Eastern white pine (Pinus strobus) is an ecologically and commercially important species in Eastern North America (Costanza et al., 2018). For example, more than 150 vertebrate wildlife species use white pine stands for habitat in Eastern USA (Yamasaki, 2003; DeGraaf et al., 2006; Leak et al., 2020). White pine stands are maintained with silvicultural treatments such as shelterwoods, crown thinnings, or patch cuts (Lancaster and Leak, 1978; Seymour, 2007; Ostry et al., 2010; Leak and Yamasaki, 2013). Shelterwoods are a series of forest cuttings to remove overstory trees and promote seedling establishment by scarifying the soil and providing partial shade before the final and complete removal of the overstory (Table 1; Figure 1) (Lancaster and Leak, 1978). Thinnings are like shelterwoods because overstory trees are removed. However, the objective of a thinning is to increase growth of residual trees, whereas the objective of a shelterwood is to regenerate the stand. Patch cuts are small clearcuts where the overstory is completely removed in 1.2 ha (Table 1; Figure 1). These silvicultural treatments also create ephemeral early successional habitat needed by some bird species (Costello et al., 2000; Thompson and DeGraaf, 2001; DeGraaf and Yamasaki, 2003; Yamasaki et al., 2014).

White pine is also commercially valuable for timber products and aesthetic values for recreational purposes, providing billions of dollars in revenue to local economies (Costanza et al., 2018). However, it is susceptible to a variety of pathogen and insect pests which include, but are not limited to, white pine weevil (Pissodes strobi), white pine blister rust (WPBR, Cronartium ribicola), Caliciopsis canker (Caliciopsis pinea), and foliar diseases such as needle casts and brown spot needle blight (Lecanosticta acicola) (Costanza et al., 2018). White pine weevil is a native insect pest to North America, where it damages Picea and Pinus species by killing the terminal leader, resulting in multi-stemmed growth form and serious lumber defects (Major et al., 2009; Ostry et al., 2010). Vigorously growing trees in full sunlight are more susceptible to damage compared to trees growing in partial shade because weevils attack succulent shoots with thicker bark produced in open conditions (Hamid et al., 1995). In addition, more open conditions increase light and temperature which stimulate the weevils (Hamid et al., 1995). White pine blister rust (WPBR) is lethal to five needle pines in North America and other parts of the world where it has been introduced (Kim et al., 2010). Partial shade and dense stands are also recommended for WPBR management because these conditions lead to self-pruning of lower branches, which are infection courts for the causal agent Cronartium ribicola (Ostry et al., 2010). A two-cut shelterwood is one of the most frequently recommended regeneration methods to avoid weevil and WPBR damage (Lancaster and Leak, 1978; Ostry et al., 2010). The first cut coincides with an abundant seed year removing 40%–60% of the overstory. Abundant seed crops (~4,429 thousand seed per hectare) may occur every three to five years, sometimes seven years (Leak et al., 2020). The second cut takes place 5–10 years later when the seedlings are growing rapidly (Lancaster and Leak, 1978).

Prior to 2010, eastern white pine was managed to reduce losses from white pine blister rust and weevil (Lancaster and Leak, 1978; Ostry et al., 2010). Since then, other native pests and diseases such as brown spot needle blight, Caliciopsis canker, and eastern white pine bast scale have caused unprecedented damage (Mech et al., 2013; Broders et al., 2015; Munck et al., 2015; Schulz et al., 2018a,b; Wyka et al., 2018; Costanza et al., 2019; Cram and Fraedrich, 2022). Although the impacts of silvicultural treatments on residual trees in stands damaged by these diseases have been evaluated (McIntire et al., 2018a,b; Costanza et al., 2019, 2020), the impact of these pests and diseases to regeneration following silvicultural treatments is less understood. For example, reducing stand density by thinning improves residual tree growth and symptom severity in stands affected by Caliciopsis canker and foliar diseases (McIntire et al., 2018a; Costanza et al., 2020). Partial shading and high seedling density recommended to control WPBR and weevil damage (Ostry et al., 2010) may increase humidity in the understory, creating conditions favorable to the reproduction and dispersal of foliar and canker fungal pathogens. In addition, inoculum from overstory trees may exacerbate foliar disease severity of understory seedlings (Wyka et al., 2018). Consequently, our objectives were to evaluate the incidence and severity of insect pests and diseases in white pine stands subjected to the following silvicultural treatments: low density thinnings, patch cuts, shelterwoods, and unmanaged or control stands. Understanding the effects of frequently recommended and implemented silvicultural treatments on insect pest and disease incidence and severity is important to maintain valuable white pine forests.

We evaluated and measured eastern white pine trees (262) and regeneration (>500 seedlings and saplings) in 36 plots, three per stand, for 12 stands, at least 2 stands per silvicultural treatment (Table 1). Other tree species present in our plots included: Abies balsamea, Acer rubrum, Betula spp., Fagus grandifolia, Prunus serotina, Picea rubens, Pinus resinosa, Quercus rubra, Q. alba, and Tsuga canadensis. Of these, A. rubrum and Q. rubra were most common (Table 1).

Silvicultural treatments in white pine stands had the intended effects in overstory white pines. These were completely removed from patch cuts, and partially removed from other treatments, thus, stem density was more than four times greater for untreated controls compared to the treated stands (Table 3). Overall, crown condition improved in treated stands compared to controls. For example, crown density of white pines in untreated control stands was significantly less (33%) compared to that in treated stands (>75%) (p < 0.001). Conversely, severity of foliar pathogens was greater for trees in untreated stands (47% crown affected) compared to that in treated stands (<25% crown affected) (p = 0.012). Resinosis, a symptom of WPBR, Caliciopsis canker and other insects and diseases, was low in all treatments (<17% trees affected) and not statistically significant among treatments.

