- McConville, Daniel J.
University of Maine
An understanding of the effects of overstory canopy structure on height development of red spruce and balsam fir saplings is critical for managing multi-cohort forests in Maine. Understory height growth responses to overstory density may be positive or negative depending on the overstory density. Clearly, dense overstory canopies inhibit understory tree growth by limiting light availability to subordinate cohorts. But as overstory density decreases the effects on understory height growth are not clear. An overstory canopy both creates an environment more amenable to tree vigor by modifying micro-climatic conditions, but competes with understory trees for limited resources thereby inhibiting their vigor. The ned affect of an overstory canopy, then, may be positive or negative for sapling height growth depending on the density level of the overstory canopy and the responses of understory trees.
To explore the influences of overstory density on spruce and fir height growth annual height increment was measured for 153 spruce and 215 fir saplings growing on fair to poor sites, beneath an eastern white pine (Pinus strobus L.) overstory ranging in gap function values from 7-100 percent. Gap fraction was measured above each of the saplings using a LI-COR LAI-2000 plant canopy analyzer. In addition to the gap fraction measurements, pine overstory density was described in terms of basal area, crown projection area, and projected leaf area. Predictive models were developed to examine the relationships between (1) gap fraction and pine overstory basal area, crown projection area, and projected leaf area; and (2) height increment of open-grown and understory red spruce and balsam fir saplings and gap fraction. The first part of the study was performed to explore the possibilities of using easily measured alternatives to gap fraction. Pine basal area, crown projection area, and projected leaf area were negatively related to gap fraction, and explained greater than 85 percent of the variation in gap fraction, suggesting that any of the predictor variables can be used as surrogates for gap fraction.
The second part of the study revealed that both red spruce and balsam fir height increment was curvilinearly related to gap fraction. From approximately 7-65 percent gap fraction height increment increased at a decreasing rate at which point additional gap fraction lead to a decrease in fir height growth. There was no difference in height increment between spruce and fir trees growing beneath 30 percent gap fraction and open-grown trees. The responses in height growth to gap fraction were related to site quality. On poorer sites both spruce and fir showed a flatter response in height growth to increases in gap fraction. For red spruce growing on poor sites, height growth continued to increase as overstory density approached zero.
The results from this study have profound implications for multi-cohort management of spruce and fir trees in Maine. These results suggest that on fair sites, spruce and fir height growth can be maximized if grown beneath approximately 65 percent gap fraction, which is equivalent to a pine leaf area index of 1.0 (m2/ha/ha) or a basal area of 10 (m2/ha). In addition, spruce and fir saplings growing beneath 30 percent gap fraction, a pine leaf area index of 2.0 (m2/ha/ha), or a basal area of 24 (m2/ha), will not suffer in decreased height growth compared with open-grown trees.