Desert Plant Adaptations (IV) ~ Leaf Adaptations

Getting back to my series on desert plant adaptations, let’s discuss some of the ways that leaves have adapted to help plants survive the hot and dry conditions of the desert wadis.

Besides dropping all of their leaves like drought-deciduous plants, some plants simply have smaller or fewer leaves. Smaller leaves means there is less surface area open to the wind and sun, and so less water is lost through transpiration. Semi-shrubs and shrubs may have large leaves in the winter that are shed during the dry season and are replaced with smaller leaves in the summer. Artemisia herba-alba and Phlomis aurea both have smaller summer leaves.

Another strategy that helps conserve water is folding or rolling up the leaves so that the stomata are facing inwards, reducing the surface area subject to transpiration. This adaptation is employed by Helianthemum species, Fumana thymifolia, and many perennial grasses.

But it’s not just transpiration that leads to water loss. Leaves also lose water through their cell walls. To combat this, the leaves of some desert plants have a waxy coating or a thick cuticle, the film covering the surface, which helps seal in and protect moisture already in the leaves. Most xerophytes have a thick cuticle.

Hair on the leaves and stems also helps reduce water loss by providing shade and trapping water vapor near the plant’s surface. Artemisia herba-alba, Phlomis spp., Majorana syriaca, and other plants utilize this adaptation.

Plants needs energy from the sun to photosynthesize. What energy they don’t use to make food is used to heat the leaf up. That is very useful for plants growing in cold climates whose leaves need to be a bit warmer to photosynthesize but not so necessary for desert plants, which must reflect some of this solar radiation or risk over-heating. How do they do this? Some plants, like Capparis sinaica, have a waxy coating on their summer leaves which gives them a lighter – and more reflective – color than their winter leaves.

Leaves of Atriplex species also have a different color depending on the season, but they don’t use a waxy coat. Instead, the leaves are covered in vesicular hairs that contain a salty solution. In the winter, the hairs are full and transparent, allowing more absorption of the sun’s energy. In the summer, the water evaporates and the hairs dry out causing the leaves to be a lighter color that reflects the sun’s rays.

You can read more about each of the species photographed above in my book. In the next post in this series, we’ll learn about stem adaptations. Stay tuned!

Desert Plant Adaptations (III) ~ Drought-escaping Plants

I’ve written about two general strategies that plants employ to survive in desert habitats – succulence and drought-deciduousness. Today I’ll introduce the most successful adapters to life in the desert – drought-escaping plants. Plants escape drought in one of two ways. They either survive only as seeds or they use taproots.

Annuals are plants that wither and die during the dry seasons but not before completing their life cycle, sometimes in just a few weeks. Their seeds are covered by a thick protective coat and are dispersed, only to wait underground as part of the desert’s “seed bank”. Contained within their seed coats are certain chemicals that prohibit germination. The seeds must wait for rain to wash these chemicals away before they can sprout. These are the plants that amaze us here in Sinai when our desert sands are suddenly covered in green after a spring rainstorm. These are also the plants that are so important to the Bedouins’ herds of goats and sheep. Some of these annuals that survive dry seasons as seeds include Astragalus spp., Artemisia spp., Diplotaxis spp., and Tribulus spp. and are pictured below.

Using taproots is also a successful strategy to avoid the problems associated with drought. A taproot is a very thick and long root that grows directly downward. Think of a carrot. The taproot is the main central root which other smaller roots grow off of. The long taproot allows the plant to reach water stored deep underground, providing an almost constant source of water. The taproots of Convolvulus lanatus, pictured below, and Artemisia monosperma are also covered with a thick bark that helps the roots to withstand the withering caused by wind.

You can learn more about these desert plants in my book, Wandering through Wadis: A nature-lover’s guide to the flora of South Sinai.

Desert Plant Adaptations (II) ~ Drought-deciduous Plants

It’s been longer than I had planned, and I’m not sure where the time went (well, a lot of it went to waiting for a new computer when my old one bit the dust), but I’m finally back with more information about how the plants in South Sinai have adapted to survive the harsh desert conditions! In the first installment of this series, I talked about succulence, the ability of plants to store water in their leaves, stems, and/or roots.

Lycium shawii, Desert Thorn

Today, I’ll discuss drought-deciduous plants. Instead of storing water like succulents, these plants use a different method to deal with drought conditions. During the dry seasons, they drop their leaves, allowing them to save water that would otherwise be lost during transpiration (similar to sweating in humans).

Because these plants don’t have their leaves to make food in the summer, their stems take over the photosynthesis process. (More on this in a future post.) Drought-deciduous plants also slow down their metabolism, making only enough energy to keep the plant alive. This means when conditions for growth are favorable again, the plants don’t have to start from zero. They’re already idling and ready to go, if we think in car terms; they don’t have to turn the ignition on first.

Lycium shawii, or Desert Thorn, is one example of a plant that is completely leafless in the summer. (It also has succulent leaves, so more than one adaptation to help it survive.) And because I don’t hike in the summertime, I don’t have photographs of this plant without leaves. But here’s a pic of their beautiful lush leaves:

In my next post, I’ll discuss the most successful adapters to life in the desert – the drought-escaping plants – so be sure to follow the blog or the Facebook page so you don’t miss out!

Desert Plant Adaptations (I) ~ Succulence

Plants that have adapted to living in dry habitats are called xerophytes, and they are the characteristic plants of deserts and semi-deserts. Over the next few weeks, I’ll be explaining some of the adaptations that allow them to survive in such a harsh desert environment. And it is harsh! Besides the limited supply of water, these plants must also survive high daytime temperatures, high levels of solar radiation, high levels of salinity, and strong winds. Water, however, is the most important ingredient in their lives and they must prevent water loss and overheating to survive. In general, to overcome these obstacles, desert plants can be classified into three main groups according to how they deal with the drought conditions – succulence, drought-deciduous, and drought-escaping. Today, I’ll talk about succulence.

Succulence is the ability of plants to store water in their leaves, stems, and/or roots. Cacti are a classic example. Succulent plants have shallow roots, allowing them to quickly absorb any available moisture, including dew. These plants can begin to grow 24 – 48 hours after rain. You may have noticed that there aren’t many cacti growing in the wadis of South Sinai, but there several species of succulents.

Species with Succulent Leaves

If you think back to your high school biology class, you’ll recall that photosynthesis is the process that allows plants to use the sun’s energy to convert carbon dioxide (CO2) and water into sugar. Basically, it’s how plants make their food. There are pores, called stomata, on the leaves that open and close to absorb carbon dioxide from the air and release oxygen. When these stomata open and close, water vapor is also released and evaporated. This release of water is called transpiration. It’s similar to sweating in humans and can help cool the plant. And we know that if we sweat a lot, we should drink more water to stay hydrated. However, desert plants do not receive a lot of rain to replace this lost water. So they have adapted to conserve as much water as possible.

Species with Succulent Stems

One way succulents do this is to use a different type of photosynthesis, one that allows the plants to make food without losing a lot of water to transpiration. It’s called crassulacean acid metabolism, or CAM photosynthesis. Plants that use CAM open their stomata at night when temperatures are cooler and keep them closed during the day. Instead of using sunlight to convert the CO2 to food right away, plants store the CO2 they absorbed at night as crassulacean acid. As the day begins and the temperature starts to rise, the acid is changed back into carbon dioxide and then used in photosynthesis. This allows the plants to conserve water and use it to make food instead of losing it to evaporation.

Species with Succulent Stems and Leaves

That’s a pretty fascinating adaptation, isn’t it? But it’s only the start! Stay tuned for my next post where I’ll discuss drought-deciduous and drought-escaping plants.