Broadleaf Tree Families Evolution and Distribution

Botanically, Broadleaf trees are classed as Angiosperms. Angio comes from the Greek for “vessel”. So an Angiosperm has its seeds in a vessel, in this case an ovary. Broadleaf trees generally have wide leaves with veins. Broadleaf trees bear flowers and fruits, not cones. Wood from Broadleaf trees is called Hardwood. Oak, Beech, Walnut, Lime, Poplar, Sycamore and Ash are all European hardwoods used in furniture manufacture.

Angiosperms first appeared around 140 million years ago during the early Cretaceous period. Throughout the late Cretaceous 100 million years ago angiosperms diversified and spread across various environments. The vast majority of modern broadleaf tree families and species have evolved over the last 66 million years. More recently species distribution has been significantly influenced by the last ice age which peaked 21,000 years ago. As the ice retreated in the Northern Hemisphere some species were able to spread back into previously occupied areas 

Broadleaf Tree Families Evolution and Distribution – Some families include a number of  trees commonly found in the UK. These families are shown in the chart below in their correct evolutionary sequence. There are about 7000 species of broadleaf trees in the these families but the worldwide total of broadleaf species may be 60,000 of which 50,000 are in tropical forests.  To find out more about the species mentioned in the family descriptions click HERE

Magnolia family (Magnoliaceae): This family includes several genera of which two are well known: Magnolia with 135 species and Liriodendron  with 2 species (Tulip Tree). The magnolia genus is named after the French botanist Pierre Magnol. It is an ancient genus with ‘primitive’ flowers adapted for pollination by beetles. The ‘petals’ for example are known as tepals because the sepals and petals are indistinguishable. This is a feature common to plants from the early evolution of Flowering Plants. Ancestral magnolias originated in North America in the Late Cretaceous 100 million years ago in a greenhouse climate. During the warm climate of 50 million years ago, ancestral Magnolias migrated via a land bridge, first to Europe and then Asia. About 40 mya global cooling began and they became extinct in Europe and southern Siberia, leaving two centres – Asia and North America. As glaciation increased, magnolias migrated south from both centres into warm upland sites where they diversified. Today there are 15 species in North and South America and 120 species in Asia, the centre of Magnolia diversity. 

Plane or sycamore family (Platanaceae): This family has only one genus, Platanus, which contains 10 species. Three species, known as sycamores (American, California and Arizona Sycamores)  are native to the USA, 1 is European (The Oriental Plane, native to Turkey, South East Europe and India), 6 are native to Mexico and 1 to South East Asia. The London Plane is probably a hybrid, between the Oriental Plane and the American Sycamore, first created in Spain or southern France and introduced to Britain in about 1680. The ancestral trees were widely distributed across the whole of the Northern Hemisphere 90 million years ago and the current distribution is the result of tectonic plate movement as Eurasia split from the Americas and climate change. Fossil evidence suggests that the first true platanus species appeared around 45 million years ago. As the climate of the Northern Hemisphere became cooler and drier tree species retreated to warmer regions and the platanus species became isolated from each other.

Pea family (Fabaceae): This is a huge family that is estimated to include around 19,000 species. It is distributed worldwide, with a significant presence in both tropical and temperate regions. It includes 4000 tree species. The reason for the ecological success of the family may be the ability to form root nodules with nitrogen-fixing bacteria. This ability to fix nitrogen has contributed to the success of legumes by enabling them to thrive in nutrient-poor soils in dry sandy regions. There are for example more than 1000 tree species in the acacia genus with many in arid regions of Australia and in African and South American savannahs. Only a few of these tree species are found in Britain: The Judas Tree and Laburnum, are native to the Mediterranean region  and the False Acacia and Honey Locust are native to North America. Note that the False Acacia has similar-looking leaves and flowers to the true acacia trees (genus Acacia) but is not closely related to it. Fossil evidence suggests that the Fabaceae family originated around 65 million years ago, and quickly diversified into a wide variety of forms. The Laburnum is thought to have originated in the Mediterranean region around 5 million years ago. The Judas Tree is in the genus Cercis which probably originated in the Mediterranean region around 20 million years ago. The Honey Locust is native to North America but it is in the genus Gleditsia which is distributed in central and Southeast Asia and North and South America. One theory is that Gleditsia originated in East Asia, crossed the land bridge across the North Pacific Ocean into North America, and then spread down to South America millions of years later.

