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1.
Microclimate of a natural pasture under planted Robinia pseudoacacia in central Appalachia, West Virginia 总被引：2，自引：0，他引：2
C. M. Feldhake 《Agroforestry Systems》2001,53(3):297-303
The conditions under which forages yield more under tree canopies than in open fields are not well understood. This study
was conducted to determine how microclimate experienced by forages in central Appalachia is modified by black locust (Robinia pseudoacacia L.) tree canopies. The effect of tree row location relative to forage growing point was evaluated for its impact on soil
water, photosynthetically active radiation (PAR), red/far-red ratio, and surface soil temperature. There was no consistent
spatial dependency relating tree rows to soil water levels. While daily PAR decreased as the time under shade increased, the
level of PAR under tree canopies nearly doubled as cloud cover increased from 0 to 25%. The red/far-red ratio decreased from
1.16 to 0.2 over forage growing between tree rows compared to forage within tree rows. Surface soil temperature remained nearly
constant (1.5–2 °C increase) during sunny days under tree canopies but increased 8–12 °C by mid afternoon at unshaded sites
depending on soil water levels. Forages under black locust trees experienced less extreme variation in both daily PAR and
temperature than unshaded forages, thus reducing the metabolic cost of adaptation to extreme conditions.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
2.
3.
We present an operational method for estimating the amount of PAR intercepted by a coniferous shoot. Interception of PAR by a shoot is divided into three components: the amount of radiation coming from the sky, the transmission of radiation through the surrounding vegetation, and the shoot' s silhouette area facing the direction of the incoming radiation. All three components usually vary with direction. Radiation incident from the sky consists of direct and diffuse radiation. The well-known equation of motion for the sun and Beer' s Law for atmospheric transmittance are used to simulate the directional distribution of direct sunlight for any given period of time. The diffuse component is assumed to be uniform. Meteorological field measurements are used to calibrate the absolute amounts of the direct and diffuse components. The gap fraction (proportion of visible sky) in different directions around a shoot is measured by analyzing a hemispherical fish-eye photograph, taken at the location of the shoot, with an image processing program. Similarly, the shoot silhouette area (SSA) is measured by photographing the shoot from many different directions. The measurements of SSA are interpolated by a method called trigonometric interpolation to obtain the directional distribution of SSA over the entire hemisphere. This distribution is then rotated according to the shoot' s position in the canopy. Multiplying incoming PAR, canopy gap fraction and SSA in different directions, and summing over all directions, gives an estimate of PAR intercepted by the shoot during the chosen period of time. The method is described step by step, and applied, as an example, to a shoot from a Scots pine (Pinus sylvestris L.) stand in central Finland. Differences in radiation interception properties between sun and shade shoots and their relevance to canopy-scale models are discussed. 相似文献
4.
Photosynthetically active radiation use efficiency (PARUE) of orchardgrass (Dactylis glomerata L.) and tall fescue (Schedonorus phoenix (Scop.) Holub) was determined. Leaf mass was harvested for canopies at different shade levels when each reached 20 cm height
with 5 cm residue for regrowth to simulate grazing. Total incident PAR at each site was summed for the growth period (S-PAR).
Values for PARUE were calculated from dried leaf mass divided by S-PAR. The more highly shaded plants reached 20 cm at lower
S-PAR. While overall leaf mass decreased linearly with shade induced decreases in S-PAR, PARUE increased exponentially. The
coefficients for the equations representing this exponential increase vary for forage species and may represent a useful index
for characterizing forage response to silvopastoral systems. 相似文献
5.
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. 相似文献
6.
Diurnal and seasonal variability in radial distribution of sap flux density: Implications for estimating stand transpiration 总被引：3，自引：0，他引：3
Daily and seasonal patterns in radial distribution of sap flux density were monitored in six trees differing in social position in a mixed coniferous stand dominated by silver fir (Abies alba Miller) and Norway spruce (Picea abies (L.) Karst) in the Alps of northeastern Italy. Radial distribution of sap flux was measured with arrays of 1-cm-long Granier probes. The radial profiles were either Gaussian or decreased monotonically toward the tree center, and seemed to be related to social position and crown distribution of the trees. The ratio between sap flux estimated with the most external sensor and the mean flux, weighted with the corresponding annulus areas, was used as a correction factor (CF) to express diurnal and seasonal radial variation in sap flow. During sunny days, the diurnal radial profile of sap flux changed with time and accumulated photosynthetic active radiation (PAR), with an increasing contribution of sap flux in the inner sapwood during the day. Seasonally, the contribution of sap flux in the inner xylem increased with daily cumulative PAR and the variation of CF was proportional to the tree diameter, ranging from 29% for suppressed trees up to 300% for dominant trees. Two models were developed, relating CF with PAR and tree diameter at breast height (DBH), to correct daily and seasonal estimates of whole-tree and stand sap flow obtained by assuming uniform sap flux density over the sapwood. If the variability in the radial profile of sap flux density was not accounted for, total stand transpiration would be overestimated by 32% during sunny days and 40% for the entire season. 相似文献
7.
