Lisa J. Samuelson

Riparian woody plant diversity and forest structure along an urban-rural gradient

Changes in riparian woody plant assemblages are anticipated in the southeastern United States due to increases in urbanization rates. Because riparian forests serve important roles in maintaining water quality and biodiversity, understanding how they respond to urbanization is crucial. The objective of this study was to examine forest structure and woody vegetation diversity indices of riparian communities in response to an urbanization gradient in West Georgia, USA. Measures of forest structure and diversity were compared to measures of urbanization and land cover. Although Liquidambar styracifluaand Quercus nigrawere dominant species in the forest stand and regeneration layer for all riparian communities, the invasive, non-native shrub Ligustrum sinense was the most dominant species observed in the regeneration layer for urban, developing, and agriculture communities. The proportion of non-native species in the forest stand and regeneration layer decreased and Shannon diversity of the regeneration layer increased with increasing distance from the urban center. Shifts in diversity indicate that anthropogenic disturbance may subdue the ability of diverse communities to resist non-native plant invasions.

Growth and physiology of loblolly pine in response to long-term resource management: defining growth potential in the southern United States

Leaf physiology and stem growth were assessed in loblolly pine (Pinus taeda L.) in response to 10 to 11 years of treatment with weed control (W), weed control plus irrigation (WI), weed control plus irrigation and fertigation (WIF), or weed control plus irrigation, fertigation, and pest control (WIFP) to determine whether increased resource availability can push productivity of loblolly pine closer to its biological growth potential expressed in favorable, exotic environments. Maximum basal area and stem biomass were 41 m 2 ha -1 and 172 Mgha -1 , respectively, in response to fertigation. Stem- wood biomass production was positively and linearly related to basal area. Belowground woody biomass was highest in the WIF and WIFP treatments and averaged 50 Mgha -1 , but the W and WI treatments exploited a greater area of soil with low-density coarse roots. Fertigation increased foliar nitrogen concentration and foliage biomass, but treatment had no effect on leaf physiological parameters or growth efficiency. Comparison with growth rates reported for loblolly pine in Hawaii revealed that loblolly pine grown in its native range can produce the high yields observed in exotic environ- ments when stands are below maximum carrying capacity.

Effects of nitrogen on leaf physiology and growth of different families of loblolly and slash pine

Leaf physiology and fractional dry weight allocation were examined in four open-pollinated families of loblolly pine (Pinus taeda L.) and slash pine (Pinus elliottii Englm. var. elliottii) in response to growth under low and high N supply for six months. Nitrogen greatly influenced dry weight allocation, total mass, leaf net photosynthesis and leaf conductance in seedlings of both species. Family variation in fine root allocation was observed under low but not high N treatment, but for the majority of physiological and growth traits, family variation was stable under varying N treatment. Family rankings based on juvenile height, diameter and biomass accretion were similar to rankings based on field performance at 22 years in slash pine but not loblolly pine. Lower leaf maintenance respiration rates were associated with families exhibiting the most rapid juvenile growth.

Growth and nutrition of Quercus rubra L. seedlings and mature trees after three seasons of ozone exposure

Seedling growth and nutritional status have been shown to be sensitive to ozone, but the influence of multi-season ozone exposure on mature tree growth and nutrition has not been examined. To determine if seedlings and mature trees were similarly affected by ozone exposure, growth and nutrient concentrations in northern red oak (Quercus rubra L.) 4-year-old seedlings and 32-year-old mature trees were examined after treatment with subambient, ambient and twice ambient concentrations of ozone for three growing seasons. SUM00 values summed over the three growing seasons were 147, 255 and 507 ppm-h, respectively, for the subambient, ambient and twice ambient exposures. For mature trees, no influence of ozone treatment on lower stem diameter growth, stem growth within the mid-canopy and foliar biomass was observed. Seedling height was increased by ozone, but biomass and diameter were unaffected. A reduction in the specific leaf weight of leaves in response to ozone coincident with the loss of recurrent flushing was observed in seedlings. Ozone exposure reduced foliar nitrogen concentrations and increased woody tissue nutrient concentrations in seedlings and mature trees at the end of the third growing season. These results suggest an influence of ozone on retranslocation processes in seedlings and mature trees.

Influence of urbanization on riparian forest diversity and structure in the Georgia Piedmont, US

Riparian forests are increasingly threatened by urban expansion and land use change worldwide. This study examined the relationships between landscape characteristics and woody plant diversity, structure, and composition of small order riparian corridors along an urban-rural land use gradient in the Georgia Piedmont, US. Riparian plant diversity, structure, and composition were related to landscape metrics and land use. Species richness was negatively associated with impervious surfaces and landscape diversity, and positively associated with forest cover and largest forest patch index. Shannon species diversity was strongly related to the biomass of non-native species, especially for the regeneration layer. Urban sites were characterized by high richness of non-native and pioneer species. Developing sites were dominated by the non-native shrub, Ligustrum sinense Lour., and several native overstory trees, mainly Acer negundo L. While agricultural and managed forest sites were composed of ubiquitous species, unmanaged forest sites had a structurally distinct midstory indicative of reduced disturbance. Urban and agricultural land uses showed decreased native stem densities and signs of overstory tree regeneration failure. Results from this study highlight the impact of the surrounding landscape matrix upon riparian forest plant diversity and structure.

