Meredith W. Cornett

Canopy feedbacks and microtopography regulate conifer seedling distribution in two Minnesota conifer-deciduous forests

Successful regeneration of some species may be linked to microsite availability, which in turn may be influenced by dominant canopy species. We investigated the relationship between canopy feedback...

Canopy type, forest floor, predation, and competition influence conifer seedling emergence and early survival in two Minnesota conifer-deciduous forests

There is concern that the conifer component of mixed conifer–deciduous forests in the Great Lakes region is on the decline, possibly the result of insufficient conifer regeneration. Limitations on conifer regeneration that occur during the first 18 months of seedling emergence and establishment were examined for Abies balsamea (L.) Mill. and Pinus strobus L. in two mixed conifer–deciduous forest types in northeastern Minnesota. We hypothesized that the removal of potential barriers to regeneration, including forest floor, seed predation, and competition with understory vegetation, will enhance conifer regeneration. We also hypothesized that the importance of the above regeneration barriers, and early regeneration success of each species, will differ beneath deciduous and conifer canopy types. The study was conducted under conifer and deciduous canopy types for two sites of each forest type: Abies balsamea – Populus tremuloides Michx. forest and Thuja occidentalis L. – Betula papyrifera Marsh. Germination experiments were conducted with seeds of A. balsamea and P. strobus. Experimental treatments included forest floor removal, caging to exclude seed predators and herbivory, and weeding to study the effects of understory competition. Abies balsamea and P. strobus responded differently to experimental treatments at different developmental stages. The best conditions for early regeneration of both A. balsamea and P. strobus included reduction of seed predation (p < 0.0001) and competition with understory vegetation (p ≤ 0.001). Abies balsamea additionally benefited from a conifer overstory (p < 0.0001) and a reduction in thickness of the forest floor (p < 0.01). Résumé: Le déclin de la composante coniférienne des forêts mixtes conifériennes–décidues dans la région des Grands Lacs devient préoccupant. Il pourrait être dû à une régénération coniférienne insuffisante. Les obstacles à la régénération des conifères, qui se manifestent pendant les premiers 18 mois de l’émergence et de l’établissement des semis, ont été examinés pour Abies balsamea (L.) Mill. et Pinus strobus L., dans deux types forestiers mixtes conifériens–décidus du nord-est du Minnesota. Les auteurs ont émis l’hypothèse que l’élimination des barrières potentielles à la régénération, incluant la litière, la prédation des semences et la compétition de la végétation du sous-bois, améliorera la régénération des conifères. Ils ont aussi émis l’hypothèse que l’importance des barrières mentionnées, ainsi que la réussite de chaque espèce, tôt au cours de la régénération, seront différentes sous les types de couvert décidu et coniférien. L’étude a été réalisée sous les types de couvert décidu et coniférien dans deux sites de chacun des types forestiers suivants : forêt d’Abies balsamea – Populus tremuloides Michx. et de Thuja occidentalis L. – Betula papyrifera Marsh. Les essais de germination ont été réalisés avec les semences de A. balsamea et de P. strobus. Les traitements expérimentaux comprenaient l’enlèvement de la litière, l’utilisation de cages pour exclure les prédateurs des semences et les herbivores, ainsi que le sarclage pour étudier les effets de la compétition du sous-bois. Abies balsamea et P. strobus ont répondu différemment à ces traitements expérimentaux, à différents stades de leur développement. Les meilleurs conditions, tôt au cours de la régénération, aussi bien d’A. balsamea que de P. strobus, comprenaient la réduction de la prédation (p < 0,0001) et la compétition de la végétation du sous-étage (p ≤ 0,001). En plus, A. balsamea a bénéficié du couvert coniférien (p < 0,0001) et de la réduction de l’épaisseur de la litière (p < 0,01). [Traduit par la Rédaction]

Seedbed and moisture availability determine safe sites for early Thuja occidentalis (Cupressaceae) regeneration

Regeneration of many late-successional tree species depends on specialized safe sites. The primary objective was to investigate the roles of seedbed and moisture retention as dimensions of safe sites for the early regeneration of drought-sensitive northern white cedar (Thuja occidentalis). We hypothesized that rates of germination, survival, and growth of T. occidentalis are unlikely to differ among seedbed types under conditions of abundant water, but that differences are likely to emerge as water becomes more limited. In a 67-d greenhouse experiment, cedar seeds were sown on logs, leaf litter, and soil of cedar and paper birch (Betula papyrifera) canopy origin. Seedbeds were subjected to three water treatments. Among the water treatments, highest germination rates occurred within the high water treatment, although germination on cedar litter was comparable to that of the low water treatment. Higher germination and survival rates occurred on decayed logs than other natural seedbeds for medium (P = 0.001) and low (P < 0.0001) water treatments. Germination on birch logs occurred at higher rates than on cedar logs within the low water treatment (P = 0.04). Seedling growth for the medium water treatment was lower on leaf litter than any other type of seedbed (P < 0.01). Results generally demonstrated that the interplay between seedbed and moisture retention is a component of safe sites for T. occidentalis regeneration.

Meeting seed demand for landscape-scale restoration sustainably: the influence of seed harvest intensity and site management

ABSTRACT Native seed is often collected en masse from remnant ecosystems to supply landscape-scale restoration. Successful large-scale restoration depends on sustained seed yields but also on donor population persistence. Native plants that reproduce solely by seed are especially sensitive to harvesting practices. We addressed the challenge of procuring sufficient seed from remnant sources to restore landscapes while also maintaining remnant populations of native plants. We evaluated: 1) the sustainability of seed harvest at varying intensities in Rudbeckia hirta, a seedreliant plant; and 2) the contribution of fire in promoting sustainability of seed donor populations. We planted seedlings of R. hirta in a field experiment that manipulated management type (burned or unburned) and harvest intensity (0, 50%, or 100% seed removed), and measured changes in seedling recruitment and seed production among treatments. Moderate intensity harvest and burning did not significantly reduce seedling recruitment, but high intensity harvest with burning reduced recruitment by 95% compared to controls. Seed production nearly doubled in burned treatments. In unburned prairie, recruitment is negligible, and harvest intensity does not have an effect on recruitment. For harvest-sensitive prairie species, a strategy incorporating moderate intensity seed harvest with burning is most likely to provide seed for large-scale restoration sustainably.