Mid-Term Effects of Forest Thinning on N Mineralization in a Semi-Arid Aleppo Pine Forest

Round 1

Reviewer 1 Report

Dear authors
How did logging affect the N content of the soil? Theoretically, a plant cover should form on an open area and its decomposition should increase the N content. However, the results presented do not indicate why? Does thinning also affect plant growth on the forest floor? I understand that drought has determined the biological activity of the soil, the survival of vegetation and pine seedlings, and the problem of N deficiency has arisen? If moisture conditions are adequate, this problem does not occur? Is it therefore advisable to fertilise with N after periods of drought? Also, was there any infestation of insect pests or pathogens during the 16-year study period? The caterpillars feed on N in the soil with their excretions. Is the soil re-covered with vegetation after the drought and is the amount of N increasing? A dendrochronological study would be useful to show the growth of trees at different periods in relation to meteorological conditions. Has it been determined how temperature affects soil N content (seasonality)? Were changes in the quantity and quality of microorganisms, including mycorrhizal fungi, noted? Mycorrhizal fungi determine soil C and N content by decomposing litter and woody debris (already in clearings) and forming extensive networks of hyphae and fungal roots. Which thinning method is recommended for Mediterranean conditions in pine cultivation? P. halepensis?

Author Response

Response to Review 1

We appreciate the reviewer’s useful suggestions, which are of great help to the revision of this manuscript and our research. We have made the modification of the manuscript based on the comments. Below you will find our point-by-point responses to the comments.

Each one of the observations in answered below.

How did logging affect the N content of the soil?

In forest ecosystems, the availability of nitrogen is mainly controlled by the coupling between nitrogen mineralization from the organic N pool and the plant uptake. Immediately after logging the pool of organic nitrogen could increase if residues are left on the soil, but plant uptake is reduced. A decoupling between mineral nitrogen produced by mineralization and plant uptake leads to surplus of mineral nitrogen subject to leaching or gaseous losses.

Theoretically, a plant cover should form on an open area and its decomposition should increase the N content. However, the results presented do not indicate why?

Assessing the impact of silvicultural treatments on biogeochemical cycles in forest ecosystems is complex, due not only to the diversity of pools and flows involves, but also to the different time scales in which they operate. In this work we did not found a clear pattern since differences in N content might be shadowed by differences in soil texture.

Does thinning also affect plant growth on the forest floor?

Decreasing stand density by thinning is traditionally used to improve residual tree growth. The positive effect of thinning on plant growth has been reported by several authors;

Reduction of intra-species competition as induced by thinning clearly benefited the remaining trees, a result that can be explained by the fact that resource availability (water, nutrients, light) increase with the growing space (Breda et al., 1995; Aussenac, 2000). In the case of P. halepensis, Fernandes et al. (2016) found that thinning made trees less sensitive to soil water shortage. Thinning enhanced tree growth on Mediterranean Aleppo pine forests (Manrique-Alba et al., 2020).

Manrique-Alba, À., Beguería, S., Molina, A. J., González-Sanchis, M., Tomàs-Burguera, M., Del Campo, A. D., ... & Camarero, J. J. (2020). Long-term thinning effects on tree growth, drought response and water use efficiency at two Aleppo pine plantations in Spain. Science of the total environment, 728, 138536.

I understand that drought has determined the biological activity of the soil, the survival of vegetation and pine seedlings, and the problem of N deficiency has arisen? If moisture conditions are adequate, this problem does not occur? Is it therefore advisable to fertilise with N after periods of drought?

Nitrogen mineralization is determined by substrate quantity and quality, microbial biomass and microbial activity which, in turn, is controlled by environmental conditions. In dry environments plant growth is restricted by water deficit rather than nitrogen availability, since the frequent drying and wetting alternation will stimulate decomposition of soil organic matter (SOM) and increase inorganic N (Birch, 1960; Sahrawat, 1980).

Birch, H.F., 1960. Nitrification in soils after different periods of dryness. Plant Soil12 (1), 81–96.

Sahrawat, K.L., 1980. Soil and fertilizer nitrogen transformations under alternate flooding and drying moisture regimes. Plant Soil 55 (2), 225–233.

Also, was there any infestation of insect pests or pathogens during the 16-year study period? The caterpillars feed on N in the soil with their excretions.

We did not report any infestation. Thank you for bringing the topic of the caterpillars to our attention.

Is the soil re-covered with vegetation after the drought and is the amount of N increasing?

