Abstract
Restorationists commonly plant overstory and understory species simultaneously at the outset of restoration, but a mature forest canopy may be necessary to facilitate survival of light-intolerant understory species. We conducted two experiments in riparian forest restoration sites along the Sacramento River to determine whether: (1) introducing understory species is more successful at the beginning of restoration or after the canopy has developed; (2) canopy cover directly (via reduced light) or indirectly (by reducing non-native competition) facilitates survival of native understory species; and (3) seeding or planting seedlings of understory species is most effective. Seven native understory species were introduced as both seeds and seedlings into an experiment that manipulated canopy cover (open or canopy) and non-native grass competition (control or grass-specific herbicide). We conducted another experiment using shade cloth to directly test the effect of different light levels on seedling survival and growth of three species. Both experiments showed that four species (Aristolochia californica, Carex barbarae, Clematis ligusticifolia, and Vitis californica) had higher survival under low-light conditions (canopy or shade cloth). In contrast, three species (Artemisia douglasiana, Euthamia occidentalis and Rubus californica) had similar survival across open and canopy conditions. Cover of unplanted understory vegetation (mostly non-native) was much lower under the canopy than in open plots treated with grass-specific herbicide. Establishment from seed was generally low and highly variable. Our results suggest that to restore understory species in riparian forests in north–central California: light-intolerant understory species should be planted after canopy closure; canopy cover is more effective than grass-specific herbicide at reducing non-native understory cover; and planting seedlings is more successful than direct seeding.
Highlights
The challenges and importance of restoring understory forest species have received increasing attention in recent years
Our results suggest that to restore understory species in riparian forests in north–central California: light-intolerant understory species should be planted after canopy closure; canopy cover is more effective than grass-specific herbicide at reducing non-native understory cover; and planting seedlings is more successful than direct seeding
Our results show that some species of understory riparian plants in north–central California have higher survival when planted under the forest canopy (Figures 2 and 4), and should be introduced after the canopy has matured; these results are consistent with those from other forest systems (e.g., Yates and others 2000; Ashton and others 2001; Parrotta and Knowles 2001; Mottl and others 2006; Pages and Michalet 2006)
Summary
The challenges and importance of restoring understory forest species have received increasing attention in recent years (reviewed by Flinn and Vellend 2005; Nilsson and Wardle 2005; Gilliam 2007). Canopy cover may facilitate understory species directly, by ameliorating abiotic stress (e.g. decreased air and soil temperatures and increased humidity, soil moisture, and litter cover) (Zou and others 2005; Pages and Michalte 2006; Barbier and others 2008), or indirectly, by reducing competition with light-demanding non-native species that are common in disturbed landscapes (Levine 1999; Pages and Michalet 2003; Pensa and others 2008). Evidence exists for both direct and indirect canopy effects. Likewise, reducing competition through removal of non-native species by herbicides or physical means can increase native species establishment (D’Antonio and others 1998; Cabin and others 2002; Denslow and others 2006)
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