Allometry and evaluation of in situ optical LAI determination in Scots pine: a case study in Belgium

We evaluated several optical methods for in situ estimation of leaf area index (LAI) in a Belgian Scots pine (Pinus sylvestris L.) stand. The results obtained were compared with LAI determined from allometric relationships established in the same stand. We found high correlations between branch cross-sectional area, diameter at breast height (DBH) and basal area as dependent variables, and leaf mass, needle area and crown projection as independent variables. We then compared LAI estimated by allometry with LAI determined by three optical methods (LAI-2000, TRAC and digital hemispherical photography) both before and after corrections for blue light scattering, clumping and non-leafy material. Estimates of stand LAI of Scots pine ranged from 1.52 for hemispherical photography to 3.57 for the allometric estimate based on DBH. There was no significant difference (alpha = 0.01) between the allometric LAI estimates and the optical LAI values corrected for blue light scattering, clumping and interception by non-leafy material. However, we observed high sensitivity of the optical LAI estimates to the various conversion factors, particularly to the clumping factor, indicating the need for caution when correcting LAI measured by optical methods.     

Influence of stand,site and meteorological variables on the maximum leaf area index of beech,oak and Scots pine

Different multiple linear regression models of maximum leaf area index (LAI_max) based on stand characteristics, site quality, meteorological variables and their combinations were constructed and cross-validated for three economically important tree species in Flanders, Belgium: European beech (Fagus sylvatica L.), Pedunculate oak (Quercus robur L.) and Scots pine (Pinus sylvestris L.). The models were successfully tested on similar datasets of experimental sites across Europe. For each species, ten homogeneous and mature stands were selected, covering the species’ entire stand productivity range based on an a priori site index classification. LAI_max was derived from measurements of leaf area index (LAI) made by means of hemispherical digital photography over the whole growing season (mid-April till end October 2008). Species-specific models of LAI_max for beech and oak were mostly driven by management practice affecting stand characteristics and tree growth. Tree density and dominant height were main predictors for beech, while stand age and tree-ring growth were important in the oak models. Scots pine models were more affected by site quality and meteorological variables. The beech meteorological model showed very good agreement with LAI at several European sites. Scots pine’s stand model predicted well LAI across Europe. Since the species-specific models did not share common predictors, generic models of LAI_max were developed for the 30 studied sites. Dominant height was found to be the best predictor in those generic models. As expected, they showed a lower predictive performance than species-specific ones.     

Estimating leaf area index in different types of mature forest stands in Switzerland: a comparison of methods

Leaf area index (LAI) was estimated at 15 sites in the Swiss Long-Term Forest Ecosystem Research Programme (LWF) in 2004–2005 using two indirect techniques: the LAI-2000 plant canopy analyzer (Licor Inc.) and digital hemispherical photography, applying several exposure settings. Hemispherical photographs of the canopy were analysed using Hemisfer, a software package that offers several new features, which were tested here: (1) automatic thresholding taking the gamma value of the picture into account; (2) implementation of several equations to solve the gap-fraction inversion model from which LAI estimates are derived; (3) correction for ground slope effects, and (4) correction for clumped canopies. In seven broadleaved stands in our sample set, LAI was also estimated semi-directly from litterfall. The various equations used to solve the gap-fraction inversion model generated significantly different estimates for the LAI-2000 measurements. In contrast, the same equations applied in Hemisfer did not produce significantly different estimates. The best relationship between the LAI-2000 and the Hemisfer estimates was obtained when the hemispherical photographs were overexposed by one to two stops compared with the exposure setting derived from the reading of a spotmeter in a canopy gap. There was no clear general relationship between the litterfall and the LAI-2000 or the hemispherical photographs estimates. This was probably due to the heterogeneity of the canopy, or to biased litterfall collection at sites on steep slopes or sites subject to strong winds. This study introduces new arguments into the comparison of the advantages and drawbacks of the LAI-2000 and hemispherical photography in terms of applicability and accuracy.     

