T E C H N I C A L R E P O R T A Systematic Review of Forest Fertilization Research in Interior British Columbia

Transcription

1 T E C H N I C A L R E P O R T A Systematic Review of Forest Fertilization Research in Interior British Columbia

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3 A Systematic Review of Forest Fertilization Research in Interior British Columbia Anya Reid, Louise de Montigny, Cindy Prescott, and Toktam Sajedi

4 The use of trade, firm, or corporation names in this publication is for the information and convenience of the reader. Such use does not constitute an official endorsement or approval by the Government of British Columbia of any product or service to the exclusion of any others that may also be suitable. Contents of this report are presented for discussion purposes only. Funding assistance does not imply endorsement of any statements or information contained herein by the Government of British Columbia. Uniform Resource Locators (urls), addresses, and contact information contained in this document are current at the time of printing unless otherwise noted. ISBN Print version ISBN Digital version Citation Reid, A., L. de Montigny, C. Prescott, and T. Sajedi A systematic review of forest fertilization research in Interior British Columbia. Prov. B.C., Victoria, B.C. Tech. Rep hfd/pubs/docs/tr/tr111.htm Prepared by Anya Reid, Toktam Sejedi, and Cindy Prescott University of British Columbia Forest and Conservation Sciences Vancouver, BC v6t 1z4 Louise de Montigny B.C. Ministry of Forests, Lands and Natural Resource Operations Resource Practices Branch PO Box 9513 Stn Prov Govt Victoria, BC v8w 9c2 Copies of this report may be obtained, depending upon supply, from: Crown Publications, Queen s Printer 2nd Floor, 563 Superior Street Victoria, BC v8w 9v For more information on other publications in this series, visit hfdcatalog/index.asp 2017 Province of British Columbia When using information from this report, please cite fully and correctly.

5 ABSTRACT The Forests for Tomorrow (FFT) Program is investing in fertilization as a mitigation strategy to address mid-term timber supply disruptions in the interior of British Columbia due to the mountain pine beetle outbreak. Forest fertilization has been shown to increase tree growth, accelerate stand development, and shorten rotation ages, but the to fertilization varies with species, and site and stand conditions. The FFT Program relies on the results of fertilization research conducted over 30 years by the B.C. Ministry of Forests, Lands and Natural Resource Operations for decision-making regarding operational fertilization. As FFT considers the expansion of the fertilization program, a review of research results to date was needed to identify knowledge gaps and information needs. As a result, a systematic review of existing published information was conducted on the growth of interior tree species lodgepole pine (Pinus contorta var. latifolia Engelm), interior spruce (Picea glauca [Moench] Voss, Picea engelmannii Parry ex Engelm, and their hybrids), and Douglas-fir (Pseudotsuga menziesii) to fertilization in relation to fertilization type and amount, damaging agents, stand density, site quality, and stand age at fertilization. Lodgepole Pine Lodgepole pine had the most variable growth to fertilization. Basal area and volume s were generally more consistently positive than were height s. The probability and magnitude of a positive growth improved by avoiding induced secondary deficiencies (particularly sulphur [S] and boron [B]), as identified through foliar nutrient analyses. Single application of 200 kg N/ha usually resulted in positive volume and basal area s if secondary nutrient deficiencies did not manifest and the stands were healthy. Lodgepole pine was susceptible to N-induced secondary B and S deficiencies, so the following recommendations were made: When pre-fertilization foliar B concentrations are < 6 mg/kg, add 3 kg B/ha, especially in areas with possible drought. When pre-fertilization foliar S concentrations are < 60 mg/kg and N/S ratios are > 13, add 50 kg S/ha. Annual fertilizations with optimum nutrition blends did not result in significant growth s. In 14 healthy lodgepole pine stands that were fertilized once with 200 kg N/ha, absolute volume s ranged from 0.5 to 20.4 m 3 /ha 6 years after fertilization, which corresponded to relative volume s of 2 to 50%. There was no clear effect of N source or season of application on growth. Fertilization increased damage and mortality from snowpress or red squirrel (Tamiasciurus hudsonicus) feeding on some sites, which resulted in poor or negative growth s. Thinning to < 1600 stems/ha reduced individual tree and total volume to fertilization. iii

6 Interior Spruce Interior spruce responded most positively to fertilization, in both magnitude and consistency of. Growth of young (< 15 years old) interior spruce responded well to repeated NB, NSB, and NPK+ fertilizations and annual optimum nutrition fertilizations. The magnitude of relative basal area and volume was greater than the magnitude of relative height to fertilization. In healthy 11- and 14-year-old stands, absolute volume after annual optimum nutrition fertilization ranged from 16 to 22 m 3 /ha 6 years after first fertilization, and 32 to 45 m 3 /ha 9 years after first fertilization. Fertilization of interior spruce can increase terminal weevil occurrence and thereby limit height growth ; therefore, recommendations have been made to avoid fertilizing spruce where terminal weevil is present. More information is needed on growth to one-time fertilizations with nitrogen in older stands. Douglas-fir Douglas-fir was generally responsive to nitrogen fertilization. In the ICH zone of British Columbia, relative growth s of Douglas-fir to fertilization were generally positive but smaller than those of spruce. In the ICH zone of British Columbia, absolute volume s ranged from 4 to 23 m 3 /ha 6 years after one-time applications of 200 kg N/ha in 19- to 34-year-old stands with 600, 800, and 1100 stems/ha. In the Inland Northwest (interior of Washington, Oregon, Idaho, and Montana), increased damage from Armillaria ostoyae root rot after fertilization limited or negated relative growth s, especially on sites with foliar K concentrations < 6 g/kg and foliar N:K ratio > 2. In the Inland Northwest, the largest growth s to fertilization occurred at sites with pre-treatment foliar N concentrations < 11.5 g/kg and foliar sulphate (SO4) concentrations > 200 mg/kg. The addition of > 200 kg N/ha per application did not appear to improve the growth of any of the species. For all three species, relative growth had no apparent relationship with site index or stand age at fertilization. Fertilization can accelerate stand development and resultant selfthinning, but this type of mortality is not a concern for stand productivity or timber production. Increased tree damage and mortality from external agents, such as small mammals, snowpress, blowdown, insects, or disease, after fertilization can result in greater losses to stand volume than growth increases from added nutrients; it is therefore necessary to identify and avoid fertilizing on sites where these conditions are likely to occur. Within the interior of British Columbia, there is a need for more information on growth on a wider range of sites (particularly for interior spruce and Douglas-fir), in older stands, and over long periods (to rotation). iv

