海拔、坡向和气候因素影响了不丹喜马拉雅地区蓝松（Pinus wallichiana）的生物量及碳动态

《Global Ecology and Conservation》：Elevation, aspect, and climate shape blue pine ( Pinus wallichiana) biomass and carbon dynamics in the Bhutanese Himalayas

　　森林碳汇能力受地形制约的机制研究——以不丹蓝松为例

　　
Bhutan's Blue Pine Forests: A Microcosm of Topographic and Climatic Interactions in Himalayan Ecosystems

The Kingdom of Bhutan, renowned for its carbon-negative status through extensive forest coverage, has become a focal point for understanding how mountainous ecosystems respond to global climate change. This comprehensive study investigates the growth dynamics and carbon sequestration patterns of blue pine (*Pinus wallichiana*), a keystone species in Bhutan's forests, by analyzing the interplay between elevation gradients (2200–3000 m) and aspect (north vs. south-facing slopes). The research provides critical insights into how topographically controlled microclimates influence forest productivity, with implications for both regional conservation strategies and global climate mitigation efforts.

**1. Study Context and Significance**
Bhutan's unique topographic configuration creates a natural laboratory for studying elevation-microclimate interactions. The country's forests, which constitute over two-thirds of its land area, play a pivotal role in maintaining carbon neutrality. However, rising temperatures and altered precipitation patterns threaten this equilibrium, necessitating a detailed understanding of species-specific responses to environmental changes.

Blue pine dominates Bhutan's inner dry valleys,展现ing remarkable adaptability across elevations (1800–3400 m). Its dual significance as a culturally vital resource and a major carbon sink makes it an ideal model species. The study addresses critical gaps in knowledge regarding:
- How topographic factors modify climate sensitivity
- Seasonal drivers of biomass accumulation
- Potential shifts in growth patterns under future climate scenarios

**2. Methodological Approach**
The research employs a multi-faceted methodology combining dendrochronology, allometric modeling, and statistical analysis:
- **Tree Ring Analysis**: Over 50 years of radial growth data were cross-dated and standardized using robust statistical methods. This enabled the identification of climate-sensitive periods while accounting for age-related growth variations.
- **Allometric Scaling**: A validated biomass equation (TAGB = 0.0621 × DBH2?.??) was derived from destructive sampling of 52 trees, providing a scalable relationship between tree size and aboveground biomass.
- **Climate Response Modeling**: Generalized additive models (GAM) were developed to disentangle the effects of elevation, aspect, and tree age on growth rates, using a 30-year climate dataset (1985–2016).

**3. Key Findings**
*3.1 Elevation-Growth Relationship*
Biomass accumulation followed a bimodal pattern with elevation, reaching peak rates at 2350 m. Lower elevations (2200–2400 m) showed significantly higher productivity, averaging 14.8 kg TAGB per tree annually, while middle elevations (2600 m) exhibited reduced growth rates (6.4 kg/year). This contradicts typical unimodal elevation-biomass relationships, suggesting complex interactions between temperature, moisture, and soil conditions at mid-elevations.

*3.2 Aspect-Driven Growth Patterns*
South-facing slopes consistently supported faster growth (17.8 kg TAGB/year vs. 11.9 kg/year on north-facing slopes). This counterintuitive result challenges established assumptions about solar radiation and moisture balance:
- South aspects receive 20–30% more solar irradiance, potentially enhancing photosynthetic efficiency
- North aspects maintain higher soil moisture due to reduced solar exposure, facilitating early growth stages
- Drier south aspects paradoxically support denser stands (380 trees/ha vs. 219 trees/ha on north slopes)

*3.3 Climate Sensitivity**
- **April Precipitation**: acting as the primary growth driver, particularly for north-facing trees at lower elevations. A 10–17% growth boost occurred when April rainfall exceeded 75–185 mm, depending on slope aspect.
- **September Minimum Temperatures**: critical for north-facing stands at mid-elevations (2400–2800 m), where each 1°C temperature increase correlated with enhanced growth. However, south-facing trees showed no significant temperature response, emphasizing species-specific microclimate adaptation.

*3.4 Age-Growth Dynamics*
Growth trajectories followed a distinct pattern:
- **Rapid Initial Growth**: First decade accounted for 40–60% of total biomass accumulation
- **Stabilized Growth**: Plateaued between ages 10–40, averaging 12–15 kg TAGB/year
- **Senescence Decline**: After age 50, growth rates fluctuated and decreased by 20–30%, potentially due to resource reallocation to reproduction and defense mechanisms.

**4. Climatic Change Implications**
Projections indicate:
- **Elevation Shifts**: Current 2350 m peak growth zone may migrate 260–900 m upward by 2100, threatening lowland forests
- **Productivity Changes**: Even with 1.3–4.5°C warming, April precipitation increases (1.7%) may not offset reduced snowpack and soil moisture at higher elevations
- **Carbon Budget Risks**: Current aboveground carbon sequestration (0.207 Mt C/year) could decline by 30–50% if growth patterns shift

**5. Management Recommendations**
The study outlines actionable strategies for Bhutan's forest governance:
- **Site-Specific Silviculture**: Prioritize south-facing slopes for afforestation (60% growth advantage) and north-facing for conservation
- **Climate Refugia Identification**: Protect lower-elevation stands (2200–2400 m) as climate buffers
- **Community Forest Adaptation**: Develop alternative resource strategies for communities dependent on shifting blue pine stands
- **Belowground Carbon Accounting**: Current estimates may underestimate total carbon storage by 25–30% if root biomass is included

**6. Broader Scientific Contributions**
This research advances three key areas:
- **Montane Ecosystem Dynamics**: Demonstrates how topographic complexity can override simple elevation gradients in controlling species responses
- **Fire Regime Insights**: South-facing slopes with higher growth rates also showed increased fire frequency, highlighting trade-offs between productivity and disturbance
- **Methodological Frameworks**: Developed a novel integration of dendrochronology and allometry for species-specific carbon accounting

**7. Limitations and Future Directions**
While groundbreaking, the study faces several constraints:
- Limited sample size for high-elevation sites (N3000 had only 10 trees)
- No belowground biomass measurements ( estimated 25% of total carbon)
- Single site analysis (Chapcha) may not generalize to Bhutan's full range
- Climate models used lack regional specificity

Future research should:
1. Extend chronology length to >100 years for better climate signal detection
2. Implement isotopic tracers for belowground carbon measurement
3. Develop high-resolution spatial models integrating LiDAR and soil moisture data
4. Monitor post-2020 climate shifts for validation of projections

**Conclusion**
This study reveals that blue pine growth in Bhutan is governed by a complex interplay of topographic microclimates rather than simple elevation gradients. The discovery of south-facing advantage despite drier conditions underscores the critical role of light availability and soil moisture dynamics. These findings directly inform Bhutan's climate adaptation strategies, suggesting prioritized conservation of lowland south aspects while preparing for upward range shifts. The research also provides a template for integrating dendrochronological data with allometric modeling in mountainous regions, offering methodological advances for global forest carbon accounting. As the Himalayas experience unprecedented climate change, understanding such species-specific responses becomes crucial for maintaining regional biodiversity and carbon sinks.