Utility of commercial high‐resolution satellite imagery for monitoring general flowering in Sarawak, Borneo

General flowering (GF), irregular synchronous mass flowering of multiple tree species across multiple families, is a unique biological phenomenon of the mixed lowland dipterocarp forest in Southeast Asia. Characterizing the spatial extent and temporal dynamics of GF is essential for an improved understanding of climate–vegetation interactions and the potential climate change impact on this species-rich rainforest. We investigated the utility of newly available high-temporal (daily) and high-spatial (3–4 m) resolution remote sensing by the PlanetScope commercial satellite constellation for detecting flowering trees in a dipterocarp rainforest at Lambir Hills National Park, Sarawak, Malaysia. Our study was focused on the latest GF event known to have occurred in the region in the year 2019. PlanetScope successfully acquired 13 clear-sky or minimally cloud-contaminated scenes over the park during a study period of January 1, 2019 to August 31, 2019 encompassing the 2019 GF event. In situ phenology observations verified that the PlanetScope images detected the flowering crowns of tree species that turned into white or orange. This multitemporal image dataset also captured the flowering peak and species differences. The correlation coefficients between the multitemporal image signatures and in situ phenology observations were moderate to very strong (0.52–0.85). The results indicated that the 2019 GF event was a whole-park phenomenon with the flowering peak in May. This study reports the first successful satellite-based observations of a GF event and suggests the possibility of regional-scale characterization of species-level phenology in the dipterocarp forest, key information for biodiversity conservation in Southeast Asia

Using canopy greenness index to identify leaf ecophysiological traits during the foliar senescence in an oak forest

© The Author(s), 2018. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Ecosphere 9 (2018): e02337, doi:10.1002/ecs2.2337.Camera‐based observation of forest canopies allows for low‐cost, continuous, high temporal‐spatial resolutions of plant phenology and seasonality of functional traits. In this study, we extracted canopy color index (green chromatic coordinate, Gcc) from the time‐series canopy images provided by a digital camera in a deciduous forest in Massachusetts, USA. We also measured leaf‐level photosynthetic activities and leaf area index (LAI) development in the field during the growing season, and corresponding leaf chlorophyll concentrations in the laboratory. We used the Bayesian change point (BCP) approach to analyze Gcc. Our results showed that (1) the date of starting decline of LAI (DOY 263), defined as the start of senescence, could be mathematically identified from the autumn Gcc pattern by analyzing change points of the Gcc curve, and Gcc is highly correlated with LAI after the first change point when LAI was decreasing (R2 = 0.88, LAI < 2.5 m2/m2); (2) the second change point of Gcc (DOY 289) started a more rapid decline of Gcc when chlorophyll concentration and photosynthesis rates were relatively low (13.4 ± 10.0% and 23.7 ± 13.4% of their maximum values, respectively) and continuously reducing; and (3) the third change point of Gcc (DOY 295) marked the end of growing season, defined by the termination of photosynthetic activities, two weeks earlier than the end of Gcc curve decline. Our results suggested that with the change point analysis, camera‐based phenology observation can effectively quantify the dynamic pattern of the start of senescence (with declining LAI) and the end of senescence (when photosynthetic activities terminated) in the deciduous forest.Priority Academic Program Development of Jiangsu Higher Education Institutions in Discipline of Environmental Science and Engineer in Nanjing Forest University; China Scholarship Council Grant Number: 201506190095; Brown University Seed Funds for International Research Projects on the Environmen