7 PREFACE This project was initiated and co-ordinated by Louise de Montigny. Toktam Sajedi reviewed the literature for lodgepole pine. Anya Reid reviewed the literature for interior spruce and Douglas-fir, analyzed the data, and led the writing of the technical report. Cindy Prescott provided expert advice, knowledge, and oversight throughout the project. ACKNOWLEDGEMENTS Rob Brockley and Monty Locke are gratefully acknowledged for insights and comments on the report. Funding for this project was provided by the Forests for Tomorrow Program. v

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9 CONTENTS Abstract... Preface... Acknowledgements... 1 Introduction Methods Results and Discussion Data Used Magnitude of Growth Response and Fertilization Frequency Lodgepole pine Interior spruce Douglas-fir Blends and Application Rate Lodgepole pine Interior spruce Douglas-fir Mortality and Damaging Agents Lodgepole pine Interior spruce Douglas-fir Stand Density Site Index Stand Age at Fertilization Information Needs Literature Cited Appendices 1 Overview of interior tree species studies Location of interior tree species studies Stand description of interior tree species Fertilization treatment descriptions of interior tree species iii v v vii

10 tables 1 For each species and fertilizer blend, mean values are summarized for cumulative nitrogen application rate, site index, and stand age at first fertilization Nutrients used in fertilizer blends, and their sources, as described in the literature Number of plots for each species, Biogeoclimatic Ecosystem Classification zone, and fertilizer blend combination from the data used in this review FIGURES 1 Relative basal area, volume, and height by fertilizer blend for lodgepole pine, interior spruce, and Douglas-fir Relative basal area, volume, and height by total nitrogen applied for lodgepole pine, interior spruce, and Douglas-fir Relative basal area, volume, and height by stand density at fertilization for lodgepole pine, interior spruce, and Douglas-fir Lodgepole pine relative basal area, volume, and height to a single fertilization of < 200 kg n/ha by stand density at fertilization Relative basal area, volume, and height by site index for lodgepole pine, interior spruce, and Douglas-fir Lodgepole pine relative basal area, volume, and height to a single fertilization of < 200 kg n/ha by site index Relative basal area, volume, and height by stand age at fertilization for lodgepole pine, interior spruce, and Douglas-fir Lodgepole pine relative basal area, volume, and height to a single fertilization of < 200 kg n/ha by stand age at fertilization viii

11 1 INTRODUCTION Forests are a valuable natural resource that is important to the environment, economy, and communities of British Columbia. In the interior of British Columbia, mountain pine beetle attack has killed approximately 731 million cubic metres of British Columbia s merchantable lodgepole pine volume (54%) to date (Government of British Columbia 2017), which has created timber supply challenges for affected communities. Although reforestation efforts are currently under way, including the extensive planting of genetically improved stock, almost all the trees that are to be harvested from interior forests within the next 50 years are already growing. To improve short- and mid-term timber supply, mitigation strategies must focus on increasing the productivity and accelerating the development of established stands of pine and other species. Forest fertilization is a silvicultural tool that can increase tree growth, accelerate stand development, and increase piece size without sacrificing total harvestable volume (Brockley 2005). The Forests for Tomorrow (FFT) Program of the B.C. Ministry of Forests, Lands and Natural Resource Operations has been investing in fertilization as a mitigation strategy to address interior mid-term timber supply disruptions. Nutrient deficiencies are widespread throughout the British Columbia interior, and decisions regarding stand selection, fertilizer blends, and application rates are based on research and experience gained from experimental fertilization trials in this region over 30 years. Early experiments demonstrated variable growth to fertilization, and research since then has improved the ability to predict which stands and sites have the highest probability of positive growth to fertilization. Growth to fertilization has been summarized for lodgepole pine (Brockley 2001), interior spruce (Brockley 2010), and Douglas-fir (Brockley (2006b). Guidance on the best locations and situations for using fertilization has been provided in the Forest Fertilization Guidebook (B.C. Ministry of Forests 1995), Stand Selection Guidelines: Forest Fertilization in BC (Land Based Investment 2016), Silviculture Funding Criteria 2016/17 to 2020/21 (Land Based Investment 2016), and Ministry growth and yield models. Despite the large body of published and unpublished research that supports the FFT Program, uncertainty remains about the extent of the biological of tree growth to fertilization by different tree species under different ecosystem, site, and stand conditions. To determine the state of knowledge and where further research and data analysis are needed, we conducted a systematic review of literature on the growth s of interior tree species to fertilization. A systematic review is a type of literature review that involves collecting and critically analyzing multiple research studies or papers (Webster and Watson 2002) with the aim of providing a complete, exhaustive summary of current peer-reviewed literature that is relevant to a research question. Systematic reviews make it possible to compare data from different studies collectively and determine if conclusions from individual studies are consistent in the collective data. An important aspect of systematic reviews is the provision of an adequate context for the results from each study to create a firm foundation for advancing knowledge. 1

