Thomas T. Lei

Variations in leaf β13C along a vertical profile of irradiance in a temperate Japanese forest

Abstract The vertical profile of stable carbon isotope ratios (δ13C) of leaves was analyzed for 13 tree species in a cool-temperate deciduous forest in Japan. The vertical distribution of long-term averaged δ13C in atmospheric CO2 (δa) was estimated from δ13C of dry matter from NADP-malic enzyme type C4 plant (Zea mays L. var. saccharata Sturt.) grown at a tower in the forest for 32␣days, assuming constant Δ value (3.3‰) in Z. mays against height. The δa value obtained from δ13C in Z.␣mays was lowest at the forest floor (−9.30 ± 0.03‰), increased with height, and was almost constant above 10␣m (−7.14 ± 0.14‰). Then leaf Δ values for the tree species were calculated from tree leaf δ13 C andδa. Mean leaf Δ values for the three tall deciduous species (Fraxinus mandshurica, Ulmus davidiana, and Alnus hirsuta) were significantly different among three height levels in the forest: 23.1 ± 0.7‰ at the forest floor (understory), 21.4 ± 0.5‰ in lower canopy, and 20.5 ± 0.3‰ in upper canopy. The true difference in tree leaf Δ among the forest height levels might be even greater, because Δ in Z. mays probably increased with shading by up to ∼‰. The difference in tree leaf Δ among the forest height levels would be mainly due to decreasing intercellular CO2 (Ci) with the increase in irradiance. Potential assimilation rate for the three tree species probably increased with height, since leaf nitrogen content on an area basis for these species also increased with height. However, the increase in stomatal conductance for these tree species would fail to meet the increase in potential assimilation rate, which might lead to increasing the degree of stomatal limitation in photosynthesis with height.

INHIBITION OF SEEDLING SURVIVAL UNDER RHODODENDRON MAXIMUM (ERICACEAE) : COULD ALLELOPATHY BE A CAUSE?

In the southern Appalachian mountains a subcanopy species, Rhododendron maximum, inhibits the establishment and survival of canopy tree seedlings. One of the mechanisms by which seedlings could be inhibited is an allelopathic effect of decomposing litter or leachate from the canopy of R. maximum (R.m.) on seed germination, root elongation, or mycorrhizal colonization. The potential for allelopathy by R.m. was tested with two bioassay species (lettuce and cress), with seeds from four native tree species, and with three ectomycorrhizal fungi. Inhibitory influences of throughfall, fresh litter, and decomposed litter (organic layer) from forest with R.m. (+R.m. sites) were compared to similar extractions made from forest without R.m. (-R.m. sites). Throughfall and leachates of the organic layer from both +R.m. and -R.m. sites stimulated germination of the bioassay species above that of the distilled water control, to a similar extent. There was an inhibitory effect of leachates of litter from +R.m. sites on seed germination and root elongation rate of both bioassay species compared with that of litter from -R.m. sites. Native tree seed stratified in forest floor material from both forest types had a slightly higher seed germination rate compared with the control. A 2-yr study of seed germination and seedling mortality of two tree species, Quercus rubra and Prunus serotina, in field plots showed no significant influence of litter or organic layer from either forest type. Incorporating R.m. leaf material into the growth medium in vitro depressed growth of one ectomycorrhizal species but did not affect two other species. Leaf material from other deciduous tree species depressed ectomycorrhizal growth to a similar or greater extent as leaf material from R.m. In conclusion, R.m. litter can have an allelopathic effect on seed germination and root elongation of bioassay species as well as some ectomycorrhizal species. However, this allelopathic affect is not manifest in field sites and is not likely to be an important cause for the inhibition of seedling survival within thickets of R.m.

Effects of Rhododendron maximum Thickets on Tree Seed Dispersal, Seedling Morphology, and Survivorship

In the southern Appalachian forests, the regeneration of canopy trees is severely inhibited by Rhododendron maximum L., an evergreen understory shrub producing dense thickets. While light availability is a major cause, other factors may also contribute to the absence of tree seedlings under R. maximum. We examined the effects of R. maximum on several life history stages of tree species, including seed dispersal, seed bank germination, seedling growth, and survivorship. We found no significant effect of R. maximum on seed reaching the forest floor for Acer rubrum, Liriodendron tulipifera, Quercus rubra, Quercus prinus, Carya spp., and Nyssa sylvatica. This indicates that either seed output of maternal trees rooted within the thicket were unaffected by R. maximum or seed dispersal from surrounding areas into thickets compensated for a lower seed production of canopy trees rooted in the thickets. Germination of tree seeds (A. rubrum, L. tulipifera, Q. rubra, and Betula lenta) from the seed bank also was not reduced by leaves and substrates within the thickets. Seedling mortality of all species (Q. rubra, Prunus serotina, and Tsuga canadensis) planted in our experimental plots was up to fivefold higher in thickets of R. maximum compared with those outside the thickets. The order of mortality under the R. maximum thickets, \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Effects of high CO2 on nodule formation in roots of Japanese mountain alder seedlings grown under two nutrient levels

