SPATIOTEMPORAL VARIATION IN DEMOGRAPHIC TRANSITIONS OF A TROPICAL UNDERSTORY HERB - PROJECTION MATRIX ANALYSIS

Our goal was to elucidate the population dynamics of the perennial und er- story herb Calathea ovandensis in a rain forest in southern Mexico using matrix projection model analysis. We emphasize the magnitude an d consequences of spatiotemporal variation in (1) basic demographic pa rameters (growth, survival, and reproduction) (2) asymptotic demograph ic properties of a given environment (the asymptotic population growth rate and the associated stable-stage distribution and reproductive va lues) and (3) demographic sensitivities associated with a given enviro nment (sensitivity and elasticity). We obtained 6 yr (1982-1987) of em pirical data from four study plots (differing in substrate, light, and density) from which we used the first 5 yr (1982-1986) to construct 1 6 plot-year and 1 pooled population projection matrices. This stage-st ructured population was characterized by a long-lived seed bank, tempo rally variable seedling recruitment (10-fold variability among years), high mortality of seedlings (>90%), very low mortality of reproductiv es (usually <10%), fertility that increased markedly with plant size, and the ability of large plants to shrink rather than die under advers ity. Within these broad outlines, the magnitudes of transitions repres enting demographic fates exhibited considerable variation through spac e and time, some parameters varying much more than others (CV from 22 to 400%). Growth and reproduction were positively correlated across en vironments. The least variable parameters were seed dormancy and stasi s of small reproductives. Observed stage distributions were reasonably close to the stable stage distributions (mean = 86.1% similar). In mo st plot-years, the stable-stage distribution was dominated by seeds, f ollowed by seedlings, and then small reproductives and the reproductiv e values increased with size class. Population growth rates, given by the dominant eigenvalue of the matrices, ranged from 0.73 to 1.25. Ana lysis of the mean dynamics gave lambda = 0.97 (using a variety of anal ytical approaches) and our analysis of the overall pooled dynamics gav e a lambda = 0.99, indicating that the habitat at the study site favor ed the persistence of Calathea ovandensis. An el Nino event coincided with the year of the highest population growth rate. Survival, growth, and reproduction varied significantly through space and time, and dif ferent plot-years were beneficial to different stages. Most interestin gly, stage-specific sensitivity parameters (sensitivity and elasticity ) also varied through space and time. Spatiotemporal variability of se nsitivity structure has important implications. Determination of stage s most ''critical'' to population dynamics will depend upon knowledge of this variation. Population growth rate was significantly positively correlated with elasticity of seed production, seed germination, and seedling growth. These results indicate that the opportunity for selec tion on plant characters affecting particular life history stages vari es through space and time even if the effect on the single-stage trans ition probability does not vary. Selection on characters affecting juv enile stages may be stronger in populations of higher growth rates.