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Plant Ecology 00: 1–5, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands.
Application of randomization methods to study the association of Trichocereus pasacana (Cactaceae) with potential nurse plants M.L. de Viana 1,*, S. Sühring 2 and B.F.J. Manly 3 1
Cátedras de Ecología, Universidad Nacional de Salta, Buenos Aires 177, 4400, Salta, Argentina; 2Cálculo Estadístico, Universidad Nacional de Salta, Buenos Aires 177, 4400, Salta, Argentina; 3Dpto Matemáticas, Universidad de Otago, Dunedin, New Zealand; *Author for correspondence (
[email protected]) Received 30 October 1998; accepted in revised form 28 August 2000
Key words: Los Cardones National Park, Nurse palnts, Randomization, Seed bank, Spatial distribution Abstract Spatial distribution of plants and seeds of Trichocereus pasacana was studied in relation to bush species abundance, and of cacti in relation to the available number of seeds under bush species, in Los Cardones National Park, Argentina. Randomization tests, carried out to find the significance level of Chi squared test statistics used to test for non-random association hypothesis, proved that T. pasacana plants, seeds, and seedling establishment is related to the different bush species that co-occur with it. Positive interactions were found mostly with L. divaricata, but also with P. ferox and A. spartioides, suggesting that they act as nurse plants. Negative interactions were found with B. boliviensis, A. spartioides, S. crassiramea, and Verbena sp, in spite of the high seed storage beneath their canopies. Cacti are more frequent than expected in relation to seed relative abundance under L. divaricata and P. ferox and less frequent under P. rougesii, B. boilviensis, Verbena sp, G. glutinosa, S. crassiramea, and A. spartioides, due to differential seed or seedling mortality when emergence occurs. Introduction A number of studies carried out on arid and semi-arid regions in Mexico, Iraq, United States and Argentina have reported the importance of biotic interactions for the recruitment and survival of annuals and cactaceae (like Carnegiea gigantea, T. pasacana, Opuntia leptocaulis, Neobuxbaumia tetetzo, Mammillaria colina), that are commonly found growing beneath the canopy of shrubs and trees: the nurse plants (Went 1942; Muller and Muller 1953; Agnew and Haines 1960; Hutto et al. 1986; McAuliffe 1984; Acreche et al. 1989; de Viana et al. 1990; Valiente-Baunet et al. 1991a). Most of the hypothesis about the role of the nurse plant, have taken into account the direct positive effects of the interaction: the microenvironment produced by the nurse plant canopy, as provider of a higher soil fertility and protection from solar radiation (heat stress), from frosts, from mechanical injury, and predation (Jordan and Nobel 1981; Niering et al.
1963; Nobel 1980; Shreve 1931; Turner et al. 1966; Steenbergh and Lowe 1977; Valiente-Baunet et al. 1991a). This interaction produces a kind of non-random spatial pattern, conditioned by the presence of the nurse plant species and has been analyzed in relation to cactii needs for establishment and survival (Turner et al. 1966; Steenbergh and Lowe 1977; Nobel 1980; Jordan and Nobel 1981; Valiente-Baunet et al. 1991a, 1991b). Studies on T. pasacana suggest that in the National Park Los Cardones, the creosote bush, Larrea divaricata is the nurse plant (Acreche et al. 1989; de Viana et al. 1990), influencing its spatial pattern. However, the need for a specific microenvironment for germination and establishment to succeed, could be met by a number of plant species that are abundant in the area, such as B. boliviensis, L. nitida, P. ferox, P. rougesii, A. spartioides and F. fiebrigii. The positive cacti-bush interaction would imply that the amount of T. pasacana renewals growing beneath bush canopies must be higher than that expected by chance. The
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2 higher or lower than expected proportion of cacti growing beneath the different bush species can aid in the determination of this positive interactions between cacti and other plant species, even as detect the presence of some kind of negative interaction. Chi-squared tests were carried out to test for nonrandom pattern. Randomization procedures were done to determine the significance level of X 2 test statistic calculated from the observed set of data by comparing the test statistic value with the distribution of values that are obtained by randomly reordering the data values in some sense. There were used in this study to overcome potential problems of incorrect test size and loss of power with chi-squared tests caused by small expected frequencies in some cells, as is commonly found with this kind of data (Potvin and Roff 1993).
Materials and methods Study area This study was carried out at National Park Los Cardones (25° 23⬘ S, 65° 51⬘ W), Salta province (North Western Argentina), in Tin Tin Valley (9000 ha). Weather is arid, mean annual rainfall at Cachi (40 Km North from study site) is 146 ± 67 mm (46 years record). More than 90% of the rain falls from December through to March. Mean maximum and minimum temperatures in January (the hottest month) are 30.7 °C and 7.8 °C, and in July (the coldest month) are 27.4 °C and −7.1 °C respectively. Throughout the year, there is a high diurnal thermal amplitude, of over 20 °C (de Viana 1995). Spatial distribution of cactii and seeds The study was conducted at Tin Tin Valley, where there are two plots, one of 3 and the other of 1 ha, located 7 km apart with all the cacti (260), identified for population studies since 1987 (Acreche et al. 1989; de Viana et al. 1990; de Viana 1999). For every cacti the height and the presence or absence of association with shrub species (when the canopy of the perennial plant includes the base of the cacti), were recorded. The composition and cover of the shrub community was established by sampling the plots with six randomly oriented strip transects (100 × 1 m, tree at each plot, as reported in (de Viana 1995, 1996–1997)).
