Using plots constructed in 2003 by a previous PHD student we were able to investigate the growth and survival of three species of dipterocarp seedlings in gaps and compare it to sites shaded by continuous canopy. In total there were six plots, half in the shade and half in gaps. There were two replicates of each plant in each plot. The plots were standardised as they were planted four years ago at the same time and at similar age and heights, reducing the bias between seedlings.
MEASUREMENTS
Several morphological traits and the chlorophyll content of each individual seedling were measured in order to be used as indicators of plasticity.
The height of each individual seedling was measured manually using a tape measure.
The number of branches of each seedling was recorded. From each seedling, two branches were selected from the top, middle and bottom of the plant in order to achieve a balanced representation of the entire seedling. The lengths of these selected branches were measured manually using a tape measure. Average branch length for each seedling was calculated using this data.
Two leaves (one from the tip and one from mid-branch) were taken from each selected branch. Specific Leaf Area (SLA) as described by Hunt (1990) and chlorophyll content was measured for each leaf. Average SLA and chlorophyll content was calculated for each seedling using this data.
The percentage herbivory was also estimated for each of the selected branches. This was calculated by counting the number of leaves on the branch, recording the number showing herbivory and estimating the herbivory damage shown on each leaf. Estimation of herbivory damage was standardised by using the same person to estimate all the seedlings. Average percentage herbivory damage was calculated using this data.
STATISTICAL ANALYSIS
Averages of each trait were then averaged across the two light environments in order to obtain two sets of data (gap and shade) for each species of dipterocarp. A Two-Way ANOVA analysis was used to test each trait for intraspecific and interspecific differences in response to the different light conditions.
Results
Plant architecture was analysed in the form of height and shape index. For this study shape index was calculated as average branch length: height. A high average branch: height describes a seedling with a wide, short shape and vice versa (Figure 1). There were highly significant differences between the heights of the three dipterocarp seedlings in response to the different light environments (ANOVA: F=17, df=1,11, p<0.005) with seedlings being taller in the gaps than the shade. However, there were no significant interspecific differences (ANOVA: F=0.4, df=2,11, p=0.68)(Figure 2). There were no significant effects of gap and shade environments on the shape of the three dipterocarp seedlings (ANOVA: F=0.1, df=1,25, p=0.955), nor were there any significant interspecific differences (ANOVA: F=1.63, df=2,25, p=0.215) (Figure 3).
There was a highly significant effect of gap and shade environments on the SLA for each dipterocarp species (ANOVA: F=31.3, df=1,11, p<0.001) with seedlings having a greater SLA in the shade then the gaps. However there were no significant interspecific differences (ANOVA: F=2.74, df=2,11, p=0.108)(Figure 4).
There was no significant difference in chlorophyll content of the three dipterocarp seedlings in response to the different light environments (ANOVA: F=0.35, df=1,11, p=0.565). However, there was a significant difference between the species (ANOVA: F=8.98, df=1,11, p=0.005) with S.macroptera having a significantly greater chlorophyll content then D.lanceolata, (Tukey multiple comparison test p<0.05). H.sangal was not significantly different from either of the other two species (Tukey multiple comparison test p>0.05)(Figure 5).
There were no significant effects of gap and non gap environment on the average herbivory percentage damage of the dipterocarp seedlings (ANOVA: F=2.3, df=1,11, p=0.158). However there were highly significant interspecific differences (ANOVA: F= 21, df=2,11, p<0.001), with S.macroptera having a significantly greater average herbivory percentage damage than the other two species (Tukey multiple comparison test p<0.05)(Figure 6).
Discussion
The result of each of the three dipterocarp seedlings having a significantly greater height in the gap then the shade was expected and supported by many studies (Augspurger 1984, Sasaki & Mori 1981, Nicholson 1960). On the other hand, the non-significant result for interspecifc height differences in response to different light environments was not expected. Valladares et al. (2000) claimed that plant species specialised for favourable environments, such as gaps, have higher plasticity then those specialised for less favourable conditions, such as shade. Therefore, as compared to shade tolerant species, such as H.sangal, less shade tolerant species, such as S.macroptera, are expected to exhibit greater plasticity, and hence have a significantly greater height than the other species in the gap environment. Further support for this idea comes from Huante & Rincon (1998). However, results from this study suggest that the dipterocarp species have equal plasticity in terms of height growth when subjected to different light conditions. Although this outcome was unexpected, similar results were found in a study by Rozendaal et al (2006).