12 In this report, we summarize existing peer-reviewed literature on growth s to fertilization of lodgepole pine (Pinus contorta var. latifolia Engelm), interior spruce (Picea glauca [Moench] Voss, Picea engelmannii Parry ex Engelm, and their hybrids), and Douglas-fir (Pseudotsuga menziesii) in the interior of British Columbia. We extracted tree growth data on height, volume, and basal area, and then calculated mean relative growth (percent difference from control plots). Data on Biogeoclimatic Ecosystem Classification (BEC) zone, site index, fertilizer blends applied, total nitrogen (N) applied, stand density, and stand age at fertilization were also extracted from all suitable publications. Together, these data formed a collective database that was used to determine patterns among growth and site variables. Using this database, we assessed growth to fertilization with respect to relative and absolute magnitude, fertilizer blends, total N application rate, mortality and damaging agents, site quality, stand density, and stand age at fertilization. 2 METHODS Relevant literature was identified using keywords in the following online databases: Web of Science, Google Scholar, Directory of Open Access Journals, and CABI directory of Forest Science. For lodgepole pine, searches included the following keywords: forest fertilization BC Canada, forest fertilization BC interior, tree growth fertilization BC, tree growth fertilization BC interior, fertilization lodgepole pine, forest fertilization BC lodgepole pine tree growth, and forest fertilization BC interior lodgepole pine tree growth. For interior spruce, searches included the following keywords: spruce or Picea, fertiliz* or fertilis*, British Columbia, and growth or height or volume. For interior Douglas-fir, searches included the following keywords: Douglas-fir or Pseudotsuga menziesii, fertiliz*, and British Columbia. In addition to the literature searches conducted using online databases, relevant literature was identified when reading papers found from the online database searches. From the numerous papers identified during these searches, the following criteria were used to determine their suitability for further review: 1. Lodgepole pine, interior spruce, or interior Douglas-fir was measured. 2. Lodgepole pine was not repressed (repressed pine stands have densities > stems/ha with permanent height growth reduction and lack of self-thinning). 3. Tree growth data were available in a data-table; data could not be interpolated from figures. 4. The study was located in the interior of British Columbia (except for one study on lodgepole pine from Alberta and six studies on Douglas-fir from the Inland Northwest [interior of Washington, Oregon, Idaho, and Montana]). 5. The study design included a fertilized treatment plot. 6. The study design included an unfertilized control plot. 2

13 7. The research was conducted in a natural forest system, not in a greenhouse or field. 8. Measured trees were older than 5 years when fertilized. 9. The fertilization treatment was not confounded by other silvicultural treatments such as scarification, herbicide application, or brushing. 10. Broadcast fertilization occurred on treatment plots. 11. There was no evidence of potential bias, such as modelling parameters, valuation decisions, publication bias, or conflicts of interest. Data on tree growth, fertilization regime, and site characteristics were extracted from suitable publications. Tree growth data included tree height (m), volume (m 3 ), and basal area (cm 2 ). Individual trees were measured, and plot averages were reported. When available, absolute volume (m 3 /ha) relative to the control (absolute volume on treatment plots minus absolute volume on control plots) was also extracted. Information on fertilizer blends, application rates, season of application, treatment intervals (years), and total number of applications was extracted. Data were also extracted on the following site characteristics: location, experimental project number, BEC zone and subzone, site series, site index (metres at age 50), soil classification, stand density (stems/ha) at establishment and at fertilization, stand age at fertilization, and time since initial fertilization (years). Growth to fertilization was measured using treatment plot means for annual growth increments. Percent difference relative to control plot means was calculated as follows: Relative growth = (treatment control)/control 100 where treatment is the mean annual growth increment from the fertilized plot, and control is the mean annual growth increment from the corresponding unfertilized plot (Edmonds and Hsiang 1987; Littke et al. 2014). This growth measure allowed for standardized comparisons of growth changes relative to the control among studies. This is referred to as relative. If the literature reported growth measures for multiple years on the same site, only the oldest measurements were used to avoid repeated measures in the data. Therefore, the collective data set has one data point for each treatment mean. This is critical for unbiased comparisons of site and fertilizer characteristics but makes it difficult to determine temporal patterns of treatment effects. It is important to differentiate between the terms experiment, installation, and plot. An experiment or experimental project includes all experimental activities associated with a project that is conducted to test a hypothesis. An experiment usually has multiple installations. An installation is a replicate within the overall experiment that has both treatment (fertilized) and control (unfertilized) plots. A plot is the basic experimental unit in which the treatment is applied (or not applied in control plots), and generally has a buffer area around the perimeter that is not measured. A plot will have a core group of research trees that are measured. 3