Validation of an efficient visual method for estimating leaf area index in clonal Eucalyptus plantations

Leaf area index (LAI) is a key ecophysiological parameter in forest stands because it characterises the interface between atmospheric processes and plant physiology. Several indirect methods for estimating LAI have been developed. However, these methods have limitations that can affect the estimates. This study aimed to evaluate the accuracy and applicability of a visual method for estimating LAI in clonal Eucalyptus grandis × E. urophylla plantations and to compare it with hemispherical photography, ceptometer and LAI-2000^® estimates. Destructive sampling for direct determination of the actual LAI was performed in 22 plots at two geographical locations in Brazil. Actual LAI values were then used to develop a field guide with photographic images representing an LAI range of 1.0–5.0 m² m^?2 (leaf area/ground area). The visual LAI estimation guide was evaluated with 17 observers in the field. The average difference between actual LAI and visual LAI estimation was 12% and the absolute difference between the two methods was less than or equal to 0.5 m² m^?2 in 77% of plots. Pearson’s correlation coefficients were high between actual LAI and hemispherical photographs (0.8), visual estimation (0.93) and LAI-2000^® (0.99) and low for the ceptometer (0.18). However, absolute values differed among methods, with the average difference between the actual and estimated LAI of [12]% for visual estimation, 28% for the LAI-2000^®, 37% for the ceptometer and ?43% for hemispherical photographs. The LAI-2000^® and ceptometer overestimated LAI in all plots, whereas hemispherical photographs underestimated the values in all measurements, showing that these methods need calibration to be used. No differences were observed between actual LAI and visual estimates across stand ages of 2–8 years and LAI of 1.5–5.3 m² m^?2 (P > 0.05). The results show that visual estimation of LAI in Eucalyptus stands is a practical method that is unaffected by atmospheric characteristics and can be used on an operational scale.     

Factors causing variation in fine root biomass in forest ecosystems

Fine roots form one of the most significant components contributing to carbon cycling in forest ecosystems. We study here the effect of variation in root diameter classes, sampling depth and the inclusion of understorey vegetation root biomass in fine root biomass (FRB) estimates. The FRB estimates for different forest biomes are updated using a database of 512 forest stands compiled from the literature. We also investigate the relationships between environmental or forest stand variables and fine root biomass (≤2 mm in diameter) at the stand (g m⁻²) and tree level (g tree⁻¹). The FRB estimates extrapolated for the whole rooting depth were 526 ± 321 g m⁻², 775 ± 474 g m⁻² and 776 ± 518 g m⁻² for boreal, temperate and tropical forests, respectively, and were 26-67% higher than those based on the original sampling depths used. We found significant positive correlations between ≤1 and ≤2 mm diameter roots and between ≤2 and ≤5 mm roots. The FRB estimates, standardized to the ≤2 mm diameter class, were 34-60% higher and 25-29% smaller than those standardized to the ≤1 mm and ≤5 mm diameter classes, respectively. The FRB of the understorey vegetation accounted for 31% of the total FRB in boreal forests and 20% in temperate forests. The results indicate that environmental factors (latitude, mean annual precipitation, elevation, temperature) or forest stand factors (life form, age, basal area, density) can not explain a significant amount of the variation in the total FRB and a maximum of 30% that in the FRB of trees at the stand level, whereas the mean basal area of the forest stand can explain 49% of the total FRB and 79% of the FRB of trees at the tree level.     

Stand and individual tree characteristics associated with Enaphalodes rufulus (Haldeman) (Coleoptera: Cerambycidae) infestations within the Ozark and Ouachita National Forests

Many oak decline events have been reported within the past century in the eastern U.S., and important causal factors often differ among them. Coincident with a recent decline event in upland oak-dominant forests of Arkansas, Missouri, and Oklahoma was an unexpected outbreak of a native cerambycid beetle, Enaphalodes rufulus (Haldeman), the red oak borer. A large range in estimates of oak mortality throughout affected forests was presumably due to variation in species composition, where oak-dominant areas experienced the greatest mortality. We chose eight sites across the Ozark and Ouachita National Forests of Arkansas, similar both topographically and by oak dominance, to determine if other stand or tree characteristics were important factors in variation of E. rufulus infestations across these forests. At each site, we sampled ∼125 dead, declining or healthy host Quercus rubra L., northern red oak. We created an estimate of the E. rufulus population level at each site during the recent outbreak using counts of dated larval gallery scars within a subset (n = 120) of all Q. rubra sampled (n = 976). We used classification tree partitioning to determine host tree characteristics that differed among dead, declining, and healthy Q. rubra. We also used classification tree partitioning, followed by logistic regression to determine stand characteristics that varied significantly among high, moderate and low infestation stands as well as between forests. Models indicated that trees which died were smallest, grew the least during the borer outbreak, and were apparently suppressed. These dying trees were likely poor competitors for resources, allowing neighboring survivors to experience a growth release during the E. rufulus outbreak. Larval survivorship was higher in trees which died, though larval densities were not greatest within these trees, which suggests that resistance in these individuals was compromised. At the stand level, differences between forests were apparently more important than those due to borer infestation. E. rufulus populations were higher at sites with lower Q. rubra basal area. This reduced basal area was likely a result of greater Q. rubra mortality at these sites during the borer outbreak in the early 2000s.     

