Observations on Seeds Fremont Cottonwood

Home , Populus deltoides, Populus fremontii

Observations on Seeds and Seedlings of Fremont Cottonwood

Item Type Article

Authors Fenner, Pattie; Brady, Ward W.; Patton, David R.

Publisher University of Arizona (Tucson, AZ)

Journal Desert Plants

Download date 29/09/2021 03:42:35

Link to Item http://hdl.handle.net/10150/552248 Fenner, Brady and Patton Fremont Cottonwood 55

where moisture is more constantly available than near the ObservationsonSeeds surface. Keywords: cottonwood, riparian, seed germination. The collection of data on natural river /floodplain ecosystems in the Southwest is of immediate concern because they are and Seedlings of rapidly being modified by construction of dams, wells and irrigation projects, channel alteration, phreatophyte control Fremont Cottonwood projects, and by clearing for agriculture. Additional information is needed on how these activities modify the environment and the subsequent effect on germination and establishment of Fremont Cottonwood. Pattie Fenner Both the importance and the diminished extent of riparian areas of the southwest have been acknowledged (Johnson and Arizona State University Jones, 1977). This has led to increased emphasis on understanding ecological characteristics of major riparian species. Ward W. Bradyl This paper describes some characteristics of one riparian Arizona State University species, Fremont Cottonwood (Populus fremontii Wats). The characteristics are: seed viability under various storage conditions, effects of moisture stress on germination, and rates of and David R. Patton2 seedling root growth. Knowledge of these characteristics is Rocky Mountain Forest and Range Experiment Station important for understanding seedling ecology of the species, USDA Forest Service which, in turn, increases understanding of the dynamics of the riparian community as a whole. Little work has been done on Fremont Cottonwood seed viability. Horton and Campbell (1974) collected and stored cottonwood seed at laboratory temperatures of 24 -27° C and found seeds to be viable for a maximum of 7 weeks. Seeds of Eastern Cottonwood (Populus deltoides Bartr.), which occurs in similar habitat in the midwestern United States, have been more extensively studied. McComb and Lovestead ( 1954 ) investigated viability of eastern cottonwood seed under various storage temperatures and humidities (-12 °, 0 °, 5° C and room temperatures; 5 %, 10 %, and 25% relative humidity). They found viability was best retained at -12° C and 25% relative humidity. Farmer and Bonner ( 1967 )created various environmental conditions in germinators, using d- mannitol solutions to mimic soil moisture stress. They found chances for successful germination and good early growth to be optimum at 27 -32° C and at moistures of less than five atmospheres tension.

Study Area Three study sites were chosen. The first was a cottonwood gallery forest in an overflow channel of the Salt River, near Phoenix, Arizona, approximately 3 km upstream from Granite Abstract Reef Dam. The age class distribution of cottonwoods at this The seeds of Fremont Cottonwood (Populus fremontii) lose location was bimodal. When the study was started, large, viability within 1 to 5 weeks after dispersal. Moisture stress decadent trees dominated the population. But during the 2 induced by osmotic solutions stronger than six atmospheres years of the study, numerous seedlings became established. both delayed and reduced total germination. Root growth rates Associated species included Willow (Salix gooddingii Ball), of young seedlings average 6 mm per day. Because of the Mesquite (Prosopis juliflora Swartz DC), and a thick carpet of limited time of seed viability, a suitable substrate for germina- annual grasses in the spring. tion must occur at or soon after seed dispersal. Also, moist The second study was near the town of Dudleyville, conditions must persist until seedling roots grow to depths Arizona, on the San Pedro River, approximately 10 km upstream of the confluence with the Gila River. Cottonwoods at this location also were in several age -classes, and the population as a whole was not decadent. Associated species IWard W. Brady, Associate Professor of Agriculture and Pattie Fenner, Graduate Assistant, Division of Agriculture, Arizona State University, Tempe, Arizona included Willow and Tamarisk (Tamarix pentandra Pall). 85287, 602- 965 -2402. The third study area was located in Tempe, Arizona. 2David R. Patton, Principal Wildlife Biologist, Rocky Mountain Forest and Range Fremont Cottonwood trees at this location were selected for Experiment Station, Forestry Sciences Lab, Arizona State University Campus, Tempe, study because they were close enough to the laboratory to Arizona85287, 602 -261 -4365. closely monitor phenological development. 56 Desert Plants 6(1) Summer 1984

Table 1. Effects of moisture stress on germination of Fremont cottonwood seeds.

