Plasticity in mesophyll volume fraction modulates light-acclimation in needle photosynthesis in two pines

Acclimation potential of needle photosynthetic capacity varies greatly among pine species, but the underlying chemical, anatomical and morphological controls are not entirely understood. We investigated the light-dependent variation in needle characteristics in individuals of Pinus patula Schlect. & Cham., which has 19-31-cm long pendulous needles, and individuals of P. radiata D. Don., which has shorter (8-17-cm-long) stiffer needles. Needle nitrogen and carbon contents, mesophyll and structural tissue volume fractions, needle dry mass per unit total area (M(A)) and its components, volume to total area ratio (V/A(T)) and needle density (D = M(A)/(V/A(T))), and maximum carboxylase activity of Rubisco (V(cmax)) and capacity of photosynthetic electron transport (J(max)) were investigated in relation to seasonal mean integrated irradiance (Q(int)). Increases in Q(int) from canopy bottom to top resulted in proportional increases in both needle thickness and width such that needle total to projected surface area ratio, characterizing the efficiency of light interception, was independent of Q(int). Increased light availability also led to larger M(A) and nitrogen content per unit area (N(A)). Light-dependent modifications in M(A) resulted from increases in both V/A(T) and D, whereas N(A) changed because of increases in both M(A) and mass-based nitrogen content (N(M)) (N(A) = N(M)M(A)). Overall, the volume fraction of mesophyll cells increased with increasing irradiance and V/A(T) as the fraction of hypodermis and epidermis decreased with increasing needle thickness. Increases in M(A) and N(A) resulted in enhanced J(max) and V(cmax) per unit area in both species, but mass-based photosynthetic capacity increased only in P. patula. In addition, J(max) and V(cmax) showed greater plasticity in response to light in P. patula. Species differences in mesophyll volume fraction explained most of the variation in mass-based needle photosynthetic capacity between species, demonstrating that differences in plastic adjustments in mass-based photosynthetic activities among these representative individuals were mainly associated with contrasting investments in mesophyll cells. Greater area-based photosynthetic plasticity in P. patula relative to P. radiata was associated with larger increases in M(A) and mesophyll volume fraction with increasing irradiance. These data collectively demonstrate that light-dependent increases in mass-based nitrogen contents and photosynthetic activities were associated with an increased mesophyll volume fraction in needles at higher irradiances. They also emphasize the importance of light-dependent anatomical modifications in determining needle photosynthetic capacity.     

Plasticity of shoot and needle morphology and photosynthesis of two Picea species with different site preferences in northern Japan

We compared shoot and needle morphology and photosynthesis in Picea glehnii (Friedr. Schmidt) M.T. Mast. and Picea jezoensis (Sieb. et Zucc.) Carr. trees planted on immature volcanic ash and well-developed brown forest soils to investigate whether species differences in morphological and physiological plasticity affected tree growth on different soil types. Height growth of P. glehnii was reduced by about 10% on volcanic ash compared with forest soil, whereas that of P. jezoensis was reduced by more than 60%. Needle morphology of P. glehnii was unaffected by soil type. In contrast, needles of P. jezoensis trees growing on volcanic ash were shorter, narrower and thicker, with less dry mass and area, than those of trees growing on forest soil, and specific needle area was lower, indicating lower foliar light-interception efficiency. In both species, changes in needle morphology with increasing irradiance were similar in trees growing on both soil types, indicating that plasticity of needle morphology was unaffected by soil type. In both species, shoot mass and shoot silhouette area were lower and needle mass per unit shoot mass was higher in trees growing on volcanic ash than in trees growing on forest soil. Trees of both species had more needles per unit shoot length, lower shoot silhouette to projected needle area ratios and lower shoot silhouette areas per unit shoot mass (SAM) on volcanic ash than on forest soil, indicating lower shoot-level light-interception efficiency. For P. glehnii, the response of shoot morphology to increasing irradiance was similar on both soil types, with the exception of SAM, which showed lower plasticity in trees growing on volcanic ash. In contrast, shoot-level morphological plasticity of P. jezoensis was reduced in trees growing on volcanic ash. Light-saturated maximum photosynthetic rate (P(max)) of P. glehnii was unaffected by soil type, whereas mass-based P(max) of P. jezoensis was lower in trees growing on volcanic ash than in trees growing on forest soil. In P. jezoensis trees growing on forest soil, area-based P(max) increased with increasing irradiance, but this response was not observed in trees growing on volcanic ash. As a result, area-based P(max) at the top of the canopy was 39 to 54% lower in trees growing on volcanic ash than in trees growing on forest soil. Our results indicate that constraints on morphological acclimation to high irradiances may contribute to reduced height growth of P. jezoensis on volcanic ash.     