Rose family (Rosaceae): The family is believed to have originated in the early Eocene, around 56 million years ago. Fossils of ancestral Rose species have been found in North America, Europe, and Asia. Today the family is found in various habitats and regions, including temperate, subtropical, and high-altitude areas. There are approximately 90 genera and over 3000 species in the family worldwide of which up to 1000 may be tree species. Asia, particularly China, is a centre of diversity. The largest genus Prunus has over 400 tree species, including plums, cherries, peaches, apricots, almonds, medlar and quince. Trees in this genus bear fruit in the form of drupes known as stone fruits.  Another group of trees bear fruit in the form of pomes and includes the genera  Malus (apples), Pyrus (pears), Sorbus (whitebeam, rowan and service tree) and Crataegus (hawthorns).

Whitebeams, Rowans and Service Trees are all members of the Sorbus genus. There are 44 species and 8 hybrids in this genus in Britain. Some of them are very rare and endangered. Today, the Sorbus genus comprises approximately 200 to 300 species  with 15 to 20 in North America and 100 to 150 in Asia.  Fossil records suggest that the earliest Sorbus species appeared in Europe during the Miocene epoch, around 23 million years ago. Hybridisation has played a significant role in generating new species within the genus. During the Pleistocene epoch, which began around 2.6 million years ago, repeated cycles of glaciation and retreat of ice sheets led to the fragmentation and isolation of populations. This, in turn, promoted speciation events as populations became genetically distinct from one another.

 Medlar and Quince – The Medlar tree is believed to be native to the region that extends from southeastern Europe to southwestern Asia. It was cultivated by the ancient Greeks and Romans and spread to other parts of Europe during the Middle Ages. Quince, has a long evolutionary history that dates back to the Tertiary period, around 65 million years ago. It is believed to have originated in the Caucasus region of Central Asia, which extends from the Black Sea to the Caspian Sea.

Elm family (Ulmaceae): The family contains several species in the Ulmus and Zelkova genera of trees. There are currently 25-30 species of trees in the Ulmus genus and 7-8 in the Zelkova genus. Fossil evidence shows that the early members of Ulmus genus were widespread and diverse 90 million years ago. By the Miocene epoch, around 23 million years ago, the genus had reached its peak diversity, with many different species of elm trees found across the Northern Hemisphere. Some of the most well-known species in the Ulmus genus are the American Elm, Slippery Elm, Wych Elm, Chinese Elm, and Siberian Elm. Zelkova species are found in East Asia, the Himalayas, and the Caucasus. The earliest Zelkova fossils are from 55 Ma in North America and even 5 million years ago they were common across the northern hemisphere but since then periods of extensive glaciation have resulted in species that are related but geographically separated – some in southern Europe and some in Asia.

Beech family (Fagaceae): This family has nearly 1000 tree species in four genera Fagus (beeches) with 10-13 species, Quercus (oaks) with over 600 species, Castanea (chestnuts), with 9-11 species and Lithocarpus known as stone oaks or tanbark oaks which are native to tropical and subtropical regions of Asia and the Pacific with around 330 species.

Beeches (Fagus genus) evolved in the Northern Pacific region based on the fossil record. In the Miocene, Fagus was established  throughout the Northern Hemisphere up to very high latitudes (e.g., Iceland). In post-Miocene times, Fagus underwent range reduction and extinction in high latitudes, Central Asia and western North America. Today beech species are found in Europe, eastern North America, the Caucasus, Iran and Turkey, Japan, Mexico southern South America and Tasmania. The European Beech (Fagus sylvatica), is native to most of Europe and parts of Asia Minor where it forms extensive forests in many regions. 

Oaks (Quercus genus). The earliest known fossils date back to the late Eocene period, around 35-40 million years ago, and were found in North America. Oaks then diversified and spread to different parts of the world, including Asia, Europe, and South America. Today, after recent ice ages, the 600 species of oaks are mainly found in temperate and subtropical regions of the Northern Hemisphere. Asia has up to 250 species, North America 100 species, Europe 60 species and Africa and South America 50 species.  