Atmospheres tension

Hours '0 0_5 2 4 6 8 10 12 14 16 Percentage germination of cottonwood seeds at successive time intervals

15 91.7 (a) 81.7 (ab) 66.7 (tic) 61.7 (bc) 48.3 (c) 25.0 (d) 15.0 (d) 0.0 (e) 1.7 (e) 0.0 (e)

25 93.3 (a) 85.0 (a) 83.3 (a) 75.0 (a) 76.7 (a) 48.7 (a) 26.7 (c) 5.0 (d) 5.0 (d) 1.7 (d)

38 95.0 (a) 88.3 (ab) 86.7 (ab) 81.7 (ab) 78.3 (b) 55.0 (c) 41.7 (c) 16.7 (dl 11.7 (d) 1.7 (e)

47 96.7 (a) 90.0 (b) 86.7 (b) 88.3 (b) 86.7 (b) 66.7 (c) 58.3 (c) 26.7 (d) 15.0 (d) 5.0 (e)

86 96.7 (a) 90.0 (ab) 88.3 (ab) 90.0 (ab) 91.7 (a) 75.0 (bc) 63.3 (c) 26.7 (d) 16.7 (d) 5.0 (e)

110 96.7 (a) 90.0 (ab) 88.3 (ab) 90.0 (ab) 91.7 (ab) 78.3 (bc) 65.0 (c) 30.0 (d) 16.7 (de) 5.0 (e)

134 96.7 (ab) 90.0 (ab) 90.0 (ab) 90.0 (ab) 93.3 (ab) 78.3 (bc) 65.0 (c) 36.7 (d) 16.7 (e) 8.3 (e)

Letters within a single hourly period indicate significant difference at the 0.05 level.

Methods five bags were placed in a more exposed, warmer site. Five to Fremont Cottonwood seeds were gathered from trees growing twenty seeds were removed weekly from each bag, brought in Tempe, immediately following dispersal. Seeds were washed back to the laboratory, and germinated. for approximately 2 minutes in a 5% Clorox/water solution In the moisture stress tests, aqueous solutions of d- mannitol (sodium hypochlorite) with a few drops of dish soap to increase were used to simulate varying amounts of soil moisture avail- wettability of fiber matrix. This liquid was drained, and the able to cottonwood seeds and seedlings in the process of seeds were sprayed with Captan. Studies by Richardson et al. germination and establishment. These solutions varied in concentra- (1982) indicated that application of Captan may reduce seed tion to simulate soil moisture tensions from 0 to 16 atmos- germination, especially when high concentrations are used. pheres. Distilled water was used for 0 atmosphere soil However, fungi problems made application necessary. The moisture tension. Twenty seeds were placed in a petri dish seeds were next placed in a petri dish containing five pieces of containing five pieces of filter paper moistened with 5 ml of filter paper moistened with 5 ml of distilled water. This same specific d- mannitol solution or distilled water. Three replica- washing and germinating procedure was used for all viability tions of each concentration were made. Values were tested by tests. Germination was noted at 24 -hour intervals, for 1 week. analysis of variance on arcsine transformation of percentage If germination did not occur within 1 week, viability was germination, in order to test for significant differences in counted as zero. number of seeds germinated at each time interval. Viability tests were performed under two different condi- Wiggans and Gardner (1959) established d- mannitol as a tions in the spring of 1978. One set of seeds was stored in a non -toxic chemical which was accurate in limiting moisture brown paper lunch bag, in the laboratory, where temperatures available to a germinating seed when mixed with water remained approximately constant at 25° C. Three dishes of 20 according to the following equation: seeds each were germinated every day for 1 week, every second PVm day for the second week, and at weekly intervals thereafter g = until no seeds germinated. The second set of seeds was stored RT in a similar brown paper bag, in a greenhouse, where temper- where g = grams of d- mannitol, P = desired osmotic pressure in ature and moisture conditions approximated those outside, atmospheres, V = volume of mixed solution in liters, m= except for the amelioration of extremes by an air cooler. Three molecular weight of d- mannitol, R = 0.0825 atm. degrees per dishes of 20 seeds each were germinated daily for 1 week, every mole, and T = absolute temperature. There have been some second day for the second week, and weekly thereafter until questions posed since 1959 concerning equivalence of osmotic seeds no longer germinated. to matric tension (Farmer and Bonner, 1967; McDonough, Seeds were stored under natural environmental conditions 1977), and a toxicity of d- mannitol. However, difficulties in in the spring of 1979. Ten 7.5 -cm- square bags made of nylon regulation of available moisture in any other manner and lack mosquito netting were filled with cottonwood seeds collected of a better chemical have led d- mannitol solutions to be widely immediately following dispersal. These bags were then placed accepted for seed germination experiments. in the Salt River study area, at sites where seeds were naturally Soil samples from the floodplains of the San Pedro and Salt accumulating and which appeared to be suitable sites for River study areas were sterilized, and textures were determined germination. Five bags were placed in a relatively cool site, by the Bouyoucos method (Foth et al., 1974). The sterilized soil shaded by a canopy of mesquite and cottonwood. The other was put into rootboxes, wooden boxes constructed so that the Fenner, Brady and Patton Fremont Cottonwood 57

