Question: Why were crosspieces made from maple rather than beech starting 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.
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. 
Much of the island remains in forest, although housing developments have made significant inroads over recent years.
Deciduous trees such as American beech, sugar maple, red maple, northern red oak, white ash, bitternut hickory and American basswood are dominant.
The vegetation is more luxuriant than one would expect from the nature of the soils. 
Beech wood is an excellent firewood, easily split and burning for many hours with bright but calm flames.
Chips of beech wood are used in the brewing of Budweiser beer as a fining agent.
Beech logs are burned to dry the malts used in some German smoked beers, giving the beers their typical flavour.
Beech is also used to smoke Westphalian ham, various sausages, and some cheeses.
Some drums are made from beech, which has a tone between those of maple and birch, the two most popular drum woods.
The textile modal is a kind of rayon often made wholly from the reconstituted cellulose of pulped beech wood.
The European species "Fagus sylvatica" yields a utility timber that is tough but dimensionally unstable.
It weighs about 720 kg per cubic metre and is widely used for furniture framing and carcass construction, flooring and engineering purposes, in plywood and in household items like plates, but rarely as a decorative wood.
The timber can be used to build chalets, houses and log cabins.
Beech wood is used for the stocks of military rifles when traditionally preferred woods such as walnut are scarce or unavailable or as a lower-cost alternative.
The fruit of the beech tree is known as "beechnuts" or "mast" and is found in small burrs that drop from the tree in autumn.
It is small, roughly triangular and edible, with a bitter, astringent taste.
They have a high enough fat content that they can be pressed for edible oil.
Fresh from the tree, beech leaves are a fine salad vegetable, as sweet as a mild cabbage though much softer in texture.
The young leaves can be steeped in gin for several weeks, the liquor strained off and sweetened to give a light green/yellow liqueur called beechleaf noyau.
Beech wood tablets were a common writing material in Germanic societies before the development of paper.
The Old English "bōc" and Old Norse "bók" both have the primary sense of “beech” but also a secondary sense of “book”, and it is from "bōc" that the modern word derives.
In modern German, the word for “book” is "Buch," with "Buche" meaning “beech tree”.
In Swedish, these words are the same, "bok" meaning both “beech tree” and “book”.
Similarly, in Russian, the word for beech is "бук" (buk), while that for letter (as in a letter of the alphabet) is "буква" (bukva).
The pigment bistre was made from beech wood soot. 
Maple is considered a tonewood, or a wood that carries sound waves well, and is used in numerous musical instruments.
Maple is harder and has a brighter sound than Mahogany, which is another major tonewood used in instrument manufacture.
The back, sides, and neck of most violins, violas, cellos, and double basses are made from maple.
Electric guitar necks are commonly made from maple, having a brighter sound than rosewood.
The necks of the Fender Stratocaster and Telecaster were originally an entirely maple one piece neck, but later were also available with rosewood fingerboards.
Les Paul desired an all maple guitar, but due to the weight of maple, only the tops of Gibson's Les Paul guitars are made from carved maple, often using quilted or flamed maple tops.
Due to its weight, very few solid body guitars are made entirely from maple, but many guitars have maple necks, tops or veneers.
Maple is also often used to make bassoons and sometimes for other woodwind instruments like maple recorders.
Many drums are made from maple.
From the 70s to the 90s, maple drum kits were a vast majority of all drum kits made, but in recent years, Birch has become popular for drums once again.
Some of the best drum-building companies use maple extensively throughout their mid-pro range.
Maple drums are favored for their bright resonant sound.