Effects of Restoration Thinning Treatments on Water Relations and Photosynthesis of Four Size Classes of Pinus ponderosa Kjerstin R. Skov, Kimberly F. Wallin, Thomas E. Kolb Northern Arizona University—School of Forestry

Introduction Northern Arizona, like much of the Southwest, has a history of logging, grazing and fire suppression that contributed to the current condition of Arizona forests (1, 2). Forests have changed from open, grassy stands with frequent fire (2-12 years in N. Arizona) to dense collections of stressed trees with little understory vegetation and high fuel loading (1, 2, 3). Present conditions present greater risks of large fires and insect outbreaks (2). While most stakeholders agree that the forest condition needs improvement, the use of restoration thinning and the appropriate intensity is a topic of zealous debate. We are evaluating the effects of different thinning intensities on tree water stress and photosynthesis of four size classes. We expect heavy thinning to increase water potential and photosynthetic rate more than light thinning or no thinning. Additionally, smaller trees may respond more vigorously than larger, older trees. Here we report results from the first year of our research.

Methods  Trees size classes: small (13-19 cm dbh), medium (23-29 cm dbh), large (33-39 cm dbh), and presettlement (mature crown, yellow bark and at least 60 cm dbh)  Measurements of leaf water potential on one-year-old needles using a pressure chamber (Model 1000, PMS Instruments, Corvallis, OR) made at predawn (0500 hrs), mid-morning (0800 hrs) and midday (1100 hrs).  Gas exchange measured on one-year-old needles with the LI-6200 portable photosynthesis system (Li-cor Inc., Lincoln, NE) and at mid- morning and midday.  Physiological response variable analyzed with fixed-effects analysis of variance (ANOVA) using SAS package (SAS Institute Inc. Cary, NC), regression, and correlation.

Ft. Valley Experimental Forest is located five miles northwest of Flagstaff, AZ. Elevation is approximately 2200 m. Soils are fine montmorillonitic complex of frigid Typic Argiboroll and Mollic Eutroboralf (4). Mean annual precipitation is 57 cm with half falling in winter months and half during the summer monsoon season (4). Late May and June are typically dry and late August is wet.

Thinning Treatments Unthinned control maintained dense collection of pole-sized post-settlement trees with some larger interspersed presettlement trees. Heavy thinning: retention of replacement trees for each evidence of a presettlement tree Light thinning: retention of 3-6 trees for each evidence of a presettlement tree (Evidence included snags, downed trees, stumps, stumpholes and presettlement trees.)

Figure 1. Predawn leaf water potential (ψ leaf ) was not significantly different between tree sizes for any treatment. Midmorning and midday ψ leaf were only significantly different between sizes at June midday and August mid-morning. Presettlement and Large trees had lower ψ leaf than small- and medium-size trees at these times.

Figure 2. Net photosynthetic rate (P n ) was not significantly different between tree sizes for any time or treatment. P n was significantly higher for heavily thinned compared with unthinned or lightly thinned at almost all measurement times.

Figure 3. Net photosynthetic rate (P n ) as a function of Vapor Pressure Deficit (VPD). Treatment effects p = 0.246, VPD effects p < , Treatment and VPD interaction p =

Figure 4. Stomatal Conductance rate (G w ) Pressure Deficit (VPD). Treatment Effects p < , VPD effects p < , Treatment and VPD interaction effects p = as a function of Vapor.

Figure 5. Predawn leaf water potential (ψ leaf ) for each thinning intensity in June (dry) and August (wet). In June, predawn ψ leaf in the unthinned control plots was significantly lower than in the lightly or heavily thinned treatments (p< for each). In August, predawn ψ leaf in the lightly thinned treatment was significantly less than both control and heavily thinned treatments (p = and p = respectively).

Conclusions  Heavy thinning increased soil water availability (predawn ψ leaf ) and photosynthetic rate (P n ) of trees within two years of treatment. Light thinning treatments had little effect on either water availability or photosynthetic rate. Therefore, light levels of restoration thinning may not satisfy goals to improve tree vigor within two years of treatment.  Stomatal conductance (G w ) was negatively related to vapor pressure deficit (VPD) for all treatments with higher conductance values in heavily thinned than lightly thinned or control treatments. Higher soil water availability in heavily thinned treatments— especially in June when VPD was highest—may mitigate the effects of atmospheric water stress on G w.  Tree size class differences (pole-size through large presettlement trees) did not significantly affect soil water availability or photosynthetic rate in most cases.  Restoration thinning can effectively increase tree vigor and growth rate even in a drought year and thinning intensity is an important consideration.

References 1. Cooper, C. F Changes in vegetation, structure, and growth of southwestern pine forests since white settlement. Ecol. Monogr. 30: Covington, W. W., P.Z. Fulé, M. M. Moore, S.C. Hart, T.E. Kolb, J.N. Mast, S. S. Sackett, and M. R. Wagner Restoring Ecosystem Health in Ponderosa Pine Forests of the Southwest. J. For. 95(4): Dietrich, J. H Chimney Spring forest fire history. USDA For. Serv. Res. Pap. RM-RP Mast, J.N., P.Z. Fulé, M. M. Moore, W. W. Covington and A. E. M. Waltz Restoration of presettlement age structure of an Arizona ponderosa pine forest. Ecological Applications 9(1):