Phenology and Vegetation Growth in Prospect Hill Soil Warming Experiment at Harvard Forest 1992-1993

As the mean annual temperature of northeast North America rises as a component of global climatic change, it is important to understand how the predominant vegetation of the region will be affected. Existing experimental and correlative evidence from field sites suggests that temperature rise will significantly modify soil processes, nutrient availability, and plant growth. We investigated the responses of temperate deciduous forest vegetation to artificial soil warming at 20 sampling dates during the 1992 and 1993 growing season. We explored whether soil warming measurably altered growth and the temporal dynamics of leaf and fruit production in 26 species of three contrasting plant growth forms (herbaceous perennials, shrubs, and canopy trees). We hypothesized that soil warming would exert differential effects on emergence, phenology, leaf expansion rates, growth, photosynthesis, and vegetative and sexual reproduction among species, with implications for changing community structure in these forests. Timing of leaf emergence and flower production was not affected by treatment in saplings; however, mature trees and shrubs leafed out slightly earlier and in larger numbers in heated plots. Soil warming significantly enhanced relative growth in stem diameters of woody plants, especially shrubs, in 1992. This effect was less pronounced in 1993. Species richness was lower in heated plots than in intact control plots in both years; disturbed but unheated control plots showed the lowest species richness of all plots. Changes in relative abundance of herbaceous species from 1992 to 1993 were not significantly affected by treatment. Rank abundances of species were more stable between years in the heated and disturbance-control plots than in the intact plots. Total density of herbaceous species was highest in heated plots during April and May of both years, reflecting greatly accelerated emergence of two dominant species, Maianthemum canadense and Uvularia sessilifolia, due to heating. Stem densities declined later in the season, however, and were lower in the heated plots than in the intact control plots. Photosynthetic rates of the two dominant herbaceous species were not affected by soil warming. Of all life forms, herbaceous species (spring ephemerals) were most sensitive to soil warming. Their early appearance could influence carbon and nitrogen acquisition by later-emerging woody species in these deciduous forests. Early leaf-out in woody shrubs and trees can also influence season-long dynamics of water use and carbon sequestration in forests. Soil warming may select for certain tolerant species and reduce species richness in temperate deciduous forests.作为全球气候变化的组成部分，北美东北部的年平均气温正逐步升高，因此厘清该区域优势植被所受的影响具有重要意义。现有来自野外样地的实验与关联研究证据表明，气温升高会显著改变土壤过程、养分有效性及植物生长状况。本研究于1992年和1993年的生长季内，在20个采样时间点开展了温带落叶林（temperate deciduous forest）植被对人工土壤增温（soil warming）的响应实验。我们探究了土壤增温是否会显著改变3类不同植物生长型（多年生草本、灌木以及冠层乔木）共26个物种的生长情况，以及叶、果生产的时间动态。我们提出假说：土壤增温会对不同物种的萌发、物候、叶片扩展速率、生长、光合作用以及营养繁殖与有性繁殖产生差异化影响，进而可能改变该类森林的群落结构。 幼树的展叶与开花时间未受增温处理的影响；但成熟乔木与灌木在增温样地中展叶时间略早，且展叶个体数量更多。1992年，土壤增温显著促进了木本植物（尤其是灌木）的茎径相对生长；但该效应在1993年已不甚显著。两年间，增温样地的物种丰富度均低于未受干扰的对照样地；而受干扰但未增温的对照样地的物种丰富度为所有样地中最低。 1992年至1993年间，草本植物相对多度的变化未受增温处理的显著影响。相较于未受干扰的对照样地，增温样地与受干扰对照样地的物种多度排名在年间更为稳定。 在两年的4月与5月期间，增温样地的草本植物总密度最高，这一现象源于增温显著加速了两个优势物种——加拿大舞鹤草（Maianthemum canadense）与无柄延龄草（Uvularia sessilifolia）的萌发进程。但在生长季后期，增温样地的茎秆密度有所下降，且低于未受干扰的对照样地。 这两种优势草本植物的光合速率并未受土壤增温的影响。 在所有生活型中，草本植物（春季短命植物）对土壤增温最为敏感。它们的提前萌发可能会影响该落叶林内后续萌发的木本物种对碳、氮的获取能力。 木本灌木与乔木的提前展叶，同样会影响森林整个生长季的水分利用与碳固持动态。 土壤增温可能会筛选出部分耐受物种，并降低温带落叶林的物种丰富度。