Compared to untreated control stands, treated stands had more and generally larger regeneration (dbh and height) with better crown condition (LCR, crown density, and WPND severity) (Table 4). The untreated controls produced in average 3,042 ha⁻¹ white pine saplings and seedlings compared to >16,000 ha^–1 saplings and seedlings for treated stands. The dbh and height of seedlings and saplings in untreated control stands was 0.6 cm and 0.9 m, respectively compared to dbh > 1 cm and height > 1.8 m in treated stands (Table 4). The main effect of some treatments was only marginally (dbh, p = 0.08) or not statistically significant (height, LCR, and crown density) for least square means comparisons among treatments. However, most treatment values were significantly different than the control value in single pair-wise comparisons (α = 0.05) (Table 4).

As expected, patch cuts and thinning to 7 m² ha⁻¹, the treatments with least canopy cover (25 and 42%, respectively) or shade, had the greatest proportion of white pine seedlings with weevil damage (27 and 34%, respectively) compared to untreated control stands where regeneration did not have any weevil damage due to heavy shading (Tables 3, 4). Despite the weevil damaged regeneration, patch cuts in this study produced in average > 10,000 white pine seedlings ha⁻¹ without weevil or other pest or insect damage (Figure 3). Similarly, despite more damage from Caliciopsis canker (14%) in thinned stands compared to untreated controls (0%), because regeneration was so abundant in thinned stands (>25,000 seedlings and saplings h⁻¹) compared to untreated stands (3,042 seedlings and saplings h⁻¹), more undamaged regeneration was present in treated stands (Figure 3). White pine blister rust incidence was <6% and not statistically significant among treatments.

Host density may facilitate or slow down the development of disease epidemics or pest outbreaks (Asaro et al., 2023) and understanding effects of host density is therefore critical to management of resilient forests. Shelterwoods and low-density thinnings are the most prescribed silvicultural treatments for management of eastern white pine insect pests and diseases (Ostry et al., 2010; Livingston et al., 2019). These treatments can reduce disease incidence and severity at the stand level by improving condition of residual trees and removal of the weakest trees during harvesting operations (McIntire et al., 2018a; Costanza et al., 2020). Harvesting operations to implement these treatments can result in abundant white pine regeneration (Leak and Yamasaki, 2013).

Compared to overstory trees, the relationship between high stem density in regeneration and foliar diseases or Caliciopsis canker is not as well understood. In a previous study, stem density was associated with greater Caliciopsis canker disease severity in white pine overstory trees and regeneration (Munck et al., 2016). Similarly, in this study the thinnings to 14 m² ha⁻¹ had the greatest Caliciopsis canker incidence and the most abundant regeneration. In this study, both overstory and understory white pines across all silvicultural treatments exhibited less foliar disease severity compared to untreated controls. This finding is consistent with a study by McIntire et al. (2018a) that evaluated crown condition of overstory trees before and after thinning stands.

Foresters and land managers are reluctant to implement clear cuts and patch cuts because white pine weevil preferentially attacks regeneration growing in open conditions. Shading provided by residual trees in shelterwoods reduces white pine weevil damage in the understory (Ostry et al., 2010). In this study, patch cuts and low density thinnings had the least canopy cover and consequently, greatest incidence of weevil damage (Tables 3, 4). Both these treatments, however, yielded abundant white pine regeneration with >10,000 seedlings and sampling per hectare. Defective regeneration could be removed in precommercial thinnings when saplings are 6 m tall to a recommended stem density of 490–740 stem per hectare (Livingston et al., 2019), thus plenty of healthy stems would be available in treated stands evaluated in this study. These low-density treatments have the added benefit of creating early successional habitat preferred by some songbirds.

In Eastern USA, forested land is mostly privately owned. Obtaining support for silvicultural treatments to improve forest health is challenging because private landowners are not solely interested in commercial value of their trees. However, many are interested in wildlife management. Following treatment, shelterwoods and patch cuts produce greater average species richness of birds than unmanaged forest (Goodale et al., 2009; Duguid et al., 2016). These silvicultural treatments create early successional habitat needed by birds using young forests (Costello et al., 2000; Thompson and DeGraaf, 2001; DeGraaf and Yamasaki, 2003; Yamasaki et al., 2014). Thus, shelterwood and patch cut treatments may both increase biodiversity and improve forest production and regeneration.

In conclusion, silvicultural treatments that reduced stem density in the overstory and scarified the soil during harvesting operations resulted in better conditions of residual trees and plenty of healthy white pine regeneration compared to unmanaged stands. Our study emphasizes the importance of considering a realistic spectrum of natural enemies when designing silvicultural operations. Our findings are consistent with management current recommendations for white pine (Livingston et al., 2019; Leak et al., 2020). Given the economic and ecological value of eastern white pine, the effects of silvicultural treatments on insect and pest conditions merit further study. Results could vary in locations where insect pest and disease pressures are greater. This study was conducted in Northeastern USA, but the insect pest and diseases we evaluated have worldwide distribution. For example, white pine blister rust damages pine in Asia, Europe and North America (Geils et al., 2010). Lecanosticta acicola has a global distribution and is a threat to plantations and naturally regenerated stands (van der Nest et al., 2019; Tubby et al., 2023). Pathogenic Caliciopsis spp. damage conifer species in Europe, North America, and Eucalyptus spp. in Australia (Pascoe et al., 2018; Migliorini et al., 2020). Consequently, our findings could be useful to the management of other patho-systems.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

MY and JJ designed and implemented silvicultural treatments, respectively. IM and JJ conceived and designed the data collection. IM analyzed the data and prepared the manuscript. All authors contributed to the article and approved the submitted version.