100

80 LABORATORY GREENHOUSE 70 O OSHADE (SALT RIVER )

1 O O SUN (SALT RIVER) 60 1

1 50 1 1 s 40

30 1

1 20 i.

e- v I I I I 0 2 4 6 8 10 12 14 16 18 20

WEEKS AFTER HARVESTING

Figure1. Cottonwood seed viability under four different environmental conditions.

front could be opened to expose the roots behind a pane of The increase in germinability after 1 week displayed by glass. Two rootboxes were filled with soil from each study laboratory- stored seeds and seeds stored in shade on the Salt area. Cottonwood seeds were placed on the soil surface and River may be the result of a short after -ripening period. The were kept moist by frequent misting and the humidifying seeds may not have been totally mature at the time of dispersal effect of a piece of plastic wrap over the box. Boxe, were kept in (Koller, 1972; McDonough, 1977). an air -cooled greenhouse. Soils were maintained in a moist Under average field conditions at the time of seed dispersal, condition until seedling roots reached the bottom of the one would not expect seeds to remain viable for longer than 5 rootbox. weeks. Theoretically, however, seeds could retain their viability Root growth was measured during growth by periodic for much longer periods, given an environment of lower marking of the root -tip location on the glass, using a grease temperatures and less fluctuation in humidity than is normally pencil. In the subsequent statistical analysis, only those roots found in the field. that reached the bottom of the rootbox were counted. Variability in germinability of fresh seed may result from factors such as seed maturity, exposure of seeds to weathering Results and Discussion forces while still on the tree, presence of fungus on or in some Results of the viability tests are summarized in Figure 1. of the seeds, or differences in seed size (Crocker and Barton, Seeds stored in the laboratory suffered a reduction in germinabil- 1953). ity of 90% after 20 weeks. Germinability was lost rather Increase in osmotic tension from 0 to 6 atmospheres rapidly after the 13th week. Those seeds exposed to conditions resulted in delayed germination, but not a significant decrease in the natural environment lost all germinability after 5 in total germination percent (Table 1(. Moisture stress induced weeks. Greenhouse -stored seeds were intermediate in viability. by osmotic solutions stronger than 6 atmospheres resulted in 58 Desert Plants 6(1) Summer 1984 delayed germination, but not a significant decrease in total previously mentioned root growth rates, following a receding germination percent (Table 1). Some seeds germinated at 16 water table, most establishing seedlings could tap groundwater atmospheres (8.33 %), but hypocotyl and radicle did not continue at depths of 72 cm by the end of the summer, and could to elongate beyond the sixth day after planting. potentially reach 162 cm. Root growth rates of newly germinated seedlings did not In natural cottonwood communities of the Southwest, the vary significantly among the four rootboxes. Mean growth rate normal sequence of events involves a spring flood caused by per day was 6 mm; however, this varied greatly from day to day. melting snow on the watershed, (occurring around the time of The most rapid rate measured occurred in San Pedro floodplain cottonwood seed dispersal). This is followed by a relatively soil -13.5 mm per day, measured over a 4 -day period. continuous dropping of river water to very low levels through Rapid root growth is essential for survival of cottonwood the summer. Cottonwood seed and seedling have character- seedlings, because the moist alluvium deposited in the spring istics described in this study, which appear to adapt them to dries rapidly with the onset of the extreme summer temper- this sequence of events. atures characteristic of the Sonoran Desert. Although flow may be curtailed through the summer, in a natural riverine environment, groundwater is present in the immediate vicinity Literature Cited of the riverbed. Periodic measurement of soil moisture of an Crocker, W and L. V. Barton. 