Data from: Tree phenology responses to winter chilling, spring warming, at north and south range limits

Increases in primary production may occur if plants respond to climate warming with prolonged growing seasons, but not if local adaptation, cued by photoperiod, limits phenological advance. It has been hypothesized that trees with diffuse porous xylem anatomy and early successional species may respond most to warming. Within species, northern populations may respond most due to the fact that growing seasons are relatively short. Species most sensitive to spring temperature may show little overall response to warming if reduced chilling in fall/winter offsets accelerated winter/spring development. Because current thermal models consider only highly aggregated variables, e.g., degree days or chilling units (temperature sums for a season or year) they may not accurately represent warming effects. We show that assumptions contained in current thermal (degree-day) models are unrealistic for climate change analysis. Critical threshold parameters are not identifiable, and they do not actually have much to do with thresholds for development. Traditional models further overlook the discrete nature of observations, observation error, and the continuous response of phenological development to temperature variation. An alternative continuous development model (CDM) that addresses these problems is applied to a large experimental warming study near northern and southern boundaries of 15 species in the eastern deciduous forest of the US, in North Carolina and Massachusetts. Results provide a detailed time course of phenological development, including vernalization during winter and warming in spring, and challenge the basic assumptions of thermal models. Where traditional models find little evidence of a chilling effect (most are insignificant or have the wrong sign), the continuous development model finds evidence of chilling effects in most species. Contrary to the hypothesis that northern populations respond most, we find southern populations are most responsive. Because northern populations already have a compressed period for spring development they may lack flexibility to further advance development. A stronger response in the southern range could allow residents to resist northward migration of immigrants as climate warms. If potential invaders fail to exploit a prolonged growing season to the same degree as residents, then there is a resident advantage. Hypothesized effects of warming for xylem anatomy and successional status are not supported by the 15 species in this study.若植物通过延长生长季来响应气候变暖，则初级生产力（primary production）可能提升；但若由光周期（photoperiod）触发的本地适应（local adaptation）限制了物候提前（phenological advance），则不会出现该提升。已有假说提出，具有散孔材木质部解剖结构（diffuse porous xylem anatomy）的树木以及早期演替物种（early successional species）或许对气候变暖的响应最为强烈。在同一物种内，北方种群（northern populations）的响应或最为显著，这是因为其生长季相对较短。若秋冬季节冷激需求的减少抵消了冬春季节发育进程的加快，那么那些对春季温度最为敏感的物种，其整体对气候变暖的响应可能微乎其微。由于当前的热模型（thermal models）仅考虑高度聚合的变量，例如积温（degree days）或冷激单位（chilling units，即某一季或全年的温度累积量），因此它们可能无法准确反映气候变暖的影响。本研究表明，当前热（积温）模型所包含的假设，在气候变化分析中并不贴合实际。关键阈值参数无法被识别，且这些参数实际上与发育阈值并无太大关联。传统模型还进一步忽略了观测数据的离散性、观测误差，以及物候发育对温度变化的连续响应。本研究采用一种可解决上述问题的新型连续发育模型（continuous development model, CDM），并将其应用于一项大型野外增温实验：该实验在美国东部落叶林的北卡罗来纳州与马萨诸塞州开展，覆盖15个物种的南北分布边界区域。研究结果揭示了物候发育的详细时间进程，包括冬季的春化（vernalization）作用与春季的增温响应，并对热模型的基本假设提出了挑战。传统模型几乎未发现冷激效应的相关证据（多数结果不显著，甚至符号错误），而连续发育模型则在多数物种中均检测到了冷激效应。与“北方种群响应最为强烈”的假说相悖，本研究发现南方种群的响应最为显著。由于北方种群的春季发育周期本已较为紧凑，它们或许已缺乏进一步提前发育的灵活性。随着气候变暖，分布区南部种群更强的响应能力，可帮助本地种群抵御外来迁入者的北迁扩散。若潜在入侵物种无法像本地物种那样充分利用延长的生长季，则本地物种将具备竞争优势。本研究涉及的15个物种，并未支持“木质部解剖结构与演替状态会影响物种对气候变暖的响应”这一假说。