FOR PUBLICATION
  UNITED STATES COURT OF APPEALS
       FOR THE NINTH CIRCUIT

LEAGUE OF WILDERNESS DEFENDERS         
— BLUE MOUNTAINS BIODIVERSITY
PROJECT; CASADIA WILDLANDS
PROJECT, an Oregon nonprofit
                                            No. 06-35780
corporation,
              Plaintiffs-Appellants,
                                             D.C. No.
                                           CV-04-00982-PP
                v.
                                             OPINION
UNITED STATES FOREST SERVICE, an
agency of the United States
Department of Agriculture,
              Defendant-Appellee.
                                       
        Appeal from the United States District Court
                  for the District of Oregon
         Paul J. Papak, Magistrate Judge, Presiding

                  Argued and Submitted
            October 21, 2008—Portland, Oregon

                  Filed December 11, 2008

   Before: David R. Thompson, A. Wallace Tashima, and
            Milan D. Smith, Jr., Circuit Judges.

           Opinion by Judge Milan D. Smith, Jr.




                            16205
16208      LEAGUE OF WILDERNESS DEFENDERS v. USFS


                         COUNSEL

R. Scott Jerger, Field Jerger LLP, Portland, Oregon, for the
plaintiffs-appellants.

Leslie B. Bellas, United States Department of Justice, Wash-
ington, D.C., John Munson, United States Department of
Agriculture, Portland, Oregon, and Stephen John Odell,
Office of the United States Attorney, Portland, Oregon, for
the defendant-appellee.


                          OPINION

MILAN D. SMITH, JR., Circuit Judge:

   In their suit filed pursuant to the Administrative Procedures
Act (APA), 5 U.S.C. § 706, the League of Wilderness
Defenders — Blue Mountains Biodiversity Project and Cas-
cadia Wildlands Project (collectively, LOWD) sought declar-
atory and injunctive relief to halt the Deep Creek Vegetation
Management Project (the Project), which called for the selec-
tive logging of 12.8 million board feet of timber in the
Ochoco National Forest. LOWD claims in its suit that the
United States Forest Service (Forest Service) failed to comply
with the National Environmental Policy Act (NEPA), 42
U.S.C. § 4231 et seq., and the National Forest Management
Act (NFMA), 16 U.S.C. § 1600 et seq., in developing and
implementing the Project. The district court denied LOWD’s
motion for summary judgment and granted the Forest Ser-
vice’s cross-motion for summary judgment. Because the Final
Supplemental Environmental Impact Statement (FSEIS) may
          LEAGUE OF WILDERNESS DEFENDERS v. USFS        16209
not tier to a non-NEPA watershed analysis to consider ade-
quately the aggregate cumulative effects of past timber sales,
we reverse the district court’s grant of summary judgment in
favor of the Forest Service, and we remand this case so the
Forest Service can reissue its NEPA documentation to include
the omitted information regarding past timber sales contained
in the watershed analysis.

   FACTUAL AND PROCEDURAL BACKGROUND

  A.   1999 Deep Creek Watershed Analysis

   The Forest Service manages the Ochoco National Forest
under the Ochoco National Forest Land and Resource Man-
agement Plan (Ochoco LRMP). In 1993, the Regional For-
ester directed the Paulina Ranger District, among others, to
conduct a watershed analysis that identified “processes and
functions within the Deep Creek watershed that are key to
maintaining sustainable and resilient terrestrial and aquatic
ecosystems.” To carry out these instructions, the Forest Ser-
vice created an interdisciplinary team of employees that even-
tually documented its findings and conclusions in the August
1999 Deep Creek Watershed Analysis (Watershed Analysis).

   The Watershed Analysis collectively considered past
actions in the Deep Creek watershed and the results of those
actions to determine existing conditions and trends. Based on
this cumulative evaluation, the Watershed Analysis deter-
mined that many of the tree stands were overly dense and sus-
ceptible to high-intensity wildfires and forest diseases; that
some types of tree stands were over-represented as compared
to historical conditions, while other types of stands were
under-represented; and that riparian habitats along streams
exhibited similar problems. The Watershed Analysis accord-
ingly concluded that the Forest Service should undertake a
blend of management activities to alleviate and improve these
unsatisfactory conditions.
16210      LEAGUE OF WILDERNESS DEFENDERS v. USFS
  B.    The Project

   In October 1999, based on the results of the Watershed
Analysis, the Forest Service initiated the NEPA documenta-
tion process for the Project. In April 2001, the Forest Service
published a draft Environmental Impact Statement (EIS) and,
after receiving public comments and making adjustments,
issued a final EIS in September 2001. The Forest Service also
issued a corresponding Record of Decision (ROD).

