRODUCTION (Betancourt 1987, Betancourt et al. 1990), allowing us General circulation models project that a doubling to place current changes into a paleoecological per- spective. From a community perspective, much is of atmospheric CO2 in the near future will result in shifts in the spatio-temporal distribution of precipita- known about how environmental stress affects pinyons, tion (e.g., Cubasch et al. 1995). Complex interactions which in turn affects insect herbivores (Mopper et al. between the terrestrial biosphere and a changing cli- 1991, Cobb et al. 1997), mycorrhizal mutualists (Gehr- mate are predicted to dramatically affect vegetation ing and Whitham 1994, Gehring et al. 1998), birds and responses (Neilson and Drapek 1998). By studying cur- mammals (Christensen and Whitham 1991, 1993). rent patterns of drought-induced plant death, we may The purpose of this study was to compare growth be able to predict important population and commu- attributes of pinyons that died (``dead'') during the nity-level responses to various climate change scenar- 1996 drought with those that survived (``live''). We ios. hypothesized that, prior to death, dead pinyons were Severe droughts have the potential to greatly impact more sensitive than live trees to wet and dry events, pinyon (Pinus edulis) populations and their associated and this greater sensitivity should be expressed as communities. In northern Arizona, a severe drought in greater ring-width variation. Though variance in ring 1996 resulted in high pinyon mortality rates, exceeding widths has been adopted as an index of environmental 40% on south-facing slopes of cinder cones where the sensitivity (e.g., Fritts 1974, 1976), it does not separate effects of drought are likely to be most severe. Pinyon the components of the tree's environment that drive the is an appropriate tree for study because it is considered response (e.g., climate, herbivory, nutrients, competi- to be drought adapted, and it is widely distributed tion). We focused on the role of climate by modeling across Mexico and the western United States (Mirov ring-width responses to drought severity and tested for 1967, Lanner and Van Devender 1998). Moreover, a differences in ``climatic sensitivity'' between live and rich history of its biogeographical expansion with the dead trees. This analysis allowed us to investigate how changing climate of the last 40 000 yr has accumulated climatic sensitivity varies across the extreme water and nutrient-stressed cinder soils of Sunset Crater National Manuscript received 24 May 1999; revised 10 January 2000; Monument (358229 N, 1118329 W) and adjacent sandy accepted 13 January 2000; ®nal version received 4 February loam soils (e.g., Table 1). 2000. This study is unique in several aspects: (1) very high 1 Present address: Duke University, Department of Biol- ogy, Durham, North Carolina 27708 USA. pinyon mortality during 1996 provided large sample E-mail: [email protected] sizes; (2) we are unaware of any study that has ex- 3238 NOTES Ecology, Vol. 81, No. 11 TABLE 1. Site characteristics of cinder and sandy-loam sites For the high stress site, we sampled transects parallel and pinyon pine performance that re¯ect differences in de- gree of environmental stress. to the slope of southern exposed faces of seven cinder cones for a total of 21 transects. Length and number Sandy of transects per cone varied depending on cone size. Cinder loam We pooled transect data within each habitat to estimate Parameter² soils³ soils site-speci®c mortality. Mean air temperature (8C) 20.3 20.8 We collected tree cores to quantify growth; dead and Mean daily precipitation (mm) 1.56 1.66 live pinyons (diameter $ 8 cm) were paired by size Gravimetric soil water content (%) 3.2 5.6* Mean soil temperature (8C) 17.2 13.8* and proximity (5±50 m apart) to reduce variation in Extractable phosphate (mg/g) 4.45 12.20* growth due to differences in age, competition, and mi- Soil NO3 mineralization crosite. Due to logistics, we were limited to collecting (mg´g21´d21) 0.015 0.147* one core from each bole, as near to the base as possible Soil NH4 mineralization (mg´g21´d21) 20.012 0.062* for accurate aging; additional cores were collected if Mean number of female cones per the ®rst core was damaged or greatly missed the pith. tree 185.0 300.0* Mean xylem pressure (MPa) Trees on slopes were bored parallel to the slope to avoid Mean stem growth (length, mm) 22.83 22.38* reaction and tension wood so that growth was not over 33.36 46.32 or under estimated. We followed methods of Swetnam * Signi®cant site effect (P # 0.005). et al. (1988) for preparing cores. Cross dating was ac- ² Obtained from Christensen and Whitham (1991), Mopper complished by visually comparing skeleton plots with et al. (1991), Gehring and Whitham (1994), Cobb et al. master chronologies for nearby sites, downloaded from (1997), and Swaty et al. (1998). 2 ³ Data only available for ¯at terrain; south-facing slopes the International Tree Ring Data Bank (ITRDB). For of cinder cones were not investigated. recent years, we used 1989 as a marker year because it was characterized by a relatively severe drought and resulted in narrow rings. amined variability in growth as a predictor of drought- induced mortality; and (3) we sampled pinyons across Tree growth, climatic sensitivity, age, a wide range of ages to tease apart interactions between and drought tolerance age, sensitivity to climate, and drought tolerance. We We began by examining simple growth character- address this question: ``Can we use patterns of tree- istics of dead and live pinyon across the three different ring variation to predict who will live and die during sites. These included lifetime growth rate (mean ring a severe drought?'' Understanding how severe droughts width), recent (1986±1995) growth rate, and variance might act as bottleneck events and affect selection pres- in mean lifetime ring-width index. Ring-width indices sures is important for generating predictions of the po- were calculated to remove the inherent effect of tree tential impact of climate change on gene frequencies, age on growth. For each series, we modeled the age species composition, and ecosystem function. trend in the raw widths by ®tting linear and negative METHODS exponential functions to the data, and selected the ap- propriate model by comparing correlation coef®cients Sites, mortality transects, and tree cores and residuals. Indices were obtained by dividing the Trees were growing in three habitat types in the Co- raw widths for each series by the predicted width given conino National Forest in northern Arizona. Habitat by the model ®t. Growth attributes of dead and live types were operationally de®ned as ``high,'' ``medi- pinyon were compared in an ANOVA with site and um,'' and ``low'' stress based on slope, aspect, and soil mortality class as ®xed factors, and dead and live trees type. South-facing slopes of cinder cones within Sunset were blocked by pair number (random factor) (SAS Crater National Monument characterize the high stress 1990; mixed models procedure). habitat; the medium stress site consisted of cinder soil Our main objective was to model ring-width varia- ¯at terrain and occurred in this same area. The low tion as a function of age, climate, and habitat type to stress habitat was characterized by sandy loam soil investigate how climatic sensitivity differed between ;10±15 km to the south of the cinder soil sites. Our dead and live pinyon growing along an environmental justi®cation for these three stress classi®cations is stress gradient. We modeled growth as a function of based on previous studies at these same sites (Table 1). Palmer Drought Severity Indices (PDSIs, based on pre- From fall 1996 to spring 1997 we counted numbers cipitation, evapotranspiration, soil properties, and ac- of dead and live pinyon encountered in 40±50 m wide cumulated effects of past conditions on present weath- transects (;15 each in the low and medium stress sites). Transects were conducted away from roads, were 2±4 2 NOAA Paleoclimatology Program, Boulder, Colorado, km long, and ranged in elevation from 1740±2120 m. USA. URL: ^http://www.ngdc.noaa.gov/paleo/&. November 2000 NOTES 3239 er; see Palmer 1965); analyses showed that ring widths comparison, we also conducted an rANCOVA with were better correlated with PDSI than with rainfall ring-width indices.
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