The Desert Biome and Climate Change Kelly Barnhart Global Change Biology February 16, 2016 Pictures from

Deserts  Environmental extremes  Arid and hot  Large daily temperature fluctuations  Cold deserts  Low rainfall  Annual rainfall <250 mm (arid)  Overall biomass of plants and animals is low (Noble and Gitay, 1996).

Desert Locations  Most occur between 20 and 40° latitude  Some found in rain shadows of mountains  Atacama desert in South America  Some are very far away from oceanic moisture  Central Asian deserts (Noble and Gitay, 1996).

Desert Locations  Approximately 30% of the Earth’s surface (Noble and Gitay, 1996).

(Lioubimtseva and Adams, 2004)

Why do deserts exist?  Low moisture regions 1. Hadley Cell  Solar heating strong at equator, air rises and cools, forms clouds and rain at equator  Rising equatorial air spreads towards the poles and then descends  As the now dry air descends, it warms and creates a dry region 2. Mountain ranges  Rain shadow effect  Mountain ranges force moist air to rise and precipitate  Descending air on the other side warms and dries 3. Distance from moisture  Trade Winds blow from east to west, losing moisture as they travel 4. Cold ocean currents  Flow toward equator and cause low evaporation, precipitation, and low temperature range (Lioubimtseva and Adams, 2004)

Biodiversity and Productivity  Less diverse than many other biomes but have many endemic species  Biodiversity declines with increasing aridity  Biological productivity is limited by precipitation and nutrient availability (especially nitrogen) (Lioubimtseva and Adams, 2004)

Biological Specifications  Many plants and animals obtain their water from fog or dew  Ephemeral biota:  migrate into deserts  become active after drought (drought resistant seeds)  become active from dormant life stages (Lioubimtseva and Adams, 2004)

Biological Specifications  Many animals and plants have thick skins or cuticles to reduce water loss  Hairs or spines reflect radiation From ct-fennecfox.cfm From

 Nocturnal  Live in burrows From arietta.edu/ ~biol/biome s/desert.htm

Evapotranspiration  Potential evapotranspiration is very high  Arid zones are where evapotranspiration exceeds precipitation  Aridity Index (PET/P) where P = precipitation and PET = potential evapotranspiration  Increases in temperature will limit soil moisture through increased rates of evapotranspiration (Lioubimtseva and Adams, 2004)

Nutrient Cycles  The main sources of nitrogen in deserts are N- fixing organisms on the soil surface  lichens, cyanobacteria, moss, and fungi  Rates of decomposition are slow  Carbon takes place in two forms in desert soil: 1. Organic carbon in decaying plants and animals 2. Inorganic carbon  Calcium carbonate  Organic carbon in desert soils is generally low (Lioubimtseva and Adams, 2004)

Predictions  Most deserts are likely to become hotter, but not significantly wetter  (moister and drier predictions for different areas)  The frequency of El Niño events is expected to increase:  more rainy pulses to winter-rain deserts  more drought pulses to summer-rain deserts  Greenhouse gases are likely to persist and will not be reduced

Predictions Continued  Likely to be changes in community composition of plants and animals  More intense rainfall events (pulses)  Reduction in soil moisture and droughts are expected to lead to expansion of major deserts  Human-induced desertification may counteract any positive effects of climate change unless it is managed  Increased CO 2 will favor C 3 species  Increased desert albedo due to less vegetation cover

Potential Carbon Sinks?  Deserts are not a major store of organic carbon in either soils or vegetation.  GCMs do not generally predict dramatic changes in deserts under global warming.  If desert regions do become significantly moister under global warming (as they did during the Last Interglacial and the Holocene Optimum), they have potential to take up hundreds of gigaton (Gt) of carbon in more organic-rich soils and vegetation.

Possible Effects of CO 2 Increase on Deserts (Lioubimtseva and Adams, 2004)

Possible Effects on Desert Plants  Likely species specific  Plants are likely more resilient to changes in CO 2 and precipitation than previously thought due to specific adaptations that have already evolved in response to stress and extreme/variable climate (Tielbörger, Katja, and Roberto Salguero- Gómez, 2014)

C 3 versus C 4 plants  Desert plants often use C 4 or CAM photosynthesis as opposed to the normal C 3 photosynthesis.  Compared to C 3 photosynthesis, both C 4 and CAM photosynthesis are more efficient under hot, dry conditions, but are not as efficient overall, and therefore fix less carbon dioxide in a given period.  As a result, growth in desert plants is very slow. (Lioubimtseva and Adams, 2004)

C 3 versus C 4 plants  As CO 2 levels increase, it’s expected that plants which use the more water-efficient and CO 2 - efficient C 4 photosynthetic system will respond less strongly than C 3 plants.  C 4 plants will likely decline due to competition (Lioubimtseva and Adams, 2004)

Desertification  “Land degradation in arid, semi-arid, and dry sub-humid areas resulting from various factors, including climatic variations and human activities.” (United Nations definition)  Soil erosion  Deterioration of properties of soil  Loss of nutrients, fine soil grains, water holding capacity  Increase in salinity and toxicity  Long-term loss of natural vegetation  Shifts from perennial to annual, palatable to unpalatable, grassland to shrubland (D’Odorico, Paolo, et al., 2013)

Desertification  Important because it affects soil productivity and global food security  Generally irreversible because it is an alternative stable state  Leads to loss of biomass and soil organic carbon  Can seriously modify albedo as well as water exchange and nutrient cycles (D’Odorico, Paolo, et al., 2013)

enfacts.org/en /desertification /

Overgrazing (Lioubimtseva and Adams, 2004)

Misuse or Mismanagement of Land  Construction of irrigation systems or boreholes causing unbalanced, intensive use of the land  Increased soil salinity destroys soil structure and reduces porosity and permeability of soils  Reduces crop growth and yield  Overexploitation of land in general (D’Odorico, Paolo, et al., 2013)

Desertification Feedbacks  Nutrient loss  Salinization  Precipitation recycling  Dust emissions  Shrub encroachment  Decrease in vegetation cover (D’Odorico, Paolo, et al., 2013)

References  D’Odorico, Paolo, et al. "Global desertification: drivers and feedbacks." Advances in Water Resources 51 (2013):  Evans, R. D., et al. "Greater ecosystem carbon in the Mojave Desert after ten years exposure to elevated CO2." Nature Climate Change 4.5 (2014): 394.  Lioubimtseva, Elena, and J. M. Adams. "Possible implications of increased carbon dioxide levels and climate change for desert ecosystems." Environmental Management 33.1 (2004): S388-S404.  Noble, I. R., and H. Gitay. "Deserts in a changing climate: impacts." (1996).  Tielbörger, Katja, and Roberto Salguero-Gómez. "Some Like It Hot: Are Desert Plants Indifferent to Climate Change?." Progress in botany. Springer Berlin Heidelberg,   