Strategies for Restoring Native Riparian Understory Plants Along the Sacramento River: Timing, Shade, Non-Native Control, and Planting Method Prairie L
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JULY 2011 Strategies for Restoring Native Riparian Understory Plants Along the Sacramento River: Timing, Shade, Non-Native Control, and Planting Method Prairie L. Moore1, Karen D. Holl2, and David M. Wood1 Volume 9, Issue 2, Article 1 | July 2011 doi: http://dx.doi.org/10.15447/sfews.2014v9iss2art1 ABSTRACT light levels on seedling survival and growth of three species. Both experiments showed that four species Restorationists commonly plant overstory and under- (Aristolochia californica, Carex barbarae, Clematis story species simultaneously at the outset of restora- ligusticifolia, and Vitis californica) had higher surviv- tion, but a mature forest canopy may be necessary al under low-light conditions (canopy or shade cloth). to facilitate survival of light-intolerant understory In contrast, three species (Artemisia douglasiana, species. We conducted two experiments in riparian Euthamia occidentalis and Rubus californica) had forest restoration sites along the Sacramento River to similar survival across open and canopy conditions. determine whether: Cover of unplanted understory vegetation (mostly 1. Introducing understory species is more success- non-native) was much lower under the canopy than ful at the beginning of restoration or after the in open plots treated with grass-specific herbicide. canopy has developed; Establishment from seed was generally low and highly variable. Our results suggest that to restore 2. Canopy cover directly (via reduced light) or indi- understory species in riparian forests in north–central rectly (by reducing non-native competition) facil- California: light-intolerant understory species should itates survival of native understory species; and be planted after canopy closure; canopy cover is 3. Seeding or planting seedlings of understory spe- more effective than grass-specific herbicide at reduc- cies is most effective. ing non-native understory cover; and planting seed- lings is more successful than direct seeding. Seven native understory species were introduced as both seeds and seedlings into an experiment that KEY WORDS manipulated canopy cover (open or canopy) and non- native grass competition (control or grass-specific competition, direct seeding, facilitation, riparian herbicide). We conducted another experiment using understory, Sacramento River, seedling survival and shade cloth to directly test the effect of different growth 1 Department of Biological Sciences, California State University, Chico, CA 95929 2 Corresponding author: Environmental Studies Department, University of California, Santa Cruz, CA 95064; [email protected] SAN FRANCISCO ESTUARY & WATERSHED SCIENCE INTRODUCTION tiveness of restoration efforts. The success of sowing seeds is limited by seed predation, seed germina- The challenges and importance of restoring under- tion, and early seedling survival (Nilson and Hjalten story forest species have received increasing atten- 2003; Doust and others 2006; Jinks and others 2006). tion in recent years (reviewed by Flinn and Vellend Planting seedlings eliminates these losses but requires 2005; Nilsson and Wardle 2005; Gilliam 2007). propagation facilities and is more costly and labor Traditionally, forest restoration efforts have focused intensive. Evidence on the efficacy of seeding vs. on restoring trees and shrubs, while either overlook- planting seedlings is mixed; some studies have found ing the understory component or assuming it would greater survival and growth of planted seedlings as self assemble over time (Young and Evans 2000; compared to those germinating from seed (Drayton Hilderbrand and others 2005; Pensa and others 2008). and Primack 2000; Wallin and others 2009), whereas Many studies, however, show that understory spe- other studies have reported the opposite result (Halter cies often fail to colonize restored forests due to dis- and others 1993; Young and Evans 2000, 2005). persal and recruitment limitations (e.g., McLachlan and Bazely 2003; Whigham 2004; Flinn and Vellend Large-scale riparian forest restoration along the 2005; Shono and others 2006). A growing number of Sacramento River in north-central California presents studies, primarily in the tropics, suggest that a closed an ideal opportunity to investigate both the timing canopy is necessary to facilitate survival of late suc- and method of understory restoration. Riparian for- cessional forest species (Parrotta and Knowles 2001; ests along the middle Sacramento River have been Cabin and others 2002; Raman and others 2009). This altered by dams and levees, extensive industrial agri- result is problematic for restoration practitioners, as culture, logging, and urban development with <5% restoration budgets are commonly allocated for one of the original forest remaining (Golet and others to a few years; therefore, it is likely to be challenging 2008). Since 1988, several nonprofit organizations to find funding to introduce later successional species and governmental agencies have carried out riparian after appropriate site conditions have developed. forest-restoration projects on >2,500 hectares (ha) of land; these efforts have resulted in a valuable natural Canopy cover may facilitate understory species experiment. Before 2000, only native overstory trees directly, by ameliorating abiotic stress (e.g. decreased and shrubs were planted. Since 2000, both overstory air and soil temperatures and increased humid- and native understory species were usually planted, ity, soil moisture, and litter cover) (Zou and others the latter including a mix of herbs, vines, and low- 2005; Pages and Michalte 2006; Barbier and others growing shrubs. Surveys of many pre-2000 restora- 2008), or indirectly, by reducing competition with tion sites have showed that few native understory light-demanding non-native species that are com- species naturally colonize the restored forests (Holl mon in disturbed landscapes (Levine 1999; Pages and and Crone 2004; McClain and others 2011). Instead, Michalet 2003; Pensa and others 2008). Evidence the understories are dominated by non-native forbs exists for both direct and indirect canopy effects. and grasses. Native understory cover, where it occurs, Studies of shade-tolerant plants report that decreas- is higher in sites with lower non-native cover and ing light can facilitate survival (e.g., Sack and Grubb greater canopy cover. This correlational evidence sug- 2002; Hastwell and Facelli 2003; Sanchez–Gomez gests that canopy cover may have a positive effect on and others 2006). Likewise, reducing competition understory species in this system, but an experimen- through removal of non-native species by herbicides tal test is required to separate out the effects of over- or physical means can increase native species estab- story cover and understory competition to help guide lishment (D’Antonio and others 1998; Cabin and oth- restoration efforts. ers 2002; Denslow and others 2006). We conducted a factorial experiment in which we The method of species introduction, whether by sow- manipulated both canopy cover and competition with ing seed or planting seedlings, may influence the non-native grasses to determine whether: (1) it is bet- magnitude of canopy effects and the overall effec- 2 JULY 2011 ter to introduce understory species into a site before Species Description or after the canopy has developed; (2) canopy cover directly (by reducing light and consequent abiotic We chose seven understory species with varied stress) or indirectly (by reducing non-native competi- growth and dispersal forms that are common in the tion) facilitates the survival of native understory spe- riparian forest understory and widely planted as cies; and (3) seeding or planting understory species is part of restoration efforts (Table 1). These species more effective. We also conducted a “shade experi- (referred to by their genera) are common in rem- ment” to test separately the effect of different light nant forests, but only Artemisia frequently estab- levels on seedling survival and growth. Finally, we lishes naturally in restored sites (Holl and Crone compared survival rates of understory species in our 2004). experiments to those in actual restoration projects to Seeds for direct seeding and growing seedlings in determine the applicability of our results to larger- both the canopy-herbicide and shade experiments scale restoration efforts. were collected locally by The Nature Conservancy Sacramento River Office and The Floral Native METHODS Nursery (Chico, CA, USA). Six species were grown by Floral Native Nursery and were 6 to 12 months Site Description old and 2 to 10 cm tall at the time of planting. These studies were conducted at sites located between Carex seedlings were grown by Hedgerow Farms Hamilton City (39.7° N, 122° W) and Gerber (40° N, (Winters, CA, USA) and were 3 months old and 2 to 122.2° W) in north-central California. Average annual 5 cm tall when planted. All species except Carex rainfall is 531.6 m with high interannual variation and Rubus were dormant (no leaves) at the time of and most rain falling between November and April. planting. Average monthly temperatures range from 6 °C in December to 27 °C in July (California Department of Canopy–Herbicide Experiment Water Resources, Orland station). Sites are all located within the 2-year floodplain (i.e., land with an aver- We conducted a factorial experiment in which we age 2-year flood recurrence frequency) and are sepa- evaluated the effect of canopy cover (open or can- rated by 0.5 to 65 km. opy) and non-native grass competition