Variable retention harvest treatments alter physiological performance in three pine species

Background/Question/Methods Managed forests often have less structural complexity than their unmanaged or historical analogs, which leads to concerns over sustaining biodiversity and key ecosystem functions across managed landscapes. Variable retention silvicultural systems are encouraged to promote complexity in forests typically managed using even-aged techniques, but our limited experience with these practices makes predicting regeneration responses to different patterns of overstory retention difficult. In this study, we examined the physiological responses of three pine species to variable retention and understory release treatments to better understand how different patterns of overstory retention impact seedling development. Naturally-regenerated Pinus resinosa stands were reduced to similar levels of residual basal area using three patterns of overstory retention including dispersed retention, aggregate retention with small (0.1 ha) gaps, and aggregate retention with large (0.3 ha) gaps. Half of each stand also received a mechanical understory release treatment. We used carbon isotope ratios (δ¹³C) and oxygen isotope ratios (δ¹⁸O) from seedling foliage to study physiological performance across treatments along with gas-exchange measurements on a subset of seedlings in each treatment’s primary regeneration environment to study seedling performance in greater detail.

Results/Conclusions δ¹³C was greater in retention treatments than overstory controls and greater in understory release treatments than understory controls for Pinus banksiana, P. resinosa, and P. strobus seedlings, but did not differ among retention treatments. δ¹⁸O was lower in overstory controls than retention treatments for P. resinosa, but similar among treatments for the other species. Net photosynthesis (A_net), transpiration (E), and stomatal conductance (g_s) were greater in retention treatments than overstory controls. P. banksiana had the highest A_net, but E, and g_s were not significantly different among species. Predawn xylem water potentials were higher in retention treatments than overstory controls, and greater in understory release treatments than understory controls. These results suggest retention harvesting treatments impact seedling performance by increasing A_net and reducing moisture stress. The relationship between δ¹⁸O and δ¹³C suggests the enriched δ¹³C signature of retention treatments was driven primarily by increased A_net rather than reduced g_s, which is supported by gas-exchange and water potential data that indicate retention harvesting treatments reduced moisture stress by increasing water availability. Our results indicate little variability in seedling performance among harvest treatments, suggesting variable retention systems offer considerable flexibility for meeting diverse management goals.