   Shortly thereafter, LOWD filed an administrative appeal of
the Project on behalf of itself and a number of environmental
organizations. In response, the Forest Service withdrew the
original ROD to perform additional analyses on the Project’s
effects, and dismissed the appeal as moot.

   In July 2002, the Forest Service issued a draft supplemental
EIS for the Project. After receiving and considering public
comments, the Forest Service issued a FSEIS in January
2004. According to the FSEIS, the stated purpose and need
for the Project are to: (1) “move the landscape-level diversity
of vegetation and associated wildlife habitat closer to the [his-
toric range of variability (HRV)] . . . in terms of species com-
position and structure,” given that “forest vegetation is
outside the [HRV] for 57% of the [watershed]”; (2) “increase
the amount of single strata late and old structure (LOS)
stands” and move “the overall abundance of LOS closer to the
[HRV]”; (3) “reduce the forest’s susceptibility to moderate
and high severity fires” by lowering fuel levels, reducing
stand densities, “increasing the relative abundance of fire tol-
erant species, and re-introducing fire into the watershed”; (4)
“reduce the [forest’s] susceptibility . . . to insects, diseases,
and wildfires by reducing their stocking levels”; (5) “enhance
vegetative conditions in the aspen, riparian, upland shrub, and
meadow communities” that have gradually declined over
time; and (6) “improve water quality and enhance the vegeta-
tion aspect of aquatic/riparian areas to provide for long-term
          LEAGUE OF WILDERNESS DEFENDERS v. USFS        16211
sustainability of resident and anadromous fisheries by reduc-
ing stream temperatures and lowering sedimentation.”

   In January 2004, the Forest Service issued another ROD
and selected modified Alternative C, which includes commer-
cial timber harvest on 6261 acres, pre-commercial thinning on
9957 acres, 6.1 miles of new and temporary road construction,
16.3 miles of road reconstruction, and fuel-reduction treat-
ments on 5379 acres. Modified Alternative C would allow
logging of 12.8 million board feet of timber, primarily
through ground-based tractor logging. The ROD concludes
that this alternative presents “the best balance of activities
suited for meeting the identified needs of the Deep Watershed
at this time” and “balances water quality issues while improv-
ing uplands and riparian vegetation conditions and reducing
susceptibility to moderate and high severity fires.”

  C.   Procedural History

   Following issuance of the 2004 ROD, LOWD filed an
administrative appeal, which the Forest Service denied.
LOWD then filed a complaint in the District of Oregon pursu-
ant to the APA, alleging that the Forest Service’s approval of
the Project violated NEPA, NFMA, and the applicable
Ochoco LRMP, and seeking declaratory and injunctive relief.
In adopting the magistrate judge’s findings and recommenda-
tions, the district court denied LOWD’s motion for summary
judgment, granted the Forest Service’s cross-motion for sum-
mary judgment, and dismissed the action with prejudice.
LOWD timely appealed. We have jurisdiction over LOWD’s
appeal pursuant to 28 U.S.C. § 1291.

                STANDARD OF REVIEW

  We review the district court’s summary judgment ruling de
novo. Nw. Envtl. Advocates v. Nat’l Marine Fisheries Serv.,

, 1132 (9th Cir. 2006). Thus, “ ‘[v]iewing the
evidence in the light most favorable to the nonmoving party,’
16212      LEAGUE OF WILDERNESS DEFENDERS v. USFS
we must determine ‘whether there are any genuine issues of
material fact and whether the district court correctly applied
the law.’ ” Pension Trust Fund for Operating Eng’rs v. Fed.
Ins. Co., 

, 948-49 (9th Cir. 2002) (quoting State
Farm Mut. Auto. Ins. Co. v. Davis, 

, 1417 (9th
Cir. 1991)).

   In making this determination, “we must remember that the
APA provides the authority for our review of decisions under
NEPA and NFMA.” Lands Council v. McNair (Lands Council
II), 

, 987 (9th Cir. 2008) (en banc). Under the
APA, the court may set aside only Forest Service actions that
are “arbitrary, capricious, an abuse of discretion, or otherwise
not in accordance with law.” 5 U.S.C. § 706(2)(A).

