Size‐dependent flowering and costs of reproduction affect population dynamics in a tuberous perennial woodland orchid

Abstract

Summary1. In woodland herbs, the probability of flowering and costs associated with reproduction may strongly depend on environmental context (shade vs. light habitats) and on plant size. This may be particularly true for tuberous orchids that inhabit woodlands, as the amount of incoming radiation and total leaf area strongly determine photosynthetic capacity and hence the amount of carbohydrates that can be relocated to below-ground storage organs that form next year's rosette and flowering stalk. 2. To fully comprehend the impact of size-dependent reproduction on population dynamics under varying light conditions, life cycle models should therefore include plant size in a continuous manner. In this study, annual changes in plant size and demographic behaviour of the tuberous perennial orchid Orchis purpurea were monitored during seven consecutive years (2003)(2004)(2005)(2006)(2007)(2008)(2009)) in open and shaded woodland. Integral projection models (IPMs) and life table response experiments (LTRE) were used to investigate the extent to which variation in plant size affected the overall population dynamics of this species and to decompose differences in population growth rates between populations of open and shaded woodland into contributions from growth, survival and reproduction. 3. Both plants in shaded and light environments needed to be a certain size to initiate flowering, but this threshold size was almost three times as large in shaded environments as in light environments. Plants in open woodlands flowered more frequently over the years, showed less size regression after flowering and produced significantly more fruits than plants in shaded environments, resulting in significantly larger population growth rates. 4. Our life cycle models revealed that costs of reproduction, measured at the population-level, were small in the light environment, and more than buffered by the increase in survival of flowering plants compared to non-flowering plants. In the shade environment, however, the costs of reproduction were significant and made the difference between a stable (current) and a growing (without reproduction costs) population. 5. Synthesis. Light penetration to the soil is a key variable determining population dynamics of woodland orchids. Our analyses show that differences in vital rates related to size-dependent reproduction (flowering) and growth are essential drivers of changes in orchid population dynamics in different light environments. The combination of IPMs and LTREs thus proves to be very useful when studying the fitness consequences of size-and context-dependent phenomena like flowering strategies and costs of reproduction.

Evolutionary demography of iteroparous plants: incorporating non-lethal costs of reproduction into integral projection models

“…The asymptotic population growth rates (l) were 1.091 in the light habitat and 1.002 in the shade habitat, in agreement with previous results from these populations [40]. The predicted reproductive delay in the shade environment was driven entirely by the growth cost of flowering, as this was the only cost we detected and included in the IPM.…”

Impact of herbivory on flowering behaviour and life history trade-offs in a polycarpic herb: a 10-year experiment

“…death (e-f) in two natural populations that were exposed to a grazing treatment and a control mowing treatment from 1999 to 2008. Data for both study populations are pooled per disturbance regime Such a relationship has previously been documented in several short-lived monocarpic species and some longlived polycarpic plants (see Bonser and Aarssen 2009;Weiner et al 2009;Jacquemyn et al 2010). However, flowering plants in grazed plots were significantly smaller than flowering plants in control plots, indicating that this grazing-mediated reduction in plant size was associated with a significant decrease in the vegetative size needed to realise investment in flowering.…”

“…Méndez and Karlsson (2004) documented differences in vegetative plant size at flowering among several Pinguicula vulgaris populations, which depended on the altitudes and soil moisture contents of these populations. In the woodland orchid Orchis purpurea, Jacquemyn et al (2010) also found significantly smaller threshold sizes for flowering in light environments. Although our data showed that P. veris was able to flower at significantly smaller sizes when herbivores were present, plants produced a significantly smaller number of flowers when they were grazed.…”

Inter-annual variability in flowering of orchids: lessons learned from 8 years of monitoring in a Mediterranean region of France

“…We attribute this result to the disparity of microclimate and light availability between habitats. Higher light availability can increase probability of flowering (Diez et al 2007;Jacquemyn et al 2010). Stable microclimatic conditions, e.g.…”