Detecting large-scale diversity patterns in tropical trees: Can we trust commercial forest inventories?

In this paper we seek to identify the floristic determination biases contained in large-scale commercial inventories conducted by logging companies and to determine whether this impacts on the observed patterns of alpha and beta diversity. The study focused on floristic data recently collected by industrial timber companies in the tropical forests of the Central African Republic (28,229 0.5-ha plots spread over 14,000 km²). A subset of these plots (n = 1107) was later re-sampled for controlling purposes by experienced botanists. The proportion of agreement between the two samplings was assessed for each species and independently for small and large trees, and at genus and family resolutions. Unsurprisingly, large trees and common species were more accurately identified than small trees and rare species. We found that the quality of the floristic determination increased slightly from species to families. We also detected a significant variation between concessions in the quality of the floristic determination that was more dependent on working conditions during forest inventories than on field workers. Contrary to a widespread belief, we did not find a strong bias toward commercial species, showing that commercial inventory data could also be valid for non-commercial species in ecological studies. Finally, we found that both alpha and beta diversity patterns in commercial inventories were highly consistent with those of the re-sampled inventory. This latter result shows that commercial inventories are well suited to detect large-scale patterns of floristic variation. Large-scale commercial inventories could thus play an important role in the identification of large-scale patterns in tropical tree diversity. This could enhance our ability to manage tropical forests by designing representative reserve networks and developing management plans that integrate diversity patterns at the landscape scale.     

Why do Pinus species have different growth dominance patterns than Eucalyptus species? A hypothesis based on differential physiological plasticity

It has been observed that Eucalyptus stands show high growth dominance levels while Pinus stands show null or low growth dominance levels. We hypothesized that this differential behaviour is linked to a higher degree of physiological-biochemical plasticity in Eucalyptus than in Pinus species related to photosynthetic capacity. This leads to an increment in growth efficiency (GE) difference between the largest and the smallest trees of a stand, and therefore to high growth dominance levels in Eucalyptus. To test our hypothesis we carried out a bibliographical survey and reanalyzed data from Pinus ponderosa and Pinus taeda plantations in Argentina. We found that some species within the genus Eucalyptus present higher growth dominance levels, physiological plasticity and GE differentiation than Pinus species. The mean maximum values of these traits reported for any Eucalyptus species were: growth dominance coefficient, 0.48; photosynthetic capacity increment when resource availability increases, 175%; GE difference between the largest and the smallest trees of a stand, 300%. Mean maximum values for the same traits reported for any Pinus species were 0.13, no phostosynthetic plasticity as the most frequent pattern, and 51%, respectively. In Pinus species the most frequent response to an increase in resource availability is characterized by an increase in leaf area or biomass, maintaining a similar photosynthetic capacity per unit area. However, it appears that in P. ponderosa there are some situations, characterized by a high degree of intraspecific competition, leading to a very high degree of GE differentiation which deserve future research. Although we did not find any study reporting simultaneously all variables concerning our hypothesis (growth dominance, growth efficiency differentiation and physiological plasticity) any of the circumstantial evidence found in the bibliography contrasts our hypothesis.     

Selection of roosting habitats by Nyctalus noctula and Nyctalus leisleri in Białowieża Forest—Adaptive response to forest management?

Tree dwelling bats select cavities in large, old, dying or dead trees. This inevitably brings them into direct conflict with the interests of forest managers, who are trained to fell such trees. Therefore the identification of forest stands providing optimal roosting opportunities for bats is crucial, in order to provide appropriate guidelines for forest management. It is also important to identify the extent to which the roosting ecology of bats changes in response to habitat modification. Bia?owie?a Forest (BF) offers a unique opportunity, in the temperate zone, to observe differences between areas with no direct human intervention and managed areas and in particular to reveal the effect of forest management on the roosting ecology of forest dwelling bat species. We used GIS techniques to evaluate bats’ spatial response to changes in forest structure and to test the hypotheses that the forest dwelling bats Nyctalus noctula and Nyctalus leisleri prefer roost sites within old deciduous or wet woodlands over young and coniferous ones and that roost site preferences reflect the extent to which dead and dying trees are removed. There was a significant difference in the selection of roosting habitat between the managed and pristine areas of the forest. Within the pristine forest, both species displayed a strong preference for roost trees located within old deciduous stands (>100 years), whereas in the managed part of the forest old wet woodland was preferred while all medium and young forest stands were avoided. Our data reveal a high degree of lability in the selection of roosting habitat by bats. It appears that bats are able to respond to changes in their environment by changing their roost site preferences and could therefore occupy habitat previously considered less suitable.     

