Thomas R. Sim was the son of John Sim, a farmer, plant collector and authority on mosses, and his wife Isabella Robertson. Thomas attended school in Aberdeen until 1872 and attended some classes in botany and agriculture at Marischal College in that city, but was largely self-taught. He was apprenticed as a gardener in the Royal Horticultural Society's garden at Chiswick, London, in 1874, and in February 1878 started work at the Royal Botanical Gardens at Kew, near London, where he received some training in botany under Sir Joseph Hooker*. In January 1879 he went to the United States, working for a year in the botanic gardens of Harvard University in Cambridge, Massachusetts, and then at a private establishment. At Harvard he attended lectures in botany by Professor Asa Gray and others. The influence of Hooker and Gray, both great systematists, was later clearly reflected in Sim's published work. He returned to Scotland in 1881 to farm with his father in Aberdeenshire. From 1884 to 1888 he was a fruit farmer at Inchmark. In 1882 he married Margaret R. Wilson, with whom he had two sons and a daughter.
Sim came to South Africa in 1889 to take up the post of curator of the King William's Town botanical gardens. His interest initially centred around the ferns of the region and from 1891 he presented ferns and other botanical specimens to the Albany Museum in Grahamstown. Only two years after his arrival he published a Handbook of the ferns of Kaffraria... (Aberdeen, 1891). This was soon followed by a more comprehensive work, The ferns of South Africa, containing descriptions and figures of the ferns and fern allies of South Africa, with localities, cultural notes, etc (Cape Town, 1892; 2nd ed., Cambridge, 1915). The book was illustrated with his own line-drawings. Two years later he published Sketch and check-list of the flora of Kaffraria... (Cape Town, 1894), based on a paper he read before the King William's Town Naturalists' Society in July 1893. He participated in the society's activities and in 1896 contributed specimens to its museum (later the Kaffrarian Museum).
In September 1894 Sim accepted a position in the Forestry Service of the Cape Colony and was soon appointed as superintendent of plantations, stationed at Fort Cunnynghame, just north of Stutterheim in the Eastern Cape. During 1897 he sent plant specimens to the Colonial Botanist, P. MacOwan*. In November 1898 he rose to district forest officer at King William's Town. The next year he made a botanical tour through the Egossa Forests, which resulted in his Botanical observations of forests of eastern Pondoland (1899, 36p), published as Pamphlet No. 28 of the Cape Department of Agriculture. He also gave lectures on forestry, horticulture and fruit culture on various occasions.
On the basis of his work in the Eastern Cape Sim was transferred to Natal as conservator of forests for that colony in September 1902, a post that had been abolished with the resignation of F. Sch?pflin* in 1893. His brief was to re-establish a Forestry Department in Natal. He soon re-organised the forest service, issued regulations for the protection and utilisation of crown forests, and planted and named an arboretum of about 400 species of trees and shrubs at Cedara, the agricultural research station near Howick. In 1907 he published The forests and forest flora of the Colony of the Cape of Good Hope (Aberdeen, 1907, 361p), the first comprehensive work on South African trees, with his own line drawings of 312 species. It provided a useful, though cumbersome handbook for the Forestry Department and is a monument to Sim's industry and perseverance. In 1908 he presented herbarium specimens of the family Thymelaeceae (small trees and shrubs) to the South African Museum. That same year, as an advisor to the government of Portuguese East Africa (now Mozambique), he made an extensive and arduous exploration of the forests of that country and wrote Forest flora and forest resources of Portuguese East Africa (Aberdeen, 1909), again illustrated with his own sketches. He also published papers and notes on various topics in the Agricultural Journal of the Cape of Good Hope and the Natal Agricultural Journal. His other activities included membership of the Natal Technical Education Commission, appointed in October 1904, and representing Natal at the South African Products Exhibition in London in 1907.
