Search

High severity fire is far outpacing post-fire reforestation capacity across the American West, highlighting the need to better understand when and where natural regeneration is sufficient to meet short- and long-term reforestation goals. The vast majority of available post-fire data represents regeneration in the first few years following a fire, raising the question of how well recovery trajectories can be predicted from early post-fire snapshots. We utilize a unique dataset from seven wildfires and 78 plots surveyed twice following stand-replacing fire in two California ecoregions, the northern Sierra Nevada and Klamath Mountains, to ask (1) how are post-fire vegetation and conifer densities changing over time and (2) how well do relatively early post-fire sampling efforts and biophysical factors (availability of seed source, shrub competition, and microclimate) explain longer-term (12–23 year) conifer recovery in California dry forests. Change in conifer seedling density between survey periods was highly variable. Overall, densities of all conifer species during the first decade after fire provided reliable estimates of longer-term conifer recovery in northern California dry forests, though in different ways for different species. Early post-fire seedling density readily explained longer-term densities for less shade-tolerant species, yellow pine (Pinus ponderosa and Pinus jeffreyi) and Douglas-fir (Pseudotsuga menziesii). In contrast, early seedling density was a poor predictor for shade-tolerant species, predominantly white fir (Abies concolor), with site conditions better explaining longer-term density. In general, the probability of a site experiencing net seedling mortality between surveys was highest in sites with high shrub cover and the probability of net recruitment increased with decreasing heat load, suggesting a continued role of microclimate and competition in forest recovery long after the initial post-fire period. Our results lend support to the use of relatively early post-fire surveys to infer longer-term forest recovery trajectories for forest management planning and can help refine reforestation prioritization tools. • Long-term post-fire conifer seedling densities can be reliably predicted from early post-fire data. • Early post-fire seedling density readily explained longer-term densities for yellow pine and Douglas-fir. • Site conditions are a strong predictor of long-term fir seedling density. • Long-term seedling mortality was more likely in sites with high shrub cover. • Long-term seedling recruitment was more likely in sites with low heat load. [ABSTRACT FROM AUTHOR]

Highlights • Current reforestation often focuses on planting regularly-spaced pine seedlings. • 'Course correction' depends on increasingly rare follow-up treatments. • This approach may not build early resilience to fire and drought stress. • An alternative is suggested that includes dividing replanting areas into three zones. • We also emphasize including cluster planting in mesic microsites. • Using prescribed fire to reduce fuels, competing vegetation and build resilience. • Limitations and areas that need further research are highlighted. Abstract The increasing frequency and severity of fire and drought events have negatively impacted the capacity and success of reforestation efforts in many dry, western U.S. forests. Challenges to reforestation include the cost and safety concerns of replanting large areas of standing dead trees, and high seedling and sapling mortality rates due to water stress, competing vegetation, and repeat fires that burn young plantations. Standard reforestation practices have emphasized establishing dense conifer cover with gridded planting, sometimes called 'pines in lines', followed by shrub control and pre-commercial thinning. Resources for such intensive management are increasingly limited, reducing the capacity for young plantations to develop early resilience to fire and drought. This paper summarizes recent research on the conditions under which current standard reforestation practices in the western U.S. may need adjustment, and suggests how these practices might be modified to improve their success. In particular we examine where and when plantations with regular tree spacing elevate the risk of future mortality, and how planting density, spatial arrangement, and species composition might be modified to increase seedling and sapling survival through recurring drought and fire events. Within large areas of contiguous mortality, we suggest a "three zone" approach to reforestation following a major disturbance that includes; (a) working with natural recruitment within a peripheral zone near live tree seed sources; (b) in a second zone, beyond effective seed dispersal range but in accessible areas, planting a combination of clustered and regularly spaced seedlings that varies with microsite water availability and potential fire behavior; and (c) a final zone defined by remote, steep terrain that in practice limits reforestation efforts to the establishment of founder stands. We also emphasize the early use of prescribed fire to build resilience in developing stands subject to increasingly common wildfires and drought events. Finally, we highlight limits to our current understanding of how young stands may respond and develop under these proposed planting and silvicultural practices, and identify areas where new research could help refine them. [ABSTRACT FROM AUTHOR]

Carbon uptake and tree growth are important factors for assessing productivity and long-term carbon storage. Measurements of radial stem growth are mainly performed at the individual tree scale and can be used to infer ecosystem net primary productivity (NPP). However, these measurements are spatially limited, and remote sensing provides a promising tool to track vegetation function and productivity across spatial scales, making it a viable technique for assessing variation in interannual tree growth and carbon storage. In this study we examined the correspondence between in-situ annual tree-ring width across four dominant evergreen species in the Sierra Nevada and a wide range of remotely sensed products linked to carbon uptake including NPP, gross primary productivity (GPP), net photosynthesis (PSN), normalized difference vegetation index (NDVI), near infrared reflectance of vegetation index (NIR V), and chlorophyll/carotenoid index (CCI) from MODIS (Moderate resolution Imaging Spectroradiometer) as well as downscaled solar-induced fluorescence (SIF) products, using a 14 year dataset (2000–2014) across 62 forest sites. We show that variation of annual tree-ring width was best captured by the annual sum of MODIS GPP, with a legacy effect (5-month backwards shift). Across all forest sites, MODIS GPP with a 5-month legacy effect showed moderate correspondence with tree-ring width (r = 0.60). Within each individual site, however, the interannual correspondence between MODIS GPP with a legacy effect and tree growth was stronger (median r = 0.70 vs 0.14 without a legacy effect). The importance of legacy effects in explaining tree growth variation within sites indicates that tree growth each year is influenced by carbon uptake during the latter part of the previous growing season. Additional local environmental factors also explained annual variation in tree-ring width, including (in descending order of importance) local tree density, latitude, slope, DBH, elevation and aspect. • GPP, NDVI, NIRv, CCI and SIF products were evaluated in Sierra Nevada conifers. • Relationships with tree-ring width improved upon consideration of a legacy effect. • MODIS GPP best reflected the interannual variation of tree-ring growth. • Site-specific environmental factors are also important considerations. [ABSTRACT FROM AUTHOR]