Northern white-cedar (Thuja occidentalis
L.) is widely recognized for ecological, cultural, and commodity-production values. This species contributes to biodiversity through its association with rare plants such as the showy lady slipper (Cypripedium reginae
], is a preferred winter browse for white-tailed deer (Odocoileus virginianus
Zimmerman), and has traditionally been used by Native American peoples for ceremonies, tools, and medicinal healing. In addition to these non-commodity values, white-cedar’s unique wood properties make it highly resistant to decay after harvesting and thus desirable for production of house siding, log homes, fence posts, and specialty products such as outdoor furniture [2
]. However, our understanding of white-cedar ecology and management across its range is incomplete, especially with regard to the conditions that favor successful regeneration establishment and growth.
A long-lived, shade-tolerant tree, white-cedar has a native range that spans across the northeast portion of the United States into Canada and as far west as the Great Lakes [2
]. White-cedar is an ecologically versatile species that can grow in both early and secondary successional forests, and in well-drained upland or poorly-drained wetland settings. In the Great Lakes region, where this study is focused, most white-cedar harvested for timber products grows in alkaline wetlands as nearly-pure stands [2
Establishment and growth of white-cedar regeneration are problematic in many parts of its range [4
]. As a result, there is concern that harvested white-cedar will not be maintained in the future forest. Regeneration failures have been linked to several factors and combinations of factors including germination substrate, overstory, and stand attributes, understory competition, browsing by whitetail deer, and hydrologic conditions in wetlands [5
Substrate requirements for germination by white-cedar are thought to be very specific. This light-seeded species germinates best in moist environments, which on drier sites may include decaying wood or exposed mineral soil [9
]. On wetland sites, seed rot and seedling mortality from seasonal flooding can occur and therefore white-cedar regeneration is positively correlated with the number of hummocks or proportion of area with hummocks [8
]. While substantial work on white-cedar substrates in the eastern portions of the range has been conducted [11
], the importance of substrate traits may not be consistent across the entire range of the species. Therefore, further understanding of substrate traits that are important for regeneration success in the Great Lakes region is needed.
White-cedar regeneration may also be affected by other stand-level attributes including overstory stocking, composition, and competition. In the overstory, white-cedar is the seed source for regeneration, but higher light transmittance in thinned canopies can lead to increased growth rates for white-cedar [7
]. As a slow-growing species, white-cedar is vulnerable to competition from other tree species and shrubs, especially in gaps and larger openings [8
In many areas, white-cedar seedlings rarely make it to the sapling height class [7
]. White-tailed deer browsing is thought to be a main impediment to white-cedar regeneration. Since the mid-1990s, white-tailed deer populations in northern Wisconsin have been 2–3 times higher than they were in the 1950s and 1960s and as much as 12 times higher than pre-settlement populations [15
]. This long period of high deer populations has likely significantly affected sapling recruitment of white-cedar and other palatable species [17
]. Browsing is considered a significant bottleneck to white-cedar regeneration across its range [5
Assessing the level of deer browsing pressure on tree regeneration can be challenging, especially for foresters and land managers with limited time and resources. Deer population estimates based on deer harvest data collected on a county level are too coarse to determine browsing pressure within a particular stand. Alternatively, stem browsing indices can be collected for one or multiple tree species, such as the sugar maple (Acer saccharum
Marshall) browse index [19
], but these data are only applicable when there are enough stems and/or species to browse. Many dense stands lack understory regeneration and especially palatable species due to their stage of stand development rather than browsing pressure. At the stand level, regeneration tallies by species and height class may show that some species are not reaching heights tall enough to escape browse pressure [e.g., 8]. In this case, deer exclosures often show dramatic differences from unprotected areas but represent an unnaturally altered condition and are often impractical on a large scale [20
]. The use of indicator plants such as Trillium
spp. has been shown as an effective measure of deer browse pressure in some locations [21
], but this practice may be time-consuming for a forester and can only be used during a limited part of the year in stands where these species are present. Additionally, Kirschbaum and Anacker found that indicator species characteristics were not correlated with signs of browse in a study in McKean County, Pennsylvania, USA, possibly because of the effects of additional environmental factors or legacy effects of historically high deer populations [23
The objectives of this study were to (1) quantify natural regeneration in mature white-cedar stands and (2) assess relationships between density and height of white-cedar regeneration and explanatory factors such as deer population density, browsing intensity, available regeneration substrate, and competitive environment. We hypothesized that the amount of bare soil and deadwood substrate would be positively related to white-cedar regeneration, while local deer population size and browsing intensity would be negatively related. In addition, we anticipated that stand-level browsing intensity would be a stronger predictor of white-cedar regeneration than other metrics such as county deer population estimates.