Effects of planting spacing and site quality on 25-year growth and mortality relationships of Douglas-fir (Pseudotsuga menziesii var. menziesii)

Growth and mortality of coast Douglas-fir (Pseudotsuga menziesii var. menziesii) were studied for 25 years after planting seedlings at 1–6-m spacings on a site of moderate quality in the western Cascade Mountains of Washington. Responses were compared to those from two other studies representing high and low site qualities. Third-year height did not differ among spacings (P = 0.80), providing no evidence that close spacing stimulated early growth. Piecewise regression identified the onset of competition-induced mortality when stand density index (SDI [Reineke, L.H. 1933. Perfecting a stand density index for even-aged forests. Journal of Agricultural Research 46, 627–638]) exceeded 52% (S.E. = 4.6) of the species’ maximum or when average crown ratio (CR) declined below 52% (S.E. = 0.9). For a range of SDI values, CR averaged 2–7% points greater at the high-quality site than at the moderate-quality site. In a regression analysis of combined data from the moderate- and high-quality sites, relative values of average stem diameter and stand volume (% of maximum values observed per site) 23–25 years after planting increased and decreased with planting spacing, respectively (R² = 0.97 and 0.91, respectively). Intersection of these relationships at 3-m spacing indicated a point of equivalent relative development of tree size and stand yield. For a range of site qualities, stands planted at 3-m spacing: (1) maintained tree vigor (CR ≥ 50%) and stability (average height:dbh ratio <90), (2) experienced little or no competition-induced mortality through age 25 years, and (3) allocated 25-year growth equitably to development of tree size and stand yield, thereby providing a desirable starting point for subsequent management.     

Tree height in Brazil's ‘arc of deforestation’: Shorter trees in south and southwest Amazonia imply lower biomass

This paper estimates the difference in stand biomass due to shorter and lighter trees in southwest (SW) and southern Amazonia (SA) compared to trees in dense forests in central Amazonia (CA). Forest biomass values used to estimate carbon emissions from deforestation throughout, Brazilian Amazonia will be affected by any differences between CA forests and those in the “arc of deforestation” where clearing activity is concentrated along the southern edge of the Amazon forest. At 12 sites (in the Brazilian states of Amazonas, Acre, Mato Grosso and Pará) 763 trees were felled and measurements were made of total height and of stem diameter. In CA dense forest, trees are taller at any given diameter than those in SW bamboo-dominated open, SW bamboo-free dense forest and SA open forests. Compared to CA, the three forest types in the arc of deforestation occur on more fertile soils, experience a longer dry season and/or are disturbed by climbing bamboos that cause frequent crown damage. Observed relationships between diameter and height were consistent with the argument that allometric scaling exponents vary in forests on different substrates or with different levels of natural disturbance. Using biomass equations based only on diameter, the reductions in stand biomass due to shorter tree height alone were 11.0, 6.2 and 3.6%, respectively, in the three forest types in the arc of deforestation. A prior study had shown these forest types to have less dense wood than CA dense forest. When tree height and wood density effects were considered jointly, total downward corrections to estimates of stand biomass were 39, 22 and 16%, respectively. Downward corrections to biomass in these forests were 76 Mg ha⁻¹ (∼21.5 Mg ha⁻¹ from the height effect alone), 65 Mg ha⁻¹ (18.5 Mg ha⁻¹ from height), and 45 Mg. ha⁻¹ (10.3 Mg ha⁻¹ from height). Hence, biomass stock and carbon emissions are overestimated when allometric relationships from dense forest are applied to SW or SA forest types. Biomass and emissions estimates in Brazil's National Communication under the United Nations Framework Convention on Climate Change require downward corrections for both wood density and tree height.     

