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Patterns of Photosynthesis Under Natural Environmental Conditions John D Reprinted from ECOLOGY, Vol. 48, No. 2. Early Spring 1967. Purchased by the Forest Service, U.S. Department of Agriculture, for official use. PATTERNS OF PHOTOSYNTHESIS UNDER NATURAL ENVIRONMENTAL CONDITIONS JOHN D. HODGES1 Ulli'Vcrsity of rVashington, Seattle, T,Vas/lillgton (Accepted for publication August 6, 1966) Abstract. Net photosynthesis in six different conifers was studied under various natural environmental conditions. Changes in the pattern of photosynthesis on clear days, especially the midday decrease, are apparently primarily controlled by changes in leaf water potential. In noble fir and Scots pine, water potential probably acts mainly through its influence on stomatal movement. In grand fir, Douglas-fir, hemlock, and Sitka spruce, however, some other mechanism, probably mesophyll resistance to CO2 diffusion, seems to play a more important role. Both mechanisms probably operate concurrently in all species. Daily variations in leaf water potential seem to occur primarily in response to changes in atmospheric moisture, or, more precisely, vapor pressure gradient from leaf to atmosphere. Variation in carbohydrate content, through its influence on solute concentration, may also influence leaf water potential. INTRODUCTION tion in explaining ecological differences between The pattern of assimilation in forest trees dif­ species. Results of the measurements of photo­ fers markedly on days with different local weather synthetic rates and efficiency will be presented in conditions and, most often, the highest rates of a subsequent paper. assimilation occur on cloudy or overcast days MATERIALS AND METHODS (Polster 1950, Pisek and Tranquillini 1954, Gentle 1963, Helms 1963). On overcast days, Plant material and environment net photosynthesis normally reaches a peak about Seedlings (2-0 stock) of six species-Douglas­ midday and then gradually decreases throughout fir (Pseudotsuga menziesii (Mirb.) Franco) , the remainder of the day. On clear days, there is grand fir (Abies grandis (Lindley)), western normally a morning peak followed by a marked hemlock (Tsuga heterophylla. (Rafinesque) Sar­ depression in photosynthesis and a secoGd, lower gent), Sitka spruce (Picea sitchensis (Bongard)), peak (Rabinowitch 1945, Polster 1950, Stocker noble fir (Abies procera), and Scots pine (Pinus 1960). The objective of this research was to silvestris (L.) )-were out-planted in plots along study intensively the pattern of net photosynthe­ north-south transects extending from deep within sis in conifer seedlings of the Pacific Northwest a 35- to 40-year-old Douglas-fir stand into an and, if possible, to explain these patterns by mea­ adjacent open area cleared of all vegetation in­ surement of various plant and environmental fac­ cluding herbaceous plants. The four most inter­ tors. The research was part of a larger investiga­ esting environmental situations were those des­ tion which was also designed to study and compare ignated as open (full exposure), outside stand photosynthetic rates and efficiency of seedlings of border, inside stand border, and deep inside stand west coast conifers under a wide range of natural (deep shade). The location designated as "out­ environmental conditions and to use the informa­ side stand border" was immediately adjacent to l Present address: Southern Forest Experiment Sta­ the outer margin of the Douglas-fir stand, while tion, Alexandria, Louisiana. the one designated as "inside stand border" was Early Spring 1967 PATTERNS OF PHOTOSYNTHESIS 235 approximately 20 ft inside the stand. Thus the sample. The total needle mass sampled was kept envirollment at both locations was influenced by at about 0.5 g, ovendry weight. For all samples, the Douglas-fir stand, but the influence of the a single branch from the uppermost whorl of stand was mllch greater at the location just inside branches which developed during the second grow­ the stand border. The location deep inside the ing season in the field was used. Thus, seedlings stand was approximately 125 ft fr0111 the margin llsed fr0111 June 1963 to about June 1964 were on the stand. planted in January 1962 and those used from June Light intensity was often greater than 11,000 1964 until the end of the study were planted in ft-c in the open on clear summer days. Intensity January 1963. In all cases, the needles were fully at the stand border (outside) was normally in the developed and, depending on the species, 2-4 range of 1,000- 2,000 ft-c except in late afternoon inches of the branch tip were placed in the cuvette. when the area received direct sunlight for a short An automatic switching mechanism at the CO2 period. Inside the stand border the plants re­ analyzer permitted cyclic sampling at six separate ceived little direct sunlight and intensity was nor­ points. Since at least one point (line) was always mally less than 1,000 ft-c. Light intensity deep used to monitor ambient CO2 concentration, up inside the stand was normally less than 