Chestnuts (Castanea genus) are believed to have originated in Asia, with the first fossil records dating back around 60 million years ago. They then diversified and spread to different parts of the world, including Europe and North America. Today most of the 12 species are found in Asia with a few in North America and Europe. Fossil records indicate that the Sweet Chestnut (Castanea sativa) was present in Europe during the Pliocene epoch, around 5 million years ago. Today it is native to southern Europe and parts of Asia Minor. 

Walnut family (Juglandaceae): This family contains around 60 species of trees. Its evolutionary history can be traced back to the Late Cretaceous period, around 90 million years ago. By the Miocene epoch, around 23 million years ago, the family had reached its peak diversity, with many different species of walnut and hickory trees found across the Northern Hemisphere. After repeated ice ages the Juglandaceae family is today represented by only a small number of species. There are 60 species in seven genera, the three largest being  Walnuts 21, Hickories 19 and Wingnuts 7, 

Walnuts (genus Juglans) are native to different regions of the Northern Hemisphere, primarily in North America, Central America, and Eurasia. These include: The Common Walnut, also known as the Persian Walnut  is native to regions in Central Asia (e.g., Iran, Kyrgyzstan, Tajikistan) and the Himalayas but widely cultivated in many temperate regions worldwide. The Black Walnut is native to eastern North America. The Butternut is native to eastern North America. 

Hickories (genus Carya) are predominantly found in North America, with a few species extending to East Asia. Shagbark Hickory is native to the eastern United States and Canada. Pecan is native to the southern United States, particularly Texas, Oklahoma, and parts of Mexico. The Pignut Hickory) is found in the eastern and southeastern United States.

Wingnuts (genus Pterocarya) primarily occurs in East Asia, including parts of China, Japan, and Korea. They include the Caucasian Wingnut native to the Caucasus region  Caucasus region, including areas in Georgia and Azerbaijan, the Japanese Wingnut): and the Chinese Wingnut)

Birch family (Betulaceae): This family has around 150 species of trees and shrubs in 5 genera divided into two subfamilies, with Betula (birch) and Alnus (alder) in one subfamily and Carpinus (hornbeam), Corylus (hazelnut), and Ostrya (hop-hornbeam) in the other. There are around 60 to 70 species of birches, 30 to 40 species of alders. 15 to 20 species of hazels, 25 to 40 species of hornbeams and 8 to 10 species of hop-hornbeams. The Birch family first appeared in the fossil record around 60 million years ago in the Northern Hemisphere. The family diversified during the Eocene epoch, around 50 million years ago, when the climate was warm and moist. During this time, birches and alders evolved and spread across Europe, Asia, and North America. The two subfamilies had a common ancestor at this time but subsequently diverged. Today Birches are typically associated with cooler climates and are often found in forests, woodlands, and montane regions in North America, Europe, Siberia, Russia, China and Japan.  Alders are typically found in moist habitats, such as along rivers, streams, and wetlands across north America, Europe, Asia and Central and South America particularly the Andes. Hazels are often found in woodlands, hedgerows, and forest understories across North America, Europe, Asia and North Africa and the Middle East. Hornbeams are often found in woodlands, forests, and mixed deciduous habitats in Europe, North America and Asia. Hop-hornbeams are usually found in woodlands, forests, and mixed deciduous habitats in North America , Europe and Asia. They prefer well-drained soils and are often associated with limestone-rich areas.

Willow family (Salicaceae):  This family is currently divided into two main genera Willows (Salix genus) and Poplars (Populus genus).  

Willows (Salix genus) This genus includes about 450-500 species and hybrids of willow trees and shrubs found throughout the Northern Hemisphere. There are several factors that have contributed to the high diversity of the Salix genus. Adaptability to a wide range of environments, their ability to hybridise with one another and the fact that they can reproduce by cuttings. This allows them to rapidly spread and colonise new areas, leading to the development of new species over time. The earliest fossils of the  Salix genus are from around 40 to 30 million years ago. Based on genetic analyses and molecular dating techniques, scientists estimate that the split between Salix and Populus occurred around 45 to 60 million years ago