城市绿地园林树种叶面积指数测定方法研究——以小叶榕为例

分别采用半球面影像技术和LAI-2000冠层分析仪对华南地区最常用园林树种小叶榕(Ficus microcarpa)的叶面积指数(Leaf Area Index,LAI)进行测定、比较和分析。研究表明,2种测量仪器所测LAI值存在极显著正相关(P<0.001);用半球面影像技术测量的LAI值与冠幅、胸径和树高之间也存在极显著一元线性关系,构建回归模型分别为:LAI=0.0444Cw+1.6526,LAI=0.0088D+1.8327,LAI=0.0543H+1.6404;通过模型可估测小叶榕单株的叶面积指数,达到95%的置信区间的估测值范围。     

Comparison of 5th- and 14th-year Douglas-fir and understory vegetation responses to selective vegetation removal

The effects of early vegetation management on the survival and growth of Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] were examined 5 and 15 years after planting in the Oregon Coast Range. Our first objective was to document the effects of vegetation species competition upon key ecosystem properties. The second objective was to document the effects of vegetation removal during early Douglas-fir stand establishment upon long-term tree growth and on biomass production by vegetation components. Seven levels of manual vegetation removal were maintained for the first 5 years after planting: 0%, 25%, 50%, 75%, and 100% shrub removal; and 100% shrub removal combined with 50% or 100% herbaceous vegetation removal. Shrub and herb removal did not affect Douglas-fir survival at year five, but treatments providing less than 75% shrub removal significantly reduced Douglas-fir survival by year 15. Removing shrubs and herbs completely (100S + 100H) during the 5 years following tree planting allowed successful tree establishment, with a 366% increase in biomass accumulation per hectare for Douglas-fir in that treatment at the end of 14 years of growth. At 15 years stand age, even with shrub removal alone, a 304% gain in tree biomass per hectare was obtained compared to no vegetation removal (NVR). By stand age 15 years, any increase in the degree of understory removal beyond 75% did not contribute significantly to additional tree survival and growth. The understory vegetation on NVR treatment plots and the herbaceous vegetation on 100% shrub removal (100S) treatment plots, contained >90% and >80% of aboveground biomass N at 5 years, respectively, indicating possible competition for soil N. Soil moisture was not different among treatments at 5 years. Complete vegetation removal (100S + 100H) for 5 years resulted in a significant increase in soil bulk density (P < 0.05), a significant decrease in total soil C (P < 0.05) and no change in total soil N in the upper 15 cm of the mineral soil. By 14 years, however, only the soil bulk density remained greater (P < 0.05) on the 100S + 100H treatment. We conclude that greater tree survival and growth occurred with at least 75% shrub removal. Our results suggest that managers may have substantial flexibility in maintaining a partial understory component suitable for ecosystem productivity, canopy cover and wildlife habitat, while maintaining forests productive for timber resources.     

Alternative silvicultural practices with variable retention improve bird conservation in managed South Patagonian forests

Alternative silvicultural approaches to timber management, such as regeneration treatments with different degrees of stand retention, may mitigate negative effects of clear-cutting or shelterwood cuts in forested ecosystems, including changes in old-growth forest bird communities. The aims of this work were: (a) to compare bird species richness and densities among different silvicultural designs with variable retention (dispersed and/or aggregated) and unmanaged primary forests, and (b) to assess temporal changes at community and species levels before and after treatments. A baseline avian survey was conducted prior to harvesting to evaluate canopy gap presence and forest stand site quality influences. Subsequent to harvesting, data on bird species richness and density were collected by point-count sampling during the summer season for 5 consecutive years (4 treatments × 5 years × 6 sampling points × 5 counts). Bird species richness and density (15 species and 9.2 individuals ha⁻¹) did not change significantly with forest site quality of the stands and canopy gap presence in unmanaged forests. However, both variables were significantly modified in managed forests, increasing over time to 18 species and reaching to 39 individuals ha⁻¹. Inside the aggregated retention, bird communities were more similar to unmanaged primary forests than those observed within the dispersed retention or in clear-cuts. Opting for a regeneration method with dispersed and aggregated retention has great potential for managing birds in Nothofagus pumilio forests. This method retained enough vegetation structure in a stand to permit the establishment of early successional birds (at least in dispersed retention), and to maintain the bird species of old-growth forests which could persisted in the retention aggregates.     

