The influences of arboraceous layer on spatial patterns and morphological characteristics of herbaceous layer in an arid plant community

ACTA ECOLOGICA SINICA Volume 27, Issue 4, April 2007 Online English edition of the Chinese language journal Cite this article as: Acta Ecologica Sinica, 2007, 27(4), 1265í1272.

RESEARCH PAPER

The influences of arboraceous layer on spatial patterns and morphological characteristics of herbaceous layer in an arid plant community Zhang Qinmei1, Zhang Cheng1, Liu Maosong1,*, Yu Wen1, Xu Chi1, Wang Hanjie2 1 School of Life Science, Nanjing University, Nanjing 210093, China 2 Laboratory of Regional Climate-Environment Research for Temperate East Asia, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100085, China

Abstract: The objectives of this study were to examine the effects of arboraceous layer on the spatial pattern and morphological characteristics of herbaceous layer in Elaeagnus angustifolia–Achnatherum splendens community in Ningxia Hui Autonomous Region, China. The analyses of community composition and structural characteristics as well as the investigation of soil moisture and salinity showed that different life forms of plants differ in the soil depth at which they absorb and utilize soil moisture. Wavelet analysis showed that there were differences between the spatial patterns of A. splendens in the canopy-projected regions and other regions, and the intrinsic scales were detected. The results from the buffer analysis showed that the control of arboraceous layer on the herbaceous layer on the spatial patterns and the morphological characteristics were influenced not only by canopy shading but also by other causes such as distribution patterns of roots as the morphological characteristics did not monotonically change with distance. Key Words: gradient belts analysis; wavelet analysis; pattern; morphological characteristics; Ningxia Hui Autonomous Region

In arid and semiarid areas, the uneven distribution of precipitation and lack of nutrition often lead to declined coverage in a community composed of a continuous herbaceous layer [1,2] and a discontinuous shrub-arborescent layer . Complex functional relationships between the plants with different life [3] forms in the arid and the semiarid areas , especially the interactions between the arboraceous layer and the herbaceous layer, were extensively studied. The interests are attracted by scholars to research the relationship among the different life form and adaption strategies in this kind of community which was sensitive to the accelerated climatic changes,. In general, arbors can effectively use water, minerals, and organic matters, and exhibit considerable influences on the spatial heterogene[4] ity of nutrition and the productivity of the understory her[5] baceous vegetation in the arid communities . Moreover, arbors can reduce soil salinity and increase soil fertility to a

[6,7]

certain extent . Understory plants often show some adaptations to the microhabitat, with specific spatial patterns and morphological characteristics under the influences of arbors. There existed inconsistency in the effects of trees on grasses in arid communities. In certain instances, the productivity of the herbaceous layer is lower under tree canopies than that in the nearby open grasslands, whereas in other instances grass [6] productivity is higher under tree canopies . There are only few researches referring to the extent of the influences of the arboraceous layer on the understory herbaceous vegetation. The objective of this study was to investigate the interrelationship between the spatial patterns and the morphological characteristics of the herbaceous layer and the top arboraceous layer in a typical temperate savanna community in the central part of Ningxia Hui Autonomous Region, China.

Received date: 2006-03-27; Accepted date: 2006-11-28 *Corresponding author. E-mail: [email protected] Copyright © 2007, Ecological Society of China. Published by Elsevier BV. All rights reserved.

