Opposite Tree-Tree Interactions Jointly Drive the Natural Fir Treeline Population on the Southeastern Tibetan Plateau

The long-term stability of alpine treeline positions and increased stem density are frequently reported by recent studies; however, whether a denser treeline forest is relevant to competitive tree–tree interactions remain unclear. Herein, we mapped and surveyed individual trees in two undisturbed Smith fir (Abies georgei var. smithii) treeline plots (with a size: 30 m × 200 m; plot NE1: 4477 m, NE2: 4451 m) near Ranwu Lake (RW) on the southeastern Tibetan Plateau. The surface pattern method and spatial point pattern analysis were used to detect the spatial distribution patterns of three size classes (seedlings, juveniles, adults) and spatial associations between the pairwise size classes. We also compared our results to the spatial patterns of the five other treeline forests (Deqin, Linzhi, Changdu, Yushu, Aba) reported from the Tibetan Plateau. Young trees dominated the two fir treeline plots. Both positive and negative spatial autocorrelations for all of the trees were detected in two study plots. Intraspecific facilitation and competition coexisted at the fir treelines in three forest regions (RW, Linzhi, Aba) characterized by a mild moist climate, whereas intraspecific facilitation dominated the other three forest regions (Changdu, Deqin, Yushu), which featured seasonal climatic stress or high disturbance pressure. Thus, increased stem density at alpine treeline can be linked to competitive interactions in relatively favorable environmental conditions. Overall, the spatial patterns of the treeline population are mainly shaped by the combination of thermal and moisture conditions and are also modulated by non-climatic variables (e.g., disturbance history and microtopography).

Spatial Analysis as a Tool for Plant Population Conservation: A Case Study of Tamarix chinensis in the Yellow River Delta, China

Saltcedar (Tamarix chinensis) is undergoing population declination and fragmentation due to climate change and human disturbance. The existing restoration strategies usually focus on improving the environmental conditions based on the environment–saltcedar relationship, while they ignore the role of spatial autocorrelation resulting from biological interaction and ecological processes. This oversight limits the efficiency and sustainability of the restoration. Here, we explored the spatial pattern of the saltcedar population in the Yellow River Delta, China, and its relationship with environmental factors, incorporating spatial autocorrelation. The plant and soil parameters were extracted by an airborne LiDAR system integrated with fixed soil environment measurements. The environment–saltcedar relationship incorporating spatial autocorrelation was evaluated with different regression models. Results showed that saltcedars aggregated at small scales (2–6 m), resulting from intraspecific facilitation and wind dispersal of seeds, while intraspecific competition was responsible for the random distribution at large scales (>10 m). The long-distance dispersal of seeds through water explained the significant positive spatial autocorrelation of saltcedars at distances up to 125 m. Consequently, resulting from intraspecific facilitation and seed dispersal, aggregation distribution and positive spatial autocorrelation within the saltcedar population improved the adaptability of saltcedar to environmental stress and thereby reduced the impact of environmental factors on the abundance of saltcedar.

Intraspecific facilitation by allelochemical mediated grazing protection within a toxigenic dinoflagellate population

Dinoflagellates are a major cause of harmful algal blooms (HABs), with consequences for coastal marine ecosystem functioning and services. Alexandrium fundyense (previously Alexandrium tamarense ) is one of the most abundant and widespread toxigenic species in the temperate Northern and Southern Hemisphere and produces paralytic shellfish poisoning toxins as well as lytic allelochemical substances. These bioactive compounds may support the success of A. fundyense and its ability to form blooms. Here we investigate the impact of grazing on monoclonal and mixed set-ups of highly (Alex2) and moderately (Alex4) allelochemically active A. fundyense strains and a non-allelochemically active conspecific (Alex5) by the heterotrophic dinoflagellate Polykrikos kofoidii . While Alex4 and particularly Alex5 were strongly grazed by P. kofoidii when offered alone, both strains grew well in the mixed assemblages (Alex4 + Alex5 and Alex2 + Alex5). Hence, the allelochemical active strains facilitated growth of the non-active strain by protecting the population as a whole against grazing. Based on our results, we argue that facilitation among clonal lineages within a species may partly explain the high genotypic and phenotypic diversity of Alexandrium populations. Populations of Alexandrium may comprise multiple cooperative traits that act in concert with intraspecific facilitation, and hence promote the success of this notorious HAB species.