Why do leaves change colour in autumn?

Emma Chaplin
, 20 October 2021
Why do leaves change colour in autumn?
Halnaker © Emma Varley

By Charlotte Owen

WildCall Officer

With the changing of the season, we are treated to a spectacular show of autumn colour as the natural palette shifts from leafy green to a golden blaze of red and yellow. This dramatic display can be incredibly beautiful but it also serves an important function.

Deciduous trees prepare for winter by shedding their leaves, partly to conserve water - which can be in short supply when the ground is frozen - and partly to reduce the risk of being blown down or damaged by winter storms. Growth is halted and the trees enter a dormant state to ‘sleep’ through the worst of the wintry weather.

Autumn beech trees Roger Wilmshurst
The reason the leaves change colour before they fall is linked to chlorophyll production - the green pigment that’s vital for photosynthesis. Each leaf is like a solar panel, using the energy from sunlight to produce the sugars that fuel new growth, and chlorophyll levels are regularly topped up throughout the spring and summer to make the most of the growing season. But as the days shorten and there is less solar energy available, chlorophyll production slows down and eventually stops altogether. As the green fades, the leaf pigments that were previously masked by chlorophyll are gradually revealed: the yellows and oranges are produced by carotene pigments, while anthocyanins create reds, pinks and purples.

The drop in temperature also triggers structural changes within each leaf, so that a wall of cells is built across the base of each stem. This will eventually sever the leaf to send it drifting to the ground but it also blocks the usual flow of sugars out of the leaf. With nowhere to go, the excess sugar is converted into more anthocyanin pigments, producing a deeper red blush. The colours are most impressive when the weather is favourable: a combination of low (but above freezing) temperatures, dry weather and plenty of sunshine all serve to reduce chlorophyll levels and enhance the conversion of leftover sugars, producing more intense shades of red.

Leave a comment

Comments

  • Tina Hutchinson:

    Great article, really clearly explained! Thank you. (I am a SWT member).

    16 Nov 2023 11:33:00

  • Laura Tyler:

    Although I’ll never be able to appreciate the changing of colours due to total blindness, it’s still very interesting to learn about, and to feel the leaves as they slowly turn from supple, softer textures to harder, drier, less flexible ones. I now know more about why that happens even before they fall, so thank you.

    16 Nov 2023 12:15:00

  • jane holbrook:

    That is truly remarkable, and makes perfect sense, when explained so simply.

    16 Nov 2023 16:27:00

  • Jacqui Cuff:

    Fab and succinct explanation thanks. I so enjoy autumn and am a proud member of swt.

    16 Nov 2023 16:44:00

  • Jan Osborne:

    As always, helpful and informative with detail at just the right level

    17 Nov 2023 07:56:00

  • Sophie Carnell:

    Thank you for a concise yet insightful summary of the reasons behind the change of colours.