Seed bank Seed bank was determined in bare soil (18 random points) and under the canopy of the more abundant shrub species that were intercepted in the six randomly oriented strip transects (100 × 1m). Three soil samples were taken beneath the canopy of each shrub species (a total of 174 soil samples). Samples were taken with an aluminum core (10 cm diameter, 2 cm depth). Soil samples were sieved through different size wire mesh, in order to search and count T. pasacana seeds (de Viana 1999). Randomization Our hypothesis of interest are alternatives to null hypothesis of randomness, that is to say, we expected a certain type of distribution pattern to appear in cactii, seeds, and seedling establishment. We tested three null hypothesis: 1) The observed number of cacti under each nurse plant is a function of the relative cover of the different bush species, implying that cactii have a random distribution pattern with respect to the bush species. 2) The abundance of cacti seeds under each nurse plant is a function of the relative cover of the different bush species implying that cactii seed storage has a random distribution pattern with respect to the bush species. 3) The observed number of cacti is a function of the relative abundance of seeds found beneath the different bush species implying that cacti seedling establishment is random, the probability of seed establishment is the same for all microenvironments provided by the different bush species. For each of the three hyothesis we calculated the usual Chi-square test statistic. To determine the significance level of a hypothesis we generated 10,000 random sets of data (using the total number of observed cactii) and calculated for each of the random data sets the usual Chi- Square test statistic. The significance level of the test statistic was then taken as the proportion of Chi-squared values for the randomized data that are equal to or greater than the chisquared value for the observed data (Manly 1997). The randomization involved allocating the observed total number of cactii to the different microenvironments.
3 Table 1. Composition and percentage cover of the different bush species present in the study area; Percentage of cacti associated to each one (assoc). Observed number of cacti (obs) under each bush species and expected frequencies (exp) assuming that there are in relation to bush species cover. Other spp: Compositae, Salvia sp., Senecio sp. and Verbena sp. Bush species
cover
assoc
obs
exp
L. divaricata P. rougesil F. fiebrigll B. boilviensis P. ferox A. spartloides G. glutinosa S. crassiramea Other spp
50 17 5.7 5.9 2.8 1.7 9 2.6 5.3
73.08 0.76 0.76 3.08 10.77 7.70 2.32 1.15 0.38
190 2 2 8 28 20 6 3 1
130 44 15 15 7 4 24 7 14
Table 2. Observed (obs) number of cacti seeds under each bush species present in the study area and expected frequencies (exp), assuming that there are in relation to bush cover (%). Other spp: Compositae, Salvia sp. and Senecio sp. Bush species
cover
obs
exp
L. divaricata F. fiebrigii B. boliviensis P. rougesii Verbena sp. P. ferox A. spartioides S. crassiramea G.glutinosa Other spp
50 5.7 5.9 17 1.7 2.8 17 2.6 9 3.6
27 5 31 15 10 51 34 11 18 0
101 12 12 34 4 6 3 5 18 7
X 2 = 780.03; Estimated P value = 0.0006
X 2 = 237.17; Estimated P value = 0.01
Results The proportion of the population associated to other plant species, was 96%. The composition and cover of the shrub community is showed in Table 1, together with the cactii percentage distribution associated to each species. The randomization tests carried out for each of the hypothesis are summarized in Tables 1, 2 and 3 respectively; X 2 test statistic was found highly significant for every case, implying that:1) the distribution of cacti is not random with respect to bush species; 2) the distribution of seeds is not random with respect to bush species; and 3) the probability of seed germination and seedling establishment is not the same for every microenvironment. That is to say, spatial pattern of plants and seeds, and seedling establishment of T. pasacana is specifically conditioned by the different bush species that co-occur with it.
Discussion In arid communities, the factors that influence species aggregation have been studied with relation to both, physical restrictions and biotic interactions (Turner et al. 1966; Steenbergh and Lowe 1977; Yeaton 1978; McAuliffe 1984; Franco and Nobel 1989; ValienteBaunet et al. 1991b; Vetaas 1992). Most of the studies agree on the importance of microsites for seed germination and seedling establishment. However controversies still persist about the specificity of the interaction cactaceae-nurse (Arriaga et al. 1993). The
Table 3. Observed number of cactii (obs) found beneath the different bush species and expected frequencies (exp) assuming that there are in relation to seed relative abundance (seed). Bush species
seed
obs
exp
L. divaricata F. flebrigii B. boliviensis P. rougesii Verbena sp. P. ferox A. spartioides S. crassiramea G. glutinosa
0.13 0.03 0.15 0.13 0.09 0.14 0.16 0.06 0.11
190 2 8 2 0 28 20 3 6
34 8 39 34 23 36 41 15 29
X 2 = 838.40; Estimated P value = 0.0001
low percentage of cacti without association (4%), suggest the importance of the nurse plant, as was reported for other cactaceae. The results of the tests help to confirm the specificity of the interaction cactaceae-nurse. These cacti-shrub species interactions are summarized in Table 4. L. divaricata is the shrub with the higher proportion of cacti associated. However the interaction was also found with other shrubs like P. ferox and A. spartioides in spite of their low cover. The amount of cacti associated with B. boliviensis, is lower than the expected in relation to its relative abundance in spite of the abundance of cacti seeds under its canopy. The leaves of this species are perennial and with resinous compounds which could have allelopathic effects. In germination experiments with ground obtained under the canopy of B. boliviensis, cacti seeds viability was reduced (de Viana 1995). Moreover, in recent studies, two allelochemicals compounds extracted from B. bo-
4 Table 4. Spatial association ( than expected by chance, Haberman test, 5% significance level) of cactii (C) and seeds (S) in relation to plant species cover; cactii in relation to seed abundance (c/s), and shrub species characterization (Sp Ch). GF: growth form; DM: differential mortality; A?: allelopathy; D: deciduous. Bush Species
C
S
C/S
Sp Ch
L. divaricata P. ferox A. spartioides B. boliviensis S. crassiramea Verbena sp F. fiebrigii P. rougesii G. glutinosa
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