Environmental Studies Program, Institute of Ecology & Evolution, University of Oregon, Eugene, OR, USA.
Climate and Ecosystem Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA.
Glob Chang Biol. 2021 Jan;27(1):136-150. doi: 10.1111/gcb.15382. Epub 2020 Oct 31.
Tropical ecosystems are under increasing pressure from land-use change and deforestation. Changes in tropical forest cover are expected to affect carbon and water cycling with important implications for climatic stability at global scales. A major roadblock for predicting how tropical deforestation affects climate is the lack of baseline conditions (i.e., prior to human disturbance) of forest-savanna dynamics. To address this limitation, we developed a long-term analysis of forest and savanna distribution across the Amazon-Cerrado transition of central Brazil. We used soil organic carbon isotope ratios as a proxy for changes in woody vegetation cover over time in response to fluctuations in precipitation inferred from speleothem oxygen and strontium stable isotope records. Based on stable isotope signatures and radiocarbon activity of organic matter in soil profiles, we quantified the magnitude and direction of changes in forest and savanna ecosystem cover. Using changes in tree cover measured in 83 different locations for forests and savannas, we developed interpolation maps to assess the coherence of regional changes in vegetation. Our analysis reveals a broad pattern of woody vegetation expansion into savannas and densification within forests and savannas for at least the past ~1,600 years. The rates of vegetation change varied significantly among sampling locations possibly due to variation in local environmental factors that constrain primary productivity. The few instances in which tree cover declined (7.7% of all sampled profiles) were associated with savannas under dry conditions. Our results suggest a regional increase in moisture and expansion of woody vegetation prior to modern deforestation, which could help inform conservation and management efforts for climate change mitigation. We discuss the possible mechanisms driving forest expansion and densification of savannas directly (i.e., increasing precipitation) and indirectly (e.g., decreasing disturbance) and suggest future research directions that have the potential to improve climate and ecosystem models.
热带生态系统正承受着土地利用变化和森林砍伐的日益压力。预计热带森林覆盖的变化将影响碳和水的循环，对全球气候稳定性产生重要影响。预测热带森林砍伐如何影响气候的主要障碍是缺乏森林-稀树草原动态的基线条件（即在人为干扰之前）。为了解决这一限制，我们对巴西中部亚马逊-塞拉多过渡带的森林和稀树草原分布进行了长期分析。我们使用土壤有机碳同位素比值作为代理指标，反映随着降水波动，木本植被覆盖的变化，这些降水波动是根据洞穴石氧和锶稳定同位素记录推断出来的。基于稳定同位素特征和土壤剖面有机质的放射性碳活性，我们量化了森林和稀树草原生态系统覆盖的变化幅度和方向。我们使用在 83 个不同位置测量的森林和稀树草原的树木覆盖变化数据，开发了插值图来评估植被区域变化的一致性。我们的分析揭示了至少在过去 1600 年中，木本植被向稀树草原扩张和森林及稀树草原内密集化的广泛模式。在不同采样地点，植被变化的速度差异很大，这可能是由于限制初级生产力的局部环境因素的变化。树木覆盖下降的情况很少见（所有采样剖面的 7.7%），与干旱条件下的稀树草原有关。我们的结果表明，在现代森林砍伐之前，该地区的湿度增加和木本植被扩张，这可能有助于为缓解气候变化的保护和管理工作提供信息。我们讨论了直接（即降水增加）和间接（例如，干扰减少）驱动森林扩张和稀树草原密集化的可能机制，并提出了未来有潜力改进气候和生态系统模型的研究方向。
