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Variable retention harvests are used to enhance the development of structural complexity in managed forests by retaining living trees and other structural legacies from the pre-harvest ecosystem. While harvesting should increase resource availability to residual trees, greater crown exposure may also increase environmental stress, which makes it difficult to predict growth in different structural environments. We used stable carbon isotope ratios (δ 13 C) of annual rings from red pine trees ( Pinus resinosa  Ait.) as an index of intrinsic water use efficiency (iWUE), the ratio of photosynthetic carbon assimilation ( A ) to stomatal conductance ( g  s ), to better understand how differences in physiological performance relate to growth responses following harvests that left residuals dispersed, aggregated between small (0.1 ha) gaps or aggregated between large (0.3 ha) gaps. Stable oxygen isotope ratios (δ 18 O) were used as an index of  g  s  to investigate the drivers behind changes in iWUE. Retention harvesting did not appear to affect δ 13 C or δ 18 O at the stand scale when compared to unharvested control stands, but there was a significant, negative correlation between residual tree δ 13 C and plot basal area in the second and third years after harvesting that suggests declining iWUE as overstory competition increases. Residual tree δ 18 O was similar across treatments and basal areas. Trees in variable retention harvests showed small but positive increases in radial growth from the pre-treatment to post-treatment measurement periods, while radial growth declined in unharvested control stands. There were no significant differences in radial growth among retention treatments. Our results suggest residual red pine in relatively open environments benefit from greater  A  but do not show evidence of changes in  g  s  that would indicate altered water relations.

Wood δ 13 C, δ 18 O and radial growth responses of residual red pine to variable retention harvesting