Vegetation patterns in heterogeneous landscapes: The importance of historyand environment

Throughout the eastern United States, plant species distributions and commu nity patterns have developed in response to heterogeneous environmental con ditions and a wide range of historical factors, including complex histories of natural and anthropogenic disturbance. Despite increased recognition of the importance of disturbance in determining forest composition and struct ure, few studies have assessed the relative influence of current environmen t and historical factors on modern vegetation, in part because detailed kno wledge of prior disturbance is often lacking. Ln the present study, we inve stigate modern and historical factors that control vegetation patterns at H arvard Forest in central Massachusetts, USA. Similar to the forested upland s throughout the northeastern United States, the site is physiographically heterogeneous and has a long and complex history of natural and anthropogen ic disturbance. However, data on forest composition and disturbance history collected over the past > 90 years allow us to evaluate the importance of historical factors rigorously, which is rarely possible on other sites. Soil analyses and historical sources document four categories of historical land use on areas that are all forested today: cultivated fields, improved pastures/mowings, unimproved pastures, and continuously forested woodlots. Ordination and logistic regressions indicate that although species have re sponded individualistically to a wide range of environmental and disturbanc e factors, many species are influenced by three factors: soil drainage, lan d use history, and C:N ratios. Few species vary in accordance with ionic gr adients, damage from the 1938 hurricane, or a 1957 fire. Contrary to our ex pectation that the effects of disturbance will diminish over time, historic al land use predicts 1992 vegetation composition better than 1937 compositi on, perhaps because historical woodlots have become increasingly differenti ated from post-agricultural stands through the 20th century. Interpretations of modem vegetation must consider the importance of histori cal factors in addition to current environmental conditions. However, becau se disturbances such as land use practices and wind damage are complex, it is often difficult to detect disturbance effects using multivariate approac hes, even when the broad history of disturbance is known.