webrolintree.jpg (15288 bytes)  In the lab.

    

             Dr. Robert O. Lawton                                         Biological Sciences


 

Ecology & Biogeography of Cloud Forest

 

 

 

 

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Simulated cloud water content above deforested (top)and forested (bottom) tropical landscapes
 

 

Deforestation, Cloud Formation and Cloud Forest Biogeography:

Deforestation of tropical lowlands alters surface energy budgets, decreasing evapotranspiration and increasing sensible heat transfer from the ground to the atmosphere by conduction and convection. This results in hotter and drier air over deforested areas, air which must be lifted higher to cool to the point of cloud formation.  This may have striking consequences for cloud forests in mountain ranges downwind of deforested areas.  Lifting the orographic cloud banks, which define the distribution of cloud forests, will reduce (1) the area covered by cloud forest, (2) the ranges and population sizes of cloud forest organisms, and (3) the hydrological inputs of cloud moisture.

Wind & the Ontogeny of Elfin Forest Stature:

Strong winds rake exposed peaks, ridges and saddles in tropical mountain ranges.  Forests in these areas thicket-like, with  short trees (4-10 m in the photo), the crowns of which are packed into a canopy with a upper surface.   Tree trunks are thick relative to the heights of trees, and limbs and twigs, too, are thick for their length.  Wood density of elfin forest tree species are higher than those of more sheltered habitats nearby.  These attributes suggest adaptations which both avoid and resist the mechanical stresses applied by wind.

 

 

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Elfin forest on the windswept headwall of the Peñas Blancas valley,

 

 

 

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Early regeneration in large gap in elfin forest. Monteverde Cloud Forest Preserve, Cordillera de Tilarán, Costa Rica
 
 

 

Cloud Forest Dynamics -- Treefall & Forest Regeneration: 

The Monteverde cloud forests are exposed to regular violent winds associated with the impact of strong winter cold fronts sweeping down from from the North American plains.  These storms cause most treefall in the wind-exposed forests along the crest of the Cordillera, opening ~1.4% of the canopy each with ~2 treefall gaps > 5m2/ha/yr.  Treefall gaps in this forest are clumped, with more gaps with centers within 20 m of other gap centers than would be expected if trees fell in a spatially random fashion.  This probably occurs because the fall of one tree exposes its downwind neighbor to increased wind stress, and thus an increased probability of  windthrow in the near future.  Demographic analysis of growth,   survivorship and gap colonization of the dominant shade-intolerant tree of Monteverde elfin forests suggest that 10 ha watersheds are in quasi-equilibrium with current rates of treefall.  

 

Stranglers, Hemiepiphytes & Other Climbers -- Host and Habitat Preferences:    The prolonged and  immersion in cloud that characterizes  tropical montane cloud forests, provides conditions in which epiphytes, hemiepiphytes and stranglers thrive.  These range in size from free-living unicellular algae to stranglers which become among the largest trees in the forest, and often have unusual life histories.   For instance,  Ficus crassiuscula, a strangling fig in the subgenus Pharmacosycea, has evolved the strangling habit independently of the better known lowland strangling figs in the subgenus Urostigma.  F. crassiuscula germinates in mats of bryophytes on tree trunks and major limbs, and develops into a sparsely branched vine, with a delicate stem without secondary growth.  It roots at the leaf nodes and sprawls loosely appressed to the epiphytic mat, sometimes covering 5 m2.  Eventually one stem begins to grow erectly to form a trunk, which in time sends a major root to the ground.  Gradually the trunk of the host is encased at the base, the host dies, and the fig, if successful, takes its place in the forest.  F. crassiuscula viny saplings are spread upon potential host species  in proportion to the trunk surface those species offer.  Juveniles with erect trunks and adult figs are more abundant than expected upon species of Guarea, and less abundant than expected upon Eugenia.

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  Looking for hemiepiphytes in the Monteverde cloud forests

 

Forest Ecology of the Southern Cumberland Plateau               

 

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Storm Damage to Mature Forests:  Hurricanes, Tornados, Thunderstorms, and Glazing Ice:  Storms of various kinds leave their distinctive footprints on forests.  We are examining factors influencing the likelihood of treefall and canopy disruption in mature deciduous hardwood forests.   In the sustained high winds and heavy rainfall accompanying the passage of Tropical Storm Opal in 1995 n red oaks, (Quercus rubra, Q. velutina and Q. shumardii) were more likely to fall than white oaks (Q. alba, Q. prinus, and Q. muhlenbergii), and other tree species. 

tornado damage, Monte Sano Mt.

 

Population Biology of Ginseng

Panax quinquefolius is widespread in mature deciduous forests in eastern North America, and apparently was once fairly common.  Over harvesting has reduced abundance drastically, and eliminated many natural populations.  Ginseng remains on the dissected slopes of the Southern Cumberland Plateau, despite active legal harvesting and considerable poaching.  We are monitoring a population of tagged ginseng in a mature mixed mesophytic forest stand in Monte Sano State Park, Alabama, using a late summer/early fall annual census.  

 

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Sexual Activity of Eastern Red Cedar

Juniperus virginiana is widespread in north Alabama in two contexts:  as trees colonizing a wide variety of anthropogenically disturbed settings, and as natural populations on rocky slopes of the Southern Cumberland Plateau.  Prior to agricultural colonization of the area, cedar was probably competitively excluded from mature hardwood forests on deep soils in the Tennessee Valley, but present as one of the dominant species in xeric woodlands on rocky, south- and west-facing slopes.   In populations established via secondary succession on abandoned agricultural lands with deep clay soils, all cedars >10cm dbh are reproductively active, and the sex ratio is 1:1.  On the Cumberland Plateau slopes, many cedars > 10cm dbh are not reproductively active and the sex ratio is 2:1 (male:female).

 

© 1998 The University of Alabama in Huntsville

Last Updated: November 1, 2002