Plant species richness increases with light availability, but not variability, in temperate forests understorey
Abstract
Background: Temperate forest understorey vegetation poses an excellent study system to investigate whether increases in resource availability lead to an increase in plant species richness. Most sunlight is absorbed by the species-poor tree canopy, making the much more species-rich understorey species inhabit a severely resource-limited habitat. Additionally, the heterogeneity of light availability, resulting from management-moderated tree composition and age structure, may contribute to species coexistence. One would therefore expect that the diversity in the herb layer correlates positively with either the overall light availability, or the light heterogeneity, depending on whether resource availability or heterogeneity are more important drivers of diversity. To test this idea, we assessed variability of light conditions in 75 forest plots across three ecoregions with four different methods. Results: We correlated these data with vegetation relevés and found light availability to be strongly positively correlated with understorey plant species richness, as well as with understorey cover. Light variability (assessed with two approaches) within plots was positively correlated with transmittance, but did not improve the relationship further, suggesting that the main driver of species richness in this system is the overall resource availability. Two of the three beech-dominated regions exhibited near-identical effects of light transmittance, while the third, featuring pine alongside beech and thus with the longest gradient of transmittance and lowest species richness, displayed a weaker light response. Conclusions: While site conditions are certainly responsible for the trees selected by foresters, for the resulting forest structure, and for the differences in plant species pools, our results suggest that light transmittance is a strong mediating factor of understorey plant species richness.
What the paper shows and why it matters
(AI-generated)
Two ideas are usually offered for why some places hold more species than others. More resources can support more individuals and so more species, and a more varied supply of resources lets more specialised species coexist. Both are plausible for plants beneath a forest canopy, where light is most likely the scarcest resource. Yet few studies recorded both average light and its variability, and the two tend to rise together. It was therefore unclear whether the number of species follows the amount of light or its patchiness (a question posed by Bartels and Chen 2010). Many studies also used proxies such as canopy closure, which match measured light poorly. The paper asks this question for understorey plant species richness, across forest types.
The paper argues that overall light availability, not its heterogeneity, is the main light-related correlate of understorey plant species richness in the temperate forests studied. The case rests on 75 plots in three German regions, with light quantified in four ways: sensor readings of the proportion of light that reaches the forest floor, observers’ estimates of canopy cover, canopy openness derived from laser scans, and crown areas computed from tree inventories. Richness rose strongly with the first three, not with crown area. In the sensor data, two regions responded almost identically; the dry, sandy region with pine alongside beech responded less strongly. Once average light was accounted for, light variability within plots did not raise richness: its effect was negative for sensor-measured light and absent for laser-scan openness. Plant cover, used as a stand-in for the number of individuals, explained only a small part of the light effect. As a side result, observers’ cover estimates and laser-scan openness explained richness better than the one-off sensor readings.
The paper presents light at the forest floor as a strong link between forest structure, which management shapes, and understorey plant diversity, and recommends measuring it as routinely as soil nutrients or moisture. The data are correlational and light is entangled with soil, climate, tree species and management, so the paper leaves the causal pathways open. A later study of 135 plots reported higher species richness with greater light heterogeneity where soil C:N variability was also high (Helbach et al. 2022), so the question of heterogeneity remains open.
References
- Bartels & Chen (2010). Is understory plant species diversity driven by resource quantity or resource heterogeneity? Ecology 91, 1931–1938. doi:10.1890/09-1376.1
- Helbach et al. (2022). Light heterogeneity affects understory plant species richness in temperate forests supporting the heterogeneity–diversity hypothesis. Ecology and Evolution 12, e8534. doi:10.1002/ece3.8534