14 3 RESULTS AND DISCUSSION 3.1 Data Used For lodgepole pine, initial searches resulted in 3368 publications that contained some of the search words. Through scanning titles and further search revisions, 40 publications were selected for in-depth review. Data were extracted from 11 publications (Table A1.1) that reported on 28 unique installations, which resulted in a total of 136 data points for analysis (not including control plots). Installations were located in the Sub-Boreal Spruce (SBS), Montane Spruce (MS), Sub-Boreal Pine Spruce (SBPS), Engelmann Spruce Subalpine Fir (ESSF), and Interior Cedar Hemlock (ICH) BEC zones (Table A2.1). Stand age at fertilization ranged from 8 to 41 years (Tables 1 and A3.1). blends included B, N, NB, NPK+, NPS, NS, NSB, optimum nutrition (ON), and PK+ (Tables 1 and A4.1). Nutrients and their sources are described in Table 2. Optimum nutrition treatments received annual applications of a fertilizer blend that was designed to address all nutrient deficiencies based on foliar nutrient analysis. NPK+ treatments are also referred to as complete fertilizer blends, which include N, P, K, B, S, Mg, and other minerals (Table 2). Cumulative N fertilizer application rates during the experiment ranged from 0 to 1600 kg N/ha (Table 1). TABLE 1 For each species and fertilizer blend (n = number of treatment means), mean values (and their range) are summarized for cumulative nitrogen (N) application rate, site index, and stand age at first fertilization. Growth s (% difference relative to controls) of height, volume, and basal area are also summarized. Species Blend a (n) Cumulative N rate (kg N/ha) Site index (m at age 50 yr) Stand age (yr) Height Volume Basal area Lodgepole B (2) (4 6) pine N (49) 193 ( ) 20 (16 25) 21 (8 30) 10 ( 14 47) 25 ( 5 50) 28 ( 10 69) NB (8) 213 ( ) 19 (16 23) 16 (10 22) 4 ( 4 12) (2 39) NPK+ (16) 341 ( ) 19 (16 21) 14 (8 22) 5 ( 6 18) 40 (14 63) 31 (5 55) NPS (6) 360 ( ) 41 4 ( 20 20) 61 (24 125) 51 (4 121) NS (37) 203 ( ) 20 (18 23) 22 (15 30) 5 ( 20 24) 29 (3 53) NSB (6) 217 ( ) 20 (16 23) 14 (10 18) 4 ( 3 14) (12 28) ON b (12) 1181 ( ) 19 (16 21) 13 (10 16) 1 ( 23 20) 33 ( 12 70) 38 (3 67) PK+ (1) Interior N (1) spruce NB (3) (19 21) 12 (10 14) 14 (9 20) (27 62) NPK+ (6) 150 ( ) 20 (19 21) 13 (10 15) 59 (19 109) (35 91) NS (2) (57 91) NSB (3) (19 21) 12 (10 14) 18 (9 24) (33 99) ON (6) 1213 ( ) 20 (19 21) 12 (10 14) 26 (14 38) 235 ( ) 101 (30 203) Douglas-fir N (43) 284 ( ) 23 (12 39) 36 (10 71) 15 (2 34) 14 ( 8 41) 56 (0 200) NK (1) NPKSB (1) NS (6) (24 39) 25 (19 34) 15 (1 28) 23 ( 3 39) 30 (19 42) a Nutrients used in fertilizer blends are described in Table 2. b ON: Optimum nutrition is the annual application of a unique fertilizer blend based on deficiencies detected from foliar nutrient analysis (Table 2). 4

15 TABLE 2 Nutrients used in fertilizer blends, and their sources, as described in the literature Nutrient Boron (B) Nitrogen (N) Potassium (K) Phosphorus (P) Sulphur (S) NPK+ or complete source Solubor, granular borate Urea, ammonium sulphate, ammonium chloride, ammonium nitrate, ammonium phosphate Muriate of potash (KCl) Calcium phosphate, triple super phosphate, ammonium phosphate Ammonium sulphate, elemental sulphur Granulated complete mix including granular dolomite, urea, mono-ammonium phosphate, ammonium nitrate, sulphate potash magnesia, potassium chloride, ammonium sulphate, ProMag 36, and granular borate Optimum nutrition (ON) Customized nutrient blends to maintain foliar N of 13 or 16 g/kg and other nutrients in balance with foliar N Interior spruce refers to white spruce (Picea glauca [Moench] Voss), Engelmann spruce (Picea engelmannii Parry ex Engelm), and their hybrids. Literature searches for interior spruce growth to fertilization identified 156 potentially relevant publications. Data were extracted from three publications (Table a1.2) that reported on four installations, which resulted in a total of 21 data points used for analysis (not including control plots). Installations were located in the SBS and ESSF (Table a2.2), and stand age at fertilization ranged from 10 to 15 years (Tables 1 and a3.2). blends applied included N, NB, NSB, NPK+, NS, and optimum nutrition (Tables 1 and a4.2). Cumulative N fertilization rates ranged from 100 to 1600 kg N/ha. For Douglas-fir, literature searches produced 115 publications for further review. Thirty-eight were relevant for full review, and data were extracted from seven publications (Table a1.3) that reported on nine installations, which resulted in a total of 51 data points used for analysis (not including control plots). Detailed descriptions of study locations, stands, and fertilization treatments are shown in Tables a2.3, a3.3, and a4.3, respectively. Only one published study on Douglas-fir growth to fertilization originated from the interior of British Columbia (Brockley 2006b). This study included fertilizer blends of N, NS, or NPK+ (Table a1.3). For this reason, we allowed the use of data published from the Inland Northwest, including those reported by Coleman et al. (2014), Balster and Marshall (2000), Binkley and Reid (1984), Garrison et al. (1997), and Shafii et al. (1989). Data from Strand and DeBell (1979) were also used, although detailed location information was missing; the report stated only that the study was conducted in the Pacific Northwest. blends included N, NK, NPKSB, and NS (Table 1). Cumulative N fertilizer application rate during the experiment ranged from 155 to 535 kg N/ha (Table 1). Stand age at fertilization ranged from 10 to 71 years, and site index ranged from 12 to 39 m (Table 1). 3.2 Magnitude of Growth Response and Fertilization Frequency Lodgepole pine Relative growth of lodgepole pine to fertilization ranged from 23 to 47% for height, 12 to 125% for volume, and 10 to 121% for basal area (Table 1). Relative basal area and volume s were generally more consistently positive than relative height (Figure 1), as also reported by Brockley (1991, 2000). Negative growth s 5

