XGBoost Tree-Generated Graph Construction

• XGBoost-based tree-generated graph construction is a method that organizes decision tree ensembles into a directed acyclic graph for distributed message passing and hierarchical representation learning.
• The approach reinterprets RandomForest and GBDT as structured node layers with edge weights, enabling explicit flow of residuals and enhanced model explainability.
• Empirical findings suggest that using small graph widths achieves competitive performance while facilitating fine-tuning and integration with graph neural network operations.

XGBoost-based tree-generated graph construction is a formal methodology by which decision tree ensembles—specifically those derived from XGBoost or similar gradient boosting frameworks—are organized into directed acyclic graphs that encode their computational and data-flow structure. This approach is centered on the Distributed Gradient Boosting Forest (DGBF) formalism, under which traditional bagging (RandomForest), boosting (GBDT), and XGBoost are unified as different motifs of a general graph-structured tree-ensemble. DGBF provides distributed representation learning between trees naturally, without requiring back-propagation, and permits downstream graph-neural-network (GNN) operations on the resulting graph. This methodology allows explicit hierarchical organization and facilitates new forms of analysis and learning, including representation aggregation, node-level explainability, and fine-tuning by message passing.

1. Mathematical Foundations of DGBF and XGBoost

The XGBoost-based tree-generated graph construction builds upon two canonical tree ensemble methods: bagging and boosting. Bagging is typified by RandomForest, where $T$ CARTs ($\{h_1,\ldots,h_T\}$) are independently trained on bootstrap subsets and combined by a simple arithmetic mean,

$$F_{\mathrm{RF}}(x) = \frac{1}{T} \sum_{t=1}^T h_t(x)$$

with each $h_t$ obtained via minimization over its bootstrap sample: $$h_t = \arg\min_{h \in \mathrm{CART}} \sum_{i \in \mathcal{B}_t} L(y_i, h(x_i))$$ Boosting, as in gradient-boosted decision trees (GBDT), involves sequentially training trees $\{g_1,\ldots,g_L\}$ where each tree fits pseudo-residuals: $r_i^{(\ell)} = y_i - F_{\ell-1}(x_i)$

$$g_\ell = \arg\min_{g \in \mathrm{CART}} \sum_{i=1}^n \left(r_i^{(\ell)} - g(x_i)\right)^2$$

with overall prediction

$F_{\mathrm{GBDT}}(x) = \sum_{\ell=1}^L g_\ell(x)$

DGBF generalizes by training not a single tree per boosting step, but a forest of $T$ trees per each boosting layer $\ell$, distributing the residual to all trees in that layer: $F_0(x) = \bar{y} = \frac{1}{n}\sum_i y_i$

$$F_\ell(x) = F_{\ell-1}(x) + \frac{1}{T} \sum_{t=1}^T h_{\ell,t}(x)$$

Each tree $h_{\ell,t}$ at layer $\ell$ receives and fits the full residual $g_i^{(\ell)} = y_i - F_{\ell-1}(x_i)$, solved over a (growing) data subsample $\mathcal{S}_{\ell,t}$: $$h_{\ell,t} = \arg\min_{h \in \mathrm{CART}} \sum_{i \in \mathcal{S}_{\ell,t}} (g_i^{(\ell)} - h(x_i))^2$$ RandomForest and GBDT are recovered in limiting cases: $L=1$, $T \gg 1$ yields RandomForest; $T=1$, $L \gg 1$ recovers GBDT. Generally, DGBF forms an $L \times T$ lattice of trees.

2. Tree Ensemble as Directed Graph: Nodes and Edges

The DGBF formulation regards the $L \times T$ trees as nodes in a directed acyclic graph (DAG).

• Nodes:
  • The graph contains a single input node $v_0$ encoding the mean target value $F_0(x)$.
  • Each tree $h_{\ell,t}$ in layer $\ell$ forms a node $v_{\ell,t}$, carrying CART function parameters: split rule indices, leaf values, and optionally learning-rate $\eta$.
• Edges:
  • Directed edges are drawn between all nodes in layer $\ell-1$ to every node in layer $\ell$. Explicitly, for each $t=1\ldots T$,

  $$\forall t' \in 1\ldots T,\;\;\; v_{\ell-1, t'} \to v_{\ell, t}$$ - Edge attributes may annotate sample subsampling schedules or local learning-rates $\lambda_{\ell,t}$. - Edges encode the flow of pseudo-residuals from upstream to downstream trees.

The resulting graph is dense (all-to-all between layers), but in the special case $T=1$ (pure XGBoost), it reduces to a simple chain of nodes,

$v_0 \to v_1 \to v_2 \to \ldots \to v_L$

with node $v_\ell$ corresponding to the $\ell$th tree $g_\ell$ and edge weight $\eta_\ell$.