Monitoring mega-crown leaf turnover from space

Spatial and temporal patterns of tropical leaf renewal are poorly understood and poorly parameterized in modern Earth System Models due to lack of data. Remote sensing has great potential for sampling leaf phenology across tropical landscapes but until now has been impeded by lack of ground-truthing, cloudiness, poor spatial resolution, and the cryptic nature of incremental leaf turnover in many tropical plants. To our knowledge, satellite data have never been used to monitor individual crown leaf phenology in the tropics, an innovation that would be a major breakthrough for individual and species-level ecology and improve climate change predictions for the tropics. In this paper, we assessed whether satellite data can detect leaf turnover for individual trees using ground observations of a candidate tropical tree species, Moabi (Baillonella toxisperma), which has a mega-crown visible from space. We identified and delineated Moabi crowns at Lopé NP, Gabon from satellite imagery using ground coordinates and extracted high spatial and temporal resolution, optical, and synthetic-aperture radar (SAR) timeseries data for each tree. We normalized these data relative to the surrounding forest canopy and combined them with concurrent monthly crown observations of new, mature, and senescent leaves recorded from the ground. We analyzed the relationship between satellite and ground observations using generalized linear mixed models (GLMMs). Ground observations of leaf turnover were significantly correlated with optical indices derived from Sentinel-2 optical data (the normalized difference vegetation index and the green leaf index), but not with SAR data derived from Sentinel-1. We demonstrate, perhaps for the first time, how the leaf phenology of individual large-canopied tropical trees can directly influence the spectral signature of satellite pixels through time. Additionally, while the level of uncertainty in our model predictions is still very high, we believe this study shows that we are near the threshold for orbital monitoring of individual crowns within tropical forests, even in challenging locations, such as cloudy Gabon. Further technical advances in remote sensing instruments into the spatial and temporal scales relevant to organismal biological processes will unlock great potential to improve our understanding of the Earth system

Multi-Scale Phenology of Temperate Grasslands: Improving Monitoring and Management With Near-Surface Phenocams

Grasslands of the Australian Southern Tablelands represent a patchwork of native and exotic systems, occupying a continuum of C3-dominated to C4-dominated grasslands where composition depends on disturbance factors (e.g., grazing) and climate. Managing these complex landscapes is both challenging and critical for maintaining the security of Australia's pasture industries, and for protecting the biodiversity of native remnants. Differentiating C3 from C4 vegetation has been a prominent theme in remote sensing research due to distinct C3/C4 seasonal productivity patterns (phenology) and high uncertainty about how C3/C4 vegetation will respond to a changing climate. Phenology is used in northern hemisphere ecosystems for a range of purposes but has not been widely adopted in Australia, where dynamic climate often results in non-repetitive seasonal vegetation patterns. We employed time-lapse cameras (phenocams) to study the phenology of twelve grassland areas dominated by cool season (C3) and warm season (C4), native or exotic grasses near Canberra, Australia. Our aims were to assess phenological characteristics of the functional types and to determine the drivers of phenological variability. We compared the fine-scale phenocam seasonal profiles with field sampling and MODIS/Landsat satellite products to assess paddock-to-landscape functioning. We found C3/C4 species dominance to be the primary driver of phenological differences among grassland types, with C3 grasslands demonstrating peak greenness in spring, and senescing rapidly in response to high summer temperatures. In contrast, C4 grasslands showed peak activity in Austral summer and autumn (January-March). Some sites displayed primary and secondary peaks dependent on rainfall and species composition. We found that the proportion of dead vegetation is an important biophysical driver of grassland phenology, as were grazing pressures and species-dependent responses to rainfall and temperature. The satellite and field datasets were in general agreement with the phenocam results. However, the higher temporal fidelity of the cameras captured changes in vegetation not observed in the coarser satellite or field results. Our phenocam data shows consistent periods of increasing and decreasing greenness over as little as 5 days. Applications for management of grasslands in temperate Australia include the identification of remnant native grasslands, tracking biosecurity issues, and assessing productivity responses to climate variability