Additional information is provided about forest evolution after thinning. Galiana, F., Palacio, A., Lidón A., Martínez-Campos, F.M., Tortajada, L., Bautista, I., Lull, C. (2017). Evolución de la estructura de la vegetación y la regeneración del pinar en un tratamiento selvícola experimental en Tuéjar (Valencia). In 7º Congreso Forestal Español. Gestión del monte: servicios ambientales y bioeconomía. (1 - 5). Plasencia, España: Sociedad Española de Ciencias Forestales.  https://secforestales.org/publicaciones/index.php/congresos_forestales/article/view/19338/19053

A dendrochronological study would be useful to show the growth of trees at different periods in relation to meteorological conditions.

Thank you for pointing out the importance of dendrochronological studies to better study and understand the relation between tree growth and meteorological conditions. We will take it into consideration in future studies.

Has it been determined how temperature affects soil N content (seasonality)?

We have studied the seasonal variation of soil mineral nitrogen, but soil mineral nitrogen is a highly variable property. The mineral nitrogen content is the result of a balance between inputs (mainly organic nitrogen mineralization) and outputs (plant uptake and volatilization or percolation losses).  Changes in the concentration of soil mineral nitrogen could be related to variations in temperature, moisture and quality of organic matter, which control microbial processes such as mineralization and immobilization.

Were changes in the quantity and quality of microorganisms, including mycorrhizal fungi, noted?

In this study we didn’t measure fungi nor microorganism’s quantity or quality.

Mycorrhizal fungi determine soil C and N content by decomposing litter and woody debris (already in clearings) and forming extensive networks of hyphae and fungal roots. Which thinning method is recommended for Mediterranean conditions in pine cultivation? P. halepensis?

Under Mediterranean conditions with Pinus halepensis forest, nitrogen availability is improved with medium pine density which favours an understory plant cover that increased nitrogen availability.

Author Response File: Author Response.docx

Reviewer 2 Report

Authors studied the effects of silvicultural treatments on nutrient mineralization in Aleppo pine forests. The research looks well designed and the analyses looks ok. I have a few comments;

1) L17: "treatments showed a twofold increase in net mineralization and nitrification rates in both blocks (TU and CH)" specify that by treatment you mean T60, T75 and T100 and T0 is not considered treatment but a control.

2) L18: Rephrase this "Within plots, mineralization was higher in sites with no plant cover and in those under undergrowth"

3) L118-119: how did you select which of the 5 out of the 9 points to analyze for C and N?

3) L181: insert "they" into "The soil samples were transported to the laboratory in a portable refrigerator where were sieved"

4) L204-205; how long was the data collection period? I am trying to understand how 6-bimonthly measurements can be called annual net mineralization.

5) please define clearly in the methods section what net mineralization and nitrification are, including how these were computed

6) L210-211; thickness of organic horizon was computed from from 9-points and C,N from 5-points, were these aggregated to the plot level or was the analysis done using point level data. also did authors test for random effects of block, sites in the linear regression model?

7) were the differences in soil temperature by the treatments groups statistically significant ?

Author Response

Response to Review 2

Comments and Suggestions for Authors

Authors studied the effects of silvicultural treatments on nutrient mineralization in Aleppo pine forests. The research looks well designed and the analyses looks ok. I have a few comments;

We value your professional opinion on our work, and we believe that your suggestions and comments have improved the overall quality of our article. The point-by-point responses to your comments are listed below; all line numbers are in the file named "Revised manuscript."

1) L17: "treatments showed a twofold increase in net mineralization and nitrification rates in both blocks (TU and CH)" specify that by treatment you mean T60, T75 and T100 and T0 is not considered treatment but a control.

Thank you for the suggestion. The paragraph specifies the treatments as following:

Lines 17-19 and 514-516: “Thirteen years after the experimental clearings T100, T75 and T60 treatments showed a twofold increase in net mineralization and nitrification rates respect to T0 in both blocks (TU and CH)”

2) L18: Rephrase this "Within plots, mineralization was higher in sites with no plant cover and in those under undergrowth"

We appreciate your correction. The statement has been rewritten.

Line 19-20: Within plots, the highest mineralization was found in sites with no plant cover followed by those under undergrowth.

3) L118-119: how did you select which of the 5 out of the 9 points to analyze for C and N?

