RESPONSE OF LODGEPOLE PINE GROWTH TO CO2 DEGASSING AT MAMMOTH MOUNTAIN, CALIFORNIA.
In 1989, a series of long-period volcanic earthquakes were reported under Mammoth Mountain, at the western margin of the Long Valley Caldera in California's Sierra Nevada, USA (Pitt and Hill 1994). At that time, a sudden uplift (after a four-year hiatus) of the resurgent dome, located in the central portion of the caldera, was also noted by Yamashita et al. (1992). These events suggested that a magmatic intrusion had occurred in the subsurface structure beneath the mountain. Beginning in 1992, localized tree stress, detected as browning of the leaves, was observed at Horseshoe Lake (Thomas Heller, USDA Forest Service, Mammoth Mountain Ranger Station, personal communication). This was originally attributed to drought conditions from previous years or to pest infestations. However, high carbon dioxide levels near the ground and further expansion of species-independent tree death suggested that soil C[O.sub.2] concentrations could be responsible for the forest decline. Subsequent analyses on soil gas revealed 3He/4He ratios indicative of mantle-derived volatiles (Sorey et al. 1993), and soil C[O.sub.2] emission rates were anomalously large, ranging from 4 x [10.sup.5] kg/d (Rahn et al. 1996) to 12 x [10.sup.5] kg/d (Farrar et al. 1995). Farrar et al. (1995) then proposed that the observed tree death was caused by impairment of root systems following concentrated soil C[O.sub.2] of magmatic origin. Observations in 1995 have shown that the area of tree mortality has expanded to [approximately]42 ha of land, separated into seven distinct localities, along the base of the mountain (Rahn et al. 1996).
As annual tree rings incorporate environmental signals, we used a dendroecological approach (Fritts and Swetnam 1989) to test if xylem formation at Mammoth Mountain had responded to magmatic C[O.sub.2] emissions, and, if so, to date the onset of the phenomenon and to quantify its temporal evolution. Persistent anaerobic soil conditions can cause root inhibition, nutrient deficiency, dehydration, root death, and finally tree death (Kramer 1951, McKee et al. 1984, Topa and McLeod 1986, Palta and Nobel 1989). Besides possible signals in ring width, root impairment may also lead to altered stable carbon isotope ratios in wood holocellulose (Francey et al. 1984). In this paper we report the ring-width patterns of lodgepole pines (Pinus contorta) in areas of tree kill at Red's Lake and Horseshoe Lake. Sampled pines were categorized as live, stressed, and dead trees, depending on their crown vigor. Relationships with local climate, as well as with degassing phenomena, are investigated. We also employed the [[Delta].sup.13]C signatures of soil, atmosphere, and ring holocellulose in a live, a stressed, and a dead tree, to formulate hypotheses on the underlying physiological processes.
We sampled a total of 21 overstory lodgepole pines on Mammoth Mountain in the summer of 1996 [ILLUSTRATION FOR FIGURE 1 OMITTED]. Sampling was concentrated on dominant trees growing in areas with [greater than]20 [degrees] slope. Some dead and stressed pines were sampled at sites with a large amount of tree mortality, such as Red's Lake (RL, [ILLUSTRATION FOR FIGURE 1 OMITTED]), where no slope was steeper than 5[degrees]. Based on the percentage of live foliage at the time of sampling, six trees were dead, eight were alive and healthy, and seven showed signs of stress (browning foliage). Two increment cores were taken from the trunk, at points 180 [degrees] from each other and [approximately]1.3 m above ground. In the laboratory, cores were vertically aligned, then glued to grooved wooden mounts. Cores were surfaced using a belt sander and progressively finer sandpaper, until tracheid walls were clearly visible under a stereo-zoom binocular microscope. Ring patterns were visually cross dated (Stokes and Smiley 1968) to assign preliminary dates to each annual growth layer. Ring width was then measured to the nearest 0.001 mm using a sliding stage interfaced with an image analysis system. We performed numerical quality control using the COFECHA software (Holmes 1983, Grissino-Mayer et al. 1996). Dating accuracy was first tested among all measured cores from Mammoth Mountain. Then, cores used in this study were also checked against nearby Pinus contorta tree-ring records that are available from the International Tree Ring Data Bank (ITRDB; National Oceanic and Atmospheric Administration 1997a). Ring-width measurements from two sites, Lone Lake and Yosemite Park (Briffa and Schweingruber 1992; [ILLUSTRATION FOR FIGURE 1 OMITTED]), were combined with those from Mammoth Mountain and numerically tested for dating errors.