Annual changes in structural attributes and seasonal dynamics in water content, photosynthetic rate and light-use efficiency (LUE) were assessed by spectral transmittance for 4 years (1999-2003) in six stands of a Mediterranean holm oak forest. Green biomass, total biomass and leaf area index (LAI) were determined. In 1999, seasonal dynamics of net carbon dioxide (CO2) exchange and water content were measured. We recorded photosynthetically active radiation (PAR) transmittance and hyperspectral transmittance in the 400-1100 nm region and derived reflectance-based vegetation indices. Transmittance over the PAR region derived from either ceptometer or spectroradiometer measurements (PART and TPAR, respectively) was related to green and total biomass. Both PART and TPAR were also related to LAI (r=0.79 and r=0.70, respectively, P <0.001) and were appropriate for comparison among stands, whereas subtle changes in LAI within a stand were better assessed by the transmittance amplitude in the red edge region (TRE) (within a stand, r=0.77-0.99, P <0.001). Spectral transmittance-based indices successfully captured physiological processes that occurred on temporal (seasonal) and spatial scales. The transmittance-based water index (TWI) was related to both foliage and canopy water content (r=0.69, P <0.001). Estimates of foliage and canopy water content improved in dense (closed) stands (r=0.84 and r=0.87, respectively, P <0.001) compared with low-density stands. Under non-drought conditions, transmittance-based photochemical reflectance index (TPRI) was related to LUE (r=0.58, P <0.05) and net CO2 exchange (r=0.72, P <0.01), and the combined TPAR x TPRI index greatly improved these relationships (r=0.93 and r=0.84, respectively, P <0.01), indicating that both structural and physiological adjustments modified CO2 fixation capacity in these forest stands. Our novel approach to the study of transmitted radiation provides a tool for estimating structural and functional variables such as LAI, LUE and water content, which are key determinants of terrestrial productivity. 相似文献
8.
2013年7月,在小兴安岭凉水国家级自然保护区云冷杉林风倒区1.5 hm2的样地内,选取由掘根倒木形成且包含坑和丘微立地的大、中、小3个代表性林隙。在每个林隙中心安装HOBO自动气象站,测定了7—9月林隙中心以及林隙内丘顶和坑底的总辐射、光合有效辐射(PAR)、空气温度和相对湿度,并在郁闭林分和空旷地设置对照。分析了不同大小林隙之间以及林隙内丘顶和坑底之间小气候在生长季的动态变化及其差异。结果表明:林隙总辐射、PAR以及空气温度依照大林隙、中林隙和小林隙的次序依次降低;同一林隙内,丘顶的总辐射、PAR和空气温度大于坑底,丘顶空气相对湿度小于坑底;月均气温和月均PAR均为7月>8月>9月,绝大多数按照空旷地、大林隙、中林隙、小林隙和郁闭林分的次序递减,仅9月份小林隙的气温大于中林隙。同一林隙的平均气温日较差均为7月>9月>8月,相同微立地(林隙中心、丘顶、坑底)气温日较差均为大林隙>中林隙>小林隙。相同月份丘顶的气温日较差均大于坑底。各林隙空气相对湿度均为8月>9月>7月,并按照空旷地、大林隙、中林隙、小林隙和郁闭林分的次序依次递增。7月份大林隙的总辐射、PAR与8、9月份相比均差异显著。9月不同大小林隙之间的总辐射、PAR差异均不显著。7—9月,大林隙和小林隙中心的空气湿度均差异显著。同一林隙相同月份丘顶和坑底的PAR、空气相对湿度差异均显著,空气温度差异不显著;无论是在林隙内还是林隙内的丘顶和坑底,同一林隙不同月份的温度差异均显著。 相似文献
9.