Fine root production and turnover in forest ecosystems in relation to stand and environmental characteristics

The production and turnover of fine roots (diameter ?2 mm) contributes significantly to carbon cycling in forest ecosystems. We compiled an up-to-date global database covering 186 stands from the literature and estimated fine root production (FRP) and fine root turnover (FRT) for boreal, temperate and tropical forests in order to study the relationships between FRP or FRT and environmental and stand variables. FRP for all plants (trees + understorey) was 311 ± 259 (n = 39), 428 ± 375 (n = 71) and 596 ± 478 g m⁻² a⁻¹ (n = 32) in the boreal, temperate and tropical forests, respectively, and the corresponding annual FRT rates were 0.77 ± 0.70, 1.21 ± 1.04 and 1.44 ± 0.76, respectively. When the FRP and FRT of trees were estimated separately for boreal and temperate forests the differences between the two biomes were insignificant. The mean FRP of trees for the two biomes combined was 306 ± 240 g m⁻² a⁻¹ (n = 86) and the annual FRT was 1.31 ± 1.43. Fine root biomass (FRB) was the most significant factor explaining the variation in FRP, and more so at the tree level than at the stand level, explaining 53% of the variation in FRP for trees at the tree level. The corresponding proportions at the stand level were 21% for all plants and 12% for trees. Latitude, mean annual temperature and annual precipitation each explained <20% of the variation in FRP or FRT. Fine root production and FRT estimates are highly dependent on the species included in the sampling, the sampling depth and the methods used for estimating FRP or calculating FRT. The results indicate that the variation in FRP on a global scale can be explained to a higher degree if we focus on tree roots separately from the roots of the understorey vegetation and on FRP at the tree level instead of FRP at the stand level or on FRT.     

The role of gaps and tree regeneration in the transition from dense to open black spruce stands

Black spruce forests growing on clay soils in northwestern Quebec change structure from dense even-aged stands to open uneven-aged stands such that almost all forests older than 200 years have an open canopy. These forests become unproductive over time because they are prone to paludification. The main goal of our study was to document the transition between dense and open stands in terms of gap dynamics, with a focus on tree regeneration. Our objective was to determine whether forests remain open due to a lack of regeneration, a lack of growth or both. Nine stands along a 50–250-year-old time since fire gradient were sampled with the line intersect sampling method. Gap fraction increased with stand age and reached a maximum of 77% in the oldest site. In old-growth stands, gaps were interconnected due to the low density of these forests. Most of the gap makers were found with broken stems. Regeneration was dominated by black spruce layers and was relatively abundant (1.71 stems/m²). However, the majority of gap fillers were smaller than 1 m in height in stands of all ages. Instead of a lack of regeneration, the opening of the forests is due to a lack of growth associated with cold and wet organic deposits. Partial harvesting could be implemented on the most productive sites, while management techniques including soil disturbances will be required on low productivity sites to recreate good growth conditions.     

Long-term response to area of competition control in <Emphasis Type="Italic">Eucalyptus globulus</Emphasis> plantations

Numerous studies have quantified the responses to vegetation management in Eucalyptus plantations but most publications have reported early responses in tree growth and a gap in knowledge exist about the magnitude and duration of growth responses throughout the whole rotation. We evaluated the long-term response (9 years-old) of E. globulus across a gradient of sites to different intensity levels of free area of competing vegetation around individual tree seedlings. Competing vegetation intensity levels considered free areas ranging between 0 (control) to 2.54 m² plus a treatment with total weed control. Competing vegetation biomass production during the first growing season was 2.9, 6.5, 2.2 and 12.9 Mg ha^?1, for sites ranging from low to high annual rainfall. Across sites, maximum response in stand volume ranged between 58 and 262 m³ ha^?1 at age 9 years and was proportional to the amount of competing biomass controlled during the first growing season. Total competing vegetation control showed the largest response in stand volume at sites with 2.9 and 12.9 Mg ha^?1 of competing vegetation. However, the 2.54 m² vegetation control treatment showed the maximum response for sites with 2.2 and 6.5 Mg ha^?1 of competing vegetation. The duration of response for vegetation control treatments ranged between 5 and 9 years. However, at the site with the largest accumulation of competing vegetation biomass the response to vegetation control showed a sustained and divergent response. Our results suggest that vegetation control improved site resources acquisition increasing long-term stand productivity by reducing environmental limitations to tree growth differentially at each site.     

