Exploring Alternate Conceptions of Flowering Phenology With a Systems Model of Alfalfa Pollination

1. Exploring Alternate Conceptions of Flowering Phenology With a Systems Model of Alfalfa Pollination K. Strickler, Pollinator Paradise, 31140 Circle Dr., Parma, ID 83660www.pollinatorparadise.com Introduction Model Design A model of alfalfa seed production (1,2) can be used to explore the implications of alternative underlying assumptions about the dynamics of pollination and seed set. The model was designed for alfalfa pollination, but is also relevant to other flowering plants with indeterminate growth. Question: How does the timing of foraging by leafcutting bees relative to initiation of bloom impact flowering phenology and seed yield over a range of bee population sizes? In the first version of the model, the rate of bud opening is rapid initially and declines exponentially over time. In the second version of the model, rate of bud opening is rapid initially, but it declines exponentially in response to the accumulation of seed pods(1,2). Model Flow Chart - Uses the software program Stella from iseesystems.com. For details, explore the model demo. Many authors treat flowering phenology as a fixed input into the pollination system. In the systems model presented here, flowering phenology (Open Flowers) is an accumulation, an output that is the net result of changes in the rate at which buds open (buds becoming flowers), the rate of flower pollination (visiting flowers, also known as “tripping” in alfalfa), and the rate at which unpollinated flowers wilt (wilting). Bud Opening Declines Exponentially As Pods Increase Bud Opening Declines Exponentially Over Time What is Flowering Phenology? Strickler, unpublished data for alfalfa plants in a greenhouse The pattern of number of open flowers blooming over time. For example, many plants show a right skewed flowering distribution: flowering begins rapidly and tails off slowly [3]. The graph of exponential decline in the number of buds opening over time. The pollination model omits a connector between Mature_Pods and buds_becoming_flowers The graph of exponential decline in the number of buds opening as mature pods increase. The pollination model includes a connector between Mature_Pods and buds_becoming_flowers Results Bud Opening Declines Exponentially Over Time Bud Opening Declines Exponentially As Pods Increase Blue curves: No bees present Red curves: Many bees present, bee foraging synchronized with bloom initiation. Consider two possible assumptions about flowering phenology:The pattern of bloom is a fixed genetic response to abiotic factors such as temperature and day length, independent of fruit set. The pattern of bloom is dependent on the pollinator population because bloom shuts down as fruit or seeds mature. Flowering Phenology Alfalfa Production • Bee boards in a shelter in an alfalfa seed field in Idaho provide nests for 100,000 female alfalfa leafcutting bees (Megachile rotundata) or more. Bees in the shelter pollinate a hectare of alfalfa. At peak bloom the alfalfa field has a purple cast and a sweet floral scent from numerous open flowers. Bloom declines even in the absence of bees. Bloom remains high in the absence of bees In both cases: Right-skewed bloom curve; peak bloom is lower and pollination is completed sooner when bee foraging is synchronized with bloom than when foraging is delayed. Pink curves: Many bee present, bee foraging delayed relative to initiation of bloom Cumulative Seed Yield Alfalfa is grown for seed production in the Pacific Northwest USA. The alfalfa leafcutting bee, Megachile rotundata F. is managed for alfalfa pollination. Tripped (pollinated) flowers are easy to recognize by the raised anthers and style against the upper petal. The amount of bloom declines over time, and the field takes on a brown cast as seed pods mature. Seed yield by the end of the season is greater when bee foraging is synchronized with initiation of bloom compared with delayed foraging. Seed yield by the end of the season is the same whether foraging is synchronized with initiation of bloom or delayed. Conclusions If pollination is delayed relative to the start of bloom, seed set is reduced when bud opening is independent of fruit set.  However, if bud opening delayed by fruit set, then seed set is delayed but not reduced (everything else being equal) by delayed pollination.  To determine the impact of the pollinator population and timing on seed yield, it is not enough to monitor bloom and pollinator population size over a season.  Rather, it is important to determine how the rate of bloom varies in the absence of pollination.  More generally, the faster the rate of bloom when pollination begins, the higher the seed yield. Alfalfa Flowering Phenology Bloom in alfalfa fields typically follows a right-skewed curve, reaching an early peak, followed by a slower decline (5, 6). Peak nesting activity of the bees was delayed by two or three weeks relative to peak bloom, so that most bee nesting occurred under conditions of intense competition for flower resources. Strickler and Freitas, 1999 Seasonal change in flowers around 8 alfalfa leafcutter bee shelters. Each symbol represents actual values for a given bee shelter. Lines were fit with the parameter estimates to an exponential function. • Left: Number of total racemes with open flowers per 3 m transect increase during the 1st wk after bee release, then decrease exponentially • Right: Open flowers per raceme decline more rapidly (Fig. 1C) close to bee shelters than at a distance from the shelters. Explore the Pollination Model Yourself Instructions: OPEN A MODEL When the computer is available and the slide show is running, wait for the appropriate slide, and then use the mouse to click on the button next to the model that you wish to explore. The Model interface will look like the figure. FOLLOW THE INSTRUCTIONS PROVIDED – Click on the “Instructions” button. “Unfurl” the model to examine its underlying structure, Run a base-line version, Examine how the model parameters change over time. Vary the model parameters to determine the impact on flower and bee populations. “The development and maturing of new flower racemes shuts down quickly as flowers are pollinated. … A more rapid decline in open flowers per raceme close to bee shelters than at a distance from the shelters is consistent with this interpretation. “ Strickler and Freitas, 1999 Bosch and Kemp, 2005 Flower standing crop (untripped flowers), percentage of pollinated flowers (tripped flowers), percentage of cavities with M. rotundata (nesting females), and percentage of cavities with newly plugged nests (plugged nests) in shelters. What assumptions underlie the conclusions about flowering phenology in these studies? References 1 Strickler, K. (1996) J. Kansas Entomol. Soc. 69, 201-215. 2 Strickler, K. (1997) In, K.W. Richards, ed. Proc. Int’l Symp. on Pollination Acta Horticulturae 437, ISHS pp. 109-113. 3 Thomson, J.D. (1980) Ecology 61:572-79 4 Rathcke, B. and E.P. Lacey (1985) Ann. Rev. Ecol. Syst. 16:179-214. 5 Strickler, K., and S. Freitas, (1999) Environ. Entomol. 28, 178-187. 6 Bosch, J and W. Kemp (2005) J. Econ. Entomol. 98:1077-1086. “pollination could have been accomplished on younger flowers, with smaller bee populations whose nesting was better timed with peak bloom”. Bosch and Kemp, 2005