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Dendroclimatology of Torrey pine (Pinus torreyana Parry ex Carr.).

INTRODUCTION

The southern and central California coast are characterized by the presence of several endemic conifer species (Barbour and Major, 1977). Widespread urban development in recent decades has generated both great interest and concern over the fate of such coastal endemic conifers because of the limited size of their natural habitat. In particular, the survival of those species within their current distribution range is intimately linked to the health and climatic sensitivity of the adult tree population. Gathering information on how climate variability affects growth of overstory trees is then helpful to devise management strategies that sustain ecosystem integrity by protecting native biodiversity. Since controlled experiments tend to be indoors and to regard early portions of a tree life span, dendroclimatological studies (Fritts, 1976) provide accurate information on the interannual and interdecadal response of mature trees to climate. In turn, because of the dominant role of tree species in land-surface vegetation, dendroclimatological research is useful to quantify the sensitivity of terrestrial ecosystems to climate change (Cook and Cole, 1991).

As part of a large-scale study on long-term climate-tree growth relationships in coastal areas, we present here the dendroclimatology of Torrey pine (Pinus torreyana Parry ex Carr.). Torrey pine has one of the most limited geographical ranges (a few square kilometers) and population size (about 10,000 individuals) in the Pinus genus. Its natural distribution is limited to a mainland population on the southern California coast between La Jolla and Del Mar, and to a disjunct population 280 km to the NW on Santa Rosa Island, approximately 50 km offshore from Santa Barbara (Haller, 1986). Torrey pine has persisted in those areas throughout the Holocene (Cole and Liu, 1994) and was much more widespread in ancient times (Kellogg et al., 1927). The two populations have been isolated from each other for several thousand years, but they do not seem to differ genetically (Waters and Schaal, 1991), even though morphological and environmental differences exist (Haller, 1986). Despite the ease of access to its mainland population, and the high biodiversity value of the species, little is known on the life history and ecological relationships of Torrey pine in its natural range (Vogl et al., 1977).

Dendroclimatological studies at coastal sites have been relatively rare in the past because of the widespread belief that trees from such sites are not sensitive to climate variability, not easily datable using dendrochronological techniques, and short-lived. Recently, Buckley et al. (1992) and Wiles et al. (1995) have shown that coastal species in the Gulf of Alaska and along the Northeastern Pacific coast incorporate strong summer temperature signals in their xylem layers, and could be used to reconstruct climate variability in that region over the last few centuries. Further to the S along the California borderland, drought-sensitive tree-ring chronologies from near-coastal and interior mountain ranges have been used to reconstruct precipitation (Schulman, 1947; Meko et al., 1980; Michaelsen et al., 1987; Haston and Michaelsen, 1994; Meko et al., 1995), sea surface temperature (Douglas, 1980), fire history (Brown and Swetnam, 1994), and El Nifio events (Michaelsen, 1989). With the exception of Brown and Swetnam (1994), the above-mentioned studies did not incorporate tree-ring records from endemic conifer species growing along the southern and central California coast. Furthermore, those species have been the subject of no previously published dendroclimatic research.

For studying the climatic response of Torrey pine, one has to consider that, besides precipitation and temperature regimes, the climate of the American West Coast is characterized by the marine fog layer, which can occur throughout the year. Coastal fog is generated offshore, then spread inland by sea breezes, and usually reaches land areas below 250-400 m elevation (Leipper, 1994; Filonczuk et al., 1995). Even though fog occurrence is widespread and frequent, the relationship between fog and tree growth is not well understood, especially on interannual and longer time scales. The present study was then devised to address the following questions: (a) Is it possible to crossdate Torrey pine tree rings?; (b) what is the maximum age of currently living trees?; (c) what are the most prominent seasonal climatic features that affect Torrey pine growth?; (d) is fog beneficial to tree growth?; (e) have climate-tree growth relationships changed over time?

