Root reinforcement and extracellular products reduce streambank fluvial erosion.

A detailed understanding of the factors that impact bank erodibility is necessary to effectively model changes in channel form. This study evaluated the combined contributions of roots and soil microorganisms to soil resistance against fluvial erosion. To do this, three flume walls were constructed to simulate unvegetated and rooted streambanks. Unamended and organic material (OM) amended soil treatments with either no-roots (bare soil), synthetic (inert) roots, or living roots (Panicum virgatum) were created and tested with the corresponding flume wall treatment. OM stimulated the production of extracellular polymeric substances (EPS) and appeared to increase the applied stress required to initiate soil erosion. Synthetic fibers alone provided a base reduction in soil erosion, regardless of the flow rate used. When used in combination, synthetic roots and OM-amendments reduced erosion rates by 86 % or more compared to bare soil; this reduction was identical to the live rooted treatments (95 % to 100 %). In summary, a synergistic relationship between roots and organic carbon inputs can significantly reduce soil erosion rates due to fiber reinforcement and EPS production. These results indicate that root-biochemical interactions, like root physical mechanisms, play an important role in influencing channel migration rates due to reductions in streambank erodibility.

Post-fertilization physiology and growth performance of loblolly pine clones.

The physiological processes leading to enhanced growth of loblolly pine (Pinus taeda L.) following fertilization are not clearly understood. Part of the debate revolves around the temporal response of net photosynthetic rate (A(n)) to fertilization and whether the A(n) response is always positive. We measured light-saturated photosynthetic rate (A(sat)), dark respiration rate, growth and crown silhouette area in eight clones of loblolly pine before and after nitrogen (N) fertilization (112 kg ha(-1)) to track the initial physiological changes prior to any changes in growth. Overall, there were positive photosynthetic and growth responses to fertilization; however, there were pronounced physiological and growth differences among clones, even among clones with the same parents. Clones 4, 6 and 7 showed large volume growth and A(sat) responses to fertilization. Clone 1 and Clone 8 (a full-sibling of Clone 7) mainly showed a volume growth response, whereas Clone 2 (full-sibling of Clone 1) showed an A(sat) response only. Clone 5 (full-sibling of Clone 6) showed little response to fertilization, whereas Clone 3 (full-sibling of Clone 4) showed a negative A(sat) response. Thus, within-family variation warrants further study to ensure that relatively expensive clonal material is used efficiently.

A comparison of root growth dynamics of silver maple and flowering dogwood in compacted soil at differing soil water contents.

Many bottomland tree species are tolerant of compacted soil and perform well in urban environments; however, the mechanism underlying this tolerance is unknown. Increased soil water content has been shown to alleviate some of the effects of soil compaction on plant growth, presumably because increasing soil water reduces soil strength. We hypothesized that tree species tolerant of very wet soils would have opportunities for root growth in compacted soil when high soil water contents reduced soil strength, whereas species intolerant of bottomland conditions would not. We tested this hypothesis on flowering dogwood (Cornus florida L.), a mesic species intolerant of inundation, and silver maple (Acer saccharinum L.), a bottomland species. Seedlings of both species were grown in pots for 21 and 30 days, respectively, in a growth chamber in native loam soil maintained at various combinations of soil strength and soil water tension. Downward root growth rate decreased in response to increasing soil strength in both species. At low soil strength (0.6 MPa), downward root growth rate of dogwood seedlings slowed when soil was either excessively wet or dry, whereas root growth rate of silver maple seedlings increased linearly with soil water content. In moderately compacted soil (1.5 g cm(-3) bulk density), silver maple seedlings had greater root growth rate, root length per plant, and ratio of root length to root dry weight in wet soil (0.006 MPa soil water tension) than in moist and dry soils (0.026 and 0.06 MPa, respectively), even though mean oxygen diffusion rate (ODR) was only 0.28 &mgr;g cm(-2) (SE = 0.05). No such effect was detected in highly compacted soil (1.7 g cm(-3) bulk density) in either species. Mean ODR showed a weak positive correlation with soil water tension (r = 0.40, P = 0.07), but was unrelated to soil strength. We conclude that silver maple roots can grow in moderately compacted soil when high soil water content decreases soil strength, whereas dogwood is unable to take advantage of this opportunity.

