Unthinned slow-growing ponderosa pine (Pinus ponderosa) trees contain muted isotopic signals in tree rings as compared to thinned trees

Julia A. Sohn, J. Renée Brooks, Jürgen Bauhus, Martin Kohler, Thomas E Kolb, Nathan G. McDowell

Research output: Contribution to journalArticle

11 Scopus citations

Abstract

Key message The muted wood isotopic signal in slow-growing trees of unthinned stands indicates lower responsiveness to changing environmental conditions compared to fast-growing trees in thinned stands. Abstract To examine the physiological processes associated with higher growth rates after thinning, we analyzed the oxygen isotopic values in wood (δ<sup>18</sup>O<inf>w</inf>) of 12 ponderosa pine (Pinus ponderosa) trees from control, moderately, and heavily thinned stands and compared them with wood-based estimates of carbon isotope discrimination (∆<sup>13</sup>C), basal area increment (BAI), and gas exchange. We found that (heavy) thinning led to shifts and increased inter-annual variability of both stable carbon and oxygen isotope ratios relative to the control throughout the first post-thinning decade. Results of a sensitivity analysis suggested that both an increase in stomatal conductance (g<inf>s</inf>) and differences in source water among treatments are equally probable causes of the δ<sup>18</sup>O<inf>w</inf> shift in heavily thinned stands. We modeled inter-annual changes in δ<sup>18</sup>O<inf>w</inf> of trees from all treatments using environmental and physiological data and found that the significant increase in δ<sup>18</sup>O<inf>w</inf> inter-annual variance was related to greater δ<sup>18</sup>O<inf>w</inf>responsiveness to changing environmental conditions for trees in thinned stands when compared to control stands. Based on model results, the more muted climatic response of wood isotopes in slow-growing control trees is likely to be the consequence of reduced carbon sink strength causing a higher degree of mixing of previously stored and fresh assimilates when compared to faster-growing trees in thinned stands. Alternatively, the muted response of δ<sup>18</sup>O<inf>w</inf> to climatic variation of trees in the control stand may result from little variation in the control stand in physiological processes (photosynthesis, transpiration) that are known to affect δ<sup>18</sup>O<inf>w</inf>.

Original languageEnglish (US)
Pages (from-to)1035-1051
Number of pages17
JournalTrees
Volume28
Issue number4
DOIs
StatePublished - Apr 24 2014

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Keywords

  • Gas exchange
  • Oxygen isotopes
  • Pinus ponderosa (ponderosa pine)
  • Sensitivity analysis
  • Thinning

ASJC Scopus subject areas

  • Forestry
  • Plant Science
  • Physiology
  • Ecology

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