MATERIALS AND METHODS

Tree-ring records. - Field collections took place within Torrey Pines State Reserve and Extension, next to Los Penasquitos Marsh Natural Preserve, on the coast between La Jolla and Del Mar, N of San Diego [ILLUSTRATION FOR FIGURE 1 OMITTED]. The mainland native population of Torrey pines was first protected in 1899 as a city park; it became a reserve in 1921 and has belonged to the California State Park system since 1959 (Hubbs et al, 1991). Torrey pine is the only overstory species at the study area, usually 10-15 m in height and open grown, occupying ridgetops, slopes and gullies on eroded marine terraces and sandstone bluffs. The understory varies widely in terms of abundance and species composition, mostly resembling coastal chaparral communities, sometime with few herbaceous species (Vogl et al., 1977). The area is characterized by maritime climate with small temperature excursions, limited winter rainfall, and frequent coastal fog.

Dominant trees were sampled by taking two increment cores from the lower stem at ca. 1 m above ground level, in a direction parallel to the topographic contour and ca. 180 [degrees] from each other. Wood cores were 4.3 mm wide and less than 50 cm long; all holes were closed and sanitized. Standing trees were selected across the area according to dendrochronological criteria, as first outlined by Douglass (1919). These criteria favor steep, open sites occupied by trees with large branches and flat crown top. Every effort was made to avoid trees affected by nonclimatic factors, e.g., human disturbance, past grazing, fire, insect outbreaks, fungi, mistletoe. Information on location, size and health status of sampled trees was recorded in the field and entered into a computer database for future reference.

All wood samples were transported to the Scripps laboratory, air-dried, and glued to wooden mounts after vertically aligning the xylem tracheids. Mounted cores were mechanically sanded, then polished by hand with progressively finer sandpaper until the smallest rings were clearly visible at 10x magnification. Ring patterns were visually crossdated (Douglass, 1941; Stokes and Smiley, 1968), using a binocular microscope; then ring widths were measured to the nearest 0.001 mm by means of a sliding stage interfaced with an image analysis system. Dating accuracy was numerically verified using the computer program COFECHA (Holmes, 1983; Grissino-Mayer et al., 1996).

Climatic records. - Local climatic data came from different sources. Monthly precipitation (1850-1990) and temperature (1852-1990) records for San Diego were obtained from the Global Historical Climatology Network (Vose et al., 1993). The most recent years of San Diego records (up to December 1994) were taken from the National Climatic Data Center on-line dataset of U.S. Cooperative and National Weather Service stations. Regional averages of monthly precipitation and temperature were provided by time series (1895-1993) for California Climate Division 6, available from the National Climatic Data Center (NOAA, 1983). Seasonal fog records for San Diego (1948-1990), as well as averaged over two marine regions (1949-1991), one S and one N of San Diego along the Southern California coast [ILLUSTRATION FOR FIGURE 1 OMITTED], were derived from the dataset compiled by Filonczuk et al. (1995). Global datasets of sea surface temperature and sea level pressure compiled by Scripps Climate Research Division were used to map correlation fields with the Torrey pine tree-ring chronology.

Statistical methods. - Ring-width measurements were standardized to remove individual trends as well as age- and size-related differences in growth rates (Cook and Kairiukstis, 1990). The Torrey pine tree-ring chronology was produced by means of a statistical model that incorporates both deterministic and stochastic components (Biondi, 1992, 1993). The computer program ARSTAN (Cook and Holmes, 1996) was used for data processing, and the final chronology could be written as follows:

[Mathematical Expression Omitted]

with [Mathematical Expression Omitted] = chronology value at year t; w = crossdated ring width; k = constant added to avoid taking the logarithm of 0; y = modified negative exponential or straight line; [n.sub.t] = number of measured specimens that include year t (in this study, 4 [less than or equal to] [n.sub.t] [less than or equal to] 25); [cross product.sub.i] = biweight robust mean (Mosteller and Tukey, 1977) of the i-values, i = 1, . . ., [n.sub.t]; [Alpha] = difference between 1.000 and the arithmetic mean of the robust-mean chronology; 1 - [Phi] B = first-order autoregressive operator (Box and Jenkins, 1976).