Growth and photosynthetic responses of four Virginia Piedmont tree species to shade.

To determine the effects of shade on biomass, carbon allocation patterns and photosynthetic response, seedlings of loblolly pine (Pinus taeda L.), white pine (Pinus strobus L.), red maple (Acer rubrum L.), and yellow-poplar (Liriodendron tulipifera L.) were grown without shade or in shade treatments providing a 79 or 89% reduction of full sunlight for two growing seasons. The shade treatments resulted in less total biomass for all species, with loblolly pine showing the greatest shade-induced growth reduction. Yellow-poplar was the only species to show increased stem height growth in the 89% shade treatment. The shade treatments increased specific leaf area of all species. Quantum efficiency, dark respiration and light compensation point were generally not affected by the shade treatments. Quantum efficiency, dark respiration, maximum photosynthesis and light compensation point did not change consistently between the first and second growing seasons. We conclude that differences in shade tolerance among these species are not the result of changes in the photosynthetic mechanism in response to shade.

Growth, shoot phenology and physiology of diverse seed sources of black spruce: I. Seedling responses to varied atmospheric CO(2) concentrations and photoperiods.

We conducted a greenhouse experiment to determine: (1) if diverse provenances of black spruce (Picea mariana (Mill.) B.S.P.) respond similarly in growth, phenology and physiology to an approximately 300 ppm increase in atmospheric CO(2) concentration, and (2) the influence of photoperiod on both provenance and provenance x CO(2) interaction effects. Seedlings from provenances that originated from the Yukon (63 degrees 34' N, 135 degrees 55' W), British Columbia (58 degrees 47' N, 123 degrees 38' W), Alberta (52 degrees 22' N, 115 degrees 15' W), Newfoundland (50 degrees 54' N, 56 degrees 06' W) and Ontario (48 degrees 59' N, 80 degrees 38' W and 45 degrees 10' N, 77 degrees 10' W) were subjected to growth analysis in greenhouse growth chambers supplied with 712 +/- 93 (SD) ppm CO(2) (elevated) or 394 +/- 59 ppm CO(2) (ambient). Seedlings from Provenances 7000 and 6901 were also subjected to an extended photoperiod treatment and periodically measured for shoot and root gas exchange. In response to a natural photoperiod, southern provenances grew more, broke and set bud later, and partitioned more biomass to shoot versus root than northern provenances. These differences among provenances were influenced by the extended photoperiod treatment but not by the elevated CO(2) treatment. Averaged across all provenances, elevated CO(2) increased seedling final weights by 55%; however, the elevated CO(2) treatment had no effect on the provenance differences in any measured trait. We conclude that the large differences in physiology, phenology and growth among these diverse provenances of black spruce were expressed similarly in both ambient and elevated atmospheric CO(2) concentrations.

Growth, shoot phenology and physiology of diverse seed sources of black spruce: II. 23-year-old field trees.

Four sources of 23-year-old black spruce (Picea mariana (Mill.) B.S.P.) from a provenance test at the Petawawa National Forestry Institute (46 degrees N, 77 degrees 30' W) were assessed for height growth, shoot phenology and seasonal gas exchange. The provenances were designated 7000 (Yukon, 63 degrees 34' N, 135 degrees 55' W), 6979 (Alberta 52 degrees 22' N, 115 degrees 15' W), 6908 (Ontario, 48 degrees 59' N, 80 degrees 38' W) and 6901 (Ontario, 45 degrees 10' N, 77 degrees 10' W). Trees of southern provenances (6901 and 6908) were considerably taller, and broke bud and ceased growth later than trees of northern provenances (6979 and 7000). In early spring, trees of northern provenances had higher net photosynthetic rates (P(n)) than trees of southern provenances (6908 and 6901). During midsummer, trees of Provenance 7000 generally had the highest P(n) as a result of low rates of shoot dark respiration (R(d)). Trees of northern provenances displayed an earlier autumn decline in P(n) than trees of southern provenances. Provenance differences in growth, shoot phenology and physiology agreed well with results from a greenhouse study of seedlings from the same provenances. We conclude that the poor growth performance of trees of northern provenances in Ontario was associated with: (1) a short period of shoot growth, (2) a high rate of dry matter partitioning to roots, (3) low rates of late-season P(n) in response to decreasing photoperiod, and possibly, (4) a high rate of root R(d).

