dry forest ecology

7/27/2019 Dry Forest Ecology

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Dry Forest Ecology

A Seasonal Climate

Dry Tropical Forest is found in regions where there are several months of

severe to absolute dry season, with most rain falling during a (usually) brief wet season. More than any other factor, the absence of precipitation during aprolonged portion of the year is what produces true dry forest, an ecosystemtype characterized by plants and animals possessing specific adaptations tosurvive the dry season. Dry forests typically are found in very warm regions inthe tropics, where the mean annual temperature is greater than 17C (63F),and where rainfall is in the range of 250 to 2000 mm per year (10 to 80inches).

Adaptations

The single most important adaptation, among plants, to the extended droughts of the dry forest isdeciduousness, the shedding of leaves. Plants drop their leaves after the rains end, and essentially halt

photosynthesis, which otherwise produces water losses the plant cannot sustain during the dry season.Some plants, like the Ceiba trichastandra, have an alternative source of photosynthetic energy: they havegreen bark rich in chlorophyll, that lets them continue to photosynthesize even when they have no leaves.Other plant adaptations include the development of water storage tissues, such as swollen roots or stems, that allow them to draw on saved water to survive the dry season. Many plants have an extra,waxy layer on the outside of their leaves, which also slows water evaporation. Some plants even engagein "nyctinasty," a fabulous botanical word that means they close their leaves at night: clasping leaves

together reduces the amount of exposed surface area, also slowing water loss.

Animals, too, have some fascinating adaptations to the dry forest's longrainless weeks and months. Foremost among these is "estivation," thesummer-time equivalent of hibernation. Many frogs and insects simply burrow

deep into damp mud, or their own excavated chambers, and go to sleep,reducing their metabolism (and thus their water needs). When the rains return,the increased moisture awakens these animals, and they return to the surfaceto breed. Other animals, such as birds and monkeys, show a remarkabledegree of mobility during the dry season, retreating to damp areas such asstream beds, where year-round moisture enables them to survive. Howler

monkeys, who eat nothing but green leaves and occasional fruits, have been known to cram into smallstream side forests at amazingly high densities, yet they do not fight over territory as they would duringthe rest of the year.

Synchronicity

It all starts with the first rains. Like a desert blooming in spring, thefirst thundershowers in the dry forest produce an outbreak of freshgreen leaves. This habitat has literally set its watch by the dry season,and many key ecological processes all take their cue from the arrivalof the rains. As the new leaves begin catching sunlight, plant growth,stalled during the dry months, shifts into high gear. The precipitationmoistens the leaf litter accumulated on the forest floor, where bacteriaand insects (many just awakened from estivation) start thedecomposition process that releases nutrients for use by the growingplants. As the rainy season nears its end, trees and other plants drop



their leaves and produce a profusion of showy flowers in a fireworks-like display designed to attract asmany pollinators as possible. Those that are lucky, who get the timing just right, succeed in having someof their flowers pollinated before the dry season commences.

During the dry season plants sit mostly dormant, drawing on their stored water, perhaps using their greenbark to generate some energy, and waiting, waiting for the rains to fall again. Leaf litter dries, and insects,

frogs and many others return to their chambers to wait as well. As this dry season reaches its conclusion,those pollinated flowers now produce fruits, which arrive with the first rains. Now there are fruits, andleaves, in abundance, and animals from birds to mammals to insects and lizards begin foragingvoraciously to recover weight lost during the dry season, and begin looking for mates. Young areproduced quickly, in order to take advantage of the surfeit of fruit and other foods. Soon the forest isteeming with new life, bright green and full of the sounds of animals stocking up in preparation for yetanother long, hot dry season.

Conservation

Dry tropical forest once occupied more land area than rainforest, at42% of all intra-tropical vegetation. However, it is easily converted tocattle pasture by logging and burning, and now very little dry tropical

forest remains. In Ecuador less than 2% of the original extent of thisforest type remains, a statistic which is characteristic of most tropicaldry forest regions in the world; however, in Central America sadly lessthan one-tenth of one percent remains. Because of these tremendousrates of loss, organisms that once were common in these forests nowface extinction, merely for lack of habitat. Furthermore, because fewfunctioning dry forest ecosystems remain (the forest is reduced to

small, isolated patches in most parts of the world), their ecology is poorly studied, and their fauna andflora are far less well understood than in the much better-studied rainforests.

Some positive developments can be reported, however, particularly in Costa Rica and Africa. In thewestern Costa Rican province of Guanacaste, the tireless efforts of Dr. Dan Janzen led to theestablishment of the Santa Rosa National Park, later reformed as the much more extensive Guanacaste

Conservation Area. Janzen has succeeded, virtually single-handedly, in resuscitating the nearlyextirpated dry forest ecosystem in Costa Rica, an area which now enjoys visits by hundreds of thousandsof tourists, students and researchers each year.

