Masting is shaped by tree-level attributes and stand structure, more than climate, in a Rocky Mountain conifer species

Many tree species mast, meaning seed production is highly variable from year to year and synchronous within a stand, but this phenomenon remains poorly understood. To better understand how a changing climate, altered disturbance regimes, or novel management strategies might affect future seed production, we quantified the joint influence of both biotic (tree size, age, and neighborhood competition) and abiotic factors (climate and weather) on seed production in a widespread conifer species, Rocky Mountain ponderosa pine (Pinus ponderosa var. scopulorum). We reconstructed individual-level annual cone production across a large portion of this species' range using the cone abscission scar method, and mixed models were used to test hypotheses related to the causes and drivers of masting in this species. Our results suggest that masting in ponderosa pine is a process shaped at the individual-level, and this leads to high, local-scale variation in annual cone production. The effects of weather were strongest at climatically marginal sites, but overall, the joint effects of weather and climate only weakly described individual-level patterns of annual cone production in ponderosa pine (R2m = 1.6%, R2c = 30.1%). Rather, we found that masting was strongly influenced by tree- and stand-level factors such as diameter, age, and local neighborhood density, all of which were associated with the mean, interannual variability, and between-tree synchrony of cone production at the individual-level. Larger and older trees produced more cones, more frequently, and with less synchrony than smaller and younger trees. Open-grown trees experiencing lower levels of neighborhood competition also produced more cones with less interannual variability, but with higher between-tree synchrony. Because tree- and stand-level traits appear to regulate seed production more strongly than climate or weather in this species, management interventions targeting these factors could be powerful tools to manage future tree recruitment. Thus, current efforts to reduce stand density and conserve large trees in some ponderosa pine forests may enhance tree-level seed production and reduce variability in seed crops among years.

诸多树木物种均存在结实周期性（masting）现象，即种子产量年际波动剧烈且林分内个体间结实同步性较强，但目前学界对该现象的认知仍较为匮乏。为明晰气候变化、扰动格局改变或新型经营策略如何影响未来种子产量，本研究针对广布针叶树种落基山黄松（Pinus ponderosa var. scopulorum），量化了生物因子（树木尺寸、树龄与邻体竞争）与非生物因子（气候与天气）对其种子产量的联合影响。本研究通过球果脱落痕法，重建了该物种分布区内大片区域的单株年球果产量序列，并采用混合效应模型检验了与该物种结实周期性成因及驱动机制相关的假说。研究结果表明，黄松的结实周期性是受单株水平调控的过程，这也导致了年球果产量在局域尺度上存在显著变异。天气因子的影响在气候边际生境中最为显著，但整体而言，天气与气候的联合效应仅能较弱地解释黄松单株年球果产量的变化规律（边际决定系数R²_m=1.6%，条件决定系数R²_c=30.1%）。与之相反，本研究发现结实周期性强烈受单株及林分水平因子调控，例如胸径、树龄与局域邻体密度，这些因子均与单株水平下球果产量的均值、年际变异性以及株间同步性显著相关。相较于小型幼树，大型老树的球果产量更高、结实频次更强，且株间同步性更低。邻体竞争强度较低的散生木同样可产出更多球果，且年际变异性更低，但株间同步性更高。鉴于相较于气候与天气因子，单株及林分性状对该物种种子产量的调控作用更强，因此针对这些因子的经营干预措施可成为调控未来林木更新的有效手段。因此，当前部分黄松林分中开展的降低林分密度、保护大型林木的营林措施，或可提升单株种子产量，并降低年度种子产量的变异性。