Physiological, morphological and allocational plasticity in understory deciduous trees: importance of plant size and light availability

In a 4-year study, we investigated changes in leaf physiology, crown morphology and whole-tree biomass allocation in seedlings and saplings of shade-tolerant sugar maple (Acer saccharum Marsh.) and intermediate shade-tolerant yellow birch (Betula alleghaniensis Britt.) growing in natural understory light (0.5 to 35% of full sunlight) or in understory light reduced by 50% with shade nets to simulate the effect of gap closure. Leaf physiological parameters were mainly influenced by the light gradient, whereas crown morphological and whole-tree allocational parameters were mainly influenced by tree size. No single physiological, morphological or allocational trait was identified that could explain the difference in shade tolerance between the species. Yellow birch had higher growth rates, biomass allocation to branches and leaf physiological plasticity and lower crown morphological plasticity in unmodified understory light than sugar maple. Sugar maple did not display significant physiological plasticity, but showed variation with tree size in both crown morphology and whole-tree biomass allocation. When sugar maple was small, a greater proportion of whole-tree biomass was allocated to roots. However, physiological differences between the species decreased with decreasing light and most morphological and allocational differences tended to disappear with increasing tree size, suggesting that many species differences in shade-tolerance are expressed mainly during the seedling stage. Understory trees of both species survived for 4 years under shade nets, possibly because of higher plasticity when small and the use of stored reserves when taller.     

Development of canopy stratification during early succession in northern hardwoods

Canopy development on a 6-year-old strip cut was analyzed by measuring the heights to terminal buds and bud scale scars of the tallest individuals of each species present on 50 plots of radius 6 m. Phenology of height growth was monitored during the following growing season. Pin cherry (Prunus pensylvania L.), an intolerant short-lived tree, had the fastest growth rate and was on the average the tallest species from the second to the sixth year of regrowth. Although advance regeneration of sugar maple (Acer saccharum Marsh.) and beech (Fagus grandifoloa Ehrh.) were the tallest trees during the first growing season, their slower growth rate insured that they would not keep up with the pin cherry. Trembling aspen (Populus tremuloides Michx.), striped maple (Acer pensylvanicum L.) and yellow birch (Betula alleghaniensis Britt.) occupied an intermediate position in the canopy by the end of the sixth growing season, and showed relatively greater annual height increment than beech or sugar maple. Height growth phenology differed slightly for each species. Beech, ash (Fraxinus americana L.) and sugar maple commenced growth early, grew rapidly and set buds all by 1 August (beech by 15 June). Yellow birch, pin cherry and trembling aspen started growing as early as the others, grew more slowly at first but then grew for a longer period of time. Striped maple seemed to be somewhat intermediate. Growth phenology and growth rate are related to the tolerance and growth form type (e.g. determinate or indeterminate) of the species. The most tolerant species tend to be determinate in growth form, have slower growth rates and complete height growth earlier. The intolerant species tend to be indeterminate, have a faster growth rate and continue to grow for a longer period. These may be mechanisms by which many species can grow together and avoid adverse effects such as suppression.     

Frost tolerance and bud dormancy of container-grown yellow birch, red oak and sugar maple seedlings

Container-grown seedlings of red oak (Quercus rubra L.), sugar maple (Acer saccharum Marsh.) and yellow birch (Betula alleghaniensis Britton) in their first year of growth were overwintered outdoors. Tolerance of roots and stems to freezing was compared from late summer to the following spring. Mitotic activity in the apical bud was related more closely to air temperature than to bud dormancy as defined by days to bud break. In all species, stem hardening was observed before days to bud break reached a maximum. Dormancy release (days to bud break equal to zero) of yellow birch coincided with loss of stem hardening in the spring. Roots hardened more slowly, had a lower frost tolerance than stems in fall and winter, and dehardened earlier than stems in the spring. There were differences in stem and root hardiness among the species, with yellow birch being the most tolerant, followed by sugar maple and red oak. Primarily because of root sensitivity to frost, winter was a critical period for all three species, but particularly for red oak.     

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.     