Constraints on light interception efficiency due to shoot architecture in broad-leaved Nothofagus species

Shoot architecture may significantly alter mean quantum flux on foliage and thus, photosynthetic productivity. There is currently only limited information about plastic alterations in shoot structure caused by within-canopy variation in mean integrated irradiance (Q(int)) in broad-leaved trees. We studied leaf and shoot structure, and nitrogen and carbon content in late-successional, widely distributed, temperate, broad-leaved Nothofagus taxa to determine the architectural controls on light harvesting and photosynthetic performance. Nothofagus fusca (Hook. f.) Oersted has larger leaves and less densely leaved shoots than the N. solandri varieties. Nothofagus solandri var. solandri (Hook. f.) Oersted is characterized by rounder leaves that potentially have a larger overlap than the ovate-triangular leaves of N. solandri var. cliffortioides (Hook. f.) Poole. Leaf dry mass (M(A)) and nitrogen content (N(A)) per unit area increased with increasing Q(int) in all species, demonstrating enhanced investment of photosynthetic biomass in high light. Although M(A) differed between species at a common irradiance, there was a uniform relationship between N(A) and Q(int) across species. Leaf carbon content per dry mass and leaf dry mass to fresh mass ratio also scaled positively with irradiance, suggesting greater structural investments in high light. In all species, shoots became more horizontal and flatter at lower Q(int), implying an enhanced use efficiency of direct irradiance in natural leaf positions. In contrast, irradiance effects on leaf aggregation varied among species. Across the data, leaf overlap or leaf area density was often greater at lower irradiances, possibly as a result of limited carbon availability for shoot axis extension growth. In N. fusca, leaves of which were more aggregated in high light, the shoot silhouette to total leaf area ratio (S(S)) declined strongly with increasing irradiance, demonstrating a lower light harvesting efficiency at high Q(int). This effect was only moderate in N. solandri var. cliffortioides and S(S) was independent of Q(int) in N. solandri var. solandri. Although the efficiency of light interception at high irradiances was lowest in N. fusca, this species had the greatest nitrogen content per unit shoot silhouette area (2N(A)/S(S)), indicating superior shoot-level photosynthetic potential. These data collectively demonstrate that shoot architecture significantly affects light interception and photosynthesis in broad-leaved trees, and that structural carbon limitations may constrain leaf light harvesting efficiency at low irradiance.     

Acclimation to low irradiance in Picea abies: influences of past and present light climate on foliage structure and function

Leaf retention time increases with decreasing irradiance, providing an effective way of amortizing the costs of foliage construction over time. To elucidate the physiological mechanisms underlying this dependence, I studied needle life span, morphology, and concentrations of carbon, nitrogen and nonstructural carbohydrates along a gradient of relative irradiance in understory trees of Picea abies (L.) Karst. Maximum needle life span was greater in shaded trees than in sun-exposed trees. However, irrespective of irradiance, needles with maximum longevity were situated in the middle rather than the bottom of the canopy, suggesting that needle life span is determined by the irradiance to which needles are exposed during their primary growth. Morphology and chemistry of current-year needles were adapted to prevailing light conditions. Current-year needles exposed to high irradiances had greater packing of foliar biomass per unit area than shaded needles, whereas shaded needles maximized foliar area to capture more light. Nitrogen concentrations were higher in shaded needles than in sun-exposed needles. This nitrogen distribution pattern was related to the high nitrogen cost of light interception and was assumed to improve light absorptance per needle mass of shaded needles. In contrast, in both 1- and 2-year-old needles, morphology was independent of prevailing light conditions; however, needle nitrogen concentrations were adjusted toward more effective light interception in 2-year-old foliage but not in 1-year-old foliage, indicating that acclimation of sun-adapted needles to shading takes more than one year. At the same time, needle aging was accompanied by accumulation of nonstructural carbohydrates (NSC), and increasing concentrations of needle carbon, suggesting a shift in the balance between photosynthesis and photosynthate export. The accumulation of NSC and carbon resulted in a dilution of the concentrations of other needle chemicals and explained the decline in needle nitrogen concentrations with increasing age. Thus, although morphological inadequacy to low light availabilities may partly be compensated for by modifications in needle chemistry, age-related changes in needle stoichiometric composition progressively lessen the potential for acclimation to low irradiance. A conceptual model, advanced to explain how environmental factors and age-related changes in the activities of needle xylem and phloem transport affect needle longevity, predicted that adaptation of needle morphology to irradiance during the primary growth period largely determines the fate of needles during subsequent tree growth and development.     

