, Volume 159, Issue 2, pp 185-199

Crown illumination limits the population growth rate of a neotropical understorey palm (Geonoma macrostachys, Arecaceae)

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Abstract

The importance of light as a limiting factor for growth and reproductionin tropical understorey plants is well known, but the population-levelconsequences of light limitation remain little explored. Here, I usedemographicmodelling to examine if the effect of canopy gaps on individual-levelperformance in a tropical understorey palm translates into an effect on thepopulation growth rate. The demographic effects of heterogeneity in crownillumination (index of hemispherical canopy openness), forest-phase, and othermicroenvironmental parameters were quantified using data on 2592 individuals ofall sizes in permanent plots followed over a 1.5 year period. Supplementaryexperiments investigating the effects of canopy conditions on seedlingrecruitment were also conducted. Among the microenvironmental parameters onlycrown illumination and forest-phase had strong and consistent demographiceffects. Growth and fecundity generally increased with increasing crownillumination. Under open forest-phase conditions growth tended to increase andsurvival to decrease (due to increased physical damage). Seedlings transplantedinto gap centres were damaged by solarization. The population-level effect ofheterogeneity in crown illumination and forest-phase was investigated usingtransition matrix models. The overall population growth rate was 0.999, and notsignificantly different from 1. Illumination-specific models showed thatG. macrostachys would be unable to persist at permanentlylow illumination, but would increase in density under higher illumination.Forest-phase-specific models gave similar, but weaker results. Thus, inG. macrostachys effects of canopy openness onindividual-level performance strongly limit the population growth rate. Thepresent study thereby shows that fine-scale spatiotemporal variation in canopyopenness in the tropical rain forest understorey can play a central role in thepopulation ecology of shade-tolerant understorey plants.