Mangrove Phenology and Water Influences Measured with Digital Repeat Photography

The intertidal habitat of mangroves is very complex due to the dynamic roles of land and sea drivers. Knowledge of mangrove phenology can help in understanding mangrove growth cycles and their responses to climate and environmental changes. Studies of phenology based on digital repeat photography, or phenocams, have been successful in many terrestrial forests and other ecosystems, however few phenocam studies in mangrove forests showing the influence and interactions of water color and tidal water levels have been performed in sub-tropical and equatorial environments. In this study, we investigated the diurnal and seasonal patterns of an equatorial mangrove forest area at an Andaman Sea site in Phuket province, Southern Thailand, using two phenocams placed at different elevations and with different view orientations, which continuously monitored vegetation and water dynamics from July 2015 to August 2016. The aims of this study were to investigate fine-resolution, in situ mangrove forest phenology and assess the influence and interactions of water color and tidal water levels on the mangrove–water canopy signal. Diurnal and seasonal patterns of red, green, and blue chromatic coordinate (RCC, GCC, and BCC) indices were analyzed over various mangrove forest and water regions of interest (ROI). GCC signals from the water background were found to positively track diurnal water levels, while RCC signals were negatively related with tidal water levels, hence lower water levels yielded higher RCC values, reflecting brownish water colors and increased soil and mud exposure. At seasonal scales, the GCC profiles of the mangrove forest peaked in the dry season and were negatively related with the water level, however the inclusion of the water background signal dampened this relationship. We also detected a strong lunar tidal water periodicity in seasonal GCC values that was not only present in the water background, but was also detected in the mangrove–water canopy and mangrove forest phenology profiles. This suggests significant interactions between mangrove forests and their water backgrounds (color and depth), which may need to be accounted for in upscaling and coupling with satellite-based mangrove monitoring

Reviews and syntheses: Australian vegetation phenology: New insights from satellite remote sensing and digital repeat photography

Phenology is the study of periodic biological occurrences and can provide important insights into the influence of climatic variability and change on ecosystems. Understanding Australia's vegetation phenology is a challenge due to its diverse range of ecosystems, from savannas and tropical rainforests to temperate eucalypt woodlands, semiarid scrublands, and alpine grasslands. These ecosystems exhibit marked differences in seasonal patterns of canopy development and plant life-cycle events, much of which deviates from the predictable seasonal phenological pulse of temperate deciduous and boreal biomes. Many Australian ecosystems are subject to irregular events (i.e. drought, flooding, cyclones, and fire) that can alter ecosystem composition, structure, and functioning just as much as seasonal change. We show how satellite remote sensing and ground-based digital repeat photography (i.e. phenocams) can be used to improve understanding of phenology in Australian ecosystems. First, we examine temporal variation in phenology on the continental scale using the enhanced vegetation index (EVI), calculated from MODerate resolution Imaging Spectroradiometer (MODIS) data. Spatial gradients are revealed, ranging from regions with pronounced seasonality in canopy development (i.e. tropical savannas) to regions where seasonal variation is minimal (i.e. tropical rainforests) or high but irregular (i.e. arid ecosystems). Next, we use time series colour information extracted from phenocam imagery to illustrate a range of phenological signals in four contrasting Australian ecosystems. These include greening and senescing events in tropical savannas and temperate eucalypt understorey, as well as strong seasonal dynamics of individual trees in a seemingly static evergreen rainforest. We also demonstrate how phenology links with ecosystem gross primary productivity (from eddy covariance) and discuss why these processes are linked in some ecosystems but not others. We conclude that phenocams have the potential to greatly improve the current understanding of Australian ecosystems. To facilitate the sharing of this information, we have formed the Australian Phenocam Network (http://phenocam.org.au/)

Ecologie comparative des écosystèmes tropicaux (en Afrique)