ZHANG Qinmei et al. / Acta Ecologica Sinica, 2007, 27(4): 1265–1272

1

Materials and Methods

1.1 Study site The study site is located in Xidatan, Pinluo County, Ningxia Hui Autonomous Region (hereafter, will be referred to in short as Ningxia), China (106°20'E, 38°45'N). It is 56 km northwest to Yinchuan, the provincial capital city of Ningxia. The elevation is about 1100 m above the sea level, and the mean annual evaporation and precipitation are 1755 mm and 172.5 mm, respectively. Approximately 70% of the annual precipitation falls between July and September. The mean annual temperature ranges between 4.7ć and 8.1ć. The mean annual relative humidity is 56%. The annual duration of sunshine varies between 2 800 h and 3 200 h. The main soil type is solonetz with high alkalescence and low salinization. The soil is fragile and poor with respect to the amount of usable calcium, or[8] ganic content, and aerial and hydraulic permeability . 1.2 Field work and data collection The community in the study site is dominated by a woody plant (Elaeagnus angustifolia) and a grass (Achnatherum splendens), with a few scattered shrubs (Sophora alopecuroides and Nitraria tangutorum) and grasses (Phragmites australis). E. angustifolia was artificially planted in the 1960s. With a natural succession of approximately 40 years, the community is relatively stable and can be viewed as a representative vegetation type in the arid and the semiarid areas of Northwestern China. Field investigation was carried out in a 50 m × 50 m sampling plot, within which all the plants were measured based on the number of plant species. The record items included the individual number (clump number for grasses), crown diameter (clump diameter for grasses), diameter at breast height (DBH), height, and location. A 100cm soil profile in the community was dug and divided into five strata at 0–10, 10 –30, 30–60cm, and 60–100 cm in depth and sampled to measure the soil water content, salinity, and physical and chemical properties in triplicates. The laboratory analysis included the following parameters: soil water content, soil organic content (estimated using the potassium dichromate oxide-heating method), and soil salinity (estimated using the conductance method). 1.3 Wavelet analysis Wavelet analysis, which is derived from the Fourier analysis, is a useful technique to identify the spatial structure in transect data. Its main advantages over other spatial analysis methods are the ability to preserve and display hierarchical [9] information while allowing for pattern decomposition . Because there are no requirements with respect to constant mean value or variance of the data, wavelet analysis has been widely used in ecological fields, such as the structure of the canopy [9,10] [11] gaps and the multistructure of urban heat island effect . Let f(t) is a squared integral function. Wavelet transform is

defined in the continuous case as:

W f ( a , b)

a



1 2

³

R

f (t )M (

t b )dt a

The wavelet transform is a collection of convolutions of the data function Wf(a,b) with a windowing function (or “wavelet”) for a given range of scales a centered at location b along the transect, and M is the mother wavelet function. Scale features can be identified through the wavelet variance. The wavelet variance is defined as (where n is the length of the transect data):

V( a )

1 n 2 ¦W f ( a , b j ) nj1

Morlet wavelet, which is one of the mostly used mother wavelet, was chose in this study. It is defined as:

M ( x) Ce



x2 2

cos5 x

Three 24m lines were marked as A, B, and C in the canopy-projected areas, and two 32-m lines marked as D and E in the noncanopy areas were randomly placed for measuring individuals that overlap with the lines (Fig. 1). Buffers with a width of 1 m along each line were prepared by the authors of this study, and then they were divided into small quadrats with the size of 1 m2 to construct five transects such that they can be analyzed by the Morlet wavelet analysis. All data processing was completed with MATLAB 6.5 software. 1.4 Gradient belt analysis To study the influences of the arboraceous layer on the herbaceous layer, the canopy-projected region of one single tree was assumed to be a circle centered at the base of the stem. Then, 20% of diametric crown was taken as an expanding unit, and buffers were prepared from these centers. With respect to the expanding unit, relative distance is more appropriate than

Fig. 1 The sketch of the crown gradient belts analysis

ZHANG Qinmei et al. / Acta Ecologica Sinica, 2007, 27(4): 1265–1272

absolute distance as because the height and the crown size vary with trees. With the extension of buffer zones, crown gradient belts covered the entire plot, and 15 crown gradient belt zones were made and named zone 1, zone 2,Ă, zone 15, respectively. The mean diameter, crown size, height, and density of A. splendens within each buffer zone were calculated using intersection analysis. To remove the edge effect, the largest diameter of the tree crown subtracted from the length of the plot (Fig. 1). All data processing was completed with ArcGIS 9.0 software.

2

Results and discussion

2.1 Composition, structure, and soil condition The plant community was located in the irrigation areas of the Yellow River with a relatively shallow water table and a considerably high coverage. The E. angustifolia trees had a mean height of 7.9 m, a mean crown diameter of 6.5 m, and a mean DBH of 29.0 cm. The total coverage of the arbor layer is up to 38.5%. The dominant species of the herbaceous layer (A. splendens) contributed to 97.8% of the total abundance. The mean diameter and the mean height of the clumps of A. splendens in the plot were 7.2 cm and 43.8 cm, respectively. Soil moisture increased with the depth of the soil. At the depth of 30–100 cm, soil moisture showed considerable dif-

