Plantation management to restore early and late-successional bird habitat under various climatic and seasonal conditions

Although agriculture and plantation forestry have decreased natural open habitats and old-growth forests, conservation in managed lands is considered essential for achieving “nature-positive” goals that reverse biodiversity trends from negative to positive. From subboreal to temperate regions, mature conifer plantations with broadleaved trees (BLTs) offer suitable habitats for species preferring mature natural BLT forests, whereas young plantations harbor species depending on early successional (ES) habitats. However, the functional forms of stand age and BLT, and their context dependency, remain unknown. We quantified the effects of stand age (3–98 years), BLT proportion (0%–97.5%), and their interaction on ES and BLT bird species in plantations, as well as the dependency of these effects on regional/seasonal climates. For both groups, we also explored whether plantations could be comparable habitats to BLT-dominated natural forests (stand age: 6–134 years) across Hokkaido (78,420 km²), northern Japan. ES bird species' richness and abundance decreased exponentially with stand age in plantations. This pattern was not evident in natural forests although only two ES stands were surveyed due to the rarity of natural forest harvesting. ES plantations in cooler regions showed higher habitat values, reflecting a climate-dependent species composition. No ES species occurred in winter. Both stand age and BLT proportion increased BLT bird species richness and abundance in a concave manner, except for age in stands with few BLTs. The positive effects of BLT were more evident in younger stands with fewer BLTs. Mature plantations with 25% BLTs supported 62% of breeding BLT bird abundance in old natural forests. In winter, lower regional temperatures weakened the positive effects of stand age and strengthened the positive effects of BLT proportion, reflecting temperature-dependent habitat selection across species. Our results suggest that regular plantation harvesting can play a critical role in restoring ES bird species across Hokkaido. To conserve BLT bird species, retaining even small amounts of BLTs within plantations may be more cost-effective than long-rotation plantation forestry or only protecting existing natural old-growth forests. Our study shows that on-site conservation within plantations across regions and seasons, when coupled with nature reserve management, can contribute to restoring biodiversity.