After a short presentation of my career trajectory in academia, and of the PhD students and post-doctoral fellows I had the chance to supervise, I present a synthesis of the research work I conducted over the last decade on the comparative ecology of tropical ecosystems in Africa. This work is anchored into applied forest sciences and the data that were accumulated to answer practical questions also helped answer more fundamental questions in ecology. In my work, trees are used as the starting point in the understanding of tropical ecosystems, mostly moist forests but also drier formations, such as woodlands and savannas. With a background in community and functional ecology and a position in Gembloux Agro-Bio Tech, University of Liège, targeting tropical tree allometry and forest carbon, I derived two types of comparative approaches in my research activities, I compare either sites (trees or stands) or lineages (species or genera, mostly). For the site comparison, I used either the angle of tree architecture and stand structure or that of diversity and composition, at different spatial and temporal scales, from tree allometry and biomass estimates, up to the landscape scale for the structural approach, and from diversity recovery after logging, the delineation of forest types for management and up to biogeography studies, including cross-taxonomic and cross-continent comparisons for the diversity approach. For the lineage comparison, the concept of functional traits has been central and transversal since it allowed relating the structure and diversity approaches. It was however first adapted to tropical trees for which leaves are difficult to access, and size can vary tremendously over the tree life span and among tree life histories. Allometric or size-controlled traits were notably derived from tree measurements in the field and computed at a certain diameter to compare species of contrasted morphologies. Wood anatomical traits were also investigated and notably related to tree hydraulics. In this line, I finally propose a research project on tree and forest seasonal functioning, and response to drought. Tropical forests of central Africa are indeed found under drier and more seasonal climates than their south-eastern Asian and south American counterparts, and their resilience to climate (change) is a timely topic. These research perspectives will complement ongoing work on (i) the biogeography of Africa using species occurrence derived from herbarium records instead of checklists, (ii) carbon and biodiversity changes over the last decade by re-census a set of existing plots in the Congo basin, and on (iii) the seasonality in tree and forest functioning, using tree dendrometers and phenological cameras (PhenoCams) to monitor in depth how trees cope with the dry season. This project entitled CANOPI has been accepted for founding and offer the opportunity to collect unique ground-based measurements of tree and forest functioning in central Africa.15. Life on lan

Timing is everything: Within-plant flowering phenology impacts fruit production in the Arctic-Alpine cushion plant Silene acaulis (L.) Jacq.

Timing is everything for Arctic flowering plants. Early flowers might be destroyed by frost, while late flowers have less time and resources to mature fruit. With climate change, Arctic flowering phenology is shifting. Yet for many species, phenology studies only encompass the onset of flowering and lack baseline data on within-plant flowering times. I used the gynodioecious cushion plant Silene acaulis (L.) Jacq. to investigate how within-plant flowering phenology impacts fruit production in one growing season. In 2019, time-lapse cameras were used to daily observe flowers within two populations in the Low-Arctic (Narsarsuaq, Greenland in the Low-Arctic/Sub-Arctic transition zone; 7851 flowers, 21 plants) and the High-Arctic (Bjørndalen, Svalbard; 1587 flowers, 11 plants). Plants flowered for approximately three weeks, with a positively skewed peak floral display. In the LowArctic site, most investigated individuals were females dependent on pollinator visits for fruit production. Within these Low-Arctic females, flowers blooming during peak floral display had a higher probability of fruit set than flowers blooming outside peak floral display. In addition, flowers blooming before peak flowering were more likely to produce fruit than flowers blooming after peak flowering, both at the individual level and between individuals within the whole population. Hermaphrodites, however, can self-pollinate, and preliminary results indicate higher fruit set outside peak flowering within individuals and populations. In contrast to the Low-Arctic site, all plants in the High-Arctic site were females and a frost event occurred during flowering. Despite the frost event, females in the High-Arctic site had twice as high fruits per flower proportions as females in the Low-Arctic site. For flowers not exposed to frost, similarly to the Low-Arctic site, flowers blooming during peak floral display were more likely to produce fruit than flowers blooming outside peak floral display. Also similar to the Low-Arctic site, early flowers, both within individuals and the population, had a higher probability of fruit set than late flowers. For frost exposed flowers, however, the degree of frost damage was likely more important for fruit set than flower timing, indicating that late flowers can be part of a bet-hedging strategy. Altogether, these results portray how a long flowering period, combined with a peak floral display and early flowering, can be a strategy to ensure fruit production in the unpredictable Arctic growing season