Sim's regulations for the protection of the indigenous forests of Natal were unpopular and led to conflict with other government departments, particularly Native Affairs. As a result he was retired in 1907 and his post abolished. He then set up in business as a nurseryman and consultant on tree-planting in Pietermaritzburg. Later he also acquired a timber factory, became involved in wattle growing, and was a foundation member of the Wattle Growers' Association. His advice on afforestation, fruit growing and wattle culture was sought from all over South Africa and even from neighbouring states. His expertise extended to the ornamental use of trees and shrubs and he played a major role in promoting tree planting in Natal. This work led to the publication of Flowering trees and shrubs for use in South Africa (Johannesburg, 1919), which dealt wit both indigenous and exotic species. The next year a monograph by him on Soil erosion and conservation... was published as Memoir No. 47 (1920) of the Department of Mines and Industries. His Native timbers of South Africa (Memoir No. 3, Department of Mines and Industries, 1921) dealt with the economic possibilities of every South African tree and shrub. He repeatedly stressed the value of timber production, both by means of planting exotic species and by working indigenous forests on a permanent yield basis. More than anyone he was responsible for the foundation of a timber industry in Natal. As part of the annual meeting of the South African Association for the Advancement of Science held at Bulawayo, Zimbabwe, in 1920, he led a botanical party to the Victoria Falls and Zimbabwe Ruins. While in that country he suffered a stroke that partly paralysed him for the rest of his life, though he was able to continue his botanical studies. He also suffered the discomforts of rheumatism during his later years.
Sim was more botanist than forester, devoting himself mainly to the study of forest flora, including ferns, mosses and liverworts. In 1905 and 1906 he read papers on his more recent studies of South African ferns at the joint meeting of the British and South African Associations for the advancement of science (Addresses and papers..., 1905) and the South African Philosophical Society (Transactions, Vol. 16, pp. 267-300). In 1915 he compiled a check-list of over 1000 mosses, and the next year a mimeographed Handbook of the Bryophyta of South Africa, with keys and descriptions of the genera of mosses. That year he and his brother, J.M. Sim, sent many specimens of Natal fungi to I.B. Pole-Evans* in Pretoria, for inclusion in his mycological herbarium. After Sim's retirement in 1922 he again turned his attention to the mosses and liverworts and expanded his earlier checklist into The Bryophyta of South Africa, published in the Transactions of the Royal Society of South Africa (1926, whole Vol. 15, pp. 1-475) and also as a monograph (Cape Town, 1926). This standard work contained descriptions of 671 species of mosses, most of them illustrated by the author. Between 1915 and 1931 he published a variety of papers in the Report of the South African Association for the Advancement of Science and its successor, the South African Journal of Science. These dealt with, among others, "South African Hepaticae, or Liverworts" (1915), "Geographical distribution of the South African Bryophyta" (1917), "South African fern notes, with lists of ferns and fern-allies found in Southern Rhodesia" (1920), "Notes on South African ferns and Rhodesian mosses" (1923), "The mosses of the south-west portion of South Africa" (1924), and "Further notes on the distribution of the ferns of South Africa" (1931). His last book was Tree planting in South Africa (Pietermaritzburg, 1927, 452p), which also covered present Zimbabwe and Mozambique.He disposed of his business interests in 1928. At the time of his death he was busy writing a major work on the trees of southern Africa.
Sim had an excellent memory for plant names, a keen interest in systematic relationships and was untiring in his search for new species. His important contributions to botany were recognised by the South African Association for the Advancement of Science, which awarded him its South Africa Medal (Gold) in 1916. In 1920 the University of South Africa conferred an honorary Doctor of Science (DSc) degree upon him. He was a Fellow of the Linnean Society of London, the Royal Horticultural Society, and (from 1927) the Royal Society of South Africa; also a member of the South African Association for the advancement of Science, serving as president of Section C in 1919/20, and a foundation member of the South African Biological Society in 1916. Around 1906 he was president of the Horticultural Society of Natal. His specimens of cryptogams are in the National Herbarium in Pretoria, with copies in many other local herbaria. He also collected the whole range of flowering plants, resulting in an herbarium that is now housed at the University of Natal. His library was bought by the National Botanical Institute, Pretoria. He is commemorated in the names of the genus Simia of liverworts, and the species Rhus simii (one of the taaibos species), Royena simii (tolbos), Eugenia simii, and Sapium simii.