La Niña
A condition where the central and eastern tropical Pacific Ocean turns cooler than normal.
Last Glacial Maximum
The period of time, approximately 18,000-22,000 years ago, during the last great ice age when glaciers, ice sheets and sea ice reached their maximum thikness and areal extent.
Laurentide ice sheet
The massive North American ice sheet of the last ice age that covered over five million square miles (16 x 106 km2)and stretched from the Arctic through eastern Canada to the northern half of the United States. Its weight was so great that it depressed the earth's crust by as much as 700 meters.
Leaf area ratio
An expression of leaf area per ground area or per total biomass.
Leaf gas exchange
The net result of leaf uptake of CO2 from the air due to photosynthesis and leaf loss of CO2 to the air due to respiration. Leaf loss of water vapor to the air due to transpiration is also sometimes implied by these words.
Leaf water potential
A common physiological measurement used to assess the general water status of a plant. A value of zero indicates the absence of water stress, while increasingly negative values depict increasing severity of water stress.
Legume species
Plants that maintain symbiotic relationships with various soil bacteria that make atmospheric nitrogen available for plant use.
Lepidoptera
An order of insects including butterflies and moths.
Lichenometry
A dating technique that utilizes the linear growth rate of certain lichen species, whereby the diameter of the largest thallus found at a site is considered to be representative of the age of the substrate on which it is growing.
Light-dependent reactions
The initial phase of photosynthesis in which sunlight is absorbed and chemically converted into usable cellular energy in the form of ATP. In addition, this phase of photosynthesis also generates oxygen and NADPH.
Light-independent reactions
The last phase of photosynthesis in which ATP and NADPH produced in the initial photosynthetic phase are used to convert CO2 into sugars for supporting plant growth and development.
Light compensation point
The level of sunlight at which the amount of CO2 being incorporated into sugars during photosynthesis is equal to that being released during respiration. In other words, it is the lowest level of sunlight that supports a net gain of carbon uptake.
Lipid
A class of hydrophobic molecules consisting mainly of fats, oils and waxes that generally possess long non-polar hydrocarbon groups arranged in chains (commonly termed fatty acid chains). One type, the phospholipids, is a major component of plant and animal membranes.
Little Ice Age
The period between the 16th and 19th centuries when global temperatures were between 1.0 and 2.0°C cooler than now.