Transpiration and forest structure in relation to soil waterlogging in a Hawaiian montane cloud forest

Transpiration, leaf characteristics and forest structure in Metrosideros polymorpha Gaud. stands growing in East Maui, Hawaii were investigated to assess physiological limitations associated with flooding as a mechanism of reduced canopy leaf area in waterlogged sites. Whole-tree sap flow, stomatal conductance, microclimate, soil oxidation-reduction potential, stand basal area and leaf area index (LAI) were measured on moderately sloped, drained sites with closed canopies (90%) and on level, waterlogged sites with open canopies (50-60%). The LAI was measured with a new technique based on enlarged photographs of individual tree crowns and allometric relationships. Sap flow was scaled to the stand level by multiplying basal area-normalized sap flow by stand basal area. Level sites had lower soil redox potentials, lower mean stand basal area, lower LAI, and a higher degree of soil avoidance by roots than sloped sites. Foliar nutrients and leaf mass per area (LMA) in M. polymorpha were similar between level and sloped sites. Stomatal conductance was similar for M. polymorpha saplings on both sites, but decreased with increasing tree height (r(2) = 0.72; P < 0.001). Stand transpiration estimates ranged from 79 to 89% of potential evapotranspiration (PET) for sloped sites and from 28 to 51% of PET for level sites. Stand transpiration estimates were strongly correlated with LAI (r(2) = 0.96; P < 0.001). Whole-tree transpiration was lower at level sites with waterlogged soils, but was similar or higher for trees on level sites when normalized by leaf area. Trees on level sites had a smaller leaf area per stem diameter than trees on sloped sites, suggesting that soil oxygen deficiency may reduce leaf area. However, transpiration per unit leaf area did not vary substantially, so leaf-level physiological behavior was conserved, regardless of differences in tree leaf area.     

Spatial variability in stand structure and density-dependent mortality in newly established post-fire stands of a California closed-cone pine forest

Fire is an important process in California closed-cone pine forests; however spatial variability in post-fire stand dynamics of these forests is poorly understood. The 1995 Vision Fire in Point Reyes National Seashore burned over 5000 ha, initiating vigorous Pinus muricata (bishop pine) regeneration in areas that were forested prior to the fire but also serving as a catalyst for forest expansion into other locales. We examined the post-fire stand structure of P. muricata forest 14 years after fire in newly established stands where the forest has expanded across the burn landscape to determine the important factors driving variability in density, basal area, tree size, and mortality. Additionally, we estimated the self-thinning line at this point in stand development and compared the size-density relationship in this forest to the theorized (−1.605) log-log slope of Reineke’s Rule, which relates maximum stand density to average tree size. Following the fire, post-fire P. muricata density in the expanded forest ranged from 500 to 8900 live stems ha⁻¹ (median density = 1800 ha⁻¹). Post-fire tree density and basal area declined with increasing distance to individual pre-fire trees, but showed little variation with other environmental covariates. Self-thinning (density-dependent mortality) was observed in nearly all stands with post-fire density >1800 stems ha⁻¹, and post-fire P. muricata stands conformed to the size-density relationship predicted by Reineke’s Rule. This study demonstrates broad spatial variability in forest development following stand-replacing fires in California closed-cone pine forests, and highlights the importance of isolated pre-fire trees as drivers of stand establishment and development in serotinous conifers.     

Methodology comparison for slope correction in canopy leaf area index estimation using hemispherical photography

The slope effect and correction methods for estimation of canopy gap fraction, leaf area index (LAI), mean leaf angle and clumping index using hemispherical photography, were investigated. The evaluation was carried out in tropical cloud forest and plantations in South-East Kenya in order to consider a range of canopy architecture and slopes up to 65%. The aim was to compare two acquisition techniques and various correction procedures. All estimates assume uniform slope, canopy parallel to ground and homogeneous canopy structure at the photo site level.

(1) Photographs oriented to local zenith (levelled acquisition). Calculation and removal of sky parts of the hemisphere obstructed by topography. Azimuthal inversion of gap fraction without prior averaging, deriving local LAI estimates (quasi-random model). (i) Fixed path lengths over azimuths. Zenith reference axis. LAI referred to horizontal and corrected for topographic shading. (ii) Variable path lengths over azimuths. Normal to slope reference axis. LAI adjusted to horizontal by dividing by the slope cosine.