   “Review under the arbitrary and capricious standard ‘is nar-
row, and [we do] not substitute [our] judgment for that of the
agency.’ ” Lands Council 

(quoting Earth
Island Inst. v. U.S. Forest Serv., 

, 1156 (9th Cir.
2006)) (alterations in original). “Rather, we will reverse a
decision as arbitrary and capricious only if the agency relied
on factors Congress did not intend it to consider, ‘entirely
failed to consider an important aspect of the problem,’ or
offered an explanation ‘that runs counter to the evidence
before the agency or is so implausible that it could not be
ascribed to a difference in view or the product of agency
expertise.’ ” 

Island 

).
In other words, there must be “ ‘a clear error of judgment.’ ”
Marsh v. Or. Natural Res. Council, 

, 378 (1989)
(quoting Citizens to Preserve Overton Park, Inc. v. Volpe, 

, 416 (1971)).

                        DISCUSSION

  LOWD argues that, in developing the Project, the Forest
Service violated NEPA because the FSEIS fails to consider
adequately the cumulative effects of the Project, as required
by 40 C.F.R. § 1508.7. LOWD also argues that the Forest Ser-
           LEAGUE OF WILDERNESS DEFENDERS v. USFS          16213
vice violated NFMA and the applicable Ochoco LRMP by:
(1) failing to assure that Forest Service employees properly
mark large-diameter trees for avoidance of harvest; and (2)
failing to maintain connective habitat corridors in the Project
planning area.

  A.   Cumulative Effects Analysis

   LOWD first claims that the Forest Service failed to con-
sider adequately the cumulative environmental effects of
other timber sales and grazing in the Project planning area.
The Forest Service responds that its responsibility is to con-
sider cumulative effects in the aggregate rather than on an
individual basis, and that the FSEIS contains a sufficient dis-
cussion of cumulative effects to satisfy the agency’s duty
under NEPA. We conclude that the Forest Service may aggre-
gate its cumulative effects analysis, and that it properly did so
with respect to grazing and future timber sales. However, we
find the aggregated cumulative effects analysis of past timber
sales deficient because the Forest Service failed to include the
relevant inputs in the FSEIS itself.

   [1] NEPA provides that a federal agency that proposes a
“major Federal action[ ] significantly affecting the quality of
the human environment” must prepare a detailed EIS on the
proposed action, including an analysis of “alternatives to the
proposed action” and a discussion of the significant environ-
mental impacts. 42 U.S.C. § 4332(2)(C). To comply with this
requirement, the Forest Service must consider, among other
things, the “cumulative impacts” of the proposed action,
which NEPA’s implementing regulations define as the “im-
pact on the environment which results from the incremental
impact of the action when added to other past, present, and
reasonably foreseeable future actions regardless of what
agency (Federal or non-Federal) or person undertakes such
other actions.” 40 C.F.R. § 1508.7.
16214              LEAGUE OF WILDERNESS DEFENDERS v. USFS
    1.        Timber Sales

         a.        Past Timber Sales

   LOWD contends that the cumulative effects analysis in the
FSEIS regarding past timber sales is insufficient because it
“only mentions one . . . past timber sale, the Summit timber
sale,” and otherwise generally “states that timber harvest has
occurred in the past.” The Forest Service counters that, under
its reading of Department of Transportation v. Public Citizen,

(2004), “agencies can conduct an adequate
cumulative effects analysis by focusing on the current aggre-
gate effects of past actions.”

              i.     Aggregation

   In rejecting LOWD’s past-timber-sales arguments, the dis-
trict court agreed with the Forest Service’s interpretation of
Public Citizen:

    The Supreme Court recently construed [the cumula-
    tive effects] regulation and found it required an eval-
    uation of the incremental impact of the project at
    issue rather than focusing on the incremental impacts
    attributable to each of the past, present and reason-
    ably foreseeable future actions in the planning area.
    Dep’t of Transportation v. Public Citizen, 

, 769-70 (2004). Thus, the Court in Public Citi-
    zen approved an agency’s approach that considers
    the effects of past actions in the aggregate, and
    explained that actions need not be considered sepa-
    rately and distinctly when analyzing cumulative
    effects.

The district court then concluded, without elaboration or cita-
tion to the record, that “the FSEIS discusses cumulative
effects from past vegetation management activities in the
watershed to a sufficient degree.”
           LEAGUE OF WILDERNESS DEFENDERS v. USFS         16215
   This reading of Public Citizen is inaccurate. In that case,
the Court addressed the narrow issue of whether NEPA “re-
quire[d] the Federal Motor Carrier Safety Administration
(FMCSA) to evaluate the environmental effects of cross-
border operations of Mexican-domiciled motor carriers” when
FMCSA merely promulgated administrative rules implement-
ing a presidential order “allow[ing] such border-crossing
activities to occur.” Public 

, 766. In
concluding that NEPA lacked such a requirement, the Court
did state that “[t]he ‘cumulative impact’ regulation required
FMCSA to consider the ‘incremental impact’ of the safety
rules themselves, in the context of the [presidential order] and
other relevant circumstances,” not “to treat the [presidential
order] itself, or consequences from the [order].” 

