Factors contributing to the superior growth and N nutrition of 11-year-old lodgepole pine compared with Sitka spruce on a N-poor cedar-hemlock cutover

We tested several hypotheses to explain the superior growth and nitrogen (N) status of lodgepole pine (Pinus contorta Dougl. ex Laws. var. contorta) compared with Sitka spruce (Picea sitchensis (Bong.) Carrière) on a N-poor site by comparing N distribution, N retranslocation, rooting distribution, and mycorrhizal fungi in plots of 11-year-old trees on a cedar–hemlock cutover. Aboveground N content was nine times greater in pine than in spruce of the same age, and thus, we conclude that pine acquired more N during the 11 years than spruce. Greater N acquisition by pine was not related to rooting depth, as both species rooted primarily in the residual forest floor. There were differences in mycorrhizal fungal associates: a high proportion of pine roots were infected with Suillus-like fungi. Pine produced more aboveground biomass per unit N (388 compared with 292 g·g–1 in spruce) and distributed more N to young foliage. Nitrogen retranslocation efficiency (based on foliar N contents in July and October) was higher in pine (50–52%) than in spruce (24–36%). These characteristics all appear to contribute to pine's abilities to both acquire more N and use it more efficiently and, thus, outperform spruce on this N-poor site.

Biomass partitioning and nitrogen retranslocation in black spruce seedlings on competitive mixedwood sites: a bioassay study

Black spruce (Picea mariana (Mill.) BSP) seedlings were exponentially nutrient loaded by applying 3 or 6 times more fertilizer than recommended for conventional seedling production in the nursery. Loaded seedlings were similar in height and biomass to nonloaded seedlings after nursery culture, but their tissue N, P, and K content was almost twice as much as that of nonloaded seedlings. The seedlings were transplanted on intact potted substrates (bioassays) retrieved with natural vegetation from two boreal mixedwood sites and grown for one season in a greenhouse to study early- and late-season growth and nutrient dynamics, and nutrient loading and herbicide effects on N retranslocation processes. After transplanting, height and biomass growth of loaded seedlings were, respectively, 9-14% and 24-49% more than nonloaded seedlings in herbicide-treated plots, and 14-32% and 42-85% more in untreated plots, resulting in 32-39% biomass reduction in natural vegetation. The effect of nutrient loading on growth was attributed to earlier and greater biomass and N partitioning to current needles and roots that promoted N uptake (up to 20.9 ± 1.7 mg) compared with nonloaded seedlings (up to 11.8 ± 1.2 mg). Although both loading and herbicide treatments stimulated seedling growth and N uptake, N retranslocation from older to actively growing tissues was promoted by loading but reduced by herbicide treatment. The results demonstrate the high dependence of seedlings on internal nutrient reserves when planted in competitive environments, and that nutrient retranslocation is mainly driven by current growth, nutrient uptake, and internal nutrient reserves.

Litter quality of native herbaceous legumes in a burned pine forest of the Georgia Piedmont

The objectives of this study were to compare the nitrogen retranslocation efficiency and the litter substrate quality of eight native and one naturalized herbaceous legume species growing in a periodically burned pine forest of the Georgia Piedmont. A survey of the nine species revealed a wide range of nitrogen retranslocation (8.3–51.9%) from senescent leaf tissue, as well as differences in nitrogen concentrations (0.9–2.5%), C:N ratios (17.7–51.2), and initial lignin contents (8.6–20.1%) of senescent leaves. Intraspecific differences in Desmodiumviridiflorum L., Lespedezahirta L., and Lespedezaprocumbens Michx. were found in the leaf, stem, coarse- and fine-root tissue C:N ratio and in the lignin concentrations. Overall, the litter of these nitrogen-fixing herbaceous legumes is predicted to decompose at moderate rates and may have a significant effect upon the long-term soil organic matter and soil nitrogen accumulations of frequently burned forest sites.