Do commercial forest plantations reduce pressure on natural forests? Evidence from forest policy reforms in Uganda

This paper investigates if and how the establishment of private commercial forest plantations in degraded forest reserves can conserve natural forests in Uganda. It uses difference-in-difference and decomposition analyses on household data collected from intervention and control villages in the neighborhood of forest reserves. We find that commercial forest plantations are weakly effective in conserving natural forests. The reduction in forest use is unevenly distributed across households depending on location and resource endowments such as farmland and livestock. The results suggest that the conservation effectiveness can be enhanced by complementary interventions that change characteristics that reduce forest use, such as more education for forest users.     

Transpiration of forest trees and stands at different altitude: consistencies rather than contrasts?

Compared to lowland trees and forests, evidence on water relations at mountainous and sub-alpine sites with their climatic and topographic peculiarities is scarce. On such limited grounds, the outcome of three pilot studies is reviewed, intended to launch working hypotheses for initiating integrative research across the altitudinal gradient of the European Alps. Highlighted are tree transpiration and structural differentiation as well as stand water balance of mono-specific and/or mixed forests with Picea abies, Pinus cembra, Larix decidua and Fagus sylvatica at sub-alpine (timberline ecotone), mountainous and colline elevation. Given the preliminary evidence, tree-allometric structures of relevancy for water transport appeared to be independent of elevation and forest type, although timberline trees under open-canopy conditions did not buffer transpiration by internal water storage and had enhanced foliage/sapwood area ratios. Transpiration appeared to depend on site conditions rather than site-dependent adaptation. Canopy transpiration approached 20% of the high seasonal precipitation at the mountainous site, with about 60% being ascribed to run-off, whereas the water budget of the colline site was balanced during summer. The water balance of the subalpine site resembled, at lower precipitation, that of the mountainous site. The derived hypotheses focus on mixed-stand transpiration under altitude-specific topography and moisture regimes, hydraulic adaptation and water demand versus uptake capacity, as this latter ratio is crucial at high altitude in view of expected warming. The clarification of consistencies relative to contrasts indicated by the pilot studies will be challenging across altitudes in view of tree species, forest types and topography.     

Do forest decision-makers in Southeastern Norway adapt forest management to climate change?

To study whether, why, and how forestry decision-makers in Southeastern Norway adapt to climate change, we conducted a series of semi-structured interviews in focus groups consisting of non-industrial private forest owners, forest managers, and forest advisors. Our results show that a majority of the participants believed in climate change as a phenomenon, and had experienced events or observed changes that they attributed to climate change. However, we found little evidence of concern regarding climate change impacts on forest ecosystems and forestry among the participants. Instead, the majority regarded climate change more as an opportunity for the Norwegian forest-based sector than a threat. A minority had implemented proactive practices motivated by climate change but in all but one case, the adjustments were adaptation of forest infrastructure. In general, the participants agreed that the uncertainty associated with the effects of climate change and the (economical) uncertainty associated with adaptation of forest ecosystems were too large to change forest management practices at present. However, many participants, in particular the managers, are already adapting in response to experienced problems, such as increased frequency and duration of periods with low carrying capacity of the ground implying reduced or no accessibility within and to stands.     

What does it take to get family forest owners to enroll in a forest stewardship-type program?

We estimated the probability of enrollment and factors influencing participation in a forest stewardship-type program, Minnesota's Sustainable Forest Incentives Act, using data from a mail survey of over 1000 randomly-selected Minnesota family forest owners. Of the 15 variables tested, only five were significant predictors of a landowner's interest in enrolling in the program: compensation amount, intention to obtain a forest management plan, opposition to the program's land covenant, prior awareness of the program, and total acres of forest land owned. The estimated median minimum compensation required was approximately $24 per acre per year. One-fourth of the survey respondents were undecided about whether they would participate in the stewardship program, suggesting there may be potential to capture additional interest and participation. Marketing efforts to raise program awareness, increasing annual stewardship payments, and eliminating the land covenant are likely to be effective strategies for increasing program participation.     

