Juvenile Conifer Growth and Regeneration Across Diverse Climatic and Environmental Conditions in the Southwestern United States

Juvenile conifer germination, growth, and survival will play important roles in the persistence of coniferous forests and woodlands in the southwestern United States (SWUS), yet it is unclear how juvenile conifers allocate aboveground and belowground early growth and if growth differs between conifer and ecosystem types. Descriptions of growth characteristics may allude to the types of stress juveniles experience in their environments, how they morphologically respond to stress, and how they may be able to survive stress to persist in their current locations or expand into new areas. In Chapter 1, I described the growth characteristics of 4 juvenile conifer groups (junipers, piñon pines, ponderosa pines, firs; 1,121 juveniles sampled, 221 destructively) in 3 height classes (\u3c 150 mm, 150-300 mm, 300+ mm), across 25 sites in the SWUS. I compared growth characteristics across conifer groups and height classes, and related differences in growth to climate and landscape characteristics. I found that as tree height increased from \u3c 150 mm to 300+ mm, belowground root mass and root area increased, root:shoot ratio declined, and specific leaf area declined for all conifers except firs. Lower elevation junipers and piñon pines were frequently located in sheltered microsites, whereas mid- and higher elevation ponderosa pines and firs were frequently unsheltered. Across all sites, herbaceous cover was positively correlated with aboveground growth. At study locations comprised of multiple sites, differences in aboveground growth were explained by variation in landscape characteristics. Thus, growth characteristics were best explained by ecosystem type (piñon pine-juniper woodland, ponderosa pine forest, mixed conifer forest) and local landscape characteristics. Low- and mid-elevation juvenile conifer groups had similar magnitudes of proportional growth, displaying limited capacity to acclimate growth to differences in climate that control ecosystem type. If juvenile conifers also do not acclimate physiologically to their environment, our findings suggest that local environmental variation will play a primary role in regulating forest and woodland persistence and modify the effects of climate change in the SWUS. Our findings in Chapter 1 indicate that mid-elevation juvenile ponderosa pines do not appear to acclimate their proportional growth differently than low-elevation juveniles and unlike low-elevation juveniles, had limited use of sheltered microsites to regulate understory microclimates where regeneration occurs. These findings from Chapter 1 suggest that the areas where successful ponderosa pine regeneration can occur may decline as conditions in the SWUS region become more arid and disturbance regimes increase in severity, extent, and duration due to climate change. To better understand how the range and distribution of ponderosa pine forest ecosystems may change in the future and what climate and environmental conditions can inform successful regeneration and juvenile growth performance, I compared ponderosa pine regeneration and aboveground growth across a subset of sites in Chapter 1 with ponderosa pine overstory cover and diverse climatic and environmental conditions (2487 juvenile ponderosa pines sampled and counted). I found that ponderosa pine regeneration was higher in sites characterizing the lower elevational range of ponderosa pine distributions, including positive linear correlations of regeneration to measures of temperature and negative correlations to measures of precipitation, elevation and slope. Differences in regeneration and aboveground growth across sites were explained by variation in climate and landscape characteristics. Differences in aboveground growth within study locations comprised of multiple sites were explained by variation in landscape characteristics. Thus, site landscape characteristics best explained differences in regeneration and juvenile growth at multiple spatial scales. These findings suggest that under current climate conditions, sufficient ponderosa pine regeneration and juvenile growth performance remain in the lower elevational limits of ponderosa pine distributions and indicate the need to isolate the climate and landscape factors in shaping early juvenile processes