The relationship between climate and tree growth was investigated 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). Multiple correlation with monthly climatic variables requires more advanced statistical techniques when predictors are intercorrelated (Fritts, 1976, 1991). These techniques are based on multivariate regression between the pre-whitened 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-too dendroclimatic window, going backwards from October of the current growth year to the previous September.

The spatial coherency between Torrey pine and other tree-ring chronologies for western North America was analyzed using the International Tree-Ring Data Bank (ITRDB, NOAA, 1992; [ILLUSTRATION FOR FIGURE 2 OMITTED]). The ITRDB is a collection of accurately dated tree-ring records developed by dendrochronologists over many years, and is continuously expanding as new data sets become available. In addition, unpublished chronologies for Southern California were provided by Dave Meko, Laboratory of Tree-Ring Research, University of Arizona, and by Laura Haston, formerly at Geography Department, California State University-Northridge. All tree-ring chronologies began prior to 1800 and ended between 1960 and 1993.

RESULTS AND DISCUSSION

Torrey pine tree rings could be crossdated, even though faint latewood boundaries in certain years and specimens required careful specimen preparation and long hours of microscope work for correct identification. based on crossdated series, sampled trees reached a maximum age of 170 yr at coring height, i.e., ca. 1 m above ground level. Considering an estimated time of 10-30 yr to reach coring height, sampled trees were no older than 200 yr, which is almost three times the maximum age reported by Vogl et al. (1977). The possibility that older trees still exist, albeit unlikely, cannot be ruled out. From the collected samples, we developed a continuous chronology spanning the 1827-1994 period using overlapping segments of well-defined annual rings [ILLUSTRATION FOR FIGURE 3 OMITTED]. Chronology sample depth is maximum in 1917-1978, when the mean index is averaged over 2025 cores from 17 trees. The early years (1827-1838) are based on four cores from two trees; sample depth rises to 10 or more cores after 1867, and the most recent years (1981-1994) are based on 18 cores from 14 trees.

Response functions based on 28 monthly predictors (14 for precipitation and 14 for temperature) were consistent using either San Diego station data or Climate Division 6 average data. Similarly, response functions based on 12 seasonal predictors (four each for precipitation, temperature, and fog) were consistent using station data or regional averages. In both cases, however, regression statistics were better using San Diego station data; hence those results are reported in the following paragraphs. The major climatic signal in Torrey pine tree rings consists of winter and spring rainfall, as shown by the significant coefficients for November through April precipitation [ILLUSTRATION FOR FIGURE 4 OMITTED]. Temperature is not a significant predictor of Torrey pine annual growth, and summer fog has a weak, positive association with Torrey pine annual growth [ILLUSTRATION FOR FIGURE 4 OMITTED]. These results were confirmed by linear correlation coefficients between the Torrey pine chronology and climatic time series [ILLUSTRATION FOR FIGURE 5 OMITTED]. From 1850 to 1994, total rainfall from November through April had the highest correlation (0.77, n = 145) with the Torrey pine tree-ring chronology. Among seasonal variables, winter and spring precipitation had, respectively, the second (0.63, n = 145) and third (0.43, n = 145) highest correlation with the tree-ring chronology. The only other seasonal variable significantly correlated (0.37, n = 43) to Torrey pine annual growth was summer fog. Most likely, summer fog benefits Torrey pine growth by reducing the evapotranspiration stress during the warmest and driest season of the year. However, when combined with winter and spring precipitation in a multiple regression model, summer fog was not a significant predictor (t = 0.951, p-value = 0.348) of Torrey pine tree growth.