ference. There was a high electrical conductivity at the 60–100 cm depth, which was up to 11.1×103 s/cm, and a special water potential gradient (water potential underlay was lower than the upper portion) was shown. In the arid areas, the precipitation is much lower than the evaporation, and the groundwater is the main water source for the plant communities, especially [12] for the species with high transpiration like E. angustifolia . The root system of E. angustifolia is widespread. The taproots of E. angustifolia can access the groundwater source in the deep soil (>100 cm), and they also block the upward moving groundwater, which cause high salinity and low soil moisture of soil at the 60–100 cm depth. Salinity at the depth of 10–30 cm was higher than that at 0–10 cm, which showed that the herbs with shallow root system, such as A. splendens, absorbed water mainly at the depth of 10–30 cm. This explains the preferential exploitation at certain soil layers by grass and others by trees. From the research on the heterogeneity of soil moisture and the vegetation in semiarid regions, concluded that the species with different [13] life forms used soil water at different depths . However, the conclusions drawn only based on the soil moisture and the salt content were inadequate, and therefore, further researches are needed. The potential gradient caused by trees against the transport of water would affect the growth and the spatial patterns of grasses.

Table 1 The physical and chemical properties of the soil Depth of soil (cm)

Soil water content (%)

Electrical conductivity (s/cm)

Soil organic matter content (%)

0–10

4.67

5.00 × 103

1.12

10–30

11.79

6.88 × 103

0.80

30–60

14.50

6.16 × 103

0.10

60–100

14.54

11.1 × 103

0.61

Fig. 2 Wavelet variance of each transects

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2.2 Influences of the arbors on the spatial patterns of the herbs One-dimensional Morlet wavelet transform was completed for five transects. In Fig. 2, each peak of wavelet variance [14] represents a specific analysis scale , where the corresponding curves of wavelet transform can be obtained to detect the characteristic scale with respect to the distribution patterns of A. splendens (Fig. 3). From the curves of wavelet variance for lines A, B, and C, wavelet peaks appeared, where a equaled to five. The curves of wavelet transform showed an obvious periodicity, and the periodic lengths were 6, 6 m, and 6.5 m, respectively, with an average of 6.2 m. For lines B and C, peaks of wavelet variance appeared, where a equaled to 13 and 14, but the waveforms were too simple to provide any available information. For lines D and E, peaks appeared, where a equaled to 6, 11, and 10, and the characteristic scales of 8 m, 13 m, and 12 m were

detected from the corresponding curves of wavelet transform. Consequently, A. splendens showed a characteristic scale of approximately 6.2 m in the canopy-projected areas, which was close to the average crown size of E. angustifolia (6.5m). Whereas in the open areas, A. splendens showed characteristic scales of 1 and 1.5 times of the average height of the trees. Such difference reflected the remarkable influence of the arboraceous layer on the spatial patterns of the herbaceous layer. 2.3 Influence of arbors on the morphological characteristics of the herbs The results from the investigation of the community and the wavelet analysis showed that the distribution of E. angustifolia had dominant effects on the spatial patterns of A. splendens of the understory layer. Furthermore, the relationship between the morphological characteristics of A. splendens and the distance from E. angustifolia was studied using crown gradient belts analysis (Fig. 4).

Fig. 3 Curves of Morlet wavelet transform at the specific analysis scales

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Fig. 4 Mean diameter (a) crown size (b) height (c) density, and (d) of A. splendens along the zones

The results showed that the curve fluctuated to a small degree from zone 1 to zone 5, which suggested that A. splendens had no obvious differences in morphology. From zone 6 to zone 10, the average crown size and the height of A. splendens gradually increased along the crown gradient, and the average diameter also showed an increasing tendency, despite some fluctuation. While this trend changed in zone 10: the average diameter and the crown size began to decrease, and rapidly increased in zone 14, and then the average diameter, the crown size, and the height reached the maximum, which was 12.73, 71.75cm, and 88.16 cm, respectively. In contrast, the density showed a decreasing tendency. Some studies showed that trees had considerable influences, which was determined by the canopy shape, the canopy structure, and the tree species, on the growth and the distribution of [15ˉ17] the understory vegetation . In general, such influences can be viewed as a result of the effect on the light condition produced by tree canopies. Whereas in the semiarid regions, because of the abundant sunshine, strong radiation, rare vegetation, and drought conditions, water becomes the most important stress factor and has a significant effect on the survival, [18ˉ20] the shape, and the net primary productivity of the plants . Previous researches indicated that the tree canopies could reduce solar radiation by 45%–65%, soil temperatures by 5–11ć, increase the content of soil total nitrogen, total phos[4,5] phorus, and total kalium compared with the open areas . In the study site, soil water content in the top and shallow soil layers were 6.77% and 4.67%, whereas in the open area it was 0.92% and 2.25%, respectively. These results agreed with those of Akpo’s (1997) study on the arid plants in Sahelian savannas. He reported that the soil moisture content was