Pseudocode for graph construction:

Input: trained XGBoost/GBDT model = sequence of trees [h1,…,hM] choose L, T so that L × T = M Let G = (V, E) be empty graph add node v0 with attribute F0(x) = mean(y) for l in 1…L: for t in 1…T: tree_index = (l-1)*T + t instantiate node v_{l,t}, store CART parameters (splits, leaf values, η) add v_{l,t} to V for each u in layer l–1: # layer 0 is v0 add directed edge (u→v_{l,t}) with weight η/T return G

3. Message-Passing, Distributed Representation, and Hierarchy Learning

DGBF dispenses with iterative back-propagation by replacing it with forward message-passing: residual targets are computed and dispatched in a one-shot fashion to all trees at a given layer.

For each $(\ell, t)$, the arriving message for node $v_{\ell, t}$ consists of all $(x_i, g_i^{(\ell)})$, and the tree $h_{\ell,t}$ is refit accordingly. There is no chain-rule gradient descent; instead, distributed representation emerges by the lattice of trees aggregating and propagating predictions. The forward pass computes predictions as in GBDT: $$F_L(x) = F_0(x) + \sum_{\ell=1}^L \frac{1}{T}\sum_{t=1}^T h_{\ell,t}(x)$$ A plausible implication is that, by recasting tree ensembles as computation graphs, one may deploy GNN-style modules (aggregation, attention, message passing) for end-to-end learning or representational refinement atop tree outputs.

4. Construction from XGBoost Trees to Forest-Graph

The process for turning a trained XGBoost or GBDT model into a DGBF-style computation graph follows:

1. Train an XGBoost/GBDT model for $M$ boosting rounds, yielding trees $\{g_1,\ldots,g_M\}$.
2. Partition trees into layers: choose width $T$ (XGBoost corresponds to $T=1$), and set $L=\lceil M/T\rceil$.
3. Instantiate nodes $v_0, v_1,\ldots,v_L$ (each with $T$ trees except possibly the final layer) with tree parameters as node attributes.
4. For each node in layer $\ell-1$, add directed edges to all nodes in layer $\ell$, weighted by $\eta/T$.
5. Optionally, attach further node features (sample indices, histogram statistics) for subsequent GNN processing.

The resulting graph $G = (V,E)$ may be consumed by graph neural network architectures to yield node-level or global representations, or to enable fine-tuning and multimodal fusion.

Typical uses for the forest-graph include:

• Explaining individual tree outputs via neighboring influences.
• Fine-tuning leaf weights by learnable message-passing.
• Integration of tree-ensemble outputs with other data modalities.

5. Specialization to Standard Ensembles and Computational Properties

The DGBF graph architecture subsumes both RandomForest and XGBoost as particular cases:

• RandomForest: $L=1$, $T=M$ yields a single layer of $T$ trees, matching the bagging formulation.
• GBDT/XGBoost: $T=1$, $L=M$ produces a chain of $M$ nodes, exactly reflecting the boosting iteration structure.

Complexity analysis is as follows:

Model	Training Cost	Node Count	Edge Count
DGBF (L×T)	$O(LTn\log n)$	$LT+1$	$T^2(L-1)+T$
XGBoost (T=1)	$O(Mn\log n)$	$M+1$	$M$
RF (L=1)	$O(Tn\log n)$	$T+1$	$T$

Empirically, small $T$ (5–20) DGBF models match or outperform RF/GBDT in 7 out of 9 datasets, incurring only modest cost increases.

6. Applications and Extensions

The primary construction allows direct application of graph-based learning methods: graph embeddings, attention modules, and end-to-end gradient-based fine-tuning. Leaf weights of tree-nodes can be refined through message-passing layers, and outputs from distributed tree graphs can be concatenated or fused with representations from other modalities (text, images) in a unified GNN framework. Explanatory analyses may be performed by examining the neighborhood structure of tree-nodes, quantifying the flow and aggregation of prediction and residual information.

A plausible implication is that encoding tree ensembles as computation graphs facilitates hierarchical representation learning more akin to deep neural networks, while preserving the interpretability and structural robustness of decision tree models. This encoding also permits further research into hybrid architectures, dynamic sampling, and non-backpropagation learning for unstructured data.

7. Conceptual Significance

By mapping decision tree ensembles, specifically those generated via XGBoost, onto directed computation graphs, the DGBF construction provides a rigorous formalism to unify RandomForest, GBDT, and XGBoost as message-passing architectures. This approach clarifies their functional relationships, allows explicit hierarchical and distributed representation learning, and opens avenues for GNN-style operations previously unavailable for standard tree ensembles. The avoidance of back-propagation, replaced by direct residual messaging, yields a streamlined and interpretable learning dynamic, and the method provides empirical performance improvements on benchmark datasets for small-to-moderate graph widths.