Re-evaluating pattern and process to understand resilience in transitional mixed conifer forests.

A key challenge to maintaining resilient landscapes is adapting to and maintaining dynamic ecological processes. In fire-dependent ecosystems, this includes identifying and defining mechanisms through which fire influences forest structure and functionality. Interpretations of tree patterns via land survey records in the Lake States have often highlighted the importance of infrequent moderate to extreme disturbance events. However, historical survey methods are limited to observing higher severity disturbances and over large landscapes, thus it is not clear if the origin, structure, and forcing factors for either patterns or processes are adequately quantified by these methods. We used dendrochronological methods to determine how fire history and stand structure, including cohort structure, tree density, and spatial patterning, are linked within Lake States mixed conifer forests in Wisconsin. We found relatively short mean fire return intervals (MFRIs) ranging from 6 to 13 yr with little variation in fire frequency among sites. Current densities of red-pine-dominated forests are 4-37 times historical (ca. 1860) densities (mean 12×) and almost entirely spatially random, whereas historically forests were spatially aggregated at stand scales. Stands also contained multiple and/or loosely defined cohort structures suggesting very different controls operating historically than currently. Heterogeneity that helped maintain ecosystem resilience in these ecosystems historically came from frequent fire disturbance processes that affected stand-scale forest resistance. This was likely the historical dynamic across fire-adapted transitional pine forests of the Lake States.

Tree recruitment in relation to climate and fire in northern Mexico.

Extensive changes in montane forest structure have occurred throughout the U.S. Southwest following Euro-American settlement. These changes are a product of confounding effects of disturbance, climate variability, species competition, and modern land use changes. Pronounced forest reproduction events in the Southwest have generally occurred in climatically wet periods but have also followed widespread fire exclusion. Understanding the ecological processes driving such events has important implications for forest restoration, although these efforts remain difficult due to confounding factors. Separation of these interacting factors was possible in the Sierra San Luis of northern Mexico where we investigated climate, fire, and tree recruitment in areas with continued frequent fires or where fire exclusion came relatively late (1940s). Fires were strongly tied to interannual wet-dry cycles of climate, whereas recruitment peaks were more closely tied to local processes, namely, fire-free periods, than to broad-scale climatically wet conditions. The greatest pulse of tree recruitment coincided with a pronounced mid-century drought (1942-1957) and a period of reduced fire frequency. The second largest pulse of recruitment (ca. 1900) preceded a well-documented period of recruitment (and an anomalously wet period) elsewhere across the Southwest in the 1910s-1920s, and also coincided with specific fire-free periods during below-average precipitation. We also found greater spatial dependence and clustering in older age classes of trees. This spatial pattern indicates a legacy of fire-induced mortality in shaping stand structure, underscoring the importance of frequent fire effects on spatial variability in forests.