Sugar maple and yellow birch regeneration in response to canopy opening,liming and vegetation control in a temperate deciduous forest of Quebec

We examined how the density, growth and survival of sugar maple (Acer saccharum Marsh.) and yellow birch (Betula alleghaniensis Britton) regeneration are influenced by gap size, soil nutrient availability and understory vegetation. We used a factorial combination of (1) three gap sizes (small: <100 m²; medium: 100–300 m²; large: ∼1000 m²); (2) presence/absence of liming (92% CaCO₃ at 500 kg ha⁻¹, 1st year post-harvest); and (3) presence/absence of vegetation control (weeding twice a year; 1st to 3rd year post-harvest). We monitored height increment and survival of 1500 seedlings and saplings of both species from the 3rd to the 6th year post-harvest, and assessed density 6 years post-harvest. Both species exhibited a complex set of density, growth and survival responses across the combination of treatments. Compared to sugar maple, yellow birch had an overall lower density, greater growth, and similar survival rate; the two species attained maximum values in different gap size for density, and similar gap size for growth and survival. Liming had very little or no effect on the species. The growth of yellow birch was slightly but significantly greater when understory vegetation was controlled, particularly in medium and large gaps. These results suggest that a variety of canopy gap sizes can provide the right combination of understory conditions for regenerating these two functionally different tree species.     

Poor recruitment is changing the structure and species composition of an old-growth hemlock-hardwood forest

Anthropogenic factors such as elevated deer populations, invasive earthworms or climate change may alter old-growth forests of the Upper Midwest region of the United States. We examined demographic trends of woody species across all size classes over 35 years in a late-successional forest dominated by hemlock (Tsuga canadensis), sugar maple (Acer saccharum) and yellow birch (Betula alleghaniensis) in Michigan's Upper Peninsula using two sets of permanent plots. For the duration of the study period, species that were less-preferred white-tailed deer (Odocoileus virginianus) forage, especially sugar maple, comprised a much higher fraction of all seedlings and saplings compared to overstory trees. The density of small sugar maple declined across the study period, but no other species became more abundant, creating a more open forest understory. By the most recent census, preferred species for deer browse had been nearly eliminated from the understory, and declines in unpreferred species such as sugar maple were also apparent. We found small changes in temperature (<0.5-1 °C rise in minimum and maximum temperatures depending on season) and precipitation (±28 mm depending on season) and little evidence of invasive earthworms impacts. Our results suggest that the sustained elevated deer density is shifting the structure and composition of this old-growth forest. A demographic model showed that if current recruitment, growth and mortality rates were to continue for 500 years the forest would eventually reach a new equilibrium with virtually no hemlock or yellow birch remaining.     

Canopy disturbance and intertree competition: implications for tree growth and recruitment in two yellow birch–conifer stands in Quebec, Canada

Composition, structure, and species-specific patterns of recruitment and growth were characterized in two yellow birch (Betula alleghaniensis Britt.)–conifer stands in Quebec, Canada, to improve our understanding of the dynamics of these complex ecosystems. The mixture of mid- and shade-tolerant species in the canopy, the inverse J-shape stem diameter distribution, and the age distribution were indicative that the two stands were in a late-successional stage. Recruitment of mid-tolerant species above 1.3 m in height appeared to be periodic and synchronized with historical spruce budworm (Choristoneura fumiferana Clem.) outbreaks, while the coniferous component of these mixedwood stands recruited continuously. Results suggest that recruitment of yellow birch and red maple (Acer rubrum L.) requires disturbances of a certain intensity that affect at least 25 % of the forest cover. In contrast, balsam fir (Abies balsamea (L.) Mill.) and red spruce (Picea rubens Sarg.) can recruit under the canopy without relying on moderate or large canopy disturbances. Results suggest that the historical disturbance regime, and differences in shade tolerance between species, largely govern the contemporary composition of these stands. This study improves the comprehension of mechanisms that regulate the dynamics of yellow birch-conifer stands and will be useful for the subsequent elaboration of forest management strategies.     

The impact of shade on morphology,growth and biomass allocation in <Emphasis Type="Italic">Picea sitchensis</Emphasis>, <Emphasis Type="Italic">Larix</Emphasis> × <Emphasis Type="Italic">eurolepis</Emphasis> and <Emphasis Type="Italic">Thuja plicata</Emphasis>

