Ecosystem physiology responses to global change
Ecosystem physiology responses to global change
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发表时间：
1998
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通讯作者：
H. Mooney;J. Canadell;F. Chapin;J. Ehleringer;C. Körner;R. Mcmurtie;W. Parton;L. Pitelka;E. S
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作者：
H. Mooney;J. Canadell;F. Chapin;J. Ehleringer;C. Körner;R. Mcmurtie;W. Parton;L. Pitelka;E. S
■ 大多数暴露于双倍环境CO 2 的生态系统显示出比在当前环境CO 2 下生长的生态系统更高的旺季净碳吸收量。对于草地来说，地上生物量平均增加了 14%，尽管特定系统和年份的个体反应范围从负到 + 85%。生物量响应的广泛范围显示了C 0 2 响应与其他环境因素（包括水和养分利用率以及温度）的高度相互作用的性质。例如，以低温为主的系统，如高山草原、北极苔原和凉爽气候的针叶林树，是对 CO 2 升高反应最弱的系统之一，在某些情况下，几年后没有生长反应并完全适应了旺季气体交换。在 CO 2 升高的情况下，年度净生态系统生产力 (NEP) 尚未得到可靠测量。幼树生长较快并不表明整个森林是否会吸收更多的碳。 ■ 对于一些最重要的功能群，植物生长对升高的 CO 2 的反应程度的一般预测（例如，C 4 物种相对于 C 3 物种的不敏感性；具有固氮共生体的物种的较大生物量响应）在自然生长条件下暴露于升高的 CO 2 的复杂物种组合中并不能一致地实现。 ■ 与早期预测相反，高C 02 生长的植物凋落物的分解速度不一定更慢。该结果的基础是发现在升高的CO 2 下生长的自然衰老凋落物中的碳氮比通常不高于在环境CO 2 下生长的相同组织中的碳氮比，尽管绿色组织中的C:N较高。然而，物种之间存在很大的差异。这一发现对长期生态系统反应具有重大影响。在考虑生态系统生理学对全球变化的反应时，需要区分对过程的直接影响（例如生长）和通过物种组成（生物多样性）变化产生的间接影响。在与碳循环相关的直接反应中，需要明确三个：（1）季节性植物生物量积累（通常称为“生产”）； (2)年净初级生产力（NPP，通过光合作用固定的干物质减去呼吸和建筑成本造成的损失；还必须考虑草食动物和凋落物造成的新建生物量的损失。真正的NPP很少被估计，因为地下生产和凋落物回收通常仍然未知。）； (3)年度净生态系统生产力(NEP)，即单位土地面积碳库的净变化。特别重要的是要注意，季节性生物量积累（最常研究的响应）并不是 NEP（碳封存）的衡量标准。如果考虑大面积和长时间范围，则在评估碳固存时需要考虑物种组成和干扰状况的变化。在这里，网络生物生产力（NBP）的概念是最合适的。参见第 10.4 节（尤其是图 10.2）和第 12 节。 2.2 更详细地讨论了 N PP、N EP 和 N BP 与陆地碳循环的关系。
■ Most ecosystems exposed to double ambient CO 2 show higher peak season net carbon uptake than those growing at current-ambient CO 2 . For grasslands, above-ground biomass increased by an average of 14 %, although individual responses for a given system and year range from negative to + 85 %. The wide range of the biomass response shows the highly interactive nature of the C 0 2 response with other environmental factors, including water and nutrient avail ability, and temperature. For instance, low-temperature dominated systems, such as alpine grassland, Arctic tundra, and cool climate coniferous forest trees, are among the least responsive to elevated CO 2 , showing in some instances no growth response and complete acclimation of peak season gas exchange after a few years. Annual Net Ecosystem Productivity (NEP) has not been reliably measured under elevated CO 2 . Faster growth in juvenile trees does not indicate whether forests as a whole will sequester more carbon or not. ■ The general predictions of the degree of responsiveness of plant growth to elevated CO 2 for some of the most important functional groups (e.g. insensitiv ity of C 4 species relative to C 3 species; larger biomass response of species with N-fixing symbionts) are not consistently realized in complex species assem blages exposed to elevated CO 2 under natural growth conditions. ■ Contrary to early predictions, litter of high C 02-grown plants does not necess arily decompose more slowly. The basis for this result is the finding that the ratio of carbon to nitrogen in naturally senesced litter grown at elevated CO 2 is normally no higher than in the same tissue grown at ambient CO 2 , despite the higher C : N in green tissues. A great deal of variation, however, occurs among species. This finding has major implications for long-term ecosystem responses. W hen considering responses o f ecosystem physiology to global change, direct effects on processes (e.g. growth) and indirect effects via changes in species composition (biodiversity) need to be distinguished. Am ong the direct responses related to the carbon cycle, three need to be clearly identified: ( 1 ) seasonal plant biomass accumulation (often termed ‘production’); ( 2 ) annual net primary produc tion (N PP, dry matter fixed through photosynthesis m inus losses due to respiration and construction costs; losses of newly built biomass to herbivores and litter must also be accounted for. True N PP has rarely been estimated because belowground production and litter recycling usually remain unknown.); and ( 3 ) annual net ecosystem productivity (NEP), i.e. the net change in carbon pools per unit land area. It is particularly important to note that seasonal biomass accumulation, the response most com monly investigated, is not a measure of NEP (carbon sequestra tion). If large areas and long time-frames are considered, both the changes in species com position and in disturbance regimes need to be included in evaluating carbon sequestration. Here the concept of N et Biom e Productivity (NBP) is most appropriate. See Sections 10.4 (especially Fig. 10 . 2 ) and 12 . 2.2 for a more detailed discussion of N PP, N EP and N BP in relation to the terrestrial carbon cycle.