Trees On Stamps :

This 2015, 50 kopeck definitive stamp of Ukraine features the horse-chesnut tree (Aesculus hippocastanum).

New Zealand
Native Trees
February 1, 2012
60c: Pohutukawa (Metrosideros excelsa)
$1.20: Cabbage Tree (Cordyline australis)
$1.90: Kowhai (Sophora microphylla)
$2.40: Nikau (Rhopalostylis sapida)
$2.90: Manuka (Leptospermum scoparium)

Venezuela
Bank of Venezuela - Tree and Arms
June 27, 1990
Tabebuia chrysantha, Ceiba pentandra, Myrospermum frutescens, Pithecellobium saman, Cedrela odorata, Dipteryx punctata, Licania pyrofolia,
Sterculia apetala, Tabebuia rosea, Prosopis juliflora, Erythrina poelpigiana, Copernicia tectorum

Indonesia
Fauna and Flora
November 5, 2018
A strip of three stamps depict trees: Koompassia excelsa, Eusideroxylon zwageri, Agathis borneensis.

Transkei (South Africa)
Trees
October 5, 1989
18c: Broom Cluster Fig (Ficus sur)
30c: Natal Fig (Ficus natalensis)
40c: Broad-leaved Coral (Erythrina latissima)
50c: Spiked Cabbage Tree (Cussonia spicata)

Monaco
National Species
July 12, 2014
The stamp depicts the carob tree (Ceratonia siliqua).

Here is the 15 sene value (SG #807) from Samoa's "National Program to Conserve Nature and Environment" set of 6 stamps from October 25, 1988. The small, long-leafed trees in the foreground are probably Fala, or Tahitian scrubpine (Pandanus tectorius).

Singapore
Heritage Trees
September 25, 2002
The stamp set depicts Flame of the Forest (Delonix regia), Rain Tree (Samanea saman), Tembusu (Fagraea fragrans), and Kapok Tree (Celba pentandra).

On May 4, 1977 Finland issued this stamp (Facit #812) to commemorate the 75th anniversary of co-operative banks in the country. Of the design, Facit says, "Symbolic with a tree and birds building a nest."

Here is a postage stamp of private courier TNT Post of the U.K. The large "S" on the stamp seems to indicate standard service for card or letter.

The TNT U.K. website indicates that the business has been acquired by FedEx (USA). Header says, "TNT will become FedEx."

Colombia
Departments of Colombia – Atlantic
June 15, 2010
Set of 12, one stamp depicts Tabebuia rosea), a neotropical tree that can grow up to 30 m.

Lebanon
Independence of Lebanon
November 19, 2018
The stamp depicts a Lebanon Cedar, the national emblem of Lebanon.

1931 - Turkey - 2nd Balkan Cnference

On June 7, 2006 Denmark issued its first series of Self-adhesive, thermally-printed stamps as a set of three "Nature Views" designs. One of these stamps (Mi #34) depicts a young tree in a storm.

Fall trees in full color are depicted on this large (55mm x 55mm) cinderella of Germany advertising "Finest oil pastels in 60 colors" by the Faber Company. It is printed by chromolithography, a process invented by American Louis Prang after studying the lithography process in Germany. Chromolithography was a popular printing process for better German postcards from the 1900-10 period, so the cinderella probably dates from the same period. The close-up pic shows the "stippled"/speckled effect of the colored areas, a characteristic of chromolithographic printing.

From wikipedia, Faber-Castell was founded in 1761 at Stein near Nuremberg by cabinet maker Kasper Faber as the A. W. Faber Company, and has remained in the Faber family for eight generations."