To determine the effect of shade on morphology, growth and biomass allocation in Picea sitchensis, Larix × eurolepis and Thuja plicata, seedlings were grown in the open or under shadehouses providing 25%, 50% and 75% reductions of full-light for two growing seasons. For most of the characteristics assessed there was no significant interaction between species and shade indicating that the morphological responses to changing shade treatments were not species-dependent. After two growing seasons the mean height increment for the three species was significantly greater in 25% (76.1 cm) and 50% shade (74.9 cm) than in the open (69.5 cm). Root collar diameter increment, shoot, root and total biomass declined significantly with increasing shade while the opposite was true for the height:diameter ratio. In both western red cedar and hybrid larch the shoot:root ratio was significantly greater in the shade while in Sitka spruce this characteristic was not influenced by shade. While all species had significantly greater specific shoot areas in 75% shade than in 0% shade, this trend was particularly pronounced in hybrid larch. In hybrid larch and western red cedar, the normalised specific projected shoot area increased significantly with increasing shade. The opposite trend was observed for Sitka spruce. We conclude that in the main the species studied demonstrated similar shade acclimation responses despite their reported differences in shade tolerance.     

Water relations and gas exchange of Acer saccharum seedlings in contrasting natural light and water regimes

Field measurements were made of leaf photosynthesis (A), stomatal conductance (g) and leaf water relations for sugar maple (Acer saccharum Marsh.) seedlings growing in a forest understory, small gap or large clearing habitat in southwestern Wisconsin, USA. Predawn water status, leaf gas exchange and plasticity in field and laboratory water relations characteristics were compared among contrasting light environments in a wet year (1987) and a dry year (1988) to evaluate possible interactions between light and water availability in these habitats. Leaf water potentials (Psi(leaf)) at predawn and midday were lower for clearing than gap or understory seedlings. Acclimation of tissue osmotic potentials to light environment was observed among habitats but did not occur within any of the habitats in response to prolonged drought. During a summer drought in 1988, decreases in daily maximum g (g(max)) and maximum A (A(max)) in clearing seedlings were correlated with predawn Psi(leaf), which reached a seasonal minimum of -2.0 MPa. Under well-watered conditions, diurnal fluctuations in Psi(leaf) of up to 2.0 MPa in clearing seedlings occurred along with large midday depressions of A and g. In a wet year, strong stomatal responses to leaf-to-air vapor pressure difference (VPD) in sunny habitats were observed over nine diurnal courses of gas exchange measurements on seedlings in a gap and a clearing. Increasing stomatal limitations to photosynthesis appeared to be responsible for the reduction in A at high VPD for clearing seedlings. In understory seedlings, however, low water-use efficiency and development of leaf water deficits in sunflecks was related to reduced stomatal limitations to photosynthesis relative to seedlings in sunny habitats. Predawn Psi(leaf) and VPD appear to be important factors limiting carbon assimilation in sugar maple seedlings in light-saturating irradiances, primarily through stomatal closure. The overall results are consistent with the idea that sugar maple seedlings exhibit "conservative" water use patterns and have low drought tolerance. Leaf water relations and patterns of water use should be considered in studies of acclimation and species photosynthetic performance in contrasting light environments.     

Effects of shade on growth and mortality of maple (Acer pseudoplatanus), ash (Fraxinus excelsior) and beech (Fagus sylvatica) saplings

We compared shade tolerance of maple, ash and beech in the saplingstage from two sites with rich soils differing in water supply,growing in dense thickets underneath a beech shelterwood ofvarying canopy densities. Shade tolerance was described by twocomponents: mortality in shade and height growth in high light.At low light, beech showed the least mortality, maple the highestand ash in between on both sites. The decline with increasinglight was steepest in beech and more gradual with ash and maple.At 15 per cent above canopy light, all three species approachedzero mortality. Beech as the most shade-tolerant species hadthe highest survival rate under low light and the least lengthgrowth rate under high light (>17 per cent). Ash had a lowersurvival rate at low light than beech and a highest growth rateat high light. Maple showed a bit weaker trade-off with thelowest survival rate but a growth rate inferior to ash. On thebetter water-supplied site, height growth was significantlysuperior in all three species only under high light. On thebasis of these results, silvicultural conclusions are drawnwith respect to appropriate light levels and cutting types.     

