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

Abstract

Ecologists often invoke interspecific facilitation to help explain positive biodiversity–ecosystem function relationships in plant communities, but seldom test how it occurs. One mechanism through which one species may facilitate another is by ameliorating abiotic stress. Physiological experiments show that a chronic excess of light can cause stress that depresses carbon assimilation. If shading by a plant's neighbours reduces light stress enough, it may facilitate that plant's growth. If light is instead most often a limiting factor for photosynthesis, shading may have an adverse, competitive effect. In a temperate tree diversity experiment, we measured stem growth rates and photosynthetic physiology in broadleaf trees across a gradient of light availability imposed by their neighbours. At the extremes, trees experienced nearly full sun (monoculture), or were shaded by nearby fast‐growing conifers (shaded biculture). Most species had slower growth rates with larger neighbours, implying a net competitive effect. On the other hand, the two most shade‐tolerant species (Tilia americana and Acer negundo) and the most shade‐intolerant one (Betula papyrifera) had faster stem growth rates with larger neighbours. The two shade‐tolerant species had the greatest increases in photoinhibition (reduced dark‐acclimated Fv/Fm) across the gradient of increasing light availability, which suggests they are more vulnerable to chronic light stress. While most species had lower carbon assimilation rates in the shaded biculture treatment, T. americana had rates up to 25% higher. T. americana also dropped its leaves 3–4 weeks earlier in monocultures, curtailing its growing season. We conclude that although large neighbours can cause light limitation in shade‐intolerant species, they can also increase growth through abiotic stress amelioration in shade‐tolerant species. Finally, in shade‐intolerant B. papyrifera, we find a pattern of stem elongation in trees with larger neighbours, which suggests that a shade avoidance response may account for the apparent positive trend in stem volume. Synthesis. Both positive and negative species interactions in our experiment can be explained in large part by the photosynthetic responses of trees to the light environment created by their neighbours. We show that photosynthetic physiology can help explain the species interactions that underlie biodiversity–ecosystem function relationships. The insights that ecologists gain by searching for such physiological mechanisms may help us forecast species interactions under environmental change.

Changes in the direction of the diversity–productivity relationship over 15 years of stand development in a planted temperate forest

“…For example, Kothari et al (2021) recently found that the growth of shorter shade-tolerant species was facilitated by taller more lightdemanding neighbours, and identified the mechanism as the reduction of chronic photoinhibition in shade-tolerant species. Hence, we can argue that several mechanisms, for example niche partitioning and perhaps also facilitation, allowed higher biomass increment in even plots in the later period of the experiment.…”

“…In addition to niche partitioning, facilitation can be responsible for higher biomass in diverse communities (Wright et al, 2021). For example, in diverse communities, large neighbours can increase the growth of shade-tolerant trees by improving microclimatic conditions (Kothari et al, 2021). Hydraulic redistribution can also facilitate the growth and survival of neighbouring trees (Grossiord, 2020), while addition of nitrogen by N-fixing trees can accelerate the growth of other trees (Richards et al, 2010).…”

The shape of trees: Reimagining forest ecology in three dimensions with remote sensing

“…Combined with ray tracing approaches it will also be possible to directly test species-specific competitive and facilitative shading effects (Kothari et al, 2021), and the relative importance of shade persistence and structural plasticity as mechanisms driving higher crown packing in mixtures (Williams et al, 2020).…”

Plant spectra as integrative measures of plant phenotypes

“…The way plants interact with light is one of the key determinants of their success in particular environments and one of most important processes structuring plant communities (Canham et al, 1994;Pacala & Tilman, 1994;Williams et al, 2020). The balance between striving for and avoiding excess light (Kothari et al, 2018;Kothari et al, 2021) influences plant growth and architecture (Jucker et al, 2015;Williams et al, 2017), water and resource use (Ellsworth & Reich, 1993), creating complex environments that shape plant communities. The processes surrounding plant light use are tightly bound with a variety of chemical, morphological and anatomical adaptations, ranging from trade-offs between fast and slow return on investment traits (Wright et al, 2004), to cuticular structures reducing water loss through evaporation, and leaf thickness altering heat dissipation.…”