100 ft-c to five seedlings could be sampled at any time. and seldom over 200 ft-c except during occasional The sampling procedure was varied so that at "sun flecks." Environmental factors other than times all sample lines were placed in one location light were also modified by the Douglas-fir stand. in order to compare photosynthesis in different The effect of the stand on air temperature, as com­ species under the same environment. At other pared to the open area, was to reduce the maxi­ times the sample lines were placed along the en­ mum temperatures and to slightly increase the vironmental transect, and rates and patterns of minimum temperatures. At locations influenced assimilation were compared for the same species by the stand, relative humidity, especially on clear over a range of environments. Net photosynthe­ days, was higher than in the open. A more de­ sis was normally determined for a period of 3 tailed description of the environment at the dif­ days on each sample seedling after which the ferent locations will be presented in a subsequent foliage sample was removed and needle dry weight paper dealing with ecological differences between determined. Seedlings were normally used only the six species. once except at the location deep inside the stand Seedlings were also planted at locations inter­ where survival was poor. The investigation ex­ mediate in position between the open area and tended over a 2-year period beginning in June the stand border (outside), and between the 10­ 1963. Thus measurements were made in all sea­ cation inside the stand border and deep shade. sons of the year. A total of 390 seedlings was These seedlings were not studied intensively, how­ ' used in the investigation. ever, since their environment and rates and pat­ Plant factors which were measured in the terns of net photosynthesis were almost identical spring, summer, and fall of 1964, included leaf either to seedlings in the open area or in deep water potentiaJ,2 sugar content of the foliage, leaf shade. temperature, and relative stomafal aperture. Leaf Experimental plantings were made in January water potential and sugar concentrations were 1962 and January 1963. Two transects of plots determined from foliage samples of the same.seed­ were established in 1962 and one transect in 1963. ling taken at periodic intervals throughout the The 1962 plots contained six seedlings of each day. An electric psychrometer, similar to the one species, while those established in 1963 contained described by Spanner (1951), was used for water 20 seedlings of each species. Thus 32 seedlings potential measurements. Sugars, reducing and of each species were planted at each experimental non-reducing, were determined colorimetrically by location. The study area was located on Charles a. modification of the method given by Bernfield Lathrop Pack Demonstration Forest which is (1955). Leaf temperature was measured con­ owned by the College of Forestry of the Univer­ tinttously by a thermocouple attached to the under sity of Washington and located near LaGrande, surface of the needle. On two occasions, field Washington. measurements of relative stomatal aperture were Measu,remen! of plant factors and processes made throughout the day and into the night. In addition, stomatal measurements were made in the An infrared gas analyzer was used for the mea­ surement of net photosynthesis during the day­ • Leaf water potential is the difference in free energy or chemical potential between pure water and the water light hours and of respiration at night. Gas contained in the leaf cells. The potential of pure water samples were collected from a small plastic (poly­ is zero. Therefore, the potential of water in leaf cells is propylene) cuvette which surrounded the foliage less than zero (a negative number). 236 JOHN D. HODGES Ecology, Vol. 48, No.2 laboratory under controlled conditions of light, 0 50 12 Light temperature, and humidity. All stol11atal measure­ ments were made by an infiltration technique 40 10 " described by Fry (1965). Measurements listed .... ., 8 above as well as the measurement of environmen­ Q. E tal factors were made concomitantly with mea­ ., 2 6 surements of CO2 assimilation. I- 4 MeaSlIre111ent of environmental factors Continuous measurements of light intensity, air 2 temperature, and relative humidity were made for the duration of the study. Light intensity was measured by means of selenium photocells and/or radiometers. Temperature and humidity were ... measured with wet-dry bulb thermocouples and with hygrothermographs. All light intensity and co ;;. thermocouple temperature measurements were re­ E c corded on a 24-point millivolt recorder. Soil 0 moisture was determined by gravimetric sampling. 'E -I. RESULTS AND DISCUSSION 'iii .. < Patterns of net photosynthesis Under fully exposed conditions, the typical pat­ 12 2 4 6 8 10 12 2 4 6 8 10 12 tern of photosynthesis on overcast or cloudy days M N M was one in which net photosynthetic rates in­ Time creased to a maximum about noon, then either FIG.
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