Trees In Pure Stands Research Articles

Radial increment dynamics of Maritime pine (Pinus pinaster Ait.) in pure and mixed stands with Scots pine (Pinus sylvestris L.) under changing environmental conditions

Forest dynamics are undergoing profound alteration due to the fact that climate change is increasing the frequency and severity of abiotic and biotic forest disturbances. Because of the unpredictable nature of the drought periods and the variation in their severity, Mediterranean forests are typically more vulnerable. Mediterranean Pinus pinaster forests are showing decay symptoms linked to climate change. There is clear evidence that promoting mixtures can serve as an effective forest adaptation strategy. In this regard, we sought to better understand the responses of P. pinaster radial growth dynamics to various factors, in both mixed and pure forest stands, and provide valuable insights into P. pinaster dynamics when mixed with P. sylvestris. In this study, we examined the variation of intra-annual cumulative radial increment patterns in response to the climate of P. pinaster between pure and mixed stands with P. sylvestris. Using data from band dendrometers collected over five consecutive climatically distinct years (2016–2020), a nonlinear mixed-effect model approach was used to analyze the differences in intra-annual cumulative radial increment patterns for P. pinaster between years in mixed and pure stands. The intra-annual radial increment pattern of P. pinaster showed significant year-to-year variation and varied with tree size, with greater increment in larger trees. Trees in mixed stands had a higher mean radial increment compared to corresponding ones in pure stands. Increased summer maximum temperatures negatively affected tree cumulative annual increment regardless of composition, but with a lower impact on trees in pure stands. Spring precipitation increased the length of the growing season, while higher spring maximum temperatures triggered an earlier inflection point. Our results highlight the high plasticity of P. pinaster in adapting to varying intra- and inter-annual environmental conditions and competition with other species and suggest that promoting mixtures with P. sylvestris may be an interesting management strategy for adaptation to climate change.

Mixing increases drought exposure through a faster growth in beech, but not in oak

Promoting mixed forests is often seen as a management option to increase the resilience of forests to droughts. However, mixing can also improve tree growth, and if faster-growing trees use more water, they may be exposed to higher risks of drought. Across Belgium, we tested whether increased tree growth in mixed sessile oak/common beech stands results in increased drought exposure of these forests in severe dry summers. We sampled wood cores from 222 trees in mixed and corresponding pure stands at eight different sites and measured ring widths and stable carbon isotope composition (δ13C) in the wood formed during 2001 (wet summer as a reference year) and 2003 (severe dry summer). Pre-drought growth (PG), an index of tree growth in normal climatic conditions, was calculated as the mean ring width in the five years preceding the drought, and drought exposure was assessed as the difference in δ13C between the dry and the reference year. Growth of individual beech trees was faster in mixtures and both mixing and pre-drought tree growth were positively related to beech drought exposure. These two effects were not independent: the increased drought exposure in mixed stands was an effect of the faster growth. Analysis of δ13C values suggests that in reference conditions, beech trees in mixtures had a less conservative water use than trees in pure stands, but that they shifted to a functioning similar to that in pure stands under drought. By contrast, growth and drought exposure of oak trees did not differ between pure and mixed stands. Our results emphasize the different strategies of these two species regarding their response to mixing and drought. Beech maximizes its growth as a result of greater resource capture in mixed stands, but this increases the risk of suffering from severe droughts. Oak, on the other hand, exhibits lower variability in growth and in drought exposure across pure and mixed stands. We should stress that if mixing is used as a management strategy to mitigate the negative effects of climate change on temperate forests, forest managers should be aware that the performance benefits of mixed forests in non-drought years could in fact hinder the drought resistance of these forests in severe drought years.