Stoichiometry of foliar carbon constituents varies along light gradients in temperate woody canopies: implications for foliage morphological plasticity

Foliar morphology and chemical composition were examined along a light gradient in the canopies of five deciduous temperate woody species, ranked according to shade-tolerance as Populus tremula L. < Fraxinus excelsior L. < Tilia cordata Mill. = Corylus avellana L. < Fagus sylvatica L. Foliar carbon was divided between structural (cell-wall polysaccharides, lignin) and nonstructural (proteins, ethanol-soluble carbohydrates, starch) fractions. Foliar morphology of all species was strongly affected by irradiance. Both leaf dry mass per area (M(A)), a product of leaf density and thickness, and leaf dry to fresh mass ratio (D(w)), characterizing the apoplastic leaf fraction, increased with increasing relative irradiance (I(sum), calculated as the weighted mean of fractional penetration of diffuse and direct irradiance). Though the relationships were qualitatively identical among the taxa, more shade-tolerant species generally had lower values of M(A) than shade-intolerant species, and their morphological relationships with irradiance were curvilinear; however, there were no signs of saturation even at the highest irradiances in shade-intolerant species. In all species, lignin concentrations increased and cell-wall polysaccharide concentrations decreased with increasing irradiance. Consequently, biomass investment in structural leaf components appeared to be relatively constant along light gradients. The relationship between irradiance and structural compounds tended to be asymptotic in the more shade-tolerant species, whereas M(A) was linearly correlated with concentrations of structural leaf components, suggesting that similar factors were responsible for the curvature in the morphological and chemical relationships with irradiance. Because lignin increases tissue elastic modulus thereby rendering leaves more resistant to low leaf water potentials, we conclude that changes in stoichiometry of cell wall components were related to foliage acclimation to the gradients of water deficit that develop in the canopy and inherently accompany light gradients. We also conclude that increased lignification decreased leaf expansion growth, and that species differences in lignification were partly responsible for the observed interspecific variability in morphological plasticity. Analysis of structural leaf compounds provided no indication of how shade-intolerant species with low investments in lignin acclimated to gradients of water availability in the canopy. Because shade-intolerant species generally had higher capacities for photosynthesis than shade-tolerant species, we postulated that they should also have a greater ability for osmotic adjustment of leaf water potential with photosynthates. The concentrations of soluble carbohydrates increased with increasing irradiance in all species; however, the osmotic adjustment achieved in this way was similar in all species, except for shade-intolerant F. excelsior, which had a lower potential for osmotic adjustment with carbohydrates than the other taxa. Although we did not determine whether the gradients of stem water potential and leaf water deficits were similar in canopies of different species, we demonstrated that water relations play a central role in determining foliar structure and composition along light gradients in the canopy.     

An analysis of light effects on foliar morphology,physiology, and light interception in temperate deciduous woody species of contrasting shade tolerance

Maximum Rubisco activities (V(cmax)), rates of photosynthetic electron transport (J(max)), and leaf nitrogen and chlorophyll concentrations were studied along a light gradient in the canopies of four temperate deciduous species differing in shade tolerance according to the ranking: Populus tremula L. < Fraxinus excelsior L. < Tilia cordata Mill. = Corylus avellana L. Long-term light environment at the canopy sampling locations was characterized by the fractional penetration of irradiance in the photosynthetically active spectral region (I(sum)). We used a process-based model to distinguish among photosynthesis limitations resulting from variability in fractional nitrogen investments in Rubisco (P(R)), bioenergetics (P(B), N in rate-limiting proteins of photosynthetic electron transport) and light harvesting machinery (P(L), N in chlorophyll and thylakoid chlorophyll-protein complexes). On an area basis, V(cmax) and J(max) (V(a) (cmax) and J(a) (max)) increased with increasing growth irradiance in all species, and the span of variation within species ranged from two (T. cordata) to ten times (C. avellana). Examination of mass-based V(cmax) and J(max) (V(m) (cmax) and J(m) (max)) demonstrated that the positive relationships between area-based quantities and relative irradiance mostly resulted from the scaling of leaf dry mass per area (M(A)) with irradiance. Although V(m) (cmax) and J(m) (max) were positively related to growth irradiance in C. avellana, and J(m) (max) was positively related to irradiance in P. tremula, the variation range was only a factor of two. Moreover, V(m) (cmax) and J(m) (max) were negatively correlated with relative irradiance in T. cordata. Rubisco activity in crude leaf extracts generally paralleled the gas-exchange data, but it was independent of light in T. cordata, suggesting that declining V(m) (cmax) with increasing relative irradiance was related to increasing diffusive resistances from the intercellular air spaces to the sites of carboxylation in this species. Because irradiance had little effect on foliar nitrogen concentration, the relationships of P(B) and P(R) with irradiance were similar to those of V(m) (cmax) and J(m) (max). Shade-intolerant species tended to have greater P(B) and P(R) and also larger V(a) (cmax) and J(a) (max) than more shade-tolerant species. However, for the whole material, P(B) and P(R) varied only about 50%, whereas V(a) (cmax) and J(a) (max) varied more than 15-fold, further emphasizing the importance of leaf anatomical plasticity in determining photosynthetic acclimation to high irradiance. Leaf chlorophyll concentrations and fractional nitrogen investments in light harvesting increased hyperbolically with decreasing irradiance to improve quantum use efficiency for incident irradiance. The effect of irradiance on P(L) was of the same order as its effect in the opposite direction on M(A), leading to either a constant model estimate of leaf absorptance with I(sum) or a slightly positive correlation. We conclude that leaf morphological plasticity is a more relevant determinant of foliage adaptation to high irradiance than foliage biochemical properties, whereas biochemical adaptation to low irradiance is of the same magnitude as the anatomical adjustments. Although shade-tolerant species did not have greater chlorophyll concentrations and P(L) than shade-intolerant species, they possessed lower M(A), and could maintain a more extensive foliar display for light capture with constant biomass investment in leaves.     

