UNITES STATES—Soil saturation is detrimental to trees for a few reasons. Excessive irrigation is almost as problematic, even without saturation. Chronic excessive moisture compromises stability by inhibiting deep root dispersion. Instead, it promotes shallower root buttressing. It may compromise structural integrity, too. It is generally the cause of spontaneous limb failure.
Spontaneous limb failure is exactly what it implies. Its spontaneity is what makes it more hazardous than limb failure caused by wind. It happens while the weather is calm, warm and perhaps humid, when least expected. Limbs might sag and produce cracking noises immediately prior to falling. However, they quite often fall suddenly and without warning.
Spontaneous limb failure is associated more with gravity than wind. It occurs as vigorous stems literally grow faster than they can support. Warm weather accelerates foliar growth that increases weight. A lack of wind and perhaps enhanced humidity inhibit evaporation from foliar surfaces. Inhibition of evapotranspiration limits weight loss during weight gain.
Some trees are more vulnerable than others.
Deciduous trees are generally more susceptible to spontaneous limb failure. Sweetgum, willows, poplars and elms are particularly vulnerable. A few evergreen trees are likewise susceptible, though. They include coast live oak, Monterey cypress and a few eucalypti. Fruit trees exhibit spontaneous limb failure if they cannot support the weight of their fruit.
The potential for spontaneous limb failure is rarely obvious. Limbs of some types of trees may visually appear to be too heavy or floppy. Limbs of Monterey pine, for example, may lean to one side if they sag prior to failure. However, valley oak, carob and sycamore are notorious for concealing their weaknesses. Even arborists cannot predict all limb failure.
Wild trees within new landscapes are particularly vulnerable to spontaneous limb failure. They are not accustomed to irrigation through otherwise arid summers. Some can adapt as landscapes with irrigation develop around them. Others overindulge and become too heavy for their own trunks or limbs to support. New trees adapt to irrigation as they grow.
Highlight: Douglas Fir
Within home gardens, Douglas fir, Pseudotsuga menziesii is very rare. So is the bigcone Douglas fir, Pseudotsuga macrocarpa of Southern California. However, they are not rare in the wild. Sometimes, suburban or rural home gardens develop around them. They can be awkward to accommodate. They are magnificent trees but simply grow much too big.
Contrary to its rarity among home gardens, Douglas fir is actually common within homes. after all, most houses are constructed mostly of Douglas fir lumber. Most Christmas trees here are plantation grown Douglas fir. It is a major lumber plantation commodity in Chile, Argentina, New Zealand and Europe. Douglas fir is the Official State Tree of Oregon too.
Douglas fir is a grand tree in the wild. Old trees can grow more than two hundred feet tall with trunks eight feet wide! Their limbs may extend more than thirty feet from their trunks. It is obviously disproportionate for home gardens. Its soft evergreen foliage is delightfully aromatic, though. Its flattened, two-ranked needles are less than an inch and a half long. The female cones exhibit uniquely lacy scales.