Growth, allocation and tissue chemistry of Picea abies seedlings affected by nutrient supply during the second growing season

One-year-old Norway spruce (Picea abies (L.) Karst.) seedlings were grown hydroponically in a growth chamber to investigate the effects of low and high nutrient availability (LN; 0.25 mM N and HN; 2.50 mM N) on growth, biomass allocation and chemical composition of needles, stem and roots during the second growing season. Climatic conditions in the growth chamber simulated the mean growing season from May to early October in Flakaliden, northern Sweden. In the latter half of the growing season, biomass allocation changed in response to nutrient availability: increased root growth and decreased shoot growth led to higher root/shoot ratios in LN seedlings than in HN seedlings. At high nutrient availability, total biomass, especially stem biomass, increased, as did total nonstructural carbohydrate and nitrogen contents per seedling. Responses of stem chemistry to nutrient addition differed from those of adult trees of the same provenance. In HN seedlings, concentrations of alpha-cellulose, hemicellulose and lignin decreased in the secondary xylem. Our results illustrate the significance of retranslocation of stored nutrients to support new growth early in the season when root growth and nutrient uptake are still low. We conclude that nutrient availability alters allocation patterns, thereby influencing the success of 2-year-old Norway spruce seedlings at forest planting sites.     

Element accumulation patterns of deciduous and evergreen tree seedlings on acid soils: implications for sensitivity to manganese toxicity

Foliar nutrient imbalances, including the hyperaccumulation of manganese (Mn), are correlated with symptoms of declining health in sensitive tree species growing on acidic forest soils. The objectives of this study were to: (1) compare foliar nutrient accumulation patterns of six deciduous (sugar maple (Acer saccharum Marsh.), red maple (Acer rubrum L.), red oak (Quercus rubra L.), white oak (Quercus alba L.), black cherry (Prunus serotina Ehrh.) and white ash (Fraxinus americana L.)) and three evergreen (eastern hemlock (Tsuga canadensis L.), white pine (Pinus strobus L.) and white spruce (Picea glauca (Moench) Voss.)) tree species growing on acidic forest soils; and (2) examine how leaf phenology and other traits that distinguish evergreen and deciduous tree species influence foliar Mn accumulation rates and sensitivity to excess Mn. For the first objective, leaf samples of seedlings from five acidic, non-glaciated field sites on Pennsylvania's Allegheny Plateau were collected and analyzed for leaf element concentrations. In a second study, we examined growth and photosynthetic responses of seedlings exposed to excess Mn in sand culture. In field samples, Mn in deciduous foliage hyperaccumulated to concentrations more than twice as high as those found in evergreen needles. Among species, sugar maple was the most sensitive to excess Mn based on growth and photosynthetic measurements. Photosynthesis in red maple and red oak was also sensitive to excess Mn, whereas white oak, black cherry, white ash and the three evergreen species were tolerant of excess Mn. Among the nine species, relative rates of photosynthesis were negatively correlated with foliar Mn concentrations, suggesting that photosynthetic sensitivity to Mn is a function of its rate of accumulation in seedling foliage.     

Osmotic potential of several hardwood species as affected by manipulation of throughfall precipitation in an upland oak forest during a dry year

Components of dehydration tolerance, including osmotic potential at full turgor (Psi(pio)) and osmotic adjustment (lowering of Psi(pio)), of several deciduous species were investigated in a mature, upland oak forest in eastern Tennessee. Beginning July 1993, the trees were subjected to one of three throughfall precipitation treatments: ambient, ambient minus 33% (dry treatment), and ambient plus 33% (wet treatment). During the dry 1995 growing season, leaf water potentials of all species declined to between -2.5 and -3.1 MPa in the dry treatment. There was considerable variation in Psi(pio) among species (-1.0 to -2.0 MPa). Based on Psi(pio) values, American beech (Fagus grandifolia Ehrh.), dogwood (Cornus florida L.), and sugar maple (Acer saccharum Marsh.) were least dehydration tolerant, red maple (A. rubrum L.) was intermediate in tolerance, and white oak (Quercus alba L.) and chestnut oak (Quercus prinus L.) were most tolerant. During severe drought, overstory chestnut oak and understory dogwood, red maple and chestnut oak displayed osmotic adjustment (-0.12 to -0.20 MPa) in the dry treatment relative to the wet treatment. (No osmotic adjustment was evident in understory red maple and chestnut oak during the previous wet year.) Osmotic potential at full turgor was generally correlated with leaf water potential, with both declining over the growing season, especially in species that displayed osmotic adjustment. However, osmotic adjustment was not restricted to species considered dehydration tolerant; for example, dogwood typically maintained high Psi(pio) and displayed osmotic adjustment to drought, but had the highest mortality rates of the species studied. Understory saplings tended to have higher Psi(pio) than overstory trees when water availability was high, but Psi(pio) of understory trees declined to values observed for overstory trees during severe drought. We conclude that Psi(pio) varies among deciduous hardwood species and is dependent on canopy position and soil water potential in the rooting zone.     

