The Effects of Neighbors on the Demography of a Dominant Desert Shrub (Ambrosia dumosa)

Abstract

Abstract. We used patch-specific matrix models to test the influence of neighboring plants on the demography of Ambrosia dumosa, a dominant perennial shrub of the Colorado Desert in southern California. In the desert literature, the presence or absence of neighboring plants is reported to influence plant growth and survival and has long been associated with plant-plant interactions that range from extreme competition to facilitation. Here we consider the less addressed question of how neighbor-defined population subsets contribute differently to overall population dynamics.Demographic data collected from Ͼ6000 individual plants from a permanently mapped hectare over 10 yr were used to divide this A. dumosa population into subsets that were defined by the presence or absence of neighbors. By partitioning the population in this way, we documented differences in population growth, elasticity structure, and stable size structure among the distinct patch types, and evaluated the contributions of each patch type to overall population dynamics. Population growth was consistently higher for population subsets of plants that were isolated throughout their ontogeny, compared to those with close neighbors throughout their ontogeny. Further, overall population growth was proportionally more sensitive to perturbations involving isolated adults, despite the projected persistence of plants with close neighbors. In short, changes in survival of isolated adults had a greater influence on population growth than changes in survival of adults with neighbors.We used life table response experiments (LTREs) to test for spatiotemporal effects of neighbors. The LTRE was consistent with the elasticity analyses in showing that dynamics among adult members made the greatest contribution to the observed differences in population growth among the neighbor-defined population subsets during each census period. Neighbor effects among adults were greater and more important than temporal variation in drought levels in decreasing population growth relative to a pooled 10-yr reference matrix that ignored neighbor effects. Although neighbor effects contributed greatly to differences in population growth among the models projected, the LTRE revealed that, relative to the reference matrix, transitions among juvenile size classes decreased population growth for the predominantly drought free 1984-1989 census interval and increased population growth for the 1989-1994 interval, which included four winters of extreme drought. We hypothesize that higher mortality during periods of high rainfall was due to increased competition among plants, especially in favor of adults at the expense of juveniles. The drought period was also characterized by increased recruitment of new adults, probably reflecting diminished competition from adults for well-established juveniles capable of growing into reproductive condition after elimination of smaller juveniles during 1984-1989. Our habitat-specific partitioning of this population revealed dramatic differences in the ...

The plot thickens: does low density affect visitation and reproductive success in a perennial herb, and are these effects altered in the presence of a co-flowering species?

“…In addition, factors that affect individual fitness may not alter population growth rates. In long-lived perennial plants, increases in fecundity tend to have little effect on population growth rate (Silvertown et al 1993), but may increase the contributions of these individual plants to the population growth rate relative to other individuals (Miriti et al 2001). In these long-lived species, facilitative or competitive effects on growth rates or seedling recruitment may have larger effects on the growth rate of the population than any effects of co-flowering species on pollination.…”

Population dynamics and clonal growth of Spartocytisus supranubius (Fabaceae), a dominant shrub in the alpine zone of Tenerife, Canary Islands

“…Dendroclimatological analysis shows that the growth of adult Spartocytisus is controlled mainly by winter precipitation (T. Kyncl unpublished data) and it likely affects seedling recruitment as well. This phenomenon is common for shrubs in dry climates (Watson et al 1997;Miriti et al 2001). While winters with precipitation below 300 mm are common in the caldera (quartiles being 266 and 555), winter precipitation could exceed 1000 mm in some years.…”

“…Still, population dynamics of shrubs are much less known than those of two other major growth forms, herbaceous plants and trees (but see Watson et al 1997;Hoffmann 1999;Wiegand et al 2000;Miriti et al 2001;Lantz and Antos 2002). Shrubs are usually too long-lived to be studied by common demographic techniques used with herbaceous plants.…”

Analysis of interspecific competition in perennial plants using Life Table Response Experiments

“…The magnitude of the contribution made by a particular vital rate depends upon both its observed variation and the effect of this variation on population growth rate. LTRE analyses have been widely used for a variety of purposes in the last few years, both in fixed designs where one or more factors are manipulated, or in random designs where demographic parameters are analysed in natural unmanipulated conditions over time or space ͑Horvitz et al 1997; Guàrdia et al 2000;Miriti et al 2001;Kiviniemi 2002͒. Fixed designs have been used in animal populations to investigate the demographic consequences of pollutant exposure ͑Levin et al 1996; Hansen et al 1999͒, food manipulation ͑Walls et al 1991Dobson and Oli 2001͒ …”