(quoting 40 C.F.R. § 1508.7). Nevertheless, the Court empha-
sized twice that its analysis and holding were limited to the
“critical feature” of the case—i.e., that FMCSA lacked
authority to countermand the presidential order allowing
Mexican carriers into the United States, 

770—and
subsequent Ninth Circuit cases have limited their application
of Public Citizen on that basis. See, e.g., Or. Natural Res.
Council Fund v. Brong, 

, 1134 n.20 (9th Cir.
2007) (holding that “Public Citizen’s limitation on NEPA
does not apply” where an agency has statutory authority to
prevent the relevant effects). Accordingly, because the Forest
Service has statutory authority to regulate the environmental
consequences of the Project, Public Citizen does not support
the agency’s position.

   [2] We conclude, however, that a different source does per-
mit the Forest Service to consider cumulative effects in the
aggregate. During the summary judgment proceedings, the
Forest Service introduced a June 24, 2005 memorandum
issued by the Chairman of the Council on Environmental
Quality (CEQ), entitled “Guidance on Consideration of Past
Actions in Cumulative Effects Analysis.” This CEQ memo-
randum advises that “[a]gencies are not required to list or ana-
lyze the effects of individual past actions unless such
16216      LEAGUE OF WILDERNESS DEFENDERS v. USFS
information is necessary to describe the cumulative effects of
all past actions combined.” Instead, the memorandum
explains, “agencies can conduct an adequate cumulative
effects analysis by focusing on the current aggregate effects
of past actions without delving into the historical details of
individual past actions.”

   The magistrate judge struck the CEQ memorandum from
the record, characterizing it as a “post-hoc rationalization”
with no relevance to judicial review of the Forest Service’s
compliance with NEPA. Based on Supreme Court precedent,
however, this determination constituted an abuse of the mag-
istrate judge’s discretion. See Golden Gate Hotel Ass’n v. City
& County of S.F., 

, 1485 (9th Cir. 1994) (“We
review a district court’s decision to grant a motion to strike
unscheduled supplementary material for abuse of discre-
tion.”). In Auer v. Robbins, 

, 461 (1997), the
Court granted deference to an agency’s interpretation of its
own regulation even though the agency offered the interpreta-
tion for the first time as a litigation position. The Court noted
that the agency’s interpretation was “controlling unless
plainly erroneous or inconsistent with the regulation,” and
that “[t]here [was] simply no reason to suspect that the inter-
pretation d[id] not reflect the agency’s fair and considered
judgment on the matter in question.” 

(internal
quotation marks omitted).

   [3] Similarly here, CEQ’s interpretation that 40 C.F.R.
§ 1508.7 permits consideration of all past impacts in the
aggregate is not plainly erroneous or inconsistent with the lan-
guage of the regulation, and CEQ is the agency charged with
interpreting NEPA and that adopted the regulation. See Jones
v. Gordon, 

, 827 (9th Cir. 1986). In addition,
LOWD points to no evidence to suggest that CEQ’s interpre-
tation does not reflect the agency’s fair and considered judg-
ment on the cumulative effects issue, and CEQ’s
interpretation is just as plausible as LOWD’s position that
analysis of “past effects” requires evaluation of discrete past
           LEAGUE OF WILDERNESS DEFENDERS v. USFS          16217
events. The CEQ memorandum is therefore entitled to Auer
deference and, as a result, we hold that the Forest Service may
aggregate its cumulative effects analysis pursuant to 40
C.F.R. § 1508.7.

   Our circuit’s precedent further supports this conclusion.
Although LOWD argues that Lands Council v. Powell (Lands
Council I), 

, 1028 (9th Cir. 2005), requires a
complete cataloguing of all prior timber sales in all cases, this
interpretation is incorrect. Lands Council I instead merely
reaffirms the general rule that “NEPA requires adequate
cataloguing of relevant past projects in the area.” 