    18 Nov 2023 12:21:00

Time Δ Debug Message - Perch Runway 3.2
1792327328.99550 [907] SELECT p.pagePath, pr.routePattern, pr.routeRegExp, p.pageTemplate, pr.routeOrder, s.settingValue AS siteOffline FROM swt_pages_staging p LEFT JOIN swt_page_routes pr ON p.pageID=pr.pageID LEFT JOIN swt_settings s ON s.settingID='siteOffline' UNION SELECT NULL AS pagePath, pr2.routePattern, pr2.routeRegExp, pr2.templatePath AS pageTemplate, pr2.routeOrder, NULL AS siteOffline FROM swt_page_routes pr2 WHERE templateID!=0 ORDER BY routeOrder ASC, pagePath ASC
1792327329.03570.0402Matched route: news/[slug:s]
1792327329.03580.0001Server address: 216.73.216.236
1792327329.03590Using master page: /templates/pages/blog/post.php
1792327329.03590Page arguments: 
Array
(
    [0] => /news/why-do-leaves-change-colour-in-autumn
    [s] => why-do-leaves-change-colour-in-autumn
    [1] => why-do-leaves-change-colour-in-autumn
)
1792327329.03610.0003 [1] SELECT * FROM swt_pages_staging WHERE pagePath='/news/post' LIMIT 1
1792327329.03740.0013 [86] SELECT DISTINCT settingID, settingValue FROM swt_settings WHERE userID=0
1792327329.03980.0024 [1] SELECT * FROM swt_shop_cart WHERE cartID=25090336
1792327329.04070.001 [1] SELECT * FROM swt_shop_cart WHERE cartID=25090336
1792327329.04190.0012 [nil] SELECT * FROM swt_shop_sales WHERE saleFrom<='2026-10-18 12:42:00' AND saleTo>'2026-10-18 12:42:00' AND saleActive=1 AND saleDeleted IS NULL ORDER BY saleOrder ASC
1792327329.04360.0017 [1] SELECT SQL_CALC_FOUND_ROWS DISTINCT tbl.* FROM ( SELECT idx.itemID, main.*, idx2.indexValue as sortval FROM swt_blog_index idx JOIN swt_blog_posts main ON idx.itemID=main.postID AND idx.itemKey='postID' JOIN swt_blog_index idx2 ON idx.itemID=idx2.itemID AND idx.itemKey='postID' AND idx2.indexKey='_id' WHERE 1=1 AND ((idx.indexKey='postSlug' AND idx.indexValue='why-do-leaves-change-colour-in-autumn')) AND idx.itemID=idx2.itemID AND idx.itemKey=idx2.itemKey GROUP BY idx.itemID, idx2.indexValue, postID ) as tbl WHERE (postStatus='Published' AND postDateTime<='2026-10-18 12:42:00' ) GROUP BY itemID, sortval ORDER BY sortval ASC LIMIT 0, 10
1792327329.04820.0046 [1] SELECT FOUND_ROWS() AS `count`
1792327329.04920.001 [1] SELECT * FROM swt_blog_posts WHERE postID=2648 AND postStatus='Published' AND postDateTime<='2026-10-18 12:42:00'
1792327329.05040.0012 [13] SELECT * FROM swt_blog_sections ORDER BY sectionTitle ASC
1792327329.05160.0012 [1] SELECT * FROM swt_blogs ORDER BY blogTitle ASC
1792327329.05290.0013 [1] SELECT * FROM swt_categories WHERE catID=181 LIMIT 1
1792327329.05410.0012 [1] SELECT * FROM swt_categories WHERE catID=51 LIMIT 1
1792327329.0550.001Using template: /templates/blog/post.html
1792327329.05530.0002 [162] SELECT catID, catPath FROM swt_categories
1792327329.05650.0012 [1] SELECT * FROM swt_blog_posts WHERE postStatus='Published' AND postDateTime<='2026-10-18 12:42:00' AND postSlug='why-do-leaves-change-colour-in-autumn'
1792327329.08910.0326Using template: /templates/blog/post.html
1792327329.08980.0007 [250] SELECT country, countryID FROM swt_shop_countries WHERE countryActive=1 ORDER BY country ASC
1792327329.09220.0024Using template: /templates/pages/attributes/seo.html
1792327329.09250.0003 [1] SELECT regionID, regionTemplate, regionPage, regionRev AS rev FROM swt_content_regions WHERE regionKey='Banner image' AND (regionPage='/news/post' OR regionPage='*')