Effects of shade on morphology,chlorophyll concentration,and chlorophyll fluorescence of four Pacific Northwest conifer species

Four species of Pacific Northwestconifer seedlings (ponderosa pine [Pinusponderosa Dougl. ex Laws.], Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco], westernredcedar [Thuja plicata Donn ex D. Donn], andwestern hemlock [Tsuga heterophylla (Raf.)Sarg.]) were planted in individual pots and grownunder shade-cloth shelters that provided four levelsof shade (0% [full sunlight], 35%, 55%, and 75%)for approximately 30 weeks. Height growth wasrecorded every 2 weeks. Initial and final seedlingmorphology and chlorophyll concentrations weremeasured. Chlorophyll fluorescence was measuredmonthly. All species responded similarly to shade. Although height growth was greatest under 75% shadeand least in 0% shade, total biomass production wassignificantly lower and shoot:root ratio significantlyhigher in 75% shade than in 0% shade. As thetreatment shade level increased from 0% to 75%,F_v/F_m was significantly lower whilechlorophyll concentrations were significantly higher. These results indicate that photochemical efficiencyof all four species was lower under higher shade. Morphological and physiological responses differedgreatly among species and corresponded with theirdegree of shade tolerance.     

Effects of soil temperature on biomass and carbohydrate allocation in Scots pine (Pinus sylvestris) seedlings at the beginning of the growing season

We studied effects of soil temperature on shoot and root extension growth and biomass and carbohydrate allocation in Scots pine (Pinus sylvestris L.) seedlings at the beginning of the growing season. One-year-old Scots pine seedlings were grown for 9 weeks at soil temperatures of 5, 9, 13 and 17 degrees C and an air temperature of 17 degrees C. Date of bud burst, and the elongation of shoots and roots were monitored. Biomass of current and previous season roots, stem and needles was determined at 3-week intervals. Starch, sucrose, glucose, fructose, sorbitol and inositol concentrations were determined in all plant parts except new roots. The timing of both bud burst and the onset of root elongation were unaffected by soil temperature. At Week 9, height growth was reduced and root extension growth was much less at a soil temperature of 5 degrees C than at higher soil temperatures. Total seedling biomass was lowest in the 5 degrees C soil temperature treatment and highest in the 13 degrees C treatment, but there was no statistically significant difference in total biomass between seedlings grown at 13 and 17 degrees C. In response to increasing soil temperature, below-ground biomass increased markedly, resulting in a slightly higher allocation of biomass to below-ground parts. Among treatments, root length was greatest at a soil temperature of 17 degrees C. The sugar content of old roots was unaffected by soil temperature, but the sugar content of new needles increased with increasing soil temperature. The starch content of all seedling parts was lowest in seedlings grown at 17 degrees C. Otherwise, soil temperature had no effect on seedling starch content.     

Forest regeneration 50 years following partial cutting in mixedwood ecosystems of southern Quebec,Canada

The impact of winter harvesting on regeneration 50 years after an experimental diameter-limit cutting was examined in mixed deciduous–coniferous ecosystems of southern Quebec, Canada. The study was conducted in La Mauricie National Park, Quebec, Canada. Regeneration data in two balsam fir (Abies balsamea (L.) Mill.), red spruce (Picea rubens Sarg.), sugar maple (Acer saccharum Marsh.), and yellow birch (Betula alleghaniensis Britt.) ecosystem types were analyzed. Comparisons between uncut and cut stands were obtained from a total of 63 sample plots. For both ecosystems, there were no significant differences between uncut and cut plots for regeneration density and stocking. The most abundant regeneration species were balsam fir, red spruce, sugar maple, red maple (Acer rubrum L.), yellow birch and American beech (Fagus grandifolia Ehrh.). The type of diameter-limit cutting described in the study did not affect regeneration density and stocking but its impact on productivity, timber quality and genetics is still unknown.     