Comparison of litterfall and nutrient return in a Vaccinio uliginosi–Betuletum pubescentis and an Empetro nigri–Pinetum forest ecosystem in northern Poland

Litterfall abundance and composition of organic and inorganic nitrogen and phosphorus forms were studied for 34 months in two different forest ecosystems (Vaccinio uliginosi–Betuletum pubescentis (Vu–Bp) and Empetro nigri–Pinetum (En–P)) in S?owiński National Park (northern Poland). Including broadleaves, litterfall abundance in Vu–Bp was above 25% higher than in En–P. In Vu–Bp the mean litterfall abundance was equal to 4050 kg ha⁻¹ year⁻¹ with needles (31%) and broadleaves (32%) dominating while in En–P the mean litterfall abundance was equal to 2828 kg ha⁻¹ year⁻¹ and needless made up 59% of total. Almost all litterfall fractions in both forest ecosystems followed a clear, coherent seasonal pattern with maximal values in Autumn and minimal values in the growing season. In general, inter-annual fall variation of needles, branches, broadleaves and seeds was not significant in both ecosystems, however, for the fall of needles and branches a decreasing tendency was observed. In Vu–Bp and in En–P total litterfall (Vu–Bp: T-N 97.5%, N_org 96.8%, En–P: T-N 95.6%, N_org 97.9%) as well as the needless (Vu–Bp: T-N 31.9%, N_org 46.4%; En–P: T-N 53.5%; N_org 53.6%) were the most abundant with nitrogen with the supremacy of organic forms. Besides needles fall, the highest nitrogen return in Vu–Bp was contributed by broadleaves, seeds and branches fall, while in En–P with branches and bark fall. The lowest nitrogen return was contributed by inflorescences and flower buds fall. Similar to organic nitrogen, organic phosphorus was dominant in Vu–Bp (91%) and in En–P (77%) ecosystems. In Vu–Bp total phosphorus contributed by broadleaves was comparable to deposited with needles. In the fallen needless phosphorus contribution was significantly higher in En–P than in Vu–Bp. Nearly 40% of total phosphorus return was contributed by branches, seeds and other litterfall material in Vu–Bp. Litterfall mass in Vu–Bp showed positive correlation with air humidity, total nitrogen content in the rooting zone, and negative with underground water level and air temperature indicating lower resistance towards climatic stress. At both forest ecosystems total litterfall mass, total nitrogen and organic nitrogen were positively correlated with air humidity, while neither total litterfall, nor total nitrogen (T-N) and total phosphorus (T-P) at both forest ecosystems were correlated directly with precipitation volume. This is probably because air humidity is partially related to precipitation (r_Sp = 0.31), however it could be also an effect of sea sprays contribution. Homogeneous Pinus sylvestris stand indicates higher resistance towards unfavorable climatic conditions than Betula pubescens, and this is why it should be preferably used in afforestation campaigns in seashore.     

Can native vegetation recover after slash pine cultivation in the Brazilian Savanna?

There is a widespread view that forest plantations with exotic species are green deserts, unable to sustain biodiversity. Few studies have demonstrated, however, that planted stands of exotic trees have a greater negative effect on the plant diversity of savanna vegetation. We compared the native woody flora under four stands of slash pine of about 45 years old with four stands where the previously existing native Cerrado vegetation was preserved and protected from disturbances for the same period, has changed into dense vegetation - the “cerradão”, at Assis municipality, São Paulo State, Brazil. Aiming at understanding the potential ecological filters driving these communities, we assessed air and soil humidity, light availability and classified the native species on the basis of shade tolerance, dispersal syndrome and biomes in which they occur (Atlantic Forest or Cerrado). We recorded an average of 70 (±13) species under pine stands and 54 (±16) species in cerradão. Of the total of 136 species recorded, 78 occurred in both habitats, eight were exclusive to the “cerradão” (shade tolerant and also occurring in forest ecosystems) and 18 were recorded only under pine stands (82% heliophytic, exclusive to the Cerrado biome). Among the functional attributes and abiotic variables analyzed, only light availability explained the floristic differences found. Since richness was higher under pine, we refuted the hypothesis that exotic species constrain the establishment of the native species richness in the understory. On the other hand, the dark environment under the closed-canopy of the “cerradão” acts as a filter inhibiting the establishment of typical Cerrado species. Since pine stands, if managed in long cycle, maintain a reasonable pool of Cerrado endemic species in the understory pine plantations may be a good starting point for savanna restoration.     

