Leaf size, along with LMA ( Specific leaf area SLA or Leaf mass per area LMA) has been shown to vary with environment (McDonald et al. 2003). In general, leaf size declines with increasing altitude, decreasing rainfall and soil nutrient content. However, there is considerable variation both within plant communities growing on a given site. There is also significant variation within species, and even within a plant (Sack et al. 2006). The ecological significance of leaf size has been explored from economic and thermal balance perspectives (Givnish 1979; Parkhurst and Loucks 1972; Westoby and Wright 2003). A small leaf size is expected to be advantageous in hot or dry environments because a smaller leaf will have a smaller boundary layer and better convective heat exchange with the environment. Leaf shape, like leaf size is highly variable between species. But leaf shape has proven more difficult to explain using environmental factors (McDonald et al. 2003). The leaf temperature argument given above can be extended to leaf shape: for a given leaf area a more divided leaf will have a yet thinner boundary layer and thus better convective heat exchange (Nobel 1983; Schuepp 1993). Yet, one finds remarkable variation in leaf shape within a given community(McDonald et al. 2003) and it is not always the case that smaller or more dissected leaves are cooler under a given set of conditions (Hegazy and El Amry 1998; Smith 1978). Within some lineages taxa vary only minimally in leaf shape (e.g. Protea) whereas in others such as Papaveraceae and the genus Pelargonium variation is tremendous (Jones et al. 2009).
Terminology and equations
Leaf size is generally measured in two ways. The total area of the lamina or the diameter of the largest circle that can be drawn inside the leaf surface. The latter is referred to as functional leaf size or effective leaf size and it is this dimension that is the major deteriminant of leaf boundary layer (Parkhurst and Loucks 1972). Leaf shape can be described in a variety of ways. These include complex measures such as fractal indices (Siso et al. 2001), and simple descriptors of perimeter as a function of area (LDI, leaf dissection index sensu McLellan and Endler 1998).
In PROTOCOL: Leaf shape analysisRoyer describes measures of leaf shape to investigate both variations in leaf margin (e.g. dentate margins or leaf teeth) and leaf lobing. Variation in leaf toothiness has been shown to be correlated with mean annual temperature and has been used in conjunction with reconstructions of paleo-climates (Royer et al. 2007; Royer and Wilf 2006; Royer et al. 2005).
It is generally the case that measures of single leaf area and shape are made using digital photography or scanners and image analysis programs. Occasionally one find measures of leaf size that are a function of total plant leaf area and leaf number – an average leaf size.
Ranges of values
Leaf sizes range from tiny scales along stems less than 1mm long, to the leaves of Welwitschia that are several meters long.
Health, safety and hazardous waste disposal considerations
Short of working with spiny or urticating leaves, measuring leaf size and shape is not particularly dangerous. Don’t look at flatbed scanners when using them. Be sure not to give yourself a repeated strain injury by doing too much image analysis at any one time.
LMA (leaf mass per unit area) or it’s inverse Specific leaf area SLA or Leaf mass per area LMA, are related areas.
Givnish T (1979) On the adaptive significance of leaf form. In: Solbrig OT, Jain S, Johnson GB, Raven PH (eds) Topics in Plant Population Biology. Columbia University Press, New York
Hegazy AK, El Amry MI (1998) Leaf temperature of desert sand dune plants: perspectives on the adaptability of leaf morphology. African Journal of Ecology 36:34-43
Jones CS, Bakker FT, Schlichting CD, Nicotra AB (2009) Leaf Shape Evolution in the South African Genus Pelargonium L’ Her. (Geraniaceae). Evolution 63:479-497
McDonald PG, Fonseca CR, Overton JM, Westoby M (2003) Leaf-size divergence along rainfall and soil-nutrient gradients: is the method of size reduction common among clades Functional Ecology 17:50-57
McLellan T, Endler JA (1998) The relative success of some methods for measuring and describing the shape of complex objects. Systematic Biology 47:264-281
Nobel PS (1983) Biophysical plant physiology and ecology. W. H. Freeman, San Francisco
Parkhurst DF, Loucks OL (1972) Optimal leaf size in relation to environment. Journal of Ecology 60:505-537
Royer DL et al. (2007) Fossil leaf economics quantified: calibration, Eocene case study, and implications. Paleobiology 33:574-589
Royer DL, Wilf P (2006) Why do toothed leaves correlate with cold climates Gas exchange at leaf margins provides new insights into a classic paleotemperature proxy. International Journal of Plant Sciences 167:11-18
Royer DL, Wilf P, Janesko DA, Kowalski EA, Dilcher DL (2005) Correlations of climate and plant ecology to leaf size and shape: Potential proxies for the fossil record. American Journal of Botany 92:1141-1151
Sack L, Melcher PJ, Liu WH, Middleton E, Pardee T (2006) How strong is intracanopy leaf plasticity in temperate deciduous trees American Journal of Botany 93:829-839
Schuepp PH (1993) Tansley Review No. 59: Leaf boundary layers. New Phytologist 125:477-507 Siso S, Camarero JJ, Gil-Pelegrin E (2001) Relationship between hydraulic resistance and leaf morphology in broadleaf Quercus species: a new interpretation of leaf lobation. Trees-Structure and Function 15:341-345
Smith WK (1978) Temperatures of desert plants – Another perspective on adaptability of leaf size. Science 201:614-616
Westoby M, Wright IJ (2003) The leaf size-twig size spectrum and its relationship to other important spectra of variation among species. Oecologia 135:621-628