Archive for month: October, 2023
Green, yellow, red, brown leaves in all transitional phases currently call for long forest walks, providing atmospheric nature experiences and creative inspirations. However, there’s nothing magical behind this: decomposition and substance transport are responsible for the transformation. The aging process is known as senescence, and the eventual leaf fall is termed abscission. Let’s take a closer look at both:
Senescence – the Last Chapter of Phenology in the Year
Genetically controlled and dependent on available energy, leaves age in autumn. They take in less and less CO2 and eventually cease photosynthesis altogether. When the leaves fall to the ground, this is measurable through remote sensing. The so-called “browning,” the brownish appearance of the ground beneath the bare tree, marks the end of the phenological year. But why do deciduous trees shed their leaves?
Leaf Shedding – Threefold Genius
Leaves evaporate large amounts of water, around 300 to 600 liters per square meter of leaf area per year in a beech tree, for example. Roots absorb the necessary water from the soil, which is not possible when it’s frozen. The tree would dry out. By shedding leaves, this flow of water is stopped, and the tree survives the winter unscathed. But there are two more important advantages that come with leaf fall: the environmental toxins accumulated and stored in the leaves are disposed of, and bare trees withstand snow loads better.
The Chemistry of Autumn Colors
The vibrant colors of the autumn forest are present in the leaves throughout the year but masked by green chlorophyll! When chloroplasts, the storage sites of chlorophyll, are transformed into gerontoplasts within cells, chlorophyll breaks down, revealing other pigments. Carotenoids appear yellow-orange, and anthocyanins bring out red tones – incidentally, as a stress response to excessive sunlight. The red pigments act as a shield against intense sunlight and ensure that chlorophyll breakdown occurs in dying leaves even on cold, sunny autumn days.
How Does Leaf Fall Work?
When exactly a leaf falls from a branch depends on various factors. There is a genetic component for each species, but also site characteristics such as altitude, temperature, day length, and wind play a role. Decreasing availability of light and warmth activates phytohormones, and at the end of the leaf stalk, an anatomical process of change occurs: a separation tissue forms where middle lamellae, cell walls, or entire cells dissolve. Eventually, the leaf’s own weight is enough for it to fall to the ground.
This article was featured as a story in the Flora-Incognita app in autumn 2023. In the app, you can find exciting information about plants, ecology, species knowledge, as well as tips and tricks for plant identification. Why not take a look!
Leaf surfaces play a crucial role in plant biology, facilitating the evaporation of water absorbed through roots. The shedding of leaves in autumn prevents them from drying out when frost freezes the water in the soil. Late autumn marks the final phenological season before the period of dormancy, signaled by the changing color of English oak leaves and the shedding of leaves in many other deciduous trees.
Leaf Coloring in English Oak
English oaks (Quercus robur) are found from the North German Lowlands to altitudes exceeding 1000 meters in the Alps. Their habitat extends far beyond Central Europe, reaching into the Caucasus region. As chlorophyll is broken down, revealing other plant compounds (carotenoids impart yellow hues, anthocyanins create red tones, and water-soluble pigments produce brown shades after leaf death), autumnal oak leaves glow in yellow and brown hues.
Rowan Leaf Fall
According to the German Weather Service, the rowan tree (Sorbus aucuparia) shedding its leaves is a key indicator of late autumn. Rowan leaves are alternately arranged on branches and are distinct with leaf stalks and leaf blades. Their vibrant red autumn coloring makes them a popular sight. The tree’s fruits, known as rowan berries, often remain on the tree in clusters throughout winter, providing essential food for songbirds during winter.
A Touch of Green
Late autumn is also marked by the appearance of tender green fields, signifying the emergence of winter crops. Winter wheat, sown in mid-September, germinates within 15-20 days, displaying the first green shoots. However, the elongation growth and leaf development occur in spring.
This article was featured as a story in the Flora-Incognita app in autumn 2023. The app provides fascinating information about plants, ecology, species identification, as well as tips and tricks for plant identification. Feel free to explore!
Importance of Phenology
A new paper, published by the Flora Incognita Research Group, shows that the plant observations collected via Flora Incognita can support phenological monitoring initiatives. Why is that important?
Observing plant phenology helps scientists to understand how climate change affects plants, for example. If flowering periods shift due to changing climatic conditions, this can have significant consequences for ecological relationships or the spread of species. Traditionally, the phases of phenology (e.g. bud break, leaf-out, onset of flowering, and leaf senescence) are recorded manually by trained volunteers – but their numbers are steadily declining.
Traditional Phenology Monitoring
In Germany, mainly the German Weather Service conducts the “official” plant phenology monitoring (Deutscher Wetterdienst, DWD). Here, every observer is given a specific “station,” corresponding to a dedicated spot for observing one tree, shrub or herbaceous plant. The number of these stations varies depending on the observed species. This means that some species have more stations than others. During the growing season, observers need to check on the plants they are studying at least two times each week and record the day when specific pheno phases begin.
Processing Flora Incognita data
The new paper’s lead authors Negin Katal and Michael Rzanny found a way to process Flora Incognita data in a way that they resemble DWD stations: They identified the locations of the DWD stations, and for each species, created a 5 km circle around them. Within this circle, they collected all the Flora Incognita observations for that species within a certain altitude range. At least 35 of those opportunistic records were needed to create a “Flora Incognita station”. If those could not be reached, even within an additional buffer (1 km at a time, up to 55 km), no Flora Incognita station was established at that location.
Interpolation of data
In the next step, the onset of flowering in 2020 and 2021 was calculated for each of these Flora Incognita stations, and the station data was interpolated for the whole of Germany. The resulting interpolation maps show similar results for most species as the manual documentation by the DWD.
The main finding of the publication is that the onset of flowering can be inferred from our plant observations – at least for annual herbaceous species or shrubs with a conspicuous flowering phase. This means that Flora Incognita data can supplement the traditionally collected phenological surveys on a broad scale, but also for many new species. This allows us to make a valuable contribution to the documentation of phenological shifts, which is of vital importance for the documentation and understanding of climate change, among other things.
We want to thank the many users of Flora Incognita who contribute to this new source of data. It is your curiosity that enables research like this.
The publication is now freely available:
Katal, N., Rzanny, M., Mäder, P., Römermann, C., Wittich, H. C., Boho, D., Musavi, T. & Wäldchen, J. (2023). Bridging the gap: How to adopt opportunistic plant observations for phenology monitoring
Frontiers in Plant Science, 14. doi: 10.3389/fpls.2023.1150956