While ring width was measured on all collected cores, time and budget constraints allowed the analysis of stable isotope features in three trees: one live, one stressed, and one dead lodgepole pine. The live tree was located on a southeast-facing slope, within an open stand of lodgepole pine and mountain hemlock. The stressed tree was taken at Red's Lake tree kill and was within an open stand of lodgepole pine and mountain hemlock, on almost flat terrain. The dead tree was sampled near Horseshoe Lake (HSL; [ILLUSTRATION FOR FIGURE 1 OMITTED]), within a fairly dense stand of dead lodgepole pines. Stable carbon isotopes were measured on both increment cores taken from a tree, using extreme caution to avoid chemical contamination while dating and measuring tree rings. Annual values were obtained by means of Leavitt and Danzer's (1993) method of separating the earlywood portion of the annual ring, homogenizing it, extracting the holocellulose, and combusting it. The C[O.sub.2] produced was cryogenically isolated, and the [[Delta].sup.13]C was measured on a VG Prism II Mass Spectrometer (Micromass, Beverly, Massachusetts, USA) at Scripps Institution of Oceanography, La Jolla, California. The 13C/12C ratios were given as relative deviations from the PDB (peedee belemnite) isotope standard (Craig 1957):
[[Delta].sup.13]C = 1000([R.sub.sample]/[R.sub.standard] - 1)
where R is equal to 13C/12C ratios of the C[O.sub.2], and values are expressed in %[per thousand] units. We estimated overall reproducibility, including cellulose preparation, combustion, and mass-spectrometric analysis, as [+ or -]0.1%[per thousand], based on replicate analyses of a wood cellulose standard taken from a lodgepole pine sampled at Horseshoe Lake.
Atmospheric and soil C[O.sub.2] concentrations and isotopic signatures were measured at the study sites. Evacuated 1-L glass flasks were used to collect air and soil gas samples. Atmospheric C[O.sub.2] was sampled 2 m above the ground and into the wind to avoid contamination due to human respiration. Soil gas samples were taken at depths of 30 cm by using a stainless steal soil probe. We measured concentrations at the laboratory using a gas chromatograph with a flame ionization detector (FID) (after the conversion of C[O.sub.2] to methane). Carbon dioxide was cryogenically isolated and isotopically measured on the VG Prism II Mass Spectrometer.
We computed the Mammoth Mountain tree-ring chronology in order to minimize individual variability in ring-width series (Douglass 1914, Fritts 1976), as follows:
[Mathematical Expression Omitted]
with [Mathematical Expression Omitted], chronology value at year t; w, crossdated ring width; y, modified negative exponential or straight line with slope [less than or equal to]0; [[symmetry].sub.i], biweight robust mean (Mosteller and Tukey 1977) of the i values, i = 1, . . ., [n.sub.t]; and [n.sub.t], number of measured specimens that include year t. To remove differences in data processing, the Lone Lake and Yosemite Park ring-width series obtained from the ITRDB were combined in a tree-ring chronology using the same method applied to the Mammoth ring-width series. The computer program ARSTAN (Autoregressive Standardization Program; Cook and Holmes 1996, Grissino-Mayer et al. 1996) was used for data processing. To test for differences among trees of different vigor, separate tree-ring chronologies were also computed for live, stressed, and dead trees sampled at Mammoth Mountain.
