Resource diversity, abundance, and quality shape soil communities and food webs. In this study, we evaluated the effects of tree diversity, density, and traits on ground beetle diversity, density, and community composition. Rather than increased resource diversity and abundance leading to increased beetle diversity and abundance, we found that tree traits shaped ground beetle diversity and density. Specifically, as the relative abundance of evergreen trees increased, ground beetle diversity and density declined. As such, our study demonstrates that tree leaf habit has a greater influence on ground beetle communities than stand diversity or tree density.
Tree traits better predict ground beetle diversity and density than stand diversity and density
Data from HARV and TALL did not support our hypotheses that higher tree diversity and density would support higher ground beetle diversity and density. Instead, ground beetle diversity and density consistently responded to tree traits; as the relative abundance of evergreen species increased, ground beetle diversity and density decreased. Similarly, across all four NEON sites, ground beetle density was lower in evergreen than deciduous forests. Tree species identity is often more important than tree diversity in predicting soil food web diversity (Cesarz et al. 2013; Eissfeller et al. 2013; Mueller et al. 2016). For instance, while increased tree diversity enhanced soil macrofauna diversity at coarse taxonomic resolution, tree functional types alone shaped diversity at fine taxonomic scale (e.g., species of Carabidae) in experimental plantings (Ganault et al. 2021). Similarly, in subtropical forests where litter quality was consistent across stands, plant diversity did not predict bacterial diversity (Chen 2019). Thus, resource quality may generally be a better predictor of soil community properties than resource diversity and density.
Leaf habit is a better predictor of ground beetle diversity and density than mycorrhizal association
As the relative abundance of evergreen trees decreased, ground beetle diversity and density increased, supporting our hypothesis that low litter quality negatively affects ground beetle diversity and density. This is consistent with the findings of Mueller et al. (2016) who found that soil invertebrate diversity generally and ground beetle diversity specifically were lower in evergreen tree monocultures than in deciduous tree monocultures. Similarly, ground beetle species richness decreases as soil N content decreases (Vician et al. 2018), which is common in evergreen stands. Evergreen trees typically produce litter with high lignin content and create acidic soils (Augusto et al. 2015). High lignin litters and low soil pH are generally associated with low net N mineralization (Scott and Binkley 1997). These harsh environmental conditions may favor specialist organisms, resulting in low diversity and density soil communities (Büchi et al. 2014). While these conditions may also prevent competitive exclusion, other properties of evergreen stands outweigh these effects resulting in less diverse ground beetle communities in evergreen forests than those found in deciduous forests.
Deciduous stands might have a larger resource base resulting in a broader range of niches to support a more diverse ground beetle community. An alternative explanation is that deciduous canopies increase litter diversity which begets beetle diversity directly by fostering diverse food resources for ground beetles. Our data support the former hypothesis given that deciduous forests clearly correlated with higher ground beetle abundance and diversity while tree species diversity had no effect on ground beetle diversity. Overall, leaf habit was an important driver of ground beetle diversity and density likely due to high evergreen abundance creating lower resource quality environments.
In contrast, mycorrhizal association was not a significant predictor of ground beetle diversity or density. Deciduous and AM species have high litter quality and soil N availability, while evergreen and ECM forests are associated with low soil inorganic N, high lignin content, and acidic soils (Augusto et al. 2015; Keller and Phillips 2019). However, along a spectrum of litter quality, we expect AM deciduous trees to have the highest quality litters, ECM evergreen trees to have the lowest quality litters, and ECM deciduous litter to be of intermediate quality. Therefore, differences in litter quality between deciduous and evergreen trees may be greater than those found between mycorrhizal types (Cornelissen et al. 2001). Leaf habit appears to be a more important driver of ground beetle community density and diversity than mycorrhizal association.
Ground beetle diversity and density patterns are not driven by shifts in community composition
While we detected shifts in community composition at TALL and some species-level preferences for deciduous-rich sites at HARV, general relationship between ground beetle diversity and density do not appear to be driven by ground beetle species composition or preferences. Past studies have suggested that leaf palatability traits have a larger effect on detritivore and carnivore community composition than leaf litter diversity (Brousseau et al. 2019). We observed that Synuchus impunctatus (496 total individuals) at HARV tend to prefer deciduous forests, which may contribute to general ground beetle density trends. However, other species at both HARV and TALL show little preference for deciduous or evergreen stands. In fact, while two of the five most abundant species at TALL, Cyclotrachelus convivus and Dicaelus dilatatus, tended to decrease in density as the relative abundance of evergreen trees increased, Anisodactylus merula, Selenophorus opalinus, and Anisodactylus haplomus seemed to favor mixed forest stands. The most abundant species found at HARV and TALL have been documented in past studies in all forest types with thick leaf litter (Lariviere and Larochelle 2003), which may be found across stands varying in leaf habit. Furthermore, the most abundant species in our study are omnivores and carnivores, feeding on larva, smaller insects, and seeds; as such, species-level habitat preferences may be limited.
Caveats and Future Directions
Our study suggests strong relationships between tree leaf habit and soil community density and diversity. However, our ability to extrapolate our results and identify the drivers underlying these patterns are limited by site characteristics and data availability. While we attempted to survey a total of four Eastern temperate forest sites that met our criteria, two datasets were incomplete. As such, our more detailed analyses were limited to HARV and TALL. Additionally, evergreen trees are largely Pinus at both HARV and TALL which may not be broadly representative of all evergreen tree species. However, general trends across all four sites based on stand-level NLCD classifications suggest that ground beetle diversity and density are generally lower in evergreen sites than deciduous sites. Future studies should leverage newly collected NEON vegetation data with additional litter and soil property measurements to assess the generality of our findings.
In addition to sample size limitations, the observational nature of this study limits our ability to disentangle potential stand-level drivers. We used stand-level tree metrics as indices of litter diversity, density, and quality, but litter properties are not the only factors that vary between evergreen and deciduous stands. For instance, light availability - commonly lower in evergreen forests than deciduous forests - has been shown to strongly shape ground beetle diversity across forest stands (Mueller et al. 2016). Increased light availability under deciduous trees may lead to increased soil temperature and understory plant diversity, thereby increasing resource availability and diversity. As such, studies that directly manipulate tree communities and assess the environmental conditions created by these communities will facilitate linking tree communities to soil insect communities in a mechanistic way (Mueller et al. 2016; Ganault et al. 2021).