A new figure has been added and the sampling points to analyze for C and N are specified as follow:

Line 131: in five points per plot (the center and the corners of the square)

4) L181: insert "they" into "The soil samples were transported to the laboratory in a portable refrigerator where were sieved"

We appreciate your correction. The statement has been rewritten.

Line 209-210: The soil samples were transported to the laboratory in a portable refrigerator where they were sieved.

5) L204-205; how long was the data collection period? I am trying to understand how 6-bimonthly measurements can be called annual net mineralization.

Thank you for bringing this to our attention.

Lines 152-155: In total, six sequential bimonthly incubations were performed throughout a year (August 2011- July 2012). Annual net N mineralization and nitrification were calculated as the sum of net N mineralization and nitrification over the six incubation periods.

6) please define clearly in the methods section what net mineralization and nitrification are, including how these were computed.

Thank you for your advice. The explanation is in the text:

Lines 160-165 At each measuring point, a soil sample was taken. An aliquot of the sample was used to fill a tube with the coupled resin that was placed back in the place where the sample was taken. The rest of the sample was transported to the laboratory to analyze as soon as possible its mineral nitrogen content (hereinafter, initial content). After an incubation period of two months, the tube with resins was collected and transported to the laboratory to analyse the mineral N content of the soil (final content) and of the resin.

Lines 165 -168. The net nitrification was obtained as the sum of the final nitrate N plus the nitrate N content in the resin, subtracting the initial nitrate N. Similarly, net mineralization is obtained as the sum of final mineral nitrogen plus resin mineral nitrogen minus initial mineral nitrogen.

7) L210-211; thickness of organic horizon was computed from 9-points and C, N from 5-points, were these aggregated to the plot level or was the analysis done using point level data. also did authors test for random effects of block, sites in the linear regression model?

The analysis was done only using point level data as spatial variability is very high.

7) were the differences in soil temperature by the treatment groups statistically significant?

The differences in temperature among treatments as analyzed by Lado Monserrat (2015) were not statistically significant.

Lado-Monserrat, L. (2015). Efecto de tratamientos selvícolas de diferente intensidad sobre el microclima del suelo y los ciclos de nutrientes de un bosque de pino carrasco. Doctor Thesis. Universitat Politécnica de València.

Author Response File: Author Response.docx

Reviewer 3 Report

Mid-term effects of forest thinning on N mineralization in a semi-arid Aleppo pine forest

Inmaculada Bautista et al.

In this manuscript the authors studied effect of different thinning intensities on soil nitrogen mineralization. The topic is interesting no doubt, but the hypothesis and experimental design have some basic weaknesses, as mentioned below. Overall I am not strongly convinced with the interpretation of N mineralization varied due to thinning treatments at the background of current studies. The experiment was conducted after 13 years of thinning, by this time juniperus and quercus trees were grown as understory, contributed to soil N mineralization and could dilute the impacts of thinning. 

Major comments:

1. The hypothesis fixed for the study was not tested. “Our hypothesis is that stand density control could be an adequate management practice to improve nutrient use efficiency and C sequestration in unmanaged pine forests with high stand density (Line 82-83). “Stand density”, “nutrient use efficiency”, “C sequestration” none of these was measured in the study!
2. Experiment with two replicates. Experiment to evaluate impacts of tree density/thinning should have sufficient replicates (at least three) for each treatment to minimize spatial variations. The authors used four treatments with only two replicated locations.  
3. Thinning affects N mineralization mainly through organic matter inputs (i.e. litterfall and rhizosphere activities which in turn affect temperature, soil moisture and pH etc.), but at the same time, understory vegetation (grass, moss, herbs and shrubs) can influence significantly. Therefore, to study the impact of thinning treatment (i.e. cutting only trees) on soil biogeochemical process, understory vegetation must be more or less uniformly distributed between treatment plots. My major concern is the nine sampling/incubation points were not uniformly covered when compared between treatments (Table-3). For example, treatment T0 had no shrub but T100 has 78% shrub cover, so the variation between these two cannot be due to thinning. Similarly, nine sampling areas cannot be evenly categorized into four cover-types. That means mineralization from bare soil of T75 plot was considered as influenced by 75% thinning and mineralization from similar bare soil of T60 by 60% thinning!
4. The authors studied soil N mineralization in different plots of two locations without analyzing general properties of soils before and after treatments. Particularly, soil pH which influence N mineralization in soil, can vary within short distance but this was not checked between treatments.
5. The manuscript is well written except  the methods section needs to write precisely.

