Bamboo phenology and life cycle drive seasonal and long-term functioning of Amazonian bamboo-dominated forests

1. Bamboo-dominated forests (BDF) extend over large areas in the drought-prone Southwestern Amazon, yet little is known about the dynamics of these ecosystems. Here, we investigate the hypothesis that bamboo modulates large-scale ecosystem dynamics through competition with coexisting trees for water. 2. We examined spatio-temporal patterns of remotely sensed metrics (Enhanced Vegetation Index [EVI], Normalized Difference Moisture Index [NDMI]) in >300 Landsat images as proxies for canopy leaf phenology and water content at two time scales: (1) a complete bamboo life cycle (~28 years), and (2) the seasonal cycle; and at two spatial scales: (a) comparing adjacent areas of BDF vs. Terra-firme forests (TFF) to investigate regional dynamics, and (b) comparing the vegetation classes of bamboo, trees in BDF, and trees in TFF to investigate the effects of bamboo on coexisting trees. 3. At the regional scale, BDF showed higher EVI (leaf area density) and lower NDMI (water content) than nearby TFF but these differences disappeared as bamboo died, suggesting a strong influence of bamboo life-stage in the functioning of these forests. BDF seasonal cycle showed a bimodal EVI pattern as trees and bamboos had asynchronized leaf production peaks. 4. At the scale of vegetation classes, trees in BDF showed lower NDMI (i.e., water content) than trees in TFF except after bamboo mortality, indicating a release from competition with bamboo for water. Canopy water content of trees in BDF was also reduced during bamboo dry-season greening (increased EVI ~ leaf production) due to increased water demands. Nevertheless, long-term and seasonal phenology of trees in BDF did not differ from that of trees in TFF suggesting a potential selection for drought-tolerant trees in BDF. 5. Synthesis. Bamboo-dominated forests have received less attention than other Amazonian forests and their functional dynamics are commonly ignored or misinterpreted. Using remote sensing to characterize forest phenology and water content, we show the distinctive seasonal and long-term dynamics of BDF and coexisting trees and the importance of bamboo competition for water in shaping this ecosystem. Our results suggest a potential selection for drought-tolerant trees in BDF since they maintain the same EVI as trees in bamboo-free forests but with lower water content. A better characterization of BDF and their cyclical dynamics is crucial for accurately interpreting Amazonian forests’ responses to extreme climatic events such as high temperatures and droughts.

1. 竹占优势森林（Bamboo-dominated Forests, BDF）在干旱易发的亚马逊西南部广泛分布，但学界对这类生态系统的动态特征仍知之甚少。本研究旨在验证如下假说：竹子通过与共存树木竞争水分，调控亚马逊大型生态系统的动态过程。
2. 我们基于超过300景陆地卫星（Landsat）影像的遥感指标（增强型植被指数（Enhanced Vegetation Index, EVI）、归一化差异水分指数（Normalized Difference Moisture Index, NDMI））的时空格局展开分析，以两个时间尺度下的冠层物候与水分含量作为替代指标：其一为完整的竹子生命周期（约28年），其二为季节周期；同时设置两个空间尺度的对比：(a) 对比相邻的BDF与未受淹陆地森林（Terra-firme Forests, TFF）区域，以探究区域尺度的生态系统动态；(b) 对比竹类、BDF内树木以及TFF内树木这三类植被类型，以解析竹子对共存树木的影响效应。
3. 在区域尺度上，BDF的EVI（叶面积密度）高于邻近的TFF，而NDMI（水分含量）则更低，但这类差异会随着竹子死亡而消失，这表明竹子的生活史阶段对这类森林的功能运转具有显著影响。BDF的季节周期呈现出双峰型EVI模式，这是由于树木与竹子的叶片生产峰值存在异步性。
4. 在植被类群尺度上，除竹子死亡阶段外，BDF内树木的NDMI（即水分含量）均低于TFF内树木，这表明BDF内树木摆脱了与竹子的水分竞争。在竹子旱季“变绿”（即EVI升高，对应叶片生产增加）的时期，BDF内树木的冠层水分含量也会因水分需求上升而降低。尽管如此，BDF内树木的长期与季节物候特征与TFF内树木并无显著差异，这暗示BDF环境可能对耐旱树木存在选择作用。
5. 综合讨论。相较于其他亚马逊森林类型，竹占优势森林受到的关注较少，其功能动态常被忽视甚至误读。本研究通过遥感手段表征森林物候与水分含量，揭示了BDF及其共存树木独特的季节与长期动态，以及竹子水分竞争在塑造该生态系统中的重要性。研究结果表明，BDF环境确实存在耐旱树木的选择倾向：这类树木与无竹森林内的树木拥有相同的EVI水平，但水分含量更低。精准表征BDF及其周期性动态，对于准确解读亚马逊森林对高温、干旱等极端气候事件的响应至关重要。