higher under tree canopies than that in the open area. Some researchers showed that in the arid and the semiarid regions, the soil resource such as soil water, nutrient, and microbe al[21ˉ25] ways accumulated under tree canopies . Because of the understory condition, the density of A. splendens was much higher in the canopy-projected areas of E. angustifolia than in the open areas, but the growth of the grasses was reduced because of the sunlight deficiency. The mean leaf area index (LAI) of the tree canopies, which was measured using “SunScan canopy analysis system”, reached 2.9 in the study site. Thus the canopy could reduce the direct sunlight and produce some influence on the growth of A. splendens. With the coverage of the trees, the grasses were small and tender, meanwhile, evapotranspiration decreased because of the poor sunlight, which could resist stress of drought to some extent, thereby resulting in higher plant density. From zone 6, with the increasing distance from E. angustifolia, shadow effect of canopies gradually decreased and sunlight became abundant. The average diameter, crown size, and height began to increase, which inevitably led to the competition for soil water in the root systems. Although the distance from E. angustifolia was slightly more, soil water was not able to afford the high density of plants, so the coverage of the community dropped and the plant density decreased. Plants could form some microhabitat through clumping to decrease wind speed, increase humidity, improve efficiency of water use, and release water stress above the ground. From zone 10 to zone 13, rising tendency of A. splendens was interrupted and then increased again from zone 14. The region between zone 10 and zone 13 was the area that had a distance of 13–17m from the tree canopy. Some studies showed

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that the root hair zone of mature E. angustifolia was mostly distributed within the region that corresponds to 1–1.5 times the height of the tree canopy[26]. Numerous root fibers were found toward the end of root system mainly for absorption of water and nutrients. Competition could limit the growth of A. splendens and finally led to the decrease in the average diameter, crown size, and height.

3

search Program of China 2006CB400505 and the National Science Foundation of China (NSFC) Project 40675040.

References [ 1 ] Scholes R J, Archer S R. Tree-grass interactions in savannas. Annu. Rev. Ecol. Syst, 1997, 28: 17– 44. [ 2 ] Akpo L E. Phenological interactions between tree and understory herbaceous vegetation of a Sahelian semi-arid savanna.

Conclusions

Plant Ecology, 1997,131 (2): 241–248.

In this article, a typical savanna community was studied in an arid region of Northwest China. The analyses of the community composition and the structural characteristics as well as the investigation of soil moisture and salinity showed that the plants with different life forms absorbed and utilized soil water at different soil depths, thereby influencing the water potential gradient. The analyses of the community structure showed that the arbor projection had significant influences on the spatial patterns and the morphological characteristics of the herbs. In general, the density of A. splendens decreased with the increment of distance from E. angustifolia, and the density of A. splendens was much higher in the canopy-projected areas than that in the open areas. Morlet wavelet analysis showed that there were remarkable differences in spatial patterns of A. splendens between the canopy-projected areas and the open areas. Furthermore, the characteristic scales close to the mean size of the tree canopy were detected in the canopy-covered areas, whereas in noncanopy-covered areas, the characteristic scales were approximately 1–1.5 times the height of the tree canopy. The difference between the covered and the uncovered areas indicated that arboraceous layer had obvious dominant effects on the spatial patterns of the understory vegetation. The morphological characteristics of A. splendens showed obvious changes along the distance from E. angustifolia, but the changing trend was different in the regions that corresponded to the scales of 1.5 times the mean height of the trees. This indicated that the arbors had remarkable effects on the spatial pattern and the morphological characteristics of the understory herbs, and the complexity of the interaction of plant communities was revealed. The distribution of the arbor’s root system, the relationship between the morphological characteristics and the spatial pattern of the understory vegetation as well as the water potential gradient in the soil need to be further studied.

Acknowledgments The authors thank Mei Xu and Sheng Sheng of Nanjing University and Xiaoyan Cui and Yongmao Ju of Chinese Academy of Sciences for their support in field survey and data collection. This study was supported by the National Basic Re-

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