CARBON DIOXIDE FLUX ACROSS A FOREST-FIELD ECOTONE FRANK GOLLEY, SANNE BAKKER and T O S H I H I D E HAMAZAKI Institute of Ecology, Univers#y of Georgia, Athens, Georgia 30602, U.S.4. (Received November 1990)
Abstract. CO2 flux of a deciduous forest and an old-field surface, and the ecotone between these two typical southern U.S. ecological communities, was studied in July, 1989. Rates of CO; flux were greatest in the old field and least in the forest plots. The ecotone showed the greatest variation in CO2 flux. These differences appear to be due to differences in soil surface temperature, the old field being more exposed to direct solar radiation. The ecotonal community represents a landscape property which should be considered in studies of the transfer of carbon across the soil-atmosphere boundary.
1. Introduction In large-scale environmental studies, such as the global change studies, it is necessary to classify landscape units, and then to attach to these units a variety of processes and properties. For example, if we are concerned with atmospheric levels of CO2, then we need to know the rate of CO2 flux from soil and vegetation surfaces of each landscape unit. We then multiply the area of the landscape unit by the rate process to estimate the surface flux over a specified time period. Ecologists and geographers are well aware that landscape units do not have precise boundaries. Rather, each junction between two units is a gradient of properties and processes, called an ecotone (Di Castri et al., 1988). Ecotones vary in width depending upon the property or process under study. If one is counting presence or absence of plant species, the ecotone may be relatively narrow. If one is interested in a physical or chemical process, the ecotone may be wider. Ecologists have, in some cases, proposed that an ecotone is a landscape unit with its own unique properties (Wiens et al. 1985; Gosz and Sharpe, 1989). The object of this study was to determine if the flux of CO2 from the soil surface on an ecotone between a hardwood forest and old field has unique properties. Our hypothesis was that CO2 flux per point would be more variable in the ecotone than in the forest or the field, with the average CO2 flux of the ecotone falling between forest and field average values. Hardwood forest and old field are two major landscape types in the southeastern region. Any broad, regional study will necessarily consider the ecotone between them. 2. Methods Field plots were located in a forest, old field and ecotone in Madison County, Georgia in July 1989. Each plot was 20 by 20 meters square. The forest plot was Environmental Monitoring and Assessment 22: 117-122, 1992. @ 1992 Kluwer Academic Publishers. Printed in the Netherlands.
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dominated by oaks (Quercus alba, Q. rubra), with pine (Pinus taeda), persimmon (Diospyros virginiana), hickory (Carya ovata) as codominants. The understory was open and the soil was covered by a litter of leaves and a humus layer of about 5 cm thickness. The old field was dominated by patches of lespedeza (Lespedeza striata), blackberry (Rubus) and mixed forbs such as Ambrosia and Solidago. The soil surface was covered by a light and patchy litter less than a centimeter in thickness. The old field was approximately eight years old and had formerly been a soybean field. The ecotone was located so that forest and field were equally represented. Oak branches extended into the ecotone providing some shade and oak litter occurred in patches. Lespedeza and forbs grew where full light conditions were present. An animal trail crossed the ecotone at a point where a fox or deer could walk unimpeded by oak branches. Thus, on a microscale the ecotone was characterized by patches of forest, field and mixed vegetation and litter. Subplots of 22 m 2 were chosen randomly each day, for four days, in each plot. A plastic chamber, with an area of 154 cm 2, was placed in each subplot at a location where the chamber could be forced through the litter into mineral soil. A glass jar was placed on a screen platform under each plastic chamber so that the jar did not disturb the litter surface. The glass jar had a diameter of 4.5 cm and held 20 ml of 1.0 N NaOH solution, made up daily. Following the methods of Kirita and Hozumi (1966) and Hendrix et al. (1988), we determined that this method would provide sufficient alkali to take up CO2 over 72 h. The actual time for CO2 uptake in the field was 24 h. The chamber method is useful for comparable measurements across habitats, as required in this study, because many samples can be collected and processed inexpensively. The chamber method does not measure CO2 flux rates in an atmospherically open environment. For such estimates the eddy correlation method which considers the resistences in the soil, the surface layer and free atmosphere is required. Each day the glass jars were removed from the chambers in the rooming and new jars containing fresh alkali were placed in the chambers. Alkali was titrated, using thymolphthalein indicator, after 10 ml of BaCl2 was added to the jars of NaOH. Titration was with 1M HC1. The final calculation gave mg CO2 per 24-h period per chamber. Two chambers were closed with plastic lids and represented blanks. Twenty chambers measured CO2 flux from the soil-litter surface. In a few cases, it was necessary to omit a sample from the calculations because some of the alkali was spilled when the jars were collected in the field.
3. Results Carbon dioxide flux from the soil litter was statistically different for the three plots (Figure 1) when data for all four days were combined. These data formed welldefined statistical distributions (Figure 1) for the approximately 80 determinations per plot. The highest average rate were observed in the old field and the lowest rates were in forest.
C A R B O N DIOXIDE FLUX
~2s-" , 20 15
Fig. 1. CO2 flux in mg CO2 per 24 h over four days in a deciduous forest, and an old-field plot of 20 by 20 meters square, and the ecotone between forest and field. The sample described by the data represent 80 chambers per plot, made up of 20 chambers per plot per day for four days.
