Herbivory and Soil Properties as Ecological Constraints to Willow Growth in a Restored Riparian Ecosystem in Coastal California

Active revegetation of native riparian species is a critical component of most riparian restoration projects. Native woody riparian species typically grow quickly and support the ecosystem by stabilizing streambanks [1], increasing root density for erosion prevention [2], and establishing channel vegetation structure for faunal habitats [3]. Woody riparian species provide valuable ecosystem services by increasing retention of flood water, reducing sedimentation, regulating temperature by providing shade, and improving water quality [4,5]. Willows (Salix spp.) are commonly utilized for revegetation of riparian ecosystems due to their simple propagation, demonstrated high survivorship, and rapid growth rate. Willows can reach heights of over 10 meters (m) and may grow approximately 2 to 3 m after a single growing season, thereby rendering most branches inaccessible for browsing by herbivores [6]. While their rapid growth makes willows resistant to browsing, cuttings are highly susceptible to intense browsing during the first few years after being planted [6]. Excessive grazing has been shown to negatively affect plant growth and reproduction [7-9], completely eradicate populations of woody plants [10], and has contributed to the worldwide degradation of riparian ecosystems [11,12]. Over time, frequent removal of woody stems and foliage by herbivores can result in a reduction in the carbon reserves below ground surface, thereby preventing natural defense mechanisms, including chemical resistance and rapid vertical growth, rendering the plants more susceptible to browsing [13].

Disturbed soil conditions can also adversely affect willow growth and overall survivorship. Construction activities commonly required for riparian restoration projects include: channel realignment; bank stabilization; reconstruction of geomorphic features such as floodplain benches, riffle pools, and depositional basins; placement of streambed material and large woody debris; and grade control structures. The use of heavy machinery and intensive ground disturbance required for these restoration activities can result in heavily compacted soils and reduced quantities of topsoil and organic material. Although increased soil compaction reduces the potential for soil erosion, it can also reduce the total porosity of the soil [14], effectively decreasing the potential for water to infiltrate the soil. Soil compaction may also reduce carbon and nitrogen cycling, soil microbial biomass [15,16], or impede plant growth, specifically root length, depth, and penetration through the soil [17]. Stunted willow growth may be indicative of an ecological stress which could negatively influence the overall success of a restoration project if not resolved. Due to the dynamic complexity of ecosystems, any number of ecological factors (e.g., excessive grazing by livestock, natural predation, poor soils, or low water availability) could detrimentally affect vegetation health [18]. 

Based on existing site conditions observed in the restored riparian ecosystem, we hypothesized that the leading ecological constraint contributing to stunted willow growth was either browsing by large herbivores or decreased soil moisture due to increased soil compaction resulting from ground-disturbing restoration activities. The objectives of our study were to 1) evaluate the effects of browsing by large herbivores on willow cuttings through the use of exclusionary fencing and 2) evaluate the effects of soil characteristics on willow growth through the collection of soil compaction, moisture, and composition data and comparison with data from a healthy riparian reference ecosystem.

Study location

Experimental design

Figure 1: Location of restoration and reference plots along Redwood Creek at Muir Beach, California.

Willow growth and herbivory

Soil characteristics

Reference plots

Statistical analysis

Legend: * = 0.05 ≥ P > 0.01 = significant, ** = 0.01 ≥ P > 0.001 = highly significant, *** = P ≤ 0.001 = very highly significant based on a 95% confidence level.

Willow height

Figure 2: Mean change in height over the growing season between fenced and unfenced and second-year versus third-year willow cuttings on the right bank.

Figure 3: Mean change in height over the growing season between fenced and unfenced and second-year versus third-year willow cuttings on the left bank.

Willow canopy diameter

Figure 4: Mean change in canopy diameter (cm) over the growing season between fenced and unfenced and second-year versus third-year willow cuttings on the right bank.

Figure 5: Mean change in canopy diameter (cm) over the growing season between fenced and unfenced and second-year versus third-year willow cuttings on the left bank.

Willow aerial percent cover

Figure 6: Mean change in percent aerial cover over the growing season between fenced and unfenced and second-year versus third-year willow cuttings on the right bank.

Figure 7: Mean change in percent aerial cover over the growing season between fenced and unfenced and second-year versus third-year willow cuttings on the left bank.

Willow volume

Figure 8: Mean change in volume (cm³) over the growing season between fenced and unfenced and second-year versus third-year willow cuttings on the right bank.

Figure 9: Mean change in volume (cm³) over the growing season between fenced and unfenced and second-year versus third-year willow cuttings on the left bank.

Deer herbivory

Figure 10: Mean number of deer visits per day for unfenced willows in their second growing season.

Soil characteristics

Soil characteristics vs willow growth

Our analysis of soil characteristics concluded that restoration activities created significantly different soil conditions within the restored portions of the study area compared to undisturbed reference site conditions. Significant differences in soil moisture and compaction were observed between the study and reference plots, with mean soil moisture within the study plots measured at nearly half that of the soil within the reference plots and mean soil compaction measured at more than double that of the soil within the reference plots. These variations could be attributed to the difference in distance from the stream or the difference in soil texture between our study and reference plots [21].