Climatic data came from different sources to obtain comparative results and to increase the number of years available for analyses. Monthly precipitation (1904-1994) and temperature (1906-1994) records for Yosemite Park Headquarters were obtained from the United States Historical Climatology Network (National Oceanic and Atmospheric Administration 1997b). Monthly precipitation data for Gem Lake (1931-1994) and Huntington Lake (1931-1994) were taken from the National Climatic Data Center online data set of U.S. Cooperative and National Weather Service stations (National Oceanic and Atmospheric Administration 1997c). Regional precipitation and temperature records were provided by the National Climatic Data Center online data set of divisional data, compiled at monthly intervals for the period 1895-present (National Oceanic and Atmospheric Administration 1997d). Statistical significance of monotone temporal trends was tested using the univariate, nonparametric, Mann-Kendall test (Kendall and Gibbons 1990).
We investigated the relationship between climate and tree growth by means of correlation analysis and response function analysis. Correlation of tree-ring chronologies with annual or seasonal climatic variables is straightforward (Douglass 1914, 1919). Correlation with monthly climatic variables requires more advanced statistical techniques to account for possible collinearity of predictors (Fritts 1991). These techniques are based on multiple regression between the tree-ring index and the principal components of the monthly climatic predictors, as well as on bootstrapped confidence intervals to test significance of each monthly variable (Fritts et al. 1971; Guiot 1990, 1991). To account for numerical and biological persistence in the tree response to climate, we employed a 14-mo dendroclimatic window, going backwards from October of the current growth year to the previous September.
Lodgepole pines at Mammoth Mountain were characterized by "complacent" ring patterns, with little year-to-year variability and a relatively small amount of common variance. Of all collected samples, a total of 21 cores from 12 trees could be cross dated, and these were then measured to develop 'tree-ring chronologies. Tree-ring series span the period 1720-1995, with a mean series intercorrelation of 0.44 and a mean sensitivity of 0.21. For comparison, mean series intercorrelation and mean sensitivity at Lone Lake (23 series) were 0.63 and 0.23, and, at Yosemite Park (22 series), they were 0.59 and 0.23. Because of the low level of variability in ring width series, the final chronology for Mammoth Mountain was restricted to the period 1815-1995, when each annual value was computed using 10-21 different core samples. The mean chronology [ILLUSTRATION FOR FIGURE 2 OMITTED] shows extremely low growth in the 1990s, superimposed on a declining growth trend over the 20th century. The reliability of cross dating was confirmed by the agreement with the nearby Yosemite chronology (Briffa and Schweingruber 1992; [ILLUSTRATION FOR FIGURE 2 OMITTED]), independently developed and publicly available from the International Tree Ring Data Bank. Correlation with the Yosemite chronology is 0.63, and it is the highest correlation between the Mammoth Mountain tree-ring chronology and all other chronologies included in the International Tree Ring Data Bank.
Climatic signals in the Mammoth Mountain tree-ring chronology were weak, as already pointed out, and correlations with the Yosemite Park Headquarters station were marginally better than correlations with National Oceanic and Atmospheric Administration climate division data. The Yosemite data have a high degree of reliability, because they are part of the Historical Climate Network, whose records were adjusted to account for missing values, changes in time of reading, and other sources of bias (National Oceanic and Atmospheric Administration 1997b). The annual cycle is characterized by very dry summers, with little rainfall falling from June to September (the warmest period of the year), and August precipitation averaging only 4.9 mm. The wet period for the year is from October to April, and precipitation falls mostly as snow. A significant positive trend during the 20th century was found in the mean temperature of every month except March, October, and November. Because of this trend, the warm season has increased in length and intensity during the 1900s, causing a progressively early onset of the spring-summer transition, an intensification of the summer warm peak, a delay in the summer-fall transition, and a milder winter. To eliminate such trend, monthly temperature series were filtered using a smoothing spline with a 50% variance response at a period of 10 yr (Cook and Peters 1981).
