R-3 Repository :: Browsing by Author "Harcombe, Paul A."

Browsing by Author "Harcombe, Paul A."

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Carbon sequestration in the forests of East Texas
(2003) Almaguer-Reisdorf, Joyce Lynn; Harcombe, Paul A.; Few, Arthur A., Jr.
Increasing levels of atmospheric CO2 threaten to change the earth's climate and diversity in numerous adverse ways. This thesis explores aspects of two potential types of terrestrial sinks in East Texas, plantation rotation management and reforestation. I used a simple method of employing government GIS and tabular data for calculating and visualizing the size of those sinks, which could store an additional 2.3 to 98 million Mg C aboveground. The uncertainty of these values is high because of data inadequacies and also uncertainty about future land use trends. The mitigative powers of these sinks are discussed, as is their potential application in newly forming carbon credit programs such as the Chicago Climate Exchange.
Effects of fire on vegetation in the Big Thicket of southeastern Texas, United States of America
(1995) Liu, Changxiang; Harcombe, Paul A.
This study shows that fire had a strong short-term effect on the small-sized individuals in the understory of two dry upland vegetation types, sandhill and upland pine. Large trees were less affected than saplings and small trees. The changes in these types were mainly structural rather than compositional. The influence of fire diminished toward the wet end of the vegetation gradient. Post-fire recruitment via resprouting and germination from seeds was rapid. The interaction of fire and vegetation type was apparent in the comparison of the two dry upland types with slope types. Simulation of fuel and fire provided additional, although not very strong, support for the existence of differences in fire behavior corresponding to an environmental gradient from dry upland forests to mesic and wet slope forests. Ordination using pre- and post-fire data suggests that fire had limited effects on current vegetation patterns and there was no indication that any present pre-fire type would be converted to another type. Furthermore, there were no clear convergent or divergent patterns in overall vegetation change; hence it was difficult to predict long-term changes. An eight-year record of vegetation change at a tornado damaged savanna site in Hickory Creek Unit demonstrates that the tornado had resulted in the succession of the vegetation toward a midslope forest, but two prescribed fires partially reversed such a trend. Phytolith analysis revealed that upland pine and some upperslope forests had abundant grass phytoliths. This indicates that these types had a grass component in the past under an open canopy which might have disappeared as a result of fire suppression, logging, and encroachment of shrubs and hardwoods.
Fluctuations in abundance and mortality of Carpinus caroliniana (American hornbeam) and the invasion of Sapium sebiferum (Chinese tallow)
(2006) Mann, Lisa Erin; Harcombe, Paul A.
A bottomland hardwood forest stand located in east Texas experienced a hurricane-related flood event in 1989. After this event, Carpinus caroliniana Walt. had a large pulse in mortality, decreasing dramatically in abundance (47%) over a 5-year period. The decline was greater at low elevations suggesting that at least part of the mortality was related to flooding. The flood was the longest summer flood on record. The duration of this growing season flood may have been partly a consequence of upstream dam management. Linear regression showed that in areas where Carpinus death was greater, several sapling species had increases in basal area growth. The invasive species Sapium sebiferum (L.) Roxb. had the greatest increase. This suggests that the Carpinus death and concomitant increase in light availability resulted in accelerated invasion of this forest by Sapium.
Patterns of vegetation in fire-prone habitats, southeastern Texas, United States
(1992) Liu, Changxiang; Harcombe, Paul A.
Patterns of vegetation in fire-prone habitats were analyzed by ordination. The results showed that vegetation was highly related to soil texture, but was not obviously related to fire history. I concluded that either fire and soil effects are confounded or fire effects have been reduced by long-time fire suppression. Indicators of site productivity--height and volume increment of loblolly pine, and stand basal area--were weakly related to vegetation type, but not related to soil texture. The lack of a relationship between these indicators and soil texture suggests that site productivity may not be related simply to soil texture and that interactions with other species may obscure the response of loblolly pine to site productivity. The flammable portion of fine fuel was higher in dry types than in wet types which is consistent with the hypothesis that fire effects will be greater in dry types than in wet types.
Prairie and forest vegetation of the Armand Bayou Nature Center, Harris County, Texas
(1990) Oliver, Mary Elizabeth; Harcombe, Paul A.
The prairie and bordering woodlands of the Armand Bayou Nature Center, Harris County, Texas were sampled as an example of Texas Upper Coastal Prairie. The prairie is homogeneous and species-rich but shows very low dominance. Paspalum plicatulum, Dichanthelium spp., Carex cherokeensis, Andropogon virginicus, and Schizachyrium scoparium are the dominant graminoids. This prairie resembles the typical Upper Coastal Prairie of Texas but contains a greater number of eastern species with few southern or southwestern influences. Baccharis halimifolia has invaded the prairie and prairie climax species are no longer dominant. There is little evidence of a reduction in diversity due to brush encroachment. The woodlands are an oak and elm dominated riparian forest of the Upper Coastal Prairie with an important shrub component. Quercus phellos, Q. falcata, Ulmus americana, U. alata and U. crassifolia are common canopy species. Ilex vomitoria, Viburnum dentata, Callicarpa americana and Ligustrum sinense are the dominant shrubs.
Sapling growth and recruitment as affected by flooding and canopy gap formation in a river floodplain forest in southeast Texas
(1993) Hall, Rosine Blount Wilson; Harcombe, Paul A.