Above- and below-ground biomass and nutrient distribution of a paper birch and subalpine fir mixed-species stand in the Sub-Boreal Spruce zone of British Columbia

Stand structure, height and diameter growth, above- and below-ground biomass, and nutrient concentrations and content were determined for a 35-year-old fire-origin paper birch (Betula papyrifera) and subalpine fir (Abies lasiocarpa) mixed-species stand in the Sub-Boreal Spruce (SBS) zone of British Columbia. Paper birch, which formed the dominant overstory following the 1961 fire, had normal distributions of height and diameter classes. Subalpine fir, which dominated the understory, had the reverse J-shaped height and diameter distributions that are expected of a shade tolerant, climax species. Paper birch grew more than three times the height of subalpine fir. Growing in the summer shade of the birch, subalpine fir had slow but steady height growth during the first 10–15 years, after which height growth declined somewhat. Allometric equations, relating dry weight of foliage, branches, stemwood, stembark, roots, and total biomass to diameter at breast height (DBH), were developed to estimate above- and below-ground biomass. Total biomass of paper birch reached 83.2 t ha⁻¹, while subalpine fir biomass was 26.7 t ha⁻¹. Subalpine fir allocated more biomass to foliage and branches compared to paper birch. Foliage of paper birch had higher nutrient concentrations of N, P, K, Ca, and Mg than subalpine fir foliage. Branches and stembark of subalpine fir had higher P, and Ca concentrations than paper birch. Subalpine fir branches contained more of all examined nutrients than paper birch branches. This is a significant component in nutrient cycling of the mixed-species forest.     

Effects of root trenching of overstorey Norway spruce (Piceaabies) on growth and biomass of underplanted beech (Fagussylvatica) and Douglas fir (Pseudotsugamenziesii) saplings

In Central Europe, the conversion of pure Norway spruce stands (Picea abies [L.] Karst.) into mixed stands with beech (Fagus silvatica L.) and other species like e.g. Douglas fir (Pseudotsuga menziesii [Mirb.] Franco) is accomplished mainly by underplanting of seedlings beneath the canopy of overstorey spruce trees after partial cutting treatments what means exposure to shade and below-ground root competition by the overstorey to the seedlings. Particularly about the second factor, our knowledge is limited. Therefore, we carried out a below-ground competition exclusion experiment by root trenching and investigated the effects on soil resources, growth, and biomass partitioning of underplanted beech and Douglas fir saplings under target diameter and strip cutting treatments. The exclusion of overstorey root competition by trenching increased the soil water potential in the second year that had a fairly dry growing season and led to significantly higher foliar concentrations of most nutrients, particularly in Douglas fir, indicating an amended nutrient supply. Both improvements were accompanied by an increase in length and diameter increment of the underplanted saplings, appearing in both species only after having surpassed a species-specific threshold light value (Douglas fir 16% of above canopy radiation, beech 22%). We also found significant interactions between trenching and light for specific fine root length and further biomass and morphological parameters. Judged by the much steeper increase in height and diameter growth with increasing light after release from below-ground competition, Douglas fir saplings appeared to be more sensitive to root competition than beech saplings what conforms to older findings for beech. According to our results, a strip cutting seems to be more appropriate than a target diameter cutting treatment to replace a pure spruce stand by a mixed stand with beech and Douglas fir.     

Shade tolerance, photoinhibition sensitivity and phenotypic plasticity of Ilex aquifolium in continental Mediterranean sites

Shade tolerance, plastic phenotypic response to light and sensitivity to photoinhibition were studied in holly (Ilex aquifolium L.) seedlings transported from the field to a greenhouse and in adult trees in the field. All plants were growing in, or originated from, continental Mediterranean sites in central Spain. Seedlings tolerated moderate but not deep shade. Mortality was high and growth reduced in 1% sunlight. Survival was maximal in 12% sunlight and minimal in full sunlight, although the relative growth rate of the seedlings surviving in high light was similar to that of plants in moderate shade. Maximum photochemical efficiency at predawn was significantly lower in sun plants than in shade plants in the field, revealing chronic photoinhibition that was most pronounced in winter. Plasticity in response to available light varied according to the variable studied, being low for photosynthetic capacity and stomatal conductance, and high for specific leaf area, root:shoot ratio and leaf area ratio, particularly in seedlings. Differences in water relations and hydraulic features between sun and shade plants in the field were marginal. High water potential at the turgor loss point of field-grown plants suggested that holly is sensitive to drought during both the seedling and the adult stage. Low relative growth rates in both high and low light with low physiological plasticity in response to light indicate the existence of a stress-tolerance mechanism. We conclude that holly is a facultative understory plant in areas of oceanic and relatively mild climate, but an obligate understory plant in dry continental areas such as the study site. The impact of abandonment of traditional management practices and climate change on these Mediterranean populations is discussed.