> ## Documentation Index
> Fetch the complete documentation index at: https://pyfia.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# Forest change

> Forest area change over time using the subplot condition-change matrix and remeasurement linkages.

Change analysis links current and previous conditions through `PLOT.PREV_PLT_CN` and the `SUBP_COND_CHNG_MTRX` table, using a change evaluation (EVALID ending in `03`).

## Missouri — forest area change by type group

<Info>**EVALID:** 292403 (Missouri 2024 change) · **Focus:** net forest area by type group (oak/hickory dominates at \~11.8M acres) · [source](https://github.com/mihiarc/pyfia/blob/main/reference/queries/forest_change/missouri_forest_type_change.sql)</Info>

The change matrix tracks the proportion of each condition that changed between measurements (`SUBPTYP_PROP_CHNG`, divided by 4 for a per-plot basis).

<Accordion title="View SQL">
  ```sql theme={null}
  SELECT
      GRP1,
      GRP2,
      SUM(ESTIMATED_VALUE * EXPNS) AS ESTIMATE
  FROM (
      SELECT
          pop_stratum.estn_unit_cn,
          pop_stratum.cn AS STRATACN,
          plot.cn AS plot_cn,
          plot.prev_plt_cn,
          cond.cn AS cond_cn,
          plot.lat, plot.lon,
          pop_stratum.expns AS EXPNS,
          '`' || printf('%04d', COALESCE(REF_FORTYGPCDSQ.FORTYGPCD, 999)) || ' ' ||
          COALESCE(REF_FORTYGPCDSQ.MEANING, 'Nonstocked') AS GRP1,
          CASE COALESCE(pop_stratum.evalid, -1)
              WHEN 292403 THEN '`0029 292024 Missouri 2024'
              ELSE '`99 Unavailable'
          END AS GRP2,
          SUM(COALESCE(
              SCCM.SUBPTYP_PROP_CHNG / 4 *
              CASE cond.PROP_BASIS
                  WHEN 'MACR' THEN pop_stratum.ADJ_FACTOR_MACR
                  ELSE pop_stratum.ADJ_FACTOR_SUBP
              END, 0
          )) AS ESTIMATED_VALUE
      FROM POP_STRATUM pop_stratum
      JOIN POP_PLOT_STRATUM_ASSGN pop_plot_stratum_assgn ON pop_plot_stratum_assgn.STRATUM_CN = pop_stratum.CN
      JOIN PLOT plot ON pop_plot_stratum_assgn.PLT_CN = plot.CN
      JOIN COND cond ON cond.PLT_CN = plot.CN
      JOIN COND pcond ON pcond.PLT_CN = plot.PREV_PLT_CN
      JOIN SUBP_COND_CHNG_MTRX sccm
          ON sccm.PLT_CN = cond.PLT_CN
          AND sccm.PREV_PLT_CN = pcond.PLT_CN
          AND sccm.CONDID = cond.CONDID
          AND sccm.PREVCOND = pcond.CONDID
      LEFT JOIN (
          SELECT ref_forest_type.VALUE AS FORTYPC,
                 ref_forest_type_group.VALUE AS FORTYGPCD,
                 ref_forest_type_group.MEANING
          FROM REF_FOREST_TYPE ref_forest_type
          JOIN REF_FOREST_TYPE_GROUP ref_forest_type_group
              ON ref_forest_type.TYPGRPCD = ref_forest_type_group.VALUE
      ) REF_FORTYGPCDSQ ON REF_FORTYGPCDSQ.FORTYPC = cond.fortypcd
      WHERE
          cond.CONDPROP_UNADJ IS NOT NULL
          AND ((sccm.SUBPTYP = 3 AND cond.PROP_BASIS = 'MACR')
               OR (sccm.SUBPTYP = 1 AND cond.PROP_BASIS = 'SUBP'))
          AND COALESCE(cond.COND_NONSAMPLE_REASN_CD, 0) = 0
          AND COALESCE(pcond.COND_NONSAMPLE_REASN_CD, 0) = 0
          AND cond.COND_STATUS_CD = 1
          AND pcond.COND_STATUS_CD = 1
          AND pop_stratum.rscd = 23
          AND pop_stratum.evalid = 292403
      GROUP BY
          pop_stratum.estn_unit_cn, pop_stratum.cn, plot.cn, plot.prev_plt_cn,
          cond.cn, plot.lat, plot.lon, pop_stratum.expns,
          REF_FORTYGPCDSQ.FORTYGPCD, REF_FORTYGPCDSQ.MEANING, pop_stratum.evalid
  ) subquery
  GROUP BY GRP1, GRP2
  ORDER BY GRP1, GRP2;
  ```
</Accordion>

<Tip>The Python equivalent is [`area_change()`](/api/pyfia-estimation-estimators-area_change).</Tip>