Archive for Saturday, February 17, 2018

Garden Variety: Many factors influence the life spans of trees

February 17, 2018

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Like all living things, trees have a life span. How long that life span might be is dependent on a combination of factors including species, site, environment, weather events, pest infestations and others. Understanding how these factors work together can help you determine why a tree died, why it is dying, how to extend the life of existing trees and how to choose wisely when planting for the future.

In general, tree species and location are the two factors that have the greatest influence on a tree life span. An extreme example is the coastal redwood tree which can survive for hundreds of years in California but would have difficulty surviving a winter in Kansas.

With native Kansas tree species and other species that are better suited to the state, the equation becomes more complicated. Let’s use white oak as an example, and look beyond the rare old trees that are outliers of the species.

A white oak tree living in a forest or partially wooded area will survive longer than a white oak living in a park or a yard. Within the forest or wooded area, life span will depend more on weather events. Think about trees that survived the historic droughts of the 1930s and 1950s. Although they survived, the stress surely shortened the lives of many of them. Genetic variability among the species also plays a role.

In a yard, park or other urban/suburban environment, trees are commonly planted in lawns. Tree roots compete with turf for water and nutrients, or in irrigated areas, they are more likely to be overwatered and overfertilized. A better environment would be a mulched area with trees close together to simulate a wooded area, manually irrigated as needed or that have irrigation set on separate zones from the lawn.

How much does the lawn shorten the life span of the oak? The answer depends on individual circumstances, plus the previously mentioned weather events and genetic variability. The life span gets even shorter when adding in the potential for wounds from weedeaters, mowers and pruning (often necessary but still wounding); herbicide damage from treatment of broadleaf weeds in the lawn; soil compaction in the root zone from mowers and people; poor soil types; the potential for construction damage from sidewalk and underground utility repair and replacement; poor planting practices and more.

Established trees that die slowly almost always die from a combination of these factors. One white oak might survive 15 years in a very harsh environment while another might survive 100 years in a stress-free forest.

Trees that die within the first few years of planting are more likely dying from an inability to adapt to the new site. This is referred to as transplant stress but still involves some of the same factors, such as poor planting practices, too much or too little water, competition for turf, etc.

Once a tree exhibits signs of dying — branches dying back or breaking off, signs of decay or damage in the main trunk, visible fungal fruiting bodies, hollow spots, thinning of canopy, etc. — the tree is nearing the end of its life span. Removing dead or dying branches and watering the tree over extended dry periods may extend the life just a little longer, but there is no silver bullet to save it.

To extend the life span of newly planted and established trees in urban/suburban settings, mulch a large area around it covering the root zone. Avoid piling the mulch against the trunk. Water deeply and infrequently over extended dry periods. Avoid overwatering and overfertilizing. Avoid damage to any part of the tree, especially the trunk and roots, except for corrective pruning.

When selecting new trees, life span is a really a secondary factor but is something to keep in mind. For example, a redbud tree may only survive 20 years in a yard (and maybe a decade or two longer in the wild). But if you want a redbud tree, you are likely planting for the aesthetic value rather than the durability. When the years go by and the tree starts breaking down, be ready to replace it.

A few of the longest-living tree species in Kansas, in the right environment, are baldcypress, oak, hickory, pecan, red cedar, sugar maple and tulip poplar. Some of the shortest-living are birch, fruit trees, pine, redbud and willow.

Finally, planting trees is really about planning for the future. If your yard is full of beautiful mature trees, consider adding in some youngsters to look over the place when the old ones are gone. Have a few young trees in the yard? They could use some companions to create more of a forest-type setting. For inspiration, think about the wind they will block, the shade they will provide and the beauty they will create.

— Jennifer Smith is a former horticulture extension agent for K-State Research and Extension and horticulturist for Lawrence Parks and Recreation.

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