Mowing Frequency Influences Number of Flowering Stems but not Population Age Structure of Asclepias viridis, an Important Monarch Host Plant.
There is a well-documented decline in the size of the eastern North American migratory population of the monarch butterfly (Danaus plexippus L.) in overwintering sites in central Mexico over the last 20 y (Brower et al., 2012; Vidal and Rendon-Salinas, 2014; Semmens et al., 2016). A decline in milkweed (Asclepias spp.) availability in the Midwest is often cited as contributing to the population decline (Pleasants and Oberhauser, 2013; Pleasants, 2017; Thogmartin et al., 2017). Milkweeds are the sole host plant for monarch larvae (Malcolm et al., 1993) and also provide nectar for adult monarchs during spring and summer breeding periods, as well as during the spring and fall migrations (Alonso-Mejia et al., 1997; Brower et al., 2006).
Because of the decline of milkweed in the Midwest region primarily due to agricultural practices (Zaya et al., 2017), understanding how management practices influence the structure of milkweed populations in other land use contexts is important for maintaining and enhancing milkweed availability for monarch butterflies. Natural and semi-natural grasslands and roadsides represent a majority of current milkweed habitat (Kasten et al, 2016; Zaya et al, 2017). Mowing is a common management practice in grasslands and roadsides, and may influence monarch-milkweed interactions. For example monarchs prefer new growth (or regrowth) compared to older growth (Zalucki and Kitching, 1982; Fischer et al, 2015). The occurrence of regrowth (resprouting) depends on the timing of mowing or other disturbance (Baum and Mueller, 2015; Fischer et al, 2015). In addition, though monarchs are considered nectar generalists (Robertson, 1928; Tooker et al, 2002), milkweed flowers are an important nectar source for monarchs. The liming and frequency of mowing may influence flowering, given the amount of reproductive effort is influenced by the availability of carbon resources in a plant (Southwick, 1984; Bazzaz et al, 1987; Kozlowski, 1992), especially when a disturbance removes a large portion of reproductive biomass (Howe, 1994; Iwasa and Kubo, 1997; Nofal et al, 2004). Moreover, continual removal of biomass could deplete carbon reserves, negatively affecting plant longevity and, consequently the mean age of a population (Dietz and Ullmann, 1998; Hautekeete et al, 2002). Given the timing of peak reproduction in a plant's life history, younger populations may have reduced overall nectar availability compared to populations with older individuals (Pergl et al, 2006).
Managed milkweed habitats in Texas and Oklahoma represent important spring and late summer/early fall breeding sites for monarch butterflies in which the availability of milkweed biomass and nectar are likely important for overall monarch success. Recent population modeling suggests the spring migration through Texas and Oklahoma is a critical time for the monarch's annual life cycle given the importance of the first generation to the overall population size (Flockhart et al, 2015; Inamine et al., 2016; Oberhauser et al, 2017). Recent stable isotope research also indicates monarchs produced in this region in the late summer/early fall can be an important component of the overwintering population (Flockhart et al, 2017). In the Texas and Oklahoma area, A. viridis mirrors the importance of A. syriaca in the Midwest as a host plant (Malcolm and Brower, 1989; Baum and Mueller, 2015). Our previous research has shown the condition of A. viridis throughout the growing season, peak nectar availability, and plant regrowth all have important impacts on opposition, adult food availability, and support of monarch butterfly larvae during critical stages of development and different monarch generations across seasons (Baum and Sharber, 2012; Baum and Mueller, 2015). In light of these findings, a more holistic consideration of management effects on milkweed reproduction and population age structure is required. For instance mowing after peak flowering could promote nectar availability in early summer and reduce stress for A. viridis, given more carbon reserves are available belowground after peak flowering (Dee and Palmer, 2018). Furthermore, plants with reduced carbon reserves in late summer/early fall may be less likely to resprout (Cruz et al, 2003; Pratt et al, 2014), negatively affecting production of the last generation of monarchs. Because A. viridis less than 5 y old have significantly fewer stems and a reduced chance of flowering than older age classes (Dee and Palmer, 2018), timing of mowing can impact population age structure and influence population longevity.
