Shade Acclimation Research Articles

Vertical gradients and tree-to-tree variation in shoot morphology and foliar nitrogen in an old-growth Pinus strobus stand

Acclimation to ambient light has been assumed to be the principal cause of vertical gradients in leaf nitrogen, foliar morphology, and related traits in forest canopies. We examined the relative role of crown exposure, damage, and reproduction as correlates of vertical gradients in shoot and needle morphology, anatomy, and chemistry in a ~120-year-old stand of Pinus strobus L. (eastern white pine) in central Ontario. Internodes at the top of trees were longer, wider, and produced more fascicles, but had lower fascicle survivorship than lower-canopy shoots. Needles on upper-canopy shoots also had higher allocation to resin ducts than mid- or lower-canopy needles, contained less nitrogen on a mass basis, and showed a higher leaf mass per area, C/N ratio, and chlorophyll a/b ratio. Tree-to-tree variation among the 26 trees measured traits was high, owing, in part, to differences in crown damage and reproductive status. Crown damage was associated with reduced leaf mass per area and other traits associated with shade acclimation, while high cone production was associated with reduced leaf nitrogen and chlorophyll in the upper canopy. Our results suggest that factors other than light acclimation play an important role in determining vertical gradients in foliar morphology, nitrogen, and leaf production in forest canopies.

Exposed red (anthocyanic) leaves of Quercus coccifera display shade characteristics

Young leaves in some plants are transiently red due to the presence of anthocyanins, which disappear upon maturation. We investigated the hypothesis that light attenuation by anthocyanins may lead to a shade acclimation of the photosynthetic machinery in red leaves. We took advantage of the intra-species variation in anthocyanin levels of young, exposed leaves of Quercus coccifera. Thus, photosynthetic and photoprotective characteristics were compared in young green and red leaves of the same age, sampled from the corresponding phenotypes occupying the same habitat. Red leaves displayed several shade attributes like thinner laminae, lower Chl a/b ratios and lower levels of the xanthophyll cycle components and β-carotene. In addition, although both leaf kinds had the same area based levels of chlorophylls, these pigments were excluded from the sub-epidermic anthocyanic cell layers, leading to a further reduction of effective mesophyll thickness and an increase in chlorophyll density. Accordingly, red leaves had higher absolute chlorophyll fluorescence signals. In spite of these apparent shade characters, red leaves were less prone to photoinhibition under mild laboratory conditions and displayed slightly but significantly higher PS II photochemical efficiencies at pre-dawn in the field. No differences in all the above measured parameters were found in mature green leaves of the two phenotypes. The results confirm the light acclimation hypothesis and are also compatible with a photoprotective function of anthocyanins.

Shade acclimation of rainforest leaves to colonization by lichens

Summary Lichens that live epiphytically on leaves (foliicolous lichens) are one of the most abundant groups of epiphytes in tropical rainforests, with lichen cover on individual leaves often exceeding 50%. In this study we quantified the shading of host leaves by foliicolous lichens, and investigated the capacity of palm leaves growing under contrasting light regimes to photo‐acclimate to lichen colonization. Spectral analyses of 11 lichen species with differing thallus colouring and morphology demonstrated that the lichens exhibited absorptance spectra similar to their host leaves. The lichens showed mean light interceptions of 50%, with some species reducing the photosynthetically usable light by up to 70%. Despite such shading, our observations indicated that leaves photo‐acclimate to the cover of lichens. The rate of photosynthesis of leaf patches covered by lichens was saturated at lower irradiances, and these patches had higher chlorophyll concentrations than uncolonized leaf areas. Monte Carlo estimates of total daily carbon gain, based on assimilation–irradiance curves and continuous light records, indicated that leaf areas covered by lichens compensated fully for the shading by lichens. These findings do not support the widely held opinion that shading by foliicolous lichens and other epiphylls generally reduces the rate of photosynthesis by their host leaves.

