The interaction of tree size and restoration thinning on growth and use of winter versus summer precipitation in northern Arizona ponderosa pines

Background/Question/Methods

Climate change and shifting management paradigms present new challenges for ecohydrology and carbon cycle science in southwestern semi-arid conifer forests. Forecasting the potential impact of future reductions in winter snowpack requires understanding the current reliance of trees on winter versus summer precipitation, while projecting changes in forest productivity with landscape-scale restoration thinning requires knowledge of how trees respond to thinning. Using 36 large and 36 small ponderosa pine trees in a forest near Flagstaff, AZ that was thinned in 1998 to four different tree densities, we asked the following questions:

1) How does reliance on summer versus winter precipitation vary with tree size and tree density?

2) What is the relationship between tree density and subsequent growth of residual trees?

To investigate the first question, we analyzed the isotopic composition (dD) of precipitation, soil, and xylem sap water samples in all seasons for two years. To address the second question, we used standard dendrochronological methods to compare post-thinning growth response among tree densities and across a series of marker wet and dry years.

Results/Conclusions

Xylem sap water (-93.2±0.45 dD‰) isotopic analyses showed that ponderosa pines in our study did not use summer precipitation (-29.3±5.8 dD‰), but rather relied exclusively on winter precipitation (-89.6±7.9 dD‰), regardless of tree size or tree density. Large trees (over 200 years old) used a more depleted water source (-94.6±0.4 dD‰) than small trees (-91.9±0.5 dD‰), indicating that although the small trees were over 75 years old, they still had shallower root systems than large trees. Trees in low-density stands used a more depleted water source (-97.0±1.0 dD‰) than trees in denser stands (-91.1±0.91 dD‰), likely due to reduced canopy interception enabling greater winter snowpack accumulation.

            Dendrochronological analyses showed that trees in the lowest density stand had the highest post-treatment radial growth (1.0±0.1 mm/yr versus 0.8±0.0 mm/yr in control treatments) and grew 78% and 59% more in post-thinning marker wet and dry years, respectively, compared to control treatment trees. Thus, heavy thinning increased tree growth under both wet and drought conditions. Compared to small trees, residual large trees had a 26% larger relative growth increase ten years after heavy thinning, consistent with recent studies indicating that large old trees retain a high capacity to respond to environmental change.