In this study we use herb-chronology, the aging of herbaceous individuals by counting annual growth rings in their perennial woody belowground taproot, to evaluate if mowing frequency influences population age structure, number of stems by age class, and number of flowering stems by age class. Given the expected stress on carbon reserves from frequent mowing, especially when the timing removes much of the carbon pool through clipped flowering stems, we predict an overall younger age structure of A. viridis in roadsides that could lead to fewer stems and fewer flowering stems per plant compared to grasslands mowed less frequently and outside of peak flowering season.
MATERIALS AND METHODS
BIOLOGY OF ASCLEPIAS VIRIDIS
Green milkweed, Asclepias viridis (Walter) A. Gray, (Apocynacae), has annual growth rings in the belowground tuber-like taproot identified by a semi-porous ring structure with a thick latewood band of parenchyma cells (Dee and Palmer, 2017). False ring occurrence is unlikely, as rings correlate well with climate and among individuals across a population (Dee and Palmer, 2017). Furthermore, Dee and Palmer (2018) found no false rings when several dozen A. viridis individuals were clipped of aboveground shoots to simulate disturbances similar to mowing. In Oklahoma, near the western edge of the species' range, a typical growing season for A. viridis consists of shoot emergence in late March to early April, peak flowering between May and June, and peak fruiting from July through as late as November with plant senescence following. Asclepias viridis averages 1.6 to 2.5 stems per plant depending on land use (Kersten, 2017). Stems emerge from a root collar (root to shoot transition zone near soil surface) 5-10 cm below ground and are fairly ascending to decumbent and are usually not longer than 50 cm (Dee and Palmer, 2017).
FIELD AND LABORATORY METHODS
Study sites were located in the general vicinity of Stillwater, Oklahoma (36.1156[degrees]N, 97.0584[degrees]W), which is in the north central part of the state. We collected A. viridis from four sites, including two roadsides and two grasslands. Roadside sites were located on state highway 51 about 14 mi west of Stillwater (south side of highway) and on Interstate 35 near mile marker 164 (west side of highway). Both sites consisted of a mixture of typical tallgrass prairie forbs and grasses in full sun. Roadsides were managed by the Oklahoma Department of Transportation and were traditionally mowed at least four times per year (before Memorial Day, Fourth of July, Labor Day, and following the first freeze), but the mowing regime shifted in 2016 to delay the first mowing until mid-June or July. Grasslands were located at Oklahoma State University's Cross Country Course and also consisted of tallgrass prairie vegetation located in full sun. Grasslands were mowed once or twite per year. Prior to 2015 these sites were typically mowed in early July around the time that A. viridis seedpods dehisce, but in 2015, and subsequent years, these sites were not mowed until after seedpod dehiscence when a majority of plants were in various stages of senescence. A second mowing, if it occurred, was during the dormant season.
During a 3 wk period between mid-June and the first week of July 2017, we collected A. viridis along a 45 m transect at each site. Within each 5 m section of the transect, we selected the closest four individuals for sampling, for a total of 36 individuals per transect, 72 individuals per mowing frequency, and 144 individuals total. We also counted the number of stems based on above ground separation, and recorded the number of these stems with reproductive structures (flowers or seedpods). In some cases reproductive parts were partially consumed by herbivores but were still visible. Individuals were excavated and the top 5 cm of the root was saved, comprising the root to shoot transition zone, which occurs just below the root crown where annual stems originate. This area of the root should contain all growth rings since germination. Roots were stored in glass jars containing 75% ethanol between the time they were excavated in the field and processed in the lab. We used a mini sledge microtome to section roots to about 20-30 [micro]m. We then stained sections with a 1:1 mix of Astra blue and Safranin (Schweingruber et al, 2011), which colors parenchyma cells blue and lignified cells red to enhance ring borders. Pictures of each stained section were taken under a Leica dissecting microscope. Resulting images were analyzed with ImageJ (Abramoff et al., 2004) to demarcate each ring and record their number.
We used an independent t-test to evaluate if mean age of A. viridis differed between roadsides and grasslands (alpha = 0.05). A posteriori, to test age related trends in number of stems and flowering stems between roadsides and grasslands, we created age classes containing a relatively even frequency of individuals (5-11 y old, 12-14 y old, and 15 and above y old) and applied a t-test using each age class separated by site type (roadside or grassland). No individuals younger than 5 y old were found in our study. All statistical analyses were performed using SPSS software (SPSS, Chicago, IL, U.S.A.).
