Weather fluctuations and the daily rating of growth of pure stands of three grass species

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<ul><li><p>125 </p><p>WEATHER FLUCTUATIONS AND THE DAILY RATE OF GROWTH OF PURE STANDS OF </p><p>THREE GRASS SPECIES </p><p>By R. W. BROUGHAM* AND THE LATE A. C. GLENDAYt </p><p>(Received 29 July 1968) </p><p>ABSTRACf </p><p>Fluctuations in the growth rates of pure stands of perennial and short-rotation ryegrass and cocksfoot attributable to daily changes in weather factors were determined during five periods of the year. </p><p>The technique of measurement included replication in time as well as space. </p><p>Marked differences in seasonal growth rates obtained were attri-butable to seasonal changes in temperature, light, and rainfall. </p><p>Significant correlations were also obtained between daily fluctuations in growth rate of the species and short-term (daily) changes in weather. Time-lag correlations of as much as 4 days were also shown for some of the weather factors. </p><p>Fluctuations in growth rate attributable to components of the daily weather were generally of a greater magnitude than growth increments attributable to smoothly changing weather in each experimental period. </p><p>The results are discussed in terms of the relative importance of the various weather factors on growth in this environment and of the length of the periods of adaptation required before changes in weather factors are expressed as changes in dry matter increments in the field. </p><p>The technique of measurement is also assessed. </p><p>INTRODUCTION </p><p>The experimental data available on the comparative growth of pasture and crop species as influenced by short-term changes in climatic factors have been obtained from controlled environment studies such as those described by Mitchell (1954, 1955), Went (1958), Evans (1963), Thorne et al. (1967), and others; from measurements and calculations of photosynthetic capacities and energy balances of leaves and leaf canopies (Gaastra 1958; Lemon 1963; Muramoto et at. 1965; and others); and from field m~asurements of variations in growth com-ponents such as Net Assimilation Rate and Relative Growth Rate (Blackman et aZ. 1955; Black 1955; Watson 1958; and others. These data have then been used to estimate yield potentials and climatic optima of the different species (Mitchell 1956; De Wit 1959, 1965; Loomis and Williams 1963; Monteith 1965). </p><p>* Grasslands Division, D.S.I.R., P.B., Palmerston North. t Late of Applied Mathematics Division, D.S.I.R., P.B., Palmerston North. </p><p>N.Z. JI agric. Res. (1969),12: 125-36 </p></li><li><p>126 Weather fluctuations and grass growth </p><p>As a complement to the above approaches to evaluating weather influences on plant growth a field measurement technique based on a method of constant fitting applied via a suitable mathematical model and experimental design (Glenday 1955, 1959) was used to determine the magnitude of the effects of weather changes on the rate of growth of pasture species when grown in association in the field (Brougham 1955, 1959). In these investigations positive correlations were obtained b~tween weekly fluctuations in growth rate and weather factors and between seasonal trends in growth rate and light and temperature. </p><p>The study reported in this paper was an attempt to extend the scope of this field measurement technique to evaluate the effects of short-term changes in some weather factors on the daily growth rates of pure stands of three pasture species commonly used in New Zealand. </p><p>EXPERIMENTAL </p><p>The experimental work was carried out at the Grasslands Division, Department of Scientific and Industrial Research, Palmerston North, on an area of land that had previously been in single-spaced plants for more than 5 years. </p><p>Pure stands of perennial ryegrass (Latium perenne L. var. 'Grass-lands Ruanui'), short-rotation ryegrass (Lotium perenne X L. multi-florum var. 'Grasslands Manawa'), and cocksfoot (Dactylis glomerata L. var. 'Grasslands Apanui') were established after sowings in March 1959. Because of the high degree of sward uniformity required, the seed-bed was intensively prepared and levelled to prevent surface water from accumulating, pre-emergence weed spraying was carried out, and the method of sowing was such that each plot area received four coverages of seed. </p><p>Grazings by sheep and mowing were employed during the 12-month establishment period, the objective being to obtain dense and evenly dispersed tiller populations of the different grass species in the plot areas. Over this period and throughout the experiment, nitrogen (as nitro-lime: 20%N) and phosphate (as superphosphate) were applied in excess of plant requirements so that growth of all species was not limited by lack of these ell(ments. At the beginning of the investigation all stands had reached the desired uniformity and were densely tillered. </p><p>The experimental lay-out included replication in time as well as in space (see Brougham 1955, 1959; Glenday 1955, 1959), there being 4 space replicates of each species and 3 time replicates. Each time replicate was of sufficient area to allow for 14 daily cuts of herbage from separate plot sizes of 2 ft X 20 ft, with due allowance for margins between each day plot to prevent edge effects. This lay-out was used for each of 5 experimental periods of 16 days, the first beginning on 21 April 1960 and the last ending on 18 December 1960. In the intervals between measurement periods the stands of each species were brought back to uniformly dense swards by frequent mowings. The total experimental area was IVr; acres. </p></li><li><p>R. W. BROUGHAM AND A. C. GLENDAY 127 </p><p>Before measurements began in each experimental period all areas were carefully pre-trimmed to a base height of herbage of 1 in. with a Dennis motor-mower. After one week of regrowth the first time replicate of each space replicate was re-trimmed. This was designated day 0 of that experimental period. On day 1 the second time replicate was trimmed and the first plot of the first time replicate was cut. On day 2 the third time replicate was