Physiological responses to light explain competition and facilitation in a tree diversity experiment

Abstract

14Author contributions: SK conceived the project with JCB, conducted physiological measurements, 15analyzed and interpreted the data with JCB and RM, and wrote the first draft of the paper. JCB and RM 16designed the FAB experiment, coordinated annual biomass surveys, and contributed to subsequent 17 revisions of the paper. 19Summary 20  Interspecific facilitation is often invoked in explanations of biodiversity-ecosystem function 21 relationships in plant communities, but it is seldom clear how it occurs. Physiological 22 experiments show that excess light causes stress that may depress long-term carbon assimilation. 23If shading by a plant's neighbors reduces light stress, it may facilitate that plant's growth. On the 24 other hand, when light is a limiting factor for growth, shading will often have a net negative, 25 competitive effect. 26  In a tree diversity experiment, we measured growth rates and photosynthetic physiology of 27 broadleaf tree species across a gradient of light availability imposed by their neighbors. At the 28 extremes, trees experienced nearly full sun (monoculture), or were shaded by nearby fast-growing 29 conifers (shaded biculture). 30  Although most species had lower growth with larger neighbors, implying a net competitive effect, 31 the two most shade-tolerant species (Tilia americana and Acer negundo) had positive responses 32 to neighbor size. Compared to the others, these two species were especially susceptible to 33 photoinhibition (reduced dark-acclimated Fv/Fm) in full sun. While most species had lower 34 assimilation rates in shaded bicultures, T. americana had carbon assimilation rates up to 25% 35 higher. T. americana also dropped its leaves 3-4 weeks later in the shaded biculture, extending its 36 growing season. We conclude that although large neighbors can cause light limitation in shade-37 intolerant species, they can also increase growth through abiotic stress amelioration in shade-38 tolerant species. 39  Both positive and negative species interactions in our experiment can be explained by the 40 photosynthetic responses of trees to the light environment created by their neighbors. We show 41 that physiological measurements can deepen our understanding of the species interactions that 42 underlie biodiversity-ecosystem function relationships. 43 44

Biodiversity and ecosystem functioning: Have our experiments and indices been underestimating the role of facilitation?

“…In the first, the papers explore the facilitative mechanisms that underlie the effect that biodiversity has on productivity. For example, Kothari et al (2021) isolate the suppressive effects of photoinhibition in monoculture to help explain biodiversity – productivity relationships in forested ecosystems. Guimarães‐Steinicke et al (2021) use hyperspectral data to examine how diversity and functional identity affect leaf surface temperatures in grasslands.…”

Enhanced light interception and light use efficiency explain overyielding in young tree communities

“…Subordinate species' LUE may increase in mixture if they intercept less light without a proportional decrease in growth; a phenomenon anticipated by the photosynthetic light-response curve if the species shifts (to lower irradiance) along a flat part of their curve or if shading reduces photoinhibition (Kothari et al 2020) (Fig. S8).…”

“…Enhanced productivity in species mixtures relative to monocultures (i.e. overyielding) may arise in many ways, including competitive relaxation via resource partitioning (Richards & Schmidt 2010;Sapijanskas et al 2014; Barry et al 2019), competitive imbalance leading to the dominance of a highly productive species (Yachi & Loreau 2007), facilitation that ameliorates nutrient, microclimatic or light stress (Tilman et al 1997;Cowles et al 2016;Wright et al 2017;Kothari et al 2020), plant-soil interactions that enhance soil resource availability (Reich et al 2012;Cowles et al 2016), and trophic interactions that increase plant fitness (Laforest-Lapointe et al 2017) or reduce herbivory or disease (Jactel & Brockerhoff 2007;Jactel et al 2017). Here we focus on how interactions among trees for a single resourcelightcan lead to overyielding in mixed-species forests.…”

Interplay of above‐ and belowground resource limitations: a competition–facilitation shift maintains species coexistence

“…After this, competitive interactions should prevail even in the originally resource-poor sites (Olsen et al 2016). Alternatively, previous studies also indicated that light disturbance may lead to higher stress for understory shade-tolerant plant species due to extreme temperatures, higher air vapour pressure deficits and light excesses, that might support facilitation after canopy opening (Wright et al 2015, Kothari et al 2021). However, we did not observe any changes in the competition-facilitation balance either towards competition or towards facilitation over time.…”