Small Appalachian hill farms may benefit economically by expanding grazing lands into some of their under-utilized forested
acreages. Our objective was to study the forage production potential of forest to silvopasture conversion. We thinned a white
oak dominated mature second growth forested area establishing two orchardgrass-perennial ryegrass-white clover silvopasture
replications for comparison with two nearby open pasture replications. After thinning trees, silvopastures were limed, fertilized
and seeded. Sheep were fed hay and corn scattered across the area to facilitate removal of residual understory vegetation,
disruption of litter layer and incorporation of applied materials into surface soil. Each area was divided into multiple paddocks
and rotationally grazed by sheep. Two 1 m2 herbage mass samples were taken from each paddock prior to animal grazing. There was no significant difference in soil moisture
between silvopastures and open pastures however, there was adequate rainfall to prevent drought all 3 years. The two silvopastures
received 42 and 51% of total daily incident PAR compared to the open field. Total dry forage mass yield from open pasture
over the 3 years averaged 11,200 kg ha−1 y−1 and from silvopasture 6,640 kg ha−1 y−1. Silvopastures, however, had a higher PAR use efficiency (PARUE) than open pasture. Hill farms could increase grazing acreages
without sacrificing all benefits from trees on the landscape by converting some areas to silvopasture. 相似文献
10.
Balandier P Sinoquet H Frak E Giuliani R Vandame M Descamps S Coll L Adam B Prevosto B Curt T 《Tree physiology》2007,27(8):1073-1082
Two-year-old Fagus sylvatica L. saplings were planted under the cover of a Pinus sylvestris L. stand in the French Massif Central. The stand was differentially thinned to obtain a gradient of transmitted photosynthetically active radiation (PAR(t); 0-0.35). Eighteen Fagus saplings were sampled in this gradient, and their growth (basal stem diameter increment) was recorded over six years. Over the same period, morphological parameters (leaf area, number and arrangement in space) were monitored by 3D-digitization. Photosynthetic parameters were estimated with a portable gas-exchange analyzer. Photosynthesis was mainly related to light availability, whereas sapling morphology was mainly driven by sapling size. Annual stem diameter increment was related to the amount of light-intercepting foliage (silhouette to total leaf area ratio (STAR) x total sapling leaf area (LA)) and light availability above the saplings (PAR(t)). However, light-use efficiency, i.e., the slope of the relationship between STAR x LA x PAR(t) and stem diameter increment, decreased over time as a result of a relative decrease in the proportion of photosynthetic tissues to total sapling biomass. 相似文献
11.
We measured sap flux in Pinus ponderosa Laws. and Pinus flexilis James trees in a high-elevation meadow in northern Arizona that has been invaded by conifers over the last 150 years. Sap flux and environmental data were collected from July 1 to September 1, 2000, and used to estimate leaf specific transpiration rate (El), canopy conductance (Gc) and whole-plant hydraulic conductance (Kh). Leaf area to sapwood area ratio (LA/SA) increased with increasing tree size in P. flexilis, but decreased with increasing tree size in P. ponderosa. Both Gc and Kh decreased with increasing tree size in P. flexilis, and showed no clear trends with tree size in P. ponderosa. For both species, Gc was lower in the summer dry season than in the summer rainy season, but El did not change between wet and dry summer seasons. Midday water potential (Psi(mid)) did not change across seasons for either species, whereas predawn water potential (Psi(pre)) tracked variation in soil water content across seasons. Pinus flexilis showed greater stomatal response to vapor pressure deficit (VPD) and maintained higher Psi(mid) than P. ponderosa. Both species showed greater sensitivity to VPD at high photosynthetically active radiation (PAR; > 2500 micromol m-2 s-1) than at low PAR (< 2500 micromol m-2 s-1). We conclude that the direction of change in Gc and Kh with increasing tree size differed between co-occurring Pinus species, and was influenced by changes in LA/SA. Whole-tree water use and El were similar between wet and dry summer seasons, possibly because of tight stomatal control over water loss. 相似文献
12.
