Monday, July 16, 2012

Life of a Cactus Part 10: Nurse Plants

The pulp within a cactus fruit contains hundreds to thousands of seeds.  Unfortunately, very few of these seeds will ever germinate, and probably less than 0.1 percent of these seed will ever grow to mature plants.   Life is just too dangerous and for the seed and young plants.  The first problem a cactus seed faces is also a blessing.  The succulent flesh of a cactus fruit is like a magnet during the bitterly dry and hot summer.  Birds and animals gorge themselves on these fruits as a rich source of moisture and nutrients.  As they eat the fruit though, many, if not most of the seeds are also eaten.  This isn't such a bad thing, as long as the seeds are not crushed by chewing.  The seeds are especially adapted to remaining intact and passing directly through the digestive tract unharmed.  With most cactus fruit being brightly colored and located high on the plant, these fruits are especially enticing to birds, which may be the only organisms capable of reaching the fruits on taller cacti.  For example, the saguaro cactus holds its fruits tens of feet off of the ground, only allowing birds to access it.  Being birds do not chew food, instead swallowing it whole by the beak-full, most of the seeds can pass through the digestive tract unharmed and be deposited in their fecal matter.  Kind of a disgusting start to life but true never-the-less. 
Young Saguaro Cactus growing under the canopy of a Palo Verde.
Being birds of course frequently perch and sleep in shrubs and trees, most of their fecal matter, and therefore cactus seeds, will be deposited below.  This is the ideal environment for a cactus seed to germinate and grow in.  The shrub or tree provides shade which creates a slightly cooler and moister environment for the seed to germinate and grow in, something much needed in the desert.  The shrub or tree also provides cover and protection from predators which might eat recently germinated seeds.  Soil also is slightly more rich in these locations also.  This environment under the shrub or tree is termed a microenvironment and the plant that makes it is called a nurse plant.  Of course, the nurse plant is termed such because it helps, or nurses, young plants such as cacti to maturity in the microenvironment they create.  A microenvironment is a small area, such as under a tree canopy, that has slightly different conditions than the surrounding environment.  Within the Sonoran Desert Triangle-Leaf Bursage is the most important nurse plant.  Very few plants are able to grow without the nursing aid of a Bursage microenvironment.  This is sort of odd considering Bursage is such a small desert shrub, usually only reaching 20 or so inches in height.  Bursage is however one of the few plants capable of establishing itself without a nurse plant and is extremely abundant across the desert.  Other plants such as Palo Verde trees are well known nurse plants but not as important as Bursage.  This probably is because Palo Verde can't become established without a nurse plant and are not as common as Bursage.  Interestingly, the most common nurse plant for Palo Verde is also Triangle-Leaf Bursage. 

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