Prunus> Quercus> Tsuga

Increased susceptibility to photoinhibition in leaves of Japanese elm (Ulmus davidiana var. japonica) with high manganese concentrations

\end{document} , was consistent with the shade tolerance ranking of these species. Loss of Tsuga seedlings was attributed to burial by litter rather than shade. Surviving seedlings of Quercus and Prunus in R. maximum thickets were taller than those outside the thickets, but the seedlings in R. maximum thickets produced significantly fewer leaves, smaller total leaf area, leaf mass, and stem mass. Leaf N (%) was significantly higher in Quercus seedlings in R. maximum thickets compared with seedlings outside the thickets. Moreover, no difference was found in leaf N (%) between forest types for Prunus and Tsuga, indicating that seedlings in R. maximum thickets were not N limited. Rather, light limitation, herbivory, and litter fall contributed to the lack of tree regeneration under R. maximum thickets.

Susceptibility to photoinhibition of three deciduous broadleaf tree species with different successional traits raised under various light regimes

The phenology and leaf traits ofDaphne kamtschatica Maxim. var.jezoensis (Maxim.) Ohwi, the only summer deciduous shrub (20–40 cm) in the temperate forest of northern Japan, are examined. This plant carries through the winter mature leaves and well formed flower buds. It flowers in early spring during snowmelt and begins photosynthesis under relatively high irradiance under an open forest canopy. Our results show that there is significant carbon gain during the period when new leaves and fruit maturation also take place. Beginning in June, as the forest canopy closes, leaves onDaphne shoots senesce acropetally and the plants become completely bare in mid-July. After a period of 20-day dormancy, the shoots begin to resprout. Leaves become mature in early October and remain on the stem over winter. Leaf traits and photosynthesis measurements suggest as follows. 1) By becoming summer deciduous,D. kamtschatica avoids the cost of maintaining leaves inefficient under deep shade. 2) The onset and breaking of the summer dormancy is triggered by photoperiod since plants at the forest edge also become dormant even when light remained relatively high. However, the decreased duration of dormancy with higher light levels suggests that there is a tendency towards shorter dormancy where summer shade is absent and this could eventually lead to an evergreen habit such as that found in the alpine speciesDaphne miyabeana.

Effects of manganese toxicity on photosynthesis of white birch (Betula platyphylla var. japonica) seedlings

Japanese mountain alder (Alnus hirsuta Trucz.) is a typical species symbiotic with Frankia and invades disturbed sites as a pioneer. This species responded with an increase in photosynthetic rates when provided with CO2 enrichment at low soil fertility. Leaf color of alder under high CO2 was slightly yellowish and similar among nutrient treatments. Specific leaf area increased at high nutrient level independent of CO2 levels. Increasing nitrogen levels were associated with higher photosynthetic rate in ambient CO2-grown seedlings but had no effect on seedlings grown in high CO2. Nodule formation was accelerated by high CO2 under infertile conditions.

Temperature response and photoinhibition investigated by chlorophyll fluorescence measurements for four distinct species of dipterocarp trees

We investigated whether dynamic photosynthesis of understory Quercus rubra L. (Fagaceae) seedlings can acclimate to the altered pattern of sunflecks in forest patches with Rhododendron maximum L. (Ericaceae), an understory evergreen shrub. Maximum photosynthesis (A) and total CO2 accumulated during lightflecks was greatest for 400‐s lightflecks, intermediate for 150‐s lightflecks, and lowest for 50‐ and 75‐s lightflecks. For the 400‐s lightflecks only, maximum A and total CO2 accumulated were significantly lower for seedlings in forest patches with shrubs present (SF) than for seedlings in forest patches without shrubs (F). These effects were found only when A was calculated on a leaf‐area basis because the specific leaf area of seedlings in F patches was 16% lower than it was for seedlings in SF patches. Photosynthesis reached 50% induction in 159 s for seedlings in F patches compared with 226 s for seedlings in SF patches. The faster induction of A for seedlings in F patches resulted in a significantly higher lightfleck use efficiency than for seedlings in SF patches. The inefficient use of lightflecks by Q. rubra seedlings in SF patches may be a primary mechanism by which Q. rubra seedlings are inhibited by subcanopy thickets of R. maximum.