L. Sánchez-Jardón B. Acosta-Gallo A. del Pozo M. A. Casado C. Ovalle J. M. de Miguel 《Agroforestry Systems》2014,88(3):397-411
In order to develop a general model of aboveground net primary production (ANPP) of herbaceous communities in grazing systems that combine forested and open grasslands in temperate areas, biomass production and a set of biotic and abiotic variables were measured at four adjacent forest and grassland sites in Chile’s northern Patagonia for two consecutive growing seasons. At each site, one transect of 80 m long (40 m in open grassland and 40 m in forest) × 10 m wide was established. ANPP was significantly higher in open grasslands but no gradual change in biomass production was observed from inside the forest towards the open grassland. In open grasslands ANPP was spatially uniform but highly variable between years of contrasting weather conditions, whereas in forests it was more spatially heterogeneous and less variable over time. ANPP was highly correlated with cattle consumption. Structural equation models developed for the whole system confirm that ANPP was driven mainly by photosynthetically active radiation (PAR) and available nitrogen. However, we found important differences between forests and the adjacent open grasslands. In forests ANPP was enhanced by positive feedbacks between the amount of transmitted PAR through the canopy and soil nutrient input via cattle dung deposition. In open grasslands nitrogen availability appeared to be the main limiting factor but also influenced by weather conditions (dryer or wetter years). The coexistence of forests and grasslands patches, with different susceptibility of ANPP to meteorological and soil nutrient availability, highlights the importance of implementing an integrated silvopastoral system with lenga (Nothofagus pumilio [Poepp. & Endl.] Krasser) in northern Patagonia. 相似文献
13.
Lucero Medinilla-Salinas Mónica de la Cruz Vargas-Mendoza Silvia López-Ortiz Catarino Ávila-Reséndiz William Bruce Campbell María del Carmen Gutiérrez-Castorena 《Agroforestry Systems》2013,87(4):891-899
The effect of tree canopy on the growth, productivity and forage quality of Megathyrsus maximus and changes in soil properties were evaluated over three seasonal periods. Four adjacent plots (15 m × 17 m each) in a tropical secondary deciduous forest having 12 years of growth and dominated by Gliricidia sepium were randomly assigned to two treatments: removal of trees (SCA) in two of the plots and leaving trees intact (COA) in the other two. In all plots, M. maximus was planted with 50 cm spacing among plants. Tree removal significantly increased the incident photosynthetically active radiation (PAR, P < 0.001) and grass size (12.5 % in height, P < 0.01, and 16.5 % in clump diameter, P < 0.05), but did not significantly affect any other variable. Season significantly affected grass height (P < 0.003), tiller number (P < 0.001), clump diameter (P < 0.001), net CO2 assimilation rate (P < 0.001), forage biomass production (P < 0.003), and acid detergent fiber content (P = 0.033). Primary soil changes after 1 year of establishment of the grass were the decline by 3 % in organic carbon (P = 0.03), and qualitative changes in soil structure, regardless of tree presence. Results are consistent with the ability of M. maximus to tolerate shade. We conclude that under the conditions of the study there was no evidence for a negative effect of tree canopy on M. maximus mediated by a reduction in PAR. 相似文献
14.
Light distribution,photosynthetic rate and yield in a Paulownia-wheat intercropping system in China 总被引：3，自引：3，他引：0
Measurements of photosynthetically active radiation (PAR), leaf photosynthesis, canopy leaf area index (LAI) and crop yield
were carried from flowering to maturity to study the effects of tree shading on crop yield in a Paulownia-wheat intercropping
system in China. We found that the tree shading reduced the amount of incoming PAR within the intercropping system by 22%,
44% and 56% during flowering, grain-filling and maturing, respectively. The amount of PAR intercepted by the wheat crop inside
the intercropping system was 34%, 55% and 68% less than the crop outside the intercropping system during flowering, grain-filling
and maturing. Estimates of PAR-saturated leaf photosynthetic rate were not affected by tree shading, and the differences between
the wheat yield inside the intercropping system and outside the intercropping system can be explained by the difference in
the amount of PAR intercepted. Total grain numbers and grain dry weight per 1000 grains were linearly correlated with the
amount of PAR intercepted during 7 days prior to anthesis and during anthesis and grain-filling, respectively. As compared
with the wheat crop outside the intercropping system, grain numbers and grain dry weight were reduced by 36% and 25%, respectively.
As a result wheat yield inside the intercropping system was 51% lower than that outside the intercropping system. Our results
suggest a much higher reduction in crop yield and much smaller economic gain of the Paulownia-wheat intercropping system than
previous studies on the similar Paulownia-wheat intercropping systems in China. 相似文献
15.