16 Lodgepole pine Species Interior spruce Douglas-fir 40 Basal area Volume Height Treatment interval One-time Repeated Annual 0 B N NB NK NS PK+ NPK NPS NSB NPK+ ON B N NB NK NS PK+ NPK NPS NSB blend NPK+ ON B N NB NK NS PK+ NPK NPS NSB NPK+ ON FIGURE 1 Relative basal area, volume, and height (% difference from control plots) by fertilizer blend for lodgepole pine, interior spruce, and Douglas-fir. blends are ordered from fewest to greatest number of nutrients. Nutrients used in fertilizer blends are described in Table 2. Each point represents a unique treatment mean. relative to controls resulted from location-specific increased mortality or damage caused by red squirrel (Tamiasciurus hudsonicus) feeding, snowpress, insects, or disease after fertilization. The sites where negative growth s could be expected were subsequently excluded when new fertilization studies were established. Additionally, the recommendation to exclude operational fertilization of these sites was incorporated into FFT fertilization guidance documents. Response of lodgepole pine in the interior of British Columbia was not significantly enhanced by repeated additions of N fertilizer or annual optimum nutrition fertilization. Repeated N fertilization resulted in negative relative height and basal area s (Figure 1), likely caused by induced B and S deficiencies (see Section 3.3). These results do not match results for Pinus species in eastern Canada and Europe, where repeated fertilization resulted in sustained growth yields (Malkonen and Kukkola 1991; Weetman et al. 1995; Tamm et al. 1999; Albaugh et al. 2004). Lodgepole pine in British Columbia may be more prone to secondary nutrient limitations. In British Columbia, annual additions of optimum nutrition blends often resulted in negative relative height, neutral relative volume, and slightly positive relative basal area (Figure 1). Reductions in lodgepole pine growth in British Columbia following annual and repeated fertilization has been attributed to increased morality, changes in growth allocations from stem to 6

17 branches, shifted foliar nutrient balance, or interactions with soil and understorey vegetation after fertilization (Brockley 2007). In 14 undamaged lodgepole pine stands that were fertilized once with 200 kg N/ha, absolute volume ranged from 0.5 to 20.4 m 3 /ha (mean of 9.2 m 3 /ha) 6 years after fertilization (Brockley 2001). This corresponds to a relative volume ranging from 2 to 50% (Brockley 2001). The author of this study removed three of the installations because of serious mortality and damage caused by snowpress and red squirrel feeding (Brockley 2001). Stand densities at these sites were 1100, 1600, and 2200 stems/ha, and the stands were years old at the time of first fertilization (Brockley 1991) Interior spruce Interior spruce growth to fertilization was consistently positive for all three parameters, and spruce responded well to repeated and annual fertilizer applications (Figure 1). These trends are consistent with research from British Columbia (Brockley and Simpson 2004) and with Swedish optimum nutrition studies of spruce (Tamm 1991; Tamm et al. 1999). Relative growth of interior spruce to fertilization ranged from 9 to 109% for height, from 75 to 277% for volume, and from 27 to 203% for basal area (Table 1). Most of the spruce data were from plots that received repeated fertilization with nutrient blends that would alleviate secondary deficiencies (Figure 1). Only one plot received one-time N fertilization (Figure 1). In healthy interior spruce stands in the SBS zone that were 11 and 14 years old when fertilized, absolute volume after annual optimum nutrition ranged from 16 to 22 m 3 /ha 6 years after first fertilization and from 32 to 45 m 3 / ha 9 years after first fertilization (Brockley and Simpson 2004). Annual optimum nutrition fertilizations of interior spruce were considered in this review Douglas-fir Relative growth of Douglas-fir to fertilization ranged from 1 to 34% for height, from 8 to 41% for stand volume, and from 0 to 200% for basal area (Table 1). Five plots had negative growth due to losses from increased mortality after fertilization. The effect of repeated fertilization on Douglas-fir growth remains unknown because of the lack of data on repeated fertilization of this species (Figure 1). In the ICH zone of British Columbia, absolute volume s ranged from 4 to 23 m 3 /ha 6 years after one-time applications of 200 kg N/ha in 19- to 34-year-old stands with 600, 800, and 1100 stems/ha (Brockley 2006b). Absolute volume of Douglas-fir was similar to that of interior spruce, even though the relative of spruce was much larger. For example, a relative of 209% in spruce volume corresponds to an absolute of 32 m 3 /ha, whereas a 41% relative in Douglas-fir volume corresponds to an absolute of 23 m 3 /ha. One explanation for this trend is that the interior spruce trees were younger and therefore smaller than the Douglas-fir trees that were studied. 3.3 Blends and Application Rate Lodgepole pine Although N is generally the most limiting nutrient in lodgepole pine forests, secondary S and B deficiencies can significantly limit growth of lodgepole pine to N fertilization. Symptoms of B deficiency can develop rapidly and result in reduced stem volume increment and wood quality (Brockley 1996). Stands with foliar B concentrations < 6 mg/kg were susceptible to fertilizer-induced B deficiency (Brockley 2003). Adding 7