The lack of correlation between the Torrey pine chronology and temperature variables was investigated further with regard to possible bias caused by temporal trends in the data. Temperature records from San Diego are indeed affected by a positive, time-dependent trend attributed to the "urban heat island" phenomenon (Cayan and Douglas, 1984). From visual inspection of time series plots, the temperature trend is monotonic from 1900 to 1994; no such trend is present in the precipitation records or in the Torrey pine chronology. A linear fit to the monthly and seasonal temperatures from 1900 to 1994 was used for trend removal, then the residual temperature values were re-tested for trend by means of the Mann-Kendall statistic (Kendall and Gibbons, 1990), which is able to detect both linear and nonlinear trends. According to the Mann-Kendall test, no significant trend was present in the residual temperatures, and no significant correlation was found between the Torrey pine chronology and the detrended temperature records (monthly and seasonal) at San Diego.

According to Mallows' [C.sub.p] criterion (Mallows, 1973; SAS Institute, 1990), from 1850 to 1994 November-April precipitation is the best predictor of Torrey pine tree growth ([C.sub.p] = 0.03, [[R.sup.2].sub.a] = 0.59) among all possible combinations of seasonal variables. The consistency of climate-tree growth relationships through time was then investigated by comparing correlation results between November-April precipitation and Torrey pine annual growth during three non-overlapping periods, 1850-1899, 1900-1949, and 1950-1994. The amount of explained variance was higher in the 20th century (64% in 1900-1949, 70% in 1950-1994) than in previous decades (48% in 1850-1899). This difference, albeit suggestive of a temporal change in climate-tree growth relationships, could also be attributed to other factors. Assuming that the tree-ring standardization effectively removed age- and size-related effects on annual growth, and that the number of samples used to develop the Torrey pine chronology was large enough throughout the entire period of interest to obscure the effect of time-varying sample depth, temporal differences in climate-tree growth relationships could still be generated by changes in the quality of rainfall data. As an example, the largest value in the whole Torrey pine chronology occurs in 1862, which may be the wettest winter in recent Southern California history (Engstrom, 1996), but is not a very large value in the San Diego precipitation record included in the Global Historical Climatology Network [ILLUSTRATION FOR FIGURE 5 OMITTED]. The Torrey pine chronology could then provide an estimate of precipitation during the early and pre-instrumental period. As such, it reveals a very dry period in the early and mid-1840s, followed by a wet spell until the early 1850s and by another wet period in the late 1860s [ILLUSTRATION FOR FIGURE 5 OMITTED].

Spatial correlation between the Torrey pine chronology and seasonal precipitation (1895-1994) over the Climate Divisions identified by NOAA (1983) revealed a connection with the lower half of the West Coast and the American Southwest in winter, and with the American Southwest in the spring [ILLUSTRATION FOR FIGURE 6 OMITTED]. Correlation maps with gridded winter sea surface temperature and sea level pressure were consistent using different near-global datasets. Prominent features were a strong positive association with sea surface temperature S of San Diego and in the eastern tropical Pacific, and an equally strong negative association with sea level pressure directly above the California Current, in an area centered approximately over 30-40 [degrees] N and 120-130 [degrees] W, just off the Patton Escarpment [ILLUSTRATION FOR FIGURE 7 OMITTED]. These correlation fields indicate that Torrey pine annual growth is favored, on average, by a weakening or a displacement of the high pressure cell that normally develops off Southern California in winter (Duxbury and Duxbury, 1991), hence by a southerly displaced winter storm track. Annual tree growth is also favored by the development of warm anomalies in the southern portion of the California Current region and in the eastern tropical Pacific. Both of these patterns indicate that Torrey pine growth is sensitive to moisture transport from the eastern tropical Pacific driven by a southerly extension of the westerlies caused by the lower-than-average pressures off Southern California. Such a mechanism is enhanced by the El Nino-Southern Oscillation (ENSO) phenomenon: during warm events, surface winds and upper-level jets move from the eastern tropical Pacific into the American Southwest, whereas during cold events that airflow is blocked or reversed (Murphree and Reynolds, 1995).

