Forest management practices induce long-lasting changes in the distribution of stand types in forested landscapes and the distribution of biota within them [1
]. Besides effects on α-diversity, management disturbances could alter community heterogeneity (β-diversity) by imposing more stringent environmental filters [2
] or increasing selection for disturbance-tolerant species [3
]. In Europe and in North America, clear cuts were the main harvesting practice until the 1980s [4
]. Concerns about biodiversity conservation, soil protection, and tree regeneration led to the development of alternative retention patterns (e.g. partial harvesting, careful logging, continuous-cover forestry) that have less conspicuous effects on forest ecosystems than clear cuts [5
]. Generally, retention forestry is defined as an approach to forest management based on the long-term retention of structures and organisms, such as live and dead trees and small areas of intact forest, at the time of harvest [8
]. The variation in understory light availability, generated by different retention patterns in a region, has been shown to be a dominant filter affecting post-harvest understory plant composition and diversity [9
Light availability may not completely explain post-harvest plant diversity. As following harvesting, different site preparation techniques may be applied in order to create favorable microsite conditions (e.g., soil temperature and moisture regulation, competition control) for reforestation. Two common examples in Europe and North America are mechanical site preparation [13
] and prescribed burning after harvest [15
]. Site preparation techniques have varying levels of soil disturbance (e.g., exposed mineral soil, a mixture of organic matter and mineral soil) and have been shown to impact tree survival and growth, as well as understory composition [13
]. Previous studies have focused on retention patterns or site preparation disturbance effects on the composition or diversity of understory plant communities [17
]. Few studies have investigated diversity changes due to both retention patterns and post-harvest site preparation, or identified whether the combined effects of retention pattern and site preparation better explained understory diversity than retention pattern.
Furthermore, inconsistent conclusions on the effect of a specific retention pattern or a site preparation technique on understory community are found in different studies. Many previous studies have suggested that forest management strategies should be based on knowledge of the basic characteristics of stand type and time since last fire or harvesting disturbance [21
], because the density and composition of canopy trees can modify resource availability [23
] and competition in the understory (e.g., shade-tolerant and intolerant species) [25
]. Moreover, the responses of the understory plant community to disturbance can vary over time since harvest or fire disturbance. For example, while some species colonize habitat patches rapidly, other species might need a long and continuous period of time to exploit a suitable habitat patch.
The understory plant community represents a substantial proportion of overall plant diversity in most boreal or temperate forests [26
], and plays essential roles in biodiversity and ecosystem structure and function [27
]. Due to its sensitivity to a variety of factors such as overstory characteristics [23
], soil properties [28
], and forest disturbances or management practices [31
], understory diversity might also be an important indicator of forest site quality and of the environmental impact of management [33
]. Furthermore, simplifying species composition and diversity to functional trait diversity can provide a synthetic view of a plant community [34
]. Plant functional traits are characteristics of plants which reflect their abilities to adapt to a habitat or influence their responses to environmental changes [36
]. Hence, the co-occurrence of species with similar traits, such as shade-intolerant species co-occurring in open canopy habitats, is considered to be evidence that communities are limited by environmental filters [28
The present study aimed to analyze how retention patterns and post-harvest site preparation act as filters that explain local understory plant communities using the boreal forest as a case study ecosystem. We also assessed whether these effects were dependent on forest attributes (stand type, time since fire, and time since harvest). Here, retention patterns were clear cut, careful logging, plus unharvested control. Careful logging is defined as the harvest of commercial trees (i.e., diameter at breast height >9.1 cm) with the retention of non-commercial trees and with the protection of tree regeneration and soils [39
]. The three site preparation techniques included plow or disk trenching after careful logging, and prescribed fire after clear cut. A large database from seven separate studies conducted in the Canadian Clay Belt region was used in the present study. We collected 10 functional response traits of dominant species that reflect plant morphology, regeneration strategy, and resource utilization.
The research questions were: 1) How does the functional diversity of the understory community vary among retention patterns, as well as in relation to the combined effect of retention pattern and site preparation? 2) Does the combined effect of retention pattern and site preparation better explain functional diversity than retention pattern only? 3) How does the combined disturbance of retention pattern and site preparation correlate with functional trait groups? We hypothesized that the functional diversity increased after harvest, and the combined effect of retention pattern and site preparation better explained diversity than the effect of retention pattern only. We also hypothesized that traits favored by site preparation were those reflecting species’ resistance or resilience to disturbance (e.g., clonal guerilla species, geophytes). Therefore, traits favored by careful logging or clear-cut forests without site preparation would be those related to their resource utilization, particularly light. Finally, we also assessed whether the relationships between retention pattern/site preparation techniques and traits are affected by forest attributes.
Our study systematically investigates the combined effect of retention pattern and post-harvest site preparation in understory community assembly using a functional trait approach in boreal forests. We found that strong trait filtering occurred, from broad-scale light environment filtering due to the retention pattern, to fine-scale niche partitioning due to the soil disturbance caused by site preparation for tree regeneration. However, the combined effect of retention pattern and post-harvest site preparation in our boreal ecosystem was the most powerful explanatory factor for understory functional diversity, when compared to retention pattern only and forest attributes (stand type, time since fire, and time since harvest). Our results indicate that the compounding effect of light variation and soil disturbance more than light alone best explains the functional trait diversity after disturbance. Among the three post-harvest site preparation techniques studied here, only prescribed burning after clear cut can achieve the goal of improving understory functional richness while promoting tree regeneration. The combined disturbance of retention pattern and site preparation affected the understory mainly by filtering for plant resistance or resilience abilities after disturbance. Yet harvested forests without subsequent site preparation mainly filtered species based on their light requirements. Finally, since our study is in boreal ecosystems, more studies on other ecosystems are needed for understanding the mechanisms behind the relationship between forest management operations and understory functional diversity.
Maintaining or improving biodiversity is an important goal of sustainable forest management. One of the forester’s most fundamental acts is the choice of retention pattern. In our study, careful logging and clear cut respectively represent the recent and traditional harvesting choices, and the selection of either of those two retention patterns induces different degrees of variation in functional diversity. However, the trend in diversity variation caused by harvest management is more complicated when incorporating the role of post-harvest site preparation. Site preparation is often neglected in plant diversity study, due to its primary goal of promoting timber production. However, our results suggest that in a boreal forest ecosystem, the choice of post-harvest site preparation techniques, e.g., the prescribed fire or mechanical site preparation that applied to retention patterns, plays an important role in understory functional composition and diversity. For example, prescribed burning after clear cut maintains higher functional richness than the two mechanical site preparation techniques after careful logging. Meanwhile, the two mechanical site preparation techniques after careful logging increase the resource utilization efficiency compared to unharvested forests, which could not be achieved by prescribed burning after clear cut. Besides, by using trait-based approach, the “indicator” traits that are favored by a specific combination of site preparation techniques with retention pattern could be identified. For example, plow and disk trenching after careful logging were respectively favored by clonal guerilla species and geophytes, while prescribed burning was favored by higher seed weight. Therefore, the trait-based approach would help researchers or forest managers predict plant diversity patterns when planning which site preparation to select, or help assess the stability of understory communities under some specific forestry practices.