Impact of tree leaf phenology on growth rates and reproduction in the spring flowering species Trillium erectum (Liliaceae)

We investigated the impact of overstory tree leaf phenology on growth rates, carbon allocation pattern, and fruit characteristics in the spring flowering species, Trillium erectum (Liliaceae). Air temperature, overstory canopy closure, and T. erectum phenology were monitored at three locations following a latitudinal gradient in Québec, Canada. Northern sugar maple trees leaf out at cooler temperatures than more southern populations, while Trillium development was initiated at the same soil temperature irrespective of the latitude. Therefore, in northern areas, the time between initiation of T. erectum leaf expansion and canopy closure was shorter than in southern areas, which left less time for northern plants to accumulate reserves before canopy closure. Differences in growth patterns were noted between T. erectum populations. From a south-north gradient, investment to reproduction, total plant biomass, and annual growth rate decreased, while specific leaf area and stem height increased, indicating shade acclimation. The length of the high light period in early spring seems to be a determinant for spring flowering plants' growth and reproduction and may explain the northern distribution limit of some of these species.

Diurnal and acclimatory responses of violaxanthin and lutein epoxide in the Australian mistletoe Amyema miquelii

This study investigated the chloroplast pigment content of the Australian mistletoe Amyema miquelii (Lehm. ex Miq.) Tiegh. over diurnal periods in sun- and shade-acclimated leaves. Amyema miquelii exhibited the typical higher plant complement of neoxanthin, the xanthophyll cycle pigments, lutein, chlorophylls a and b and β carotene. Substantial levels of lutein epoxide were also present. Interestingly, diurnal light exposure elicited a decrease in lutein epoxide that paralleled the decrease in violaxanthin. Compared with shade-acclimated leaves, sun leaves exhibited reduced lutein epoxide and violaxanthin levels and higher chlorophyll a/b ratios. It is clear that the pools of violaxanthin and lutein epoxide respond in parallel to both diurnal light changes and sun–shade acclimation, although there seemed to be some differences in the recovery characteristics. These results raise a question as to whether lutein and lutein epoxide cycling may provide an auxiliary means of energy dissipation for some species.

Light acclimation, CO2 response and long‐term capacity of underwater photosynthesis in three terrestrial plant species

ABSTRACTTo characterize underwater photosynthetic performance in some terrestrial plants, we determined (i) underwater light acclimation (ii) underwater photosynthetic response to dissolved CO2, and (iii) underwater photosynthetic capacity during prolonged submergence in three species that differ in submergence tolerance: Phalaris arundinacea, Rumex crispus (both submergence‐tolerant) and Arrhenatherum elatius (submergence‐intolerant). None of the species had adjusted to low irradiance after 1 week of submergence. Under non‐submerged (control) conditions, only R. crispus displayed shade acclimation. Submergence increased the apparent quantum yield in this species, presumably because of the enhanced CO2 affinity of the elongated leaves. In control plants of the grass species P. arundinacea and A. elatius, CO2 affinities were higher than for R. crispus. The underwater photosynthetic capacity of R. crispus increased during 1 month of submergence. In P. arundinacea photosynthesis remained constant during 1 month of submergence at normal irradiance; at low irradiance a reduction in photosynthetic capacity was observed after 2 weeks, although there was no tissue degeneration. In contrast, underwater photosynthesis of the submergence‐intolerant species A. elatius collapsed rapidly under both irradiances, and this was accompanied by leaf decay. To describe photosynthesis versus irradiance curves, four models were evaluated. The hyperbolic tangent produced the best goodness‐of‐fit, whereas the rectangular hyperbola (Michaelis‐Menten model) gave relatively poor results.

PHOTOSYNTHESIS IN AN EXTREME SHADE ENVIRONMENT: BENTHIC MICROBIAL MATS FROM LAKE HOARE, A PERMANENTLY ICE‐COVERED ANTARCTIC LAKE