Terrestrial laser scanning reveals differences in crown structure of Fagus sylvatica in mixed vs. pure European forests

Competition with neighboring trees of different species can affect crown size and shape. However, whether intra-specific differences in crown characteristics in mixed stands compared to pure stands are dependent on site conditions remains poorly understood. We used terrestrial laser scanning (TLS) to examine the differences in Fagus sylvatica crown characteristics at four sites, each of which contained pure stands of F. sylvatica and their mixture with Pinus sylvestris. These sites covered the area where the mixture occurs in Europe from south to north, representing a gradient of F. sylvatica productivity, defined as the mean increment of annual volume growth in pure F. sylvatica stands. Despite the large range in productivity, F. sylvatica trees in mixtures had larger crowns regardless of site conditions, with a higher proportion of their crown volume in the lower canopy compared to trees in pure stands. Larger crown volumes were related to higher live crown ratios and greater crown expansion, depending on the site. The magnitude of the mixing effect was variable among the crown characteristics evaluated, but overall our findings provide evidence that for a given species combination and density, the effect of mixture increased in the two most productive sites. TLS-derived novel crown metrics revealed that the mixing effect was affected by productivity, which was not captured by traditionally measured crown variables.

Productivity, pest tolerance and carbon sequestration of Khaya grandifoliola in the dry semi-deciduous forest of Ghana: a comparison in pure stands and mixed stands

In order to restore biodiversity in the degraded forest landscape and to use forest plantations for climate change mitigation, experimental plantations of indigenous trees (including mahogany species) and important exotic trees species like Tectona grandis have been established in pure and in mixed stands in the degraded Tain Tributaries Block II Forest Reserve in the dry semi-deciduous forest zone of Ghana. This study assessed the performance of an important indigenous species, Khaya grandifoliola, in pure and in mixed stands, and compared its performance to the exotic tree species, T. grandis. The results from the study indicated that after 4 years, there was a significant difference in the diameter of K. grandifoliola (P = 0.001) between pure and mixed stands with the pure stands having an average diameter of 9.15 ± 0.19 cm compared with 7.81 ± 0.33 cm for mixed stands. Pure stands had a correspondingly higher basal area at breast height for individual trees in pure stands compared with mixed stands. K. grandifoliola also recorded average total height of 5.50 ± 0.13 m and merchantable height 3.63 ± 0.09 m in pure stands, compared to total height of 5.04 ± 0.24 m and merchantable height of 3.52 ± 0.18 m in mixed stands. However, these values were not significantly different between the stands (P > 0.05). Basal area at breast height for K. grandifoliola grown in pure stands was 5.5 ± 0.3 m2/ha at age four, which was significantly higher than the basal area of 1.1 ± 0.4 m2/ha at breast height for K. grandifoliola in mixed stands. Also total volume per hectare was higher in pure stands (17.8 ± 0.9 m3/ha) than in mixed stands (3.4 ± 0.6 m3/ha). Consequently, K. grandifoliola accumulated more carbon in pure stands (10,126 ± 557.2 kg/ha) than in mixed stands (1,976 ± 335.1 kg/ha), but the mixture of the tree species, including K. grandifoliola, accumulated more carbon (11,929 ± 401.3 kg/ha) than the K. grandifoliola in pure stands though not statistically different. Overall, K. grandifoliola performed better in pure stands than in mixed stands. With regards to the tolerance to pest attacks on K. grandifoliola, it was more tolerant to pests’ attacks in mixed stands than in pure stands. There was no statistical difference in diameter growth between K. grandifoliola (9.15 ± 0.19 cm) and T. grandis (9.61 ± 0.26 cm) in pure stands. The values of total height, merchantable height and total volume for T. grandis were 8.22 ± 0.20 m, 5.38 ± 0.18 m and 22.5 ± 1.7 m3/ha respectively which differed significantly from 5.50 ± 0.13 m, 3.63 ± 0.09 m and 17.8 ± 0.9 m3/ha for K. grandifoliola for the same parameters (P < 0.05). However, there was no significant difference between the two species with respect to basal area per hectare (P = 0.189); K. grandifoliola grew to 5.5 ± 0.3 m2/ha and T. grandis grew to 4.8 ± 0.3 m2/ha. T. grandis in pure stands accumulated slightly more aboveground biomass than K. grandifoliola in pure stands after 4 years though not statistically different.

