Plant Ecology

Factors modulating introgressive hybridization between the red mangrove species Rhizophora mangle and R. racemosa in spatially defined sites are poorly understood. To investigate this, we evaluated the reproductive phenology and the nutrient and physiological traits in those two species and their F1 hybrids genotyped with microsatellite data across a natural hybrid zone from the Pacific coast of Panama. We found no evidence that reproductive phenology represents a barrier to gene flow, because R. mangle and the F1 hybrids produced flowers and propagules throughout the annual cycle, while R. racemosa flowered only in the dry season. Soil nutrient concentrations decreased landward, while soil salinity varied only slightly. Foliar nutrients and δ15N signatures varied according to the soil nutrient gradient, but only foliar phosphorus and carbon varied among species. In contrast, two structural variables (height and trunk diameter) and leaf variables related to salinity tolerance (Na, Cl:Na, K:Na, cation:anion) and water-use efficiency (i.e., δ13C) differed among species, suggesting higher salinity tolerance for R. mangle and F1 hybrids compared with R. racemosa. We conclude that parental species and F1 hybrids differ in salinity tolerance and water-use efficiency, which could be associated with adaptive evolution of the red mangrove hybrid complex.

Spatial distribution of plants and seeds of Trichocereuspasacana was studied in relation to bush species abundance, and ofcacti in relation to the available number of seeds under bush species, in LosCardones National Park, Argentina. Randomization tests, carried out to find thesignificance level of Chi squared test statistics used to test fornon-random association hypothesis, proved that T.pasacana plants, seeds, and seedling establishment is related to thedifferent bush species that co-occur with it. Positive interactions werefound mostly with L. divaricata, but also withP.ferox and A. spartioides, suggesting that theyact as nurse plants. Negative interactions were found with B.boliviensis, A. spartioides, S.crassiramea, and Verbena sp, in spite of thehigh seed storage beneath their canopies. Cacti are more frequent than expectedin relation to seed relative abundance under L. divaricataand P. ferox and less frequent under P.rougesii, B. boilviensis,Verbena sp, G. glutinosa,S.crassiramea, and A. spartioides, due todifferential seed or seedling mortality when emergence occurs.

The way competition structures plant communities has been investigated intensely over many decades. Dominance structures due to competitive hierarchies, with consequences for species richness, have not received as much experimental attention, since their manipulation is a large logistic undertaking. Here the data from a model system based on bryophytes are presented to investigate competition structure in a three-species system. Grown in monocultures, pairwise and three-species mixtures under no and high nitrogen supply, the three moss species responded strongly to treatment conditions. One of them suffered from nitrogen fertilisation and hence performed better in mixtures, where the dominant species provided physical shelter from apparently toxic nitrogen spray. Accordingly, no linear competitive hierarchy emerged and qualitative transitivity remains restricted to the unfertilised treatments. Faciliation also affected other properties of the competition structure. The reciprocity of competition effects could not be observed. Moreover, the performances in three-species mixtures were not well predictable from the knowledge of monocultures and pairwise mixtures because competitive effects were not additive. This had implications for community stability at equilibrium: all two-species systems were stable, both fertilised and unfertilised, while the three-species system was only stable when fertilised. This stability under fertilisation has probably to do with the facilitative effect of the two dominant species on the third. In this experiment, little support for commonly held ideas was found about the way competition in plant communities is structured. On the other hand, this study shows that moss communities are ideal model systems to test predictions of theoretical models concerning properties and consequences of competition in plant communities.