We investigated the composition of benthic microbial mats in permanently ice‐covered Lake Hoare, Antarctica, and their irradiance vs. photosynthetic oxygen exchange relationships. Mats could be subdivided into three distinct depth zones: a seasonally ice‐free “moat” zone and two under‐ice zones. The upper under‐ice zone extended from below the 3.5 m thick ice to approximately 13 m and the lower from below 13 m to 22 m. Moat mats were acclimated to the high irradiance they experienced during summer. They contained photoprotective pigments, predominantly those characteristic of cyanobacteria, and had high compensation and saturating irradiances (Ec and Ek) of 75 and 130 μmol photons·m−2·s−1, respectively. The moat mats used light inefficiently. The upper under‐ice community contained both cyanobacteria and diatoms. Within this zone, biomass (as pigments) increased with increasing depth, reaching a maximum at 10 m. Phycoerythrin was abundant in this zone, with shade acclimation and efficiency of utilization of incident light increasing with depth to a maximum of 0.06 mol C fixed·mol−1 incident photons under light‐limiting conditions. Precipitation of inorganic carbon as calcite was associated with this community, representing up to 50% of the carbon sequestered into the sediment. The lower under‐ice zone was characterized by a decline in pigment concentrations with depth and an increasing prevalence of diatoms. Photosynthesis in this community was highly shade acclimated and efficient, with Ec and Ek below 0.5 μmol·m−2·s−1 and 2 μmol·m−2·s−1, respectively, and maximum yields of 0.04 mol C fixed·mol−1 incident quanta. Carbon uptake in situ by both under‐ice and moat mats was estimated at up to 100 and 140 mg·m−2·day−1, based on the photosynthesis–irradiance curves, incident irradiance, and light attenuation by ice and the water column.

Shoot structure, light interception, and distribution of nitrogen in an Abies amabilis canopy.

We studied the effects of variation in shoot structure and needle morphology on the distributions of light and nitrogen within a Pacific silver fir (Abies amabilis (Dougl.) Forbes) canopy. Specifically, we investigated the role of morphological shade acclimation in the determination of resource use efficiency, which is claimed to be optimal when the distribution of nitrogen within the canopy is directly proportional to the distribution of intercepted photosynthetically active radiation (PAR). Shoots were collected from different heights in the crowns of trees representing four different size classes. A new method was developed to estimate seasonal light interceptance (SLI, intercepted PAR per unit needle area) of the shoots using a model for the directional distribution of above-canopy PAR, measurements of shoot silhouette area and canopy gap fraction in different directions. The ratio SLI/SLI(o), where the reference value SLI(o) represents the seasonal light interceptance of a spherical surface at the shoot location, was used to quantify the efficiency of light capture by a shoot. The ratio SLI/SLI(o) doubled from the top to the bottom of the canopy, mainly as a result of smaller internal shading in shade shoots than in sun shoots. Increased light-capturing efficiency of shade shoots implies that the difference in intercepted light by sun shoots versus shade shoots is much less than the decrease in available light from the upper to the lower canopy. For example, SLI of the five most sunlit shoots was only about 20 times greater than the SLI of the five most shaded shoots, whereas SLI(o) was 40 times greater for sun shoots than for shade shoots. Nitrogen content per unit needle area was about three times higher in sun needles than in shade needles. This variation, however, was not enough to produce proportionality between the amounts of nitrogen and intercepted PAR throughout the canopy.

Diverse Responses of Maple Saplings to Forest Light Regimes

Seedlings of 11 species of forest maples (AcerL.) were grown outdoors from budburst to senescence under three light regimes: ‘gap centre under clear skies’ (approx. 20% open sky irradiance; red:far-red ratio=1.12); ‘gap centre under cloudy skies’ (1.5%, ratio=1.03); and ‘gap edge’ (2.5%, ratio=0.6). Seedlings grown under the gap centre (clear sky) regime had significantly greater height growth, greater specific leaf mass, higher root:shoot ratio, greater investment in roots, higher leaf nitrogen concentrations, greater chlorophylla:bratio, lower photosynthetic rates under dim light, higher maximum photosynthetic rate, higher stomatal conductance, and lower leaf internal CO2concentrations compared with those grown in either gap edge or gap centre (cloudy) regimes. Responses to the gap edgevs.gap centre (cloudy) treatments differ little, suggesting that shade acclimation in forest maple seedlings is mainly a response to light intensity rather than spectral quality. The ubiquitous and, except for leaf internal CO2concentration, highly significant interspecific variation in traits was broad-ranging and continuous. These results suggest that (1) the responses to light quality found in shade intolerant herbaceous and woody species growing in more open habitats may not have a selective advantage in seedlings of shade tolerant forest trees, and (2) the adaptive plastic response to understoreyvs.gap environments in forest maples, which is qualitatively consistent across species, is founded on co-ordinated, small shifts in sets of functionally inter-related traits.

