Forest carbon allocation modelling under climate change

Abstract

Carbon allocation plays a key role in ecosystem dynamics and plant adaptation to changing environmental conditions. Hence, proper description of this process in vegetation models is crucial for the simulations of the impact of climate change on carbon cycling in forests. Here we review how carbon allocation modelling is currently implemented in 31 contrasting models to identify the main gaps compared with our theoretical and empirical understanding of carbon allocation. A hybrid approach based on combining several principles and/or types of carbon allocation modelling prevailed in the examined models, while physiologically more sophisticated approaches were used less often than empirical ones. The analysis revealed that, although the number of carbon allocation studies over the past 10 years has substantially increased, some background processes are still insufficiently understood and some issues in models are frequently poorly represented, oversimplified or even omitted. Hence, current challenges for carbon allocation modelling in forest ecosystems are (i) to overcome remaining limits in process understanding, particularly regarding the impact of disturbances on carbon allocation, accumulation and utilization of nonstructural carbohydrates, and carbon use by symbionts, and (ii) to implement existing knowledge of carbon allocation into defence, regeneration and improved resource uptake in order to better account for changing environmental conditions.

Plant respiration: Controlled by photosynthesis or biomass?

“…(2) (Merganičová et al, 2019;Schiestl-Aalto et al, 2019). However, the ability of trees to prioritize storage over growth depends on the role of NSC in allowing temporal asynchrony between carbon demand and carbon supply (Fatichi, Leunzinger, & Kӧrner, 2014).…”

“…Note that this assumption departs from the notion that NSC is a mere reservoir for excess supply of carbon relative to growth demand ('passive' storage: Kozlowski, 1992). In the model, carbon allocation to all tree structural and nonstructural pools is computed here daily and is controlled by functional constraints due to direct and lagged C-requirements (Huang et al, 2019;Merganičová et al, 2019). It is assumed that a minimum NSC threshold level concentration (11% of sapwood dry mass for deciduous and 5% for evergreen species: Genet, Bréda, & Dufrêne, 2010) has to be maintained for multiple functions including osmoregulation, cell turgor, vascular integrity, tree survival (reviewed in Hartmann & Trumbore, 2016) and organ-specific phenology (leaf and fine-root formation).…”

Forest production efficiency increases with growth temperature

“…At the stand scale, closing canopies may contribute further to reducing or stabilizing GPP 32 at the level of individual trees. It is also likely that young trees allocate more carbon to biomass growth as they compete spatially for light and nutrients; while older trees invest more in maintenance of their existing biomass, and prioritize the chemical defence of that biomass, relative to acquisition of new biomass 33,34 .…”

Frost and drought: effects of extreme weather events on stem carbon dynamics in a Mediterranean beech forest

“…Until then, net absorbed carbon was allocated mostly to foliage growth. This can be related to C allocation hierarchy that identifies newly developing leaves as the main C sink as at the beginning of the growing season (Campioli et al , 2013; Collalti et al , 2018; Merganičová et al , 2019, and references therein). Interestingly, and counter to our expectations, in 2016 the cambium remained active at low rates even after complete canopy defoliation.…”

“…Stem radial growth of beech in Selva Piana was greatly affected by extreme late spring frost event in 2016 because of the premature cessation of cambial cell production and the lower growth rate during the active period, which resulted in 82% narrower annual xylem increments if compared to 2015 and 2017. This can be related to a somewhat hypothesised, genetically controlled, form of hierarchy in C allocation (composed by old C reserve and recently fixed photosynthetates) that identifies newly developing leaves as the main C sink rather to radial growth (Campioli et al , 2013; Collalti et al , 2018; Merganičová et al , 2019). Some new insights on the genetic control (rather than the environmental ones) on cambial growth have only emerged recently (see Zhang et al 2019; Greb, 2019) but a clear picture still missing.…”