森林自然更新过程中地上氮贮量与生物量异速生长的关系

Abstract

研究植物氮贮量与生物量(M)之间的异速生长关系对于开展生态系统碳收支和氮循环研究具有重要意义。目前大量对氮贮量与生物量关系的研究主要集中在个体水平,对于群落水平的氮贮量和生物量之间的异速生长关系,以及群落自然更新过程如何影响该关系仍有待深入研究。利用3种森林类型皆伐后20多年自然更新过程中氮贮量和生物量的数据,采用简化主轴回归方法(reduced major axis, RMA)对不同自然更新阶段的森林氮贮量和生物量之间的异速生长指数和常数进行比较。结果表明:在不同的更新阶段,3种森林类型的植物氮贮量和生物量之间的异速生长指数均接近于1.0 (即N∝M^{0.91 -1.07})。异速生长常数随更新时间的增加而逐渐降低,导致3种森林类型整体上氮贮量正比于生物量的0.85次幂。异速生长常数的降低可能是由于在更新过程中叶生物量占整体生物量的比例逐渐下降,导致其对N吸收的生态化学计量制约所造成。;Exploring the relationship between tissue nitrogen content (N) and its biomass (M) is crucial for understanding ecosystem carbon balance and N cycling because N is one of the limiting nutrients for the primary production in most terrestrial ecosystems. Although much has been learnt about N utilization and distribution within plant body, how N and M are coupled at the community level, especially for ecosystems after disturbance, remains inadequately understood. Prior work indicates that N and M often co-vary in a manner that can be expressed as a power law, mathematically taking the form N = β^Mα, where β is an allometric constant and α is the scaling exponent. Although it has been demonstrated that N scales as approximately the 3/4 power of M across a wide diversity of plant communities, considerable uncertainty exists as to whether this 3/4 scaling relationship is valid for all forested communities or whether it changes as a function of forest successional stage. To fill this gap, data on N and M were collected from different woody species of three different forest community-types (i.e. cove hardwoods, mesic, mixed-oak community, and the dry, mixed-oak community) over 20-years of succession following commercial clear-cutting in a watershed (WS7) located in the Coweeta Hydrologic Laboratory. Total stand nitrogen content scaled nearly isometrically with respect to total stand biomass (i.e. N∝M^≈1.0) for 16 out of the 18 successional stages of the three community types. The mean scaling exponents of N vs. M for the cove hardwood community, the mesic, mixed-oak community, and the dry, mixed-oak community were 0.96, 0.99 and 0.97, respectively. Our observations make it reasonable to suppose that the isometric scaling relationship of N vs. M should scale up from the level of individual trees to the level of an entire forest stand at each particular successional stage. Despite this homogeneity in N vs. M scaling exponents across the different successional stages and the three community-types, all three forest communities had allometric constants that decreased systemically with increasing stand age. The highest <em>Y</em>-intercept was observed immediately after clear-cutting, whereas the lowest <em>Y</em>-intercept was observed approximately 20 years after cutting. Specifically, N scales as the 0.85-power of M across entire data set as a consequence of the systemic decrease in the numerical value of the allometric constant as succession progresses. Thus, the 3/4 scaling relationship of N vs. M likely results from neglecting the effects of shifting allometric constant with increasing stand age. Our data provide an example of how a systemic change in allometric constants could result indirectly in a significant change in the scaling exponent controlling the relationship between total N content and biomass at the level of an entire forest. Furthermore, the decrease in the allometric constants over stand development may be due to the concurrent decrease in the ratio of leaf biomass to total aboveground biomass (LAR) as succession proceeded. Such results suggest ways to bridge the gap between plant physiological constraints and stand (community) allometry, and highlight the importance in understanding and modeling the nutrient allocation patterns at the level of stand.