Evette et al. [22] found that all growth metrics of willow cuttings are significantly related to available soil moisture and drought conditions generally increase willow growth stress compared to moist, well-drained conditions [23,24]. Highly-compacted soils may result in increased soil strength, reduced bulk density, and a reduction in water-holding capacity, all of which can restrict root growth and hinder plant health [25]. Furthermore, natural recovery of compacted soils may require many years depending on the regional climate, soil texture, and the level of compaction. Contrary to findings of similar studies, we only found weak correlations between soil characteristics and willow growth. 

Our study identified a clear effect of willow growth within the exclusionary fencing along the banks of Redwood Creek. This effect was especially well pronounced in the second growing season. Fenced willow cuttings generally exhibited increased growth and increased biomass compared to cuttings located outside of fenced plots during the growing season. These results indicate that grazing contributed to stunted growth of planted willow cuttings. Although many studies have found a correlation between unrestricted browsing by herbivores and adverse effects to vegetation growth, the findings of our study were unique in that the effect of fencing was different between second- and third-year cuttings and the sides of the bank.

Overall, most fenced second-year cuttings demonstrated an increased change in growth for all metrics compared to unfenced cuttings. However, fenced third-year cuttings demonstrated no significant difference in growth metrics compared to unfenced third-year cuttings. These results suggest that the effects of browsing have a greater detrimental effect on second-year willows compared to third-year willows in the same habitat. Other studies have also found the effects of herbivory to depend on the age of the willows [26-29]. Several studies have shown that the nutritional value of willow leaves and shoots in their first two years of growth is valuable to large herbivores [29,30]. Additionally, juvenile willows are likely to be more accessible to herbivores due to their smaller stature and may have reduced chemical defenses as they prioritize early rapid growth [27]. Therefore, it may be possible that willows at Redwood Creek were not only more accessible to herbivores but may have also been higher in nutritional value if they continued to prioritize the production of photosynthetic material over chemical defenses.

The effectiveness of exclusionary fencing between second- and third-year willows suggests resilience to the effects of herbivory was different between the two age groups. These results differ from studies which conclude that younger willows may demonstrate increased compensatory responses to herbivory than older willows due to greater accumulation of stored reserves. However, Hanley and Fegan [31] found that younger willows were more susceptible to the detrimental effects of herbivory since biomass removal can greatly reduce the reserves necessary for compensatory responses. Therefore, younger willows with less biomass may have a reduced ability to recover under conditions of unrestricted browsing. Our results indicate that willow cuttings at this site become more resistant to the effects of herbivory by the third growing season following initial planting; however, the mechanism (chemical or reserves) for resistance remains unknown.

Willows are a common preferred food source for herbivores in riparian ecosystems [32]. Observations of increased growth among willows protected from browsing in this study were consistent with many studies, including De Jager et al. [33], who demonstrated improved survivorship and growth among willow communities following the removal of large herbivores. However, several studies used fencing to restrict access by livestock, not wild herbivores [34,35]. Our study corroborates findings from other studies that the use of exclusionary fencing can be a successful tool for protecting restored vegetation from wild herbivores as well as livestock [28,36].

Our study suggests that the use of exclusionary fencing may be an effective method for facilitating recovery of riparian vegetation at riparian restoration sites where herbivory is prevalent. Understanding the timing and frequency of browsing is particularly valuable information for installing exclusionary fencing treatment [37]. Fencing has a greater impact if implemented during the first two years following initial planting when willows are most accessible by herbivores, or until willows demonstrate sufficient tolerance to effects of herbivory. High levels of browsing found in our study may be more detrimental during the growing season because the tearing of leaves directly affects the ability of the plant to photosynthesize; whereas during the winter, browsed willows have demonstrated increased shoot growth, branch frequency, and bud formation [38].

While the use of exclusionary fencing to restrict access by large herbivores and reduce the stress of browsing proved to be an effective method for protecting newly planted willow cuttings in this study, it is not a universally applicable solution and may not be appropriate for all restoration projects. Depending on site conditions, fencing could result in additional adverse effects such as inadvertent entrapment of wildlife or obstruction of wildlife corridors. In this study, fenced plots were originally intended to remain in place for at least two consecutive years; however, during the winter season, water levels rose substantially in Redwood Creek. Due to concerns that fencing could inadvertently trap fish utilizing the riparian corridor, fencing was removed before final measurements were taken for the remainder of the wet season and re-installed in April. This is why we did not show results for first year willow cuttings. While the removal of exclusionary fencing was not anticipated, it was a necessary action and should be taken into consideration for similar riparian restoration projects. Therefore, while the use of exclusionary fencing can be an effective, low-cost, low-maintenance method for restricting browsing by large herbivores; it is important to consider the potential indirect effects of implementing this method.

The authors gratefully acknowledge the NPS and GGNRA for allowing us to conduct this study at Redwood Creek throughout ongoing restoration efforts. We greatly appreciate the University of San Francisco, Environmental Science Department and Masters of Science in Environmental Management Program.

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