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

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 (delta13°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 (delta). Cellulose-delta13°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 delta13°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, delta increased by 0.89 ppt (± 0.15 ppt). The trees in the un-thinned stands showed no change in delta (0.00 ppt ± 0.04 ppt). 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 ppt 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 delta response, peaked 1 year after the delta peak, and then BAI and delta oscillated in unison. The lag between BAI and delta was not due to slow changes in anatomical properties of the sapwood, because tracheid dimensions and sapwood-specific conductivity remained unchanged after disturbance. The delta 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 delta post-thinning (R² = 0.91). There was a strong relationship between BAI and 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.