In the Philippine legal system, all lands of the public domain belong to the State. This is the Regalian Doctrine, a fundamental principle enshrined in Section 2, Article XII of the 1987 Constitution. Under this doctrine, any land not clearly appearing to be within the context of private ownership is presumed to belong to the State.

For public land to be subject to private appropriation or ownership, it must first be declared Alienable and Disposable (A&D). Until such a formal classification occurs, the land remains part of the inalienable public domain (such as forest lands, mineral lands, or national parks) and cannot be acquired through prescription or administrative legal titles.

I. The Primary Authority: The President of the Philippines

The power to classify or reclassify lands of the public domain is an executive prerogative. Historically and legally, the President holds the ultimate authority to declare lands as alienable and disposable.

Commonwealth Act No. 141 (The Public Land Act): Under Sections 6 and 7 of this Act, the President is empowered to classify lands of the public domain into alienable or disposable, timber, and mineral lands.
Administrative Naturalization: The President exercises this power through Executive Orders or Proclamations, often acting upon the recommendation of the relevant administrative agency.

II. Delegated Authority: The DENR Secretary

While the President holds the ultimate power, the day-to-day administrative authority to classify lands has been delegated to the Secretary of the Department of Environment and Natural Resources (DENR).

Section 13 of Presidential Decree No. 705 (Revised Forestry Code): This law mandates that the DENR Secretary shall study, devise, determine, and identify which lands of the public domain are needed for forest purposes and which are available for agriculture (A&D).
DENR Administrative Order (DAO) No. 2019-08: This clarifies that the Secretary (or their authorized representative) has the authority to certify whether a specific parcel of land is A&D based on existing land classification maps.

III. The Role of the Legislative Branch

While the Executive branch classifies land, the Congress of the Philippines holds the power to determine the specific limits of forest lands and national parks. Once Congress passes a law defining the boundaries of a forest reserve, the Executive branch cannot unilaterally declare portions of that specific area as A&D without a subsequent act of Congress or a specific delegation of power.

IV. Requisites for a Valid Classification

For a parcel of land to be legally considered Alienable and Disposable, the following "Positive Acts" of the government must be proven:

A Presidential Proclamation or Executive Order: Declaring the land open to disposition.
A Legislative Act: In specific instances where Congress has reserved the land for a particular purpose.
A Certification of A&D Status: Issued by the DENR. In recent jurisprudence (notably Republic v. T.A.N. Properties), the Supreme Court ruled that a mere certification from a Community Environment and Natural Resources Officer (CENRO) is insufficient. It must be accompanied by a certified true copy of the original classification plan approved by the DENR Secretary.

V. Key Legal Principles and Jurisprudence

Presumption of State Ownership: The burden of proof to overcome the presumption that land is public forest land rests on the applicant. They must prove a "positive act" of the government.
Non-Prescription against the State: No matter how long a person occupies a piece of land (even for 50 or 100 years), if the land is not yet classified as A&D, the period of possession does not count toward the acquisition of title. Prescription only begins to run once the land is declared A&D.
The Watershed Rule: Lands situated in "critical watersheds" or those with a slope of 18% or more are generally restricted from being classified as alienable and disposable to preserve ecological balance and prevent erosion.

VI. Summary Table of Authority

Authority	Legal Basis	Scope of Power
The President	Constitution / C.A. No. 141	Ultimate power to classify and declare A&D via Proclamation.
DENR Secretary	P.D. No. 705 / E.O. 192	Administrative authority to identify, survey, and certify land status.
Congress	1987 Constitution	Power to set specific boundaries of forest lands and national parks.
CENRO / PENRO	Administrative Delegations	Issuance of initial certifications (subject to strict verification of Secretary-approved maps).