Forest Structure and Pollinator Diversity: A Review on Forest Ecological Functions

Academic Background

In recent years, the global diversity of pollinators has been declining rapidly, posing a serious threat to ecosystem functions and agricultural production. Forests, as critical ecosystems, not only provide habitats for pollinators but also play a key role in maintaining regional pollinator diversity. However, the capacity of different types of forests to support pollinators varies significantly. To better understand the relationship between forest structure and pollinator diversity and to provide scientific guidance for forest management, researchers have conducted a systematic review of how forest structure, composition, and age affect pollinator communities.

The primary objectives of this paper are: (1) to review how changes in forest structure, composition, and age influence pollinator communities; (2) to explore the differences in pollinator diversity between conifer forests and broadleaf forests; and (3) to provide recommendations for forest managers to optimize pollinator habitats.

Source of the Paper

This paper was co-authored by Michael D. Ulyshen, Kimberly M. Ballare, Christopher J. Fettig, James W. Rivers, and Justin B. Runyon, affiliated with various research stations of the USDA Forest Service and Oregon State University. The paper was published online on July 9, 2024, in the journal Current Forestry Reports, titled The Value of Forests to Pollinating Insects Varies with Forest Structure, Composition, and Age.

Key Points

1. Impact of Forest Structure on Pollinators

Forest structure refers to the horizontal and vertical distribution of vegetation within a forest, including the canopy, shrub layer, and ground vegetation. The paper highlights that the complexity of forest structure directly affects pollinator habitats and food resources. For example, an open canopy increases the diversity and abundance of understory herbaceous plants, thereby providing more nectar and pollen resources for pollinators. Studies have shown a positive correlation between canopy openness and pollinator diversity and abundance.

Additionally, forest management practices such as thinning and prescribed burning can effectively improve forest structure and enhance pollinator habitat quality. For instance, in ponderosa pine (Pinus ponderosa) forests in Colorado, USA, thinning significantly increased bee diversity and plant-bee interactions. However, in tropical or temperate broadleaf forests, thinning may reduce the number of flowering trees, negatively impacting pollinators.

2. Impact of Forest Composition on Pollinators

Forest composition refers to the distribution of different plant species within a forest. The paper notes that broadleaf forests generally support higher pollinator diversity than conifer forests because the canopy of broadleaf trees provides more nectar and pollen resources. For example, in the southeastern United States, bee diversity is positively correlated with the proportion of broadleaf trees. In contrast, the canopy of conifer forests offers fewer nectar resources, and pollinator diversity primarily depends on the richness of understory vegetation.

Furthermore, the paper discusses the impact of plantation forests on pollinators. Globally, the area of plantation forests is rapidly increasing, but these plantations often consist of single tree species and lack diversity, limiting their ability to support pollinators. Research shows that even flowering plantation tree species (e.g., eucalyptus and pine) have limited attractiveness to pollinators due to their short flowering periods and lack of other nectar resources.

3. Impact of Forest Age on Pollinators

Forest age is another critical factor influencing pollinator diversity. The paper points out that as forests age, changes in forest structure and composition affect pollinator habitat quality. In conifer forests, pollinator diversity typically declines with increasing forest age, especially when the canopy becomes closed. However, in broadleaf forests, as trees mature and begin to flower, pollinator diversity may increase, even surpassing that of open habitats.

For example, in pine forests in the southeastern United States, frequent fires maintain an open canopy, thereby increasing pollinator diversity. In contrast, pine forests without fire management experience significant declines in pollinator diversity due to canopy closure. In broadleaf forests, flowering mature trees provide essential food resources for pollinators, particularly when understory vegetation is sparse.

4. Forest Management and Pollinator Conservation

The paper concludes with recommendations for forest managers to optimize pollinator habitats. First, managers should aim to mimic natural disturbances, such as fires and windstorms, to maintain open forest structures. Second, in plantation forests, planting a variety of native tree species and preserving open spaces within the forest can enhance pollinator habitat diversity. Additionally, managers should control invasive plants, which may competitively exclude native plants and reduce pollinator food resources.

Significance and Value of the Paper

This paper provides scientific guidance for forest managers to optimize pollinator habitats through a systematic review of the effects of forest structure, composition, and age on pollinators. It highlights the differences between broadleaf and conifer forests in supporting pollinator diversity and identifies potential threats posed by plantation forests and invasive plants. Furthermore, the paper proposes specific forest management measures, such as thinning and prescribed burning, to enhance pollinator diversity.

This paper not only offers new perspectives for forest ecology research but also provides important practical guidance for global pollinator conservation. As global forest degradation and pollinator diversity decline continue to intensify, the findings of this paper hold significant scientific and practical value.