Nutrient contents and concentrations in relation to growth of Picea abies and Fagus sylvatica along a European transect

Mineral nutrition of Norway spruce (Picea abies (L.) Karst.) and beech (Fagus sylvatica L.) was investigated along a transect extending from northern Sweden to central Italy. Nitrogen (N) concentrations of needles and leaves in stands growing on acid soils did not differ significantly between central Italy and southern Sweden (1.0 +/- 0.1 mmol N g(-1) for needles and 1.9 +/- 0.14 mmol N g(-1) for leaves). In both species, foliar N concentrations were highest in Germany (1.2 mmol N g(-1) for needles and 2.0 mmol N g(-1) for leaves) and decreased by 50% toward northern Sweden (0.5 mmol N g(-1)). Both species showed constant S/N and P/N ratios along the transect. Calcium, K and Mg concentrations generally reflected local soil conditions; however, Mg concentrations reached deficiency values in Germany. Leaf area per unit dry weight varied significantly along the transect with lowest values for Norway spruce recorded in northern Sweden and Italy (3.4 m(2) kg(-1)) and a maximum in central Europe (4.7 m(2) kg(-1)). A similar pattern was observed for beech. Despite the low variation in foliar N concentrations on the large geographic scale, local and regional variations in N concentrations equalled or exceeded the variation along the entire continental transect. Furthermore, nutrient contents (i.e., nutrient concentration x dry weight per needle or leaf) showed a greater variation than nutrient concentrations along the transect. Nitrogen contents of Norway spruce needles reached minimum values in northern Sweden (2.4 micro mol N needle(-1)) and maximum values in Denmark (5.0 micro mol N needle(-1)). The N content of beech leaves was highest in Denmark (242 micro mol N leaf(-1)). At the German site, foliar N content rather than N concentration reflected the seasonal dynamics of foliar growth and N storage of the two species. During foliage expansion, there was an initial rapid increase in N content and a decrease in N concentration. This pattern lasted for about 2 weeks after bud break and was followed by 6 weeks during which dry weight and N content of the foliage increased, resulting in a further decrease in N concentration. During summer, dry weight and N content of mature needles of Norway spruce increased further to reach a maximum in autumn, whereas N concentration remained constant. In spring, reallocation of N from 1- and 2-year-old needles was 1.5 and 1.0 micro mol N needle(-1), respectively. This remobilized N was a major source of N for the development of new needles, which had an N content of 1.5 micro mol N needle(-1) after bud break. The seasonal remobilization of N from old foliage decreased with increasing needle age. Needle N content and dry weight decreased progressively with age (1 micro mol N needle(-1) between age classes 2 and 5), whereas N concentrations remained constant. For Norway spruce, annual stemwood production was correlated with needle N content but not with foliar N concentration or with the total amount of N in the canopy. Interspecific and geographical differences in plant nutrition are discussed on the basis of competitive demands for C and N between growth of foliage and wood.     

Biomass investment in leaf lamina versus lamina support in relation to growth irradiance and leaf size in temperate deciduous trees

Foliar biomass investment in support and assimilative compartments was studied in four temperate deciduous tree species along a natural light gradient across the canopy. The species ranked according to shade tolerance as Betula pendula Roth. < Populus tremula L. < Fraxinus excelsior L. < Tilia cordata Mill. Long-term light conditions at sampling locations were characterized as seasonal mean integrated quantum flux density (Q(int), mol m(-2) day(-1)) estimated by a method combining hemispherical photography and light measurements with quantum sensors. Leaf morphology was altered by Q(int) in all species. Both lamina and petiole dry mass per lamina area (LMA and PMA, respectively) increased with increasing Q(int). Shade-tolerant species had lower LMA at low Q(int) than shade-intolerant species; however, PMA was not related to shade tolerance. Across species, the ratio of petiole dry mass to lamina dry mass (PMR) varied from 0.07 to 0.21. It was independent of Q(int) in the simple-leaved species, but decreased with increasing Q(int) in the compound-leaved F. excelsior, which also had the largest foliar biomass investment in petioles. Differences in leaf mass and area, ranging over four orders of magnitude, provided an explanation for the interspecific variability in PMR. Species with large leaves also had greater biomass investments in foliar support than species with smaller leaves. This relationship was similar for both simple- and compound-leaved species. There was a negative relationship between PMR and petiole N concentration, suggesting that petioles had greater carbon assimilation rates and paid back a larger fraction of their construction cost in species with low PMR than in species with high PMR. This was probably the result of a negative relationship between PMR and petiole surface to volume ratio. Nevertheless, petioles had lower concentrations of mineral nutrients than laminas. Across species, the ratio of petiole N to lamina N varied from only 3 to 6%, demonstrating that petiole costs are less in terms of nutrients than in terms of total biomass, and that the petiole contribution to carbon assimilation is disproportionately lower than that of the lamina contribution.     

