Forests in countries like Brazil and the Congo get a lot of attention from environmentalists, and it is easy to see why. South America and sub-Saharan Africa are experiencing deforestation on an enormous scale: every year almost 5 million hectares are lost. But forests are also changing in rich Western countries. They are growing larger, both in the sense that they occupy more land and that the trees in them are bigger. What is going on?

        Forests are spreading in almost all Western countries, with the fastest growth in places that historically had rather few trees. In 1990 28% of Spain was forested; now the proportion is 37%. In both Greece and Italy, the growth was from 26% to 32% over the same period. Forests are gradually taking more land in America and Australia. Perhaps most astonishing is the trend in Ireland. Roughly 1% of that country was forested when it became independent in 1922. Now forests cover 11% of the land, and the government wants to push the proportion to 18% by the 2040s.

        Two things are fertilising this growth. The first is the abandonment of farmland, especially in high, dry places where nothing grows terribly well. When farmers give up trying to earn a living from farming or herding, trees simply move in. The second is government policy and subsidy. Throughout history, governments have protected and promoted forests for diverse reasons, ranging from the need for wooden warships to a desire to promote suburban house-building. Nowadays forests are increasingly welcome because they suck in carbon pollution from the air. The justifications change; the desire for more trees remains constant.

        The greening of the West does not delight everyone. Farmers complain that land is being taken out of use by generously subsidised tree plantations. Parts of Spain and Portugal suffer from terrible forest fires. Others simply dislike the appearance of forests planted in neat rows. They will have to get used to the trees, however. The growth of Western forests seems almost as unstoppable as deforestation elsewhere.

解析：C。根据题干中的increasing desire for forests可定位到第三段第六句。定位句指出，如今，森林越来越受到欢迎，因为森林可以从空气中吸收碳污染。也就是说，森林能提高空气质量。C项与此内容相符，其中improving air quality是对定位句中的suck in carbon pollution from the air的同义转述，故C项正确。

错项排除：原文中没有提到森林的美景和种植果树的用途，故A、B两项排除。第三段第五句中提到了植树造林是为了满足对木制战船的需求以及促进郊区房屋的建设，与D项中的供应建材（supply of building materials）有些关联，但这是政府支持植树造林的原因，不是我们希望森林增长的原因，故D项排除。