I used sapling population data collected since 1980 to investigate how canopy disturbance interacts with chronic understory disturbance to determine regeneration patterns. I reconstructed flooding history using a combination of river gauging, a local water-level recorder, and contour maps. 1979 was an extreme flood year for this site, as was 1989. Reduction in flooding frequency since dam construction in 1965 was significant for all elevations, while reduction in flooding duration was significant only for the upper half of the site. Using constrained ordination, I showed that sapling occurrence varies primarily along a flooding/soil moisture gradient, and secondarily along a canopy-openness gradient. This confirms that both flooding and light influence local variation in species composition. Small sapling density increased by more than five times during the decade, while large sapling density increased over 70%. I suggest that this increase is related to the decline in frequency and duration of flooding, and specifically, to the pattern of flooding from 1979-1989, characterized by severe flooding in 1979 and 1989. Sapling survivorship also responded to temporal variation in flooding over the decade. A damage survey revealed a size component to flooding damage, with small individuals making up a disproportionate share of those in the highest damage classes. In addition, there is a relationship between river flow and the elevation of saplings that died, suggesting that the effects of elevation are not simply related to flooding, but perhaps to soil moisture as well. Analyses of sapling growth suggest that there is a trade-off between tolerating flooding in wet years and being subject to moisture stress in dry years. I presented evidence that canopy gaps play an important role in determining sapling growth. Gaps appear to be crucial to the continued success of large saplings, and the fastest-growing individuals are found in them. This may also confer a measure of flood-tolerance related to sapling size.
Taxonomic revision of Clematis L. subgenus Clematis (Ranunculaceae) for Latin America and the Caribbean
(1993) Moreno, Nancy Pearson; Harcombe, Paul A.
A revised classification of the Latin American and Caribbean species of Clematis L. subgenus Clematis is presented. Twenty-six species are recognized based on morphological and geographical data. Eight taxa not considered in recent floristic treatments (C. acapulcensis, C. affinis, C. grahamii, C. guadeloupae var. guadeloupae, C. guadeloupae var. medusaea, C. populifolia, C. polygama, C. rhodocarpa) are included. A cladistic analysis of all of the American species of the subgenus supports the division of the group into two subsections within the Section Clematis.
Testing shade tolerance as a mechanism of dynamics in three forests of Big Thicket National Preserve, southeast Texas
(2002) Lin, Jie; Harcombe, Paul A.
I investigated the role of shade tolerance in the dynamics of three forests of Big Thicket National Preserve, southeast Texas. In a mature mesic forest, shade-intolerant species had higher high-light growth and lower low-light growth than tolerant species. Results suggested that there was a tradeoff between high-light growth and low-light growth across species. Moreover, low-light survival and high-light growth were negatively correlated across species. In contrast to northern hardwood forests where survival in low light may be achieved at the expense of growth, my results suggested that shade-tolerant species in this southern mixed forest can grow faster as well as survive better than shade-intolerant species in low light. I conclude that both juvenile growth and survival are important components of shade tolerance and their relationships may be system-specific. In a floodplain forest, growth responses to light were consistent with the expectation that shade-intolerant species grow faster than shade-tolerant species in high light and vice versa. But mortality risks of some shade-tolerant species were unexpectedly high. The increased flooding during one of the study time periods may be responsible for the high mortality risks of shade-tolerant species. The results further supported that the success of shade-tolerant species in this forest may be limited by flooding as previous studies suggested. Compared with the mesic site, common species showed little intraspecific differences in shade tolerance. In a sandy upland pine-oak forest, low-light growth responses of saplings corresponded to shade tolerance expectation. However, shade-intolerant species did not show faster growth than shade-tolerant species in high light possibly because these species are drought-tolerant. The correspondence between increasing stem density and increasing death rates of shade-intolerant xeric dominants suggested that recent increases in canopy shading may be responsible for high death rates of these species. Cross-site comparison showed that shade-tolerant species had better performance at the dry site than at the mesic site. Consistent with the facilitative effect found at dry sites, these species may benefit from the less competition from surrounding vegetation.
Testing two measures of shade tolerance in a mesic forest in southeast Texas
(1999) Lin, Jie; Harcombe, Paul A.
I used sapling demographic data to investigate the relationship between shade tolerance and parameter estimates of a mortality-growth model and a height-diameter model. The study site is at Wier Woods, a mesic forest in southeast Texas. The results show that species order for probability of mortality at zero growth corresponds closely to the standard shade tolerance classification: the probability of mortality at zero growth decreases as shade tolerance rank increases. Also, the probability of mortality decreases rapidly as growth increases for shade-intolerant species, while showing little variation for shade-tolerant species. Therefore, this study provides strong support for the assertion that the mortality-growth relationship is a key life-history characteristic that determines shade tolerance. The results of a linear regression of height against DBH show that shade-intolerant species have steeper slopes than shade-tolerant species. This implies that a trade-off of photosynthate allocation between height growth and diameter growth may be an additional mechanism that influences shade tolerance.
Within-season woody plant growth: Patterns, characteristics, and sensitivity to weather variation as an indicator of vegetation response to climate change
(1996) Winters, Karin; Harcombe, Paul A.
Aluminum spring-band dendrometers were used to measure within-season tree growth at three sites (dry, mesic, and wet) in the Big Thicket National Preserve, Texas to determine what affects within-season tree growth and if patterns of growth vary among species, sites, and years. Growth patterns were found to differ among sites when the variation among species was removed, and among years and species. Significant interactions implied that trees were responding differentially to weather variation. Temperature, rain, soil moisture, and deficit were found to affect the within-season growth of trees. However, site patterns seemed to mediate the effects of weather, and intrinsic seasonal growth patterns defined how different species responded to weather variation. My results suggest that climate change will differentially affect tree growth, depending on species and how within-season weather patterns change.

Browsing by Author "Harcombe, Paul A."

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