(citing Muckleshoot Indian Tribe v. U.S. Forest Serv., 

, 809-10 (9th Cir. 1999)) (emphasis added). An
aggregated cumulative effects analysis that includes the rele-
vant past-timber-sale inputs comports with this standard, and
also furthers NEPA’s purpose of “concentrat[ing] on the
issues that are truly significant to the action in question.” See
40 C.F.R. § 1500.1(b).

   Moreover, sitting en banc, this court recently clarified that
Lands Council I does not require the Forest Service to con-
duct any particular test or to use any particular method, so
long as “the evidence . . . provided to support [the Forest Ser-
vice’s] conclusions, along with other materials in the record,”
ensure that the agency “made no clear error of judgment that
would render its action arbitrary and capricious.” Lands
Council 

(internal quotation marks omit-
ted). In Lands Council II, the en banc court reasoned that this
approach not only “respects our law that requires us to defer
to an agency’s determination in an area involving a high level
of technical expertise,” but also “acknowledges that “[w]e are
not free to impose on the agency [our] own notion of which
procedures are best or most likely to further some vague,
undefined public good.” 

marks omit-
ted) (alterations in original).

  [4] Accordingly, to the extent that 40 C.F.R. § 1508.7 does
not explicitly provide otherwise, the Forest Service is free to
16218      LEAGUE OF WILDERNESS DEFENDERS v. USFS
consider cumulative effects in the aggregate or to use any
other procedure it deems appropriate. It is not for this court
to tell the Forest Service what specific evidence to include,
nor how specifically to present it.

         ii.   Tiering

   Notwithstanding our conclusion that the Forest Service
may aggregate its cumulative effects analyses, the Forest Ser-
vice’s almost complete failure to include the relevant past-
timber-sale inputs in the FSEIS itself fails to survive arbitrary
and capricious review. See Lands Council 

(noting that “we will reverse a decision as arbitrary and capri-
cious . . . if the agency . . . ‘entirely failed to consider an
important aspect of the problem’ ”) (quoting Earth Island

)).

   [5] As LOWD observes, the FSEIS itself mentions only one
past timber sale, the Summit Timber Sale, and otherwise
notes generally that other timber sales occurred in the past.
The Forest Service counters that no deficiency exists because
the omitted information about the past timber sales appears in
the Watershed Analysis:

    [T]he Deep Creek Watershed Analysis . . . collec-
    tively considered past actions in the watershed and
    the results of those actions . . . to determine existing
    conditions and trends in the watershed. This compre-
    hensive analysis served as the basis for the FSEIS.

       While each timber sale may not have been identi-
    fied by name, the Deep Creek Watershed Analysis
    evaluated the cumulative impacts of several human
    activities, including past timber harvests, to inform
    the Forest Service and the public about baseline con-
    ditions within the Deep Creek Watershed.

The FSEIS states, in turn, that “[t]he purpose and need for
action is based on the analysis and conclusions regarding the
           LEAGUE OF WILDERNESS DEFENDERS v. USFS         16219
conditions described in the Deep Creek Watershed Analysis,”
and that the Watershed Analysis “is tiered to the [FSEIS] . . .
and its applicable [ROD]. The management direction, along
with standards and guidelines, for activities proposed are
based, in part, on these documents.”

   [6] The problem with the Forest Service’s approach, how-
ever, is that the FSEIS cannot “tier” to the Watershed Analy-
sis to analyze sufficiently the cumulative effects of the
Project. The NEPA implementing regulations define “tiering”
as:

    [T]he coverage of general matters in broader envi-
    ronmental impact statements (such as national pro-
    gram or policy statements) with subsequent narrower
    statements or environmental analyses (such as
    regional or basinwide program statements or ulti-
    mately site-specific statements) incorporating by ref-
    erence the general discussions and concentrating
    solely on the issues specific to the statement subse-
    quently prepared.

40 C.F.R. § 1508.28. Meanwhile, this court “ha[s] previously
interpreted the regulations to allow tiering only to another
environmental impact statement.” Muckleshoot Indian 

; see also Or. Natural Res. Council v. U.S.
BLM, 

, 823 (9th Cir. 2006) (holding similarly
proposed tiering impermissible because “the Watershed Anal-
ysis is not a NEPA document”); Kern v. U.S. BLM, 

, 1073 (9th Cir. 2002) (holding that “tiering to a docu-
ment that has not itself been subject to NEPA review is not
permitted”).

