Dividing the pie: A quantitative review on plant density responses

Abstract

Plant population density is an important variable in agronomy and forestry and offers an experimental way to better understand plant-plant competition. We made a meta-analysis of responses of even-aged mono-specific stands to population density by quantifying for 3 stand and 33 individual plant variables in 334 experiments how much both plant biomass and phenotypic traits change with a doubling in density. Increasing density increases standing crop per area, but decreases the mean size of its individuals, mostly through reduced tillering and branching. Among the phenotypic traits, stem diameter is negatively affected, but plant height remains remarkably similar, partly due to an increased stem length-tomass ratio and partly by increased allocation to stems. The reduction in biomass is caused by a lower photosynthetic rate, mainly due to shading of part of the foliage. Total seed mass per plant is also strongly reduced, marginally by lower mass per seed, but mainly because of lower seed numbers. Plants generally have fewer shoot-born roots, but their overall rooting depth seems hardly affected. The phenotypic plasticity responses to high densities correlate strongly with those to low light, and less with those to low nutrients, suggesting that at high density, shading affects plants more than nutrient depletion.

Biomass partitioning in response to intraspecific competition depends on nutrients and species characteristics: A study of 43 plant species

“…In our study, plants growing at high densities allocated more biomass to roots, indicating that competition was more intense for below-ground resources than for light, even at high nutrient levels. In contrast, a recent meta-analysis found that at high densities most plants allocate more biomass to stems, and concluded that higher density predominantly increases competition for light (Postma et al 2020).…”

“…However, plants can also respond to different environments by changing morphological or physiological traits , Freschet et al 2015, Kramer-Walter & Laughlin 2017, Sandner & Matthies 2018, Poorter et al 2019). For example, in response to shade by competitors, plants may increase their specific stem length (Poorter et al 2019, Postma et al 2020 and in response to nutrient shortage due to competition may increase their specific root length (Schippers & Olff 2000, Trubat et al 2006, Dybzinsky et al 2019, but see Freschet et al 2015, Kramer-Walter et al 2017).…”

“…There are several possible explanations for these conflicting results. Approximately half of the studies on stem mass fraction reviewed by Postma et al (2020) were carried out in the glasshouse, where light availability is lower than in the field, whereas our experiment was performed in a common garden during summer. In three studies, below-ground competition had explicitly been prevented (Casper et al 1998, Maliakal et al 1999, Aphalo & Riskala 2006.…”

“…Similar patterns have been found in agricultural systems (Hecht et al 2016, Shao et al 2018, but see Kamel 1959), where biomass allocation to roots (Jackson et al 1996) and below-ground competition is strongly reduced by the addition of high amounts of fertiliser. Similarly, Postma et al (2020) suggested that the high levels of soil fertility in agronomic field experiments were responsible for the fact that in their review they found a reduction of root mass fraction (RMF) in response to density only in crops but not in wild species.…”

“…Studies examining the effects of intraspecific competition on biomass allocation have given inconsistent results: with increasing density plants either increased their RMF (Lentz 1999, Berendse & Möller 2009, their LMF or SMF (Puri et al 1993, Meekins & McCarthy 2000, Forster et al 2011, Hecht et al 2016, or did not change their biomass allocation at all (Kamel et al 1959, Flint & Patterson 1983. In a review of the results of the available studies, Postma et al (2020) found that there was large variation among studies and species in the response to density,…”