Western Larch Regeneration Responds More Strongly to Site and Indirect Climate Factors Than to Direct Climate Factors

Abstract

Substantial shifts in the distribution of western larch (Larix occidentalis Nutt.) are predicted during the coming decades in response to changing climatic conditions. However, it is unclear how the interplay between direct climate effects, such as warmer, drier conditions, and indirect climate effects, such as predicted increases in fire disturbance, will impact fire-adapted species such as western larch. The objectives of this study were (1) to compare the relative importance of stand, site, and indirect versus direct climatic factors in determining western larch seedling recruitment; (2) to determine whether seedling recruitment rates have changed in recent years in response to disturbance, post-fire weather, and/or climate; and (3) to determine whether seedlings and mature trees are experiencing niche differentiation based on recent climatic shifts. We addressed these objectives using data collected from 1286 national forest inventory plots in the US states of Idaho and Montana. We used statistical models to determine the relative importance of 35 stand, site, and climatic factors for larch seedling recruitment. Our results suggest that the most important predictors of larch seedling recruitment were indicative of early-seral stand conditions, and were often associated with recent fire disturbance and cutting. Despite indications of climatic niche compression, seedling recruitment rates have increased in recent decades, likely due to increased fire disturbance, and were unrelated to post-fire weather. Compared to sites occupied by mature trees, seedling recruitment was positively associated with cooler, drier climatic conditions, and particularly with cooler summer temperatures, but these climatic factors were generally less important than biotic stand variables such as stand age, basal area, and canopy cover. These results suggest that, for fire-dependent species such as western larch, increased heat and drought stress resulting from climatic change may be offset, at least in the near term, by an increase in early-seral stand conditions resulting from increased fire disturbance, although localized range contraction may occur at warm, dry extremes.

Vegetative and Edaphic Responses in a Northern Mixed Conifer Forest Three Decades after Harvest and Fire: Implications for Managing Regeneration and Carbon and Nitrogen Pools

“…The ability of L. occidentalis to readily regenerate when presented with early seral stand conditions as the result of fire has allowed this species to respond favorably to short-term changes in climate [119]. The continuing shift toward a hotter and drier climate, which facilitates more frequent and intense fires, has benefitted seedling recruitment, indicating that climatic factors are apparently less important to regeneration than those associated with overstory competition [119].…”

“…Our results appear to concur with that conclusion, and confirm that L. occidentalis regeneration can be supported by harvest and prescribed fire in mixed conifer forests, particularly on southern and western aspects. As the climate niche for L. occidentalis continues to retract within its current range and regeneration shifts to the cool, dry subset of the sites now occupied by this species [119], land managers may need to place additional focus on eastern and northern aspects. Our results show that L. occidentalis can be regenerated with harvest and prescribed fire in these cooler, moister sites as well, although additional treatment(s) may be necessary to control competition that would otherwise prevent this seral, shade-intolerant species from establishing its renowned place in western USA landscapes and from fulfilling desired ecosystem functions.…”