Ozone-exposure responses of black cherry and red maple seedlings

Abstract Field foliar injury surveys have been used to identify ozone injury in deciduous tree species growing in forested areas in the southeastern United States. Whether ozone-induced foliar symptoms are indicative of leaf photosynthetic and growth reductions is not well understood. This research tested the hypothesis that ozone-induced foliar injury is accompanied by reductions in leaf gas exchange and growth in 1-year-old black cherry ( Prunus serotina Ehrh.) and red maple ( Acer rubrum L.). Seedlings were grown under a 75% reduction of ambient light and fumigated with sub-ambient, ambient or twice-ambient concentrations of ozone on a 24-hr basis for one growing season (5 months). For both species, only leaves exposed to the twice-ambient ozone treatment displayed visible symptoms. Net photosynthesis and leaf conductance of red maple and black cherry leaves declined, and internal CO 2 concentrations increased in response to ozone treatment. Ozone reduced the height growth and the root/shoot ratio of black cherry, but ozone did not influence red maple growth. These results indicate that ozone-induced foliar injury may be accompanied by reductions in leaf gas exchange in black cherry and red maple, and by growth reductions in black cherry.

Comparison of the response of red oak (Quercus rubra) seedlings and mature trees to ozone exposure using simulation modeling

Field studies have determined that the photosynthetic rates of mature northern red oak (Quercus rubra L.) trees are sensitive to ozone exposure whereas those of seedlings are not. We used a model of tree physiology to determine the consequences of these differences in photosynthetic response to carbon allocation and tissue growth in seedlings and mature trees over a 2-year period. In the seedling simulations at twice ambient ozone, only the total non-structural carbohydrate (TNC) storage pool was affected. The effects in the simulated mature tree were much greater, with large decreases predicted for TNC, fine root, leaf, stem, branch, and coarse root tissues. The model produced many ozone-induced responses in the mature tree that were similar or consistent with observations made in a field study, but the simulations overestimated the effect of twice ambient ozone on root TNC and growth. The discrepancy between field and simulated results suggests that the field study trees exposed to elevated ozone levels may use carbon at a reduced rate, particularly through reduced respiration.

Fraser fir seedling gas exchange and growth in response to elevated CO2

Abstract Growth and gas exchange characteristics were examined in Fraser fir ( Abies fraseri (Pursh.) Poir.) seedlings grown from seed in elevated (713 ppm) or ambient (374 ppm) CO 2 for 1 year (two artificial growing seasons) to determine the potential influence of a twice-ambient CO 2 concentration on this species. A subset of seedlings was transplanted from 172 cm 3 pots into 1000 cm 3 pots at 7 months to determine if CO 2 effects were dependent on rooting volume. At 5 and 12 months, net photosynthesis ( P net ) and leaf conductance ( g l ) were lower in elevated CO 2 -grown seedlings grown in 172 cm 3 pots than in ambient CO 2 -grown seedlings when measured at either 346 or 796 ppm CO 2 . For 12-month-old seedlings grown in 1000 cm 3 pots, P net was reduced by an elevated CO 2 growth environment only when measured at 346 ppm CO 2 , although g l was lower in these seedlings when measured at either CO 2 measurement level. Seedlings grown in both pot sizes and in elevated CO 2 for 1 year had greater height, diameter, and leaf, stem, root and total dry weights than seedlings grown in ambient CO 2 . Specific leaf weight ( SLW ) was greater in elevated than in ambient CO 2 -grown needles only in the large pot size treatment. These results suggest that Fraser fir seedling growth will increase in a future elevated CO 2 environment despite changes in gas exchange characteristics.

Gas exchange and canopy structure of 9-year-old loblolly pine, pitch pine and pitch x loblolly hybrids

SummarySeasonal gas exchange and canopy structure were compared among 9-year-old loblolly pine (Pinus taeda L.), pitch pine (Pinus rigida Mill.), and pitch x loblolly hybrids (Pinus rigida x taeda) growing in an F2 plantation located in Critz, Va., USA. Leaf net photosynthesis, conductance, internal CO2 concentration (ci), water use efficiency (WUE; photosynthesis/conductance), dark respiration and the ratio of net photosynthesis/respiration did not vary among or within the three taxa. Significant differences in volume production, crown length, total crown leaf surface area and the silhouette area of shade shoots among the taxa were observed. The loblolly-South Carolina source had greater volume and crown surface area than the pitch pine, and the hybrid taxa were intermediate between the two. Although the silhouette area ratio of shade foliage varied among taxa, it was not related to volume. A strong relationship between total leaf surface area and volume was observed. Leaf conductance, ci, WUE and leaf water potential were the physiological parameters significantly and positively correlated with volume. This study suggests that the amount of needle surface in the canopy is more important in early stand volume growth than the leaf carbon exchange rate and the degree of needle self-shading in the lower canopy.

Maximum growth potential in loblolly pine: results from a 47-year-old spacing study in Hawaii

Growth, allocation to woody root biomass, wood properties, leaf physiology, and shoot morphology were examined in a 47-year-old loblolly pine (Pinus taeda L.) density trial located in Maui, Hawaii,...