science

On July 8 a small crowd turned out at the Tupper Auditorium to hear biologist Egbert Leigh talk about heirarchies and productivity limits in very different ecological niches. It seems that hard and fast rules are hard to come by.

In the tropical rainforest, the plants grow in layers. Some don’t get enough light to reproduce until the shading trees die and fall. The whole arrangement has given plants an evolutionary choice, which Leigh describes as “keeping up with the Joneses in the race for the canopy, or adjusting to various levels of shade.” The biomechanical advantage goes to the tall, at least up to the point when they become too top-heavy for their roots and are easily felled by storms.

The challenges and options are quite different for marine plants along Washington’s Olympic Peninsula. “Where the waves beat hard, starfish have a hard time being active.” Mussels spread down the intertidal zone “like a glacier,” until storms wash them away patches at a time, then sea palms usually move in. The plants don’t jostle one another to grow closer to the light, but have adapted themselves to resist the pounding waves. Hollow stalks that bend rather than break are the biomechanical advantage here.

Kelp fronds “get a lot more chance at the light than leaves in a rainforest,” Leigh noted, adding that the wave action creates a flashing aspect to the marine plants’ light exposure that enhances photosynthesis. The kelps thus have high productivity, but then whole stands tend to get replaced by less productive but more aggressive species. It’s a succession of single or limited species patches. In this setting equality of access to light doesn’t encourage either productivity or species diversity.

In the forest setting, on the other hand, access to light isn’t equal and diversity and productivity are both favored. Some plants, however, defend themselves not by producing more leaves than the insects or other predators can eat, but by chemicals that make their foliage unpalatable to grazers big or small.

At the forest’s grassland edge, things are different again. “There are very different rules of competition” between forests and grasslands, Leigh pointed out. Grassland plants produce more biomass to sustain the herbivores that eat the competition. Forest plants defend themselves against herbivores, and how well they do that is a key to how competitive they are with other plants.

So what general trend does Leigh see in these different pictures? In the plant world, he says, “the abolition of heirarchy plays a big role in higher productivity.” Think about that when you’re weeding your garden.

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