Lecture 12 - Plant Phenology Flashcards
Definition of Phenology
The study of the periodicity of biological phenomena in plants or animals triggered by biotic and abiotic phenomena in plants or animals triggered by biotic and abiotic elements of their envionment.
While phenology is as old as agrivulture, the study of it is fairly new.
Plant phenological events as envionmental metrics
Plants are finely tunned to the seasonality of their environment, and shifts in the timming of plant activity (i.e. phenology) provide some of the most compelling evidence that species and ecosystems are being influenced by global environmental change.
Consider the following:
* Higher temperature induce earlier leafing out and delay senescence, thus stretching the growing season and resulting in more carbon uptake. More carbon fixation in a single season can lead to earlier flowering teh following season-at least from some species.
Plant phenological events are therfore strong bio-indicator for global change. They can also tell us of the rates of change.
Plant phenology, as it turns out, has all the attributes of a good metric. Environmental philosophers have argued that to advance public and political discourse on the environment we need metrics with the following attributes:
- capture an important dimension of the system
- change over time
- respond to the effectiveness of interventions
- link to widely help human social value
Pooled long term phenological data across taxa, space and time can provide powerful objective metrics of global change.
Biotic and abiotic components of the environment that affect the timing of phenological events include the seasonal distribution of:
- Environmental conditions: temp. , photoperiod, irradiance
- Resources: water, nutrients, pollinators
- Hazards: frost, herbivory
The seasonnal distribution of these element**s generates selection pressures on the timing of the different phases which compose the vegetative and reproductive cycles of plants. **
Earlier events will affect the timing of subsequent phenological events.
Environmental conditions: photoperiod and temperature
Temperature and photoperiod are the best studied components of the environment affecting plant phenology:
* For herbaceous species - the main control is photoperiod.
Photoperiod provides more accurate information about time of year. It is therfore far more reliable than temperatures.
- For woody plants - th emain control is temperature
Herbaceous plants need to leafing out before the canopy closes
Herbaceous plants have a short window of full light before the canopy leaves out.
Herbaceous plants **rely on the photochrome system to know the time of year. **
When daylength increases, more of the phytochrome is in Pfr for. This induces leafing out response.
Woody Plants Respond to Temperature or Thermal Time
All metabolic processes increase with temperature. At freezing cold temp, metabolism of most cells stops.
For trees, thermal time is a better predictore of budbreak than calendar time.
The metric growing degree day (GDD) measures thermal time.
Calculating GDD
Calculating GDD is a way of assigning a heat value to each day. It is calculated at the difference between the daily mean temperature and some base temperature. Over the course of the year, daily degree days are summed. Phenological events are triggered when a critical sum is exceeded.
From Reamur to Hopkin’s Law
In 1938 Andrew Hopkins laid down some generalizations that have since been referred to as Hopkins Law, or the bioclimatic law.
He argued that in the central part of North America, very roughly, the date of some plant event (flowering, petal abscission, leaf fall, ect…)will occur:
* 4 days later with each increasing degree of northward latitude.
* For alpine species, it will occur 4 days later for each 120 meters of elevation (altitude).
* And finally, given that the land is rising slowly from the east coast to the Rockies, an event will occur 4 days later for each 5 degrees of westward longitude.
The Green Wave of North America: First leaf date (bud break) travels from Tennessee-Kentucky border to north shore of Lake Superior at a velocity of 21 km/day.
The lines on a map depicting an isophene indicate the occurence of a phenological.
Rephrasing Hopkin’s law: an isophene in North America will travel northward about one degree every 4 days. Given 111km per degree, this is 28km per day for the isophene velocity.
Annual records for the flowering of cherry trees in Japan date back to the 9th century.
Isophene in Japan travels roughly 25km/day for the northward component.
Other Examples:
1. In Britain, mean flowering dates of 385 plant species advanced by 4.5 days during the 1990s compared with the 1954-1990 mean.
2. In the northeastern United States, changes in lilac, apple, and grape phenology suggest that spring has advanced by ~2.8 days over the last three or four decades, consistent with pattern across North America as a whole.
