Northern White-Cedar Stand Structure, Composition, and Ecophysiology: Implications for Management and Climate Change Resilience

Low rates of regeneration and recruitment for northern white-cedar (Thuja occidentalis) have been observed in lowland forests in some parts of its range, especially in comparison to its main tree competitor, balsam fir (Abies balsamea). In addition, climate projections suggest that suitable habitat conditions for the two species will decline in the northeastern United States and adjacent Canada due to widespread warming and increased variation in the timing and amount of precipitation. As such, future climate conditions may alter the competitive dynamics of these two species, which poses a major challenge for mitigating the negative effects of climate change through stand management. Therefore, we investigated the lowland stand conditions associated with cedar and fir regeneration and quantified how those conditions changed after a partial harvest. Additionally, we determined how drought and flood treatments affected cedar and fir sapling survival, growth, and physiology in a greenhouse. In the field assessment of stand conditions associated with regeneration, we found that species composition at the study sites was dominated by cedar in the overstory, fir in the sapling 3 layer, and both cedar and fir in the seedling layer. Although there was no significant difference between the density of cedar and fir seedlings, density of fir saplings was significantly greater than that of cedar. These results, consistent with the findings of other research, suggest that there is a recruitment bottleneck between the seedling and sapling class for cedar but not for fir. Cedar and fir seedlings were associated with many of the same site conditions, such as microtopographic mounds and greater proportion of overstory cedar. Except for decreasing canopy closure, the harvesting operations had no significant impact on microsite conditions typically associated with cedar regeneration at the whole-stand scale, such as the microtopographic variability of the soil surface and the amount of coarse woody debris that can be used as regeneration sites. These data suggest that current conditions at these sites are equally suitable for cedar and fir regeneration, but are more suitable for fir recruitment, which may allow fir to outcompete cedar in the sapling recruitment stage. In the greenhouse experiment, we used estimates of percent brown foliage as a proxy for mortality and found that drought treatments were more detrimental to cedar, and flood treatments were more detrimental to fir. The percentage of brown foliage for cedar increased at a greater rate in comparison to fir because cedar experienced drought conditions more quickly and for a greater duration than fir. This pattern was driven by cedar's high water use in comparison to fir, which was likely at least partly due to its larger size despite being the same age. However, cedar was also found to have a greater ability than fir to survive and recover from extreme drought and flood stress than fir. Previously documented physiological drought-resistance characteristics such as radially sectored hydraulic pathways and the capacity to maintain conductivity at low water potentials likely explain this ability of cedar to recover. Important physiological indicators of drought and flood stress were also identified for both species. These data suggest that although 4 cedar was negatively affected by drought and flood conditions, cedar may be more resilient to those conditions than fir. Although the hydrologic conditions of lowland systems are predicted to have a ‘thermal buffering’ effect on climate-growth responses, the moisture regimes to which cedar and fir are adapted may be subjected to periods of increased drought or flooding. Our results suggest that both cedar and fir may be sensitive to future climate conditions and thus are at risk due to climate change. If future moisture regimes favor cedar over fir, climate change may reduce the need for population management strategies for reducing density of fir competition. Currently, competition control to release desired submerchantable cedar stems may be warranted, but continued monitoring is needed to determine longer-term effects of management and climate change on regeneration and recruitment.

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