DOT Project Number:  90-00-LRTF-300

Fiscal Year:  2003

Award:  $25,000.00

Principal Investigator:  Dr. Thomas Rosburg, Department of Biology, Drake University, thomas.rosburg@drake.edu

Summary Report:

DEMOGRAPHY STUDY OF PRAIRIE PLANTS

Prairie reconstructions provide great opportunities to study the assemblage of prairie communities and prairie plant biology. In particular, demographic studies of prairie plant populations can provide information on plant growth, reproduction and survival that is relatively unknown for many species. As a follow-up to the demographic study investigating the effects of late spring fire on prairie forbs, a longer term study was initiated in 2000 to investigate the demography of a group of forbs on the I-35 project. Its goals were to mark a group of plants at each of the sites and monitor them over time to assess their survival and life span, their growth and reproduction patterns, and the herbivory they incur.

Demography Methods
Plants were selected for the demography study in May and June of 2000. Approximately 10 to 16 individuals representing 23 species were marked at 13 study sites for a total of 150 marked plants. Site 739N and 767S were not included because of low forb diversity. Except for four plants marked in the spring of 1998 on the control plots of two study sites, all the plants selected at a site were adjacent to the study plots but not located on the plots. Plants were marked with a steel electric fence post and numbered tag. The fence post was placed 10 cm north of the center of the genet. Species and individuals were selected that were relatively distinct from the rest of the prairie community so that the same genet could be reliably relocated in the future. The following measurements were made for each genet:
1- The number of ramets present was determined. For caulescent species, the ramets were their individual stems; for acaulescent species, the ramets were distinguishable caudices. The area encompassing a 20 cm radius around the center of the original marked genet was included in the count to allow some movement of the individual by vegetative growth.
2- The height (cm) of each ramet was determined by measuring to the highest point of leaf attachment or location of other meristematic growth like flowers. For acaulescent species, the caudex was typically recorded as 1 cm. For some large individuals of typically acaulescent species, a short stem was present and measurable. The flowering stems of acaulescent species were measured as any other stem.
3- For acaulescent individuals lacking a flowering stem, the height of the tallest leaf was measured from the ground to its tip.
4- The number of ramets producing flowers was determined. Anything recognizable as a flower bud or beyond was counted as evidence of reproduction.
5- The number of ramets that showed evidence of herbivory in the form of stem or leaf removal. Herbivory of this nature was most likely caused by small mammals. Insect herbivory was not included.

The marked plants were measured once during the growing season in 2000, four times in 2001, three times in 2002 and twice during 2003. Data were compiled and summarized by calculating the ramet frequency, mean ramet height, and the number of ramets in flower or exhibiting herbivory for each genet.

Results – Survival and Growth
Because the seeding occurred in 1996 and the genets were selected and marked in 2000, the exact age of the genets cannot be known for sure. In most cases, the genets probably germinated in 1996 or 1997 and therefore were either 3 to 4 years old at the time they were marked and initially measured. For the sake of simplicity, it is assumed that the genets in this study were 3 to 4 years old in 2000, thus their current age is the maximum age possible. For some species, individuals may not have germinated until after 1997, thus their current age would be one or two years less.

Only two individuals of leadplant were marked, but both are still alive and apparently 6-7 years old. Over the last four years, the number of ramets per genet has remained consistently in the 1 to 3 range. No growth was observed on the earliest sample date of May 5, but by May 15 the average ramet height was nearly 20 cm.

Virginia windflower has an 83% survival, one of the original 6 genets marked died at 4-5 years of age. Two of the genets have increased in size from 4-6 ramets to 9-12 ramets over the last four years, while the others have remained steady at less than 3 ramets. An average of about 1 cm of ramet growth was observed as early as May 5; average maximum ramet height has been about 20 cm.

Only 40% of the butterfly milkweed initially marked is still alive. Nine individuals made it to 4-5 years age and six made it to 5-6 years of age. Genet size has been in the 1-3 ramet range. Of the four surviving genets, three have two or fewer ramets. One vigorous individual has increased to six ramets. No growth was observed on the early sample date of May 5. In 2001, average ramet height was about 10 cm on May 15, but in 2002 and 2003, average ramet height was less than 5 cm as late as May 25. Average maximum ramet height on the project area has been in the 40 to 50 cm range.