18 1.5 3 kg B/ha to N fertilizers can prevent top dieback and improve growth to fertilization (Brockley 2003). Additions of B to N fertilizer are especially recommended at sites with possible drought conditions (Brockley 2003) because B availability is closely linked to soil moisture content (Mengel and Kirkby 1987). Stands with foliar sulphate (SO 4 ) concentrations < 60 mg/kg and N/S ratio > 13 were susceptible to fertilizer-induced S deficiency (Brockley 2000). Adding 50 kg S/ha to N fertilizer was effective in promoting growth of lodgepole pine (Brockley 2004). Induced S deficiencies were most common in Luvisolic soils on the northern Interior Plateau and in fire-origin stands in the SBS and IDF (Brockley 1996), but S additions are not considered necessary when fertilizing in the MS zone (Brockley 1996). The incremental growth increases from a complete fertilizer over a more basic NS fertilizer were too small to justify the added cost (Brockley 2001). Furthermore, foliar nutrient analyses suggested that S was the main nutrient responsible for increased growth to complete fertilizers (Kishchuk et al. 2002). The rate of N fertilizer application that had the highest probability of positive growth in lodgepole pine was 200 kg N/ha (Figure 2). Average individual tree volume was 7% greater when trees were fertilized with 200 kg N/ha compared to 100 kg N/ha (Brockley 1991). There was no evidence of increased of lodgepole pine to N fertilization at rates > 200 kg N/ha, even when additional nutrients were applied (Figure 2) (Brockley 2001). The Forest Fertilization Guidebook (B.C. Ministry of Forests 1995) Lodgepole pine Species Interior spruce Douglas-fir 40 Basal area Volume Height Treatment interval One-time Repeated Annual 0 FIGURE Total N applied (kg/ha) Relative basal area, volume, and height (% difference from control plots) by total nitrogen (N) applied for lodgepole pine, interior spruce, and Douglas-fir. Each point represents a unique treatment mean. 8

19 recommends applications of kg N/ha for pine (and all tree species) in interior British Columbia. Growth of lodgepole pine was not significantly affected by the source of N fertilizer (urea versus ammonium nitrate) (Brockley 1995, 2006a) or season of application (spring versus fall) (Brockley 1995) Interior spruce In young spruce stands in the interior of British Columbia, there was an incremental volume from adding 100 kg S/ha to an NB blend that was applied every 6 years (Brockley 1992, 2010). However, there was little added volume from complete (NPK+) fertilizer application compared to the NSB blend (Brockley 1992, 2010). Optimum nutrition treatments produced the greatest relative s in basal area and volume, but relative height s were similar to or less than those associated with the NB, NSB, and NPK+ fertilizer blends (Figure 1). Little is known about the relative growth of spruce to a fertilizer blend with only N because there was only one plot with this treatment. Ballard and Carter (1986) provide nutrient deficiency thresholds for interior spruce: slight deficiencies started when foliar concentrations were < 1.30% for N, < 0.14% for P, < 0.45% for K, < 0.10% for Ca, and < 0.06% for Mg. Optimum nutrition experiments described by Brockley (2010) determined that the type and quantities of fertilizers that would maintain foliar N concentration were 13 or 16 g/kg, and foliar nutrient ratios below critical thresholds that were determined for Norway spruce were 10 for N/P, 3 for N/K, 15 for N/S, 20 for N/Mg, 20 for N/Ca, and 1000 for N/B (Ingestad 1979; Linder 1995; Brække and Salih 2002). Young interior spruce stands in the interior of British Columbia showed consistent positive relative growth s to annual fertilizations with optimal nutrient blends of or kg N/ha applied per year (Figure 2) Douglas-fir Potassium and sulphur are the most common secondary limiting nutrients that can reduce relative growth of Douglas-fir after N fertilization. Potassium deficiencies can occur when foliar K concentrations are < 6 g/kg and foliar N:K is > 2 (Mika and Moore 1990). Induced K deficiencies after N or NS fertilizer applications have been linked to increased Armillaria (Armillaria ostoyae) root rot (Mika and Moore 1990; Brockley 2006b). In the interior of British Columbia, N fertilization can exacerbate S deficiencies when pre-fertilization foliar SO 4 concentrations are < 200 mg/kg (Brockley 2006b), which is lower than the 400 mg/kg threshold suggested by Turner et al. (1979). At the one site in the ICH zone where Douglas-fir to fertilization has been studied, all fertilized treatment plots received 200 kg N/ha and generally had relative growth s (Brockley 2006b). However, these results may not be indicative of relative growth s of Douglas-fir in other BEC zones or site conditions in British Columbia. The collective database also has a narrow range of total N applied (Figure 2), which makes conclusions about growth relevant only to application rates from 200 to 400 kg N/ha. 3.4 Mortality and Damaging Agents Fertilization has often been reported to increase tree mortality relative to unfertilized trees. In some cases, increased tree mortality after fertilization was caused by accelerated stand development and resultant self-thinning (Yang 9

