Limited sink but large storage: biomass dynamics in naturally developing beech (Fagus sylvatica) and oak (Quercus robur, Quercus petraea) forests of northwestern Germany

1. Currently, the dynamics underlying the storage and acquisition of biomass, and thus carbon, in naturally developing forests are under debate. A better understanding of the biomass dynamics of forests is needed to clarify the role played by naturally developing forests in the mitigation of climate change. 2. Long-term monitoring data from unmanaged strict forest reserves (SFRs) in northwestern Germany were used to analyze the biomass dynamics of pure beech, mixed beech, and mixed oak forests. A complete balance of aboveground woody biomass (biomass) and growth, density-dependent and -independent mortality, as well as deadwood decay was derived. Density-independent mortality served as a proxy for disturbance severity. 3. After a time of abandonment (TSA) of 50 years, the average biomass ranged between 334 t ha-1 in mixed oak and 478 t ha-1 in pure beech stands. The net change in biomass was positive in all forest types. Density-independent mortality and decay rates were much lower than the growth rates. Pure beech forests reached higher levels of biomass, a higher net change in biomass, and more growth than either of the mixed forest type. Biomass increased linearly with TSA in pure beech stands but followed an asymptotic course in the mixed forests. In the latter, the net change in biomass and growth were consistent with a unimodal development pattern. The development of biomass could not be explained by the aging of the tree communities. 4. Synthesis: We hypothesized that the observed biomass dynamics is a result of the interaction between resource supply within a limited growing space and the resource-use efficiency of the tree stand in conjunction with disturbances. The still-linear increase in the biomass of pure beech forests was assumed to reflect the high resource-use efficiency of beech, especially its use of light. The aboveground capacity of naturally developing broadleaved forests to store and acquire biomass, and thus carbon, is substantial. Accordingly, allowing broadleaved forests to develop naturally can contribute substantially to carbon storage and sequestration. However. our study also suggests that the aboveground carbon sink decreases after several decades.

1. 当前，天然更新森林的生物量乃至碳储量的积累与获取动态仍存在争议。为阐明天然更新森林在减缓气候变化中所发挥的作用，亟需深化对森林生物量动态的认知。
2. 本研究采用德国西北部未受人为干扰的严格森林保护区（Strict Forest Reserves, SFRs）的长期监测数据，分析了纯山毛榉林、混交山毛榉林与混交栎林的生物量动态。研究完整核算了地上木质生物量（以下简称生物量）及其增长、密度依赖型与非依赖型死亡率，以及枯木分解过程，并推导得到各参数间的完整平衡关系。其中，密度非依赖型死亡率被用作干扰强度的替代指标。
3. 在弃耕时间（Time Since Abandonment, TSA）达50年后，混交栎林的平均生物量为334 t·ha⁻¹，纯山毛榉林则为478 t·ha⁻¹。所有林型的生物量净变化均为正值。密度非依赖型死亡率与枯木分解速率均远低于生长速率。相较于两类混交林，纯山毛榉林的生物量水平更高、生物量净变化更大，且生长量更可观。纯山毛榉林的生物量随TSA呈线性增长，而混交林的生物量增长则呈现渐近趋势。在后一类林型中，生物量净变化与生长量符合单峰发育模式。研究发现，生物量动态无法通过树木群落的衰老过程加以解释。
4. 综合讨论：本研究提出假说，认为观测到的生物量动态是有限生长空间内的资源供给、林分资源利用效率与干扰三者共同作用的结果。纯山毛榉林仍呈线性增长的生物量，被认为反映了山毛榉较高的资源利用效率，尤其是对光照的利用效率。天然更新阔叶林的地上生物量乃至碳储量积累与获取能力极强。因此，放任阔叶林自然发育可大幅提升碳储存与固碳能力。然而，本研究同时表明，地上碳汇在数十年后会出现衰减。