In Africa, the tremendous revenue potential of international tourism has been harnessed for decades, andnow provides sufficient income to support the protection of dry forest and savannah (seasonal, but grass-dominated) ecosystems. The famed safaris to the Serengeti National Park are just one example of howthe interest of the world has been used to protect ecosystems.

Diversity

Although the dry tropical forest is not as species-rich as the famed

rainforests of Amazonian South America, a remarkable diversity of plants and animals calls this habitat home. Between 50 and 70 woodyplants (trees and large shrubs) can be found in only one-tenth of ahectare (0.25 acres), far more than in most temperate ecosystems,where hundreds of acres can be populated by only a handful of species. Even epiphytic plants can be abundant in dry forests,including bromeliads and a stunning variety of orchids. These forestscan be very rich in bird life, often numbering over 200 or 300 species,and are important as stop-overs for migratory fowl, especially thosebirds that follow the Pacific coast to the temperate breeding grounds.

http://www.acguanacaste.ac.cr/




http://www.serengeti.org/



http://www.ceiba.org/loorflora.htm


http://www.ceiba.org/loorfauna.htm










Despite the annual dry season, these forests can support a surprising diversity of amphibians, and of course, reptiles can be very abundant in dry forest, where they tough skin and hard-shelled eggs givethem an advantage.

http://www.ceiba.org/loorecology.htm

http://www.marietta.edu/~biol/biomes/tropdry.htm

www.slideshar.net/mdonoohue/tropical-dry-forest-biome

found in regions where there are several months of severe to absolute dry season, with most rain fallingduring a (usually) brief wet season.

annual temperature is greater than 17C (63F),

250 to 2000 mm per year (10 to 80 inches).

deciduousness, the shedding of leaves. Plants drop their leaves after the rains end, and essentially haltphotosynthesis, which otherwise produces water losses the plant cannot sustain during the dry season.

like the Ceiba trichastandra, have an alternative source of photosynthetic energy: they have green barkrich in chlorophyll, that lets them continue to photosynthesize even when they have no leaves.

Other plant adaptations include the development of water storage tissues, such as swollen roots or stems, that allow them to draw on saved water to survive the dry season

The Tropical Seasonal Forest

Tropical Dry Forest, Tropical Deciduous Forest, Savanna

Climate:distinguish between two closely related biomes, the tropical seasonal forest and the savanna.The tropical seasonal forest has more or less densely growing trees which lose their leaves during the dryseason. The savanna is a grassland with individual trees or groups of trees dotting the landscape. If





http://www.slideshar.net/mdonoohue/tropical-dry-forest-biome







you look at the climate diagrams to the right, you will see that the temperature regime is the same for both; with average annual temperatures over 20�C. Rainfall can vary from less than 50cm/year toalmost 300 cm/year. Savannas would occupy the dryer part of this range.

The figure to the left was taken at the Santa Rosa National Park in Costa Rica. This park has a typicaltropical seasonal forest, and gets about 130 cm of rain per year. The rainfall is highly seasonal; withmost of the rain falling in the months from May to November, May being both the end of the dry seasonand the wettest month overall, at least in the year pictured here. This strong seasonality of rainfall ischaracteristic of the tropical seasonal forest. During the rainy season(s), trees can easily maintain leavesand productivity is high. During the dry season(s), evaporation from the leaves is too high for the tree tosustain, so the leaves are dropped, much the way temperate deciduous forest plants lose their leaves inthe winter.

World Distribution:

Tropical seasonal forests and savannas are, as one might suspect, largely restricted to the tropics;actually a little further as most are found between 30�N and South latitudes. Much of Mexico falls intothis biome, as does a stretch of forest down the west coast of Central America. South America hasthese biomes in the northeast, in a band across Brazil to Argentina, and a smaller band on the west

coast. Africa has a large extent of tropical seasonal forests and savannas; a band runs through sub-saharan Africa and runs down the eastern coast with another band extending across the continent southof the tropical rainforest. There are extensive areas of these biomes in India, southeast Asia, andnorthern Australia as well.

Tropical Seasonal Forest Distribution



Indicator Plant Species:

Indio Desnuda, Bursera simarouba - Santa Rosa, Costa Rica

Acacia and Ants, Santa Rosa, Costa Rica

At Santa Rosa, one of the more noticeable trees at the end of the dry season was Indio Desnuda (thenaked Indian), Bursera simarouba (above). This plant has a photosynthetic green bark that can carry onsome photosynthesis even during the dry season when the leaves have dropped. Acacias (above right)are another important tree of this type of biome; they are found both in the new world and in Africa. Thethorns deter many herbivores (but not giraffes); to increase the deterrent factor many species have hollowthorns which are home to colonies of ants which attack herbivores, insects, competing plants, andcareless hikers.

I have no idea what the big tree to the right is, but we saw a number of them in Santa Rosa. They hadleaves at the end of the dry season and were very big and very impressive.