Stomatal conductance alone does not explain the decline in foliar photosynthetic rates with increasing tree age and size in Picea abies and Pinus sylvestris

Foliar light-saturated net assimilation rates (A) generally decrease with increasing tree height (H) and tree age (Y), but it is unclear whether the decline in A is attributable to size- and age-related modifications in foliage morphology (needle dry mass per unit projected area; M(A)), nitrogen concentration, stomatal conductance to water vapor (G), or biochemical foliage potentials for photosynthesis (maximum carboxylase activity of Rubisco; V(cmax)). I studied the influences of H and Y on foliage structure and function in a data set consisting of 114 published studies reporting observations on more than 200 specimens of various height and age of Picea abies (L.) Karst. and Pinus sylvestris L. In this data set, foliar nitrogen concentrations were independent of H and Y, but net assimilation rates per unit needle dry mass (A(M)) decreased strongly with increasing H and Y. Although M(A) scaled positively with H and Y, net assimilation rates per unit area (A(A) = M(A) x A(M)) were strongly and negatively related to H, indicating that the structural adjustment of needles did not compensate for the decline in mass-based needle photosynthetic rates. A relevant determinant of tree height- and age-dependent modifications of A was the decrease in G. This led to lower needle intercellular CO2 concentrations and thereby to lower efficiency with which the biochemical photosynthetic apparatus functioned. However, V(cmax) per unit needle dry mass and area strongly decreased with increasing H, indicating that foliar photosynthetic potentials were lower in larger trees at a common intercellular CO2 concentration. Given the constancy of foliar nitrogen concentrations, but the large decline in apparent V(cmax) with tree size and age, I hypothesize that the decline in Vcmax results from increasing diffusive resistances between the needle intercellular air space and carboxylation sites in chloroplasts. Increased diffusive limitations may be the inevitable consequence of morphological adaptation (changes in M(A) and needle density) to greater water stress in needles of larger trees. Foliage structural and physiological variables were nonlinearly related to H and Y, possibly because of hyperbolic decreases in shoot hydraulic conductances with increasing tree height and age. Although H and Y were correlated, foliar characteristics were generally more strongly related to H than to Y, suggesting that increases in height rather than age are responsible for declines in foliar net assimilation capacities.     

Atmospheric carbon dioxide, irrigation, and fertilization effects on phenolic and nitrogen concentrations in loblolly pine (Pinus taeda) needles

Concentrations of total soluble phenolics, catechin, proanthocyanidins (PA), lignin and nitrogen (N) were measured in loblolly pine (Pinus taeda L.) needles exposed to either ambient CO(2) concentration ([CO(2)]), ambient plus 175 or ambient plus 350 micromol CO(2) mol(-1) in branch chambers for 2 years. The CO(2) treatments were superimposed on a 2 x 2 factorial combination of irrigation and fertilization treatments. In addition, we compared the effects of branch chambers and open-top chambers on needle chemistry. Proanthocyanidin and N concentrations were measured in needles from branch chambers and from trees in open-top chambers exposed concurrently for two years to either ambient [CO(2)] or ambient plus 200 micromol CO(2) mol(-1) in combination with a fertilization treatment. In the branch chambers, concentrations of total soluble phenolics in needles generally increased with needle age. Concentrations of total soluble phenolics, catechin and PA in needle extracts increased about 11% in response to the elevated [CO(2)] treatments. There were no significant treatment effects on foliar lignin concentrations. Nitrogen concentrations were about 10% lower in needles from the elevated [CO(2)] treatments than in needles from the ambient [CO(2)] treatments. Soluble phenolic and PA concentrations were higher in the control and irrigated soil treatments in about half of the comparisons; otherwise, differences were not statistically significant. Needle N concentrations increased 23% in response to fertilization. Treatment effects on PA and N concentrations were similar between branch and open-top chambers, although in this part of the study N concentrations were not significantly affected by the CO(2) treatments in either the branch or open-top chambers. We conclude that elevated [CO(2)] and low N availability affected foliar chemical composition, which could in turn affect plant-pathogen interactions, decomposition rates and mineral nutrient cycling.     

