AN EXPERIMENTAL TEST OF THE CAUSES OF FOREST GROWTH DECLINE WITH STAND AGE
AN EXPERIMENTAL TEST OF THE CAUSES OF FOREST GROWTH DECLINE WITH STAND AGE
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DOI：
10.1890/03-4037
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发表时间：
2004-02
影响因子：
6.1
通讯作者：
M. G. Ryan;D. Binkley;J. Fownes;C. Giardina;R. Senock
中科院分区：
文献类型：
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作者：
M. G. Ryan;D. Binkley;J. Fownes;C. Giardina;R. Senock
同龄林郁闭后地上木材产量下降是森林中的一种常见模式，但缺乏导致下降的机制的明确证据。这个问题对森林生物学、商业林业（衰退决定了轮伐期）和森林的碳储存都是至关重要的。我们测试了三个假设的机制，导致木材生长下降，通过量化的完整的碳预算的发展代表了6年（一个完整的旋转）在复制的人工林的柳桉附近Pepeekeo，夏威夷。我们的第一个假设是，总初级生产力（GPP）不随林龄下降，木材生长的下降是由于从木材生产到呼吸（树木生物量积累），地下总碳分配（由于土壤养分供应下降），或这些或其他汇的组合的变化。另一种假设是，GPP下降与林分年龄和地上木材生产量的下降是成比例的GPP下降。GPP的下降可能是由于营养限制增加、树冠间磨损增加、膨压降低导致叶片膨胀或水通量的水力限制导致树冠叶面积和光合能力降低而导致的。最后一个假设是前两个假设的结合：全球生产力下降，但木材产量的下降幅度不成比例地更大，因为分配也发生了变化。我们测量了整个年度碳收支（地上生产和呼吸，地下总碳分配[TBCA]和GPP）从0.5年后的幼苗种植到6 1/2年（当树木高约25米）。复制的样地包括两种密度的树木（1111棵树/公顷和10 000棵树/公顷），以改变个体树木的树冠叶质量与木材质量的比例，以及三种施肥制度（最小，密集，以及三年后最小接着密集），以评估营养在塑造GPP和地上木材产量下降中的作用。森林在1-2年内关闭了树冠，地上木材产量达到峰值，与树冠关闭一致，为1.2-1.8 kg C.m-2 yr-1。林下木材产量从2龄时的1.4 kg C.m-2 yr-1下降到6龄时的0.60 kg C.m-2 yr-1。假设1失败：GPP从2岁的5.0 kg C.m-2 yr-1下降到6岁的3.2 kg C.m-2 yr-1。林地木质呼吸从2龄的0.66 kg C.m-2 yr-1下降到6龄的0.22 kg C.m-2 yr-1，TBCA从2龄的1.9 kg C.m-2 yr-1下降到6龄的1.4 kg C.m-2 yr-1。我们的数据支持假设3：地上部木材生产的下降（峰值的42%）成比例大于冠层光合作用的下降（峰值的64%）。分配到地下的GPP和叶片呼吸的分数随着林龄的增加而增加，并导致地上木材生产量的下降。GPP的下降不是由营养限制，叶面积或光合能力下降，或（从同一网站上的相关研究）由水力限制。营养与GPP和地上部木材产量的下降相互作用，因为具有高营养有效性的处理比我们的对照处理下降得更慢，对照处理仅在林分建立期间施肥。
The decline in aboveground wood production after canopy closure in even-aged forest stands is a common pattern in forests, but clear evidence for the mechanism causing the decline is lacking. The problem is fundamental to forest biology, commercial forestry (the decline sets the rotation age), and to carbon storage in forests. We tested three hypotheses about mechanisms causing the decline in wood growth by quantifying the complete carbon budget of developing stands for over six years (a full rotation) in replicated plantations of Eucalyptus saligna near Pepeekeo, Hawaii. Our first hypothesis was that gross primary production (GPP) does not decline with stand age, and that the decline in wood growth results from a shifft in partitioning from wood production to respiration (as tree biomass accumulates), total belowground carbon allocation (as a result of declining soil nutrient supply), or some combination of these or other sinks. An alternative hypothesis was that GPP declines with stand age and that the decline in aboveground wood production is proportional to the decline in GPP. A decline in GPP could be driven be reduced canopy leaf area and photosynthetic capacity resulting from increasing nutrient limitation, increased abrasion between tree canopies, lower turgor pressure to drive foliar expansion, or hydraulic limitation of water flux as tree height increases. A final hypothesis was a combination of the first two: GPP declines, but the decline in wood production is disproportionately larger because partitioning shifts as well. We measured the entire annual carbon budget (aboveground production and respiration, total belowground carbon allocation [TBCA], and GPP) from 0.5 years after seedling planting through 6 1/2 years (when trees were ~25m tall). The replicated plots included two densities of trees (1111 trees/ha and 10 000 trees/ha) to vary the ratio of canopy leaf mass to wood mass in the individual trees, and three fertilization regimes (minimal, intensive, and minimal followed by intensive after three years) to assess the role of nutrition in shaping the decline in GPP and aboveground wood production. The forest closed its canopy in 1-2 years, with peak aboveground wood production, coinciding with canopy closure, of 1.2-1.8 kg C.m-2yr-1. Aboveground wood production declined from 1.4 kg C.m-2yr-1 at age 2 to 0.60 kg C.m-2yr-1 at age 6. Hypothesis 1 failed: GPP declined from 5.0 kg C.m-2yr-1 at age 2 to 3.2 kg C.m-2yr-1 at age 6. Aboveground woody respiration declined from 0.66 kg C.m-2yr-1 at age 2 to 0.22 kg C.m-2yr-1 at age 6 and TBCA declined from 1.9 kg C.m-2yr-1 at age 2 to 1.4 kg C.m-2yr-1 at age 6. Our data supported hypothesis 3: the decline in aboveground wood production (42% of peak) was proportionally greater than the decline in canopy photosynthesis (64% of peak). The fraction of GPP partitioned to belowground allocation and foliar respiration increased with stand age and contributed to the decline in aboveground wood production. The decline in GPP was not caused by nutrient limitation, a decline in leaf area or in photsynthetic capacity, or (from a related study on the same site) by hydraulic limitation. Nutrition did interact with the decline in GPP and aboveground wood production, because treatments with high nutritient availablity declined more slowly than did our control treatment, which was fertilized only during stand establishment.