The Torrey pine chronology was highly correlated to tree-ring chronologies developed for Southern California and the Colorado Plateau [ILLUSTRATION FOR FIGURE 8A OMITTED]. The contoured correlation field showed a pronounced, V-shaped, SW-to-NE gradient extending into a large portion of the American Southwest [ILLUSTRATION FOR FIGURE 8B OMITTED]. Interestingly, the correlation with other regions of the western U.S. was consistently near-zero [ILLUSTRATION FOR FIGURE 8 OMITTED]. Torrey pine and Southern California are linked to the Colorado Plateau mostly via winter precipitation patterns, as revealed by the spatial correlation between San Diego winter precipitation and tree-ring chronologies [ILLUSTRATION FOR FIGURE 9A OMITTED]. On one hand, the high coherency between Torrey pine and tree-ring chronologies much further inland is remarkable, as it confirms that coastal species can be highly sensitive to climate. On the other hand, the extremely good agreement between the correlation maps shown in Figures 8a and 9a is not surprising, considering the previously discussed relationship between winter precipitation and Torrey pine annual growth.

In winter, El Nino events are associated with increased rainfall in a region longitudinally stretched from Southern and Baja California to Western Texas and the southeastern United States (Schonher and Nicholson, 1989; Diaz and Kiladis, 1992; Stahle and Cleaveland, 1993). The linear correlation between the seasonal Southern Oscillation Index (SOI Ropelewski and Jones, 1987) and the Torrey pine chronology is maximum in winter (-0.34, n = 103). However, the linkage between Torrey pine and the American Southwest can only in part be attributed to ENSO, as revealed by the correlation maps between SOI and the western North America tree-ring chronologies [ILLUSTRATION FOR FIGURE 9B OMITTED]. The Colorado Plateau tends to have greater correlations with SOl than Southern California, and some large correlations are also found in the northern portion of the North American West Coast. Persistence effects in tree-ring response to climate, as well as other ocean-atmosphere interactions, may play a role in shaping the Southern California-Colorado Plateau connection, as suggested by correlation maps between tree-ring chronologies for western North America and previous summer sea surface temperature at the coastal stations of La Jolla, next to Torrey Pine State Reserve, and of Pacific Grove, near San Francisco (Biondi et al., in press).

CONCLUSION

This study provided evidence that Torrey pine tree rings are datable using dendrochronological techniques, that maximum tree age should not exceed 200 yr, and that annual growth is highly sensitive to winter and spring precipitation. Summer fog was positively correlated with Torrey pine annual growth, but its predictive ability was minimal when compared with winter and spring rainfall. There is little evidence that climate-tree growth relationships have changed over the 20th century. The lower association between tree growth and November through April precipitation observed in 1850-1899 than in more recent decades may have different causes, including lower accuracy of rainfall records for the San Diego station. The Torrey pine chronology is then well suited to provide an estimate of early and pre-instrumental climate records for San Diego. Largescale climatic signals in Torrey pine tree rings extend to the California Current region off Southern California and to the eastern tropical Pacific, and show a pronounced gradient from the Southern California coast into the Colorado Plateau. Resource managers, the scientific community, and the general public could benefit from these results, Torrey Pine State Reserve receives an average of one million visitors per year (Mike Wells, Biologist, Torrey Pine State Reserve, California State Parks and Recreation Department, San Diego, pers. comm.), and the recreational value alone of conifer species endemic to the California coast justifies detailed studies on the climatic sensitivity and possible impact of future climate change on the growth and survival of those species within their natural habitat.

Acknowledgments. - We are grateful to the California State Park System for permission to sample within Torrey Pines State Reserve, and to M. Wells and K. Yanow for help during field collections. L. G. Riddle participated in assembling the fog data used for statistical analyses. Funding was provided by the "Premio Balzan" to W. H. Berger by National Science Foundation Grant ATM-9509780 to W. H. Berger, D. R. Cayan, E Biondi and M. K. Hughes, and by a Scripps Post-Doctoral Fellowship to E Biondi made possible by a G. Unger Vetlesen Foundation grant.

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Author:Biondi, Franco; Cayan, Daniel R.; Berger, Wolfgang H.
Publication:The American Midland Naturalist
Date:Oct 1, 1997
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