Effects of postfire climate and seed availability on postfire conifer regeneration.

Large, severe fires are becoming more frequent in many forest types across the western United States and have resulted in tree mortality across tens of thousands of hectares. Conifer regeneration in these areas is limited because seeds must travel long distances to reach the interior of large burned patches and establishment is jeopardized by increasingly hot and dry conditions. To better inform postfire management in low elevation forests of California, USA, we collected 5-yr postfire recovery data from 1,234 study plots in 19 wildfires that burned from 2004-2012 and 18 yrs of seed production data from 216 seed fall traps (1999-2017). We used these data in conjunction with spatially extensive climate, topography, forest composition, and burn severity surfaces to construct taxon-specific, spatially explicit models of conifer regeneration that incorporate climate conditions and seed availability during postfire recovery windows. We found that after accounting for other predictors both postfire and historical precipitation were strong predictors of regeneration, suggesting that both direct effects of postfire moisture conditions and biological inertia from historical climate may play a role in regeneration. Alternatively, postfire regeneration may simply be driven by postfire climate and apparent relationships with historical climate could be spurious. The estimated sensitivity of regeneration to postfire seed availability was strongest in firs and all conifers combined and weaker in pines. Seed production exhibited high temporal variability with seed production varying by over two orders of magnitude among years. Our models indicate that during droughts postfire conifer regeneration declines most substantially in low-to-moderate elevation forests. These findings enhance our mechanistic understanding of forecasted and historically documented shifts in the distribution of trees.

Post-fire forest regeneration shows limited climate tracking and potential for drought-induced type conversion.

Disturbance such as wildfire may create opportunities for plant communities to reorganize in response to climate change. The interaction between climate change and disturbance may be particularly important in forests, where many of the foundational plant species (trees) are long-lived and where poor initial tree establishment can result in conversion to shrub- or graminoid-dominated systems. The response of post-disturbance vegetation establishment to post-disturbance weather conditions, particularly to extreme weather, could therefore provide useful information about how forest communities will respond to climate change. We examined the effect of post-fire weather conditions on post-fire tree, shrub, and graminoid recruitment in fire-adapted forests in northern California, USA, by surveying regenerating vegetation in severely burned areas 4-5 yr after 14 different wildfires that burned between 2004 and 2012. This time period (2004-2016) encompassed a wide range of post-fire weather conditions, including a period of extreme drought. For the most common tree species, we observed little evidence of disturbance-mediated community reorganization or range shifts but instead either (1) low sensitivity of recruitment to post-fire weather or (2) weak but widespread decreases in recruitment under unusually dry post-fire conditions, depending on the species. The occurrence of a single strong drought year following fire was more important than a series of moderately dry years in explaining tree recruitment declines. Overall, however, post-fire tree recruitment patterns were explained more strongly by long-term climate and topography and local adult tree species abundance than by post-fire weather conditions. This observation suggests that surviving adult trees can contribute to a "biological inertia" that restricts the extent to which tree community composition will track changes in climate through post-disturbance recruitment. In contrast to our observations in trees, we observed substantial increases in shrub and graminoid establishment under post-fire drought, suggesting that shifts in dominance between functional groups may become more likely in a future with more frequent and intense drought.

From the stand scale to the landscape scale: predicting the spatial patterns of forest regeneration after disturbance.

Shifting disturbance regimes can have cascading effects on many ecosystems processes. This is particularly true when the scale of the disturbance no longer matches the regeneration strategy of the dominant vegetation. In the yellow pine and mixed conifer forests of California, over a century of fire exclusion and the warming climate are increasing the incidence and extent of stand-replacing wildfire; such changes in severity patterns are altering regeneration dynamics by dramatically increasing the distance from live tree seed sources. This has raised concerns about limitations to natural reforestation and the potential for conversion to non-forested vegetation types, which in turn has implications for shifts in many ecological processes and ecosystem services. We used a California region-wide data set with 1,848 plots across 24 wildfires in yellow pine and mixed conifer forests to build a spatially explicit habitat suitability model for forecasting postfire forest regeneration. To model the effect of seed availability, the critical initial biological filter for regeneration, we used a novel approach to predicting spatial patterns of seed availability by estimating annual seed production from existing basal area and burn severity maps. The probability of observing any conifer seedling in a 60-m2 area (the field plot scale) was highly dependent on 30-yr average annual precipitation, burn severity, and seed availability. We then used this model to predict regeneration probabilities across the entire extent of a "new" fire (the 2014 King Fire), which highlights the spatial variability inherent in postfire regeneration patterns. Such forecasts of postfire regeneration patterns are of importance to land managers and conservationists interested in maintaining forest cover on the landscape. Our tool can also help anticipate shifts in ecosystem properties, supporting researchers interested in investigating questions surrounding alternative stable states, and the interaction of altered disturbance regimes and the changing climate.