The Elements of Evolution (51-3-2) Plant Life-History Variables continued


More than 1,000 tree species may be in a small area of a tropical rainforest. While competition is inevitable, the trees must get along with their neighbors to some degree.

Wood density and leaf morphology are exemplary traits that influence a tree’s capacity to compete. There are trade-offs. Lighter wood trees can grow more quickly than those with dense wood, but the lightweights tend to die sooner and are poor competitors. Trees with dense tissues are more imposing on their neighbors.

While the density of forest diversity is not a fully understood dynamic, it is more than avoiding being different from the neighbors in resource use and life-history strategy. Certain traits are more advantageous at the various stages of forest succession regardless of whether they are different from nearby trees. In young forests, where trees are more spread out and there is little competition, fast-growing trees have an advantage. As a forest matures and the neighborhood becomes more crowded, slow growers do better in getting the resources they need: minerals, water, and light.

Larger leaves can be helpful in gathering light and thereby garnering energy but trees that can readily vary leaf size based upon prevailing conditions are more competitive.

Certain life-history selections preclude adaptations to other biomes. Grasses’ adjustment to relative aridity precludes their appearance in rain forests. Geophytes, with their underground storage organs, shun Arctic heathlands, as long-frozen soil is too much to bear.

Secondary metabolites are a plant life-history variable, both in compound selection and deployment. These are driven not only by ecology interactions with microbes, other plants, and animals, but also by the ready availability of ingredients and the economy of biosynthesis.

Unlike animals, being stuck in one spot for a lifetime creates a fundamentally different scenario by which life-history facets are favored. For plants, seasonal light, air (e.g., moisture, temperature), and substrate (e.g., soil) conditions are primary drivers.

Climate determines the seasonal growth patterns of annuals. The customary practice of blooming in late spring from seeds dormant through the winter is belied if summer is unwelcoming. In habitats with moist winters and dry summers, many annuals germinate in autumn, producing a rosette of overwintering leaves. Rapid growth starts early in the year, with flowering in early spring, and ripened seeds before summer scorching.

While winter annuals avoid the attentions of most invertebrate herbivores, they are especially vulnerable to vertebrates that forage throughout the winter. Young, green foliage at a time when many other species are largely leafless is irresistible.

As with animals, allometric relationships indicate constraints which determine which plant architectures are possible and those that are likely to be successful. Such trade-offs are as much ecological as they are internal.

A host of plant traits conform to the neighborhood in which plants live and how plants conduct their lives. Interspecific ecology is a major influence on plant life-history variables.

The life-history spectrum of perennials presents different strategies. Both trees and herbaceous perennials maintain tenure of their plot of land for many years. The most obvious lifestyle distinction between the two is their occupancy of the aerial environment. Trees maintain their presence year-round while the shoots of renascent herbs die back every growing season.

Unlike trees, herbs must reestablish each year their position in the canopy of plants competing for access to light. This is offset by lessened vulnerability to drought, cold, or pests during the inclement season, though herbs must have various defensive measures in place. Herbs being spared the considerable investment in wood is a mixed blessing.

Herbs are also more mobile than trees: able to spread radially via rhizomes, stolons or rooting shoots. This is something of a hedging stratagem for having a less commanding presence than trees.

Obtaining sufficient sunlight is the great challenge for shrubs and trees. Reproduction is delayed until a homestead can be established. Trees unable to obtain a place in the canopy die without progeny.

The race for a place is the sun creates options for various growth regimes, particularly branching patterns and shoot rates. Regardless of the chosen pattern, a ubiquitous mitigation is to cut losses by culling branches that do not prosper.

Conversely, trees tend to grow only as tall as necessary to compete and meet reproductive goals. Growth is a series of decision-laden stages.

Above all, both above ground and below plants are opportunistic. The overarching life-history strategy of plants is to optimize the possibilities for success in ways that have no analogues with animals.

Phenotypic and growth pattern variability in plants exceeds that of animals. This may seem odd, as plants are sessile. But heterotrophy is an overwhelming burden as a life-history variable, dictating motility. There is no need for ambulation if sustenance can be manufactured on the spot.

From a life-history perspective, autotrophy spells freedom. The flexibility afforded by photosynthesis has yielded tremendous diversity, as creative design is relatively unconstrained. Overall, plant life-history variables are selected from countless compromises, and so are not easily characterized.