Mulching of Ornamental Trees: Effects on Growth and Physiology

RESULTS AND DISCUSSION

Tables 1 and 2 summarize growth data for both species. In horsechestnut, height increment was higher in trees mulched with compost only in the first year, whereas no difference emerged either in the second year or considering the total height increment (Table 1). Similarly, trunk diameter was positively affected by compost mulching in the first year compared with pine bark and control trees. This parameter was also higher considering the total increment after 3 years from planting. Shoot length was generally higher in trees mulched with compost.

In linden trees, the effects of mulching were more evident in the second year after planting when trees mulched with compost generally showed higher values of the parameters measured than control trees (Table 2). Total diameter increment was higher in trees mulched with compost, whereas plant height was not statistically affected.

In 2005, coarse compost was more effective to increase mean annual net photosynthesis of Tilia × europaea than pine bark and control. On Aesculus × carnea, no statistical differences were found among the different management techniques (Table 3). However, because mulched trees were larger with longer shoots, whole-plant leaf gas exchange was probably greater.

Mulching with both compost and pine bark increased transpiration of horsechestnut in 2005 compared with chemical weeding, whereas no effect was found in 2006. No differences were found on Tilia × europaea in both years for this parameter (Table 4).

Net photosynthesis also showed a declining trend over time in both species in 2005, whereas this trend was not observed in 2006. An explanation for this behavior is the higher rainfall during August in 2006 than in 2005. Differences were only noted in linden at the beginning and at the end of the growing season, when control trees showed lower Pn values compared with those mulched with compost (Figure 1). No differences were found in horsechestnut (Figure 2). Comparing the findings of the 2 separated years, the Pn for horsechestnut trees was higher in 2006, probably as a result of the different environmental conditions. In this regard, it needs to be stressed that individual leaf photosynthesis has demonstrated high variability compared with whole plant gas exchange, and it is sometimes complicated by a poor relationship of dry matter production to yield (Klingemann et al. 2005), but it is the only way to measure leaf gas exchange in adult trees.

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Figure 1.

Net photosinthesis (μmol/m⁻²/s⁻¹) in Tilia × europaea young trees under different cultivation techniques. Different letters indicate statistically differences at P < 0.05 using Duncan test.

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Figure 2.

Net photosinthesis (μmol/m⁻²/s⁻¹) in Aesculus × carnea young trees under different cultivation techniques. Different letters indicate statistically differences at P < 0.05 using Duncan test.

Water use efficiency, in this research considered as the ratio of carbon gain during CO₂ assimilation (A, μmol/m⁻²/s⁻¹) to water loss during transpiration (E, mmol/m⁻²/s⁻¹), is of major importance to the survival, productivity, and fitness of individual plants and it was higher in control trees only in 2005 in Tilia and in 2006 in Aesculus. Actually, the two species showed different patterns in the 2 years. Tilia × europaea had higher Pn and E values in 2005 compared with Aesculus × carnea, which in 2006 showed higher values of photosynthesis but not a corresponding increase in transpiration, which was actually lower. As a consequence, this species showed higher values of WUE.

It has been generally demonstrated that increased WUE can be a water conservation measure indicating that more carbon can be accumulated for growth with the use of less water (Nilsen and Orcutt 1996; Jones 2004).

Stomatal conductance (Gs) was higher in trees mulched with compost in both species and it showed superior values in 2005 than 2006. The relation of WUE and Gs clearly indicates that these two variables are, in this case, inversely correlated. A decrease in stomatal conductance causes a proportionally larger decrease in transpiration than in carbon assimilation, with the net result being a higher WUE.

In this regard, it also has to be underlined that instantaneous WUE refers to a comparison of CO₂ assimilated versus H₂O transpired by individual leaves (or canopies) and is affected by environmental and genetic factors (Glenn et al. 2006) and it also varies among years considered (Fageria et al. 2006). Therefore, studies of instantaneous WUE by gas exchange techniques are difficult in field situations because environmental conditions around the leaves are always changing and it is hard to determine an average plant WUE (Nilsen and Orcutt 1996). Also, differences in WUE determined at the leaf level may be reduced substantially at the canopy level (Lambers et al. 1998), and it can be finally hypothesized that, being mulched trees bigger and with longer shoots, they would be expected to have a higher leaf gas exchange on a whole plant basis.

Leaf internal CO₂ concentration was not affected in both species in 2005, whereas it was higher in mulched Tilia trees in the second year and, in general, showed superior values in 2006.

Photochemical efficiencies of PSII (Fv/Fm) of the two species, measured during both growing seasons from June to September, were similar on an annual basis, and their values were comprised between 0.74 and 0.77 (Tables 1 and 2). No differences emerged among the different treatments, even when considering the single sampling date, except for the last measurements in 2006 on Aesculus, when mulched trees showed higher values. The highest Fv/Fm values were observed in July and were 0.8 on average among the treatments (data not shown).