Effects of climate, biotic factors, and phylogeny on allometric relationships: testing the metabolic scaling theory in plantations and natural forests across China

Abstract

Background: Metabolic scaling theory (MST) is still in debate because observed allometric exponents often deviate from MST predictions, and can change significantly depending on environment, phylogeny, and disturbance. We assembled published scaling exponents from literatures for three allometric relationships linked to biomass allocation: leaf biomass-diameter (L-D), stem biomass-diameter (S-D), and root biomass-diameter (R-D). We used data from natural forests and plantations across China to test the following hypotheses: 1) the allometric relationships of trees support the predictions of MST on a broad scale; 2) the observed deviations from MST predictions are caused by climate, biotic factors, and/or phylogeny; 3) abiotic and biotic factors influence allometric relationships in plantations and natural forests differently, and different allometric relationships (i.e. L-D, S-D, and R-D) are affected differently. We related these scaling exponents to geographic climate gradient, successional stage, stand density, leaf form and phenology, and phylogeny. We used mixed-effect models to examine the major factors affecting tree allometries. Results: In natural forests, S-D and R-D scaling exponents were consistent with MST predictions in primary forests, but were significantly lower in secondary forests. Both S-D and R-D scaling exponents in plantations had a medium value that fell between those of the secondary and primary forests, despite plantations being similar in species characteristics and age to secondary forests. The S-D and R-D exponents were significantly affected by factors that are not yet considered in MST, including winter coldness which explained 2.76%-3.24% of variations, successional stage (7.91%-8.20% of variations), density (a surrogate for competition, 5.86%-8.54% of variations), and especially phylogeny (45.86%-56.64% of variations explained). However, the L-D scaling exponents conformed to MST predictions in primary, secondary, and plantation forests, and was not strongly explained by most factors. Conclusion: MST is only applicable to primary (steady-state) forests, and climate, biotic factors and phylogeny are causes of the observed deviations of allometric relationships from MST predictions. Forest management practices in plantations have a strong influence on tree allometries. L-D allometry is more strongly controlled by biophysical constraints than S-D and R-D allometries, however, the mechanisms behind this difference still need further examinations.

Are fine roots ‘leaves underground' in terms of allometry? A test in a tropical forest successional series in southwest China

“…In recent years, this prediction has been supported by studies on above‐ground allometries, e.g. tree diameter versus height, tree size distribution and biomass allocation (Rüger and Condit 2012, Duncanson et al 2015, Sun et al 2017b, 2020). If fine‐root allometries also showed a similar pattern across successional stages, then it is clearly a good support to MST.…”

“…H max is a commonly‐used proxy for forest height, and Duncanson et al (2015) showed that forest height may be an important driver for changes in above‐ground allometries. Stand density and TBA are generally suggested to be related to competition for light, water and nutrients among individuals (Matsuo et al 2021), and previous studies have also reported that competition may be a critical modulator of tree allometry across successional stages (Sun et al 2017b, 2020).…”

“…For instance, it is well known that trees are slender in stands with higher stand density due to increased competition (Wang et al 2006, Lines et al 2012. Sun et al (2017bSun et al ( , 2020 further showed that stand density may be a significant cause of deviation from MST w 's predictions for several scaling relationships (diameter versus height, stem and coarse root biomass allocation). However, this study did not find any significant effect of stand density on fineroot allometries.…”

Geological Substrate Effects on Teucrium montanum L. (Lamiaceae) Morphological Traits: Geometric Morphometrics Approach

“…Allometric relationships are based on the observation that growth rates of different plant parts are not proportional to their size, but rather follow specific patterns that depend on various factors, such as genetic makeup, environmental conditions, and resource availability [ 15 ]. Allometric relationships are usually expressed in terms of scaling laws that describe the relationship between the size of one plant part and another, such as the size of leaves relative to the stem diameter [ 16 ]. These scaling laws can be used to predict how a plant’s growth and development will change in response to different environmental conditions or resource availability, and to compare the growth patterns of different plant species or individuals [ 15 , 17 ].…”

“…These scaling laws can be used to predict how a plant’s growth and development will change in response to different environmental conditions or resource availability, and to compare the growth patterns of different plant species or individuals [ 15 , 17 ]. A common example of allometry in plants is the relationship between leaf size and stem diameter [ 16 ]. As a plant grows taller and larger, its stem diameter usually increases faster than its leaf size, resulting in a lower ratio between leaf and stem size.…”