Seasonal variation in the pigment content and photosynthesis of different thallus regions of Ascophyllum nodosum (Fucales, Phaeophyta) in relation to position in the canopy

Pigment contents and photosynthetic parameters of different regions of thalli of Ascophyllum nodosum (L.) Le Jolis collected from Strangford Lough, Irish Sea, and Helgoland, North Sea, were examined on a seasonal basis and after exposure to various irradiance and nutrient treatments in the laboratory. In culture, pigment contents of young plant parts acclimated rapidly to changes in irradiance and increased after nutrient enrichment in all irradiances. Chlorophyll a levels in plant tips in situ increased during autumn and winter and were reduced by 50% during summer. In older plant parts, which were subject to less seasonal variation in irradiance and were more shaded at both high and low tide, chlorophyll a levels were higher than in the tips at all times of the year. Older plant material from the lower shore contained higher pigment levels than corresponding material from the upper shore. Fucoxanthin : chlorophyll a ratios peaked during summer in apical plant parts from the upper shore and increased in culture with irradiance and with length of daily exposure to high irradiance. Variations of pigment concentration within plants and between shore levels were probably due to differences in irradiance between microhabitats caused by water depth and self-shading and enhanced by nutrient limitation in summer. Maximum net photosynthesis (Pmax) of young plant parts was higher than that of older plant parts and increased with temperature to a maximum at 20°C, in contrast to older plant parts which had low rates at all temperatures. Pmax was highest in tips in all seasons and decreased with distance from the tip along the thallus. Older, basal thallus parts exhibited shade acclimation with high pigment content and high photosynthetic rates at low irradiances, in contrast to apical tips, which were acclimated to high light. The observed decrease in Pmax of plants tips collected in summer from the upper shore may have been caused by photoinhibition.

Interactive effects of photon fluence rates and drought on CAM-cycling in Delosperma tradescantioides (Mesembryanthemaceae).

The ability of photosynthesis and CAM to acclimate to low (220 µmol m-2 s-1 ; LL) and relatively high (550 µmol m-2 s-1 ; HL) photosynthetic photon flux densities (PPFD) was investigated in the CAM-cycling species Delosperma tradescantioides by means of CO2 gas exchange and chlorophyll fluorescence analysis. Furthermore, the influence of short-term drought on malic acid accumulation and the activity of photosystem II (PSII) was studied to assess the possible interactions between drought and the prevailing PPFD in this species. HL plants showed features of sun versus shade acclimation relative to LL plants. Nocturnal malic acid accumulation (Δ-malate) and leaf water content also tended to be higher in HL plants. Irrespective of the PPFD during growth, the weak Δ-malate doubled within 3 days of drought. Despite largely restricted CO2 uptake, photosynthetic activity as estimated from fluorescence analysis declined only ca 5%. After 7 days of drought, when plants showed CAM-idling and Δ-malate had decreased again, potential carbon assimilation was still ca 84% of that in well-watered plants and remained relatively constant throughout the day. Decarboxylation of malic acid accounted for ca 23% of potential assimilation assuming total oxidation of a maximum portion of this organic acid. Drought did not affect predawn maximum photochemical efficiency (Fv /Fm ). Nonphotochemical quenching (qN) increased (24%) in response to desiccation and resulted in a more or less constant reduction state of PSII. This increase in qN resulted mainly from the change in its fast-relaxing component (qNF), while the slow component (qNS) was significant only at or above saturating PPFD in both HL and LL plants. The photon response characteristics of PSII, which differed between LL and HL plants, were unaffected by short-term drought. Photon harvesting and photon use were always adjusted to guarantee a low reduction state of PSII. Results suggest that in both LL and HL plants CAM-cycling may help to stabilize photosynthesis but to a large extent by other means than simply providing internally derived CO2 .

