Nowadays it's obvious that nature is the most uncontrollable thing in our universe. During all years of constant studying and knowledge acquisition we have managed just to predict some natural events, but not to control them. Among the great amount of sciences dealing with natural phenomena we'll talk about phenology, namely plant phenology.
So what is phenology? There is a lot of definitions according to which phenology is the study of the relationship between climate and the timing of periodic natural phenomena such as migration of birds, bud bursting, or flowering of plants.
Phenology is an old scientific discipline. Centuries ago people already recognized that the timing of life cycle events could provide information concerning the development of plants and animals. It was useful most of all for agricultural purposes. To get a better understanding of the timing of life cycle events, several monitoring networks were set up all over the world. In the last three decades of the 20th century, however, many phenological networks were reduced in size or stopped because of a decrease in agricultural importance of these networks.
However, the importance of phenology as a science remains an undeniable fact.
Why do we need phenology and what is its importance lie in? As was mentioned above phenology is the science of recording natural regularly occurring events and for today it has already provided some of the longest written biological records. The following gaining of such valuable information on seasonal occurrences will give the possibility to demonstrate how climate change is affecting the wildlife habitats.
Plant phenology investigates the vegetative cover changes taking place nowadays and compares it with the previous years. Such investigation enables understanding and predicting the possible changes in future. Tree phenology scrutinizes the timing of periodic biological phases, the causes of their timing concerning biotic and antibiotic forces, and the interrelation among phases of the same or different species.
Plant phenology subject of study is the "phases", or "phenophases", that may be the date of first flowering, budbreak, unfolding of first leaf, etc. The timing of phenophases is very important in biological systems and processes as it influences factors like the length of the growing season, frost damage, timing and duration of pests and diseases, water fluxes, nutrient budgets, carbon sequestration and food availability.
With the global warming the phenophases have changed greatly. Plant phenology offers real evidence that climate change is happening now and that it is already having a significant effect on our wildlife. Trees are coming into leaf sooner, and some typical spring flowers are increasingly being seen coming into bloom in November and December. The same things happen to animals, namely birds and butterflies that appear earlier.
Changes in climate and calendar dates are not reliable basis for establishing certain rules and management decisions. Heat measurement accumulated over time provides a physiological time scale that is biologically more accurate than calendar days. Speaking about the temperature influence on growth and development of life creatures and plants, phenologists use two parameters: the lower developmental threshold and the upper developmental threshold.
The lower developmental threshold for a species is the temperature below which development stops. The upper developmental threshold is the temperature at which the rate of growth or development begins to decrease. Both lower and upper thresholds are defined through thorough research and are unique for different unique organisms. The amount of heat needed by an organism to develop is known as physiological time.
Physiological time is often expressed in units called degree-days. For instance: if a species has a lower developmental threshold of 52° F, and the temperature remains at 52°F (maximum 1° higher) for 24 hours, this day is defined as one degree-day.
It's impossible to speak about the exact timing of the development of plants and animals. It can vary quite markedly from year to year.
The most obvious reason for such variability and irregularity is the weather. After a warm start of the year, the growing season can easily begin one month earlier than after a cold start of the year.