Trees On Stamps :

Dragon Tree on Somali Coast # 287.
Genus Dracaena, with different species in the Arabian Peninsula and the Canary Islands.
Also known as Dragon's Blood Tree because of the crimson resin it produces.
Used since ancient time for dyes, as medicine, and in ritual magic and alchemy. (how could you not use it for ritual magic with a name and appearance like that?!)
Also used from the 18th century until present time as a stain for violins, and currently popular as incense in it's powder form.

Here is the 2 d value (Sc #41) from Nigeria's February 1, 1936 KGV pictorial definitive series.

This October 3, 1979 35c value of New Zealand is from the 1979 Christmas set of three. Stamp depicts a pohutukawa, also known as a N.Z. Christmas, or iron tree. The pohutukawa (Metrosideros excelsa) is a coastal evergreen tree in the myrtle family.

From nzhistory.gov.nz, "The pohutukawa tree with its crimson flower has become an established part of New Zealand Christmas tradition. This iconic Kiwi Christmas tree, which often features on greeting cards and in poems and songs, has become important symbol for New Zealanders at home and abroad."

Ciskei (South Africa)
Trees
February 2, 1983
The stamp set depicts Cussonia spicata, Curtisia dentata, Calodendrum capense, Podocarpus falcatus.

Here is the 60c value (Sc #450) from Mauritius's 1978 Historical Events and Maps pictorial definitive series. Stamp shows Pierre Poivre and a nutmeg tree (Myristica fragrans).

From wikipedia, "Pierre Poivre (1719-1786) was an 18th century horticulturist and botanist...On northern Isle de France - Mauritius, he is renowned for the establishment of Botanical Garden of Pamplemousse with specimens of trees and plants imported from tropical habitats worldwide...He is also remembered for introducing spice plants to Mauritius and Reunion, such as clove and nutmeg, commodities which at the time were controlled by the Dutch, who had a virtual monopoly on them in the Dutch East Indies."

Note that Poivre's right hand is missing. That is the result of a hit taken from a British cannonball.

Trees are depicted on two values from Mauritius's 1989-98 Environmental Protection series. The one rupee stamp (Sc #689) shows a mangrove tree (Rhizophora mucronata), and the 4 rupees value (Sc #693a) features a fern tree (Alsophila borbonica).

On October 27, 1945 Sweden issued a set of two identically-designed coil stamps in different colors and of different values to commemorate the 125th anniversary of the founding of the first Swedish savings bank in Gothenburg (Oct. 28, 1820). Here is the 10 ore value (Facit #363A) of the set. A mature oak tree symbolizes the Savings Bank Movement in Sweden.

Jordan
Trees
August 7, 2003
The stamp set depicts Cupressus sempervirens, Pistacia atlantica, Quercus Aegilops.

Republic of China (Taiwan)
Scenery - Taoyuan
September 10, 2021
Set of four, one stamp depicts giant red juniper trees (up to 2800 years of age) in Lala Mountain.

Switzerland
Trees
September 9, 2021
The stamp set depicts larch (85c), oak (100c), sycamore (150c), pine (200c).

Finland issued this 1st class domestic rate stamp on September 13, 2010 to commemorate the 20th anniversary of Torronsuo National Park. The Park is a large (25.5 sq km / 9.85 sq mi) raised bog standing on a thick layer of peat. Stamp depicts a Baltic, European red, or Scots pine (Pinus sylvestris).

From wikipedia, "The species is mainly found on poorer, sandy soils, rocky outcrops, peat bogs or close to the forest limit. On fertile sites, the pine is out-competed by other tree species".

Pakistan
Pakistan Tourism Convention
October 1, 1987
Set of four, one stamp depicts apricot trees.

On December 26, 1935 El Salvador issued a set of 10 stamps (5 regular postage, and 5 airmails) to commemorate the tercentenary of the town of San Vicente. The central image of these stamps shows a tempesque, or yellow danthus tree (Sideroxylon capiri). This is a species native to the Pacific slopes of Central America.

The 15c regular postage value (Sc #558) of the set is shown.

From wikipedia, "San Vicente is a municipality in the San Vicente department of El Salvador. It was founded by 50 Spanish families in 1635, under the tempesque tree which is still standing today."

On April 20, 1976 the Republic of South Africa issued a souvenir sheet with stamps of four values to commemorate the birth centenary of painter Erich Mayer (1876-1960). Each of the s. sheet's stamps shows a Mayer oil painting of a South African landscape. The 4c value (Sc #461) depicts Picnic Under Baobab Tree.

Here is Guatemala's 1963 4c airmail stamp (Sc #C273) depicting a Ceiba, or kapok tree (Ceiba pentandra) which in 1955 had been declared the country's national tree.

From wikipedia, "The Ceiba, or ya'axche' (in the Mopan Mayan language), symbolized to the Mayan civilization an 'axis mundi' which connects the planes of the underworld (Xibalba) and the sky with that of the terrestrial world."