Carbon isotope discrimination and growth response of old Pinus ponderosa trees to stand density reductions

By Nathan G. McDowell, J. R. Brooks, S. Fitzgerald, B. Bond. 2003

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Stand density reductions have been proposed as a method by which old-growth ponderosa pine ( Pinus ponderosa ) forests of North America can be converted back to pre-1900 conditions, thereby reducing the danger of catastrophic forest fires and insect attacks while increasing the productivity of the remaining old-growth individuals. However, the duration of productivity response of individual trees and the physiological mechanisms underlying such a response remain speculative issues, particularly in old trees. Treering measurements of carbon isotope ratios ( d 13 C) and basal area increment (BAI) were used to assess the response of intrinsic water-use efficiency (the ratio of photosynthesis, A to stomatal conductance, g ) and growth of individual > 250-year-old-ponderosa pine trees to stand density reductions. It was hypothesized that reductions in stand density would increase soil moisture availability, thus decreasing canopy A / g and increasing carbon isotope discrimination ( D ). Cellulose- d 13 C of annual tree rings, soil water availability (estimated from pre-dawn leaf water potential), photosynthetic capacity, stem basal growth and xylem anatomy were measured in individual trees within three pairs of thinned and un-thinned stands. The thinned stands were treated 7 to 15 years prior to measurement. The values of d 13 C and BAI were assessed for 20 consecutive years overlapping the date of thinning in a single intensively studied stand, and was measured for 3 years on either side of the date of thinning for the two other stands to assess the generality of the response. After thinning, D D D increased by 0.89‰ ( ± 0.15‰). The trees in the un-thinned stands showed no change in D D D (0.00‰ ± 0.04‰). In the intensively studied trees, significant differences were expressed in the first growing season after the thinning took place but it took 6 years before the full 0.89‰ difference was observed. BAI doubled or tripled after disturbance, depending on the stand, and the increased BAI lasted up to 15 years after thinning. In the intensively studied trees, the BAI response did not begin until 3 years after the D response, peaked 1 year after the D D D peak, and then BAI and D oscillated in unison. The lag between BAI and D was not due to slow changes in anatomical properties of the sapwood, because tracheid dimensions and sapwood-specific conductivity remained unchanged after disturbance. The D response of thinned trees indicated that A / g decreased after thinning. Photosynthetic capacity, as indexed by foliar nitrogen ([N]) and by the relationship between photosynthesis and internal CO 2 ( A ‐ C i curves), was unchanged by thinning, confirming our suspicion that the decline in A / g was due to a relatively greater increase in g in comparison with A . Model estimates agreed with this conclusion, predicting that g increased by nearly 25% after thinning relative to a 15% increase in A . Pre-dawn leaf water potential averaged 0.11 MPa ( ± ± ± 0.03 MPa) less negative for the thinned compared with the un-thinned trees in all stands, and was strongly correlated with D post-thinning ( R 2 = 0.91). There

Subfields: Atmospheric Science, Global and Planetary Change, Nature and Landscape Conservation

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Citations in the corpus

Disentangling the effect of competition, CO2 and climate on intrinsic water‐use efficiency and tree growth (2016, background): “This, together with longer growing seasons observed under reduced competition conditions (Aussenac & Granier 1988; Br eda, Granier & Aussenac 1995), translates into higher growth rates (Piutti & Cescatti 1997; McDowell et al. 2003, 2006; Mart ın-Benito et al. 2010).”
Increasing wood production through old age in tall trees (2010, background): “Indeed, wood production by old trees can clearly increase in response to elevated resource availability (McDowell et al., 2003; Soulé and Knapp, 2006; Martı́nez-Vilalta et al., 2007; Phillips et al., 2008a; Salzer et al., 2009).”
Perpetuating old ponderosa pine (2007, background): “Removing younger trees by thinning will increase water available to old trees during drought (e.g., Feeney et al., 1998; McDowell et al., 2003; Wallin et al., 2004) and likely reduce their mortality.”