Evaluating Habitat Data in Plant Culture
Introduction
More and more habitat data is being made available to slipper growers. It generally falls into one of two categories, data collected from the actual growing location or locations of a particular plant, or climate data taken from the records of the nearest available weather station and corrected for altitude. There is no doubt that both kinds of data are tremendously useful to the Slipper Orchid grower, and our gratitude is extended to those folks who go to the time and trouble and expense to gather and analyze this information, but some cautions in the interpretation and application of this information may be in order.
Obligately Adapted or Tolerant of Conditions
The first question to be considered is whether a plant is obligately adapted to the extremes it is exposed to and needs this exposure to remain healthy and bloom, or simply can tolerate these extremes but might actually perform better if not exposed to them. It would be an impossible goal to try to duplicate the complexities of the natural environment of the plants, the goal instead should be to provide them with the best environment we can within the constraints of our growing area, be it windowsill, under lights, or greenhouse, that keeps them growing strongly, pest free, and hopefully flowering well. To do this we need to take cues from the habitat and climate information available, reasonably interpreted, but then also experiment within our growing areas (as well as learn from the experiments and experiences of other successful growers) to learn what the plants really do need to grow and flower versus what they may be tolerant of in their habitat. We also need to be aware that there are some limitations on the accuracy of the habitat data we read.
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| |
| Ranges of Paphs. concolor & niveum | |
Range size, including elevation
If you look at the map shown, you will see the approximate range of Paph. niveum in red overlay, and the approximate range of Paph concolor in purple overlay. I think it is easy to see that climate from a representative weather station for Paph niveum is much more likely to be within reasonable ranges for most of its relatively small range, but any projections for Paph concolor are going to be difficult to project over such a wide area and expect reasonable accuracy. To add further variation in projecting weather conditions for Paph. concolor is the fact that it has a vertical range of 90 t o1000 meters, while Paph. niveum is restricted to a much narrower range of sea level to 200 meters.
Errors in extrapolating data from one locale and elevation to another
While data collected at the growing site needs careful interpretation due to range variation, climate data extrapolated from one site to another adds another layer of uncertainty. Correction for altitude differences between the recording station and a plants habitat altitude (which can often vary by 1000 meters or more) is made by applying what is termed the adiabatic lapse rate. The "dry" air adiabatic lapse rate (relative humidity <100%) is approximately 1° C per 100 meters change in elevation, and the wet adiabatic lapse rate (relative humidity = 100%) is approximately 0.6° C per 100 meters (note that this is an average value, the wet rate is not linear). Under ideal conditions these formulas can be used to predict temperature changes with altitude changes, but a tremendous number of factors such as wind currents, topography and fog, mists or cloud cover can greatly affect what happens. The chart below shows projected data versus actual measurements of temperature one day over Salt Lake City, Utah. On this day a layer of fog had rolled into the Salt Lake Valley, and it illustrates the variables such common weather occurrences can introduce into extrapolating data from one site to another site, and at another elevation.
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Elevation meters |
Actual Temperature ° C* |
Dry Adiabatic Prediction ° C* |
Error in Dry Adiabatic Prediction ° C* |
Wet Adiabatic Prediction ° C* |
Error in Wet Adiabatic Prediction ° C* |
|
1300 ground level |
+1 |
||||
|
1500 |
-2 |
- 1 |
+ 1 |
0 |
+ 2 |
|
1700 |
+2 |
- 3 |
- 5 |
- 2 |
- 4 |
|
1900 |
+3 |
- 5 |
- 8 |
- 3 |
- 6 |
|
2100 |
+4 |
- 7 |
-11 |
- 4 |
- 8 |
|
2300 |
+3 |
- 9 |
-12 |
- 5 |
- 8 |
|
2500 |
+2 |
-11 |
-13 |
- 6 |
- 8 |
|
2700 |
+1 |
-13 |
-14 |
- 7 |
- 8 |
|
2900 |
0 |
-15 |
-15 |
- 9 |
- 9 |
* all values rounded to nearest degree
Under these conditions, correcting data for an altitude difference of 1000 meters would have resulted in a temperature prediction lower than recorded by 8° C (14.4° F) to 12° C (21.6° C). Many other factors such as wind and geographic features such as mountains or bodies of water can greatly influence local precipitation and temperatures, adding further uncertainty when extrapolating data from one location to another.