Prior land-use history might influence damage and recovery of plant communities from natural disturbance. We examined effects of previous agricultural land use on damage and recovery of plant communities affected by Hurricane Georges. The study was conducted in the karst region of Los Haitises National Park, Dominican Republic. We compared pre- and post-hurricane stem density, basal area and diversity of woody vegetation in sites within the park that had been subject to different land use histories. The type of land uses included a wide range of histories, ranging from abandoned pastures and conucos (mixed plantings) to cacao plantations, intact forests and mogotes (hilltop communities) with no recent history of land use. Previous land use and the amount of basal area present prior to the hurricane determined effects of, and recovery from hurricane disturbance. Systems with high pre-hurricane basal area lost many large trees, whereas the basal area hardly changed in systems without large trees. Thus, basal area decreased at forested sites, mogotes, and cacao plantations and remained comparable to pre-hurricane figures in all other land uses. Shifts in species diversity paralleled to some degree reductions in basal area. Species diversity increased in mogotes and cacao plantations, perhaps as a result of hurricane damage to the forest canopy, which facilitated regeneration of heliophilic species. Finally, regeneration of cacao seedlings in former cacao plantations, and growth of pioneer species in young conucos (mixed plantings) more than doubled post-hurricane stem densities for these two types of land use. Previous agricultural land use left a lasting impression on the structure and composition on plant communities, which persisted even after hurricane damage.

Analysis of aerial photographs indicates that woody plant biomass has rapidly increased in Hluhluwe Game Reserve over the last 40 years. Open Acacia nilotica savanna is being replaced by broadleaf species, especially Euclea spp. We were interested in whether this secondary successional shift was due to high numbers of seedlings establishing and growing to maturity under acacias (facilitation) or due to the release of already established, but suppressed individuals (‘gullivers’) of the resprouting broadleaf species. We examined the recruitment patterns and size-class distributions (height, basal diameter) of important species in this savanna. Densities of euclea seedlings (<0.6 cm basal diameter) under Acacia nilotica were low (median of 0 and mean of 0.06 m2) below adult canopies and effectively zero in adjacent interspaces. No differences in numbers of other broadleaf species were found between open sites and under Acacia nilotica sites. Few large eucleas or other broadleaf species occurred under Acacia nilotica. Few Acacia nilotica recruits were found either under adults or in the open whereas Acacia karroo recruits were more common. The overall size class distribution for eucleas was dominated by individuals in the intermediate size class, suggesting that recruitment is not the dominant demographic process. We propose that the escape of intermediate sized eucleas from the fire trap has caused the increase in woody plants. Analysis of a time sequence of aerial photographs shows that invasion occurred rapidly between 1954 and 1975. A common feature for sites where woody plant invasion has taken place, was the presence of barriers to fire (especially roads). We suggest that the recent and rapid increase in woody vegetation is due to a decrease in the frequency of intense fires, rather than the recent absence of megaherbivores which allowed Acacia nilotica establishment. Few intense fires allow suppressed tree or shrub individuals, ('gullivers'), to escape the grass/fire layer and thereby become tall and fire-resistant. This release may explain the rapid rate of invasion by inherently slow growing broad-leaf species.

Through tabular and average linkage cluster analyses, 737 phytosociological relevés were classified. Based on these relevés, we described and typified the associations, alliances, orders, and classes grouping the coastal plant communities of the Baja California peninsula. Diagnostic tables, classification by average linkage clustering, and climatic, edaphic, and biogeographical data were used to establish floristic affinities among these syntaxa and to interpret their distributions. Syntaxa were characterized by their floristic composition, physiognomy, and biogeographical distribution, along with their positions in halophilous and psammophilous gradients. Thirty-three associations were identified, of which 22 are here described for the first time. Our syntaxonomical proposal includes descriptions of nine new alliances, seven new orders and four new classes: Allenrolfeetea occidentalis, Atriplici julaceae–Frankenietea palmeri, Euphorbio leucophyllae–Sporoboletea virginici and Achyronichio cooperi–Abronietea villosae.

