Influence of herbaceous and woody vegetation control on seedling microclimate, leaf gas exchange, water status, and nutrient relations of Pinus strobus L. seedlings planted in a shelterwood

The influence of herbaceous and woody vegetation control applied over four consecutive growing seasons (GS) on microclimate, leaf gas exchange, water status, nutrient relations, and growth of planted eastern white pine (Pinus strobus L.) seedlings was examined in a central Ontario shelterwood. Treatment effects on pine seedling ecophysiology were closely associated with temporal changes in the structure and species composition of the developing understory plant communities. Vegetation control had minimal influence on air temperature, but herbaceous control, sometimes in combination with woody control, improved the soil thermal regime in every GS. Herbaceous vegetation control increased soil moisture availability in GS one and two, but only during mid-summer periods of little precipitation. Light availability showed a relatively large treatment response, with highest light levels created where both herbaceous and woody vegetation were suppressed. Herbaceous and woody vegetation control had additive or interactive effects on net carbon assimilation (Aₙ) and leaf conductance to water vapour (Gwᵥ) in a given GS, while water use efficiency and midday leaf water potential (ψₘ) were largely independent of treatment. The effects of vegetation control on Aₙ, Gwᵥ, and ψₘ were often correlated with treatment-induced changes in total vegetative cover, light, and soil moisture availability. Vector analysis of leaf nutrient (N, P, K, Ca, and Mg) relations suggested that herbaceous vegetation control relieved foliar N, P, and K deficiencies in 5-year-old white pine seedlings, woody vegetation control did not affect leaf nutrient relations, and total vegetation control provided non-limiting conditions. In GS four, white pine growth responses were highest where both herbaceous and woody vegetation control had been conducted, likely in response to improved microclimate, resource availability, leaf gas exchange, and foliar nutrition.