Peter Haase

Spatial pattern analysis in ecology based on Ripley's K-function: Introduction and methods of edge correction

Spatial pattern analysis based on Ripleys K-func- tion is a second-order analysis of point patterns in a two- dimensional space. The method is increasingly used in studies of spatial distribution patterns of plant communities, but the statistical methods involved are sometimes poorly understood or have been modified without evaluating the effects on re- sults. The procedures of field data acquisition, statistical analy- sis, and the test for the null hypothesis of complete spatial randomness are described and the presentation of results is discussed. Different methods of edge correction were tested on a computer-generated random pattern and a mapped distri- bution of a Mediterranean shrubland. The inclusion of buffer zones around mapped plots describes the spatial pattern most accurately, but may not warrant the additional labour in- volved. Three variations of the weighted edge correction yielded comparable results for the distribution patterns tested. The toroidal edge correction may give biased results for non- random patterns. Recommendations for standardisation of the statistical procedures and data presentation are given.

Facilitation and succession under the canopy of a leguminous shrub, Retama sphaerocarpa, in a semi-arid environment in south-east Spain

frequently with a growth of herbs in its understorey which contrasts with the surrounding areas by its higher biomass and diversity. We examined the relationships between the shrub and the herbs underneath along a gradient of shrub age. A total of fifty individuals were selected to fit five age classes and shrub characteristics, soil properties and flora under the canopy were examined along the estimated chronosequence. All shrub size variables increased with time, as did the amount of nutrients stored by the shrub, but differences were often significant only between the three oldest classes. Concentration of nitrogen and phosphorus in photosynthetic stems remained constant, but nutrient pools in stem biomass increased with time. The shrubs changed the soil environment under their canopies with age by ameliorating soil texture, nutrient content and capacitance of water. The improvement was most pronounced between the two oldest classes (IV and V), and was probably due to the high biomass of perennial species in the understorey which increased the production of litter, and the interception of wind-blown dust. Plant diversity in the understorey increased with shrub age, likely due to a greater heterogeneity under larger canopies. Drought-resistant species, typical of the open areas between shrubs, were displaced from the centre of the understorey by taller, more mesic species over the age gradient. Retama sphaerocarpa shrubs benefited from the increase of resources in the understorey and showed a higher reproductive output with shrub age, but decreased at the beginning of senescence. The indirect interactions between R. sphaerocarpa and its understorey herbs could be considered as a two-way facilitation in which both partners benefit from their association.

Spatial patterns in a two-tiered semi-arid shrubland in southeastern Spain

Single species and bivariate distribution patterns in a semi-arid shrubland in southeastern Spain, dominated by the tall leguminous shrub Retama sphaerocarpa, were investi- gated by second-order spatial analysis based on Ripleys K- function. Shrubs were significantly clumped because of a strong association of dwarf shrubs, mostly Artemisia barrelieri, under the canopy of Retama. Retama shrubs were randomly distributed, but when different size-classes were analysed separately, the pattern changed from significantly clumped to random and then to regular with increasing canopy diameter, suggesting increasing intraspecific competition with shrub size. Artemisia was significantly clumped at all scales because of aggregation under the canopy of large Retama shrubs. The association between the species became stronger with increas- ing canopy diameter of Retama shrubs, suggesting that facili- tation prevailed over interspecific competition because of niche separation in different tiers, both above and below ground. Retama shrub size thus determined both the type of pattern for its own size class and tier, and the scale and intensity of the association with its understorey shrubs.

An investigation of rooting depth of the semiarid shrub Retama sphaerocarpa (L.) Boiss. by labelling of ground water with a chemical tracer

1. (1) Water containing lithium chloride as a chemical tracer was applied to the bottom of two survey wells 16 and 28 m deep and the concentration of Li+ in young cladodes of Retama sphaerocarpa shrubs growing around the wells was monitored for 9 days after the subterranean application. 2. (2) The mean concentration of Li+ in the young cladodes of the shrubs increased from a natural background of 1.8 ± 0.8 μg Li+ g−1 dry mass to 91 ± 18 μg g−1 within 24 h of application. 3. (3) The mean concentration of Li+ was significantly higher in shrubs around the shallower well where a maximum of 184 ± 52 μg g−1 was measured 5 days after application. 4. (4) At the deeper well, mean concentration of Li+ averaged over the 9 day period of observation was positively correlated with shrub size.

