The Janzen–Connell hypothesis proposes that specialist natural enemies, such as herbivores and pathogens, maintain diversity in plant communities by reducing survival rates of conspecific seeds and seedlings located close to reproductive adults or in areas of high conspecific density. Variation in the strength of distance- and density-dependent effects is hypothesized to explain variation in plant species richness along climatic gradients, with effects predicted to be stronger in the tropics than the temperate zone and in wetter habitats compared to drier habitats.We conducted a comprehensive literature search to identify peer-reviewed experimental studies published in the 40+ years since the hypothesis was first proposed. Using data from these studies, we conducted a meta-analysis to assess the current weight of evidence for the distance and density predictions of the Janzen–Connell hypothesis.Overall, we found significant support for both the distance- and density-dependent predictions. For all studies combined, survival rates were significantly reduced near conspecifics compared to far from conspecifics, and in areas with high densities of conspecifics compared to areas with low conspecific densities. There was no indication that these results were due to publication bias.The strength of distance and density effects varied widely among studies. Contrary to expectations, this variation was unrelated to latitude, and there was no significant effect of study region. However, we did find a trend for stronger distance and density dependence in wetter sites compared to sites with lower annual precipitation. In addition, effects were significantly stronger at the seedling stage compared to the seed stage.Synthesis. Our study provides support for the idea that distance- and density-dependent mortality occurs in plant communities world-wide. Available evidence suggests that natural enemies are frequently the cause of such patterns, consistent with the Janzen–Connell hypothesis, but additional studies are needed to rule out other mechanisms (e.g. intraspecific competition). With the widespread existence of density and distance dependence clearly established, future research should focus on assessing the degree to which these effects permit species coexistence and contribute to the maintenance of diversity in plant communities.
Data transparency is key to accounting for how local governments and the private sector are contributing to global emissions reduction, say Angel Hsu and colleagues.
The rise of sub-national actors in global climate governance underscores the need for clear alignment between these efforts and their national counterparts. As these sub-national climate actions are filling gaps in mitigation, adaptation, and financing, among other functions, a critical question is how these efforts complement or overlap with national climate pledges. This consideration is particularly important in the context of the Paris Agreement's mandate for fiveyear review cycles, where national governments will be asked to demonstrate progress towards climate mitigation goals and increase their ambition. In this paper, we argue that alignment-both vertically between multiple jurisdictions and horizontally with external networks and actors-is critical to clarifying climate actions between multiple levels of actors and to maximizing mitigation potential. We use nine case studies to demonstrate the varying degrees and modes of vertical integration between subnational and national climate actors. We find that the case studies embody different styles of vertical alignment, and exhibit significant variation in the degree and direction of vertical alignment within each of these modes. We also find that many case studies rely on horizontally-aligned international networks and coalitions to fill gaps in financial resources or technical support. As a proof of concept, we demonstrate that an additional 1 gigaton carbon dioxide equivalent (CO2e) in 2020 can be achieved in these nine case studies through stronger alignment that makes it possible to scale sub-national climate actions to the national level. These findings suggest there may be a missed opportunity to realize greater mitigation potential by fostering stronger vertical alignment, and enhancing coordination between horizontal networks of climate action and national governments.
Insect herbivores can serve as important regulators of plant dynamics, but their impacts in temperate forest understories have received minimal attention at local scales. Here, we test several related hypotheses about the influence of plant neighborhood composition on insect leaf damage in southwestern Pennsylvania, USA. Using data on seedlings and adult trees sampled at 36 sites over an approximately 900 ha area, we tested for the effects of total plant density, rarefied species richness (i.e., resource concentration and dietary‐mixing hypotheses), conspecific density (i.e., Janzen–Connell hypothesis), and heterospecific density (i.e., herd‐immunity hypothesis), on the proportion of leaf tissue removed from 290 seedlings of 20 species. We also tested for the effects of generic‐ and familial‐level neighborhoods. Our results showed that the proportion of leaf tissue removed ranged from zero to just under 50% across individuals, but was generally quite low (<2%). Using linear mixed models, we found a significant negative relationship between insect damage and rarefied species richness, but no relationship with neighborhood density or composition. In addition, leaf damage had no significant effect on subsequent seedling growth or survival, likely due to the low levels of damage experienced by most individuals. Our results provide some support for the resource concentration hypothesis, but suggest a limited role for insect herbivores in driving local‐scale seedling dynamics in temperate forest understories.
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