Population Structure

Plants constituting a population exhibit sizes that are generally far from uniform (Figure 4). The differential effect of environmental factors on plants through time and the specific effect of the spatial arrangement of plants on their performance ultimately determine the structure of a plant population. Given that plant size is a good predictor of plant performance in terms of growth, survivorship, and fecundity, population structure can provide insights into the population dynamics. For example, populations with high recruitment rates and a high population growth rate are also characterized by a high proportion of small young individuals. By contrast, declining populations are usually composed by large old plants. Furthermore, for a given plant species, population structure can also indicate the quality of the site (e.g., given by resource availability) where a plant population occurs. Hence, plant populations in good sites tend to show higher recruitment rates, and therefore a higher proportion of small young plants, than sites that became poorer and where populations are not performing so well.

Although plants may exhibit symptoms of senescence (e.g., lower growth rates, lower fecundity values, etc.), the relationship between age and size or other components of

Figure 4 Size (diameter at breast height) distributions of two tree species, holm oak (Quercus ilex) and Aleppo pine (Pinus halepensis), in a Mediterranean mixed forest. The size distribution of Aleppo pine is characteristic of a shade-intolerant species without regeneration in the understory. The distribution of holm oak corresponds to a shade-tolerant species, with many small individuals surviving in the understory of the forest.

Diameter (cm)

Figure 4 Size (diameter at breast height) distributions of two tree species, holm oak (Quercus ilex) and Aleppo pine (Pinus halepensis), in a Mediterranean mixed forest. The size distribution of Aleppo pine is characteristic of a shade-intolerant species without regeneration in the understory. The distribution of holm oak corresponds to a shade-tolerant species, with many small individuals surviving in the understory of the forest.

plant performance is usually inaccurate. In fact, age distributions of plant populations provide valuable insights into population history (e.g., abundance of different cohorts or the pattern of past effective recruitment in the population), but say little about the overall demographic behavior of the population. For this reason, size and age distributions of plants populations are poorly correlated and trying to infer one from the other can be misleading.

Finally, the genetic structure of plant populations also deserves some attention. The genetic structure is given by the proportion of the different genotypes present in the population. Among the several factors that can determine the genetic structure of plant populations, the proportion of self- and cross-pollinations occurring in the population and the mating system of plants (i.e., self-compatibility or self-incompatibility) are two of the most important. The genetic variability of populations also plays a role in determining overall population dynamics. Populations with reduced genetic variability often show lower performance and lower population growth rates. Different genetic factors, such as the expression of inbreeding depression or genetic drift, may account for this result.

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