(2) Photographs oriented parallel to slope (tilted acquisition). Fixed path lengths over azimuths. Normal to slope reference axis. LAI adjusted to horizontal by dividing by the slope cosine. Azimuthal inversion of gap fraction without prior averaging, deriving local LAI estimates (quasi-random model).

Gap fractions present a stronger upslope/downslope asymmetry if retrieved from levelled acquisition. As a result, gap dispersion index and clumping index proved to be significantly higher for levelled acquisition (P < 0.001). LAI estimates adjusted to horizontal are not significantly different, whether retrieved from levelled or tilted acquisitions, up to 30% slopes. From levelled acquisition, fixed and variable path length do not yield significantly different LAI estimates along the whole slope gradient. From tilted acquisition, LAI values were systematically higher than from levelled acquisitions, the stronger the slope, the higher the difference. Mean leaf angles do not differ significantly (P > 0.05) for fixed vs. variable path lengths along the slope gradient up to 30%. For more severe slopes, variable path lengths yield lower mean leaf angle values. The interpretation of results from tilted acquisition remains uncertain. As a preliminary study, no preference is suggested for the levelled or tilted acquisition technique. Further investigation is needed and indirect optical derived estimates should be checked against direct reference measures, which are almost entirely lacking for mountainous areas.     

Structural and environmental characterization of old-growth temperate rainforests of northern Chiloé Island,Chile: Regional and global relevance

Old-growth forests are ecologically relevant reservoirs of biodiversity and provide valuable and unique ecosystem functions in the landscape. However, what constitutes an old-growth stand is confusing because the definition depends largely on the forest type under study. Despite the ecological importance of old-growth temperate rainforests in southern Chile in comparison to other global forests, no attempts have been made to characterize them as a way to assess their structural variability. Here, we characterized old-growth stands of Valdivian and North Patagonian rain forest types located in Chiloé Island (Chile, 42°30′S) using inventory data from 23 permanent plots (0.1 ha each) located in rural landscapes and protected areas of northern Chiloé Island. For each stand, its age (average age of the oldest trees present in each stand) and disturbance regimes (evidence of recent human impact, e.g. cuttings or fires, and tree growth rates) were used as defining old-growth criteria. We characterized the structure (tree species richness, size-density distributions, vertical stratification and presence of snags) and floristic composition of each stand. Environmental variables (i.e. temperature, distance to coastline and elevation) were related to stand structure using multivariate constrained correspondence analysis. Old-growth forests were commonly characterized by (a) tree basal areas >80 m²/ha; (b) density of shade-tolerant tree species in the emergent and dominant canopy layer >36%; (c) higher tree species richness (>7 tree species) than successional stands; (d) presence of large canopy emergents (>80 cm dbh, >25 m tall); (e) high vertical heterogeneity; and (f) minimum stand ages older than 200 years. Old-growth forests showed a distinctive structural variability and floristic diversity influenced both by stand age and disturbance history of the stands. Structural variability was also related to environmental differences among sites (e.g. air temperature, distance to coastline, soil types). Old-growth forest features described here can offer a baseline for managers interested in maintaining and restoring old-growth forest structure in southern temperate rain forests.     

Lianas and self-supporting plants during tropical forest succession

Lianas (woody vines) are an important component of tropical forests, with a strong impact on forest dynamics, but their responses during forest succession have received relatively little attention. Here, we present an analysis of the changes in stem density, biomass, and species richness of lianas and self-supporting plants during tropical forest succession. We surveyed lianas ≥0.5 cm diameter at breast height (dbh) and self-supporting plants ≥2.5 cm dbh in 0.1 ha inventory plots in a chronosequence of 30 sites in northeastern Costa Rica, 23 sites on abandoned pastures 10–44 years of age, and seven sites in old-growth forest. Stem density of self-supporting plants showed no predictable chronosequence trend, but liana stem density declined significantly with forest age. Aboveground biomass of self-supporting vegetation increased rapidly during succession, with forests 31–44 years exhibiting higher levels of biomass than old-growth forests. Liana biomass accumulated more slowly, with the highest levels in old-growth sites. Species richness of self-supporting vegetation increased significantly during succession, but species richness of lianas showed no change or a slight decline with forest age, depending on the method of assessment. The differences between tree and liana responses during succession stem from the unique physiology and life history traits of lianas.