For response functions, it was necessary to estimate any missing value in the temperature or precipitation record. The Yosemite temperature record included no missing values during 1906-1994, and the precipitation record included only four missing values. Those missing values occurred in the 1990s and were estimated using the Gem Lake, Huntington Lake, and divisional precipitation records. Correlation between Yosemite and the three other precipitation time series is very high (0.68-0.94). No significant trend was found in monthly precipitation during the 20th century. Because of the growth suppression in the 1990s, response functions between the Mammoth Mountain tree-ring chronology and the Yosemite climate data were run for the 19071990 period. No single monthly climatic parameter was a significant predictor of tree growth. This finding agreed with the relatively little amount of common variability among tree-ring series.
Tree-ring patterns of currently live, stressed, and dead pines revealed an overall decline during the 20th century [ILLUSTRATION FOR FIGURE 3 OMITTED]. The decline was more pronounced among stressed pines (Mann-Kendall [Tau] = -0.48, P [less than] 0.0001) and dead pines (Mann-Kendall [Tau] = -0.42, P [less than] 0.0001) compared to live ones (Mann-Kendall [Tau] = -0.32, P [less than] 0.0001). Live pines did not show the pronounced 1990s growth dip that was evident in dead and stressed pines [ILLUSTRATION FOR FIGURE 3 OMITTED]. Long-term variability was removed from the tree-ring chronologies in the same way used for temperature trends. Residuals from the decadal-scale pattern were then tested for correlation with climatic parameters at Yosemite. The years after 1990 in both stressed and dead pine chronologies were excluded from such correlations. The live-tree chronology was significantly correlated only with May temperature (r = 0.33, P = 0.0015), which was also significantly correlated with the dead-tree chronology (r = 0.24, P = 0.0249). The stressed-pine chronology correlated best with July temperature (r = -0.34, P = 0.0015). March precipitation also had a significant correlation with the stressed-pine chronology (r = 0.24, P = 0.0296) and with the dead-pine chronology (r = 0.28, P = 0.0101). Overall, climatic signals were less obscure than for the mean Mammoth Mountain chronology; yet they did not reveal a clear pattern of mean climate-tree growth relationships over the whole 20th century.
In healthy lodgepole pine stands, concentrations of atmospheric C[O.sub.2] ranged 348-444 [micro]mole C[O.sub.2]/mole air, and [[Delta].sup.13]C[O.sub.2] was between -7.24%[per thousand] and -7.67%[per thousand], well within the boundaries of normal diunal cycles in mountainous, forested areas (C. Keeling, Scripps Institution of Oceanography, La Jolla, California; personal communication). Similarly, [[Delta].sup.13]C of C[O.sub.2] in the soil column averaged -18.7%[per thousand], a value similar to other western American woodlands (Mook 1980), and mean soil C[O.sub.2] concentration was 0.4%, a typical value for decomposition of organic matter and root respiration (Crill 1991, Kicklighter et al. 1994). In areas of tree kill, concentration and [[Delta].sup.13]C of atmospheric C[O.sub.2] were similar to those in healthy stands. However, [[Delta].sup.13]C of soil C[O.sub.2] exhibited a mean of -1.61%[per thousand], and soil C[O.sub.2] concentration was [greater than]12% of air volume in soil. The [[Delta].sup.13]C value suggests the presence of magmatic C[O.sub.2], because the isotopic signature from a basaltic melt is within - 1 and -4%[per thousand] (Sano et al. 1994). Even though soil C[O.sub.2] concentration may vary both spatially and temporally within one tree mortality area (Farrar et al. 1995, Rahn et al. 1996), all measured concentrations were at least 20x greater than normal respiratory soil C[O.sub.2]. Stable carbon isotopes in annual growth layers were analyzed on crossdated rings for the 1988-1995 period. The [[Delta].sup.13]C signature in tree holocellulose differed among the live, stressed, and dead pines [ILLUSTRATION FOR FIGURE 4 OMITTED]. Beginning in 1990, the rings of both the stressed and dead trees revealed enrichment in 13C when compared to rings of the live tree.
