Sweetgum

Liquidambar styraciflua

There is only one sweetgum in the Park, a beautiful tree in the southeast triangle. The Wikipedia entry is here.
The sweetgum is a deciduous tree native to warm temperate areas of eastern North America.
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Sweetgum in the southeast triangle.
Sweetgum factoids:
  • It is easily recognized by its five-pointed leaves and its spiked spherical seed pods lying underneath its canopy.
  • It is named for the fragrant resin, or gum, which the tree exudes.
  • The glossy, leathery, dark green leaves turn bright orange and purple in autumn.
  • Sweetgums grow extensively in the southeast United States and are used for furniture and plywood
Distinctive five-pointed leaves
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Spiky seed pods of sweetgum, one removed from the tree at left and a dry one on the ground that has had its seeds picked out by birds.
George Washington (on quarter) donated 13 sweetgum seeds to Alexander Hamilton to line the latter's estate in upper Manhattan. Hamilton wanted to make the sweetgum our national tree.
Some cities ban the planting of sweetgums as street trees because of the seed pods being a nuisance. If you take a walk out the gate on the north side of the Park onto the NxNW curved driveway, you can see seedless cultivars of the sweetgum, Liquidambar styraciflua 'Cherokee,'  lining the left side as you look north.
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Sweetgum in early April
The new buds are about to blossom amidst some old fruit from last year. The magnolia-like buds will become the male flowers.
Sweetgum branch fallen onto a bench on April 16
There are separate male and female flowers on sweetgums (see discussion on monoecious versus dioecious on the Red Maple page here). The male flowers are tiny, yellow, and grow in cone like clumps on thick peduncles (like stems) projecting upwards from the leaves. The female flowers are only slightly larger, and grow in spherical clumps hanging below the leaves. Each of these female flowers will generate a seed, and the spherical clump will become the spiky seed pod in the fall.
Botany 101 Bonus
 
Over the last 2 million years glaciers have periodically advanced south and then retreated. During the last glaciation, which peaked 18,000 years ago, northwestern and northeastern Pennsylvania was covered with ice. Trees cannot move, but generations can over hundreds of years shift their habitable zones north and south with the climate changes. This is still happening, and being accelerated by atmospheric CO2 elevations caused by human use of fossil fuels.
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Green marks the current range of the sweetgum. It prefers warmer weather but can extend north along the coastal plain with its milder climate. 
The Earth is getting warmer. The colors on the map below represent gradations of minimum average yearly temperatures in different parts of the United States. In just 16 years, the Philadelphia area has warmed from Zone 6 (light green) to Zone 7 (yellow). Plants like sweetgum and sweetbay magnolia are southern plants, but their ranges are extending northward. Likewise, sugar maples, a northern species, are retreating northward as the climate changes. Sugar maples are under stress in Philadelphia. Trees can live hundreds of years, so arborists are now taking these warming trends seriously when choosing plantings in an area.
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Comparison USDA Plant Zones over just 16 years, a blink of an eye in geologic time.