This paper aims to provide a broad overview of dry forests, with particular focus on the tropical dry (seasonal) forests found in Latin America as these represent the most extensive remaining contiguous areas of this type of forest in the world (Miles et al, 2006). We will consider the effects which human disturbance and climate change may have on these forests in the future and potential management options to respond to these.
Distribution
Dry forests make up 42% of all tropical forests and are found across the world (Figure 1). They consist of two main types (i) temperate evergreen forests (found in the southern USA, northern India and southern China, northern Australia, the Rift Valley of Africa and parts of South Africa) and (ii) tropical seasonal forests which are mostly deciduous (found in southern Asia, Africa, Madagascar and Central and South America).
Definition
(S. H. Bullock, H. A. Mooney & E. Medina (eds) 1995) described dry seasonal forests as those occurring in the tropical regions where there are several months of severe drought. (Van Bloem, Murphy et al. 2004b) provided a more detailed definition of dry forests which clearly illustrates the range across with they are found; tropical dry forests occur in frost-free terrains from lowlands to lower montane regions where potential evapotranspiration exceeds precipitation on an annual basis, annual precipitation is 500-2000mm and the mean annual bio-temperature is >17oc (Figure 2).
Characteristics
Typically, dry forests have a closed canopy, although this may not be the case in the driest parts of their range or if disturbance is prevalent. Dry forests are generally smaller in structure and simpler in composition than wetter forests of a given region, but there is enormous geographic variation in most features due to differences in climate, soil, biogeographic factors and disturbance history. The only unifying characteristic of the dry forest climate is the strong seasonality of the rainfall distribution resulting in dry forests being seasonally stressed by drought (Murphy & Lugo, 1995).
Tropical dry forests occur on substrates ranging from nutrient-rich alluvial soils to nutrient-poor rock outcrops. (Huxman et al, 2004; Kurc and Small, 2007 cited in Perez-Ruiz, Garatuza-Payan et al, 2009). Shallow or infertile soils tend to support evergreen forests, while deciduous forests generally grow on better soils. Tropical dry forest vegetation is mainly water rather than nutrient limited (Van Bloem, Murphy et al. 2004a). All biological activity in the underground system is strongly limited by water availability and interactions with carbon and nutrient availability. Synchronization between activity of decomposers and fine root production at the beginning of the rainy season seems to be essential for an efficient balance between nutrient uptake by the plants and nutrient immobilization by microbes. {{28 Cardoso,Irene M. 2006}} Microbial activity decreases greatly in dry seasons.
Tropical dry forests are generally less species-rich than moister forests in terms of plants and vertebrates (Gentry, 1995). They have more structure and physiological diversity in life forms. Diversity of life forms is both structural (wood specific gravity, plant habit) and physiological (photosynthetic types, water relations, growth seasonality) (Mooney et al, 1995 cited in (Kalacska, Sanchez-Azofeifa et al. 2004). The common phenological response of drought deciduousness is exhibited by many woody species, however, the number of species and individuals with this response in tropical dry forests varies with each stage of succession and topography. This mix of deciduous and evergreen species gives tropical dry forests a kind of phenological complexity not found in moister forests. (Burnam, 1997 cited in Kalacska, Sanchez-Azofeifa et al, 2004). A distinguishing feature of tropical dry forests is that they buck the trend of species richness increasing as one gets closer to the Equator - the most diverse tropical dry forests are farthest from the equator, in Mexico and South America, with the most species-rich areas being among the driest (Gentry, 1995).
Disturbance
In 1988, Janzen stated that tropical dry forests are among the most threatened ecosystems in the world as a consequence of intensive anthropogenic disturbance. Dry forests support a large fraction of the human population in the tropics as environmental constraints on human development are low compared to other ecosystems, dry climates are preferred over very wet climates, (Ewel, 1999, cited in {{15 Portillo-Quintero,C.A.}} As a result these forests are under intense pressure; in the tropics and so large population concentrations occur in dry forest areas (Murphy and Lugo, 1986).. Hoekstra et al (2005) estimated that approximately 48.5% of tropical dry forests have already been converted to other land uses at the global level; in the Americas alone, 66% had been converted {{15 Portillo-Quintero,C.A.}}. North, Central and South American dry forests have suffered intense transformations from agricultural development and cattle ranching.
