We investigated N transfers from pine saplings to nearby pines, forbs, grasses and legumes in a pine-oak woodland in northern California’s Sierra Nevada foothills. Using ¹⁵N isotopic pulse-labeling, we evaluated effects of source-sink relationships on N movement between fifteen pairs of naturally regenerated Pinus ponderosa saplings that were ~0.6 m apart.  Applied treatments were: (1) ¹⁵N donor treatment where K¹⁵NO₃^- applied to the needles of “donor” saplings; (2) K¹⁵NO₃^-  applied to donor sapling needles and K¹⁴NO₃^- applied to “receiver” sapling needles; (3) control treatment where donor and receiver pines received no ¹⁵N or ¹⁴N. After 6 weeks, entire pine saplings were harvested. Herbaceous species were sampled along a 300 cm transect that was located between the pine pairs.

Results/Conclusions

Although application of ¹⁴N to receiver pines resulted in lower ¹⁵N enrichment in receivers, this trend was not statistically significant. Effects of supplying N to receivers may have been diminished by relatively high N already present in sapling foliar tissue. Donor needles and stem had the greatest ¹⁵N enrichment. Herbaceous plants adjacent to donors were enriched with ¹⁵N.  Ectomycorrhizal colonization of donor roots was negatively correlated with δ¹⁵N found in roots of herbaceous species sampled at +100 cm. In receiver roots, ectomycorrhizal colonization correlated negatively with ¹⁵N concentrations in root tissue. Along the 300 cm transect, herbaceous plants located nearest to the pine pairs had the greatest enrichment suggesting that herbaceous species benefited from proximity to the donor pine. Grasses and forbs were more enriched than legumes. The higher ¹⁵N concentrations in forb roots harvested near the pine pairs was negatively correlated with ¹⁵N in the receiver needles; uptake by forbs corresponds with reduced transfer to the receiver pine. This study demonstrated that in a pine-oak woodland, N transfers from pine saplings to other saplings and herbaceous species are strongly affected by multiple source-sink relationships.