Temporal and spatial variability in stand structure and individual-tree growth for 10 years following commercial thinning in spruce-fir forests of northern Maine Christian Kuehne1, Aaron Weiskittel1, Arne Pommerening2, Robert Wagner1 1 School of Forest Resources, University of Maine 2 Department of Forest Resource Management, Swedish University of Agricultural Sciences SLU

All trees: spatial locations Data 6 replicates (locations): 34- to 70-year-old naturally regenerated spruce-fir stands, not pre-commercially thinned 7 treatments: control & factorial combination of thinning method (low, dominant, or crown) and level of relative density reduction (33 or 50%) All trees: spatial locations 10 years of annual post-treatment measurements Permanent 0.08 ha measurement plots with a forwarder trail established in 0.37 ha treatment plots

Research questions Are there differences in stand structural heterogeneity between treatments and how do they relate to variation in individual-tree diameter growth over the 10-year study period? Do spatially explicit statistics (inclusion of tree locations) reveal new insights into the variability patterns as well as the observed stand dynamics?

Tree diameter distributions (2-cm dbh classes) 1 (black) and 10 years (white bars) since thinning (YST) Control Crown Dominant Low Change in mean absolute ΔDBH (cm year-1)  YST Control Crown Dominant Low   1 0.9 1.9 1.3 2.4 10 1.1 2.6 2.9 3.0

DBH-based stand structural heterogeneity quantified using the Gini coefficient 1 and 10 years since thinning (YST) & ΔDBH-based tree growth variation   Stand structural heterogeneity Tree growth variation  Treatment YST 1 10 Control .23 .21 .72 .68 Crown .25 .66 .51 Dominant .17 .15 .74 .48 Low .13 .12 .43

Mean pair correlation functions g(r) & mean mark variograms 𝜸𝒎(r) for DBH Control Crown Dominant Low

Change in Growing Space Efficiency (GSE) using estimated parameter a derived from nonlinear regression models predicting GSE (mm m-2 year-1) as a function of area potentially available (APA, m2) GSE was calculated as the ratio of annual ΔDBH (mm) and APA. Higher a values depict greater GSE across the entire APA range. GSE (annual ΔDBH/APA)   Treatment Years since thinning 1 5 10 Control 0.58 0.63 0.47 Crown 0.94 2.34 1.74 Dominant 0.85 1.13 2.53 Low 1.29 1.28 0.81 GSE = a*APA^b APA Sit & Poulin-Costello 1994

Conclusions Medium-aged suppressed and intermediate balsam fir and red spruce proved to be highly responsive to newly available growing space while growth rates in the low thinning treatments remained at relatively high levels throughout the study period Spatially explicit statistics and measures helped to better understand observed structural and growth patterns and thus stand dynamics Changes in forest structure and growth highlight the interrelationship of forest management, stand structure, and tree growth: permanent feedback loop

Thank you! Funding provided by Northeastern States Research Cooperative Cooperative Forestry Research Unit of Maine German Science Foundation

Change in mean growth dominance G G values greater than 0 signal growth dominance of larger trees (i.e. contribution to stand total growth by larger trees is greater than their contribution to stand total size)   Treatment Years since thinning 1 5 10 Control .18 .22 .23 Crown .17 .09 Dominant .15 Low -.02 .08

Effect of distance to forwarder trail on relative tree species-specific diameter growth 100% = control