1792327329.09390.0014 [nil] SELECT * FROM ( SELECT idx.itemID, c.regionID, idx.pageID, c.itemJSON, idx2.indexValue as sortval FROM swt_content_index idx JOIN swt_content_items c ON idx.itemID=c.itemID AND idx.itemRev=c.itemRev AND idx.regionID=c.regionID JOIN swt_content_index idx2 ON idx.itemID=idx2.itemID AND idx.itemRev=idx2.itemRev AND idx2.indexKey='_order' WHERE ((idx.regionID=104 AND idx.itemRev=0)) AND idx.itemID=idx2.itemID AND idx.itemRev=idx2.itemRev ) as tbl GROUP BY itemID, pageID, itemJSON, sortval, regionID ORDER BY sortval ASC
1792327329.09670.0028 [82] SELECT * FROM swt_pages_staging WHERE pageNew=0 AND pageHidden=0 AND pageDepth >=0 AND pageDepth<=2 ORDER BY pageTreePosition ASC
1792327329.10170.005 [1] SELECT pageTreePosition FROM swt_pages_staging WHERE pagePath='/news/post' LIMIT 1
1792327329.10270.001 [2] SELECT pageID FROM swt_pages_staging WHERE pageTreePosition IN ('000-006-001', '000-006', '000') ORDER BY pageTreePosition DESC
1792327329.10620.0035Using template: /templates/navigation/ps_nav_template.html
1792327329.10630 [6] Using template: /templates/navigation/ps_nav_template_children.html
1792327329.10710.0009 [4] Using template: /templates/navigation/ps_nav_template_children.html
1792327329.10760.0005 [29] Using template: /templates/navigation/ps_nav_template_children.html
1792327329.11050.0029 [10] Using template: /templates/navigation/ps_nav_template_children.html
1792327329.11170.0012 [7] Using template: /templates/navigation/ps_nav_template_children.html
1792327329.11250.0008 [9] Using template: /templates/navigation/ps_nav_template_children.html
1792327329.11560.003 [1] SELECT pageTreePosition FROM swt_pages_staging WHERE pagePath='/news/post' OR pageSortPath='/news/post' LIMIT 1
1792327329.11690.0013 [1] SELECT * FROM swt_pages_staging WHERE pageHidden=0 AND pageNew=0 AND pageTreePosition IN ('000-006-001', '000-006', '000') ORDER BY pageTreePosition
1792327329.12070.0038 [1] Using template: /templates/navigation/breadcrumbs.html
1792327329.12130.0006Using template: /templates/blog/post.html
1792327329.12160.0003 [1] SELECT * FROM swt_blog_authors WHERE authorID='20' LIMIT 1
1792327329.12260.001Using template: /templates/blog/author.html
1792327329.12310.0005 [1] SELECT SQL_CALC_FOUND_ROWS DISTINCT tbl.* FROM ( SELECT idx.itemID, main.*, idx2.indexValue as sortval FROM swt_blog_index idx JOIN swt_blog_posts main ON idx.itemID=main.postID AND idx.itemKey='postID' JOIN swt_blog_index idx2 ON idx.itemID=idx2.itemID AND idx.itemKey='postID' AND idx2.indexKey='_id' WHERE 1=1 AND ((idx.indexKey='postSlug' AND idx.indexValue='why-do-leaves-change-colour-in-autumn')) AND idx.itemID=idx2.itemID AND idx.itemKey=idx2.itemKey GROUP BY idx.itemID, idx2.indexValue, postID ) as tbl WHERE (postStatus='Published' AND postDateTime<='2026-10-18 12:42:00' ) GROUP BY itemID, sortval ORDER BY sortval ASC LIMIT 0, 1
1792327329.12640.0033 [1] SELECT FOUND_ROWS() AS `count`
1792327329.12710.0007 [1] Using template: /templates/blog/post.html
1792327329.12950.0024Using template: /templates/blog/comment_form.html
1792327329.13070.0012 [6] SELECT * FROM swt_blog_comments WHERE 1=1 AND postID=2648 AND commentStatus='LIVE' ORDER BY FIELD(webmentionType, 'like', 'repost', 'comment', NULL) ASC, commentDateTime ASC
1792327329.22230.0916 [6] Using template: /templates/blog/comment.html
1792327329.22420.0019Using template: /templates/blog/post.html
1792327329.22440.0002 [1] SELECT * FROM swt_blog_authors WHERE authorID='20' LIMIT 1
