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McDowell, N., J.R. Brooks, S. Fitzgerald, and B.J. Bond. 2003. Carbon isotope discrimination and growth response of old Pinus ponderosa trees to stand density reductions. Plant, Cell and Environment 26:631-644. WED-02-168
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. Tree-ring
measurements of carbon isotope ratios (δ13C) 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 (Δ). Cellulose-δ13C of
annual tree rings, soil water availability (estimated from pre-dawn leaf water potential),
photosynthetic capacity, stern 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
δ13C and BAI were assessed for 20 consecutive ears 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 generality
of the response.
After thinning, Δ increased by 0.89‰ (± 0.15‰). The trees in the un-thinned stands
showed no change in Δ (0.00‰ f 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 Δ response, peaked 1 year after the Δ peak, and then BAI
and Δ oscillated in unison. The lag between BAI and Δ was not due to slow c
hanges in anatomical properties of the sapwood, because tracheid dimensions and
sapwood-specific conductivity remained unchanged after disturbance. The Δ 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 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 Δ post-thinning (R2 = 0.91). There was a strong relationship between BAI a
nd modelled 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 of individual trees 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.