Dwarf forest

Dwarf forest, elfin forest, or pygmy forest is an uncommon ecosystem featuring miniature trees, inhabited by small species of fauna such as rodents and lizards. They are usually located at high elevations, under conditions of sufficient air humidity but poor soil. There are two main dwarf forest ecosystem types, involving different species and environmental characteristics: coastal temperate and montane tropical regions. Temperate coastal dwarf forest is common for parts of Southern California. Montane tropical forests are found across tropical highlands of Central America, northern South America and Southeast Asia. There are also other isolated examples of dwarf forests scattered across the world, while the largest dwarf forest is found in the Philippines.

High-elevation tropical dwarf forest

Elfin forest on Mt. Kemiri, Sumatra, at an elevation of about 3000 m

High-elevation tropical locations in cloud forests contain mossy wet elfin forests due to high-elevation precipitation. These regions are characterized by low rainfall, with most of the water in the form of mist and fog. The water supplied is primarily available during the night, when clouds move from the ocean over the mountains, and are intercepted by the vegetation. During the day, water demands are increased as clouds rise over the mountain peaks without dissipating into available forms of precipitation.^[1]^[2]

The forests are characterized by small trees (5–8 m), with shallow root systems, and abundant epiphytes.^[2] The epiphytes make up a large portion of the canopy, with greater abundances in high-elevation tropical elfin forests than what is found in other, non-elfin, tropical forests.^[3]

Dwarf forests should be more commonly found on isolated mountains due to the massenerhebung effect. The massenerhebung effect is a phenomenon where treelines are typically higher among mountains in close proximity to other mountains. The mountains in close proximity affect the rate of wind and heat retention, decreasing the negative effects of climate. When the treeline is lower on isolated mountains, the climatic effects are more severe.^[4]

Flora

Within montane dwarf forests, there are relatively few species, with a small number of dominant species that make up a large portion of the population. Low, horizontally-branching, shrub-like plants, and dense populations of moss, lichen, and liverworts are found due to high wind speeds, low temperatures and light reduction from persistent clouds and fog, which limit the growth of tall plants.^[5] The high wind speeds act as the determining factor of the stature in dwarf forest flora, especially on ridges and slopes. A low stature increases the structural stability of the plants.^[3] Wind-exposed trees invest more of their resources to increasing strength than to growth, compared to non-wind-exposed trees. The increased focus on strengthening leads to thicker trunks and twigs, which increases the ability of the trees to withstand greater wind stresses near the ridge-crest, where the majority of wind-exposed trees are found.^[6] A large percentage of energy is also allocated to growing and maintaining heavy and extensive root structures, further strengthening the tree and increasing its resistance to high winds.^[1]

Plants here have leaves with moisture-tolerant characteristics, such as drip tips and waxy cuticles.^[5] They also have a slow rate of transpiration and metabolism due to low temperatures and low radiation penetration.^[3] A large percentage of plants possess alkaloids and other natural products, likely to combat a high amount of herbivory from insects. This may also account for the low leaf surface area and low transpiration rates of the plants. One study showed that leaves of ten plant species had approximately 70–98% of all leaves damaged by insects.^[5]

Fauna

Elfin forests occur at high elevations, which are generally associated with low vertebrate biodiversity. Hummingbirds and bats make up a large proportion of the vertebrates in some areas, usually as altitudinal migrants during seasonal shifts, such as for reproduction, or in response to food abundance.^[7] Other vertebrate species mostly include small rodents.

Seasonal variations

Rainfall tends to be highly seasonal, sparse, and far between, therefore fog interception is a significant water source during dry seasons.^[8] Throughout the year, wind speed, temperature and humidity are fairly consistent, with humidity usually greater than 90%.^[1] At one study site in the Guajira Peninsula, dry season precipitation ranged from 1–4 days per month, while in the wet season, although increased, it was still a relatively low 4 to 12 days per month,^[1] supporting the idea that the majority of the water in this region is held in low cloud cover and fog interception.^[8] Sunshine duration is distributed bimodally and correlates with evaporation rates.^[3]

Examples

Mount Hamiguitan pygmy forest in the Philippines

Pygmy forest of Mount Hamiguitan on Mindanao, Philippines at the elevation of 1160–1200 m a.s.l.).^[9] It is a national park and a protected area since 2004 and was declared a UNESCO World Heritage Site in 2014. It is estimated to have around 1,380 different species; with 341 endemic to the Philippines (including the Philippine eagle, the Philippine cockatoo and the Philippine tarsier). Several species are endemic to Mount Hamiguitan itself, like the Mount Hamiguitan pitcher plant (Nepenthes hamiguitanensis), the Mount Hamiguitan fern (Lindsaea hamiguitanensis) and the butterfly (Delias magsadana).^[9]^[10]^[11]^[12] At 6,834 hectares (16,890 acres) the extensive pygmy forest of Mount Hamiguitan is the largest known dwarf forest in the Philippines and possibly the world.^[13]^[14] It merges with mossy forests, dipterocarp forests, and montane forests at lower elevations.^[13]^[15]
El Yunque National Forest of Puerto Rico features dwarf forest areas at elevations above 900 m,^[16] as does Los Tres Picachos State Forest.^[17]

Coastal temperate elfin forest

Pygmy forest at Salt Point State Park in northern California

Elfin forests of California are the primary example of coastal temperate dwarf forests. They are expansive, and cover most of the mountains in the southern half of California, extending into Mexico, Nevada, and Arizona.^[18] Other expanses of elfin forest are found throughout the state, in the northern and central regions.

In northern California, Henry Cowell Redwoods State Park is home to an elfin forest with Mendocino cypress (Cupressus pygmaea), and Sargent's cypress (C. sargentii), which is partially within a section of the Zayante Sandhill Area. On the Central Coast of California, on the southeastern shore of Morro Bay, Los Osos contains the El Moro Elfin Forest Natural Area. The area is approximately 90 acres. It derives the “elfin forest” title from the short California live oaks, which range in height from 4 – 20 feet, compared to the typical 30–80 feet. This region also contains the federally endangered Morro shoulderband snail (Helminthoglypta walkeriana). At a higher elevation, on Cuesta Ridge, the Land Conservancy of San Luis Obispo County manages the San Luis Obispo Elfin Forest of dwarf cypresses.

Factors such as soil moisture, solar radiation, and rockiness of soil influence species composition along an elevation gradient, resulting in certain shrub species, such as Adenostoma fasciculatum and Arctostaphylos glauca, being present in elfin forest habitats.^[19]

Fire occurs at low-moderate frequency with high severity. Many plants have adapted to this by having serotinous seeds that open to germinate only under high heat.^[19] Because of this, they are often the first to colonize a new area.

Pygmy coast live oaks in elfin forest, California