Survival in the asters marked has been high. Sky blue aster (N=2) and New England aster (N=6) have 100% survival, while one individual of smooth aster (N=7) died at 5-6 years resulting in 86% survival. The two genets of sky blue aster exhibit a range of vigor and have slowly increased their ramet number from 1 to 3 and from 8 to 17 over the last four years. Smooth aster genets have ranged from 1 to 34 ramets per genet, but have generally been consistently in the range of 5 to 14 ramets per genet. Individuals of New England aster have also exhibited a large range of ramet number, from 6 to 26. However a slight decrease in vigor is apparent in the last two years as ramet number per genet is decreasing. The six marked plants currently have 6-18 ramets. All three aster species had initiated some growth (average ramet height less than 5 cm) by the May 5 sample date. By May 25, New England has achieved the greatest growth (about 25-30 cm ramet height), smooth aster the next most growth (about 12 cm ramet height) and sky blue aster the least growth (5-10 cm ramet height).

Only one of the six initially marked Canada milkvetch plants appears to have survived from 2000 to 2003, resulting in a 17% survival rate. Four of the genets died before reaching 4-5 years of age. When initially marked in 2000, most plants had 4 to 8 ramets per genet. A considerably larger ramet number is possible, as shown by the 15 to 20 ramets present for each of the two plants alive in 2001. It appears that the single plant alive in 2003 may not survive much longer as its ramet number has decreased from 15 to 5 over the last two years. However, it was noted during the 2003 surveys that there were numerous stems outside the 40 cm diameter area centered on the original genet. It may be that vegetative growth by rhizomes maintains Canada milkvetch genets longer than observed. An average of about 5 cm of ramet height was present on the earliest sample date of May 5; average maximum ramet height has been between 50 and 70 cm.

All four white wild indigo plants are doing well, each has maintained a single ramet since being marked in 2000. None of them were observed at any of the May sample times. The earliest observation for wild indigo is June 15, when average ramet height was 40 cm. Maximum ramet height has been 90 to 100 cm.

One to two plants of purple prairie clover have died each year since the initial 8 were marked in 2000. Three genets attained 6-7 years of age and were still alive in 2003, resulting in a 38% survival rate. One of those plants appeared to be dead in 2002 as no ramets were observed for it on any of the surveys, however in 2003 four ramets were present. A possible explanation is that herbivory removed all the ramets during 2002 making the plant appear to be dead. Purple prairie clover genets have generally had 2 to 6 ramets each, although one of the extant plants had 9 ramets in 2003. Growth was barely underway on the May 5 sample date, as average ramet height was only 1 cm. By 10 days to 2 weeks later, average ramet height was a little over 15 cm, and average maximum ramet height for the season has been about 60 cm.

Ox-eye survived well until 4-5 years of age (92%) and then 4 plants died before reaching 5-6 years (62% survival) and 3 more died before reaching 6-7 years (23% survival). Ramet number per genet was initially between 2 and 18 in 2000, but concomitant with survival they have been declining ever since. One exceptional individual with 27 ramets in 2000 grew larger and attained 69 ramets in 2002, but since then it crashed to about 5 ramets in 2003. Its decline was probably caused at least in part by high herbivory in 2002, when over 70% of its ramets were partially consumed. Ox-eye plants have averaged about 5 cm of ramet height on May 5 and about 20 cm of ramet height two weeks later. Average maximum heights have been in the 45 to 50 cm range.

Round-headed bush clover exhibited a 57% survival rate, but that may be a low estimate since it has demonstrated ability to be dormant for a season. Two individuals were not observed in 2002, but returned in 2003. Plants have typically had 4 or fewer ramets per genet. Two of the current living plants have attained a larger size with 7 and 10 ramets per genet. No evidence of growth was observed on May 5; by mid to late May average ramet height has been in the 12-15 cm range. Average maximum ramet heights have been in the range of 60 to 70 cm.

