Wood productivity in plantation forests depends on successful choices of genetic materials and silvicultural practices. Wisely defining the intitial tree spacing according to edaphoclimatic conditions is important as spacing influences the amount of water, nutrients and light available for each tree. Resource availability, in turn, influences some important processes controlling wood production such as tree survival, CO₂ uptake, and partitioning of photosynthates to stem wood and other tree compartments. Different genotypes may respond differently to tree spacing, due to possible differences in their canopy structure, physiology, and strategies for soil ressources acquisitions and carbon allocations. Our objective in this study was to investigate the interactive effects of genotype and tree spacing on wood production of fast-growing clonal eucalypt plantations in Southeastern Brazil. Tree growth, gross primary production (GPP) and partitioning of GPP to total belowground carbon flux (TBCF), wood and leaf production, and aboveground respiration were assessed in an experimental trial involving three commercial E. urophylla clones (AEC 0144, AEC 224 and COP 1404) planted at two spacings (3 m × 2 m; 3 m × 4 m). We hypotetized that (1) the most productive genotypes are those with both the highest GPP and the highest carbon partitioning to stem wood production; and (2) at closer spacing, leaf area index (LAI) and GPP are higher, but the partitioning to wood production is lower, and the belowground partitioning is higher, due to reduced soil water and nutrients availability per tree. Biomass and LAI were measured at ages 40 months (mid-rotation) and 52 months, and GPP, TBCF, wood production and leaf production were estimated between these two stand ages. Stem biomass and LAI were higher, for all clones, at the closer spacing (3 m × 2 m), as we could have expected since this spacing (corresponding to a stocking of 1667 tree ha⁻¹) is the most commonly chosen in industrial eucalypt plantations in this region. Consistently with clonal ranking established in other studies, at spacing 3 m × 2 m clones were ranked as AEC 0144 > AEC 224 > COP 1404 for their wood production, but this ranking almost vanished at wide spacing. Our first hypothesis was supported by data: at spacing 3 m × 2 m the most productive clones (AEC 0144 > AEC 224 > COP 1404) were those with both the highest GPP (6.14 > 4.63 > 4.57 kg C m⁻² year⁻¹) and the highest carbon partitioning to wood production (0.33 > 0.29 > 0.26), although the differences between COP 1404 and AEC 224 were not significant (at P = 0.05). Our second hypothesis was only partially supported by data: wood biomass at age 40 months was higher at the closer spacing, but this positive effect of tight spacing seemed to decrease with stand age, as wood production and GPP measured between ages 40 and 52 months remained higher at the closer spacing only for clone AEC 0144. There was no consistent difference in the partitioning of GPP to wood production between the two spacings. The age-related decrease of the positive effect of high stockings may result from a faster depletion of deep soil water reserves in dense stands.

人工林的木材生产率取决于遗传材料和造林实践的成功选择。根据生态环境条件明智地定义初始树的间距很重要，因为间距会影响每棵树可用的水，养分和光的数量。反过来，资源的可用性会影响一些控制木材生产的重要过程，例如树木的存活，CO ₂光合产物的吸收和分配，将其分解成茎木和其他树木。由于不同的基因型在冠层结构，生理和土壤资源获取和碳分配策略上可能存在差异，因此不同的基因型对树木的间距可能会有不同的反应。我们在这项研究中的目的是调查基因型和树间距对巴西东南部快速生长的无核桉树人工林木材生产的交互影响。在一项涉及三个商业化的E. urophylla的实验试验中，评估了树木的生长，初级总产值（GPP）以及GPP分配至地下总碳通量（TBCF），木材和叶片的产量以及地上呼吸作用以两个间距（3 m×2 m; 3 m×4 m）种植无性系（AEC 0144，AEC 224和COP 1404）。我们假设（1）生产力最高的基因型是茎木生产同时具有最高GPP和最高碳分配的基因型；（2）在较小的间距下，叶面积指数（LAI）和GPP较高，但由于每棵树的土壤水分和养分利用率降低，对木材生产的分配较低，而地下分配较高。在40个月（中度旋转）和52个月的年龄测量了生物量和LAI，并估计了这两个林龄之间的GPP，TBCF，木材产量和叶片产量。对于所有克隆，茎生物量和LAI都较高，且间隔较小（3 m×2 m），这是自该间隔以来我们可以预期的（对应于1667棵树ha ^-1的放养量））是该地区工业桉树人工林中最常用的选择。与其他研究中确定的克隆等级一致，在3 m×2 m的克隆中，其木材产量的等级为AEC 0144> AEC 224> COP 1404，但该排名在宽间距时几乎消失了。我们的第一个假设得到了数据的支持：在间隔3 m×2 m时，生产力最高的克隆（AEC 0144> AEC 224> COP 1404）是具有最高GPP的克隆（6.14> 4.63> 4.57 kg C m - ²年^-1））和最高的碳分配至木材生产（0.33> 0.29> 0.26），尽管COP 1404和AEC 224之间的差异并不显着（P = 0.05）。我们的第二个假设仅部分得到数据的支持：40个月龄的木材生物量在更近的间距处较高，但紧密间距的这种积极影响似乎随着林分龄的增加而降低，因为在40到52个月龄之间的木材产量和GPP仍然存在仅在克隆AEC 0144中，在更接近的间距处具有更高的间距。在这两个间距之间，GPP与木材生产的划分没有一致的差异。与年龄有关的高放养积极作用的下降可能是由于密集林分中较快的土壤深层水储量枯竭所致。