Effects of nutrient distribution pattern and aboveground competition on size of individuals in Ipomoea tricolor populations

We investigated whether the distribution pattern of soil nutrients interacted with aboveground competition to affect plant size in Ipomoea tricolor Cav. populations. Six plants per pot were grown in a factorial experiment with combinations of heterogeneous or homogeneous nutrient distribution pattern and presence or absence of aboveground competition. Plants were harvested and ranked by their aboveground biomasses. In analyses of plant sizes in all ranks simultaneously, mean plant size was significantly affected by nutrient distribution pattern, aboveground competition, and their interaction. In analyses of plant sizes of each rank, aboveground competition affected plant size, which was found in all ranks. Nutrient distribution pattern affected plant size in the higher ranks, but not in the lowest rank. Selective root placement into nutrient-rich patches under heterogeneous conditions was observed. Our results suggest that the magnitude of the effect of nutrient distribution pattern on plant size changed among ranks owing to changes in aboveground competition and size-dependent growth rate. Size-dependent growth rate could explain the significant effect of nutrient distribution pattern independent of the effect of aboveground competition. Nutrient distribution pattern would then interact with aboveground competition and consequently affect size structure in a population.

Unusual effect of controlling aboveground competition by Ledum groenlandicum on black spruce (Picea mariana) in boreal forested peatland

Poor growth of black spruce (Picea mariana (Mill.) BSP) has been associated with the presence of Ledum groenlandicum L. (Ledum) on some sites in the eastern boreal forest in Canada. To increase black spruce productivity on lowland sites, a study was carried out to test the effect of mechanical control of Ledum (by clipping) on black spruce growth on forested peatland in northwestern Quebec. We compared the growth and foliar nutrient concentrations of advance-regeneration black spruce seedlings with and without Ledum control. Contrary to our expectations, our results showed that 3-year control of aboveground competition by Ledum decreased rather than increased black spruce growth and had no effect on foliar nutrient concentrations. Ledum grows on a variety of site types; therefore, the mere presence of this species does not necessarily indicate that there will be a problem with conifer regeneration (growth and germination).

Light, not nitrogen, limits growth of the grass Deschampsia flexuosa in boreal forests

Increased nitrogen (N) input in boreal forests has previously been shown to induce a shift from Vaccinium myrtillus L. to Deschampsia flexuosa (L.) Trin. as the dominant understory species. We investigated the relative importance of increased light and N for this shift, in a field experiment. We increased light availability, that is, we reduced aboveground competition from V. myrtillus, and increased N by adding 50 kg N·ha–1. Increased light availability had a positive effect on both the growth rate and final biomass of D. flexuosa. Although N addition increased the uptake of fertilizer N by both species, it had no effect on the growth or biomass of either species. Thus, aboveground competition from V. myrtillus prevented expansion of D. flexuosa, regardless of N treatment. The results suggest that aboveground competition may be more important than belowground competition for structuring understory boreal forest communities. As light availability is important, both the structure and total amount of standing crop will be important for the outcome of species interactions.Key words: aboveground competition, belowground competition, fertilization, natural enemies, nitrogen deposition, vegetation change.