   [7] Accordingly, although we in no way doubt what the
Forest Service says about the Watershed Analysis, the
FSEIS’s cumulative effects analysis is insufficient because it
cannot tier to a non-NEPA document that the FSEIS fails to
include. We therefore reverse the district court’s grant of sum-
16220            LEAGUE OF WILDERNESS DEFENDERS v. USFS
mary judgment in favor of the Forest Service, and remand so
that the agency can reissue its NEPA documentation to
include the omitted, but clearly relevant, information regard-
ing past timber sales contained in the Watershed Analysis. See

(“While NEPA empowers neither the
plaintiffs nor this court to second-guess the [agency’s] man-
agement decisions, it does require the [agency] to articulate,
publicly and in detail, the reasons for and likely effects of
those management decisions, and to allow public comment on
that articulation.”).

           b.    Future Timber Sales

   [8] LOWD also argues that “there is simply no mention of
future timber sales in the FSEIS.” Regarding such projects,
however, the FSEIS states: “No other timber vegetation man-
agement activities are planned within this watershed within
the foreseeable future.” As the district court noted, “this dis-
tinguishes the . . . Project from other timber harvest cases
where courts found that multiple planned timber sales in one
area required consideration of cumulative effects as to timber
management activities.” See, e.g., Blue Mountain Biodiversity
Project v. Blackwood, 

, 1214-16 (9th Cir.
1998). Accordingly, the district court properly concluded that
the FSEIS need say nothing more regarding future timber
sales to satisfy the cumulative effects standard.1

      2.        Grazing

  LOWD further asserts that the FSEIS fails to consider ade-
quately the cumulative environmental effects of grazing in the
Project planning area. This argument lacks merit for the fol-
lowing reasons. First, the FSEIS adequately evaluates the
cumulative effects of past grazing. For example, the FSEIS
  1
    Although LOWD does not focus on “ongoing” timber sale activities in
the Project planning area, the record reflects that the FSEIS’s cumulative
effects analysis is also sufficient in that regard.
          LEAGUE OF WILDERNESS DEFENDERS v. USFS         16221
discusses how unregulated sheep grazing dating back to the
1880s, before the conversion of the allotments to cattle, con-
tributed to “the loss of [soil], stream bank degradation, and
channel erosion”; explains the past development of an allot-
ment “to draw cattle from riparian areas and improve live-
stock distribution” and its resulting “effects [on] sensitive
aquatic habitats”; and notes “[t]he cumulative effects of fire
suppression, roads, conifer invasion, and browsing by cattle
and native ungulates have had detrimental effects on hydrol-
ogy . . . and riparian vegetation.”

   Second, the FSEIS adequately evaluates the cumulative
effects of present grazing. In a section entitled “Cumulative
Effects on Range,” for instance, the FSEIS states that
“[l]ivestock plays no role in the overstory stand composition,
structures, or density aspects of the Purpose and Need” of the
Project, “[n]or does it have much influence on the risk of
moderate and high intensity fires.” Although the Forest Ser-
vice acknowledges in that section that limited areas exist
where grazing could affect the vegetative conditions of ripar-
ian communities, the agency explains that limited thinning
and no-treatment buffers along streams will mitigate any such
effects. The FSEIS also discusses grazing effects in the
cumulative-effects-analysis sections for noxious weeds, soils,
threatened, endangered, and sensitive species, riparian habi-
tats, and habitat-indicator species like deer and elk.

   [9] Finally, regarding future grazing, the FSEIS explains
that “Allotment Management Plan updates . . . were initiated
in 2002 and alternative grazing regimes will be developed and
considered under a separate environmental analysis and deci-
sion that is scheduled for completion in 2004.” A cumulative
effects analysis of those future grazing regimes was therefore
impracticable because the Forest Service had not yet desig-
nated the specific grazing allotments at the time it issued the
FSEIS. Cf. N. Alaska Envtl. Ctr. v. Kempthorne, 

, 976-77 (9th Cir. 2006) (“We conclude that the govern-
ment[, in its EIS for oil and gas leases,] was not required at
16222         LEAGUE OF WILDERNESS DEFENDERS v. USFS
this stage to do a parcel by parcel examination of potential
environmental effects. Such effects are currently unidentifi-
able, because the parcels likely to be affected are not yet
known.”). Accordingly, the district court properly held that
the FSEIS adequately considers the cumulative effects of
grazing.

  B.      Marking of Large-Diameter Trees

   Next, LOWD contends that the Forest Service’s tree mark-
ings will result in the logging of trees equal to or greater than
twenty-one inches in diameter, in violation of NFMA2 and the
applicable Ochoco LRMP.3 LOWD also asserts that the Forest
Service violated NFMA because agency personnel allegedly
did not mark the trees,4 and because the timber sale contract
language is insufficient to prevent harvesting of the large
trees.