An index, based on soil properties as defined by soil mapping unit and positively related to growth of quaking aspen (Populus tremuloides Michx.) in Minnesota (the aspen productivity index—APX), was developed. Aspen grows on a wide variety of soils and comprises the majority of roundwood harvest in the state. The APX was to be included in an existing information system, and the soil properties used were restricted to those in an established database. There were no co-collected forest measurements. Response curves relating aspen productivity to soil and site properties, scaled from 0.0 to 1.0, were initially developed using the abundant information in the literature and were then iteratively adjusted. Properties were aggregated into three categories; by their effects on water availability, nutrient availability, and by other (site) factors that affect growth. The APX was evaluated and validated by comparison to generalized forest productivity ratings in published soil surveys and with spatially referenced forest measurements collected at three different intensities, including a national inventory (FIA). The APX agreed well with the generalized forest productivity ratings, including both productivity classes and estimated aspen mean annual increment. Multiple and variable forest measurements within mapping units, but a single APX, reduced the explanatory power of relationships. Despite that, the APX differed significantly among productivity classes established by the FIA and those based on relative stocking. Regressions with APX as the independent variable and mean basal area and site index by mapping unit, weighted by frequency, as the dependent variable were statistically significant for basal area for all datasets and for site index for the FIA data. The relationships, evaluated over the range of the APX in the FIA dataset, predict differences in total aspen yield of about 50% in 50 years. The APX is being used to provide comparative forest productivity information for all soil mapping unit components in Minnesota (about 5800 units). 相似文献
16.
Riverine silvopastoral practices with native pecan (Carya illinoinensis) are a suitable land use for areas subjected to seasonal flooding in southern and central regions of the United States. Nut,
timber and forage production, and the economics of managed pecan silvopastures were examined in southeastern Kansas. During
1981–2000, annual hulled nut production varied between 50 and 1600 kg ha−1 in stands averaging 72 years of age, and ranging in density between 35 and 74 trees ha−1. The nut crop had a pattern of biennial bearing with some exceptions. Tree stem diameter and stand basal area increased linearly
with time. Nut production was not related to stand age or tree density, however, suggesting that nut production had reached
a steady state level. Merchantable timber yield ranged between 0.25 and 1.35 m3 ha−1 year−1. In pecan silvopastures with a mean tree age of 37 years, forage production varied between 1500 and 4600 kg DM ha−1 in 2001 and 2002. In 2001 only, grass production decreased with decreasing solar radiation within the range of 0.25–0.83
of fraction light transmitted. In both years, the grass understory had acceptable quality for cow-calf production with average
crude protein content between 9 and 11.8%, and no evidence of excessive levels of ergoalkaloids from tall fescue. Twenty-seven
vascular plants were identified in the understory of which nut sedge (Cyperus esculentus), tall fescue (Festuca arundinacea), wild oat (Avena fatua) and Canadian wild rye (Elymus canadensis) were the most abundant. Economic simulations obtained with the U.S. Agroforestry Estate Model indicated that pecan nut price
is the main variable driving economic outputs under current production conditions. Annual cash flows from nut sales had smaller
fluctuations than nut yields because of an inverse relation between nut price and yield. Improved timber production appears
an option for increasing profitability of pecan silvopastures. 相似文献
17.
Diurnal and seasonal variability in the radial distribution of sap flow: predicting total stem flow in Pinus taeda trees 总被引：10，自引：0，他引：10
We monitored the radial distribution of sap flux density (v; g H2O m(-2) s(-1)) in the sapwood of six plantation-grown Pinus taeda L. trees during wet and dry soil periods. Mean basal diameter of the 32-year-old trees was 33.3 cm. For all trees, the radial distribution of sap flow in the base of the stem (i.e., radial profile) was Gaussian in shape. Sap flow occurred maximally in the outer 4 cm of sapwood, comprising 50-60% of total stem flow (F), and decreased toward the center, with the innermost 4 cm of sapwood (11-15 cm) comprising less than 10% of F. The percent of flow occurring in the outer 4 cm of sapwood was stable with time (average CV < 10%); however, the percentage of flow occurring in the remaining sapwood was more variable over time (average CV > 40%). Diurnally, the radial profile changed predictably with time and with total stem flow. Seasonally, the radial profile became less steep as the soil water content (theta) declined from 0.38 to 0.21. Throughout the season, daytime sap flow also decreased as theta decreased; however, nighttime sap flow (an estimate of stored water use) remained relatively constant. As a result, the percentage of stored water use increased as theta declined. Time series analysis of 15-min values of F, theta, photosynthetically active radiation (PAR) and vapor pressure deficit (D) showed that F lagged behind D by 0-15 min and behind PAR by 15-30 min. Diurnally, the relationship between F and D was much stronger than the relationship between F and PAR, whereas no relationship was found between F and theta. An autoregressive moving average (ARIMA) model estimated that 97% of the variability in F could be predicted by D alone. Although total sap flow in all trees responded similarly to D, we show that the radial distribution of sap flow comprising total flow could change temporally, both on daily and seasonal scales. 相似文献
18.