White-cedar serves important roles in social and ecological systems. Its value as a decay resistant building material and its traditional uses by Native American peoples make it an important forest product. However, this species is often difficult to regenerate. Our study focused on inventories of mature stands of white-cedar to uncover factors related to success or failure of natural white-cedar regeneration. Based on existing studies, we anticipated white-cedar regeneration densities to increase with more exposed soil (minimal litter layer) and coarse woody debris [10
] and decrease with higher deer densities [7
]. The inventoried stands in Wisconsin did not meet our expectations but were influenced by other biotic and abiotic factors that we discuss in the following paragraphs.
One of the most surprising findings from this study was that none of the measures of deer abundance or deer browse pressure were associated with density of white-cedar regeneration. The negative effect of deer browsing on white-cedar has been well documented in many studies and is often cited by foresters as the biggest challenge to white-cedar regeneration [4
]. However, deer browsing pressure can be difficult to quantify and varies seasonally and year-to-year [39
]. While the stands in this study were not known to be used as deer yards and deer yarding is likely to be less common in this area than farther north [40
], the use of white-cedar stands as winter deer yards can further complicate attempts to quantify deer browsing pressure. This study attempted to quantify deer impacts in three ways, using county-level deer population estimates, estimating browse intensity on palatable species at the plot level, and by using surrounding land-use as a proxy for deer habitat. County-level deer estimates did not relate to stem density of white-cedar regeneration or to observed browsing level in the stand. It is not surprising because these measures of deer populations are coarse, and we would not expect them to represent the deer browsing pressure in an individual stand. While we do recognize that the chances of over-browsing increase with increasing county-level deer population, when measuring a highly palatable species like white-cedar, even stands where county-level deer populations are low may still have high white-cedar browse impact due to deer congregating in white-cedar-rich stands. Additionally, we estimated browse intensity in 25% classes (ex. 1%–25% of stems browsed) and estimated most of the plots in the study to have 51%–75% of palatable stems browsed. These estimates of browse intensity may not be fine enough to capture differences that are important ecologically. We did observe a large decrease in white-cedar stem density between the small seedling height class (which is protected from browsing by snow for much of the winter [28
]) and the large seedling height class, which is vulnerable to browsing year-round. This drop off in seedling density could have been due to browsing by deer but could also be related to many other factors such as nutritional demands and competition [8
]. Other measures of browse pressure, such as density and diversity of seedlings by height class [31
] or browsing quantified at the species level may be necessary to better understand stand-specific challenges.
Regeneration of white-cedar was poor overall and highly variable among plots within sites. Large white-cedar seedlings were found on only 19% of the subplots in this study. Management guidelines suggest that adequate stocking can be achieved with 60% milacre (4 m2
) stocking of young, seed-origin seedlings greater than 30 cm tall [43
]. In this study, no sites achieved at least 60% white-cedar stocking in the large seedling height class. The Langlade County site had the highest stocking of large white-cedar seedlings, with 46% of subplots having at least one seedling present, while at the Florence County site, only one subplot of the 24 sampled subplots (4%) had any white-cedar seedlings in the large seedling height class.
