Thursday, November 19, 2015

How many leaves has a tree ?

Sometimes ago I asked myself how many leaves a tree has (a real tree, not the homonymous informatics structure). Certainly it depends on the type of tree, the size of the leaves, and on season too. A simplification would probably be to estimate which is the maximum number of leaves a tree can have. The question was rised by the contemplation of woodland in Trentino, but also has an impact on hydrology. No leaves, no transpiration, and maybe, more leaves more transpiration, even if as a possibility ( for which hydrologists coined the infamous potential evapotranspiration concept). I started  to google around in trying to understand. 

Many internet surfers report this:

It depends on the tree's species and age, but a mature, healthy tree can have 200,000 leaves. During 60 years of life, such a tree would grow and shed 3,600 pounds of leaves, returning about 70% of their nutrients to the soil.

and cite as source the Wisconsins County Forests Association: but, on their site, I was not able to find the cited words. Anyway, anticipating the answers, this number mentioned is in the range most of leaves' counters gives, at the end (did they influences each other?).

I personally found three approaches to solve the problem. 

The first was simply to count the leaves on a tree. Probably some one really did it. But I could not find trace of it.    Some others made it indirectly Here you will find a counting exercise for kids (but that adults can enjoy).  Even a Wired's journalist got this problem to solve. Another version of the same approach is here.

These professor Morrow's students, instead, were actually interested in the weight of leaves (and I can understand they were possibly interested to estimate the gross primary production). These students of Mathematics built actually a model of plant growth. Their interesting trial, which has to do also with fractals, can be found here

The third method is based on determining the leaf area index (LAI, see Baldocchi’s Notes first), the ratio between the surface of the total area of the leaves in a canopy, divided by the projected area the canopy covers. It seems, that under many circumstances this quantity is easier to measure (it can be obtained also from satellites) that counting the the leaves (or is it a modern automatic way to do it ?)
Having the LAI and the canopy area covered by a tree (which is actually very similar to what done in the “counting methods above”) the number of leaves can be estimated, indeed even over large scale. or an entire forest. 

The problem is connected with others like; How big a tree leaf is  or how much it weights. 

See below a short bibliography on the leaf area index and on its implications. 

P.S. - Section 2.3 of G. Bonan 2019 book talks about the leaf mass per unit area and can be partially used to complement this post. 

Leaf Area Index

Asher G.P, Scurlock J.M.O, Hicke J.E., Global synthesis of leaf are index observations: implications for ecological and remote sensing studies, Global Ecology and Biogeography, 12, 191-205, 2003

Breda N.J, Ground-based measurements of leaf area index: a review of methods, instruments and current controversies, Journal of experimental Botany, 54(392), 2403-2417, 2003

Bonan, Gordon. 2019. Climate Change and Terrestrial Ecosystem Modeling. Cambridge University Press.

Colombo R., Bellingeri D., Pasolini D., Marino C.M., Retrieval of leaf area index in different vegetation types using high resolution data, (Also here) Remote Sensing and Environment, 86, 120-131, 2003

Grier, G.C., Running, S.W., Leaf area of mature northwesternconieferous forests: relation to site water balance, Ecology, 58: pp: 893-899, 1977

Norman, J.M., Campbell G.S., In: Canopy structure, Chapter Plant Physiological Ecology, pp 301-325, Springer-Verlag, 2000,  DOI:10.1007/978-94-010-9013-1_14

Pasolli L, Asam S., Castelli M, Bruzzone L, Wohlfahrt, Zebisch M, Notarnicola C,  Retrieval of Leaf area index in mountain grasslands in the Alps from MODIS satellite images, Remote Sensing of Environment, 159-174, 2015

Wang Q., Adieu S., Tenhunen J, Granier  A., On the relationship of NDVI with leaf area index in a deciduous  forest site, Remote Sensing, 94, 244-255, 2005

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