20 1998). This type of mortality is not a concern for stand productivity or timber production. However, there has also been greater tree mortality after fertilization due to damage from external agents such as small mammals, snowpress, blowdown, insects, or disease. In some cases, increased damage and mortality after fertilization incur greater losses to stand volume than growth increases due to the added nutrients. This results in negative growth s that can reduce stand productivity and timber production. It is therefore necessary to determine the main causes of damage for each species, what situations increased damage after fertilization, and how to avoid these situations Lodgepole pine Snowpress and red squirrel feeding damage were the main causes of increased mortality and damage to lodgepole pine after fertilization (Mika et al. 1992; Brockley 2001). Productivity losses in lodgepole pine stands due to treatment-related damage and mortality negated growth s to fertilization in four of 15 installations (Brockley 1991). Selecting sites with low incidence of red squirrel damage prior to fertilization minimized feeding injury (Brockley 2001). Damage is greater when thinning occurs, when application rates of N are high, and when ammonium nitrate versus urea is applied. In thinned lodgepole pine plots, Yang (1998) found that damage was greater in the fertilized plots than in unfertilized plots. Within the fertilized plots, damage was greater in plots fertilized with 540 kg N/ha compared to 180 or 360 kg N/ha (Yang 1998). Similarly, Brockley (1991) reported greater damage in plots fertilized with 200 kg N/ha compared to 100 kg N/ha. Occurrence of mountain pine beetle was higher in plots fertilized with ammonium nitrate compared to urea (Brockley 2006a). Brockley (2001) recommended delaying fertilization for 2 years after thinning in order to reduce feeding injury and snowpress damage after fertilization Interior spruce Terminal weevils were the main damaging agent in fertilized spruce stands. Near Prince George, fertilized plots had twice as many white pine terminal weevils (Pissodes strobi) as did control plots because of increased resource availability and reduced tree defence after fertilization (vanakker et al. 2004). Weevil damage was lowest in the SBSmc subzone (compared with the SBSwk and SBSmk), possibly because of colder temperatures, which could limit terminal weevil occurrence (vanakker et al. 2005). Fertilization of interior spruce has been classified as less risky in the SBSmc subzone than in the SBSwk or mk subzones (Land Based Investment 2006) Douglas-fir For interior Douglas-fir, Armillaria root rot was the main damaging agent after fertilization, although damage from wind, snow, and bark beetles can also increase after fertilization (Mika and VanderPloeg 1991). Increased mortality from Armillaria after N fertilization has been linked to aggravated K deficiencies in the Inland Northwest (Mika and Moore 1990; Moore et al. 1994; Coleman et al. 2014), which reduce phenol:sugar ratios in the roots and promote disease spread (Entry et al. 1991; Shaw et al. 1998). Stands with pre-fertilization foliar concentrations of K < 6 g/kg and foliar N:K > 2 are at greatest risk of increased mortality due to Armillaria (Mika and Moore 1990). Similarly, in the interior of British Columbia, the one installation that had significant Armillaria-induced mortality (9%) after fertilization had pre-fertilization foliar K < 6 g/kg (Brockley 2006b). 10

21 3.5 Stand Density For lodgepole pine, stand densities ranged from 250 to 6000 stems/ha, and there was no clear relationship between stand density and relative growth (Figure 3). Lodgepole pine data are also summarized for single applications of < 200 kg N/ha (Figure 4); again, there was no apparent relationship between stand density and growth. In very dense post-fire regenerating lodgepole pine stands, thinning had mixed effects on growth to fertilization. In dense, nutrient-poor, fire-origin pine stands in central British Columbia, relative and absolute growth s to fertilization were greater in unthinned plots ( stems/ha) than in thinned plots ( stems/ha) (Blevins et al. 2005). Conversely, Yang (1998) reported greater relative stand and tree volume to fertilization in thinned plots (2000 stems/ha) than in unthinned plots (5000 stems/ha) of lodgepole pine (286 and 209 m 3 /ha, respectively). In planted lodgepole pine stands, thinning to < 1600 stems/ha reduced growth to fertilization. When thinned to 600, 1100, and 1600 stems/ ha, both relative volume (39, 56, and 59%, respectively) and absolute volume (14, 29, and 34 m 3 /ha, respectively) increased with increasing stand density (Brockley 2005). The smaller fertilization in the 600 stems/ ha plots was attributed to interactions with understorey vegetation. Thinning prior to fertilization on sites that are prone to red squirrel feeding can also offset growth by causing greater red squirrel feeding damage after fertilization. In a fertilized lodgepole pine stand, red squirrel damage was Lodgepole pine Species Interior spruce Douglas-fir 40 Basal area Volume Height Treatment interval One-time Repeated Annual Stand density (stems/ha) at fertilization FIGURE 3 Relative basal area, volume, and height (% difference from control plots) by stand density at fertilization for lodgepole pine, interior spruce, and Douglas-fir. Each point represents a unique treatment mean. 11

22 Basal area Volume Height 40 Treatment 30 B N 20 NB 10 NS 0 NPS 10 NSB 20 NPK Stand density (stems/ha) at fertilization FIGURE 4 Lodgepole pine relative basal area, volume, and height (% difference from control plots) to a single fertilization of < 200 kg N/ ha (or B alone) by stand density at fertilization. Each point represents a unique treatment mean. Nutrients used in fertilizer blends are described in Table % greater in thinned plots (2500 stems/ha) than in unthinned plots ( stems/ha) (Sullivan and Sullivan 1982). All interior spruce and Douglas-fir stands in the collective data set were approximately 1000 stems/ha (Figure 3), so it was not possible to explore relationships between stand density and relative growth to fertilization of these species. 3.6 Site Index Site quality depends on nutrient and water availability, and can be measured by site index, which is the average tree height (m) of dominant trees (unsuppressed, unrepressed, and undamaged) after 50 years of growth beyond breast height age (B.C. Ministry of Forests 1999). There was no apparent relationship between site index and relative growth of lodgepole pine to fertilization when all data were considered (Figure 5) or when only single applications of < 200 kg N/ha were considered (Figure 6). This is consistent with Brockley s (1996) report of no relationship between lodgepole pine relative growth and site index in interior British Columbia. With no relationship between relative growth and site index, absolute volume would be expected to be greater at higher-productivity sites. Similarly, absolute growth is predicted to be minimal in low-productivity stands with site index < 16 m (Brockley 1996). Interior spruce fertilizations occurred on site indices of 19 and 21 (Figure 5); therefore, there are not enough data to determine if there is a relationship between site index and relative growth of interior spruce. 12