When elevational gradients include combinations of different climatic gradients, such as a decline in temperature combined with an increase in moisture, vegetation and plant trait responses are difficult to explain. Here, we used plant species richness and morphometric traits data across steep elevational gradients in the Helan Range of Northwestern China in an attempt to separate general trends (temperature related) from regional peculiarity (moisture related). Based on the floristic data of the Helan Range as well as plot-based data, both drought (at low elevation) and low temperature (high elevation) are associated with low species richness presumably also explaining the peak in diversity at mid-elevation, where climatic conditions are moderate. However, this mid-elevation peak in diversity is not mirrored in trends of plant traits such as leaf size and inflorescence size, which show either unidirectional trends or no change with elevation (with impacts of drought and low temperature perhaps gradually replacing each other). Our analysis illustrated the taxonomic and plant functional type (PFT)-related biases in functional trait studies and showed that consistent patterns only emerge after careful data stratification, with taxonomy (family level) holding more promises than PFTs. Inflorescence size increased with elevation in major insect-pollinated families, a trend not seen in wind-pollinated (graminoid) families. However, the reproductive effort expressed as inflorescence/leaf length ratio increases with elevation in the majority of plant families, irrespective of their pollination system. The fact that these biometric responses to elevation do not correlate with responses in species richness (which peaks at mid-elevation) may reflect contrasting drivers of trait selection and biodiversity. Based on our plot-based data, this analysis also confirmed the usefulness of floristic archive data for testing ecological theory related to elevational gradients.

Little is known about the importance of the forest overstorey relative to other factors in controlling the spatial variability in understorey species composition in near-natural temperate broadleaved forests. We addressed this question for the 19 ha ancient forest Suserup Skov (55°22′ N, 11°34′ E) in Denmark, one of the few old-growth temperate broadleaved forest remnants in north-western Europe, by inventorying understorey species composition and environmental conditions in 163 100 m2 plots. We use unconstrained and constrained ordinations, variation partitioning, and Indicator Species Analysis to provide a quantitative assessment of the importance of the forest overstorey in controlling understorey species composition. Comparison of the gradients extracted by unconstrained and constrained ordinations showed that the main gradients in understorey species composition in our old-growth temperate broadleaved forest remnant are not caused by variability in the forest overstorey, but are related to topography and soil, edge effects, and unknown broad-scale factors. Nevertheless, overstorey-related variables uniquely accounted for 15% of the total explained variation in understorey species composition, with the pure overstorey-related (Rpo), topography and soil (Rpt), edge and anthropogenic disturbance effects (Rpa), and spatial (Rps) variation fractions being of equal magnitude. The forward variable selection showed that among the overstorey-related variables understorey light availability and to a lesser extent vertical forest structure were most important for understorey species composition. No unique influence of overstorey tree species identity could be documented. There were many indicator species for high understorey light levels and canopy gap centres, but none for medium or low light or closed canopy. Hence, no understorey species behaved as obligate shade plants. Our study shows that, the forest overstorey has a weak control of understorey species composition in near-natural broadleaved forest, in contrast to results from natural and managed forests comprising both conifer and broadleaved species. Nevertheless, >20% of the understorey species found were indicators of high light conditions or canopy openings. Hence, variability in canopy structure and understorey light availability is important for maintaining understorey species diversity.

The role of harvester ants in Mediterranean grassland and scrubland has mostly focused on seed consumption. However, recent studies have reported their role as accidental dispersal agents of some of the collected seeds via refuse piles. The objective of this study is to examine the effect of the ant Messor barbarus on seed availability and dispersal of one of its major diet components, Lavandula stoechas subsp. pedunculata, in scrubland, grassland and the ecotones between them. After confirming and quantifying the Lavandula contribution to M. barbarus diet, we described the spatial and temporal patterns of pre- and post-dispersal seed predation, seed content and seedling occurrence in the refuse piles. Our results show that: (1) Lavandula propagules constitute a high proportion of the prey items collected by M. barbarus, with particularly intense collection activity in mid-summer, spring and autumn, in decreasing order. (2) Pre-dispersal predation rate was significantly higher in the ecotone than in the scrubland (76% and 13.5% of total seed production lost respectively). (3) Season and propagule type (seed vs. fruit) were the most significant variables explaining the post-dispersal predation probability, which approached 100% of seeds after 48 h in mid-summer. (4) Viable Lavandula seeds were found in refuse piles at densities of 0.06–0.2 per g of refuse pile material, or 58.8–207.2 per refuse pile. On the one hand, these results indicate that the ecotones are most affected by M. barbarus pre-dispersal consumption, which may locally limit Lavandula colonisation. On the other hand, the small proportion of consumed seeds that is dispersed to refuse piles may be relevant at the population level, as this dispersal implies arrival at potentially favourable sites for establishment.