NATF320 L9 Tropical deserts

defining deserts - challenging, many defintions exists

different criterai have been used

climate, vegetation phsuiognomy, vegetation cover, soil properties

border with grassland/savanna biomes is often blurry

central aspects → water scarcity (i.e. drought)

how do we quantify aridity

aridity index

aridity index = precipitation / potential evapotranspiration

potential evaportranspiration

• wind, solar radiation, air temperature, relative humidity

desert substrates

desert pavemetn and exposed rock make up most desert

what creates deserts

atmospheric circulation → subtropical belt of high pressure, with stable and dry conditions

continentality → humid sea air tends to become drier as it moves further inland

rain shadows → orographic lift causes the air to dray as it goes over mountains

cold coastal currents → associated with upwelling systems, cause cooling and drying of the air

desert biota - the abiotic stressors

drougth → minimize water loss/maximize water acquisition

high solar radiation → protect from excess sunlight and avoid overheating

soil → physiological adaptatons

• salinity, poor soil structure, scarce nutrients, special soil types

strong winds

sand abrasion

desert biota - plants

abiotic constraints

minimize water loss through transipiration

minimize damage from excessive solar radiation and overheating

some genral xeromorphic adaptions → low surface area:volume ratio, small leaves, sunken stomata, thick waxy cuticle, hairs

desert biota - plants

different ecophysiological strategies

different ecophysiologicla strategies

• drought escape → complete life cycle in favourable season → ephemeral plants

• drought avoidance → store water in tissues → succulent plants

• drought tolerance → tolerate water loss → true xerophytes

succulents

water storage

renders the plant temporrily independent of external water supply

desert biota - xerophytes

• (often evergreen shrubs/subshrubs), some lose their leaves during the driest season (drought deciduous), sometimes aphyllous, shrubs

hidden precipitation

importance of hidden precipitatoin → fog and dew

water uptake through hydathodes in crassula (crassulaceac) from the succulent karro and the namib

desert biota - animasl

abiotic constraints

minimize water loss

minimize damage from excessive solar radiation and overheating

desert biota - ecological strategies

heat evaders → shady microhabitats, hiding during hotter hours, being active during cooler hours

heat endures → limited activity during hotter hours, heterothermy, physiological adaptations to cope with water loss, increasing surface area

desert biota - animals

general adaptations

high surface area:volume ratio for heat dissipation (homeotherms/heterotherms)

waxy cover (arthropods) or hairs/feathers (vertebrates) → insulation form heat and cold

some adaptations to use hidden precipitation (fog, dew)

aridity could be driving cooperative breeding in mammals

desert biota - insects

fog basking behaviour → headstand position

ecology of deserts

in terms of plant diversity, most deserts are rather poor at regional scale

high geological, edaphic and topographic diversity → highly heterogeneous landscape with diverse microhabitats

differences between quartz islands and matrix habitats in terms of

• soil physicochemical parameters and land cover

• plant community regarding distribution ranges and life forms

interesting aspects of deserts ecohydrology

• different root distribution among plant funcitonal groups

• hydraulix redistribution or lift -

  • . passive process when transpiration decreases

vegetatoin structure in deserts

islands of fertility → shrubs accumulate moisture and nutrients, thus increasing icrobial activity in the soil → creates resource heterogeneity in the landscape

islands of fertility → some promote establishemt of seedlings of other species → nurse plant

self-organized vegetatoin patterns → often observed in ecosystems with limited resources → positive feedback between vegetation growth and distribution of moisture and nutrients

threats to deserts

desertification → biological productivity is lost due to natural processes or induced by human activities → climate change (increasing aridity and/or changes in rainfall regimes)+ land use

vegetation changes

shifts between ecosystem stable states

thresholds of aridity, leading to abrupt ecological shifts

• more than 20% of land surface, and almost 30% of dryland surface, will cross at least one of the thresholds by 2100 (sceanrio of sustained increase in CO2 emissions)

additonal factors beside changes in climate → e.g. CO2 fertilizaton and landuse/degradation

CITES

Convention on international trade in Endangered speceis of wild fauana dn flora

all parts and derivatives, live or dead, regulated