23 Lodgepole pine Species Interior spruce Douglas-fir 40 Basal area Volume Height Treatment interval One-time Repeated Annual 0 FIGURE Site index (m at 50 yr) Relative basal area, volume, and height (% difference from control plots) by site index for lodgepole pine, interior spruce, and Douglas-fir. Each point represents a unique treatment mean. Basal area Volume Height 40 Treatment B N NB NS NPS NSB NPK Site index (m at 50 yr) FIGURE 6 Lodgepole pine relative basal area, volume, and height (% difference from control plots) to a single fertilization of < 200 kg N/ha (or B alone) by site index. Each point represents a unique treatment mean. Nutrients used in fertilizer blends are described in Table 2. 13

24 There was no clear relationship between site index and relative growth of Douglas-fir to fertilization (Figure 5). Brockley (2006b) found greater growth to fertilization on poorer sites with initial foliar N concentrations < 11.5 g/kg. However, in the Inland Northwest, fertilization was generally better at good-quality sites and low at poor-quality sites, probably because moisture limitations at the poor-quality sites caused small relative growth to fertilization (Coleman et al. 2014). 3.7 Stand Age at Fertilization There was no clear relationship between stand age and relative growth for any species in the collective data set (Figure 7). Stand age (years) at fertilization ranged from 8 to 41 for lodgepole pine, from 10 to 15 for interior spruce, and from 10 to 71 for Douglas-fir (Table 1). Douglas-fir stands in British Columbia s ICH zone ranged from 15 to 35 years old at the time of fertilization (Brockley 2006b). In the subset of data on lodgepole pine that were fertilized once with < 200 kg N/ha, the highest relative height and basal area s to fertilization occurred in stands that were approximately 20 years old at first fertilization, but there was considerable variability in this trend (Figure 8). Lodgepole pine Species Interior spruce Douglas-fir 40 Basal area Volume Height Treatment interval One-time Repeated Annual Stand age at fertilization (yr) FIGURE 7 Relative basal area, volume, and height (% difference from control plots) by stand age at fertilization for lodgepole pine, interior spruce, and Douglas-fir. Each point represents a unique treatment mean. 14

25 Basal area Volume Height 40 Treatment B N NB NS NPS NSB 50 NPK Stand age at fertilization (yr) FIGURE 8 Lodgepole pine relative basal area, volume, and height (% difference from control plots) to a single fertilization of < 200 kg N/ ha (or B alone) by stand age at fertilization. Each point represents a unique treatment mean. Nutrients used in fertilizer blends are described in Table 2. 4 INFORMATION NEEDS Research on growth to fertilization that has been conducted over the last 40 years has provided considerable information, but there are critical knowledge gaps and information needs for operational fertilization programs that are targeted at increasing future timber supply. Priority information needs identified in this literature review include the growth of: interior spruce stands older than 15 years at time of fertilization; interior spruce stands to one-time fertilization of N; Douglas-fir stands at sites in zones other than the ICH circum-mesic sites, particularly in the IDF wet belt; larch stands to fertilization treatments; stands of preferred and acceptable species in the SBSmh, ICHmw, and ICHwk subzones; late-rotation fertilization in stands (all species) that are years old at time of fertilization; stands given repeated fertilization 6 10 years after the initial fertilization treatment; and all healthy stands in research experiments beyond the 6- or 9-year period to determine long-term yields from fertilization. Two other topics of concern with respect to the resilience of British Columbia forests to climate change are the interactions between forest fertilization 15

26 and (1) damage due to insects, disease, and weather, and (2) carbon fluxes and storage. Some of these information needs can be met by analyzing and reporting on fertilization research data from the B.C. Ministry of Forests, Lands and Natural Resource Operations Research Program, which cover a much wider range of ecosystems, stand ages, and treatment types than were found in the published literature (Table 3). Determining the long-term yields from fertilization can be achieved by continued monitoring and measurement of existing fertilization installations. Other information needs will require the establishment of new fertilization research experiments. TABLE 3 Number of plots (n) for each species, Biogeoclimatic Ecosystem Classification (BEC) zone, and fertilizer blend combination from the data used in this review. Blue shading indicates that no published data were available. Species BEC a (n) blends b (n) Lodgepole pine B N NB NPK+ NS NSB ON PK+ BWBS ESSF (5) ICH (4) IDF MS (36) MH PP SBPS (3) 3 SBS (83) Interior spruce B N NB NPK+ NS NSB ON PK+ BG BWBS ESSF (7) ICH (1) IDF MS MH PP SBPS SBS (15) Douglas-fir B N NB NPK+ NS NSB ON PK+ BG ESSF ICH (13) IDF MS MH PP SBPS SBS a bec zones in which the species occurs in the interior region of British Columbia ( Compendium): BG = Bunchgrass; BWBS = Boreal White and Black Spruce; ESSF = Engelmann Spruce Subalpine Fir; ICH = Interior Cedar Hemlock; IDF = Interior Douglas-fir; MH = Mountain Hemlock; MS = Montane Spruce; PP = Ponderosa Pine; SBPS = Sub-Boreal Pine Spruce; SBS = Sub-Boreal Spruce. b Nutrients used in fertilizer blends are described in Table 2. 16