Poplars (Populus genus).This genus includes about 60 species and hybrids of poplar trees distributed throughout the Northern Hemisphere. The genus Populus is believed to have originated in western North America and later spread to other parts of the Northern Hemisphere. Fossil evidence suggests that early poplars were primarily found in swampy and wetland environments. Fossil records indicate that poplars have been present on Earth for at least 60 million years. Over time, various Populus species have diverged and adapted to different ecological niches. The genus includes both fast-growing pioneer species and long-lived forest trees. Two species are native to Britain Aspen and Black Poplar and a third Grey Poplar to southern England. Populus tremuloides, commonly known as quaking aspen or trembling aspen, is native to North America, particularly the western and central parts of the continent. It is widely distributed across Canada and the United States. On the other hand, Populus tremula, known as European aspen or common aspen, is native to Europe and parts of Asia, including Scandinavia, Russia, and China. The best known poplars in Britain and Europe are cultivated varieties such as the hybrid Black Poplars and the Lombardy Poplar. 

Maple family (Aceraceae): This family has undergone significant taxonomic revision in recent years, and is now considered to be part of the larger Sapindaceae family, also known as the soapberry family which is a large and diverse family of flowering plants that includes about 140 genera and over 2,000 species distributed throughout the world. Within this larger group are two well known genera Maples (genus Acer) and Horse Chestnuts (genus Aesculus)

Maples (Acer Genus) includes about 130 species of trees. Fossil evidence suggests that the genus Acer originated in the Northern Hemisphere 66 million years ago. Today, Acer species are widely distributed across the Northern Hemisphere, particularly in temperate regions of North America, Europe, and Asia. Some species are also found in parts of North Africa and the Middle East. Over 100 species are native to Asia (Japanese Maple, Paper-bark Maple) with the rest split between Europe (Sycamore, Norway Maple, Field Maple) and North America (Sugar Maple, Red Maple, Silver Maple, Box Elder). Recent genetic evidence suggests that Acer originated and diversified in Asia and then spread by various land bridges to Europe and then to North America.

Horse Chestnuts (Aesculus genus) includes 13-19 species with one European species, five Asian species and seven North American species. The European and Asian species are known as Horse chestnuts, the North America species  as Buckeyes (named after their ‘Conkers’ which reminded the early settlers of the USA of a buck’s eye). The Red Horse Chestnut is a hybrid between the American Red Buckeye and the Horse Chestnut. A Late Cretaceous origin of the common ancestor of Aesculus in eastern Asia was followed by dispersals into western North America, Europe, and eastern North America during the Late Cretaceous and the Paleogene. 

Lime family (Tiliaceae): This  is a large family of flowering plants that originally included around 500-600 species. In recent classifications it has become a subfamily of the Malvaceae family. The subfamilies most important genus is Tilia (Limes) also known as the linden or basswood genus which has more than 20 species of trees of which 17 are found in Asia (Mongolian, Henry’s and Oliver’s Limes), 4 in Europe (Common, Small-leaved, Broad-leaved and Silver Limes) and 1 in North America (American Lime known as Basswood). The actual number of species is difficult to estimate because species are variable and difficult to differentiate. The genus is native to the temperate regions of the Northern Hemisphere, including Europe, North America, and Asia. The Tilia genus is thought to have originated in Eurasia, with the oldest known fossils dating back to the early Eocene period, around 50 million years ago. Today, the Tilia genus is widely distributed throughout the temperate regions of the Northern Hemisphere with the highest species diversity in Asia. 

Olive family (Oleaceae):  This is a family of flowering plants that includes about 25 genera and approximately 600 species. The family is distributed primarily in the temperate regions of the Northern Hemisphere, but also occurs in some tropical regions. Some of the well-known genera in the Oleaceae family include: Fraxinus (ash), Jasminum (jasmine), Ligustrum (privet), Olea (olive) and Syringa (lilac). Ash trees  (Fraxinus genus), includes 45  species of trees widely distributed across the Northern Hemisphere, particularly in temperate regions of North America (22 species), Europe (3), and Asia (20). Fossil and genetic evidence suggests that the early ancestors of Fraxinus appeared in North America. This was followed by migration to Asia and then expansion from  Asia to Eurasia and Africa. Common Ash, Narrow-leaved Ash are from Europe, White Ash, Green Ash, Black Ash from North America and Chinese, Manchurian, Japanese and Korean Ash from Asia.