1953. Physiology of Seeds: An Introduction overflow channel nursery bar (a common site for cottonwood to the Experimental Study of Seed and Germination Problems. and germination) on the Gila River indicated that moisture Chronica Botanica Company. Waltham, Mass. levels at depths of 60 cm are relatively constant throughout Farmer, R. and R. T. Bonner. 1967. Germination and initial growth of eastern cottonwood as influenced by moisture stress, temperature, the year. Soil moisture remained at nearly 10% throughout the and storage. Botanical Gazette 128(3):211-215. year, while moisture levels at shallower depths (10, 20, and 40 Foth, H. D., H. S. Jacobs, and L. V. Withee. 1974. Laboratory Manual For cm) fluctuated from 23% to 8%. (Collection of data by Paxson Introductory Soil Science. 3rd ed. Wm. C. Brown Company and Brady, 1976 -1978). Publication. The results of these studies increase understanding of the Horton, J. S. and C. J. Campbell. 1974. Management of phreatophyte sequence of conditions required for successful germination and riparian vegetation for maximum multiple use values. USDA and establishment of cottonwood in the natural environment. Forest Service Research Paper RM -48, 26 p. Rocky Mountain Forest Seeds require a moist surface to start germination. On a sandy - and Range Experiment Station, Fort Collins, Colo. loam textured soil material, moisture may be as low as 10% for Johnson, R. R. and D. A. Jones. 1977. Importance, preservation, and management of riparian habitat. A Symposium. USDA Forest a relatively high germination rate (more than 80 %). This moist Service General Technical Report RM -43, 217 p. Rocky Mountain germination medium is naturally provided for by a spring Forest and Range Experiment Station. Fort Collins, Colo. flood that deposits fresh alluvium along riverbanks. Fresh Koller, D. 1972. Environmental control of seed germination. Vol. II, p. seeds germinate more rapidly than older seeds, so that if a 447. In: Kozlowski, T. T. (ed.), Seed Biology. New York Academic moist soil material were present at time of dispersal, most Press. seeds would germinate within 24 hours. Because of the short McComb, A. L. and H. S. Lovestead. 1954. Viability of cottonwood viability period of seeds, these conditions would have to occur seeds in relation to storage temperatures and humidities. USDA within 6 to 8 weeks after seed dispersal. Forest Service Tree Planters Notes 17:9 -11. Subsequently, surface soil moisture need not be maintained, McDonough, W. T. 1977. Seed Physiology. In: Sosebee, R. E.(ed.), as long as water is available to the elongating root. Roots were Rangeland Plant Physiology. Society of Range Management. Range observed to grow at an approximate average rate of 6 mm per Science Series 4:155 -184. Richardson, J. M., R. A. Backhaus, and J. C. Stutz. 1982. Effect of day; therefore, theoretically, water table levels may recede at fungicides in fluid drilling gel media on damping -off of Parthenium this rate through the spring and summer. The declining water argentatum. Phytopathology 72:979. table tends to promote root growth to greater depth, as Wiggans, S. C. and E.P. Gardner. 1959. Effectiveness of various opposed to the situation with a perched water table, which solutions for stimulating growth conditions as measured by would cause a shallow root system to develop. At the germination and seedling growth. Agronomy Journal 51:315 -318.