Prairie blazingstar currently has 100% survival. All but one of the four marked plants have exhibited a slow increase in size from their initial 6 to 10 ramets per genet to their current 12 to 16 ramets per genet. Plants were just beginning to emerge on May 5 and by mid to late May had an average ramet height of 5 to 10 cm. Average maximum ramet height has been about 50 cm.

Of the three genets of prairie phlox marked in 2000, two are still alive (67% survival). One is fairly vigorous and has produced 4 to 10 ramets over the last three years, while the other is less vigorous and has maintained one or two ramets. Growth was well underway on May 5 with an average ramet height of 15 cm. Average maximum ramet heights have been in the 40 to 50 cm range.

Survival for grey-headed coneflower was fairly high at 78%; all nine individuals initially marked reached 5-6 years of age and seven of them are now 6-7 years. However, it appears they are gradually declining in vigor as ramet number has dropped from an initial range of 8 to 20 in 2000 to the current range of 3 to 10. Because grey-headed coneflower is strongly acaulescent and not all caudices form flowering stems, average ramet height can be misleading. Even a well-developed basal clump of leaves has a ramet height of only 1 cm. Flowering stems need to be present for heights to exceed 1 cm. An average ramet height of 1 cm was observed on May 5 and is evidence that growth was underway, but since that is too early for flower stems to be present ramet height doesn’t convey exactly how much growth had occurred. Leaf height for grey-headed coneflower on May 5 ranged from 7 to 20 cm, which is 20 to 50% of its typical leaf size, thus a substantial amount of growth had occurred by May 5. When flowering stems were present, typical heights were in the range of 75 to 100 cm.

Among all the species initially marked, black-eyed susan had the lowest survival rate. Only two of the original 8 plants marked in 2000 survived to 2001 (25% survival), and they died before 2002 (0% survival). It is clear that black-eyed susans are short-lived plants, but because none were marked during the first growing season, it is not clear from these data how long their life span is. If all the plants originally marked germinated in 1996, then most of them died at an age of 4 years. However, it is possible that some of these marked plants germinated later than 1996, perhaps as late as 1999, either from the seed planted or from seed produced by plants (i.e. were second generation plants). Seedlings of black-eyed susan were commonly observed on the plots, and at least once were observed at the location of a deceased black-eyed susan. None of the marked genets were alive when the May 5 sample was made in 2002. In 2001, the two surviving plants had an average ramet height of 6 to 7 cm on May 15, and reached an average maximum ramet height of about 35 cm.

Sweet coneflower, rosin weed, and compass plant all have 100% survival for the four years from 2000 to 2003. All three also exhibit a trend of gradually increasing vigor from 2000 to 2003. Sweet coneflower and rosin weed are extremely variable in their size. Currently some coneflower individuals have as few as 6 or as many as 56 ramets per genet, and some rosin weed individuals have as few as 1 or as many as 52 ramets per genet. Compass plant size is more consistently between 2 and 3 ramets per genet. All three species were producing growth on May 5. Sweet coneflower had just initiated growth and had an average ramet height of 1 to 2 cm. Rosin weed and compass plant were a little further along with average ramet heights of 3.5 and 4.5 cm respectively. These three species are among the tallest species in the reconstruction. Average maximum ramet heights for sweet coneflower and rosin weed are 90 to 95 cm and 100 to 110 cm respectively. For compass plant, an acaulescent species, average ramet height typically includes some non-flowering caudices that lower the mean. Heights of compass plant flowering stems were 175 to 210 cm.

Rigid goldenrod is the third species to exhibit a season of dormancy. A plant with 10 ramets in 2001 appeared to be dead in 2002 due to lack of ramet growth and then reappeared with a single ramet in 2003. Its survival has been fairly high at 73%. Most genets have maintained between 1 and 20 ramets. One individual has exhibited very good growth and has increased every year to over 50 ramets in 2003. Average ramet height on May 5 was about 3 cm, and average maximum height has been in the range of 50 to 60 cm.

