编辑“︁树”︁（章节）

== 定義 ==
[[File:General Sherman tree looking up.jpg|thumb|[[雪曼將軍樹]]]]
[[File:Tree secondary growth diagram.svg|thumb|[[真雙子葉植物]]或是[[松柏門]]{{link-en|二次生長|secondary growth}}的示意圖，每一季[[木質部]]會生長，使樹幹、樹枝及根變粗]]

雖然樹是一個常用的詞語，但沒有一個廣為認同的[[定義]]，不管是[[植物學]]上的定義或是日常生活的定義都沒有<ref name="as.miami.edu">{{cite web | url=http://www.as.miami.edu/qr/arboretum/what_is_a_tree.html | title=What is a tree? | year=2012 | work=Smartphone tour | publisher=University of Miami: John C. Gifford Arboretum | accessdate=2014-09-23 | archive-url=https://web.archive.org/web/20140420004648/http://www.as.miami.edu/qr/arboretum/what_is_a_tree.html | archive-date=2014-04-20 | dead-url=yes }}</ref>。

因為樹本意是一個人為分類法，不同的樹之間沒有血緣關係。一般稱為樹的都集中在兩類當中，便是[[雙子葉植物]]([[闊葉樹]])和[[松柏門]]([[針葉樹]])，可兩個門或網都包括了很多[[草本植物]]和灌木。但在這兩類外仍有很多類型的樹，包括了[[竹]]、[[棕櫚]]、[[蘇鐵]]、[[銀杏]]、[[買麻藤]]等其他[[種子植物]]，何況種子植物便是由[[前裸子植物]]演化而成的。

因為[[維管植物]]先演化出木質的樹幹，生育器官較晚演化出[[種子]]，四億年前廣義[[蕨類植物]]中，便有如[[鱗木]]和[[蘆木]]及其遺孑的[[桫欏目]]等，稱得上算是最早的樹了。

廣義樹是泛指中等至高大的植物。有細長莖（或樹幹）可以支持葉子或枝條離地面相當的高度<ref>{{cite web |url=http://www.newton.dep.anl.gov/askasci/bot00/bot00761.htm |title=Tree definition |author=Tokuhisa, Jim |publisher=Newton Ask a Scientist |accessdate=2014-10-05 |archive-date=2013-12-06 |archive-url=https://web.archive.org/web/20131206131101/http://www.newton.dep.anl.gov/askasci/bot00/bot00761.htm |dead-url=no }}</ref>。樹也常用高度來定義<ref name="Gschwantner, Thomas 2009">Gschwantner, Thomas, et al. "Common tree definitions for national forest inventories in Europe." Silva Fennica 43.2 (2009): 303-321.</ref>，高度在{{convert|0.5|to|10|m|ft|abbr=on}}的中至大型植物都算樹的，包括了[[灌木]]<ref name=treedictionary>{{cite web | url=http://www.treedictionary.com/ | title=Tree Biology Dictionary | author=Keslick, John A. | year=2004 | accessdate=2012-07-30 | archive-date=2021-03-19 | archive-url=https://web.archive.org/web/20210319140326/http://www.treedictionary.com/ | dead-url=no }}</ref>向小型[[喬木]]過渡與大型[[草本植物]]，所以樹木的最小高度也只是大致的定義而已<ref name="Gschwantner, Thomas 2009"/>。以此定義來看，像[[香蕉]]樹及[[番木瓜]]樹等高大的草本植物也算是樹<ref name="as.miami.edu"/><ref>{{cite web | url=http://people.umass.edu/psoil370/Syllabus-files/Agroforestry_Principles.pdf | title=Agroforestry principles | author=Martin, Franklin; Sherman, Scott | year=2007 | work=Echo technical notes | accessdate=2014-09-22 | archive-date=2013-07-28 | archive-url=https://web.archive.org/web/20130728035517/http://people.umass.edu/psoil370/Syllabus-files/Agroforestry_Principles.pdf | dead-url=no }}</ref>。

狹義是樹有木質且會{{link-en|二次生長|secondary growth}}的莖，也就是除了每年因[[分生組織]]而高度增加以外，莖也會每年往外加粗<ref name="Gschwantner, Thomas 2009"/><ref>{{cite web | url=http://warnell.forestry.uga.edu/SERVICE/LIBRARY/index.php3?docID=171 | title=Secondary Growth Anatomy and Tree Rings | author=Coder, Kim D. | publisher=Warnell School of Forest Resources, University of Georgia | date=August 1999 | accessdate=2014-09-23 | deadurl=yes | archiveurl=https://web.archive.org/web/20140908112429/http://warnell.forestry.uga.edu/SERVICE/LIBRARY/index.php3?docID=171 | archivedate=2014-09-08 }}</ref>。此定義下像[[棕櫚科|棕櫚]]樹、[[香蕉]]樹及[[番木瓜]]樹不論高度多高，都不能視為是樹。

中義常依使用方式來定義樹，例如指那些可以作[[木材]]用的植物為樹<ref name="fao.org">{{cite web | url=http://www.fao.org/docrep/006/t7540e/T7540E04.htm | title=Community forestry rapid appraisal of tree and land tenure | publisher=Food and Agriculture Organisation | accessdate=1 October 2014 | archive-date=2018-07-17 | archive-url=https://web.archive.org/web/20180717100135/http://www.fao.org/docrep/006/t7540e/T7540E04.htm | dead-url=no }}</ref>，就是堅固的大型植物了。一些廣義的樹在木材的用途過狹了，純草本的大中型植物只可以做紙或薪柴和小型草木相似，而即使是灌木除做紙或燃料外只能小的木制品(火柴、筷箸、牙簽、棍棒等)，所以指定要有相當尺寸和堅硬的植物。但有些[[單子葉植物]]下歸類為樹<ref>Lund, H. Gyde. ""A forest by any other name…." Environmental Science & Policy 2.2 (1999): 125-133.</ref>，[[短葉絲蘭|約書亞樹]]、[[棕櫚]]和[[竹子]]沒有二次生長<ref name="botany.wisc.edu">{{cite web | url=http://www.botany.wisc.edu/courses/botany_400/Lecture/0pdf/25MonocotOrigin.pdf | title=Diversity and evolution of monocots | publisher=University of Wisconsin | accessdate=2014-09-22 | archive-date=2016-10-22 | archive-url=https://web.archive.org/web/20161022091304/http://www.botany.wisc.edu/courses/botany_400/Lecture/0pdf/25MonocotOrigin.pdf | dead-url=no }}</ref><ref name="RoddStackhouse2008">{{cite book | author1=Tony Rodd | author2=Jennifer Stackhouse | title=Trees: A Visual Guide | url=http://books.google.com/books?id=Mpsc2hsYk1YC&pg=PA112 | accessdate=2012-09-27 | date=1 April 2008 | publisher=University of California Press | isbn=978-0-520-25650-7 | pages=112– | archive-date=2021-01-22 | archive-url=https://web.archive.org/web/20210122170548/https://books.google.com/books?id=Mpsc2hsYk1YC&pg=PA112 | dead-url=no }}</ref>，不過在一次成長時其莖部有形成木質組織，材質類似木材<ref>{{cite web | url=http://www.bio.miami.edu/dana/dox/stem.html | title=Monocot stems | work=The stem | publisher=University of Miami | accessdate=2014-09-22 | archive-date=2021-03-19 | archive-url=https://web.archive.org/web/20210319140404/http://www.bio.miami.edu/dana/dox/stem.html | dead-url=no }}</ref>。