ESRM 401, Spring 2010
Spring Comes to the Cascades
Instructors: Tom Hinckley & Julie Combs
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Phenological cycles: Definitions & Pictures Content
Phenology is the seasonal and diurnal changes in plant growth. It is often most dramatically displayed during periods of leaf emergence and then leaf senescence and drop (complete definition, rationale and description are found below). The figure shown immediately below is Tumwater Canyon in the fall and spring.
Go to the following site: http://www.metla.fi/metinfo/fenologia/index-en.htm for further clarification, etc. Processes associated with the regular or predictable parts of a phenological cycle Bud burst: First there is the loss of abscisic acid in the bud scales (the presence of ABA, a plant hormone or growth substance, results in true dormancy), then the mobilization of carbon compounds and their conversion to sugar, likely some mitotic activity, then bud swell, the opening of the bud scales and the emergence of the vegetative bud and the expanion of the foliage. Picture below illustrates initial leaf expansion in Populus fremontii in a riparian system which includes an evergreen conifer and another deciduous species (sycamore, Platanus spp.) (photo from Sedona, Arizona by L. Brubaker).
Male and female strobili or cone appearance, fertilization and female cone development (reproductive processes associated with gymnosperms).
Flower emergence: Now is a great time to see the appearance of flowers (from daffodils to cherry trees to the very prominent flowers on bigleaf maple (first picture below or see "video") or a Japanese maple (second picture below)(reproductive processes associated with angiosperms):
Leaf senescence: In the photograph below (Iron Peak Trail, fall 1999, ESC 221), one sees clearly two species in which senescence is occurring. The foliage of western larch (Larix occidentalis) is golden yellow -- the green color, due to chlorophyll, is gone because the chlorophyll molecule has been broken down and the nitrogen, magnesium and other mobile nutrients with chlorophyll have been at least partially reabsorbed into the twig (like the tissue around the phloem, but the ray parenchyma of the xylem may also be involved). Within a month, the foliage will abcise and fall to the ground. There are shrubs where one also sees the appearance of pigments other than chlorophyll, again indicative of the senscence process. Not readily seen; however, is the abcission of the older age classes of foliage from the subalpine and Pacific silver fir and mountain hemlock trees also seen.
Leaf Fall: Actual loss of leaves ( due to strong coupling with environmental cues or mechanical forces [wind, ice, snow] or drought). Processes associated with the irregular or unpredictable factors affecting phenology Fire: Please note the recovery of some plants via sprouting (e.g., snowbrush in the photograph below) and the presence of fireweed. See special section on fire in the Pacific Northwest (web page)
Herbivory: The removal of plant material (leaves, roots, branches, etc.) by herbivores. Morphology and Anatomy of Buds, Meristems and Epicormic Shoots
Photograph above illustrates dormant or resting buds of a conifer (likely Douglas-fir) where for each of the buds, the bud scales have been removed. Question: what function(s) do bud scales perform). The bud on the left is a male strobilus and each reddish sac is a pollen sac (will contain pollen grains when the bud bursts and expands). The bud in the middle is a vegetative bud and the greenish dots are needle primordia. The bud on the right is a female strobilus. Just based upon bud size, one can often examine a conifer branch and estimate the number and types of buds.
In the picture above, we see a series of epicormic shoots or branches on a Douglas-fir tree. Dr. Roaki Ishii has developed a wonderful story about how epicormic shoots are partially responsible for the extraordinary longevity of Douglas-fir (his Ph.D. research is found here). Also above is a picture of epicormic shoots on a noble fir branch (from westside of Mt. St. Helens, July 2001) Complete Description of Phenology, Its importance, etc. The following key elements about phenology are common to all our field trips.
Phenology: the study or description of growth events in time and space (the most obvious events associated with a phenological cycle are bud burst, cone development, flower emergence, leaf senescence, and leaf fall) . If there were a single focus point in phenology, it would be on the activity of plant meristems (perpetually embryonic tissue). Primary meristems are located in buds and these meristems may be vegetatative or reproductive. Buds are most visible at the tip of all shoots and roots; however, buds occur in very undeveloped (and, therefore, not highly visible) conditions in the axils of all leaf (or foliage) -- stem junctions. Some of these dormant or latent buds may open later and these then appear as epicormic shoots on trees. Primary meristems are also found at the tips of all roots. Secondary meristems are derived from primary meristems and are responsible for secondary thickening of branches, stems and roots. It is also important to remember that genetics play a role. In the picture below, we have two year old cuttings of black cottonwood (Populus trichocarpa) growing in a common garden in Belgium. The tree on the left is from the Willamette Valley whereas the one on the right is from the eastside of the Oregon Cascades.
There are a series of relatively predictable/regular cycles and these affect/trigger a number of growth/physiological responses in plants. A simple phenological cycle for the aboveground components of growth consists of reproductive/vegetative bud swell, bud burst, elongation of flowers (or cones), shoots and leaves, cessation of elongation, fertilization, seed development, senescence of plant parts, etc. One such cycle for a Douglas-fir seedling is illustrated below:
Steps seen include:
Questions that should arise when examining such a diagram include:
If you are interested in seeing phenological events as a series of "video strips," click on these sites:
When we go to Lake Tradition (as well as the two other sites), it is important to think about phenology and the role that macro-, micro-site, species (and associated life history as well as stage of development), and canopy position play in what we see. If one examines physiological events in a plant (i.e., its phenology), a number of predictable and rather regular cycles emerge (see below). These cycles have a range of effects and these effects may be determined by where one is in the other cycles. For example, the response to sunrise and sunset may be very different in the middle of winter vs. summer.
The above circular diagram should be used as one possible organizing principle for your first set of field trip journal notes. There are also irregular and unpredictable events such as sunflects or disturbances. Disturbances can range from wind throw to floor, avalanche or fire or even to a volcanic eruption. Again responses to these events will depend upon where other cycles may be. For example, if Mt. St. Helens had erupted in February, August or October (vs. May 18, 1980), how might this timing have affected the impact, response and recovery of plants? |
