Lake Sediment Pollen from Berry Pond in North Andover MA from 15000 BP to Present

Aim We analysed lake-sediment pollen records from eight sites in southern New England to address: (1) regional variation in ecological responses to post-glacial climatic changes, (2) landscape-scale vegetational heterogeneity at different times in the past, and (3) environmental and ecological controls on spatial patterns of vegetation. Location The eight study sites are located in southern New England in the states of Massachusetts and Connecticut. The sites span a climatic and vegetational gradient from the lowland areas of eastern Massachusetts and Connecticut to the uplands of north-central and western Massachusetts. Tsuga canadensis and Fagus grandifolia are abundant in the upland area, while Quercus, Carya and Pinus species have higher abundances in the lowlands. Methods We collected sediment cores from three lakes in eastern and north-central Massachusetts (Berry East, Blood and Little Royalston Ponds). Pollen records from those sites were compared with previously published pollen data from five other sites. Multivariate data analysis (non-metric multi-dimensional scaling) was used to compare the pollen spectra of these sites through time. Results Our analyses revealed a sequence of vegetational responses to climate changes occurring across southern New England during the past 14,000 calibrated radiocarbon years before present (cal yr BP). Pollen assemblages at all sites were dominated by Picea and Pinus banksiana between 14,000 and 11,500 cal yr BP; by Pinus strobus from 11,500 to 10,500 cal yr BP; and by P. strobus and Tsuga between 10,500 and 9500 cal yr BP. At 9500-8000 cal yr BP, however, vegetation composition began to differentiate between lowland and upland sites. Lowland sites had higher percentages of Quercus pollen, whereas Tsuga abundance was higher at the upland sites. This spatial heterogeneity strengthened between 8000 and 5500 cal yr BP, when Fagus became abundant in the uplands and Quercus pollen percentages increased further in the lowland records. The differentiation of upland and lowland vegetation zones remained strong during the mid-Holocene Tsuga decline (5500-3500 cal yr BP), but the pattern weakened during the late-Holocene (3500-300 cal yr BP) and European-settlement intervals. Within-group similarity declined in response to the uneven late-Holocene expansion of Castanea, while between-group similarity increased due to homogenization of the regional vegetation by forest clearance and ongoing disturbances. Main conclusions The regional gradient of vegetation composition across southern New England was first established between 9500 and 8000 cal yr BP. The spatial heterogeneity of the vegetation may have arisen at that time in response to the development or strengthening of the regional climatic gradient. Alternatively, the differentiation of upland and lowland vegetation types may have occurred as the climate ameliorated and an increasing number of species arrived in the region, arranging themselves in progressively more complex vegetation patterns across relatively stationary environmental gradients. The emergence of a regional vegetational gradient in southern New England may be a manifestation of the increasing number of species and more finely divided resource gradient.

研究目标
我们对新英格兰南部8个点位的湖相沉积物花粉记录展开分析，旨在解答三大核心科学问题：（1）冰期后气候变化下生态响应的区域差异；（2）不同历史时期景观尺度的植被异质性；（3）植被空间格局的环境与生态调控机制。

研究区域
8个研究点位均位于新英格兰南部的马萨诸塞州与康涅狄格州，覆盖了从马萨诸塞州东部、康涅狄格州低地到马萨诸塞州中北部及西部高地的气候与植被梯度。高地区域以加拿大铁杉（Tsuga canadensis）和美洲山毛榉（Fagus grandifolia）为优势类群，而低地区域则以栎属（Quercus）、山核桃属（Carya）和松属（Pinus）物种占比更高。

研究方法
我们从马萨诸塞州东部及中北部的3个湖泊（Berry East、Blood与Little Royalston Ponds）采集了沉积物岩芯，将这些点位的花粉记录与此前已发表的另外5个点位的花粉数据进行对比。采用多元数据分析方法——非度量多维尺度分析（non-metric multi-dimensional scaling），对不同时期各点位的花粉谱开展对比分析。

研究结果
本研究揭示了距今14000个校准放射性碳年（calibrated radiocarbon years before present, cal yr BP）以来，新英格兰南部地区植被对气候变化的一系列响应过程：在14000~11500 cal yr BP期间，所有点位的花粉组合均以云杉属（Picea）和班克松（Pinus banksiana）为主；11500~10500 cal yr BP期间以北美乔松（Pinus strobus）为主；10500~9500 cal yr BP期间则以北美乔松和铁杉属（Tsuga）为主。
但在9500~8000 cal yr BP，植被组成开始出现低地与高地点位的分化：低地点位的栎属花粉占比更高，而高地点位的铁杉属丰度更高。这种空间异质性在8000~5500 cal yr BP期间进一步增强，此时美洲山毛榉在高地区域变得丰富，低地记录中的栎属花粉占比进一步提升。全新世中期铁杉属衰减期（5500~3500 cal yr BP），高低地植被带的分化依然显著，但在全新世晚期（3500~300 cal yr BP）及欧洲殖民时期，该分化模式逐渐减弱。受栗属（Castanea）在全新世晚期不均衡扩张的影响，组内相似性有所下降；而由于森林砍伐与持续干扰导致区域植被同质化，组间相似性则有所上升。

主要结论
新英格兰南部地区植被组成的区域梯度最早形成于9500~8000 cal yr BP期间。植被的空间异质性可能正是在这一时期，响应区域气候梯度的形成或强化而产生；亦或是随着气候改善、越来越多的物种迁入该区域，物种在相对稳定的环境梯度上形成愈发复杂的植被格局，进而导致高低地植被类型的分化。新英格兰南部区域植被梯度的出现，或许反映了区域物种数量不断增加、资源梯度划分愈发精细的生态过程。