Lodgepole pines sampled at Mammoth Mountain showed little variability in patterns of ring width. Even when separate tree-ring chronologies were developed for live, stressed, and dead pines, correlation with climatic parameters was low. Most likely, microenvironmental site factors (Major 1951), such as substrate, topography, stand dynamics, and soil properties, played a more important role than climate in forcing tree growth. For instance, Fritts (1969) found that trees on south-facing slopes, which are drier and receive more solar radiation (Haase 1970), exhibited the highest ring width variability ("sensitivity"). Trees on west-facing, steep slopes also showed high sensitivity due to removal of snow by the prevailing winds. Trees on southeast-facing slopes near protecting ridges had wide rings of low variability, because the site was kept moist by the accumulation of drifting snow from the lee of the ridge (Fritts 1969). Since most precipitation at the study area falls as winter snow, it is possible that local variations in snow depth and melting rates may obscure large-scale climatic signals.
The overall decline observed during the 20th century in growth rates of live, stressed, and dead pines at the study area was associated with an increase in mean temperatures. Considering that detrended time series were being used, the negative correlation between growth rates of stressed pines and July temperature is the strongest indication we found for climatically driven growth decline. Summer is the driest time of the year, and water stress could be responsible for the negative correlation between July temperature and tree growth. If temperature increases while precipitation does not, trees are bound to experience a higher degree of moisture deficits. However, the July correlation was not significant for live and dead trees, and in general climate signals in tree-ring chronologies were neither consistent nor strong. Even though we cannot rule out the possibility that the pine decline was linked to the increased temperatures experienced by the region during the 20th century, there is not enough quantitative evidence to support such a claim.
Soil C[O.sub.2] shows [[Delta].sup.13]C enrichments from magmatic degassing at the study area, whereas atmospheric C[O.sub.2] does not. Most likely, gusty wind conditions and relatively open forest canopies throughout the study area prevent buildup of magmatic C[O.sub.2] within the atmosphere. The 2-3%[per thousand] enrichment in 13C of pines growing within the tree kill site reflects the heavy isotopic signature observed within the soil. Among the factors leading to heavier [[Delta].sup.13]C in tree rings, climatic effects were apparently negligible, because the enrichment was absent in the live tree. Even though trees may have directly incorporated magmatic C[O.sub.2], it is more likely that excess soil C[O.sub.2] from volcanic degassing impaired root systems and water uptake, thereby increasing water stress and inducing stomata closure, which usually enriches the [[Delta].sup.13]C signature of the plant (Farquhar et al. 1982, Leavitt and Long 1988, 1989).
It is remarkable that the onset of magmatic C[O.sub.2] degassing in 1990 was recorded in tree-ring series as an abrupt growth reduction. Between 1991 and 1995, ring indices of stressed and dead lodgepole pines were [approximately]46% and 39% lower, respectively, than their 19001990 means. Such extremely low growth was superimposed on an overall, gradual decline during the 20th century, possibly related to increasing air temperatures in the region. Even though trees suffering a growth decline may have been predisposed to be killed or severely damaged by magmatic degassing, our study adds a new facet to previous investigations of volcanic phenomena causing abrupt changes in tree rings (e.g., Smiley 1958, LaMarche and Hirschboeck 1984).
We would like to thank Bruce Deck for all of his patience and guidance in the laboratory and Mike Sorey for advice as well as information on Mammoth Mountain. J. Fessenden owes thanks to Martin Wahlen, her advisor, for his support and interest in this study. F. Biondi was supported by NSF grant ATM-9509780, and by a Scripps postdoctoral fellowship made possible by a grant from the G. Unger Vetlesen Foundation. The comments of R. Alan Black and two anonymous reviewers greatly improved the original manuscript.
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|Author:||Biondi, Franco; Fessenden, Julianna E.|
|Date:||Oct 1, 1999|
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