Tropical phenology in a time of change

Phenology is increasingly recognised as an important indicator to measure the impacts of global environmental change. Changes to the phenology of tropical ecosystems are likely to have wide-reaching impacts on species, human society and even feedback onto climate. However, tropical phenology data are often unavailable and analyses have been constrained by dependence on geographically limited, noncircular indicators and lack of power in statistical analyses. This thesis addresses these challenges by making available and analysing for the first time a 32-year long record of monthly focal-crown observations (>1000 individuals of >80 species) from western equatorial Africa (Lopé National Park, Gabon). In Chapter 2, I developed a novel application of Fourier analysis to objectively and quantitatively describe flowering phenology at Lopé (856 trees of 70 species). I tested the power of this approach under different scenarios of data noise (regularity of the cycle and detectability of phenological events) and data length using both simulations and field data. Most individual trees monitored at Lopé flower at regular intervals (59%) and most species have dominant annual flowering modes (88%). I showed that at least six years of data are necessary to confidently detect flowering cycles using this method. In Chapter 3, I considered how both existing, and emerging, tropical phenology monitoring programs could be made most effective for change analyses by investigating major sources of noise in data collection. Using Fourier analyses of focal crown observations from Lopé (827 trees of 61 species) I showed that regular annual cycles are more common among reproductive than vegetative phenophases. Using expert knowledge and Generalized Linear Mixed Modelling I showed that experienced field observers can provide important information on major sources of noise in data collection and that observation length, phenophase visibility and phenophase duration are all important positive predictors of cycle detectability. In Chapter 4, I assessed how local weather has changed in western equatorial Africa using Wavelet analysis and Generalised Linear Mixed Models of the long-term weather record from Lopé (34 years of rainfall and temperature observations). Lopé is characterised by a cool, cloudy, long dry season that contrasts with two bright rainy seasons. Lopé has warmed at a rate of 0.23°C per decade (minimum daily temperature) and dried at a rate of -52mm per decade (total annual precipitation) since 1984. Interannual variation in rainfall and temperature is significantly influenced by global weather patterns such as the El Niño Southern Oscillation and the Atlantic Cold Tongue. Given this context of change, in Chapter 5 I selected focal-crown observations from a representative subset of canopy tree species at Lopé (108 trees of 8 species representative of 63% of total canopy volume) to assess seasonal and interannual variation in leaf phenology. I found that the tree community is evergreen with dominant species exchanging leaves incrementally and that new leaf development is suppressed during the long dry season. Using Generalised Linear Mixed Models I demonstrated that moisture, light and leaf herbivory are all important positive predictors of new leaf production at seasonal scales. The community-wide probability of leaf flush at Lopé has declined since 1986 and is most strongly predicted by the rise in atmospheric CO2. Finally, in Chapter 6 I applied the knowledge accumulated in the previous chapters to assess the impacts of fluctuating resource availability on commercialisation of Moabi Oil, a traditional non-timber forest product in west central Africa. I combined over 15 years’ scientific monitoring of the phenology of Baillonella toxisperma at Lopé National Park with interviews of indigenous knowledge of Moabi oil producers in rural Gabon, to describe the factors that influence Moabi harvest success and explore its impacts on the rest of the Moabi oil value chain. Because of the temporal and regional variability of wild Moabi fruit availability I recommended a multi-species approach to NTFP commercialisation in the Gabonese NP buffer zones. In summary, I have shown that regularly cycling phenology is common in tropical tree communities although a wide range of strategies is evident. The evidence from Lopé supports the idea that western equatorial Africa experiences a strongly seasonal environment with a uniquely light deficient long dry season and that this seasonality in environmental conditions directly impacts the phenology of the plant community. The potential stresses on the plant community associated with the long-term warming and drying trends at Lopé appear to be compensated by CO2 fertilisation and the characteristic light deficiency of the region which improve water use efficiency. This thesis answers numerous calls for more quantitative assessment of tropical phenology data by making available evidence from a previously unpublished longterm dataset. This thesis also serves to link the cycles of tropical forest productivity and reproduction to global socio-ecological issues such as forest regeneration, climate mediation and resource availability for threatened animal species and human forest users