These rates are within those expected for forest and field habitats (Singh and Gupta, 1977). Tile variation in rates of CO2 flux were widest in the ecotone. However, there were no significant differences between plots among days (Table I) because of the variation within the plots. Standard deviation of average CO2 flux was highest in the ecotone for each day. The per day differences were not significant because the 20 samples per plot were inadequate to represent the central tendency and variation in CO2 flux, and the environment was not constant over the four days. For example, day 1, 2 and 4 were clear and warm; on day 3 the weather was cloudy, with a trace of rainfall.
FRANK GOLLEYET AL. TABLE I COz flux (mg per 24 h) in two communities and an ecotone between them in Georgia in July, 1989. CO2 flux is for a chamber of 154 cm2. Community Date N CO2 ltux Mean SD Forest 24 18 64.5 15.4 25 19 48.0 16.8 26 17 20.0 13.7 27 16 44.1 19.0 Mean
24 25 26 27
20 19 20 20
76.9 62.6 54.8 55.2
24.7 21.1 16.4 39.3
24 25 26 27
20 20 18 18
96.7 53.9 65.9 89.7
22.5 18.3 8.4 26.5
4. Discussion The results raise two questions that need to be answered before we can c o m m e n t on the significance of the differences between plots. First, why were differences in CO2 flux observed in the field mad forest? COs flux from soil represents the biological activity o f plant roots and soil organisms, which are, in turn, strongly influenced by the soil moisture, carbon in litter and soil, and nutrient levels. The litter quantity on the soil surface and the amount o f roots in the soil are well k n o w n to be higher in forest than in fields (Reichle, 1981). Our nonquantitative observations indicated that this pattern was also true for our plots. However, soils in fields b e c o m e heated to higher temperatures than do soils under forests because they are more directly irradiated from sunlight. M a x i m u m temperatures observed in these plots, with a m a x i m u m - m i n i m u m thermometer, were about 3°C higher in the field than in the forest ( 2 1 - 3 4 ° C forest, 2 1 - 3 7 ° C field) during the study. We conclude that difference in temperature probably caused the observed differences in COs flux o f field and forest overriding the differences in root and carbon biomass,
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which would tend to create higher CO2 output from forest surfaces. The second question is how are the characteristic field and forest enviroments distributed across the ecotone plot? The ecotone was carefully mapped meter by meter and dominant vegetation and quantity of litter determined. On a total of 400 m, 245 were dominated by forest, 108 by old field, and 47 were truly ecotonal. By truly ecotonal we mean that the square meter plot had some of the surface, but not all, covered by forest litter or overshadowed by the forest canopy. However, it is well known that enviromental conditions at an ecotone extend into the adjoining communities (Wales, 1972; Burgess and Sharpe, 1981). For example, here, in the late afternoon, sun shone into the forest a distance beyond the forest edge, and, conversely, the old field was shaded by the forest in early morning. Further, wind passes over the field and enters the forest edge creating different temperatures and moisture conditions in the ecotone. Thus, even those plots dominated by forest or field canopy or litter in the ecotone experienced different environments than the forest and field plots located about 50 meters from the ecotone. We conclude, then, that the CO2 flux from the soil and litter surface in the ecotone of forest and field is statistically more variable than that in the field or forest alone and represents a property of a real boundary community, the ecotone. The mean value of CO2 flux lies between the forest and field values. Ifa typological system (a vector as opposed to a raster system) is used to characterize the land surface in climate modelling, then these data suggest that it may be useful to treat the ecotone as a separate type with unique properties (that is, as a fuzzy vector). Otherwise, it is necessary to assume that the ecotone is made up fifty-fifty of the properties of the two adjacent systems. If a gradient approach is used in modelling, then the ecotone represents a gradient between forest and field, and the increased variation in CO2 flux is due to the special enviromental conditions observed on the gradient. In either case, it is clear that more research on the boundary conditions of landscape units would be useful, as well as theoretically interesting. 5. Conclusions A study of CO2 flux across a forest-field ecotone in July in northern Georgia showed that the meter-by-meter variation was greater in the ecotone than in forest or field plots located some distance from the boundary area. The mean CO2 flux per day was highest for the field plots, lowest for the forest, with the ecotone intermediate between them. Differences between forest and field appear to be the result of higher soil surface temperatures in the field. The higher variation of CO2 flux in the ecotone appears to be due to the unique mixture of environmental properties in this zone. Thus, if regional landscape studies of CO2 dynamics, or some other ecological process, are based on the classification of the region into landscape units, and then, the area of each unit is multiplied by a quantity representing a process characteristic of that unit, the regional value may be in error if the ecotones are ignored. Ecotones may
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well represent a unique unit in certain landscape that must be treated separately. Research is needed to determine how important ecotones are in different landscapes. In this particular example, the ecotone was selected to represent 50% forest and 50% field. However, a square meter-by-square meter analysis of the canopy and litter on the ecotone showed that forest conditions occurred on 61% of the area, field conditions on 27%, and mixed, ecotonal conditions on 18% of the area. A computerized mapping system may be expected to blur the boundaries even more. Ecotonal conditions may be expected to be especially important in a highly disected or highly disturbed landscape. For example, in the southeastern United States it is very common for forest and fields to lie side by side. Then, over time, forest species invade the field, creating a broader ecotone, which eventually becomes a forest, with the properties of forest. These ecotonal areas are ubiquitous in the region. If ecotones have properties intermediate to, and more variable than, those of the adjoining communities, then the ecotone should be treated as a landscape unit and be mapped as such.
Acknowledgements We appreciate the assistance and the loan of soil respiration equipment and chemicals, provided by RF. Hendrix and the Horse Shoe Bend Project, Institute of Ecology, University of Georgia. We also appreciate the comments of David Coleman on the manuscript.
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