Author Response

Response to Review 3

Comments and Suggestions for Authors

Thank you for the review and your suggestions. Each one of the observations is answered below. Methods were improved and conclusions rewritten.

Mid-term effects of forest thinning on N mineralization in a semi-arid Aleppo pine forest

Inmaculada Bautista et al.

In this manuscript the authors studied effect of different thinning intensities on soil nitrogen mineralization. The topic is interesting no doubt, but the hypothesis and experimental design have some basic weaknesses, as mentioned below. Overall I am not strongly convinced with the interpretation of N mineralization varied due to thinning treatments at the background of current studies. The experiment was conducted after 13 years of thinning, by this time juniperus and quercus trees were grown as understory, contributed to soil N mineralization and could dilute the impacts of thinning. 

Major comments:

The hypothesis fixed for the study was not tested. “Our hypothesis is that stand density control could be an adequate management practice to improve nutrient use efficiency and C sequestration in unmanaged pine forests with high stand density (Line 82-83). “Stand density”, “nutrient use efficiency”, “C sequestration” none of these was measured in the study!

Thank you for the observation. The hypothesis has been more precisely defined.

Lines 83-86: “Our hypothesis is that stand density control could be an adequate management practice to improve nitrogen availability in unmanaged pine forests with high stand density.”

 Experiment with two replicates. Experiment to evaluate impacts of tree density/thinning should have sufficient replicates (at least three) for each treatment to minimize spatial variations. The authors used four treatments with only two replicated locations.

The thinning experiment has three replicates, but we measured soil nitrogen mineralization only in two locations due to high soil stoniness in the third. The information about thinning performance was added to material and methods section.

Lines 105-115: In the spring of 1998, an experimental study began to compare different silvicultural systems on this P. halepensis stand. The main goal of these silvicultural systems was to convert the stand into a mixed forest of P. halepensis and Q. rotundifolia to increase its biological diversity and resilience. Treatments were carried out following a randomized block design, with four treatments and three blocks. The distance among the three blocks was less than 3 km. In each block, four experimental square plots (30 m x 30 m with a 7.5 width along each edge as a buffer zone) were selected, one per treatment. Treatments were: (i) T0: untreated control reference; (ii) T60: moderate shelterwood with 60% of mean basal area removed; (iii) T75: strong shelterwood with 75% of mean basal area removed; (iv) T100: clearfelling (100% of mean basal area removed).

Thinning affects N mineralization mainly through organic matter inputs (i.e. litterfall and rhizosphere activities which in turn affect temperature, soil moisture and pH etc.), but at the same time, understory vegetation (grass, moss, herbs and shrubs) can influence significantly. Therefore, to study the impact of thinning treatment (i.e. cutting only trees) on soil biogeochemical process, understory vegetation must be more or less uniformly distributed between treatment plots. My major concern is the nine sampling/incubation points were not uniformly covered when compared between treatments (Table-3). For example, treatment T0 had no shrub but T100 has 78% shrub cover, so the variation between these two cannot be due to thinning. Similarly, nine sampling areas cannot be evenly categorized into four cover-types. That means mineralization from bare soil of T75 plot was considered as influenced by 75% thinning and mineralization from similar bare soil of T60 by 60% thinning!

We didn’t perform a thinning experiment to analyse the effect of vegetation cover on soil nitrogen mineralization, but we measure the soil nitrogen mineralization on several plots subjected to thinning treatments. As the scale of soil sample to plot is very big, soil nitrogen mineralization is highly variable. We try to identify some patterns of some measurable properties associated to this variability, such as vegetative cover or organic horizon depth.

In order to clarify this a new figure (Figure 1) and a new reference have been added.

Lines 377-380: “Galán et al. [61], analyzed the vegetation structure in summer 2016 in these stands, being the shrub cover significantly lower in T0 plots. The understory species composition also differed with prevalence of Quercus coccifera in T100, T75 and T60 plots and Juniperus oxycedrus in T0 plots”.

Table 3 has been deleted.   

The authors studied soil N mineralization in different plots of two locations without analyzing general properties of soils before and after treatments. Particularly, soil pH which influence N mineralization in soil, can vary within short distance but this was not checked between treatments.

Soil pH is around 8-8,5 in all the plots. Since geologic material contain lime, pH is buffered by calcium carbonate precipitation dilution.

The manuscript is well written except the methods section needs to write precisely.

The methods section has been rewritten.

Author Response File: Author Response.docx