Effects of long-term ozone fumigations on growth and gas exchange of Fraser fir seedlings.

Fraser fir seedlings from two seed sources in the Southern Appalachians (Mt Mitchell, North Carolina, a declining population; and Mt Rogers, Virginia, a relatively healthy population) were subjected to long-term (2.5 years) intermittent ozone fumigations (0.025, 0.070, and 0.150 ppm) while being grown through five growth cycles in an accelerated-growing regime. Fumigations took place during bud break, stem elongation and bud set. Following each growing cycle, gas exchange parameters and dry weights were determined. The ozone fumigations did not produce any effect on seedling growth. The ozone fumigation effects on gas exchange parameters were inconsistent, and generally not statistically different, with no differences occurring between seed sources. There was no correlation between photosynthetic rates and seedling growth. These results provide no evidence that ozone may be contributing to the differences in decline noted between the Mt Rogers and Mt Mitchell populations of Fraser fir.

Influence of water stress on the physiology and growth of red spruce seedlings.

Two-year-old, container-grown red spruce (Picea rubens Sarg.) seedlings from a New Hampshire seed source were exposed to 10 or 11 drying cycles in which the seedlings were not watered until their midday (1400 h) xylem water potentials averaged -1.57 MPa. Control seedlings were kept well watered to maintain midday water potentials of about -0.73 MPa. After the final drying cycle, the water-stressed seedlings were rehydrated and osmotic potentials were determined by pressure-volume analysis. Gas exchange at ambient CO(2) concentration (338 ppm) and at an elevated CO(2) concentration (838 ppm) was measured on both groups of plants as they slowly dried down. No osmotic adjustment or photosynthetic acclimation occurred as a result of the water-stress treatment and both groups of seedlings maintained photosynthesis to water potentials as low as -3.0 MPa. Twenty-four hours after rehydration, the water-stressed seedlings had photosynthetic rates as high as the control seedlings. Estimated stomatal limitation to photosynthesis was approximately 30% down to water potentials of -1.4 MPa, but increased steadily as water potentials decreased further. At ambient CO(2) concentrations (338 ppm) and water potentials averaging -2.45 MPa, photosynthetic rates of water-stressed seedlings were 15% those of well-watered seedlings, whereas when the same water-stressed seedlings were measured in the presence of an elevated concentration of CO(2) (838 ppm) their photosynthetic rates were 73% those of well-watered seedlings measured at an ambient CO(2) concentration (338 ppm).

Growth and physiological responses of yellow-poplar seedlings exposed to ozone and simulated acidic rain.

Nine-week-old, yellow-poplar (Liriodendron tulipifera L.) seedlings were exposed to ozone (O(3)) 4 h day(-1), 5 days week(-1), in combination with simulated rain (pH 3.0, 4.3, or 5.6, 1 h day(-1), 2 days week(-1), at 0.75 cm h(-1)) for 6 weeks, under controlled laboratory conditions. There was no main treatment effect of O(3) (<0.02, 0.05, 0.10, or 0.15 microlitre(-1)) on height growth or total plant biomass. However, specific leaf area increased linearly (P=0.05) with exposure of plants to increasing O(3) concentrations. Exposure of seedlings to 0.10 microl litre(-1) O(3) reduced the leaf area ratio and the stomatal conductance (S(c)), but increased the mean unit leaf rate compared to control plants. This O(3) concentration also caused S(c) to decrease more rapidly, relative to non-fumigated plants, in response to increasing vapor pressure deficit. The only main treatment effect caused by simulated rain was a linear decrease in S(c) with increasing acidity. For combined pollutant effects, O(3) applied at 0.05 or 0.10 microl litre(-1) resulted in significant (P=0.05) linear decreases in dry weights and relative growth rates (RGR) of stems and leaves, and in cumulative leaf area (LAI), as the solution pH decreased. Root to shoot ratio (RSR) decreased linearly, whereas stem RGR and LAI increased linearly in response to decreasing pH for seedlings exposed to 0.15 microl litre(-1) O(3). Seedlings treated with a pH 5.6 solution exhibited a linear decrease (P=0.05) in leaf dry weight and RGR and a linear increase (P=0.05) in RSR as O(3) concentrations increased.