Age distributions for both site types were normally distributed (Fig. 1) with no difference among means (t=0.918, DF= 142, P = 0.360). The mean age for the roadsides was 11.86 y (SD = 3.67) and the mean age for the grasslands was 12.46 y (SD = 4.35). The higher variance in age from the grasslands could reflect the presence of six individuals that were older than 20 y (maximum of 25 y old). The maximum age from the roadsides was one 20 y old individual.
The site types did not differ in mean number of stems per individual plant (t= 1.032, DF = 142, P = 0.304). The mean number of stems for the roadsides was 1.96 (SD = 1.518) and the mean number of stems for the grasslands was 2.24 (SD = 1.676). Number of stems was positively related to age for roadsides (r= 0.326, P = 0.04); there were more stems for individuals 5-11 y old and 12-14 y old, but not for 15 y or older (Fig. 2). The correlation between age and number of stems for the grasslands was not significant (r= 0.151, P = 0.219); between the youngest and oldest age classes there was only a 0.5 increase in mean number of stems (Fig. 2). The only significant difference between the two site types was between the 5-11 y classes for which the average number of stems was significantly higher in the grasslands than along the roadsides (t=2.536, DF = 58, P = 0.014; Fig. 2).
Mean number of flowering stems was significantly higher in the grasslands compared to the roadsides (t = 2.006, DF = 142, P = 0.047). The mean number of stems for the roadside sites was 1.29 (SD = 1.239) and the mean number of stems for the grassland sites was 1.81 (SD = 1.806). The number of flowering stems was also positively and significantly related to age at the grassland (r= 0.267, P = 0.046) and roadside sites (r= 0.405, P < 0.001). There was a strong linear increase in number of flowering stems between the 5-11 y old age class and the 12-14 class within each site type (Fig. 3). However, the increase tapered off for the grasslands between 12-14 and 15 y or older. The increase between age classes remained constant for 12-14 and 15 y older at the roadside sites (Fig. 3). There were more flowering stems per individual in the youngest age class in the grasslands compared to the roadsides (t = 2.389, DF = 58, P = 0.020; Fig. 3), with no differences between any other age classes.
Milkweeds serve a critical role in the life history of monarchs. In addition to serving as the sole host plant for immature monarchs, nectar from milkweeds also helps fuel migration, one of the longest migrations of any insect (Brower, 1995). Due to the documented decline in the availability of milkweeds in agricultural areas of the Midwest, there is an emphasis on increasing milkweed availability across other land uses. Therefore, identifying management practices that maximize milkweed stems and nectar availability in other land uses and throughout the landscape is needed to support monarch conservation efforts (Zalucki and Lammers, 2010; Oberhauser et al, 2017). Our research area of the southern Great Plains is important for providing milkweed to support spring and late summer breeding activity, as well as nectar resources for foraging along the migration route to and from central Mexico (Malcolm et al, 1992; Baum and Sharber, 2012; Baum and Mueller, 2015). Given that many perennial herbaceous species take several years to reach reproductive maturity (Noble and Slatyer, 1980; Pergl et al, 2006) and produce more biomass with age (Dietz and Ullmann, 1998), landscape level age structure of populations of A. viridis is an important consideration for maintaining and enhancing monarch habitat.
Contrary to our expectations, mowing regimes had no effect on age structure of A. viridis. In numerous studies focused on control of invasive species, repeated mowing of invaders while they are flowering is recognized as key to the successful reduction in plant longevity and population sizes (Benefield et al, 1999; Wilson and Clark, 2001). Some of the earliest studies in herb-chronology found a negative effect of prolonged mowing on mean age of herbaceous populations (Pergl et al, 2006; Dietz and Ullmann, 1998). Because A. viridis roots experience a seasonal low in starch as they are beginning to flower (Dee and Palmer, 2018), large losses in reproductive biomass could be detrimental to a plant's internal carbon resources over time (Bazzaz et al., 1987; Kozlowski, 1992). However, given the lack of effect of mowing on age structure, there may be a physiological mechanism that enables A. viridis to withstand frequent disturbance without increased mortality. Asclepias viridis does exist in an area that historically sees growing season fires (Howe, 1994; Baum and Sharber, 2012), which are part of the natural disturbance regime that maintains the prairie ecosystem (Samson el al, 2004).