Microclimate variations
Paph. armeniacum range
Even in a structure as small as a greenhouse, you have a variety of microclimates with variations of temperature and humidity and light levels and air movement. The possibilities for large variation in microclimates in the natural habitat of our Slipper Orchids are tremendous. Geologic formations, presence of bodies of water, regular overnight mists and fogs, large areas of thermal retentive rock that the plants may be in close contact with, and vegetative cover will affect weather conditions in local situations. It is possible that plant colonies can exist only in these small pockets within the larger range, so again care must be exercised in utilizing climate data for the range as a whole.
Discussion
Temperatures
As we stated at the beginning of this article, habitat and climate data is extremely useful if interpreted carefully, looking for general trends and correlations, and realizing that for the various reasons discussed above the specific values are not necessarily the conditions that the plants are exposed to, and even if they were the plants may simply tolerate these conditions, but may actually grow and flower better under different conditions. It is necessary to weigh experimentation and experience into the equation when utilizing this data. We can take the available information for Paph. armeniacum as a good example to illustrate our points. Various reports from visitors to the habitat of Paph armeniacum indicate that it grows in close association with karst limestone, in loose detritus in cracks and crevasses on the north faces of steep limestone cliffs, with a measured substrate pH in the mid 7's. Indeed, as
these conditions would suggest, the plants in culture seem to prefer a slighltly alkaline pH which we achieve with the addition of calcareous material to the mix. The mix also needs to be open and quick draining, as indicated by their growth in loose detritus on steep slopes. Interesting climate data is suggested by the material provided by the Bakers in "The Genus Paphiopedilum" volume 1, as summarized in the following graphs.
Rainfall
The range of Paph. armeniacum is quite restricted, its elevation range is from 1200 to 2000 meters. The temperature chart is corrected to 2000 meters, so plants from the lower end of the elevation range would average about 14°
F higher than indicated. It is most striking that the data for this habitat (from a weather station approximately 95 miles from the habitat) shows an average low temperature of 32°
F for the month of December. Taken at face value as the temperatures that these plants are exposed to during that
month, and recognizing that there is going to be a range of variation around the average temperature, this would suggest that the plants are sometimes exposed to temperatures overnight in the low to mid 20's F. While this is a possibility, I believe that it is more probable that the plants are not growing at quite such a low temperature, either due to location in smaller microclimates where it stays warmer, or due to possible errors in extrapolating the data from the weather station to the habitat as discussed above. There is very little doubt, however, that the plants are subject to quite cold nighttime temperatures during the late fall and winter months, and the diurnal temperature range is quite large. The tremendous value of the data provides is in the trends as much or more than the absolute values, and looking at the data graphically helps spot these trends. In the case of Paph armeniacum we have coincident cold nighttime temperatures, greatly decreased rainfall and (not surprisingly with the decreased rainfall) clearer skies starting in the late fall and through the winter, which is coincident with the bloom period of this paph. Keep in mind that while the skies are clearer, these plants are generally found on northern exposures, so this means bright indirect light. These are the conditions we try to include in our culture of this species, but within the constraints of a mixed greenhouse collection. We have found that a night time fall and winter temperature of 55 to 58°
F is adequate both for the long term health and flowering of the plants, along with bright light (supplemented with HID in our dull winters) and greatly reduced water starting in the fall. We mist the surface of the plants and medium frequently during this period to reduce the dehydration of the plants, somewhat imitating the mists common in the area.
Percentage Days Clear at Noon
Habitat and climate data are very important and useful, and great thanks are owed to those who measure, compile and report it. It's interpretation and use in our growing programs needs careful understanding, however, of its limitations as we integrate it rationally into our growing conditions.
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