1792327329.22540.001 [1] SELECT pageID, pageParentID, pageDepth, pageTreePosition FROM swt_pages_staging WHERE pagePath='/what-we-do' OR pageSortPath='/what-we-do' LIMIT 1
1792327329.22640.001 [1] SELECT pageID, pageParentID, pageDepth, pageTreePosition FROM swt_pages_staging WHERE pageTreePosition='000-002' LIMIT 1
1792327329.22770.0013 [7] SELECT * FROM swt_pages_staging WHERE pageNew=0 AND pageHidden=0 AND pageTreePosition LIKE '000-002%' AND pageDepth >=1 AND pageDepth<=2 ORDER BY pageTreePosition ASC
1792327329.23110.0035 [1] SELECT pageTreePosition FROM swt_pages_staging WHERE pagePath='/news/post' LIMIT 1
1792327329.23190.0007 [2] SELECT pageID FROM swt_pages_staging WHERE pageTreePosition IN ('000-006-001', '000-006', '000') ORDER BY pageTreePosition DESC
1792327329.23470.0029 [6] Using template: /templates/navigation/footer-nav.html
1792327329.23550.0008 [1] SELECT pageID, pageParentID, pageDepth, pageTreePosition FROM swt_pages_staging WHERE pagePath='/make-a-difference' OR pageSortPath='/make-a-difference' LIMIT 1
1792327329.23640.0009 [1] SELECT pageID, pageParentID, pageDepth, pageTreePosition FROM swt_pages_staging WHERE pageTreePosition='000-005' LIMIT 1
1792327329.2370.0007 [38] SELECT * FROM swt_pages_staging WHERE pageNew=0 AND pageHidden=0 AND pageTreePosition LIKE '000-005%' ORDER BY pageTreePosition ASC
1792327329.24070.0036 [1] SELECT pageTreePosition FROM swt_pages_staging WHERE pagePath='/news/post' LIMIT 1
1792327329.24150.0008 [2] SELECT pageID FROM swt_pages_staging WHERE pageTreePosition IN ('000-006-001', '000-006', '000') ORDER BY pageTreePosition DESC
1792327329.24480.0034 [10] Using template: /templates/navigation/footer-nav.html
1792327329.24840.0036 [1] SELECT pageID, pageParentID, pageDepth, pageTreePosition FROM swt_pages_staging WHERE pagePath='/visit' OR pageSortPath='/visit' LIMIT 1
1792327329.24950.0011 [1] SELECT pageID, pageParentID, pageDepth, pageTreePosition FROM swt_pages_staging WHERE pageTreePosition='000-004' LIMIT 1
1792327329.25040.0009 [30] SELECT * FROM swt_pages_staging WHERE pageNew=0 AND pageHidden=0 AND pageTreePosition LIKE '000-004%' AND pageDepth >=1 AND pageDepth<=2 ORDER BY pageTreePosition ASC
1792327329.25420.0038 [1] SELECT pageTreePosition FROM swt_pages_staging WHERE pagePath='/news/post' LIMIT 1
1792327329.25510.0009 [2] SELECT pageID FROM swt_pages_staging WHERE pageTreePosition IN ('000-006-001', '000-006', '000') ORDER BY pageTreePosition DESC
1792327329.25820.0032 [29] Using template: /templates/navigation/footer-nav.html
1792327329.2610.0028 [1] SELECT pageID, pageParentID, pageDepth, pageTreePosition FROM swt_pages_staging WHERE pagePath='/make-a-difference' OR pageSortPath='/make-a-difference' LIMIT 1
1792327329.2620.001 [1] SELECT pageID, pageParentID, pageDepth, pageTreePosition FROM swt_pages_staging WHERE pageTreePosition='000-005' LIMIT 1
1792327329.26280.0007 [11] SELECT * FROM swt_pages_staging WHERE pageNew=0 AND pageHidden=0 AND pageTreePosition LIKE '000-005%' AND pageDepth >=1 AND pageDepth<=2 ORDER BY pageTreePosition ASC
1792327329.26620.0034 [1] SELECT pageTreePosition FROM swt_pages_staging WHERE pagePath='/news/post' LIMIT 1
1792327329.2670.0008 [2] SELECT pageID FROM swt_pages_staging WHERE pageTreePosition IN ('000-006-001', '000-006', '000') ORDER BY pageTreePosition DESC
1792327329.26990.0029 [10] Using template: /templates/navigation/footer-nav.html
1792327329.27190.002Request time: 0.2776
1792327329.27190Process time: 0.2774
1792327329.27190Memory: 1.6253