Though age structure was not affected, we did find a negative effect of growing season mowing frequency on the number of total stems and flowering stems produced at the youngest age class of 5-11 y old. Although the average age of first flowering for A. viridis has not been documented, previous studies found a significant increase of flowering in individuals 5 y and older (Dee and Palmer, 2018). Therefore, it is possible sustained mowing during the peak reproductive season does not stress carbon sources enough to lead to mortality for A. viridis but does impact foliar and reproductive biomass of younger individuals. Similar studies evaluating age structures and reproductive capacity of invaders find sexual maturation later in life for populations in more disturbed areas (Hautekeete et al., 2002; Pergl et al, 2006). Given A. viridis peak flowering season in Oklahoma is during May and June, reducing mowing during this period could increase the number of flowering stems overall in the population by facilitating sexual maturation at earlier ages. In addition reduced spring mowing will provide milkweed for spring breeding monarchs and reduce mortality for eggs or larvae that are present (Flockhart et al., 2015; Inamine et al, 2016; Oberhauser et al, 2017).
In addition to the season of mowing, the amount of mowing individual plants experience on an annual basis is also important for the availability of carbon reserves, which may facilitate resprouting of fresh new growth after disturbance (Baum and Sharber, 2012; Dee et al., 2018) Reduced frequency of mowing, and mowing immediately after the May through June peak reproductive season should enhance milkweed resprouting probability, given the expected availability carbon reserves (Cruz et al., 2003; Pratt et al., 2014; Dee and Palmer, 2018; Dee et al, 2018). Enhanced resprouting from early to mid-summer mowing or other disturbance would support monarch breeding in the late summer/early fall in the southern Great Plains (Baum and Sharber, 2012; Baum and Mueller, 2015). Late summer and fall availability of milkweed nectar resources for adults may also be enhanced through resprouting (Dee and Palmer, 2018). Although monarchs are nectar generalists (Robertson, 1928; Tooker et al., 2002), milkweeds are considered important nectar sources for monarchs as well as other pollinators.
The timing of mowing that generates similar patterns to those observed for this study will shift earlier or later for other latitudes, tracking shifts in A. viridis phenology. Given the importance of A. syriaca throughout the monarch's summer breeding range (Malcolm and Brower, 1989; Malcolm et al., 1993; Zaya et al., 2017), similar studies to ours are needed to identify the timing and frequency of mowing to maximize stem production and flowering for this species, as well as other important species for monarchs. Implications for recruitment of new individuals to monarch plant host populations may differ among species, especially for rhizomatous versus nonrhizomatous species. Additional research is needed to assess how other mowing characteristics (mowing height, leaving or removing hay, etc.) and management practices influence the population structure, resprouting capability, leaf tissue nutritional quality, and nectar resource availability of milkweeds.
Acknowledgments.--We thank Michael Caballero for his assistance in excavation of individuals from the field and the subsequent aging of roots. We also thank the Oklahoma Department of Transportation and Oklahoma Stale University Facilities Management for allowing access to sites and providing information on mowing regimes.
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SUBMITED 12 NOVEMBER 2018
ACCEPTED 28 FEBRUARY 2019
JUSTIN R. DEE (1) Department of Plant Biology, Ecology, and Evolution, Oklahoma Slate University, Stillwater 74078
KRISTEN A. BAUM
Department of Integrative Biology, Oklahoma State University, Stillwater 74078.
(1) Corresponding author: E-mail: email@example.com
Caption: FIG. 1.--Histogram with age frequency and normal curves for Asrlepias viridis plants excavated from grassland and roadside sites
Caption: FIG. 2.--Mean number of stems by age class for Asclepias viridis plants excavated from grassland and roadside sites
Caption: FIG. 3.--Mean number of flowering stems by age class for Asclepias viridis plants excavated from grassland and roads
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|Author:||Dee, Justin R.; Baum, Kristen A.|
|Publication:||The American Midland Naturalist|
|Date:||Jul 1, 2019|
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