Response of the tussock grass Stipa tenacissima to watering in a semi-arid environment

1. Stipa tenacissima is a perennial tussock grass of arid and semi-arid zones around the Mediterranean basin that occupies extensive areas in south-eastern Spain. The environment of this region is extreme, with low irregular rainfall, high temperature and high irradiance, so that S. tenacissima tussocks are subjected to a high degree of environmental stress, especially during the summer months. 2. The ability of S. tenacissima to use a pulse of water applied in midsummer was tested in a field experiment in which extension growth and physiological performance of leaves of watered and unwatered plants were measured. 3. In unwatered plants, when leaf extension had ceased, leaf water potential, relative water content, leaf conductance and net photosynthetic rate were low and minimum fluorescence was high. 4. Leaf conductance and net photosynthetic rate doubled, leaf extension resumed, minimum fluorescence (F 0 ) fell and photosynthetic efficiency of photosystem II (F v /F m ) rose with watering. As the soil dried out again, values of these variables approached or equalled those of unwatered plants. 5. These changes in F 0 and F V /F m suggested that the photosynthetic apparatus had not been damaged permanently, i.e. that S. tenacissima has reversible photoprotective mechanisms. 6. We conclude that the decreased photosynthetic rate of unwatered plants in summer results from non-damaging photoinhibition and low leaf conductance. The latter is partly a consequence of the folding of leaves, which was inversely proportional to relative water content. 7. The arrested development of S. tenacissima leaves was apparently not due to summer dormancy because leaves responded opportunistically within days to an applied pulse of water.

Spatial pattern in Anthyllis cytisoides shrubland on abandoned land in southeastern Spain

Abstract. Univariate and bivariate distribution patterns of small shrubs on abandoned land in semi-arid southeastern Spain were investigated by second-order spatial analysis (Ripleys K-function). All shrubs (Anthyllis cytisoides and subdominant Artemisia barrelieri) were either randomly distributed or clumped at scales of 0.25 - 1.0 m. The pattern shown by A. cytisoides shrubs alone changed from clumped to random with decreasing density. Pattern analysis and demographic data suggest a successive replacement of A. barrelieri, which had high proportions (44 - 86 %) of dead shrubs, by the dominant A. cytisoides. In two of three plots there was a positive association between A. cytisoides and A. barrelieri at a scale of 0.25 - 0.5 m. In the third plot, believed to represent a more advanced stage of colonization, there was a negative association (repulsion) between the two species, presumably as a result of interspecific competition from A. cytisoides. Dead shrubs present in spring 1994 were randomly distributed in all plots. Living and dead A. cytisoides shrubs were positively associated at a scale of 0.5 m, suggesting that the shrubs died as a result of intraspecific competition in small clumps. The shrubland previously dominated by A. barrelieri is changing to A. cytisoides shrubland with increasing biomass and ground cover.

Dynamics of cohorts of cladodes and related effects on reproduction in the shrub Retama sphaerocarpa in semi-arid south-eastern Spain

Changes in the size and age structure of the canopy of the leguminous shrub Retama sphaerocarpa in semi-arid south-eastern Spain were investigated by monitoring growth and survivorship of cladodes (photosynthetic stems) on marked shoots over a period of 26 months. Three basic morphological types of cladodes – long shoots, short shoots, and secondary short shoots – were distinguished.The canopy of the shrubs contained three annual cohorts of cladodes in spring and summer. The number and size of shoots produced each year was highly variable and was apparently related to the amount of rainfall during the preceding cool season. The spring cohort of 1994 produced only 37% of cladodes compared with 1993. Two cohorts of shoots were actually initiated in spring and late summer of each year, but the second cohort produced only 2–12% of the number of cladodes compared with the spring cohort. The proportions of the different cohorts in the canopy changed rapidly from April to August, but only slowly during the remainder of the year when only two annual cohorts remained after extensive litterfall in late summer. This late summer litterfall caused a substantial reduction in green canopy area (40–50%) which was achieved mainly be shedding of one year old cladodes. The life expectancy of cladodes decreased with increasing order of their morphological type from 850 ± 25 days in long shoots to 563 ± 4 days and 546 ± 9 days in short shoots and second order short shoots, respectively.Flowering and fruiting took place from May to July, almost exclusively on one year old cladodes, and coincided with the maximum development of the canopy. Flowering intensity was high in 1994, when individual shoots supported a mean number of approximately 150 flowers. Shoots produced an average of 12.6 ± 0.6 and 5.3 ± 1.0 fruits per shoot in 1993 and 1994, respectively. Most of the annual fruit crop (80–90%) was shed during litterfall in late summer. A proportion of 10–20% was retained in the canopy for up to 12 months, however, with some fruits persisting for more than 22 months.