Disturbance is an important factor in tropical dry forests. Hurricanes bring destructive winds andprolonged or unusually severe droughts threaten all forests. The frequency and intensity of these disturbances can have important effects on dry forest structure and function. High winds from hurricanes result in defoliation and breakage of trees, with longterm consequences for forest structure. Return intervals for hurricanes range from 5 to 100 years.
Mature dry forests in the hurricane-prone West Indies tend to have shorter canopies, higher stem densities, and a greater proportion of multiplestemmed
trees than dry forests outside of the hurricane belt.
Fire is a difficult feature to interpret in dry forests. In most locations, even in Africa, fire is not a frequent or severe aspect of the ecosystem. Lightning occasionally starts fires in dry forests, but they are low intensity, small scale, and usually doused by rains. Sparse understory and grass cover limit the intensity of the ground fires that burn leaf litter about every 5-10 years, returning some proportion of nutrients to the soil as ash, while losing others to the atmosphere. Forest floor heterogeneity caused by termite mounds, rock outcrops, and crusty surfaces inhibit the ability of fires to spread. During the last 50 000 years, humans have probably caused the vast majority of fires in dry forest in order to clear land, hunt animals, and burn fallow to encourage new growth for livestock. Frequent burning transforms forests into savanna, shrub, and open woodland - ecosystems more correctly associated with fire.
Conversely, humans prevent burning in other forests that naturally would experience ground fires. This alters natural disturbance patterns and can lead to very destructive crown fires sparked by the build-up of litter and underbrush during long fire-free periods.
Dry forests have a particularly long history of human disturbance because they occur in climate regions that are pleasant to live in and favorable for
grazing and agriculture. The dry season provides respite from rain and humidity, and reduces populations of agricultural and human pests. Livestock
survive well in dry forest areas, and the relatively small trees are easy to clear for agricultural fields.
Because of the desirability of dry forest climates, human population densities in these areas exceed 100km_2 globally and are expected to double every 20-25 years.
Human disturbance in dry forests is primarily related to land use change and extractive activities.
As in other ecosystems, there are some examples of sustainably managed dry forests, depending on political, economic, and population pressures.
Large-scale logging operations affect dry forest primarily in continental locations. Some dry forest species are suitable for plantations but others grow
poorly because of infestation by pests (e.g., shootboring insects on mahogany in monoculture). Trees are cut for firewood or charcoal production in most
regions. Firewood accounts for 80-90% of dry forest harvest. Many trees coppice - an advantage when used for firewood. Unfortunately fuelwood is not
regrowing quickly enough to sustain demand in many areas of Africa. The long-term effects of extraction depend on the amount and pattern of biomass removed and the degree of soil disturbance or compaction that accompanies tree harvests.
Conversion of forests to agricultural, industrial, or residential areas completely removes forest cover and disrupts roots and soil structure. Traditionally, shifting agriculture has been practiced in dry forest
regions in a sustainable manner, but increasingly short fallow periods prevent re-establishment of the forest. Grazing and agriculture increase nutrient loss from the system by erosion. Grazing reduces forest biomass because cattle reduce understory growth while compacting the soil and accelerating erosion.
Approximately 32% of the area that once was dry forest remains, with continuing annual losses of 0.7-1.5%. The highest deforestation rates occur in
Africa, even though population densities in dry forests there are only one-fifth of those in Asia. This trend continues in most areas, but in some locations where economies have turned from agriculture to manufacturing, land is reverting back to forest. This is the case in Puerto Rico, Cuba, and the Gambia.
Response to Disturbance
The ability of dry forest to tolerate disturbance, whether natural or anthropogenic, depends on disturbance type, frequency, duration, and severity.
The forest that regrows following disturbance may resemble the original, or differ greatly in terms of function and composition. In general, dry forests
exhibit a high rate of resilience, defined as the rate at which a forest stand recovers from large, infrequent disturbance. Compared to other forests, succession in dry forests progresses quickly, and in some cases this can lead to expansion into other systems. In Mexico, clearing of large tracts of rainforest resulted in a drier environment that was subsequently recolonized by
dry forest.
