On the Below- and Aboveground Phenology in Deciduous Trees: Observing the Fine-Root Lifespan, Turnover Rate, and Phenology of Fagus sylvatica L., Quercus robur L., and Betula pendula Roth for Two Growing Seasons

Bertold Mariën,Chao Fang,Matteo Campioli,Patrick Willems,Hòa Xuan Nguyen,Ivika Ostonen,Alice Penanhoat,Tomáš Ghisi,Páll Sigurðsson

doi:10.3390/f12121680

Abstract

We tested the relation between the below- and aboveground tree phenology, determining if beech and oak have a greater fine-root lifespan and a smaller turnover rate than birch and if thinner fine-roots or fine-roots born in spring have a shorter lifespan and greater turnover rate than thicker fine-roots or fine-roots born in another season. The fine-root phenology, bud burst, and leaf senescence in Belgian stands were monitored using minirhizotrons, visual observations, and chlorophyll measurements, respectively. The fine-root phenology and the lifespan and turnover rate were estimated using generalized additive models and Kaplan–Meier analyses, respectively. Unlike the aboveground phenology, the belowground phenology did not show a clear and repeating yearly pattern. The cumulative root surface remained stable for birch but peaked for beech and oak around summer to autumn in 2019 and spring in 2020. The new root count was larger in 2019 than in 2020. The mean lifespan of fine-roots with a diameter below 0.5 mm (308 to 399 days) was shorter than those with a diameter between 0.5 to 1 mm (438 to 502 days), 1 to 2 mm (409 to 446 days), or above 2 mm (418 to 471 days). Fine-roots born in different seasons showed a species-specific lifespan and turnover rate.

Highlights

Our results showed that the fine-root lifespan and the turnover rate in beech and oak were greater and smaller, respectively, than those of birch
The results shown in the literature indicate an extremely wide variability in the estimations of the fine-root lifespan and the turnover rate among and within deciduous tree species [13,27,50,53,54,55,56,57,58,59,60,61,63,66,171]
We found considerable variation in our estimations of the fine-root lifespan and the turnover rate within species, especially whenever the estimations were split by root diameter class or the season of root birth

Summary

Introduction

Roots should be defined based on their functional characteristics [1]. Roots with a diameter below 2 mm are usually defined as fine-roots, while lignified (i.e., woody) roots with a diameter above 2 mm and secondary development are usually defined as coarse roots [2]. Roots with a diameter below 0.5 mm may be considered mostly absorptive roots with a primary or secondary structure, while roots with a diameter between 0.5 and 2 mm may largely be considered woody transport roots [6,7]. Roots of the same diameter sometimes have a different branching structure (i.e., primary or secondary roots) or function [2,8,9]. Apart from resource uptake, resource transportation, and physical stabilization, roots can store nutrients and carbohydrates or act as sensors for exterior conditions [10,11,12,13]

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