Can forest structural diversity be a response to anthropogenic stress? A case study in old-growth fir <Emphasis Type="Italic">Abies alba</Emphasis> Mill. stands

Key message

From 1973 to 1991, Polish SO ₂ emissions above 3250 Gg/year resulted in a decline of fir Abies alba Mill. After stresses connected with these emissions, five main diameter at breast height (DBH) structural types were described. This type of heterogeneous forest structure is supposed to increase forest resistance and resilience to abiotic, biotic and anthropogenic disturbances.

Context

The analyses of forest structure are important under the current scenario of global change, since heterogeneous structures allow for better responses to disturbances. Forests with more complex structures should present greater vitality.

Aims

The main hypotheses were as follows: (1) the temporal changes of atmospheric SO₂ emissions caused (a) the abrupt changes in the tree DBH radial increment and (b) the death of fir trees; and (2) atmospheric SO₂ emissions and related fir decline and recovery processes ultimately result in the development of stands characterised by diverse DBH structures.

Methods

Radial growth trends of 49 and 215 fir trees in the older and younger generations, respectively, and 85 dead fir trees were evaluated. Using data collected in 32 stands, the DBH structural types were identified, and the shapes of these types were illustrated.

Results

The structural diversification of forest patches may influence forest resistance and resilience to disturbances; five main structural types were identified: ML1 and ML2 represent DBH distributions of multi-layered stands, and OS, TS1 and TS2 represent DBH distributions of one- and two-storied stands.

Conclusion

Structural diversity of forests was a response to SO₂ emissions; fir trees had the ability to increase their radial growth, although there were still high SO₂ emissions.

     

Why do forest owners fail to heed warnings? Conflicting risk evaluations made by the Swedish forest agency and forest owners

In the aftermath of a hurricane in Sweden that felled some 250 million trees, the Swedish Forest Agency advised forest owners to reduce forest vulnerability by planting different tree species. This paper analyses why forest owners failed to heed the Forest Agency's recommendation, thereby reproducing a forest vulnerable to storms. This paper focuses on the deliberations and risk evaluations of forest owners when deciding which tree species to plant. The analysis identifies three main categories of reasoning that guided the forest owners' decision-making process: short-term economic reasoning caused by the pressing situation they faced; an understanding of windstorms as natural catastrophes that are impossible to influence; and the uncertainties associated with alternative forest management practices. Furthermore, given their risk-averse strategy, their approach to understanding and coping with uncertainty was crucial in determining their responses. This paper concludes that the forest owners primarily employed experience-based, practical and embodied knowledge, implying that abstract risks and theoretical knowledge regarding future developments were not deemed relevant. An additional conclusion is that even if a huge storm felling shows the need to change forest management practice, it does not provide the most favourable social conditions for achieving change.     

Can random components explain differences in the height–diameter relationship in mixed uneven-aged stands?

? Context

Tree height prediction is an important issue in forest management since tree heights are usually measured only in a sample of trees. Although numerous model approaches have been used for this purpose, no agreement on which one is more appropriate has been achieved.

? Aims

To analyse the random effects of basic and generalised height–diameter (h–d) models fitted to multi-species uneven-aged forest stands, and to establish their ability to explain differences between ecoregions, plots and species.

? Methods

Height and diameter measurements for 29,084 trees from 187 sample plots located in the state of Durango (Mexico) were used. Basic and generalised h–d models were fitted in a mixed-models framework. The variability between ecoregions, plots and species was considered in the random effects definition. Model calibration for different height sampling designs and sampling sizes was also analysed.

? Results

Random components performed well in explaining the differences in the h–d relationship between the different plots and species; however, no significant variance for the random effects was found for the different ecoregions. A calibrated basic h–d model produced similar results to a fixed-effects generalised h–d model when a sufficiently large number of trees was used in the calibration process.

? Conclusion

From a practical point of view, if no calibration is carried out, different models should be used for the different species, so that at least the variation among species is captured.     

Is forest insurance a relevant vector to induce adaptation efforts to climate change?