Dry forests in Puerto Rico have provided a location to compare response to natural and anthropogenic disturbances. The forest recovered similarly
from the effects of a recent hurricane and small-scale tree cutting for charcoal production in the past. Both resulted in patchy loss of o25% of trees, while soil systems remained intact. Stem density increased after each disturbance, as broken trees grew from coppice sprouts. After 45 years, cut areas regained an average of 87% of mature forest structure. Post-hurricane
forest will probably take about 25 years to fully recover. Neither disturbance changed the species pool. Conversely, forest recovery from abandoned agricultural and residential uses has taken much longer. After 45 years, abandoned agricultural areas had only recovered an average of 71% of mature forest structure, while residential sites were 58% recovered. The disruption of root systems had a large effect - root biomass was half or less that in mature forests and coppice growth was minimal. These areas
have shifted from being dominated by native species to the invasive species Leucaena leucocephala, although native Puerto Rican species have begun
reappearing after 40 years.
Recovery following forest conversion to agriculture or housing illustrates how a disturbance can cause a system to shift toward monodominance by a
single, invasive tree species. Conversion of African dry forests to grass-dominated savanna by fire results in both a new species composition, and also dominance by a different growth form. Both of these anthropogenic disturbances have resulted in new ecosystems with different structure, though the Leucaena system more closely resembles the original dry forest than does the savanna.
The occurrence of multiple disturbances can have major consequences for dry forests. In the Mexican Yucatan, 10% of trees died following a hurricane,
but fires subsequently burned through a portion of the forest resulting in 85% mortality, among the highest mortality rates recorded for a dry forest. On
the other hand, when a hurricane hits a forest within a year following the previous hurricane, damage is usually light, as the susceptible trees have already been removed.
Characteristics of dry forests that help confer resilience include a high concentration of nutrients below ground or in litter, high root : shoot ratios, and the ability to coppice. When trees are lost, nutrient pools support regrowth and roots access nutrients and minimize erosion. Mycorrhizae and high nutrient use efficiency of dry forest trees help to keep nutrients from being leached. Dry forest trees have adapted to their natural disturbances, allowing forests to withstand drought and regrow following
wind, fire, and insect damage. Dry forests absorb human activities that mimic natural disturbances but when no natural disturbance analog exists, dry forests may shift toward other systems or become susceptible to invasion by exotic species.
Life zones. The result is that tropical dry forests not only provide space for the expanding human population, but are also used intensively as a source of fuelwood and charcoal. Grazing animals are also often allowed to roam free through dry forests. The net result of human activity in this life zone is the serious degradation or disappearance of dry forests in most tropical regions. Because succession is usually slow in these forests, chronic human use results in deforestation and modification of vegetation cover. Usually, degraded stands lose their understory to grazing animals, trees are repeatedly harvested and resprout as multiple small stems, the canopy is opened, and soils are exposed to erosion (Murphy et al. 1995). In cases of extreme use, fire is introduced. Despite these problems, the current condition of dry forests opens the opportunity for restoration and management. Tropical dry forests are resilient in terms of their ability to root and stem sprout, a characteristic that can be used to rehabilitate forests and restore biomass (Murphy and Lugo 1986b; Murphy et al. 1995; Murphy and Lugo 1995). Experience with tree plantations shows promise provided they are managed carefully (Lugo et al. 1990; Wang et al. 1991).
Climate change section
List parts of LAC that will be affected (ref IPCC 2007 report)
Predicted 2000-2010 SA and CA deforestation hotspots IPCC chapter 13 p595.jpg
Declining trend in rainfall in Western Central America (where notably there is a lot of TDF) - IPCC
Increase of tropical forest conversion to dry and very dry forests and savannahs (up to 47% shift in Holdridge life zones) at expense of temperate oak and conifers in region.
May see migration of tropical forests to higher altitudes due to changes in climate.
TDF is severely threatened
TDF has the capacity to fix a larger quantity of CO2 than other seasonally dry ecosystems
Ecosystem CO2 and water vapour exchange in monsoon period were highly influenced by precipitation (thus as precipitation falls - as predicted by IPCC - how will this influence these two factors in TDF? A further release of stored CO2?)
Land use changes causes perturbation of the ecosystem and can influence carbon stocks and fluxes.
Conversion of forest to agricultural land use invariably results in the depletion of SOC stock by 20-50% (see Covington Curve below). IPCC predicts major change in land use of LAC forests for pasture and expanding livestock production.
SOC is affected by the slope and aspect of the landscape a forest is situated in (could this mean depletion of SOC if tropical forests are forced to migrate to higher ground as climate changes?).
Frequency of weather and climate extremes and hurricane frequency and intensity will increase in LAC region - severe natural disturbances such as these have an impact on SOC.
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