Research | Hirao Lab

Ecology of Forest Biodiversity

Forests are home to a diverse range of organisms, from microorganisms in the soil to plants and mammals on the forest floor and insects and birds in the forest canopy, and their interactions form a complex network. It is said that about 80% of all terrestrial species depend on forests. Therefore, understanding forest biodiversity and the interactions between organisms will help us conserve the diversity of life on Earth and understand their ecosystem functions. The research of this laboratory focus on forest biodiversity and interactions using multifaceted approaches, such as field surveys, field experiments, genome analysis, and statistical modeling, mainly in the Okuchichibu Mountains, where a pristine natural environment remains, and in the mountainous regions of eastern Japan. Our research targets all populations of organisms that grow in forests. In particular, we focus on major environmental change events to understand how forest biodiversity and ecosystem functioning change in general. We hope to use the knowledge gained to help conserve forest flora, fauna, and ecosystem services.

Current Research Projects

Impacts of vegetation decline due to sika deer herbivory on forest biodiversity

In recent years, the population density of sika deer has increased in many parts of Japan, causing major changes to forests. Social issues in the background, such as the decline in hunting and forestry, are difficult to solve. In the Okuchichibu Mountains, deer density increased more than ten-fold in the 2000s, which resulted in the decline of understory vegetation and the death of canopy trees due to deer herbivory. There is growing concern that this decline in vegetation may lead to the loss of various ecosystem services that benefit forests. For example, the decline in invertebrates that use plants and humus as resources and the loss of habitat for higher-order predators that use invertebrates as food resources may alter forest biodiversity and biological interactions. In addition, only plants with low nitrogen content, which is not preferred by deer, remain. This reduces litter decomposition and nitrogen mineralization and slows down the biogeochemical cycle, which may affect forest regeneration. Since the effects of deer on vegetation decline are multifaceted, it is necessary to study the long-term changes in various processes in forest ecosystems. Therefore, we installed many deer exclusion fences over a wide area to understand changes in the diversity of plants, animals, and microorganisms and their interactions and to clarify the effects on ecosystem functions.

Roles of soil microorganisms in forest plants-soil feedback

The importance of plant-soil relationships for plant growth has long been known in agriculture. In recent years, it has become clear that this relationship plays a universal role in determining the dynamics of terrestrial ecosystems. For example, plants supply litter and root exudates to the soil, and decomposers that use these materials mineralize nutrients, which may promote plant growth. However, as plants grow, nearby soil attracts herbivores and pathogens that may limit plant growth. Thus, plant-soil interactions can be both facilitative and restrictive. In addition, the relationship between plants and soil is complex because there are many unknown factors that intervene. One such factor is the diversity and function of soil microorganisms. In particular, identifying the role of the functional core microbiome, whose characteristics are related to plant fitness, is expected to lead to a better understanding of plant-soil feedback. We are currently working on elucidating the mechanisms of plant-soil feedback in forest ecosystems through transplantation experiments of tree seedlings and soil and hologenomic analyses of the functional core microbiota. Our goal is to apply this knowledge to restoring vegetation under deer herbivory and converting unmanaged plantations into natural forests.

Climate change adaptation for the conservation of flora and fauna endemic to mountainous regions

Recently, extreme weather events, such as localized torrential rains and rising temperatures, have been frequently reported in various regions. These phenomena are associated with climate change, and there are concerns that they may cause changes in the phenology and distribution of organisms, affecting biodiversity and ecosystem services. In particular, flora and fauna endemic to mountainous regions are vulnerable to climate change. For example, many plant species in the Okuchichibu Mountains have adapted to limestone habitats. However, the population of limestone plants is very small and can be easily lost owing to changes in climatic conditions and the resulting expansion of the distribution areas of other plants and animals; therefore, conservation approaches are needed. One approach to counteracting the effects of climate change is to increase the adaptability of organisms to climate change by reducing other stressors. It is also necessary to assess the effects of climate change and identify high-risk areas and organisms. By considering adaptations to climate change, we expect to contribute to the conservation of biodiversity and ecosystem services in mountainous regions. We currently combine field surveys and genome analyses to elucidate the distribution and adaptability of plants and animals endemic to mountainous regions and develop climate change adaptations for their conservation.

Habitat management for mitigating human-wildlife conflicts

Conflicts between wild animals and humans, such as damage to agriculture and their approach to residential areas, have been recognized as social problems. Agricultural damage caused by wild animals has been apparent since approximately 1990 and has been characterized by a simultaneous increase in damage caused by many wild animals, not only deer. This suggests that the wildlife problem is not simply due to decreased hunting but also to structural changes in the natural environment. One such change is the replacement of natural broadleaf forests with coniferous plantations through expanded afforestation, which has created large areas of resource-poor habitats for wildlife in the backcountry. However, as the demand for firewood and charcoal diminished, the sparse forests that spread around human settlements shifted to broad-leaved forests, creating resource-rich habitats for wildlife around human settlements. Therefore, clarifying the relationships between environmental changes and wildlife dynamics is necessary. It has also been pointed out that wild animals are no longer afraid of humans because of the decreased human activity in forests. We currently use camera traps and environmental DNA analyses to elucidate the regional habitat use of wildlife from rural areas to forests and to propose approaches to mitigate conflicts between human society and wildlife.

Solutions for carbon neutrality through forest ecosystem functions

Addressing climate change has become a common global issue, and the goal of carbon neutrality by 2050 has been set. To achieve this goal, we need to take a new approach that addresses energy issues and proposes nature-based solutions that utilize ecosystem functions and implement them in society. One of the main pillars is to improve the carbon storage function of forests. The forest area in Japan covers approximately 70% of the land area, of which approximately 40% consists of plantations such as cedar, cypress, and larch. Most of these plantations were established through the so-called “expanded afforestation” policy, in which natural broad-leaved forests were cut down and replaced with economically valuable plantations. However, with the subsequent increase in demand for foreign lumber, the price of domestic lumber has slumped, and as a result, it is considered that many plantations are no longer thinned or managed. It has been pointed out that these unmanaged forests have low timber volume, which affects the economic aspects of timber production and impairs the carbon storage function. Therefore, we quantitatively evaluate the carbon sequestration function of unmanaged plantations, including the above-ground and below-ground parts. In addition, we verify the results experimentally by converting plantations to natural forests to examine effective management approaches for improving the carbon sequestration function.