Date Published: July 14, 2017
Publisher: Public Library of Science
Author(s): Fabien Hubert Wagner, Bruno Hérault, Vivien Rossi, Thomas Hilker, Eduardo Eiji Maeda, Alber Sanchez, Alexei I. Lyapustin, Lênio Soares Galvão, Yujie Wang, Luiz E. O. C. Aragão, Benjamin Poulter.
Our limited understanding of the climate controls on tropical forest seasonality is one of the biggest sources of uncertainty in modeling climate change impacts on terrestrial ecosystems. Combining leaf production, litterfall and climate observations from satellite and ground data in the Amazon forest, we show that seasonal variation in leaf production is largely triggered by climate signals, specifically, insolation increase (70.4% of the total area) and precipitation increase (29.6%). Increase of insolation drives leaf growth in the absence of water limitation. For these non-water-limited forests, the simultaneous leaf flush occurs in a sufficient proportion of the trees to be observed from space. While tropical cycles are generally defined in terms of dry or wet season, we show that for a large part of Amazonia the increase in insolation triggers the visible progress of leaf growth, just like during spring in temperate forests. The dependence of leaf growth initiation on climate seasonality may result in a higher sensitivity of these ecosystems to changes in climate than previously thought.
The Amazonian forests account for 14% of the global net primary production (NPP) and are a major component (66%) of the inter-annual variation in global NPP . While large seasonal swings in leaf area have been reported at least in parts of the Amazon basin [2–4], the environmental controls that trigger the synchronous development of new leaves are not well understood [5–7]. As a result, current earth system models inadequately represent the dynamics of leaf development, despite its major role for photosynthesis of tropical vegetation . In equatorial forests, leaf flushing correlates with increased light availability and photosynthetically active radiation during the dry season [4, 9], and is theoretically driven by a change in daily insolation . However, water availability constrains leaf phenology in southern Amazonia and most of the Congo basin, impeding the maintenance of the evergreen state during the dry season . The climate thresholds controlling the phenological responses of vegetation remain unclear , as well as the sensitivities of these responses to future climate changes .
EVI greening across the Amazon forest can be explained by seasonal increase in insolation and precipitation (Fig 1 and S8 Fig). Seasonal changes in EVI were mainly related to insolation (70.4% of the total area) in the north-western, central and eastern part of Amazonia. EVI greening is associated to precipitation seasonality (29.6% of the area) in continuous areas above 5°N (North part) and below 5°S (South-West, South and South East part). 75.0% of the pixels where the greening is associated to insolation follows the increase in solar radiation with a mean lag of one month (S7 Fig). No time lag was observed in regions where greening is associated to an increase in precipitation. The modeled EVI with precipitation and maximal temperature is consistent with MAIAC EVI (S8 Fig). Inconsistent regions (R2<0.5, 6.2% of the forested area) have either uncommon forest structures, such as bamboo dominated forests, or constant cloud cover inducing high noise in the EVI signal (S3 Appendix, S10 Fig) Despite the high diversity and different species composition of these forests, we show that EVI greening across the Amazon forest may be explained by climate seasonality (Fig 1). The predicted EVI from precipitation and insolation is highly consistent with the observed MAIAC EVI (S8 Fig), supporting the model’s biological assumptions: EVI increase is likely a proxy of leaf production triggered by climate drivers. Isolated patches where the results are not consistent (R2<0.5, 6.2% of the forested area) represent areas highly challenging for remote sensing; that is, uncommon forest structures, such as bamboo dominated forests, constant cloud cover resulting in high noise in EVI (S3 Appendix and S10 Fig); and may be micro-climate or local soil water retention characteristics. Comparing our modeled EVI to MAIAC EVI reveals limitations of the remotely sensed EVI that cannot be discovered with classic remote sensing analysis (such as classifications) which does not consider the biophysical nature of the signal. Additionally, it gives a biological interpretation of EVI derived from optical measurements of reflected electromagnetic radiation. Subtropical and tropical seasons are generally defined in terms of dry or wet season, but here we show that this definition is not correct in the Amazonian forest regions without water constraints. In 70.4% of the Amazon forest, the increase in insolation triggers the visible progress of leaf growth and this process occurs in a sufficient proportion of the trees to be observed from satellites, just like during spring in temperate forests . Direct and strong climate environmental signals trigger leaf growing season, which supports the hypothesis of a leaf production optimized for carbon gain under seasonal resource availability [56, 58, 60]. In absence of water limitation, the leaf production follows the solar insolation, while in water-limited regions, trees quickly produce leaves with the first rains to benefit from high insolation at the end of the dry season. This high dependency of seasonal leaf renewal on climate, previously largely underestimated , may indicate a high sensitivity of these ecosystems to climate change. Source: http://doi.org/10.1371/journal.pone.0180932