Laos is the only landlocked country in Southeast Asia, and it lies mostly between latitudes 14° and 23°N (a small area is south of 14°), and longitudes 100° and 108°E. Its thickly forested landscape consists mostly of rugged mountains, the highest of which is Phou Bia at 2,818 metres (9,245 ft), with some plains and plateaus. The Mekong River forms a large part of the western boundary with Thailand, where the mountains of the Annamite Range form most of the eastern border with Vietnam and the Luang Prabang Range the northwestern border with the Thai highlands. There are two plateaux, the Xiangkhoang in the north and the Bolaven Plateau at the southern end. Laos can be considered to consist of three geographical areas: north, central, and south. Laos had a 2019 Forest Landscape Integrity Index mean score of 5.59/10, ranking it 98th globally out of 172 countries.

In 1993 the Laos government set aside 21% of the nation's land area for habitat conservation preservation. The country is one of four in the opium poppy growing region known as the "Golden Triangle". According to the October 2007 UNODC fact book Opium Poppy Cultivation in South East Asia, the poppy cultivation area was 15 square kilometres (5.8 sq mi), down from 18 square kilometres (6.9 sq mi) in 2006.

Climate

The climate is mostly tropical savanna and influenced by the monsoon pattern. There is a distinct rainy season from May to October, followed by a dry season from November to April. Local tradition holds that there are three seasons (rainy, cool and hot) as the latter two months of the climatologically defined dry season are noticeably hotter than the earlier four months.

Habitat

Much of Laos is still covered by natural forest and six different species of gibbon are found here, all of which are threatened by being hunted for food and by reduction in forest cover.

The Annamite Range has a high level of endemism and is home to the critically endangered saola (Pseudoryx nghetinhensis), the Annamite striped rabbit (Nesolagus timminsi) and the Truong Son muntjac (Muntiacus truongsonensis), all of which have only been discovered in the last two decades.

In the south of the country, mostly within about 50 km (30 mi) of the Mekong River, there are wetlands and swamp forests. These include lakes and ponds, some permanent and some temporary, swamps, and seasonally-flooded grasslands, and these and the surrounding woodlands support a biodiverse community.

Flora

Much of Laos still retains its natural tropical and subtropical dry broadleaf forests, and some areas consist of secondary forests growing where timber has been extracted from the primary forest. In general, the upper parts of the canopy are dominated by the cauliflower-like crowns of tall dipterocarps with trunks devoid of branches. The middle levels consist of other hardwood trees including teak, mahogany, Heritiera javanica, Tetrameles nudiflora, Ficus and Pterocarpus. The understorey consists of smaller trees, bamboos, shrubs and grasses. Near the streams are bamboo thickets; Laos is rich with species of bamboo, and where timber has been extracted from primary forest, bamboos tend to dominate in the new-growth secondary forest. In wetter, upland regions of northern Laos, Fujian cypress is dominant, and supports a dense ground cover of mosses and ferns.

There is tropical rainforest in the Annamite Mountains because the rainfall is higher and more evenly distributed throughout the year. The dominant species here are Elaeocarpus, Podocarpus, oaks, magnolias, camellias and laurels, and many of these are endemic to this mountain range. In the higher parts of the range there are forests of pine, and the mists that often envelope these cloud forests encourage the growth of lush mosses and ferns on the deep leaf litter, and epiphytes on the branches.

In the south of the country there are tropical pine forests, and on the southern plateau are more open deciduous forests with less-dense, middle-level growth and more shrubs, herbs and grasses below. The bamboo orchid grows here, and the Vietnamese white pine is an uncommon tree that can be seen in the Nakai–Nam Theun conservation area in central Laos. It is one of about sixteen species of conifer present in the country.