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Marin IJ Articles

Unusual weather extremes impacting trees, leaves

July 29, 2017
Nanette Londeree

I love my pepper tree. With its shaggy, gnarled trunk and wide canopy of graceful, weeping foliage, it adds beauty and character to our garden along with much welcomed summer shade. It is gorgeous year round, at least in most “normal” weather years, whatever that is. Normal it hasn’t been — in the past six months we had the rainiest season in a century, and recently, the hottest temperatures in a decade. That followed years of the worst drought the state has ever seen, and even a few nights with sub-freezing temperatures.

The California pepper tree (Schinus molle), an evergreen, drought-tolerant native of Peru, handles many adverse conditions like poor soil, smog, wind and moderate frosts, but doesn’t like too much water. It generally drops 20 to 30 percent of its leaves in the spring, hardly a noticeable event other than a slight increase in leaf litter on the ground. Early this year the tree looked terrible, with the majority of its foliage grey and falling at an alarming rate. By late March, it had lost 80 percent of its leaves. Abundantly planted around the county, many pepper trees were displaying the same dismal symptoms. What was causing it? A pest? Disease? Or maybe Mother Nature changing her routine?

I took my tree concern to Steven Swain, horticultural advisor for Marin and Sonoma counties; he knew just what I was talking about as he was also getting that question from arborists. While looking into possible causes over the next months, those with ailing trees began noticing lots of new growth. By May, the majority of the trees were cloaked in lush, brilliant green foliage, pictures of good health. What happened? Swain deduced that the pepper trees were displaying an irregular pattern of leaf cycling for this species of tree, likely due to extraordinary environmental conditions — particularly years of drought followed by record-breaking rains.

What is leaf cycling? It’s basically the life cycle of plant leaves. Even though they come in a huge array of types, shapes and colors, the life cycle of leaves is pretty much the same for all. Acting as food manufacturing machines, leaves take in water, sunlight and air to produce nourishment for the tree. When water is limited, production of leaves may be slowed or stopped altogether; you might see plants with smaller or fewer leaves, or ones that change color and drop earlier in the season. When water returns after a drought, old, inefficient leaves will be replaced by young productive ones.

The life cycle of leaves is most visible with deciduous trees — ones that put out all new leaves in the spring and shed them in the autumn. Evergreen trees — those that appear to hold on to their leaves all year long — also produce new leaves to replace old ones, but the process is less obvious and doesn’t normally happen all at once. These types of trees, which include most conifers (cone-bearers with needlelike leaves) can grow year round under suitable conditions, and their new leaves can take more than a year to develop. As a result, environmental conditions may have more of an impact on evergreens than on deciduous types, and affect when and how many leaves they shed.

Back to my pepper tree. Over the past two years, while new leaves were being developed, the tree was subjected to temperatures from 26 to 102 degrees, ambient humidity from 10 to 100 percent and soil moisture, normally on the dry side, was completely saturated for an extended period during the worst of the rains. Like pepper trees around the county, it appears that because of the extreme weather conditions, the tree sped up the leaf cycling process, discarded old foliage and replaced nearly the entire tree with new leaves.

Fortunately, it looks as radiant as ever. Now let’s hope our weather returns to normal — whatever that is!

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