Santa Rosa National Park, Costa Rica

Indicator Animal Species:



Scorpion, Santa Rosa National Park, Costa Rica

White-throated Magpie-Jay (Calocitta formosa),

Santa Rosa National Park, Costa Rica

Scorpions are predatory invertebrates that are part of many world ecosystems, particularly dry ones. For some reason they are not common in temperate areas and thus seem exotic to those of us who live insuch climes. The two birds here are characteristic of the dry forests of Costa Rica. Like other jays, theMagpie-Jay caches food in the wet season for consumption in the dry season (see the article on scrub jays for a comparison). Because of the need to remember where all the food is, these birds are verysmart. The large Crested Guan lends its name to a whole region of Costa Rica - Guanacaste.

The giraffe is perhaps the most characteristic animal of this biome from Africa. Uniquely suited tobrowse leaves from trees, they are found in the savannas and dry forests of Africa. Their long blacktongues are so dexterous they can even remove leaves from thorny acacias.

A number of other African mammals make the dry forests and savannas home; in these pages most of them are covered with the grasslands. The white rhino here reminds us that animals often have little usefor our boundaries be they political or theoretical.

During the dry season the search for water is critical. Trees either have to have deep roots tounderground sources of water, or they will have to shed their leaves to reduce their water loss until therainy season comes. As in the temperate deciduous forest, the leaves laying on the forest floor willdecompose and release their nutrients back to the soil, where the tree can use them once again. Thedecomposition will proceed rather rapidly in the hot, wet conditions of the rainy season.

Some trees flower just before the rains come; at this point the flowers really stand out and are sure toattract the attention of desperate pollinators. In the picture to the right, a Ctenosaur hangs out in an earlyflowering tree. Life is tough for vegetation-eating lizards at this time as well.

The honeybees shown below were visiting a basin of water catching some overflow from the laundry.The

Ctenosaur (=



Threats:

Tropical dry forests are susceptible to all the usual threats. In the image to the right, you can see smokein the distance. This means that someone is clearing forest for use as agricultural fields or pasture.Dry forests and savannas around the world are subject to this pressure, all the more so because many of the forests are located in countries with very high population growth rates, and thus increasing pressureto grow crops to feed the people. Also, in many areas wild animals are being chased out as herders usethe land for their cattle and other domestic animals; often fences are put in place which prevent thewildlife from performing seasonal migrations or accessing critical water holes.

Another pressure put on dry forests is wood collecting. In many places wood is collected for use asbuilding material, fuel, or as stock for making charcoal. This leads to deforestation as often the collectingproceeds faster than the trees are able to grow. The fuel is needed for simple things like cooking; in many

places young children will walk miles each day to gather enough firewood for their mother to prepare asingle meal. The industrialized world, with its vast use of fossil fuels (and waster thereof driving our SUV's) effectively prices poor Africans out of the fuel market. The price for a gallon of kerosene mightexceed a poor person's monthly wage. The story is not all grim, however, many Africans are fightingback by planting trees. Read more about this response here!

Biological Field Station, Santa Rosa National Park, Costa Rica

In 2005, the Biology and Environmental Science Department at Marietta College made a trip to CostaRica. While there, we stayed for several days at Santa Rosa National Park in the Guanacaste Provinceon the western (Pacific) coast. The native habitat in this area is dry forest, and it was all highlythreatened until Costa Ricans, with the help of American biologist Daniel Janzen and others, began towork to protect this unique habitat. We stayed at the biological station in the park. The accommodationswere comfortable dormitories as seen in the pictures above and right.

We were there at the very end of the dry season. The wildlife was easy to see with the leaves down.One of my favorites was this Roadside Hawk (below) which hunted near the station. In the image belowyou can see it has caught an anole; like other hawks they also eat small rodents and other small animals.



It should also be noted that this ecosystem is not maintained by fire. Many of the forest plants areextremely susceptible to fire, which does not appear to be a natural phenomenon here.

.

One of the most interesting things to observe at Santa Rosa was the mutualistic relationship between

Acacia Ants and the Acacia trees. You would think that the big thorns on the Acacia tree would deter most herbivores, but the Acacia is not satisfied with that. It turns out that the thorns are hollow and easilymodified by the ants to serve as living chambers. To attract the ants, the Acacia trees also bear nectariesat the base of the leaves (above), these non-floral nectar sources feed and water the ants. When theleaves first form, beltian bodies, which are rich in protein, are formed as well to help feed the ants. Inreturn, the ants defend the Acacia from large browsing herbivores and insect herbivores, clip off any vinesthat try to attach, and even kill any plants growing too close to the Acacia tree. More on Acacia Ants.

Some more views of the tropical dry forest at the end of the dry season. Again, some of the trees didhave leaves, and we were uncertain if they were put out in anticipation of the rains or if those trees had awater source deep down in the soil.

The water source for the green trees in the image to the left is not in question. Those are mangroves andother trees in a small e stuary at Playa Naranjo on the coast.

dry forest ecology

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