Question: Why were the beech crosspieces switched to maple in 1970?
About the article "Mona Lisa", section "Frame".

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Because the "Mona Lisa"'s poplar support expands and contracts with changes in humidity, the picture has experienced some warping.
In response to warping and swelling experienced during its storage during World War II, and to prepare the picture for an exhibit to honor the anniversary of Leonardo's 500th birthday, the "Mona Lisa" was fitted in 1951 with a flexible oak frame with beech crosspieces.
This flexible frame, which is used in addition to the decorative frame described below, exerts pressure on the panel to keep it from warping further.
In 1970, the beech crosspieces were switched to maple after it was found that the beechwood had been infested with insects.
In 2004–2005, a conservation and study team replaced the maple crosspieces with sycamore ones, and an additional metal crosspiece was added for scientific measurement of the panel's warp.
The "Mona Lisa" has had many different decorative frames in its history, owing to changes in taste over the centuries.
In 1909, the Comtesse de Béhague gave the portrait its current frame, a Renaissance-era work consistent with the historical period of the "Mona Lisa".
The edges of the painting have been trimmed at least once in its history to fit the picture into various frames, but no part of the original paint layer has been trimmed.
The mean annual temperatures in the higher elevations in the Smokies are cool enough to support forest types more commonly found in the northern United States.
The northern hardwood forests of the Smokies constitute the highest broad-leaved forest in the eastern United States.
About of the northern hardwood forest are old-growth.
In the Smokies, the northern hardwood canopies are dominated by yellow birch ("Betula alleghaniensis") and American beech ("Fagus grandifolia").
White basswood ("Tilia heterophylla"), mountain maple ("Acer spicatum") and striped maple ("Acer pensylvanicum"), and yellow buckeye ("Aesculus flava") are also present.
The northern hardwood understory is home to diverse species such as coneflower, skunk goldenrod, Rugels ragwort, bloodroot, hydrangea, and several species of grasses and ferns.
One unique community in the northern hardwoods of the Smokies is the beech gap, or beech orchard.
Beech gaps consist of high mountain gaps that have been monopolized by beech trees.
The beech trees are often twisted and contorted by the high winds that occur in these gaps.
Why other tree types such as the red spruce fail to encroach into the beech gaps is unknown. 
The range of the beech-maple forest type extends from the Atlantic coast west to Minnesota, Michigan and from southern Canada south to Virginia and Tennessee.
It is widespread in New York and was an important component of the original vegetation of northeastern Ohio.
Instances of a beech-maple forest can be found at altitudes of to . 
Papa hōlua are composed of a pair of runners and a superstructure (platform).
The runners are made from hard native woods, traditionally that of kauila ("Alphitonia ponderosa" or "Colubrina oppositifolia"), uhiuhi ("Caesalpinia kavaiensis"), or māmane ("Sophora chrysophylla").
The runners have up-curved fore edges, straight aft edges, and rounded lower edges.
Crosspieces keep the runners apart.
The superstructure, two rails smaller in diameter than the runners and spaced apart by pieces of bamboo, sits on top of the crosspieces.
The runners, crosspieces, and rails are bound together with sennit cordage.
The rails are wrapped in white kapa cloth and the rail frame is covered in lauhala matting.
Oil from kukui ("Aleurites moluccana") nuts coats either the course or the runners to provide lubrication. 
Sandra Beech (born Sandra Millar 1942 in Ballymena, Northern Ireland) is a Canadian children's musician.
She was a member of the family music group The Musical Millars after relocating to Canada in 1953.
After her 1964 marriage to Len Beech, she specialised in recording children's music.
Her brothers, Will and George Millar plus her cousin Joe Millar were members of The Irish Rovers.
Sandra Beech has three daughters with her husband Len Beech: Carrie Beech, Heather Beech and Jennifer Beech.