Acclimation of shoot and needle morphology and photosynthesis of two Picea species to differences in soil nutrient availability

To investigate morphological acclimation to differences in nutrient availability, we compared shoot and needle morphology of Picea glehnii (Friedr. Schmidt) M. T. Mast. and Picea jezoensis (Siebold & Zucc.) Carrière trees growing on nutrient-poor volcanic ash and nutrient-rich, brown forest soil. Trees of both species were shorter and had more open canopies when growing on volcanic ash than when growing on brown forest soil. Nutrient-poor conditions limited height growth less in P. glehnii than in P. jezoensis. In both species, trees growing on volcanic ash had shorter annual increments in the previous year and more needles per shoot length and, hence, a smaller shoot silhouette area (SSA) relative to needle dry mass (NDM) than trees growing on brown forest soil. Soil type had less effect on shoot projected needle area (PNA). Total needle area (TNA) of P. glehnii shoots was similar between soil types, whereas TNA of P. jezoensis was lower in trees growing on volcanic ash than in trees growing on brown forest soil. For both species, low SSA in response to nutrient-poor conditions resulted in low shoot SSA/PNA ratios, indicating high within-shoot self-shading. Shoot SSA/TNA of P. glehnii was lower in trees growing on volcanic ash than in trees growing on brown forest soil, indicating that needles were sun-acclimated. In contrast, shoot SSA/TNA of P. jezoensis was higher in trees growing on volcanic ash than in trees growing on brown forest soil. The contrasting response of TNA to low nutrient availability was associated with species-specific differences in needle morphology. Needles of P. glehnii growing on volcanic ash were slightly shorter, wider, thicker and heavier than those of trees growing on brown forest soil, indicating morphological acclimation to high irradiance. Needles of P. jezoensis growing on volcanic ash were shorter than those of trees growing on brown forest soil, but did not show morphological acclimation to high irradiance in width, thickness or mass. For both species, nutrient-poor conditions decreased maximum photosynthetic rate (Amax) per NDM. However, when expressed per PNA, the decrease in Amax was reduced, and when expressed per SSA, Amax was higher in trees growing on volcanic ash than in trees growing on brown forest soil. On volcanic ash, Amax per NDM was lower for P. glehnii than for P. jezoensis. However, morphological changes at the shoot and needle levels reversed this trend when Amax was expressed per SSA or per PNA. The species-specific differences in morphological response to differences in soil nutrient availability suggest that P. glehnii is more tolerant of nutrient-poor conditions, whereas P. jezoensis is better at exploiting nutrient-rich soils.     

Decreased needle longevity of fertilized Douglas-fir and grand fir in the northern Rockies

Changes in nutrient availability significantly affect canopy dynamics in conifers. To elucidate these effects, we experimentally fertilized mixed conifer stands at several sites across the northern Rocky Mountains. We measured needle longevity, total branch length and foliated length along the main branch axis, and determined mean retained cohort length on mid-canopy branches of shade-intolerant Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco) and shade-tolerant grand fir (Abies grandis Lindl.). Needle longevity ranged from 6 to 8 years in Douglas-fir and from 7 to 8 years in grand fir on unfertilized plots. Fertilization significantly decreased needle longevity by 26 and 27% in Douglas-fir and grand fir, respectively. However, the foliated branch length remained unchanged following fertilization and was similar for both species, indicating a 33% increase in mean branch length per needle cohort in Douglas-fir and a 27% increase in grand fir. These data are consistent with the theory that foliated branch length and needle longevity are a result of the ecological light compensation point (ELCP), which links the inherent physiology of the leaf with the availability of resources in the leaf environment. Mid-canopy ELCP was approximately 74 and 71 cm from the branch terminus in Douglas-fir and grand fir, respectively, regardless of fertilization. We hypothesize that fertilization-enhanced needle production and annual shoot growth resulted in a higher rate of shading of older needles. The shaded needles were unable to maintain a positive carbon balance and abscised. The results demonstrate that foliated branch length of Douglas-fir and grand fir in the northern Rocky Mountains can be treated as a homeostatic response to fertilization, whereas foliar turnover is plastic.     

Pine needle holders for use in gas exchange measurements

Simple holders for positioning pine needles in a gas exchange cuvette are described. The holders make it easy to enclose a standard length of needles in the cuvette in a single plane without mutual shading. The holders also make it possible, following gas exchange measurements, to harvest for further analysis just those needle portions that were enclosed in the leaf chamber. Field observations, which were made with a gas exchange cuvette incorporating the needle holders, on the relationship between carbon exchange rate and photon flux density in loblolly pine (Pinus taeda L.) needles are reported.     

