Selected paper: Defoliation and ENSO effects on vital rates of an understorey tropical rain forest palm
The palm family has a long history of use by humans for a wide diversity of purposes, including food, fibre, clothing and shelter. One example of the many uses to which palms are put concerns the small tropical understorey forest species Chamaedorea elegans, which grows in southern Mexico, Belize and Guatemala. Its leaves are harvested from natural populations and sold for use in the horticultural and floral trade. The same plants are often repeatedly used for harvests of leaves, so that this source of income for local people is completely dependent on the establishment of a sustainable harvesting regime.
Under normal circumstances, local experience can be used to identify a harvesting intensity that will both ensure survival of most of the plants in exploited populations and result in production of a sufficient and sustainable supply of new leaves. However, if climatic conditions stray far outside the usual limits, demographic behaviour may change, and the harvesting intensity that is usually applied, whether repeated or not, may place future harvests in jeopardy. Martínez-Ramos et al. (2009) have investigated this topic by undertaking an experiment involving the use of several different intensities of defoliation treatment on C. elegans plants across several years (1997-2000), one of which, in the middle of the experiment, happened to coincide with an ENSO (El Niño Southern Oscillation) event. Characteristically, there were unusually high temperatures and a severe shortage of water during the ENSO year. Plant performance and population vital rates (i.e. mortality, growth and reproduction) were measured in the years preceding and succeeding the ENSO year, and in the ENSO year itself.
The experiment demonstrated important interactions between defoliation, the climate in different years of the study, and availability of light. All these factors had strong effects on the behaviour of this understorey palm. Predictably, defoliation was associated with less frequent reproduction and lower investment in reproduction, but it had smaller effects on growth and survival. Importantly, plants in higher light conditions suffered a much greater level of mortality following the ENSO event, especially if they had not been defoliated. The authors interpreted this as a direct consequence of greater water shortage and higher temperatures in the high light patches, with the undefoliated plants that had greater leaf area and lower root mass:leaf area ratios being more vulnerable to drought. In the year of the ENSO event, growth and flowering both increased, but survival and seed production suffered compared with years with more normal conditions.
The experiment described in this study started prior to the ENSO event, but the authors made excellent opportunistic use of the occurrence of this striking natural phenomenon. They were able to make detailed and accurate measurements of the baseline demographic properties of the study species and of the changes in these properties that were caused by different intensities of defoliation. This may have been the original aim of their experiment, but as the ENSO event unfolded they were also able to examine and interpret changes in the species’ demography caused by two different types of disturbance – defoliation and extreme climate – acting both singly and in combination. Altogether, the study reveals much about the mechanisms underlying the demography of this exploited understorey species, and the likely impacts of departures from past climatic conditions upon its future behaviour. The study also suggests that traditional leaf harvesting regimes may well need to be modified to ensure that a resource that currently makes an important contribution to the local economy can still be exploited in a sustainable fashion under future climate scenarios.
Executive Editor, Journal of Ecology
Martínez-Ramos, M., Anten, N. P. R. & Ackerly, D. D. (2009). Defoliation and ENSO effects on vital rates of an understorey tropical rain forest palm. Journal of Ecology, 97, 1050-1061.
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