Simulations of the effects of shoot structure and orientation on vertical gradients in intercepted light by conifer canopies.

Coniferous tree canopies typically carry more leaf area than is necessary to intercept most of the incoming light. I postulated that an excessively large leaf area will reduce net productivity at tree level, unless the net photosynthetic production of the most shaded shoots in the canopy remains positive. The hypothesis tested was that a coniferous tree canopy maintains a large productive leaf area by increasing the efficiency of light capture as the available light decreases. The light interception efficiency of a shoot was quantified by the ratio of shoot silhouette area to total needle surface area (STAR). The STAR depends on shoot geometry and varies with shoot orientation relative to the direction of light. Shade shoots have a larger STAR and, in particular, higher values of STAR(max) than sun shoots. In addition, shade shoots tend to be horizontally inclined, which may increase the advantage of a large STAR(max) in the lower canopy, where radiation is incident from angles closer to the zenith. Adaptation to shade (changes in STAR and shoot orientation) was described on the basis of empirical data for several coniferous species, and the vertical gradient of seasonal light interception by shoots was simulated assuming different adaptive strategies. Simulations were performed at two latitudes, to account for differences in the amount and directional distribution of light during the growing season. Results support the hypothesis that increases in STAR, shoot zenith angle and shoot asymmetry (flatness) with shading increase the efficiency of light interception by deeply shaded shoots. However, because competition for light among shoots increases progressively as soon as shade acclimation occurs, there cannot exist a deep layer of shade shoots, such that the net productivity of each shoot remains positive (i.e., irradiance is above the compensation point). Therefore, if maximization of productive leaf area is the goal, the optimal strategy is to maintain an inefficient deep canopy and to increase light interception efficiency only when shading becomes severe.

Effects of light environment and successional status on lightfleck use by understory trees of temperate and tropical forests.

Utilization efficiency (LUE) of lightflecks by leaves increases with decreasing duration of the lightfleck, and depends on photosynthetic induction. Sun and shade leaves differ with respect to photosynthetic induction. Shade leaves may become fully induced by a series of light pulses, whereas photosynthetic induction of leaves from partial shade or full sun depends on continuous light. Additionally, shade leaves maintain a higher induction state over longer periods in dim light or darkness than sun leaves. Both features are advantageous to shade leaves in a highly dynamic light environment. We determined whether pioneer plants and late-successional species differ in photosynthetic induction dynamics and LUE during the establishment phase when both plant types are growing in the shade of the understory. We also determined the effects of shade acclimation and successional position of species on photosynthetic induction and LUE. Results from temperate and tropical rain forests indicate a trade-off between leaf acclimation to shade and the successional position of species. Light acclimation is important, but in deep shade, late-successional species maintain a higher induction state over longer periods than pioneer species.

Effects of free‐air CO2 enrichment on the development of the photosynthetic apparatus in wheat, as indicated by changes in leaf proteins