All six of the Ohio spiderwort genets marked in 2000 have survived (100%) and have exhibited a gradual increase in vigor from a initial range of 1 to 4 ramets per genet in 2000 to a current range of 4 to 14 ramets per genet in 2003. About 6 cm of ramet growth had occurred by May 5 for spiderwort, and its average maximum height has typically been about 90 cm.

Golden alexander survival is also high at 90%. One of the initial 10 plants marked in 2000 died at age 5-6 years. Most of the remaining genets have maintained steady vigor with ramet numbers ranging from 1 to 6 per genet. Two more vigorous individuals are present with ramet numbers of 14 and 18. Being acaulescent, average ramet height does not convey its growth pattern very well. Its average ramet height of 4 to 5 cm on May 5 means that several flowering stems were already present. The height of the flowering stems ranged from 3 to 16 cm and leaf height for non-flowering individuals on May 5 was about 15 cm. Among all the species marked in the demography study, golden alexander was the earliest species to initiate growth.

Results – Reproduction and Herbivory
All but two of the 23 species marked in the demography study produced flowers during the 2000 to 2003 study period. Leadplant and indigo bush have not yet flowered. Reproductive capacity is a measure of the readiness and ability of individual genets to reproduce. It reflects reproduction at the population level. Reproductive effort is a measure of how much effort an individual allocates toward reproduction once it goes into reproductive mode. It reflects reproduction at the individual level. Aside from leadplant and indigo bush, other species with relatively low reproductive capacity (the proportion of genets that reproduce generally less than 40%) include Virginia windflower, purple meadow rue, grey-headed coneflower and white wild indigo. Slow growth to maturity could be a reason for the low reproductive capacity in all except grey-headed coneflower. High levels of herbivory may be stressing grey-headed coneflower and reducing its reproductive capacity. Species that exhibited medium levels of reproductive capacity (the proportion of genets that reproduce generally between 40% and 70%) included butterfly milkweed, smooth aster, compass plant, purple prairie clover, ox-eye, round-headed bush clover, rigid goldenrod and golden alexander. Those species with the highest reproductive capacity (the proportion of genets that reproduce generally greater than 70%) were prairie blazingstar, prairie phlox, New England aster, sweet coneflower, rosin weed, Ohio spiderwort, sky blue aster, Canada milkvetch and black-eyed susan.

Although reproductive effort can only be determined for individuals that reproduce, its magnitude is independent from reproductive capacity. One species had low reproductive capacity and low reproductive effort (grey-headed coneflower), others had low reproductive capacity but high reproductive effort (white wild indigo, purple meadow rue), some had high reproductive capacity but low reproductive effort (New England aster, Canada milkvetch), and one had high reproductive capacity and high reproductive effort (Ohio spiderwort). These reproductive options combine to identify four possible strategies that were all represented in the prairie species in the I-35 demography study.

All 23 species exhibited signs of small mammal herbivory at some time during the study period from 2000 to 2003. For over 65% of the species, herbivore incidence was relatively low (the proportion of genets affected generally less than 40%). Six species exhibited medium levels of herbivore incidence (the proportion of genets affected generally between 40% and 70%). These included ox-eye, indigo bush, Canada milkvetch, purple prairie clover, New England aster and smooth aster. Two species had relatively high herbivore incidence (the proportion of genets affected generally greater than 70%) – sweet coneflower and grey-headed coneflower. Both of these species had herbivore incidence over 80% at times. However, herbivore severity for these two species was relatively low (generally less than 40%). Many genets were selected for herbivory, but the herbivore effect per genet was relatively low. Four species had relatively high herbivore severity (Ohio spiderwort, prairie phlox, compass plant, and purple meadow rue), but all had relatively low herbivore incidence. Thus although some individual genets of these species were hit hard by herbivores, only a few genets were selected. Overall, there was a significant negative correlation between herbivore incidence and herbivore severity (Pearson correlation, r = -0.48, p=0.019). Thus herbivore effects were distributed such that maximum herbivore stress (high incidence and severity) did not occur.