trimmed and the second plot of the first time replicate and the first plot of the second time replicate were cut. On day 3 plots were cut in each time replicate. Plots were then cut on each succeeding day until 14 plots had been cut in each time replicate. except in the early summer, when only 11 plots were cut in each replicate. All cuttings were to the base height of 1 in. and all plots were cut within an hour of sunrise, except in the winter on the days when the herbage was frosted. On these days the plots were cut approximately 2 hours after sunrise when the frost had melted. Measurements were confined to the hour about sunrise so that the daily growth increments measured were the net gains from photo-synthesis of the previous day less respiration losses during the hours of darkness. </p><p>For each plot all the herbage cut was collected, weighed green, then sub-sampled for dry weight determinations, except in the early stages of regrowth, when all material collected was dried. The material was dried in air-draught ovens at a temperature of 180F. Weighings of all material were to an accuracy of 0.5%. </p><p>The weather records presented, with the exception of the radiation measurements, were obtained from a meteorological station approx-imately 100 yd from the experimental site. The instruments used in this station are standard equipment for New Zealand (New Zealand Meteoro-logical Official Circ. No. 18, June 1943). The radiation measurements were obtained from an instrument designed to integrate short-wave radiation incident on a horizontal surface on a relative daily scale (Allan and McCree 1954). It was sited on the experimental area and required manual reading (at the time of cutting). </p><p>It is realised that the weather measurements against which growth has been correlated in this study are not the environmental conditions that prevailed at the synthesising sites in the leaves of the different species. However. it is considered that the correlations against standard meteorological data (those available in agriculture) are important, parti-cularly when related to data such as those obtained by Lemon (1963), Slatyer (1965), and others. </p><p>RESULTS </p><p>The dry matter (D.M.) yields (not presented) obtained for each species during each experimeptal period were separated into growth parameters for smoothly ch~.nging weather over these periods and growth that was considered attributable to daily fluctuations in weather ("w" parameters) by the method of Glenday (1955, 1959). These separations, which were made by the use of an IBM 1620 computer program, are shown for each experimental period in Fig. 1. </p></li><li><p>-GJ '-0 0 </p><p>.......... ..c ........ (J') </p><p>0 -I W </p><p>&gt;-a: W I-I--a: 0 </p><p>128 W eat her fluctuations and grass growth 500 560 </p><p>LATE WINTER -</p><p>400 EARLY SPRING </p><p>400 </p><p>Growth" curves </p><p>300 300 </p><p>200 200 </p><p>100 100 </p><p>.50 </p><p>~ 50 </p><p>~ 0 . PER .. 0 '50 </p><p>~ '50 </p><p>0 0 ~ '50 ! 50 CO~ </p><p>0 0 </p><p>-50 - 50 21 AUG 4 SEPT 1B </p><p>DEC </p><p>MIDWINTER </p><p>100 .. Growth" curves. " cJc"''''' 2t)~ ,</p></li><li><p>R. W. BROUGHAM AND A. C. GLENDAY 129 </p><p>Correlation coefficients were then calculated for the "w" parameters of each experimental period and various components of the daily weather. These were maximum and minimum temperatures, relative radiation receipt, and daily rainfall for the last experimental period (early summer). At other periods of measurements soil water levels were adequate for growth. It should also be noted that no attempt was made to evaluate the combined effects of the different weather factors on the growth of the three species. </p><p>So that lag responses to weather factors could also be assessed, correlation coefficients were calculated between these and the "w" para-meters 1, 2, 3, and 4 days removed. </p><p>The correlation coefficients, where significant, are shown for each species during each experimental period in Table 1. So that correlation patterns could be better determined, coefficients significant at the 10% level or greater are presented. </p><p>To determine the seasonal changes in growth rate of the three species, average and maximum daily growth increments were calculated for each experimental period from the data presented in Fig. 1. These data and some average daily weather data for each experimental period are shown in Table 2. </p><p>DISCUSSION </p><p>The growth increments measured in this study and the fluctuations in growth attributed to daily changes in weather were determined during the early stages of regrowth after defoliation. Daily growth rates had not reached the maxima possible in any of the experimental periods (Fig. 1), as interception of incident light energy by the foliage in the different stands would not have been complete except perhaps for the last 2 or 3 days in the spring (Brougham 1956). The results obtained therefore apply to the effects of weather changes on growth during the early stages of regrowth. </p><p>For all periods except the winter and winter--early spring the smoothed growth curves of the three species conformed to the early stages of the sigmoid pattern of growth previously described (Brougham 1955, 1959). For the other two periods, and particularly in mid-winter, the relationships between growth and time were linear. This is attributed to the marked limits imposed on growth processes such as photosynthesis by the colder temperatures and low radiation that prevailed over the later days of this period (see Table 2), thus restricting daily D.M. increases to constant but reduced increments. </p><p>Marked seasonal changes in growth rate have been recorded (Table 2). These ranged from average rates of 8-11lb D.M./acre/day in mid-winter to 77-80 lb in mid-spring. The maximum rates ranged from 14-21 lb D.M./acre/day in mid-winter to 103-134Ib in mid-spring. 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These varied with time of the year. In mid-winter daily fluctuations in the "w" parameters were as great as 30 lb D.M./acre and at the other extreme in mi

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