Spruce Forest Response to Warming

Spruce seedlings colonizing the forest edge.

White spruce woodlands and open forest are an important landscape component in Lake Clark and Katmai. Warming since the 1980s has led to increased spruce beetle-related mortality in older, denser forest stands, but also to increases in spruce growth and seedling establishment in more open sites, particularly near the forest-tundra margin. New studies using monitoring and tree-ring data collected by the Southwest Alaska Network describe recent changes in forest dynamics near western treeline, represented by Boundary Creek in southern Katmai. The first uses tree-ring data to examine the effects of warmer summer temperatures on growth at sites extending from northern Lake Clark to southern Katmai. This study found that trees in all but a handful of forested sites have been growing more rapidly in recent decades and that late-summer precipitation has become increasingly important in maintaining enhanced growth (Sherriff et al. 2017). The second study found that spruce establishment since the mid-twentieth century has been greatest in warm years, but that the majority of establishment has occurred at sites near western treeline where growing season conditions are generally cooler and wetter than at sites farther north (Miller et al. 2017). This wave of new spruce establishment across southern and western Katmai could result in the creation of new forested areas over the course of decades, offsetting to some extent the dieback currently occurring on other areas of the landscape, though the increased basal area (crowding) has also shown to stunt growth rates (Wright et al. 2018). Similar studies of white spruce stand basal area and tree rings from Denali National Park and Preserve (Nicklen et al 2018) reinforce these findings.

Warming drives a front of white spruce establishment near western treeline, Alaska

Abstract

Regional warming has led to increased productivity near the boreal forest margin in Alaska. To date, the effects of warming on seedling recruitment have received little attention, in spite of forecasted forest expansion. Here, we used stand structure and environmental data from 95 white spruce (Picea glauca) plots sampled across a longitudinal gradient in southwest Alaska to explore factors influencing spruce establishment and recruitment near western treeline. We used total counts of live seedlings, saplings, and trees, representing five life stages, to evaluate whether geospatial, climate, and measured plot covariates predicted abundance, using current abundance distributions as a surrogate for climate conditions in the past. We used generalized linear models to test the null hypothesis that conditions favorable for recruitment were similar along the environmental gradient represented by longitude, by exploring relationships between per‐plot counts of each life stage and the covariates hypothesized to affect abundance. We also examined the relationship between growing degree days (GDD) and seedling establishment over a period of three decades using tree‐ring chronologies obtained from cores taken at a subset of our sites (n = 30). Our results indicated that seedling, sapling, and tree abundance were positively correlated with temperature across the study area. The response to longitude was mixed, with earlier life stages (seedlings, saplings) most abundant at the western end of the gradient, and later life stages (trees) most abundant to the east. The differential relationship between longitude and life‐stage abundance suggests a moving front of white spruce establishment through time, driven by changes in environmental conditions near the species’ western range limit. Likewise, we found a positive relationship between periods of seedling establishment and GDD, suggesting that longer summers and/or greater heat accumulation might enhance establishment, consistent with the positive relationship we found between life‐stage abundance and temperature.

Miller, A. E., T. L. Wilson, R. L. Sherriff, and J. Walton. 2017. Warming drives a front of white spruce establishment near western treeline, Alaska. Global Change Biology 23:5509-5522.

Spruce growth responses to warming vary by ecoregion and ecosystem type near the forest‐tundra boundary in southwest Alaska

Abstract

A critical concern for boreal ecosystems centres on broad‐scale responses to warming, i.e. declining growth and mortality, or enhanced growth and greater productivity. However, few studies have synthesized tree growth along biogeographic gradients to address this issue. This study develops a broader understanding of how growth of a dominant conifer has responded to recent warming near the western forest margin of Alaska.

Sherriff, R. L., A. E. Miller, K. Muth, M. Shriver, and R. Batzel. 2017. Spruce growth responses to warming vary by ecoregion and ecosystem type near the forest-tundra boundary in southwest Alaska. Journal of Biogeography 44:1457-1468.

Stand basal area and temperature interact to influence growth in white spruce in southwest Alaska

Abstract

Warming temperatures are having a disproportionate effect on boreal ecosystems, influencing the establishment and growth of tree species across their respective ranges. However, less is known about how competitive interactions influence growth–climate relationships. We used tree‐ring data from 26 study plots (836 trees) to investigate how the growth patterns of white spruce (Picea glauca) respond to variation in competition and climate in southwest Alaska. Using linear mixed‐effects models, we compared growth against covariates for stand basal area, competition, climate (temperature, precipitation), and spruce beetle mortality. We characterized competition at the tree level using two distance‐dependent competition indices: Hegyi index and a height advantage index. Furthermore, we assumed that stand‐level competition increased with basal area. We found that high summer temperatures resulted in reduced growth in stands with higher basal area (i.e., the most crowded stands), but increased growth at sites with low basal area. Our results suggest that historically productive stands may show declines in growth under warmer conditions, with important implications for future stand structure and productivity.

Wright, M., R. L. Sherriff, A. E. Miller, and T. Wilson. 2018. Stand basal area and temperature interact to influence growth in white spruce in southwest Alaska. Ecosphere 9(10):e02462 DOI: 10.1002/ecs2.2462

Stand basal area and solar radiation amplify white spruce climate sensitivity in interior Alaska: Evidence from carbon isotopes and tree rings

Abstract

The negative growth response of North American boreal forest trees to warm summers is well documented and the constraint of competition on tree growth widely reported, but the potential interaction between climate and competition in the boreal forest is not well studied. Because competition may amplify or mute tree climate‐growth responses, understanding the role current forest structure plays in tree growth responses to climate is critical in assessing and managing future forest productivity in a warming climate. Using white spruce tree ring and carbon isotope data from a long‐term vegetation monitoring program in Denali National Park and Preserve, we investigated the hypotheses that (a) competition and site moisture characteristics mediate white spruce radial growth response to climate and (b) moisture limitation is the mechanism for reduced growth. We further examined the impact of large reproductive events (mast years) on white spruce radial growth and stomatal regulation. We found that competition and site moisture characteristics mediated white spruce climate‐growth response. The negative radial growth response to warm and dry early‐ to mid‐summer and dry late summer conditions intensified in high competition stands and in areas receiving high potential solar radiation. Discrimination against ¹³C was reduced in warm, dry summers and further diminished on south‐facing hillslopes and in high competition stands, but was unaffected by climate in open floodplain stands, supporting the hypothesis that competition for moisture limits growth. Finally, during mast years, we found a shift in current year's carbon resources from radial growth to reproduction, reduced ¹³C discrimination, and increased intrinsic water‐use efficiency. Our findings highlight the importance of temporally variable and confounded factors, such as forest structure and climate, on the observed climate‐growth response of white spruce. Thus, white spruce growth trends and productivity in a warming climate will likely depend on landscape position and current forest structure.

Nicklen, E. F., C.A. Roland, A. Z. Csank, M. Wilmking, R. W. Ruess, and L. A. Muldoon. 2018. Stand basal area and solar radiation amplify white spruce climate sensitivity in interior Alaska: Evidence from carbon isotopes and tree rings. Global Change Biology 00:1–16. https://doi.org/10.1111/gcb.14511