Other surprising results were those of plant shape. It was hypothesised that seedlings in the shade would adapt to the low light conditions by invest more resources into horizontal growth, rather than vertical growth, and therefore have a high shape index. This would increase the quantity of leaves exposed to the limited light and therefore maximise light capture (Sterck et al 2006). However, results from this study suggest no significant differences in shape of the seedlings of each species in response to the different light environments, nor any interspecific differences.
The result of each of the three dipterocarp seedlings having a significantly greater SLA in the shade then the gap was expected and supported by many studies (Oguchi et al 2003, Rozendaal et al 2006). Those leaves of seedlings in the shade would adapt to the low light conditions by developing thinner and broader leaves, and therefore a high SLA, in order to maximise light capture (Evans & Poorter 2001). On the other hand, those leaves of seedlings in the gaps would develop thicker leaves, and therefore a low SLA, in order to prevent overheating due to the high light intensity (Rozendaal et al 2006). This study also found a non-significant result for interspecific SLA differences in response to the different light environments. This suggests that each of the three dipterocarp species have equal plasticity in terms of SLA when subjected to different light conditions.
It has been found that plants in the shade have higher chlorophyll content than those found in the gaps, an adaptation which is thought to increase the absorption of the limited light (Cao 2000). However, results of this study contradict this and suggest no significant differences in chlorophyll content of the seedlings of each species in response to gap and shade. Nevertheless, there was a significant interspecific difference of chlorophyll content. This suggests that cholorophyll content is primarily determined genetically, rather than the environment, and hence chlorophyll content is not a trait of plasticity. Further investigation is required if this idea is to be supported or rejected.
It has been found that plants in gaps are more herbivorised then those in shade (Richards & Coley 2007). This is because plants in gaps invest more resources into growth rather than herbivory defence (Coley 1983). However, this study found equivalent average herbivory damage between gap and shade environments for the three dipterocarp species. Nonetheless, other studies have also recorded no difference in herbivory in response to different light environments for a range of species. (Coley 1983, Aide & Zimmerman 1990)
Slow growing plants are known to invest more resources in defence and possess less palatable leaves and higher concentrations of chemical defences than faster growing species (Coley et al. 1985). The results from this study supports this, with fast growing S.macroptera being significantly more herbivorised then the other two slower growing species. Therefore overall, according to this study, herbivory is dependent on the species rather than the light environment. Eichhorn et al (2006) also found that it is the species of a seedling is the principal factor in determining the amount of herbivory suffered.
The outcome of S.macroptera being significantly more herbivorised then the other two slower growing species may help to explain the unexpected result for interspecifc height differences in response to different light environments mentioned previously. S.macroptera was expected to exhibit greater plasticity then the other two species and grow significantly higher in the gaps, yet the results did not support this. A study by Becker (1984) found that experimentally non-herbivorised seedlings of the genus Shorea had a significantly higher mean height then experimentally herbivorised seedlings. This means that those species that invest less into herbivory defence, such as S.macroptera, will have their growing success hindered by herbivores. This indicates that herbivory has a much larger role in the growth and survival of dipterocarp species in the rainforest environment, perhaps more so than plasticity. Further research into this area is required if we are to broaden our knowledge of this concept.
There are a few criticisms of this study. Firstly, as a consequences of time constrictions the sample size used was small. Furthermore, some of the S.macroptera had not survived in the shaded plots, which additionally reduced the sample size. However, averaging the data will have reduced any bias caused by the limited sample size.
Secondly there were diverse variances of the three dipterocarp species for each of the traits. For example those seedlings in the gaps had particularly large variances. This is explained by gap heterogeneity. Different gaps have differing light intensities and even across the same gap the light can vary (Brown 1983). Nevertheless, the two-way ANOVA analysis method was appropriate since it is robust enough to deal with the unequal variances between the treatments.
Another criticism is that since this study was conducted in the field it was difficult to control all conditions that have affected the results. Further research using an artificially controlled environment would be useful to compare results and widen understanding of this area.
In conclusion, all species of dipterocarp seedlings showed some degree of morphological plasticity in response to the different light environments. Each of the three dipterocarp species showed equal plasticity in terms morphological traits when subjected to different light conditions. Chlorophyll content was found to be a genetic trait rather than a trait of plasticity. Herbivory was also found to be determined by species rather than environment. Damage by herbivores was found to play a larger role in plant growth and survival than anticipated, perhaps more so than plasticity. It has a negative effect on plant fitness by repressing growth and reducing competitive ability. Further study is required to determine the extent herbivory affects the growth and survival of these seedlings.
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