On the database side, the best approach is often to store attributes common to all forests in one table, and to store unique attributes in other tables. Build updatable views for clients to use.
create table forests (
forest_id integer primary key,
-- Assumes forest names are not unique on a continent.
forest_name varchar(45) not null,
forest_type char(1) not null
check (forest_type in ('c', 'd')),
area_sq_km integer not null
check (area_sq_km > 0),
-- Other columns common to all forests go here.
--
-- This constraint lets foreign keys target the pair
-- of columns, guaranteeing that a row in each subtype
-- table references a row here having the same subtype.
unique (forest_id, forest_type)
);
create table coniferous_forests_subtype (
forest_id integer primary key,
forest_type char(1) not null
default 'c'
check (forest_type = 'c'),
pine_fir_ratio float not null
check (pine_fir_ratio >= 0),
foreign key (forest_id, forest_type)
references forests (forest_id, forest_type)
);
create table deciduous_forests_subtype (
forest_id integer primary key,
forest_type char(1) not null
default 'd'
check (forest_type = 'd'),
autumn_duration_days integer not null
check (autumn_duration_days between 20 and 100),
foreign key (forest_id, forest_type)
references forests (forest_id, forest_type)
);
Clients usually use updatable views, one for each subtype, instead of using the base tables. (You can revoke privileges on the base subtype tables to guarantee this.) You might want to omit the "forest_type" column.
create view coniferous_forests as
select t1.forest_id, t1.forest_type, t1.area_sq_km,
t2.pine_fir_ratio
from forests t1
inner join coniferous_forests_subtype t2
on t1.forest_id = t2.forest_id;
create view deciduous_forests as
select t1.forest_id, t1.forest_type, t1.area_sq_km,
t2.autumn_duration_days
from forests t1
inner join deciduous_forests_subtype t2
on t1.forest_id = t2.forest_id;
What you have to do to make these views updatable varies a little with the dbms, but expect to write some triggers (not shown). You'll need triggers to handle all the DML actions--insert, update, and delete.
If you need to report only on columns that appear in "forests", then just query the table "forests".