   At oral argument in the district court, the court urged the
Forest Service to provide further assurances of the correct
marking of trees greater than twenty-one inches in diameter
at breast height. The Forest Service responded with the decla-
rations of two Forest Service employees. The Declaration of
Lori Blackburn, “a certified silviculturist for the Forest Ser-
vice for over 12 years,” states that she “wrote the marking
  2
     See 16 U.S.C. § 1604(i) (requiring consistency between site-specific
projects and the applicable LRMP).
   3
     A set of interim wildlife, ecosystem, and riparian standards known as
the “Eastside Screens” and adopted in 1993 amended the Ochoco LRMP,
in relevant part, to proscribe “the logging of green trees larger than 21
inches at breast height.”
   4
     LOWD relies on 16 U.S.C. § 472a(g), which states:
      Designation, marking when necessary, and supervision of har-
      vesting of trees . . . shall by conducted by persons employed by
      the Secretary of Agriculture. Such persons shall have no personal
      interest in the purchase or harvest of such products and shall not
      be directly or indirectly in the employment of the purchaser
      thereof.
           LEAGUE OF WILDERNESS DEFENDERS v. USFS          16223
guides for the [Project]; participated in training the District
marking crew; and monitored their work during the imple-
mentation of [her] marking guides on the sale.” The “marking
crew consisted of seasonal Forest Service employees super-
vised by a . . . permanent Forest Service employee,” and
Blackburn “formally reviewed the work of the marking crew
three times,” ensuring that the crew properly measured and
marked the trees to avoid harvesting those over twenty-one
inches in diameter. Her declaration concludes, “my oversight
and monitoring indicate that the marking crew for the [Proj-
ect] has marked the sale in accordance with the prescriptions
and design criteria that have been adopted for the . . .
[P]roject.”

   Meanwhile, the Declaration of Dennis R. Dietrich, “a Tim-
ber Sale Contracting Officer for the Ochoco . . . National For-
est[ ],” provides the following language from the timber sale
contract regarding “Individual Tree Designation”:

    All trees less than 21.0 inches D.B.H. [diameter at
    breast height] Marked with blue paint above and
    below stump height in cutting Units 2, 3, 9, 21, 24-
    26 and 36; and all live trees 7.0 to 20.9 inches
    D.B.H. not Marked with orange paint above and
    below stump height in Cutting Units 4-8, 10-20, 22,
    23, 27-35 and 37-54 which meet the minimum tree
    diameter stated in AT2 are designated for cutting.
    Additional timber to be cut, if any, will be desig-
    nated for cutting in accordance with BT2.37.

Dietrich concludes that “[t]his combination of the marking on
the trees and the description in the contract, along with pro-
fessional contract administration, makes [him] confident that
the Forest Service has taken every reasonable measure to pro-
tect against the harvest of trees that are not authorized by con-
tract to be cut.” He also notes that, “[t]o further protect
against the possibility that a tree over 21 inches in diameter
will be cut during execution of the [Project], the contract that
16224       LEAGUE OF WILDERNESS DEFENDERS v. USFS
will be awarded for the sale will provide for liquidated dam-
ages to be assessed against the purchaser if any such unautho-
rized harvest were to occur.”

   [10] Like the district court, we hold that these declarations
sufficiently indicate that Forest Service employees marked the
trees, as required by 16 U.S.C. § 472a(g), and that the mark-
ing complied with NFMA’s and the applicable Ochoco
LRMP’s requirements.

  C.    Connective Habitat Corridors

  Finally, LOWD alleges that the Project fails to maintain
connective habitat corridors in the planning area, as required
by the NFMA5 and the applicable Ochoco LRMP. LOWD
points to the fact that many of the connective corridors in the
Project planning area already fail to meet the Ochoco LRMP,
and that further timber harvest in those corridors will only
exacerbate the problem. The Forest Service responds that
LOWD incorrectly focuses the inquiry on each corridor as a
whole rather than on the particular stands within a connective
corridor.

   The Ochoco LRMP, as amended by the Eastside Screens
interim standards, directs the Forest Service, in pertinent part,
to:

         (a) Maintain or enhance the current level of con-
      nectivity between LOS stands and between all Forest
      Plan designated “old growth/MR” habitats by main-
      taining stands between them that serve the purpose
      of connection as described below:

          (1) Network pattern — LOS stands and MR/
      Old Growth habitats need to be connected with each
  5
   See 16 U.S.C. § 1604(i) (requiring consistency between site-specific
projects and the applicable LRMP).
           LEAGUE OF WILDERNESS DEFENDERS v. USFS          16225
    other inside the watershed as well as to like stands
    in adjacent watersheds in a contiguous network pat-
    tern by at least 2 different directions.

