Tree-ring d13C and d18O responses to climate change and forest B21H-0367 Tree-ring d13C and d18O responses to climate change and forest thinning in ponderosa pine ecosystems Liang Wei1,2, Jianwei Zhang3, John D. Marshall1 1: Department of Forest Resources, University of Idaho, Moscow, ID, 83844, USA . 2: Corresponding author. Email address: liangwei@vandals.uidaho.edu. 3: USDA Forest Service, Pacific Southwest Research Station, 3644 Avtech Parkway, Redding, CA 96002, USA. Introduction: Tree rings record tree responses to changes of environment. For example, the ratios of stable carbon isotopes (d13C) in tree rings reflect the change of water and light conditions, while the ratios of stable oxygen isotopes (d18O) reflect only changes in water conditions. In this study, we looked at the information stored in the tree rings of ponderosa pine in northern California to find out how the ponderosa pine responded to the changes. Hypotheses: d13C in tree rings should decrease due to the drop of d13C in atmospheric CO2 in the last 20 decades, but the carbon isotope discrimination (D) and δ18O may not have changed 2) trees may have higher D in wet years than dry years, because stomata are more open when water is sufficient and hence the value for ci/ca is higher; and 3) thinning changed the water conditions of stands such that carbon isotope ratios would decline following the thinning. Results and Discussion: ◎The long-term trend: The d13C remained relatively stable until 1950, when d13C started dropping gradually. In the first decade of the 21st century (Figure 3a), the d13C has dropped ~1.5 per mil compared to pre-1950. D was relatively unchanged (Figure 3b) for trees 1, 2, and 3, but the D of tree 4 increased since the 1910s. There was no obvious trend for d18O (Figure 3c). Marshall & Monserud (2006) measured the d18O of ponderosa pine tree rings in northern Idaho, and there was no significant trend over eight decades since 1910s, same as this study. However, the D of ponderosa pine had a decreasing trend in northern Idaho since the 1910s (Monserud & Marshall, 2001), which differs from this study. Figure 1. Stand average ring width (a) and standard ring width chronologies (b) of three treated stands. We took 30 trees per each stand. Arrows show the time of thinning. Stands M and H were thinned in 1938, and stand C in 1996-1997. Figure 3: The change of d13C, discrimination, and d18O in 200 years. Each sample combined 10 years of tree-rings. Figure 2. The Palmer drought severity index (PDSI) for northeastern California from 1820 to presentThe wet and dry years picked for stable isotopes analysis are represented with blue dots. Data prior to 1895 are extrapolated from tree rings (green) and those after 1895 are from National Climatic Data Center (gray, http://www1.ncdc.noaa.gov/pub/data/cirs/ ). ◎Dry vs wet years: The D in wet years was significantly lower than that in dry years (P = 0.001), and d18O of dry and wet years were not different (P > 0.05) (Figure 4). This supports our hypothesis two. ◎Management: The thinning had clear effects on d13C in tree rings. The d13C decreased in the five years after thinning (Figure 5). This support our hypothesis three, that the increased moisture would dominate the response.. Conclusion: The results suggest that tree-ring d13C tracks d13C of atmospheric CO2, drought, and thinning, while the trend of d18O responses to these variables requires more data and more careful evaluation. Figure 4. (Left) The D of dry years were significantly lower than those of wet years (P < 0.001). Samples were collected from three trees. (Right) The d18O of dry and wet years were not different (P > 0.05). Samples were collected from two trees. Error bar shows SE. Methods: ◆ We examined the last 200 years of stable carbon and oxygen isotopes in tree rings. Tree ring samples were extracted from at least 100 year-old ponderosa pine grown at the Blacks Mountain Experimental Forest, which is located approximately 35 km northeast of Lassen Volcanic National Park in northeastern California, USA. ◆ Thirty trees were sampled from thinned (Stand M) and control stands (Stand C) each (Oliver, 2000). Stand M was thinned in 1938. ◆ Exact tree age was determined for each tree ring. Tree-ring widths were measured and samples were crossdated by the aid of program COFECHA (Holmes, 1983)(Figure 1). We use extractive-free wood for the measurement of d13C (Harlow et al. 2006) and holocelullose for d18O. ○ We tested hypothesis 1 by combining tree rings of each decade since 1810 and examining the long-term trends of d13C and d18O. Tree ring samples for this test were collected from stand C. ○Second, we identified six dry years and six wet years and compared d13C and d18O between them (Figure 2). ○Third, we tested hypothesis three by comparing the d13C and d18O in tree rings of the years before and after the thinning. Figure 5. The change of d13C after thinning. One tree is taken from unthinned stand C, and one tree from thinned stand M. Arrow shows the year of thinning (1938). Data were normalized to account for pretreatment differences by subtracting mean d13C of 1934-1937 from each value of each tree (Brooks & Mitchell, 2011). Note that drought in 1939 increased d13C in both trees, but the increase in the control trees was larger. References: ◆Brooks J.R. & Mitchell A.K. (2011) Interpreting tree responses to thinning and fertilization using tree-ring stable isotopes. New Phytologist, 190, 770-782. ◆Harlow B.A., Marshall J.D. & Robinson A.P. (2006) A multi-species comparison of d13C from whole wood, extractive-free wood and holocellulose. Tree Physiology, 26, 767-774. ◆Holmes, R.L. 1983. Computer-assisted quality control in tree-ring dating and measurement. Tree-ring Bulletin 43: 69-78. ◆Marshall J.D. & Monserud R.A. (2006) Co-occurring species differ in tree-ring d18O trends. Tree Physiology, 26, 1055-1066. ◆Monserud R.A. & Marshall J.D. (2001) Time-series analysis of d13C from tree rings. I. Time trends and autocorrelation. Tree Physiology, 21, 1087-1102. ◆Oliver W.W. (2000) Ecological research at the Blacks Mountain Experimental Forest in northeastern California ( Tech. Rep. PSW-GTR-179). Pacific Southwest Research Station, Forest Service, U.S.Department of Agriculture.