What's the Difference between these 4 trees?

Since I've had a bit of a frustration working out the details separating these trees in my mind, I figure it'd be fun to write a blog about their differences and similarities!

Just what's the difference between Elm, Alder, Hazel, and Birch?

Photos*: (References at the end of the entry!)


Overall Habit of each tree:

Elm: Ulmus


Alder: Alnus


Hazel: Corylus


Birch: Betula



Leaves:

Elm: Ulmus


Alder: Alnus


Hazel: Corylus


Birch: Betula



Male & Female Parts:

Elm: Ulmus


Alder: Alnus


Hazel: Corylus



Birch: Betula



* None of these images are my own. Images were taken from these websites, in order:

http://www.first-nature.com/trees/images/ulmus_glabra3.jpg
http://www.winona.edu/publications/treebook/images/Alder-1820.jpg
http://www.kurowski.pl/foto/corylus_avellana_aurea.jpg
http://www.ces.ncsu.edu/depts/hort/consumer/factsheets/trees-new/images/betula/betula_nigra-duraheat3.jpg
http://botit.botany.wisc.edu/images/130/Trees_and_Shrubs/10_common_Genera/Ulmus/Ulmus_americana_dt_MC_.low.jpg
http://www.british-trees.com/guide/images/alder_leaf_scan.jpg
http://www.discoverlife.org/IM/I_JP/0010/320/Corylus_americana_leaves_upper,I_JP1036.jpg
http://www.plant-identification.co.uk/images/betulaceae/betula-pubescens-1.jpg
http://cloudbridge.org/trees/ulmus-mexicana-leaves-1.jpg
http://upload.wikimedia.org/wikipedia/commons/1/19/Tagalder8139.jpg
http://www.nps.gov/plants/pubs/chesapeake/img/Shrubs/Corylus-americana-2-inset-U.jpg
http://www.plant-identification.co.uk/images/corylaceae/corylus-avellana-2.jpg
http://www.cnr.vt.edu/dendro/dendrology/syllabus2/picts/betula_nana_fruit.jpg
http://www.csdl.tamu.edu/FLORA/tfplab/betula.jpg