ABSTRACTA spring wheat crop was grown at ambient and elevated (550 μmol mol−1) CO2 concentrations under free‐air CO2 enrichment (FACE) in the field. Four experimental blocks, each comprising 21‐m‐diameter FACE and control experimental areas, were used. CO2 elevation was maintained day and night from crop emergence to final grain harvest. This experiment provided a unique opportunity to examine the hypothesis that CO2 elevation in the field would lead to acclimatory changes within the photosynthetic apparatus under open field conditions and lo assess whether acclimation was affected by crop developmental stage, leaf ontogeny and leaf age. Change in the photosynthetic apparatus was assessed by measuring changes in the composition of total leaf and thylakoid polypeptides separated by SDS‐PAGE. For leaves at completion of emergence of the blade, growth at the elevated CO2 concentration had no apparent effect on the amount of any of the major proteins of the photosynthetic apparatus regardless of the leaf examined. Leaf 5 on the main stem was in full sunlight at emergence, but then became shaded progressively as 3–4 further leaves formed above with continued development of the crop. By 35 d following completion of blade emergence, leaf 5 was in shade. At this point, the chlorophyll alb ratio had declined by 26% both in plants grown at the control CO2 concentration and in those grown at the elevated CO2 concentration, which is indicative of shade acclimation. The ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco) content declined by 45% in the control leaves, but by 60% in the leaves grown at the elevated CO2 concentration. The light‐ harvesting complex of photosystcm II (LHCII) and the chlorophyll content showed no decrease and no difference between treatments, indicating that the decrease in Rubisco was not an effect of earlier senescence in the leaves at the elevated CO2 concentration. Following completion of the emergence of the flag‐leaf blade, the elevated‐CO2 treatment inhibited the further accumulation of Rubisco which was apparent in control leaves over the subsequent 14 d. From this point onwards, the flag leaves from both treatments showed a loss of Rubisco, which was far more pronounced in the elevated‐CO2 treatment, so that by 36 d the Rubisco content of these leaves was just 70% of that of the controls and by 52 d it was only 20%. At 36 d, there was no decline in chlorophyll, LHCII or the chloroplast ATPase coupling factor (CFI) in the elevated CO2 concentration treatment relative to the control. By 52 d, all of these proteins showed a significant decline relative to the control. This indicates that the decreased concentration of Rubisco at this final stage probably reflected earlier senescence in the elevated‐CO2 treatment, but that this was preceded by a CO2‐concentration‐dependent decline in Rubisco.

Effects of shade on the morphology and physiology of amabilis fir and western hemlock seedlings

Successful regeneration of coastal montane sites harvested using alternative silvicultural systems may depend on the degree to which tree species can acclimate morphologically and physiologically to a variety of light environments. In a study to determine shade acclimation in montane conifers, one-year-old amabilis fir (Abies amabilis (Dougl.) Forbes) and western hemlock (Tsuga heterophylla (Raf.) Sarg.) seedlings were grown in a nursery under four shade treatments: full sunlight (0% shade), 60% and 30% shade using shade cloth, and 30% shade using lath slats. Shading influenced shoot development, foliar physiology and morphological characteristics of both amabilis fir and western hemlock but in general, the effects were small. Shade levels of 60% were required to induce significant acclimation, and western hemlock appeared to respond more positively than amabilis fir and therefore was considered more shade tolerant than amabilis fir. Light quality had little influence on growth and development, as indicated by a lack of significant differences in physiology or morphology between seedlings grown under shade cloth or lath slats. There were indications that adequate nutrition levels may mitigate the effects of shade on seedling morphology and physiology.

Variable Urban Irradiance and Shade Acclimation in Norway Maple Street Trees

Shade acclimation response of Emerald Queen Norway maple street trees to variable irradiance levels in an urban setting was investigated. Specific leaf area, trunkgrowth, and crown density were measured in thirteen sites ranging from urban canyons in the business core to open exposures in residential areas of Seattle, Washington. Percentage of potential seasonal input of global shortwave radiation for each site was modeled based on the azimuth and elevation angles of the surrounding horizon topography. Building height in the business core reduced estimated potential seasonal irradiance to 27-90% of that for an unobstructed horizon topography, while those outside the business core had 90-95% of potential irradiance. As potential irradiance decreased these maples exhibited growth responses characteristic of shade acclimation in a dose-response pattern. Specific leaf area increased and trunk growth and crown density decreased to acclimated levels between 70-85% of potential irradiance. Shade acclimation did not detract from the appearance or utility of these trees in the urban canyon.

VARIABLE URBAN IRRADIANCE AND SHADE ACCLIMATION IN NORWAY MAPLE STREET TREES

Shade acclimation response of Emerald Queen Norway maple street trees to variable urban irradiance levels was investigated. Specific leaf area, trunk growth, and crown density were measured from trees in 13 sites ranging from urban canyons in the business core to open exposures in residential areas of Seattle, Wash. Percentage of potential seasonal input of global shortwave radiation for each site was modeled based on the azimuth and elevation angles of the surrounding horizon topography. Building height in the business core reduced estimated irradiance to a range of 27% to 90% of that for an unobstructed horizon topography, while those outside the business core had 90% to 95% irradiance. As estimated potential irradiance decreased, growth of these maple street trees exhibited responses characteristic of shade acclimation in a dose-response pattern. Specific leaf area increased and trunk growth and crown density decreased to acclimated levels at -70% of potential irradiance. These acclimation responses did not degrade the function of the trees in their urban-canyon locations. Their foliage was healthy, and reduced crown density was not apparent since there were no full-sun-grown trees for comparison.

