'Bigger data' on scale-dependent effects of invasive species on biodiversity cannot overcome confounded analyses: a comment on Stohlgren & Rejmánek (2014).

A recent study by Stohlgren & Rejmánek (SR: Stohlgren TJ, Rejmánek M. 2014 Biol. Lett. 10. (doi:10.1098/rsbl.2013.0939)) purported to test the generality of a recent finding of scale-dependent effects of invasive plants on native diversity; dominant invasive plants decreased the intercept and increased the slope of the species-area relationship. SR (2014) find little correlation between invasive species cover and the slopes and intercepts of SARs across a diversity of sites. We show that the analyses of SR (2014) are inappropriate because of confounding causality.

Invasive plants have scale-dependent effects on diversity by altering species-area relationships.

Although invasive plant species often reduce diversity, they rarely cause plant extinctions. We surveyed paired invaded and uninvaded plant communities from three biomes. We reconcile the discrepancy in diversity loss from invaders by showing that invaded communities have lower local richness but steeper species accumulation with area than that of uninvaded communities, leading to proportionately fewer species loss at broader spatial scales. We show that invaders drive scale-dependent biodiversity loss through strong neutral sampling effects on the number of individuals in a community. We also show that nonneutral species extirpations are due to a proportionately larger effect of invaders on common species, suggesting that rare species are buffered against extinction. Our study provides a synthetic perspective on the threat of invasions to biodiversity loss across spatial scales.

Contribution of sunflecks is minimal in expanding shrub thickets compared to temperate forest.

Ecological consequences of shrub encroachment are emerging as a key issue in the study of global change. In mesic grasslands, shrub encroachment can result in a fivefold increase in ecosystem leaf area index (LAI) and a concurrent reduction in understory light and herbaceous diversity. LAI and light attenuation are often higher for shrub thickets than for forest communities in the same region, yet little is known about the contribution of sunflecks in shrub-dominated systems. Our objective was to compare fine-scale spatial and temporal dynamics of understory light in shrub thickets to the light environment in typical forest communities. We used an array of quantum sensors to examine variation in diffuse and direct light and determined the relative contribution of sunflecks during midday in Morella cerifera shrub thickets, a 30-yr-old abandoned Pinus taeda plantation, and a mature, second-growth, deciduous forest. Instantaneous photosynthetic photon flux density (PPFD) was measured at 1-s intervals at five sites in each community during midday. In summer, understory light during midday in shrub thickets was approximately 0.8% of above-canopy light, compared to 1.9% and 5.4% in pine and deciduous forests, respectively. During summer, PPFD was uncorrelated between sensors as close as 0.075 m in shrub thickets compared to 0.175 m and 0.900 m in pine and deciduous forests, respectively, indicating that sunflecks in shrub thickets were generally small compared to sunflecks in the two forests. Sunflecks in shrub thickets were generally short (all <30 s) and relatively low in intensity (<150 micromol photons x m(-2) x s(-1)) and contributed only 5% of understory light during midday. Sunflecks were longer (up to 6 minutes) and more intense (up to 350 micromol photons x m(-2) x s(-1)) in the two forest communities and Contributed 31% and 22% of understory light during midday in pine and deciduous forest, respectively. The combination of high LAI and relatively short-stature of M. cerifera shrub thickets produces a dense canopy that reduces both diffuse light and the occurrence of sunflecks. The lack of sunflecks may limit the number of microsites with a favorable light environment and contribute to the reduction in understory cover and diversity within the shrub thickets.

Linking leaf chlorophyll fluorescence properties to physiological responses for detection of salt and drought stress in coastal plant species.

Effects of salinity and drought on physiology and chlorophyll fluorescence were used to evaluate stress in two coastal plants, Myrica cerifera (L.) and Phragmites australis (Cav.) Trin. ex Steud. Drought and salinity stress were induced and measurements of stomatal conductance, photosynthesis, xylem pressure potential (psi) and fluorescence were conducted following treatment. The onset of stress began at 2 g l(-1) for M. cerifera, and 5 g l(-1) for P. australis, as seen by significant decreases in physiological measurements. Despite the physiological effects of salinity, there was no significant difference in dark-adapted fluorescence (F(v)/F(m), where F(m) is the maximal fluorescence in dark-adapted leaves) for either species at any salinity level. Significant decreases in the light-adapted measurement Delta F/F'(m) (F'(m) is maximal fluorescence in light-adapted leaves) occurred at 10 g l(-1) in M. cerifera and P. australis, days before visible stress was evident. The quantum yield of xanthophyll-regulated thermal energy dissipation (Phi(NPQ), where NPQ is non-photochemical quenching of chlorophyll fluorescence) increased with decreasing Delta F/F'(m). Drought studies showed similar results, with significant decreases in physiological measurements occurring by day 2 in M. cerifera and day 4 in P. australis. Differences in Delta F/F'(m) were seen by day 5 for both species, whereas F(v)/F(m) showed no indication of stress, despite apparent visible signs. Xanthophyll-cycle-dependent energy dissipation may be the underlying mechanism in protecting photosystem II from excess energy in salinity- and drought-treated plants.

Molecular phylogeny of Myricaceae: a reexamination of host-symbiont specificity.

The phylogeny of 13 species of Myricaceae, the most ancient actinorhizal family involved in a nitrogen-fixing symbiosis with the actinomycete Frankia, was established by the analysis of their rbcL gene and 18S-26S ITS. The phylogenetic position of those species was then compared to their specificity of association with Frankia in their natural habitat and to their nodulation potential determined on greenhouse-grown seedlings. The results showed that Genus Myrica, including M. gale and M. hartwegii, and Genus Comptonia, including C. peregrina, belong to a phylogenetic cluster distinct from the other Myrica species transferred in a new genus, Morella. This grouping parallels the natural specificity of each cluster with Comptonia-Myrica and Morella being nodulated by two phylogenetically divergent clusters of Frankia strains, the Alnus and Elaeagnaceae-infective strains clusters, respectively. Under laboratory conditions, Comptonia and Morella had a nodulation potential larger than under natural conditions. From this study it appears that the Myricaceae are split into two different specificity groups. It can be hypothesized that the early divergence of the genera led to the selection of genetically diverse Frankia strains which is contradictory to the earlier proposal that evolution has proceeded toward narrower promiscuity within the family.