? Key message

Insurance might be an efficient tool to strengthen adaptation of forest management to climate change. A theoretical model under uncertainty is proposed to highlight the effect, on adaptation decisions, of considering adaptation efforts in forest insurance contracts. Results show that insurance is relevant to increase adaptation efforts under some realistic conditions on forest owner’s uncertainty and risk preferences, and on the observability or not of adaptation efforts.

? Context

One of the challenges of forest adaptation to climate change is to encourage private forest owners to implement adaptation strategies.

? Aims

We suggest the analysis of forest insurance contracts against natural hazards as a vector to promote the implementation of adaptation efforts by private forest owners.

? Methods

We propose a theoretical model of insurance economics under risk and under uncertainty.

? Results

Our results indicate that when climate change makes the probability of the occurrence of the natural event uncertain, then it may be relevant to include adaptation efforts in the insurance contract, leading to an increase in the adaptation efforts of risk-averse and uncertainty-averse forest owners. In addition, we show that the relevance of insurance as a vector to promote adaptation efforts is greater when the forest owner’s effort is unobservable by the insurer as compared to a situation of perfectly observable effort.

? Conclusion

Under some realistic assumptions, the forest insurance contract seems to be a relevant tool to encourage forest owners to adapt to climate change.

     

Silvicultural alternatives to conventional even-aged forest man-agement-what limits global adoption?

Background: The development of forestry as a scientific and management discipline over the last two centuries has mainly emphasized intensive management operations focused on increased commodity production, mostly wood. This conventional forest management approach has typically favored production of even-aged, single-species stands. While alternative management regimes have generally received less attention, this has been changing over the last three decades, especially in countries with developed economies. Reasons for this change include a combination of new information and concerns about the ecological consequences of intensive forestry practices and a willingness on the part of many forest owners and society to embrace a wider set of management objectives. Alternative silvicultural approaches are characterized by a set of fundamental principles, including avoidance of clearcutting, an emphasis on structural diversity and small-scale variability, deployment of mixed species with natural regeneration,and avoidance of intensive site-preparation methods.Methods: Our compilation of the authors’ experiences and perspectives from various parts of the world aims to initiate a larger discussion concerning the constraints to and the potential of adopting alternative silvicultural practices.Results: The results suggest that a wider adoption of alternative silvicultural practices is currently hindered by a suite of ecological, economic, logistical, informational, cultural, and historical constraints. Individual contexts display their own unique combinations and relative significance of these constraints, and accordingly, targeted efforts, such as regulations and incentives, may help to overcome specific challenges.Conclusions: In a broader context, we propose that less emphases on strict applications of principles and on stand structures might provide additional flexibility and facilitate the adoption of alternative silvicultural regimes in a broader set of circumstances. At the same time, the acceptance of alternative silvicultural systems as the preferred or default mode of management wil necessitate and benefit from the continued development of the scientific basis and valuation of a variety of ecosystem goods and services. This publication is aimed to further the discussion in this context.     

Does the effect of forest roads extend a few meters or more into the adjacent forest? A study on understory plant diversity in managed oak stands

Roads are recognised as having different ecological roles such as barrier, corridor or habitat, but the spatial extent of road effects on plant communities in forests remains unclear. We studied the effect of forest road distance on plant understory diversity at 20 sites in young and adult oak stands in a French lowland forest with a long history of management and road construction. All vascular and bryophyte species were collected at five distances ranging from the road verge to 100 m into the adjacent forest stand. We analysed species composition, individual species response, a priori life-history traits response – life form, habitat preference and dispersal mode – and environmental indicator values in relation to road distance and stand age. Plant composition strongly differed between road verge and forest interior habitats. The main road effect extended less than 5 m into the forest stand. A third habitat was detected at the forest-road edge resulting from the road effect on light and soil conditions, and from edge-specific topography. Non-forest species were almost absent from the forest interior. In contrast, many bryophytes and several vascular plants kept away from the road. We identified a posteriori six species groups that better explained the variability of plant response profiles than a priori life-history traits. Plant response to road distance was also dependent on stand age: some species colonised from the road into the forest interior in young stands following regeneration cutting, while other species displayed the reverse pattern in adult stands once canopy closed above the forest road. Even if the depth of forest road effect measured in lowland managed stands was narrow, building of a new forest road has non-negligible effects on plant population dynamics. Forest managers should take into account the impacts of roads on biodiversity, since the expected intensification of silviculture in response to global changes is set to accentuate the effect of forest roads. We recommend further study on the role of dispersal by vehicles (i.e. agestochory) in road effects.     