RESPONSE OF FIELD-GROWN NORWAY MAPLE AND GREEN ASH TREES TO WHITE AND BROWN TREESHELTERS

We investigated microclimate, gas exchange, and growth of field-grown Norway maple (Acer platanoides) and green ash (Fraxinus pennsylvanica) trees in brown, white, or no treeshelters. Microclimate, tree growth, and gas exchange measurements were taken summer and winter. Treeshelter microclimate was greenhouse-like compared to ambient conditions, as short-wave radiation (S↓) was lower, and midday air temperature and relative humidity were higher. In both species, this resulted in less trunk growth and greater specific leaf area, which are growth responses characteristic of shade acclimation. Treeshelter microclimate did, however, substantially increase shoot elongation and stomatal conductance, but did not increase photosynthesis when compared to trees grown without shelters. White shelters allowed 25% more penetration of S↓ than brown shelters, but tree growth and climatic variables did not differ with treeshelter color. Stomatal conductance, however, was higher for trees in white shelters. Treeshelters also appeared to have a negative effect on plant hardiness. New shoot growth in shelters was more winter-damaged, particularly in maple, than nonsheltered trees. This may be related to winter bark (Tb) and air temperature (Ta). Winter midday Tb on trees grown in shelters was up to 15C higher than Tb on trees outside shelters, while midday Ta inside treeshelters was up to 20C higher than Ta outside treeshelters.

URBAN MICROCLIMATES AND GROWTH OF SWEETGUM STREET TREES

Summary Microclimates characteristic of urban park, plaza, and canyon spaces were related to physiology and growth of even-aged sweetgum (Liquidambar styraciflua L.) street trees. Microclimates, tree growth, and physiological responses were characterized diurnally and seasonally. Park and plaza sites received unobstructed sunlight while the canyon was limited to four hours of direct solar radiation in midsummer. Potential seasonal insolation was 44 per cent of the potential maximum at the canyon and over 90 per cent at the park. Heating from paved surfaces at the plaza resulted in cumulative pan evaporation nearly 50 per cent greater than the other sites. Tree growth at the plaza and canyon acclimated physiologically and develop-mentally to the prevailing environmental conditions. Indicative of shade acclimation, canyon trees exhibited thinner leaves and less trunk growth when compared with the park; crown size and shoot growth were unaffected. In contrast, plaza trees, growing in a very coarse and shallo...

Growth and Water Relations of Kentucky Coffee Tree in Protective Shelters during Establishment

Growth and water relations of Kentucky coffee tree [Gymnocladus dioica (L.) K. Koch] whips in translucent tubelike shelters were investigated. In a container study, 1.2-m-high shelters were placed over whips following transplanting, then diurnal microclimate, water relations, and water use were measured. Shelter air temperature and vapor pressure were substantially higher, and solar radiation was 70% lower, than ambient conditions. Sheltered trees responded with nearly three-times higher stomatrd conductance than nonsheltered trees. However, due to substantially lower boundary layer conductance created by the shelter, normalized water use was 40910 lower. In a second experiment, same-sized shelters were placed on whips following spring transplanting in the field. Predawn and midday leaf water potentials and midday stomatal conductance (g,) were monitored periodically through the season, and growth was measured in late summer. Midday gs was also much higher in field-grown trees with shelters than in those without. Sheltered trees in the field had four times greater terminal shoot elongation but 40% less stem diameter growth. Attenuated radiation in the shelters and lower specific leaf area of sheltered trees indicated shade acclimation. Shelters can improve height and reduce water loss during establishment in a field nursery, but they do not allow for sufficient trunk growth.