Effect of dothistroma needle blight on needle and shoot lengths

The needles of pine trees are indicative of the overall health of the tree, and their length is affected by many factors. This study describes the effect of high levels of pathogen infection on lengths of both needles and shoots. Dothistroma septosporum is an important foliar pathogen of pines causing necrosis and premature defoliation with successive years of high infection leading to growth reduction and in extreme cases tree death. Corsican pine trees with all foliage infected by D. septosporum had needles of primary, secondary and tertiary shoots reduced by 31.8%, 44.0% and 64.2%, respectively, compared to non‐infected trees. Needle lengths were reduced both in the upper and lower portions of the canopy, with a greater reduction lower in the canopy. Trees with high levels of infection had shorter shoots in the upper canopy with fewer, shorter needles on these shoots compared to trees with low levels of infection. The results demonstrate the substantial negative effect on needle and shoot lengths of trees with high levels of D. septosporum infection, comparable to factors such as water and nutrient availability known to have a strong influence on these parameters. The reductions in length reduce the photosynthetic capacity of the tree and compound the immediate reductions caused by necrosis and premature defoliation. These effects on needle and shoot lengths contribute to the reductions in volume growth of affected trees and, moreover, are longer lasting than the immediate effects of necrosis and premature defoliation.     

Thinning,fertilization, and crown position interact to control physiological responses of loblolly pine

To examine physiological responses to thinning, fertilization, and crown position, we measured net photosynthesis (P(n)), transpiration (E), vapor pressure difference (VPD), stomatal conductance (g(s)), and xylem pressure potential (Psi(1)) between 0930 and 1130 h under ambient conditions in the upper and lower crowns of a 13-year-old loblolly pine (Pinus taeda L.) plantation six years (1994) after the treatments were applied. Photosynthetic photon flux density (PPFD) and air temperature (T(a)) within the canopy were also recorded. Needle P(n) of thinned trees was significantly enhanced by 22-54% in the lower crown, because canopy PPFD increased by 28-52%. Lower crown foliage of thinned plots also had higher E and g(s) than foliage of unthinned plots, but thinning had no effect on needle Psi(1) and predawn xylem pressure potential (0430-0530 h; Psi(pd)). Tree water status did not limit P(n), E and g(s) during the late-morning measurements. Fertilization significantly decreased within-canopy PPFD and T(a). Needle Psi(1) was increased in fertilized stands, whereas P(n), E and g(s) were not significantly altered. Upper crown foliage had significantly greater PPFD, P(n), VPD, g(s), E, and more negative Psi(1) than lower crown foliage. In both crown positions, needle P(n) was closely related to g(s), PPFD and T(a) (R(2) = 0.77 for the upper crown and 0.82 for the lower crown). We conclude that (1) silvicultural manipulation causes microclimate changes within the crowns of large trees, and (2) needle physiology adjusts to the within-crown environmental conditions.     

Estimation of light interception properties of conifer shoots by an improved photographic method and a 3D model of shoot structure

The spherical mean of the shoot silhouette-to-total leaf area ratio (STAR) and the shoot transmission coefficient (c) are two key structural parameters in radiative transfer models for calculating canopy photosynthesis and leaf area index. The standard optical method for estimating these parameters might introduce errors in the estimates for species with flexible shoots and needles by changing shoot inclination relative to its inclination in situ. We devised and tested two methods to address this problem. First, we modified the standard optical method by designing an apparatus that allows shoots to be photographed in their original orientation. Second, we developed a faster, model-based approach to replace photography and tested the results against the established approach. We used shoots of three pine species, Pinus echinata Mill. (needle length ~50 mm), P. taeda L. (~150 mm) and P. palustris Mill. (~300 mm). Values of the parameters simulated by the model were similar to those measured from the photographs. In our data, STAR varied about twofold among the pine species and was ~40% higher in shade shoots than in sun shoots of P. taeda. The transmission coefficient for P. taeda shade shoots was also ~40% higher than that of sun shoots of all three species. We tested the versatility of the model by employing it on shoots of two other pine species (P. strobus L. and P. thumbergiana Parl.) as well as on shoots of Tsuga canadensis L. Carr. and Picea pungens Engelm. Regardless of shoot characteristics, the model generated values of shoot structural parameters similar to those estimated with the optical method. Although species-specific and vertical gradients in parameter values are best for modeling radiative transfer in conifer canopies, our results suggest that, in the absence of adequate data, STAR can be approximated as 0.16 for a wide range of shoot structures. For applications requiring angle-dependent parameterization, our new model facilitates rapid generation of these radiative transfer parameters.     

