PhD Defence Siqi Shi | Interaction between phenology and climate based on multi-source remote sensing observations

Interaction between phenology and climate based on multi-source remote sensing observations

Siqi Shi is a PhD student in the department Department of Water Resources. (Co)Promotors are dr.ir. C. van der Tol from the Faculty of Geo-Information Science and Earth Observation (ITC), University of Twente and prof. P. Yang from the Nanjing Normal University.

Plants are a dynamic component of the Earth system. Through their seasonal cycles of growth and dormancy, vegetation regulates the exchange of carbon, water, and energy between the land surface and the atmosphere. These seasonal cycles, known as vegetation phenology, determine when ecosystems absorb carbon dioxide, transpire water, and interact with climate. Understanding how phenology responds to climate variability and long-term climate change is therefore essential for predicting the future functioning of terrestrial ecosystems.

Over the past decades, increasing attention has been devoted to understanding the interactions between vegetation phenology and climate change. Numerous studies have reported earlier start of the growing season (SOS) in response to rising temperatures, particularly in temperate and boreal ecosystems of the Northern Hemisphere. However, observations in subtropical and tropical regions reveal unexpected delays in SOS despite the same warming background. This contrasting shifts of SOS challenges the conventional view that warming universally advances vegetation growth and suggests that the climatic controls of phenology may be more complex than previously assumed. From the perspective from satellite observations, most satellite-based phenological studies rely on traditional vegetation indices such as the normalized difference vegetation index (NDVI), which primarily capture seasonal variations in canopy greenness and structure rather than photosynthetic activity. In contrast, solar-induced chlorophyll fluorescence (SIF) provides information more directly related to vegetation photosynthesis. Recent studies have reported notable differences between VI-based structural phenology and SIF-based functional phenology, yet the mechanisms underlying these discrepancies remain poorly understood.

To address these questions, this dissertation investigates the interactions between land surface phenology (LSP) and climate using multi-source satellite observations.

This dissertation characterizes the spatial and temporal dynamics of structural LSP across Africa. Using long-term GIMMS NDVI datasets, the analysis reveals pronounced spatial heterogeneity in phenological events across the continent, including SOS, the peak of the growing season (POS), and the end of the growing season (EOS). Northern Africa exhibited a clear latitudinal gradients in these phenological events, whereas southern Africa showed a relatively complex spatial patterns. In terms of temporal changes, “delayed SOS”, “delayed POS”, and “delayed EOS” dominate the phenological trends in most African areas.

These findings raise the question of what drives the delayed SOS observed in Africa, which was contrast with earlier SOS widely recognized in the Northern Hemisphere. Further results reveal that both precipitation and temperature significantly affected the temporal shifts of SOS, while with opposite effects on SOS. Specifically, increased preseason precipitation () advanced SOS while warmer preseason temperature () delayed SOS. The delaying effects of SOS caused by warming overweighted the advancing effects by increased precipitation, resulting a delayed SOS in Africa.

Building on these findings, the dissertation further explores the mechanisms underlying divergent phenological responses to climate warming at the global scale. The results demonstrate that vegetation optimal temperature () plays a key role in modulating how vegetation responds to warming climate. When  below the , warming tends to promote vegetation growth, advancing SOS. Conversely, when  approach or exceed the , additional warming may instead delay SOS.

In addition to examining climate-phenology interactions, this dissertation also explores the drivers behind the discrepancies between structural and functional phenology using multiple satellite observations. We introduced the radiation-regulated vegetation indices (rrVI) to explore the role of radiation in driving the structural-functional mismatches. The results showed that the apparent discrepancies between structural and functional phenology were largely influenced by radiation, particularly in the mid- and high- latitudes.

Overall, this dissertation provides new insights into vegetation phenology from two perspectives. First, it advances our understanding of how climate factors regulate vegetation phenology across different ecosystems. In particular, it highlights the important role of vegetation optimal temperature () in regulating the responses of vegetation phenology to climate warming. Second, this dissertation improves the interpretation of satellite-based phenological observations by investigating the discrepancies between structural phenology and functional phenology. The results demonstrate that variations in radiation largely explain the structural-functional phenology differences.