Laboratory of Tree-Ring Research, University of Arizona, Tucson, AZ 85721, USA.
Department of Earth and Climate Sciences, Bates College, Lewiston, ME 04240, USA.
Tree Physiol. 2021 Oct 4;41(10):1893-1905. doi: 10.1093/treephys/tpab045.
Tracking wood formation in semiarid regions during the seasonal march of precipitation extremes has two important applications. It can provide (i) insight into the adaptive capacities of trees to drought and (ii) a basis for a richer interpretation of tree-ring data, assisting in a deeper understanding of past and current climate. In the southwestern USA, the anatomical signature of seasonally bimodal precipitation is the 'false ring'-a band of latewood-like cells in the earlywood. These occur when a particularly deep drought during the early growing season ends abruptly with timely, mid-growing season monsoonal rains. Such conditions presented in southern Arizona in 2014, enabling us to explore false-ring formation in ponderosa pine (Pinus ponderosa Lawson and C. Lawson) and Douglas-fir (Pseudotsuga menziesii Mirb. Franco) in mixed-conifer forest at 2573 m above sea level. We ask: what were the cell-by-cell timings and durations in the phases of wood cell development in 2014? How do these seasonal patterns relate to strongly fluctuating environmental conditions during the growing season? We took weekly microcores from March through November from six ponderosa pine and seven Douglas-fir trees at a well-instrumented flux tower site. Thin sections were prepared, and we counted cells in cambial, expansion, cell wall thickening and mature phases. For ponderosa pine trees forming a false ring, the first impact of intensifying seasonal drought was seen in the enlarging phase and then, almost a month later, in cambial activity. In this species, recovery from drought was associated with recovery first in cambial activity, followed by cell enlargement. This timing raised the possibility that cell division may be affected by atmospheric moisture increases before soil recharge. In both species, the last false-ring cells matured during the summer rainy season. Bimodal cambial activity coincident with moisture availability was observed in both species, whether or not they formed a false ring. This deeper knowledge of the precise timing of both developmental and environmental events should help define mechanistic connections among these factors in creating bimodal growth patterns.
追踪半干旱地区降水极值季节性推移过程中的木材形成有两个重要应用。它可以提供（i）树木对干旱的适应能力的深入了解，以及（ii）对树木年轮数据进行更丰富解释的基础，有助于加深对过去和当前气候的理解。在美国西南部，季节性双峰降水的解剖学特征是“假年轮”-早材中类似于晚材的细胞带。这种情况发生在早期生长季节特别深的干旱突然结束，随后是及时的、生长中期的季风降雨时。2014 年亚利桑那州南部就出现了这种情况，使我们能够在海拔 2573 米的混合针叶林中探索黄松（Pinus ponderosa Lawson 和 C. Lawson）和花旗松（Pseudotsuga menziesii Mirb. Franco）的假年轮形成。我们提出了以下问题：2014 年，在树木细胞发育的各个阶段，细胞的时间和持续时间是怎样的？这些季节性模式与生长季节期间强烈波动的环境条件有何关系？我们从 3 月到 11 月每周从一个装备良好的通量塔站点的六棵黄松和七棵花旗松树上采集微芯。制备了薄片，并统计了形成层、扩展、细胞壁增厚和成熟阶段的细胞数。对于形成假年轮的黄松，季节性干旱加剧的第一个影响在扩大阶段显现，然后在近一个月后，在形成层活动中显现。在这个物种中，从干旱中恢复与形成层活动的恢复有关，然后是细胞扩大。这种时间安排提出了这样一种可能性，即细胞分裂可能会受到大气湿度增加的影响，然后才是土壤补给。在这两个物种中，最后一轮假年轮细胞在夏季雨季成熟。在这两个物种中，无论是否形成假年轮，都观察到与水分供应同时出现的双峰形成层活动。这种对发育和环境事件精确时间的更深入了解，应该有助于确定这些因素在创建双峰生长模式方面的机制联系。
