Big plants make the microclimates of smaller plants

The plants that live on the forest floor—at low light levels, milder temperatures and higher humidity—are specialized to a microclimate made for them by the canopy trees that absorb most of the sunlight. Their photosynthetic chemistry is specialized to low light levels and they cannot cope with direct sunlight. These forest floor plants tend to have soft leaves, because leaves underneath the canopy have no need to be "tough"—they are not blown about by the wind, nor are they dehydrated in direct sunlight. An example of one of these forest floor plants is the African violet (Saintpaulia), a common house plant which requires shade. As many houseplant owners know all too well, it dies quickly when exposed to direct sunshine.

Some forest floor plants have peculiar adaptations to help them gather as much as possible of the light that falls upon them. Certain herbaceous plants—such as the South-East Asian vine spike moss (Selaginella willdenowii) and some species of Begonia (Figure 4.7*)—have a bluish sheen (known as iridescence) to their leaves. This is caused by little silica beads within the epidermis of the leaf. Experiments have suggested that these beads help the leaf to focus in light from a range of directions, sending it straight into the photosynthetic cells below. In Selaginella each cell underneath a silica bead has a single large chloroplast which seems to be precisely located to receive this focused beam of light.

The leaves at the top of a tree also make the microclimate for the leaves below them. Even on the same tree, leaves that are out in full sunlight develop slightly differently from those in the shaded branches down below. The "sun leaves'' are thicker with more layers of photosynthetic cells packed in, to take advantage of the abundant light. The lacquer-like cuticle on the upper surface of a sun leaf also tends

Figure 4.7. This species of Begonia lives in the understory of mountain rainforests in SouthEast Asia. The bluish metallic "sheen" of many species of rainforest understory plants is thought to come from the refractive effect of silica beads which help to gather in light for the leaves. Source: Author.

Figure 4.7. This species of Begonia lives in the understory of mountain rainforests in SouthEast Asia. The bluish metallic "sheen" of many species of rainforest understory plants is thought to come from the refractive effect of silica beads which help to gather in light for the leaves. Source: Author.

to be thicker, to help reduce unnecessary evaporation. On a sun leaf there are more stomata—the pores which open to let CO2 in—so that the leaf can take advantage of high light levels to bring in more CO2 for photosynthesis when it has enough water. As soon as evaporation through the stomata becomes too intense and the leaf is in danger of dehydrating, the stomata are clamped shut and the leaf relies on its cuticle to prevent further water loss.

The chemistry and color of sun leaves also tends to be different from shade leaves. Shade leaves tend to be a darker green because they are richer in a particular dark green form of chlorophyll (chlorophyll b) that is good at harvesting light at low intensities and at the wavelengths that get past the filtering of leaves above. Sun leaves have more of the chlorophyll a form which exploits high light intensities more effectively. The upper epidermis of sun leaves is also packed with natural sunscreen compounds such as flavenoids which absorb most UV light and prevent it from damaging the sensitive photosynthetic cells below. Just putting a shade-grown tree seedling out into direct sunlight shows how important this protection is: in a few days the shade-grown leaves are bleached and useless.

Tree seedlings often survive for years in the shaded forest floor environment. Depending on how much light they are getting, they may either stay more or less the same size, or slowly grow up into the canopy. These young trees from the forest floor can go through different phases in their life, with physiological adaptations to different light levels. For example, many of Australia's Eucalyptus trees have an "early" phase with an entirely different leaf form, suited to growing at low light levels within the darker forest interior. Typically, the juvenile leaves of such eucalypts form disk-like pairs with the stem in the center, while the adult leaves are long and strap-shaped. It is thought that the disk-like leaves—arranged along the stem like a kebab—are good for harvesting light coming down from a small gap in the canopy above; it helps to keep the photosynthetic area "all lined up'' within a shaft of sunlight as the seedling grows its stem up to follow the light.

Often a young tree on the forest floor will only really be able to start growing fast when a bigger tree—or a large branch—falls from the canopy to give a patch of sunlight that illuminates its leaves. This is the turning point that gives the young tree enough energy to fix enough carbon to lay down wood and grow tall, rather than merely surviving.

The subtle range of opportunities provided by microclimates is thought to help maintain the species diversity of forests and other plant communities. A tropical rainforest can have more than two hundred species of trees packed into a hectare, and in ecological terms it is difficult to explain how they can all manage to exist side-by-side. Simple ecological theory suggests that eventually just one species that can compete more effectively should increase its numbers and push the rest out, so that it dominates the forest. Yet, obviously this does not happen. Ecologists suspect that part of the reason such exclusion of species does not happen is that small differences in light level, as well as soil texture and nutrient levels, determine which tree species gets established in any particular spot on the forest floor. It is thought that for trees in particular a critical stage which determines whether a species grows in any one spot is its early growth as a seedling and small sapling. Each species might be adapted when it is a seedling to a narrow range of light intensities, or light of particular wavelengths or angles. If it finds itself in its forte, it will out-compete seedlings of other species. Once that critical seedling stage is passed and the tree has established itself, it is essentially guaranteed a place in the forest. Although this is quite a compelling theory, there is still only limited evidence that this sort of specialization on the forest floor is important in the competition and survival of forest trees.

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