A cross-scale assessment of historical and contemporary fire effects, forest conditions, and tree physiology in mixed-severity fire regime forests of the northern Rockies: Implications for forest restoration

Background/Question/Methods

Evaluation of restoration need is often based upon assessments of current ecosystem function or structure in relation to some reference range of conditions. Forest restoration efforts in many low-middle elevation forests are motivated by two main perceptions: 1) many low-mid elevation forests were historically dominated by low-severity fires which reduced the risk of future high-severity fire and minimized competition-related stress in surviving trees and 2) management-induced shifts in forest structure and climate change have increased the probability of high-severity fire and reduced individual tree vigor, making forests more prone to pathogen and insect related mortality. The high spatiotemporal variability of fire effects and forest conditions in mixed-severity fire regimes has, however, impeded clear assessment of restoration needs for many low-mid elevation forests. Here, we argue that evaluation of fire effects and forest conditions in mixed-severity fire regimes must incorporate multiple spatial scales (i.e. tree, stand and landscape level) and span long temporal scales if it is to capture inherent variability, identify its main drivers, and understand the consequences of past management and climate change to ecosystem function. We address these perceptions with data from forests of Pinus ponderosa/Pseudotsuga menziesii, mixed-conifer Larix occidentalis, and pure P. menziesii, three of the most widespread low-mid elevation forests in the northern Rockies that are characterized by mixed-severity fire regimes. We review the important role that data of historical fire effects provide for restoration assessment and outline the inherent methodological challenges and limitations which have impeded conceptual and practical advances in this field. Using ongoing research from forests of P. menziesii and mixed-conifer L. occidentalis, we describe a cross-scale, multiple methodology approach based on aerial photography analysis and dendroecology to construct high resolution spatiotemporal accounts of historical fire severity. 

Results/Conclusions

Preliminary results of these analyses indicate that many stands were initiated by high-severity fires, with sporadic subsequent low- and moderate-severity fire in a portion of stands. We also present field, aerial photography, and remote sensing data which show that despite shifts in forest structure and composition associated with past land management, current patterns of fire severity in low-middle elevation forests of the Rocky Mountains are not largely inconsistent with our understanding of historical fires. Finally, we present data from Pinus ponderosa/Pseudotsuga menziesii forests which show minimal impact of altered forest conditions on tree physiology. Restoration will be difficult to justify in some mixed-severity fire regimes and, where justifiable, restoration need will vary with management history.