1. As plants evolve, they devise different ways to deal with herbivory. Defense chemicals is one mechanism to prevent insect invasion. Another way plants may deal is to compensate for the damage done. One proposed mechanism is compensatory growth. McNaughton (1983) defines compensatory growth as a response to defoliation which results in no net loss of biomass as compared to undefoliated plants. This compensatory response can be measured with increased photosynthetic rates and/or changes in allocation of carbon (photosynthate) (Bassman and Dickman 1985). Previous field experiments with Poplar trees showed that even with a 40% loss of leaf area, the final height was not significantly different than those of the controls; defoliation of 75% reduced height by only 20% (Bassman and Dickman, 1982). This compensatory response is accomplished by increasing the rate of photosynthesis in the remaining leaf tissues (Vanderklein, et. al., 2001). Another study, using 14 C demonstrated that Poplars respond to defoliation by allocating less photosynthate to the root system in favor of allocating to the shoot system (Bassman and Dickman, 1985). Black Locust (Robinia pseudoacacia) is an early successional tree species that is subjected to a substantial amount of herbivory by the Locust leaf miner (Odontota dorsalis) every summer. Despite this, these trees can still remain competitive year after year. The purpose of this study is to determine whether or not compensatory growth occurs in artificially defoliated saplings of Black Locust. This will reveal one mechanism that Black Locust utilizes to stay alive when faced with intense herbivory. The Compensatory Responses by Black Locust (Robinia pseudoacacia) to Defoliation Sally L. Cohen Department of Biological Sciences, York College of Pennsylvania Black locust (Robinia pseudoacacia) is an early successional tree species that loses a substantial amount of leaf area due to Locust leaf miner (Odontota dorsalis) damage. Defoliation of a plant can induce a response of compensatory growth. My hypothesis was that the defoliated Black Locust seedlings would display a compensatory growth response as a result of increased photosynthetic rates and increased allocation of carbon to the shoot system. Groups of three plants were randomly assigned a level of 0%, 50%, or 75% removal of the total leaf area. The results of this study are: 1) Photosynthetic rates increased in response to defoliation. 2) Shoot growth also increased in response to defoliation, but at the expense of root growth. Compensation allows defoliated plants to attain the same biomasses as undefoliated plants helping them remain competitive. September, 17 : Measurements were made on the LI-6400 Gas Exchange System. Height and Diameter: measurements were taken twice: 1)Prior to defoliation (9/1) 2)Prior to biomass measurements Biomass measurements: All measurements were recorded February 17 - March 14, 2003. 1)Dry root mass 2)Dry shoot mass APRIL: Seedpods were collected from random trees at Memorial park on Edgar Street (York, PA). First Week in July: Transplanted all seedlings into tall pots. August, 2: Seedlings of similar heights were placed in groups of 3. PLANT GROWTH Each plant of a triplet was randomly assigned a whole plant defoliation level: 0% Control 50% (every other leaflet)* 75% (2 out of 3 leaflets) *Displayed in the picture to the right. Treatment Type: Leaflet Removal  (Loss of a whole leaflets on every fully expanded leaf) Gas Exchange:  Control plants CONCLUSION LITERATURE CITED ACKNOWLEDGEMENTS R Black Locust will respond to defoliation by increasing the photosynthetic rate in the remaining leaf area. R Black Locust will respond to defoliation by allocating more carbon to shoots than roots. R If compensatory growth does occur, then shoot biomasses will not differ among defoliation treatments. Bassman, J.H., Dickman, D.I. 1982. Effects of Defoliation in the Developing Leaf Zone on Young Populus X euramericana Plants. I. Photosynthetic Physiology, Growth, and Dry Weight Partitioning. Forrest Science 28(3): 599-612. Bassman, J.H., Dickman, D.I. 1985. Effects of Defoliation in Developing Leaf Zone on Young Populus X euramericana Plants. II. Distribution of 14 C-Photosynthate After Defoliation. Forrest Science 31(2): 358-366. McNaughton, S.J. 1983. Compensatory Plant Growth as a Response to Herbivory. Oikos 40: 329-336. Vanderklein, D., Daquila, E., Carrozza, E. 2001. White Pine, Japanese Larch and, Bearoak Respond Differently to Partial Defoliation. Northeastern 8(3): 319-330. STATISTICAL ANALYSIS All data was analyzed using a randomized block design test (N = 11) with a Tukey-Kramer post test (GraphPad Instat). Figure 1. Figure 1. Photosynthetic rates among treatment groups are not significantly different (P = 0.1097). Figure 3. Figure 3. Shoot means with different capital letters and, root means with different lowercase letters are significantly different from each other (P < 0.05). AA B a a b Figure 4. Figure 4. Means with different letters are significantly different (P < 0.01). A B B Figure 5. Figure 5. Heights taken after defoliation treatments are not significantly different (P = 0.2077). My hypothesis that Black Locust would have a compensatory response to defoliation was supported. This is demonstrated by defoliated plants having an increased photosynthetic rate, as well as, increased allocation of carbon to the shoots at the expense of roots. Total shoot biomasses were not equal among treatment groups. - This outcome does not fully meet McNaughton’s (1983) definition of compensatory growth. - It is observed that the defoliated plants are going towards a final result of compensatory growth. - One explanation for this lack of fulfillment is the limited time left in the growing season. - I feel that if these plants were defoliated earlier in the season a full compensatory growth response would have been observed. In defoliated plants, shoot growth was in the form of increased height, not basal area. - Defoliated plants did not have a large change in basal area compared to the controls. - This implies that the increase in shoot biomasses must have come from increased height growth (the biomass measurements were done over winter so there was no re-growth of branches or leaves). - Due to shifting soil level in the pots (weathering and tip overs) the height measurements may include some error. This response of compensatory growth allows defoliated plants to attain the same biomasses as undefoliated plants helping them remain competitive. METHODS DATA COLLECTION DEFOLIATION All defoliation was done on September 1, 2002 RESULTS ABSTRACT INTRODUCTION HYPOTHESES I would like to thank Lindsay Pulliam for help in the field and especially Dr. Kleiner for his support and advise through out the process of this study. Photosynthetic rates generally increased in response to defoliation (P = 0.109; Figure 1). Defoliation resulted in significantly greater shoot:root ratios (P=0.026; Figure 2) Increased carbon allocation to the shoot makes the defoliated plants’ final shoot biomass almost as large as the control. (P=0.0185; Figure 3) Increased carbon allocation to the shoot resulted in reduced root weights (P=0.0021; Figure 3) Basal area growth was significantly greater in the control treatment than the defoliated treatments (P=0.0003; Figure 4) There are no significant differences in height between the treatment groups (P=0.2077; Figure 5) Figure 2. A A B Figure 2. Means with different letters are significantly different (P < 0.05). B Last Week in May: All seeds were planted in flats.