         (2) Connectivity Corridor Stand Description
    — Stands in which medium diameter or larger trees
    are common, and canopy closures are within the top
    one-third of site potential. Stand widths should be at
    least 400 ft. wide at their narrowest point. The only
    exception to stand width is when it is impossible to
    meet 400 ft with current vegetative structure, AND
    these “narrower stands” are the only connections
    available; (use them as last resorts).

Standard 6(d)(3)(a)(1)-(2). The Ochoco LRMP explains that
“[h]arvesting within connectivity corridors is permitted if all
the criteria in [subparagraph] (2) can be met, and if some
amount of understory (if any occurs) is left in patches or scat-
tered to assist in supporting stand density and cover.” Stan-
dard 6(d)(3)(a)(4). The Ochoco LRMP also states that, “[t]o
reduce fragmentation of LOS stands, or at least not increase
it from current levels, stands that do not currently meet LOS
that are located within, or surrounded by, blocks of LOS
stands should not be considered for even-aged regeneration,
or group selection at this time.” Standard 6(d)(3)(a)(4)(b).

   Meanwhile, the FSEIS states that, “[o]f the 3,966 acres
identified in connectivity corridors, 23% currently are in the
upper one-third of site potential,” meaning that 77% of the
planning does not comply with the connectivity requirements.
The Forest Service—interpreting the language of Standard
6(d)(3)(a)(2) to mean that “harvesting within a particular
stand is permissible whe[n] the stand has a canopy closure
within the top one-third of site potential and if the stand is at
least 400 feet at its narrowest point”—asserts that all of the
individual stands proposed for harvest in the Project, regard-
less of which corridor they are in, meet the necessary criteria.
LOWD, by contrast, interprets Standard 6(d)(3)(a)(2) to mean
16226      LEAGUE OF WILDERNESS DEFENDERS v. USFS
that all stands within the corridor as a whole must be in the
top one-third of site potential for harvesting in that corridor.
Accordingly, LOWD argues, “[b]ecause none of the corridors
meet the requirement, none of the timber stands—which com-
prise the corridors—meet the connectivity corridor require-
ments.”

   We find the Forest Service’s interpretation persuasive for
the following reasons. First, as noted above, an agency’s
interpretation of its own regulations is entitled to substantial
deference, and our review is limited to ensuring that the agen-
cy’s interpretation is not “plainly erroneous or inconsistent
with the regulation.” 

(internal quota-
tion marks omitted). Because Standard 6(d)(3)(a)(2) refers to
“Stand” or “Stands” instead of every stand or all stands, and
because Standard 6(d)(3)(a)(4) refers to “[h]arvesting within
connectivity corridors”—as opposed to “harvesting a connec-
tivity corridor” or simply “harvesting connectivity corridors”
—the Forest Service’s interpretation does not appear plainly
erroneous or inconsistent. Rather, LOWD’s interpretation
appears plainly erroneous, as it would prohibit harvesting in
any watershed governed by the connectivity requirements, an
impractical result almost certainly unintended by the Forest
Service.

   Second, the FSEIS relies on data collection and findings
made by the Deep Creek EIS Interdisciplinary Team, which
identified and mapped the connection acreage, assessed the
harvesting proposed within each corridor, and certified that
the silvicultural prescriptions will ensure that the stands
remain within the top one-third of their site potential.

   [11] Finally, the Forest Service points out—and LOWD
does not dispute—that no change in the connectivity-
compliance percentages will result from implementation of
the Project, thereby fulfilling the connectivity requirements’
directive that the agency “maintain” connective corridors. See
Standard 6(d)(3)(a). As a result, the district court properly
          LEAGUE OF WILDERNESS DEFENDERS v. USFS       16227
concluded that the Project does not violate the connective-
corridor requirements of NFMA and the Ochoco LRMP.

                      CONCLUSION

   The Forest Service’s approval of the Project violates NEPA
because the FSEIS may not tier to the non-NEPA Watershed
Analysis to consider adequately the aggregate cumulative
effects of past timber sales. We reverse the district court’s
grant of summary judgment in favor of the Forest Service,
and we remand this case so the agency can reissue its NEPA
documentation to include the omitted information regarding
past timber sales contained in the Watershed Analysis. Each
party shall bear its own costs on appeal.

  REVERSED and REMANDED.