The effects of ultraviolet-B radiation on photosynthesis of different aged needles in field-grown loblolly pine

We examined the effect of supplemental UV-B radiation (290-320 nm) on photosynthetic characteristics of different aged needles of 3-year-old, field-grown loblolly pine (Pinus taeda L.). Needles in four age classes were examined: I, most recently fully expanded, year 3; II, first flush, year 3; III, final flush, year 2; and IV, oldest needles still present, year 2. Enhanced UV-B radiation caused a statistically significant decrease (6%) in the ratio of variable to maximum fluorescence (F(v)/F(m)) following dark adaptation only in needles from the youngest age class, suggesting transient damage to photosynthesis. However, no effects of enhanced UV-B radiation on other instantaneous measures of photosynthesis, including maximum photosynthesis, apparent quantum yield and dark respiration, were seen for needles of any age. Foliar nitrogen concentration was unaffected by UV-B treatment. However, the (13)C/(12)C carbon isotope ratios (delta(13)C-a time integrated measure of photosynthetic function) of needles in age classes II and IV were 3% (P < 0.01) and 2% (P < 0.05) more negative, respectively, in treated plants than in control plants. Exposure to enhanced UV-B radiation caused a 20% decrease in total biomass and a 4% (P < 0.05), 25% (P < 0.01), and 9% (P < 0.01) decrease in needle length of needles in age classes I, II, and IV, respectively. The observed decreases in delta(13)C, and F(v)/F(m) of the needles in the youngest needle age class suggest subtle damage to photosynthesis, although overall growth reductions were probably a result of decreased total leaf surface rather than decreased photosynthetic capacity. Needles of age class IV had lower light- and CO(2)-saturated maximum photosynthetic rates (39%), lower dark respiration (34%), lower light saturation points (37%), lower foliar nitrogen concentration (28%), and lower delta(13)C (14%) values than needles of age class I. Apparent quantum yield and F(v)/F(m) did not change with needle age. The observed changes in photosynthesis and foliage chemical composition with needle age are consistent with previous studies of coniferous trees and may represent adaptations of older needles to shaded conditions within the canopy.     

Photosynthetic activity in winter needles of the evergreen tree Taxus cuspidata at low temperatures

Photosystems harvest light energy, yet this energy cannot be efficiently employed for CO(2) assimilation at the below-freezing temperatures to which plants are typically exposed during winter in the temperate and boreal zones. To elucidate the mechanisms whereby this energy is dissipated, I evaluated performance of photosystems in winter needles of the evergreen tree Taxus cuspidata Sieb. et Zucc. Chloroplasts were localized adjacent to plasma membranes in needle cells in summer, whereas they congregated together in the centers of the cells during winter. When winter needles were acclimated to a temperature of 20 degrees C, their chloroplasts gradually dispersed to the edges of the cells, as in the summer. Acclimation-dependent relocalization coincided with changes in CO(2) uptake. Examination of photosystem II fluorescence kinetics in winter needles indicated that the quinone electron acceptor (Q(A)) reduction rate exceeded the Q(A) oxidation rate at low temperatures. The majority of Q(A) remained reduced even when winter needles were subjected to a temperature of -5 degrees C at low irradiance.     

Comparison of methods for estimating surface area of water-stressed and fully hydrated pine needle segments for gas exchange analysis

A nondestructive technique for repeatedly estimating total surface area of water-stressed pine (Pinus taeda L.) fascicle segments was evaluated. Fascicle radius was measured with a digital caliper and needle surface area calculated on the assumption that the needles were segments of a cylinder. High correlations (r >/= 0.94) were found between surface area estimated in this way and surface area calculated (a) from needle displacement, also on the assumption that needles are segments of a cylinder, and (b) from needle dry weight using a regression between dry weight and surface area estimated from needle displacement. The caliper-radius method gave slightly lower estimates of surface area than the other methods. However, differences among surface area estimates made with the three methods and between estimates made on water-stressed and fully hydrated needles were small relative to variances of leaf gas exchange measurements.     

A comparison of red spruce and balsam fir shoot structures

I compared the shoot structures of high-elevation red spruce (Picea rubens Sarg.) and balsam fir (Abies balsamea (L.) Mill.). Needle widths, thicknesses and perimeters were measured to estimate total leaf areas from measured projected leaf areas. Measured needle perimeter/needle width ratios differed significantly from estimated ratios that assumed needles were either rhomboidal or elliptical in cross section. The vertical and horizontal silhouette shoot area to total leaf area ratios (STAR(v) and STAR(h)) of the two species were negatively correlated with needle packing and canopy height. Red spruce had higher values of STAR(v) than balsam fir at each canopy height, but STAR(v) declined with canopy height at a similar rate in the two species. The STAR(h) values of the two species did not differ significantly at a given canopy height. Needle packing increased with canopy height at the same rate in the two species. Needle weight increased in red spruce and decreased in balsam fir with increased needle packing, but showed no significant dependence on canopy height. Red spruce had higher values of STAR(h) than balsam fir at low values of needle packing, but STAR(h) values converged at high values of needle packing. The generally comparable values of STAR, along with similar needle diameters, may imply that red spruce and balsam fir have similar collection efficiencies of wet and dry particles. Measurements of STAR may be used to estimate leaf area indices (LAI) more accurately when using indirect techniques.