Landscape-scale disturbance and boreal forest birds: Can large single-pass harvest approximate fires?

Boreal forest birds have adapted to changes caused by natural disturbances such as fire and this adaptation forms the basis for the Natural Disturbance Paradigm (NDP) underlying recent proposed changes in forest harvesting practices in western Canada. To date, this paradigm has been evaluated primarily at the stand level and within conventional harvesting systems. The potential for improvements in avian conservation at the landscape scale by adopting the NDP approach is largely unknown. We examined the effects of landscape-scale disturbances on forest bird communities by contrasting richness and abundance of birds in (1) 16 single-pass harvest sites with residual forest patches, (2) 29 multi-pass harvest sites with residuals; and (3) 15 salvage-logged post-fire sites with variable harvest intensity. We contrasted bird communities in these treatments with those in unsalvaged post-fire sites of similar age. Post-fire sites were used to provide a metric of the Natural Range of Variation (NRV) to be expected in bird communities. Sites were surveyed for avian community composition and abundance 1–5 years post-disturbance. Redundancy analysis indicated that bird communities differed from the NRV in all of the harvest treatments. However, single-pass harvests provided a somewhat better fit to NRV than did multi-pass harvesting. Avian community similarity was influenced by non-linear responses to area harvested, amount of residual retention, residual composition and pre-disturbance forest composition. An optimization routine created from a General Linear Model, suggests that community similarity to NRV can be maximized by using single-pass harvests over multi-pass harvests, harvesting 66–88% of the timber in the planning unit, and retaining 5–19% of the disturbance area as live residual patches, with 50% of harvests having at least 9% of the area in residuals.     

Can native tree species plantations in Panama compete with Teak plantations? An economic estimation

Panama has the highest rate of change in the area of primary forests within Central America. However, to meet growing timber demands, it became popular over the last decades to establish plantations made up of foreign species such as Tectona grandis or Pinus spp. In the majority of the cases the species used are well known; their characteristics such as growth performance have been reviewed intensively and can be accessed in numerous publications. Characteristics of Panama’s native tree species of commercial relevance such as Hieronyma alchorneoides, Swietenia macrophylla and Terminalia amazonia are largely unknown and have been investigated within the study at hand. Using valuation methods of financial mathematics, the competitive position of these three indigenous species was assessed, the results compared to those of T. grandis stands in the same area. Land costs and taxes were not considered, as they would be the same for all species. Financial estimates for indigenous species will enlarge their acceptance for use in reforestation and plantation projects. Using the NPV method and applying the standard scenario, the profitability of T. grandis is lower than that of T. amazonia and S. macrophylla and lies only slightly above the profitability calculated for H. alchorneoides. This result clearly indicates that the investigated native tree species are comparable with T. grandis regarding their economic profitability. Besides its ecological impact, growing native tree species is now also economically legitimate. By calculating land expectation values for all tree species, ideal rotation lengths could be determined. For these species, considerable flexibility exists regarding the optimal rotation length.     

Can methyl jasmonate treatment of conifer seedlings be used as a tool to stop height growth in nursery forest trees?

A plant's induced defense system can be triggered by the application of the plant hormone methyl jasmonate (MeJA), and recent research suggest that MeJA treatment may become a tool for protection of conifer seedlings against insect herbivory (e.g. by the pine weevil Hylobius abietis). A side-effect of MeJA application is temporarily reduced height growth. This has generally been considered as negative, but in forest tree nurseries this could instead be beneficial since it is commonly desired to stop the growth of nursery seedlings in late summer. Artificially longer dark periods (long nights/short days) are widely used in high-latitude nurseries to terminate height growth and induce freezing tolerance. However, long night treatment requires specialized nursery equipment and are labor intensive. Therefore alternatives are sought after. We compared long-night and MeJA treatments by following the growth of Norway spruce (Picea abies) seedlings throughout one season. The regulatory effect of MeJA on height growth was similar if not even better than that of long nights, i.e. it was terminated faster. However, MeJA treatment also reduced root growth and delayed the development of freezing tolerance. MeJA may therefore not replace long-night treatments, but it could facilitate a more flexible application of long nights by gaining a longer time interval during which this treatment can be used without risking the seedlings growing too large.