Foliar cover is often selected over biomass as the variable to sample when inventorying agroforestry systems. To assess forage production, biomass and cover must be satisfactorily correlated. Significant cover/biomass relationships were developed for four major Alabama forest types. Conversion factors varied by forest type. These relationships provide practical application of understory cover measurements taken in forest inventories for use in multiresource assessments and in designing agroforestry systems. 相似文献
19.
Ecophysiology of seedlings of three Mediterranean pine species in contrasting light regimes 总被引：2，自引：0，他引：2
Seasonal dynamics of net photosynthesis (Anet) in 2-year-old seedlings of Pinus brutia Ten., Pinus pinea L. and Pinus pinaster Ait. were investigated. Seedlings were grown in the field in two light regimes: sun (ambient light) and shade (25% of photosynthetically active radiation (PAR)). Repeated measures analyses over a 12-month period showed that Anet varied significantly among species and from season to season. Maximum Anet in sun-acclimated seedlings was low in winter (yet remained positive) and peaked during summer. Maximum Anet was observed in June in P. pinea (12 micromol m-2 s-1), July in P. pinaster (23 micromol m-2 s-1) and August in P. brutia (20 micromol m-2 s-1). Photosynthetic light response curves saturated at a PAR of 200-300 micromol m-2 s-1 in winter and in shade-acclimated seedlings in summer. Net photosynthesis in sun-acclimated seedlings did not saturate at PAR up to 1900 micromol m-2 s-1 in P. brutia and P. pinaster. Minimum air temperature of the preceding night was apparently one of the main factors controlling Anet during the day. In shade-acclimated seedlings, photosynthetic rates were reduced by 50% in P. brutia and P. pinaster and by 20% in P. pinea compared with those in sun-acclimated seedlings. Stomatal conductance was generally lower in shaded seedlings than in seedlings grown in the sun, except on days with a high vapor pressure deficit. Total chlorophyll concentration per unit leaf area, specific leaf area (SLA) and height significantly increased in P. pinea in response to shade, but not in P. pinaster or P. brutia. In response to shade, P. brutia showed a significant increase in total chlorophyll concentration but not SLA. Photosynthetic and growth data indicate that P. pinaster and P. brutia are more light-demanding than P. pinea. 相似文献
20.
We analyzed the effect of simplifying assumptions in canopy representation of radiation transfer models, comparing modeled diffuse non-interceptance and photosynthetic photon flux density with measurements at different layers of complex pine-broadleaved canopy with large seasonal variation of leaf area index. The most detailed model included clumping of trees (i.e.,?stand density) and a vertical specification of leaf angle distribution and shoot clumping. A less detailed model replaced the vertically specified variables with their means. The most parsimonious model accounted for neither shoot clumping nor stand density. The vertical specification of shoot clumping and leaf angle distribution only slightly improved vertical and seasonal openness and light estimates over using mean values. Further simplification had little effect on total absorbed light but was more risky for estimates of the vertical distributions of openness and light absorbed by the canopy, which will affect photosynthesis estimates due to the non-linearity of photosynthetic light response. Including woody surfaces in winter, when leaf area was low, was essential for reproducing the measurements correctly. A sensitivity analysis showed that ignoring (i)?shoot clumping could result in a substantial overestimation of total absorbed light with errors increasing with decreasing leaf area and (ii) stand density in sparse stands could lead to substantial overestimation of total absorbed light, and the effect is largely independent of leaf area. Also, (iii) the effect of changing leaf angle distribution increased with decreasing leaf area, and was larger and more persistent along the leaf area range with increasing shoot clumping. Overall, accounting for the effect of tree clumping on absorbed light is most important in stands composed of species where leaves are not very clumped (e.g., broadleaved). However, even in forests with highly clumped shoots (i.e., coniferous), an accurate estimation of absorbed light distribution in stands requires incorporation of stand density in the model. 相似文献