While stem densities of white-cedar regeneration were low in all height classes, they were comparable to stem densities found in other recent studies. Site-level mean stem densities ranged from 500–13,500 small white-cedar seedlings per hectare and 60–1200 large white-cedar seedlings per hectare in this study. In mixedwood stands in Quebec and Maine, Larouche and Ruel observed natural regeneration densities of about 3000–8000 white-cedar seedlings per hectare [7
]. In a lowland white-cedar stand in Wisconsin, Forester et al. observed a median stem density for small white-cedar seedlings of 1800 stems ha−1
]. A study conducted in poorly drained stands in Wisconsin in the early 1980s found much higher densities of white-cedar regeneration in unharvested stands, with mean stem densities of over 30,000 stems ha−1
Of the 30 potential factors analyzed in this study, three were found to be related to the density of at least one height class of white-cedar regeneration: soil pH, stem density of regeneration of other species, and stem density of mature white-cedar in the overstory. Soil samples in this study had pH values from 4.6 to 6.7 and stem density of small white-cedar seedlings increased with increasing soil pH. These results are similar to other studies both in the Lake States and in the Northeast. In Michigan, Nelson found lower densities of white-cedar seedlings when the soil pH was less than 6.0 [45
]. In Maine, Kell found that white-cedar growth was positively correlated with pH [46
], and Curtis found optimal white-cedar growth when soil pH was between 5.5 and 6.7 [47
Stem density of mature white-cedar in the overstory was positively related to stem density of white-cedar saplings in the study. One potential reason for this positive correlation could be increased seed rain with more white-cedar in the overstory. In some cases, seed rain may be the most limiting factor to white-cedar regeneration, or at least very high seed rain may increase the likelihood of some individuals surviving against other challenges [13
]. Cornett et al. found an effective seed rain dispersal distance of about 20 m for white-cedar [48
], which could lead to a strong correlation between overstory white-cedar and regeneration at the plot level. Alternately, since we did not age the saplings in this study, it is possible that they are older, suppressed individuals that established at the same time as the larger, mature trees in the overstory after a disturbance or extended period of low browsing intensity [49
]. Finally, layering (asexual reproduction resulting from branches rooting to the ground) has been reported as the predominant white-cedar regeneration mechanism on some wet sites [45
]; this could explain the correlation we observed between density of overstory white-cedar and regeneration of this species. Mode of regeneration was not determined in the present study.
We found both small and large white-cedar seedlings to increase with increasing stem density of regeneration of other species in the same height classes. This seems to be inconsistent with other studies that found negative impacts of competition on white-cedar regeneration [14
]. Chimner and Hart also found that white-cedar seedlings decreased when shrub density increased [8
]. The findings in this study could mean that there were additional (unmeasured) factors that made certain plots or sites more favorable to regeneration overall, not just regeneration of white-cedar or there could be associational resistance to deer browsing due to increasing stem density of other species [50
Microtopography has been found to be an important factor for white-cedar regeneration in several studies. In swamps and on wetter sites, the increased presence of hummocks has been associated with increased white-cedar survival and growth [51
] and decreased competition from shrubs and hardwoods [8
]. On drier and upland sites where white-cedar seedlings are more susceptible to desiccation, decaying logs have been found to be a better seedbed [48
] and soil moisture has been suggested as one of the most important factors affecting white-cedar germination and early seedling survival [10
]. This study took place on sites with poorly drained, organic soils. While we did not find the amount of standing water in a plot to have a significant effect white-cedar regeneration, we did observe the majority of white-cedar seedlings growing on either hummocks or decaying logs.
There are several additional factors that may be important to white-cedar regeneration that were not measured in this study. Hydrology, fire, and fire history of a stand may also play a significant role in site preparation for white-cedar regeneration [8
], but the fire history of the sites in this study is not known and hydrology data were not collected for this study. Browsing by snowshoe hares (Lepus americanus
Erxleben) was not evaluated in this study but has historically had the potential to be as or more important in affecting white-cedar regeneration than browsing from deer [54
]. We also acknowledge that this study was of limited geographic scope, with only five sites across northern Wisconsin, USA. Additional study sites across a broader geographic area would increase the robustness of this study.