Can Fertilizers Increase the Seed Yield of Two Native Herb Species in the Subarctic? Implications for Wild Seed Collection

Restoration Notes have been a distinguishing feature of Ecological Restoration for more than 25 years. This section is geared toward introducing innovative research, tools, technologies, programs, and ideas, as well as providing short-term research results and updates on ongoing efforts. Please direct submissions and inquiries to the editorial staff (ERjournal@aesop.rutgers.edu).

Local native seed is collected from the wild to restore disturbed land when local commercial seed sources are unavailable. What if wild local populations have low seed output? Low seed yields can result from: 1) pollen limitation and pollinator scarcities, especially for self-incompatible species (Burd 1994, Ashman et al. 2004); and 2) abiotic limitations, including resources such as light, water, and nutrients (Stephenson 1981). Pollen limitation and climate are difficult to circumvent. Managers may more easily manipulate certain resources, such as nutrients. In boreal and arctic biomes, N and P are the most commonly limiting nutrients (Shaver and Chapin 1995, Weintraub 2011), but fertilization studies have found variable and species-specific responses on flowering or seed output (Shaver and Chapin 1995, Grainger and Turkington 2013, Petraglia et al. 2013). In this study, we posed the question whether fertilizer addition in a natural subarctic ecosystem over two growing seasons could increase the seed yields for desirable restoration species.

We conducted our study in north-central Canada along a 1.5-km south-facing section of the Attawapiskat River floodplain (52.88° N, 83.91° W, 83 m elevation). The soils were calcareous, alkaline and poorly-developed, with low N and P within primary rooting zones (Garrah 2013). The vegetation was open as a result of annual river ice damage, dominated by herbs and scattered shrubs. We selected two perennial herbs, Potentilla anserina (silverweed), a spreading ground cover, and Vicia americana (American vetch), a N-fixing legume. Both species are self-incompatible and insect-pollinated (Gunn and Kluve 1976, Eriksson 1987, Miyanishi et al. 1991) and are regionally common but have low seed output. We hypothesized that fertilization would increase flowering, seed set, and total seed yield after two seasons and that V. americana would not need N amendment because it is a N-fixing species.

For each species, we set up ten blocks of 1-m² plots, containing one plant of either P. anserina or V. americana. In June 2015 and again in June 2016, we applied two treatments (control and NPK fertilization) to P. anserina and three treatments (control, PK, and NPK fertilization) to V. americana. We used urea [CO(NH₂)₂], superphosphate [Ca(H₂PO₄)₂] and potash [KCl (95%) NaCl (5%)] at a rate that contributed 4 g m⁻² each of N, P, and K. We watered plots after fertilization. In August 2016, we assessed: (i) the number of ramets; (ii) the number of flowers per ramet; and (iii) the number of flowers per plot. We tested for significant differences among treatments for each dependent variable using a permutation test with 10,000 random shuffles, using Resampling Stats^® (Excel add-in v. 4.0, Resampling Stats, Arlington, Virginia). For P. anserina, we resampled the difference between the two treatment means, and for V. americana, we resampled the F statistic and then the pairwise difference in the three means.

For P. anserina, we found no significant differences between the two treatments in the total number of ramets per plot (mean difference = 3.07; n_unfert = 9, n_fert = 8, p = 0.28; Figure 1A), but a borderline difference in the total number of flowers per plot (difference = 4.04, p = 0.08; Figure 1B). None of the P. anserina ramets set seed. For V. americana, we found significantly more ramets in the NPK treatment compared to the control (F_{2, 25} = 3.00, p = 0.025), and the PK treatment was intermediate (Figure 1C). Only three of all the V. americana plots produced flowers (Figure 1D), and only one plot produced seed. Although we found some differences, we were unable to increase seed output for either species, despite two years of spring fertilization.

Eriksson...