diff --git a/phpstan.dist.neon b/phpstan.dist.neon
index 139627b..f7afeec 100644
--- a/phpstan.dist.neon
+++ b/phpstan.dist.neon
@@ -4,3 +4,4 @@ parameters:
         - src
     typeAliases:
         JsonType: 'string|int|bool|float|null|array<string, mixed>|array<mixed>'
+    treatPhpDocTypesAsCertain: false
diff --git a/src/Schema/StandardRepresentativeTree.php b/src/Schema/StandardRepresentativeTree.php
index f462563..0296bb1 100644
--- a/src/Schema/StandardRepresentativeTree.php
+++ b/src/Schema/StandardRepresentativeTree.php
@@ -26,7 +26,7 @@ class StandardRepresentativeTree
 
 
     /**
-     * trees per hectare
+     * Volume per hectare
      * @var float
      */
     public $volumePerHa;
diff --git a/src/Schema/StandardTree.php b/src/Schema/StandardTree.php
index b7b72d4..36acab9 100644
--- a/src/Schema/StandardTree.php
+++ b/src/Schema/StandardTree.php
@@ -8,20 +8,25 @@ class StandardTree
 {
     /**
      * species ID
-     * todo create common list
+     * @todo create common list
+     * @todo How about the 3-letter codes? Then we do not need to rely on IDs
+     * into a common list.
      * @var int
      */
     public $speciesId;
 
     /**
-     * height of this tree
-     * if this is a representative tree, thisshould correspond to the mean height
+     * Height of this tree in metres.
+     *
+     * If this is a representative tree, this is the mean height of the representative tree.
      * @var float
      */
     public $height;
+
     /**
-     * dbh of this tree
-     * if this is a representative tree, thisshould correspond to the mean dbh
+     * dbh of this tree in metres.
+     *
+     * If this is a representative tree, this is the mean dbh of the representative tree.
      * @var float
      */
     public $dbh;
@@ -36,10 +41,10 @@ class StandardTree
      * volume calculation method: eg blue book look up
      * @var string
      */
-    public $volumeCalculationethod;
+    public $volumeCalculationMethod;
 
     /**
-     * @see    Create the Tree Species.
+     * Create the Standard Tree.
      * @param  int $speciesId     The species ID.
      * @return void
      */
diff --git a/src/Schema/StandardTreeDistributionStats.php b/src/Schema/StandardTreeDistributionStats.php
index 248b016..3f3d6de 100644
--- a/src/Schema/StandardTreeDistributionStats.php
+++ b/src/Schema/StandardTreeDistributionStats.php
@@ -7,37 +7,53 @@
 class StandardTreeDistributionStats
 {
     /**
-     * height range
-     * @var float
+     * The height range of the StandardTree expressed as a 2-tuple of floats
+     * in the order [min,max]
+     *
+     * @var array{float,float}
      */
     public $heightRange;
 
     /**
-     * dbh range
-     * @var float
+     * The dbh range of the StandardTree expressed as a 2-tuple of floats
+     * in the order [min,max]
+     *
+     * @var array{float,float}
      */
     public $dbhRange;
 
     /**
-     * height variance
+     * The height variance. This is the variance of the mean defined in
+     * the linked StandardTree.
      * @var float
      */
     public $heightVariance;
 
     /**
-     * dbh variance
+     * The dbh variance. This is the variance of the mean defined in
+     * the linked StandardTree.
      * @var float
      */
     public $dbhVariance;
 
     /**
-     * @see    Create the Tree distribution statistics.
-     * @param  float $heightRange     The Tree distribution height range.
+     * Create the Tree distribution statistics.
+     *
+     * @param array{float,float} $heightRange The Tree distribution height range.
+     * @param array{float,float} $dbhRange    The Tree distribution dbh range.
      * @return void
      */
-    public function __construct(float $heightRange)
+    public function __construct(array $heightRange, array $dbhRange)
     {
+        if (count($heightRange) != 2) {
+            throw new \Exception("heightRange must be an array of two floats");
+        }
         $this->heightRange = $heightRange;
+
+        if (count($dbhRange) != 2) {
+            throw new \Exception("heightRange must be an array of two floats");
+        }
+        $this->dbhRange = $dbhRange;
     }
 
 }