Realized growing seasons: Patterns and implications of intra-annual tree growth in temperate forests

Background/Question/Methods

Tree growth is a key component in the movement of carbon through terrestrial ecosystems.  This growth, however, is sensitive to environmental factors.  Quantifying how climate variability affects tree growth presents a challenge to predicting future atmospheric CO₂ levels.  Correlating annual growth rates to temperature and precipitation averages is the most common approach linking growth to climate, but trees respond to weather at a much finer temporal scale.  Using weekly and bi-weekly measurements of dendrometer bands on 100 trees in three sites in the eastern US (N=300) in 2011 and at one site in 2010 (N = 100), I was able to model intra-annual forest productivity and how that productivity responds to fine-scale changes in temperature and precipitation.  Fitting a non-linear function that increases sharply and then asymptotes, I estimated the day of the year when growth begins, the slope of the growth curve, and the total annual growth.  Using the residuals around this fit, weather data was used to predict deviations from this functional form.  

Results/Conclusions

The data showed consistent patterns across sites, species, and size class, and fit the process model well.  Small trees grew slower than large trees, but all showed synchronized increases and decreases in growth due to temperature and precipitation.   The growing season began sharply, usually weeks after leaf flush.  There was some variance in the start date.  Trees grew very rapidly at the beginning of the growing season and slowed down, at different rates, in mid-summer.  Most annual growth for most trees occurred in fewer than three months.  This study indicates that although temperate forest trees are able to photosynthesize over many months across the leaf-flush to leaf-fall ‘growing season’, the actual time spent adding biomass to boles is much shorter.  Sensitivity to climate variability, therefore, is acute during this realized growing season.  Future work will include teasing apart the seasonal contribution to roots and maintenance respiration and how intra-annual growth influences and is influenced by performance in previous years.