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CARBON ISOTOPE DISCRIMINATION AND GROWTH RESPONSE OF OLD PINUS PONDEROSA TREES TO STAND DENSITY REDUCTIONS
Citation:
McDowell, N., J R. Brooks, S. A. Fitzgerald, AND B. J. Bond. CARBON ISOTOPE DISCRIMINATION AND GROWTH RESPONSE OF OLD PINUS PONDEROSA TREES TO STAND DENSITY REDUCTIONS. PLANT, CELL, AND ENVIRONMENT. Blackwell Publishing, Malden, MA, 26:631-644, (2003).
Description:
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 productivity of the remaining old-growth trees. However, the duration of a productivity response and the physiological mechanisms underlying such a response remain speculative issues, particularly in old trees. We used tree-ring measurements of carbon isotope ratios ( 13C) and basal area increment (BAI) 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. We hypothesized that reductions in stand density would increase soil moisture availability, thus decreasing canopy A/g and increasing carbon isotope discrimination ( ). We measured cellulose- 13C of annual tree rings, soil water availability, photosynthetic capacity, stem basal growth and xylem anatomy in individual trees within three pairs of thinned and un-thinned stands. The thinned stands were treated seven to 15 years prior to measurement. 13C and BAI were assessed for twenty consecutive years overlapping the date of thinning in a single intensively studied stand, and was measured for three years on either side of the date of thinning for the two other stands to assess the generality of the response. After thinning, increased by 0.89? (? 0.15?). Trees in un-thinned stands showed no change in (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 six 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 three years after the _response, peaked one year after the peak, and then BAI and _oscillated in unison. The lag between BAI and was not due to slow changes in anatomical properties of the sapwood, because tracheid dimensions and sapwood specific conductivity remained unchanged after disturbance. The _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 CO2 (A-Ci curves), was unchanged by thinning, confirming our suspicion that the decline in A/g was due to a relatively greater increase in g compared to 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 to the un-thinned trees in all stands, and was strongly correlated with post-thinning (R2 = 0.91). There was a strong relationship between BAI and modeled A, suggesting that changes in water availability and g have a significant effect on carbon assimilation and growth of these old trees. These results confirm that stand density reductions result in increased growth via increased stomatal conductance. Furthermore, they show that a physiological response to stand density reductions can last for up to 15 years in old ponderosa pines if stand leaf area is not fully re-established.