Temperature-driven range expansion of an irruptive insect heightened by weakly coevolved plant defenses

Warming climate has increased access of native bark beetles to high-elevation pines that historically received only intermittent exposure to these tree-killing herbivores. Here we show that a dominant, relatively naïve, high-elevation species, whitebark pine, has inferior defenses against mountain pine beetle compared with its historical lower-elevation host, lodgepole pine. Lodgepole pines respond by exuding more resin and accumulating higher concentrations of toxic monoterpenes than whitebark pine, where they co-occur. Furthermore, the chemical composition of whitebark pine appears less able to inhibit the pheromonal communication beetles use to jointly overcome tree defenses. Despite whitebark pine's inferior defenses, beetles were more likely to attack their historical host in mixed stands. This finding suggests there has been insufficient sustained contact for beetles to alter their complex behavioral mechanisms driving host preference. In no-choice assays, however, beetles readily entered and tunneled in both hosts equally, and in stands containing less lodgepole pine, attacks on whitebark pines increased. High-elevation trees in pure stands may thus be particularly vulnerable to temperature-driven range expansions. Predators and competitors were more attracted to volatiles from herbivores attacking their historical host, further increasing risk in less coevolved systems. Our results suggest cold temperatures provided a sufficient barrier against herbivores for high-elevation trees to allocate resources to other physiological processes besides defense. Changing climate may reduce the viability of that evolutionary strategy, and the life histories of high-elevation trees seem unlikely to foster rapid counter adaptation. Consequences extend from reduced food supplies for endangered grizzly bears to altered landscape and hydrological processes.

Herbivory by Insects on Oak Trees in Pure Stands Compared with Paired Mixtures

(1) The communities of herbivorous insects and their feeding on oak (Quercus petraea) trees in pure stands or in mixtures with alder (Alnus glutinosa), pine (Pinus sylvestris) or spruce (Picea abies) were investigated. (2) The species of insect found on a particular tree species were the same whether the tree was growing in a pure or in a mixed stand. (3) The densities of insects were different in pure and mixed stands. The numbers of all herbivore groups were highest on oak in the oak/alder mixture and lowest on oak in the oak/spruce mixture. (4) Feeding damage was significantly different on trees in pure and mixed stands. The major contributors to this damage were also different. For example, leafhoppers caused little damage in the pure oak and oak/spruce but caused almost as much damage as caterpillars on oak planted with alder. (5) It is concluded that patterns of planting significantly affect both the densities and damage caused by insects.

Factors Influencing Herbivory by Insects on Oak Trees in Pure Stands and Paired Mixtures

Factors Influencing Herbivory by Insects on Oak Trees in Pure Stands and Paired Mixtures

Winter use of montane forests by porcupines in southwestern Alberta: preferences, density effects, and temporal changes

The pattern of winter use of montane forest by porcupines (Erethizon dorsatum) during a 40-year period was examined on two areas in southwestern Alberta. Of the three available coniferous communities, porcupines fed in relatively more trees in pure stands of Douglas fir (Pseudotsuga menziesii) in leeward situations. Douglas fir, limber pine (Pinus flexilis), and hybrid spruce (Picea glauca × P. engelmannii) were used with equivalent frequency when found in the same community. The number of porcupines that used both areas peaked and declined in relation to the average ages of the forests. Trees in low density stands (&lt; 11 trees/100 m2) were fed in during more years than those in denser stands and the use of dense stands was more frequent during years when porcupines were most abundant. Differences between the study areas in repeated feeding in leeward Douglas fir and in the relative proportions of trees used in low and high density stands are attributed to differences in the abundance of porcupines.